Chapter 12: Photosynthesis
In the early 1770s, an English chemist named Joseph Priestley sealed a burning candle inside a closed glass jar and waited. The flame flickered and died. When he placed a small mouse in that same trapped air, the animal quickly became distressed. Something in the air, Priestley reasoned, had been “injured” or used up, and once injured it could no longer support either a flame or an animal.
Then he tried something that changed biology forever. He placed a sprig of green mint inside a similar jar of spoiled air and left it for several days. When he came back and tested the jar again, a candle could burn in it once more, and a mouse could breathe in it. The plant had somehow repaired the air.
Priestley could not explain how. A few years later, Jan Ingenhousz showed that the plant only restored the air when it was kept in light — in darkness, green leaves did nothing of the kind. Light, it turned out, was the missing ingredient.
Those simple jar experiments were the first glimpse of photosynthesis: the process by which a green leaf takes an invisible gas, ordinary water and sunlight, and quietly builds the food and the oxygen that almost every living thing on earth depends on.
Diagram
Fig. 12.0 – Priestley’s bell-jar experiment: left jar with a burning candle and a mouse alone (flame out, mouse distressed), right jar with a sprig of green mint plus candle and mouse in sunlight (flame burning, mouse active); label jar, candle, mouse, mint sprig, sunlight
Keep these questions in mind as you read — this chapter will answer every one of them.
12.1 Chloroplast
What Photosynthesis Means
Every living thing needs energy — to grow, to move, to repair itself, even to stay alive while resting. Animals get that energy by eating. Green plants do something far more remarkable: they manufacture their own food out of simple inorganic raw materials, using nothing but sunlight as the energy source.
Photosynthesis is the only process on earth by which solar energy is trapped by green plants and converted into food. It is the doorway through which the sun’s energy enters the living world.
The word itself explains the idea: photo means light, and synthesis means building up. So photosynthesis is a “building-up with light” reaction, in which small, energy-poor molecules are joined into a large, energy-rich one.
The overall reaction is written as:
\[6CO_2 + 12H_2O \xrightarrow[\text{Chloroplast}]{\text{Light}} C_6H_{12}O_6 + 6O_2 + 6H_2O\]
Read the equation carefully, because every part of it matters. Six molecules of carbon dioxide and twelve molecules of water are used up. One molecule of glucose (\(C_6H_{12}O_6\)) is built, six molecules of oxygen are released, and six molecules of water are formed afresh. Light is not a reactant that gets consumed into the product — it is the energy input, and the chloroplast is the site where the whole reaction is carried out, which is why both are written on the arrow.
This process is unique to green plants and is the final light-energy-trapping process on which all life ultimately depends. Whether an organism is a grasshopper, a tiger or a human being, the energy in its food can be traced back to a green leaf. It is one of the most massive chemical processes going on earth.
The Scale of the Process
The atmosphere contains only about 0.03 percent carbon dioxide by volume. That sounds negligible, but the earth’s atmosphere is enormous — this small percentage represents 2200 billion tons of \(CO_2\).
The oceans hold even more. They contain over 50 times the amount of atmospheric \(CO_2\), present in the form of dissolved gas or as carbonates.
From these two sources together, about 70 billion tons of carbon is fixed by green plants annually. “Fixed” means locked from a free gaseous form into stable organic molecules — the carbon of the air being converted into the carbon of living tissue.
Where Photosynthesis Happens
Photosynthesis is carried out inside a specialised cell organelle, the chloroplast. Chloroplasts are mainly located in the mesophyll cells of leaves — the soft green tissue sandwiched between the upper and lower epidermis.
The two raw materials reach these cells by two separate routes. The \(CO_2\) reaches them through the stomata, the tiny pores on the leaf surface, while water reaches them through the veins, which are the vascular bundles running through the leaf.
Inside the Chloroplast
In higher plants, the chloroplasts are discoid or lens-shaped. Each chloroplast is bounded by a double membrane — an external membrane and an internal membrane, separated by a narrow intermembrane space.
Inside the membranes is found a ground substance, the stroma. This is a semi-fluid matrix that holds enzymes, ribosomes, chloroplast DNA and plastoglobules, and it is here that sugar is finally assembled.
Inside the stroma is found a system of chlorophyll-bearing, double-membrane sacs or lamellae. These are stacked one above the other to form grana (singular, granum). Individual sacs in each granum are known as thylakoids, and the space enclosed within a thylakoid is called the lumen. This ultrastructure is shown in Fig. 12.1.
Diagram
Fig. 12.1 – Ultrastructure of chloroplast, labelling external membrane, internal membrane, intermembrane space, stroma, granum, thylakoid, thylakoid membrane, lumen, lamella, stroma lamellae, chloroplast DNA, ribosome and plastoglobule
The stacking is not decoration. Piling thylakoids into grana packs a very large membrane area into a very small organelle, and since the pigments sit in those membranes, more membrane means more light-catching surface facing the sun.
The internal organisation of a chloroplast follows a clear hierarchy, summarised in FC 12.1.
Flowchart
FC 12.1 – Structural hierarchy of the chloroplast: Chloroplast → double membrane (external + internal) → stroma → grana (stacks) → thylakoids → thylakoid membrane → pigment molecules
Photosynthetic Pigments
All the pigments — chlorophylls, carotenes and xanthophylls — are located in the thylakoid membranes. A pigment is simply a molecule that absorbs some wavelengths of light and reflects the rest, which is why it appears coloured.
These pigments absorb light of specific wavelengths in the visible region. Each pigment has its own preferred wavelengths, so a mixture of pigments lets the plant harvest a wider slice of sunlight than any one pigment could manage alone.
The pigments are fat soluble and are located in the lipid part of the membrane. Because the thylakoid membrane is itself largely lipid, the pigments dissolve neatly into it and stay firmly anchored in place instead of drifting away into the watery stroma.
With the help of certain enzymes, they participate in the conversion of solar energy into ATP and \(NADPH_2\). The enzymes of stroma then utilize ATP and \(NADPH_2\) to produce carbohydrates. In other words, the membrane converts light into two portable chemical carriers, and the stroma spends them.
Flowchart
FC 12.2 – Energy handover inside the chloroplast: sunlight → pigments in thylakoid membrane → ATP and \(NADPH_2\) formed → carried into stroma → stromal enzymes → carbohydrate
Two Predominant Types of Chlorophylls
Chlorophyll a and b differ in the nature of side groups. Chlorophyll a has a methyl group (\(-CH_3\)) while chlorophyll b has an aldehyde group (\(-CHO\)). This single difference in one side group is enough to shift the wavelengths each one absorbs best — which is precisely why having both is useful to the plant.
Chemically, a chlorophyll molecule consists of two parts: a head, the tetrapyrrole Porphyrin ring, and a long hydrocarbon tail called phytol attached to the porphyrin group. At the centre of the porphyrin head sits a magnesium atom, and it is this light-absorbing head that actually captures photons.
Carotenoids: The Accessory Pigments
Carotenoids are lipid compounds present universally in almost all the higher plants and several micro-organisms. They are usually red, orange, yellow and brown, and are associated with chlorophyll.
They are of two types — the carotenes and xanthophylls. The carotenes (\(C_{40}H_{56}\)) are orange red, and xanthophylls contain oxygen. Notice that the two differ only by the presence of oxygen in the molecule, yet that is enough to change the shade of colour.
The light energy absorbed by the carotenoids is transferred to chlorophyll a to be utilized in photosynthesis. Carotenoids therefore do not carry out photosynthesis themselves; they act as collectors that pass their catch on to chlorophyll a.
All photosynthetic plants have these pigments that absorb light between the red and blue region of the spectrum. Carotenoids found mainly in higher plants absorb primarily in the violet to blue regions of the spectrum.
They not only absorb light energy and transfer it to chlorophyll but also protect the chlorophyll molecules from photo-oxidation. In very strong light, chlorophyll can absorb more energy than it can safely pass on, and that excess energy can chemically destroy the molecule. Carotenoids soak up and safely dissipate this surplus, acting rather like a sunscreen for the pigment system.
12.2 Nature of Light
Since photosynthesis runs on sunlight, it is worth pausing to ask what light actually is before we ask how a leaf uses it.
Light as Particles and as Waves
Light is a form of electromagnetic radiation. Interestingly, it behaves in two different ways depending on how it is studied, and both descriptions are needed to explain photosynthesis.
According to quantum theory, light travels as a stream of tiny particles called photons. A photon contains a quantum of light — a quantum being the smallest indivisible packet of energy that light can be delivered in. A pigment molecule cannot absorb half a photon; it absorbs one whole packet or none at all.
According to wave theory, light travels in the form of waves of different wavelengths having different colours. Wavelength is the distance between two successive crests of the wave, and it is measured in nanometres (nm), where \(1\ nm = 10^{-9}\ m\).
The two pictures fit together simply: shorter wavelengths carry more energy per photon, longer wavelengths carry less. Blue light is therefore more energetic than red light.
One can see electromagnetic radiation with wavelengths ranging from 390 nm to 730 nm. This part of the spectrum is called the Visible light. It lies between the wavelengths of ultraviolet and infra-red, as shown in Fig. 12.2 — with gamma rays and X-rays at the very short, high-energy end and radio waves at the very long, low-energy end.
Diagram
Fig. 12.2 – Electromagnetic spectrum of light: full range from gamma rays, X-rays, ultraviolet, visible, infrared to radio waves, with the visible band expanded from 400 nm to 700 nm and arrows marking increasing energy and increasing wavelength
Absorption and Action Spectrum
All the photosynthetic pigments have the ability to absorb different wavelengths of light. The light absorbed at each wavelength by different pigments can be shown in the form of a graph. Such a graph, showing the absorption of light at different wavelengths by different pigments, is termed as Absorption spectrum. One such graph is given as Graph 12.3.
Diagram
Graph 12.3 – Absorption spectrum: wavelength of light (400–700 nm) on the x-axis and absorption of light by pigment on the y-axis, with separate curves labelled Chlorophyll a, Chlorophyll b and Carotenoids
The absorption spectrum of chlorophyll a and b clearly shows that more light is absorbed at the blue, violet and red wavelengths of the visible spectrum. Between these two peaks, in the green region, absorption drops sharply — the green light is largely reflected and transmitted instead of being absorbed, and that reflected light is what reaches our eyes.
Now a second, quite different graph can be drawn. Instead of asking how much light a pigment soaks up, we ask how fast photosynthesis actually runs when the plant is given one wavelength at a time. A graph showing the rate of photosynthesis at different wavelengths of light is called Action Spectrum, plotted in Graph 12.4.
Diagram
Graph 12.4 – Action spectrum for photosynthesis: wavelength of light in nm (400–700) on the x-axis, with one curve for rate of photosynthesis and one curve for absorption plotted together for comparison
The relative rate of photosynthesis at different wavelengths indicates a close relationship with the absorption spectrum of chlorophyll a and b.
Comparison of the absorption spectrum for chlorophyll pigments and the action spectrum for chlorophyll pigments shows that the wavelengths of light absorbed (red and blue) by chlorophyll pigments are very similar to the wavelengths that caused photosynthesis. Thus, absorption and action spectra overlap each other. In other words, the absorption and the action spectra for chlorophyll a and b coincide.
This overlap is not a coincidence — it is powerful evidence. If photosynthesis is fastest at exactly the wavelengths chlorophyll absorbs best, then chlorophyll must be the molecule that captures the light which drives the process.
12.3 Mechanism of Photosynthesis
For a long time, the overall equation of photosynthesis was known but one basic question remained unsettled: when a plant releases oxygen, which raw material does that oxygen come from? Both \(CO_2\) and \(H_2O\) contain oxygen atoms, so either could be the source. The answer came from an unexpected direction — from bacteria that make sugar without releasing any oxygen at all.
Van Niel’s Insight from Bacteria
In 1931, Van Niel proved that bacteria used \(H_2S\) and \(CO_2\) to synthesize carbohydrates as follows:
\[6CO_2 + 12H_2S \rightarrow C_6H_{12}O_6 + 6H_2O + 12S \downarrow\]
Look closely at what these purple and green sulphur bacteria do. They use hydrogen sulphide instead of water, and instead of giving out oxygen they deposit yellow granules of sulphur. The downward arrow beside the sulphur simply indicates that it is deposited out of solution.
The chemical logic is striking. The bacterium splits \(H_2S\) to obtain hydrogen for building sugar, and the leftover atom — sulphur — is thrown out as waste.
This led Van Niel to postulate that in green plants, water is utilized in place of \(H_2S\) and \(O_2\) is evolved in place of sulphur. If the two processes are versions of the same chemistry, then the oxygen a plant releases must be the leftover atom from splitting water, not from \(CO_2\).
Proof from Heavy Oxygen
A postulate, however convincing, still needs experimental proof. Ruben (in 1941) confirmed it in Chlorella. He used water labeled with heavy isotope of oxygen (\(^{18}O_2\)), i.e. \(H_2{}^{18}O\).
A heavy isotope has the same chemical behaviour as the ordinary atom but a slightly greater mass, so it can be detected in the products. It acts like a tag tied to the oxygen atoms of water, letting the experimenter follow exactly where those atoms end up.
The oxygen evolved contained \(^{18}O_2\), thereby proving Van Niel’s hypothesis that oxygen evolved in photosynthesis comes from water. Had the oxygen come from \(CO_2\), the gas released would have been ordinary oxygen and the tag would have appeared elsewhere.
This leads to the currently accepted general equation of photosynthesis:
\[6CO_2 + 12H_2{}^{18}O \xrightarrow{\text{Light}} C_6H_{12}O_6 + 6H_2O + 6\ {}^{18}O_2\]
Hill’s Experiment
Independent proof came from an experiment done a few years earlier. In 1937, R. Hill cultured isolated chloroplasts in a medium containing water (devoid of any trace of dissolved \(CO_2\)) and a ferric compound.
Notice the two deliberate choices in the design. Carbon dioxide was completely excluded, so that if oxygen still appeared it could not have come from \(CO_2\). And a ferric compound was added to act as an artificial electron acceptor, standing in for the natural acceptor of the cell.
On illumination (exposure to light), he observed that there was a change in the colour of the ferric compound due to reduction, caused by \(H_2\) removed from water through its photolysis. Photolysis simply means splitting by light — photo for light, lysis for breaking.
The source of evolved \(O_2\) must be from water and not from \(CO_2\). This observation directly proves that \(H_2\) comes from water as a result of photolysis and \(O_2\) is evolved. Later Ruben and Kamen, by using isotopic oxygen (\(^{18}O_2\)), confirmed Hill’s findings.
By adding haemoglobin in the culture medium, one can directly prove that the source of evolving oxygen is water. This is because, due to the action of evolved oxygen, haemoglobin turns red in colour forming oxyhaemoglobin. This observation confirmed that \(H_2O\) must have been oxidized, releasing \(O_2\) that reacted with haemoglobin.
Hill indirectly proved that the source of evolving \(O_2\) is \(H_2O\) and not \(CO_2\). Therefore, it is called the Hill reaction.
Thus Hill reaction proves that:
In photosynthesis, oxygen is released from water.
Electrons for the reduction of \(CO_2\) are obtained from water.
Light Energy Becomes Chemical Energy
According to Arnon, in this process light energy is converted to chemical energy. This energy is stored in ATP, and \(NADPH_2\) is formed as a hydrogen donor. This ATP formation is known as photophosphorylation.
The names are worth unpacking. ATP (adenosine triphosphate) is the cell’s ready-to-spend energy currency, while \(NADPH_2\) carries hydrogen and electrons — the raw material for reducing \(CO_2\). “Photo” refers to light and “phosphorylation” to the addition of a phosphate group to ADP, so photophosphorylation literally means making ATP using light.
Photosynthesis as an Oxidation–Reduction Process
In the modern concept, the process of photosynthesis is an oxidation and reduction process in which water is oxidized (to release \(O_2\)) and \(CO_2\) is reduced to form sugar.
Recall the basic chemistry: oxidation is the loss of hydrogen or electrons, and reduction is their gain. In photosynthesis the two happen together — water loses hydrogen and electrons, carbon dioxide gains them.
Two Successive Stages
The process consists of two major reactions, viz., i. the light reaction (photochemical), that requires light, and ii. the other reaction is the dark reaction, which is independent of light (also called the biochemical reaction).
These two reactions occur successively, i.e. the light reaction is followed by the dark reaction (also called Blackman’s reaction). The order is not arbitrary — the dark reaction depends entirely on the ATP and \(NADPH_2\) handed over by the light reaction, so it cannot run first. This dependence is summarised in FC 12.3.
Flowchart
FC 12.3 – Two stages of photosynthesis: sunlight absorbed by pigments → light reaction (in grana) → water split, \(O_2\) released, ATP and \(NADPH_2\) formed → ATP and \(NADPH_2\) pass into stroma → dark reaction (in stroma) → \(CO_2\) reduced → carbohydrate; ADP and \(NADP^+\) returned to light reaction
12.4 Light Reaction
In the light reaction, solar energy is trapped by chlorophyll and stored in the form of chemical energy as ATP and in the form of reducing power as \(NADPH_2\). Oxygen is evolved in the light reaction by the splitting of water.
Notice that light energy leaves the stage almost immediately. Within a fraction of a second it is converted into two stable chemical products that the plant can carry into the stroma and spend later. Sunlight cannot be stored, but ATP and \(NADPH_2\) can.
Excitation of Chlorophyll
When a photon is absorbed by a chlorophyll molecule, an electron is boosted to a higher energy level. The electron does not leave the molecule at once; it simply moves outward, into an orbit further from the nucleus.
To boost an electron, a photon must have a certain minimum quantity of energy called quantum energy. This is an all-or-nothing requirement — a photon with too little energy, such as an infra-red photon, cannot excite the electron no matter how many such photons arrive.
A molecule that has absorbed a photon is in an energy rich excited state. This state is extremely short-lived, lasting only a tiny fraction of a second.
When the light source is turned off, the high energy electrons return rapidly to their normal low energy orbitals as the excited molecule reverts to its original stable condition, called the ground state. The three conditions of the pigment molecule — ground state, excited state and ionized chlorophyll-a — are shown in Fig. 12.5.
Diagram
Fig. 12.5 – Photoexcitation of chlorophyll-a: photon striking a chlorophyll molecule in the ground state, the electron shifting to an outer orbit in the excited state, and the electron leaving the molecule to give ionized chlorophyll-a; label photon, original orbit, ground state, excited state, ionized chlorophyll-a and electron
Reaction Centre
The light absorbing pigments are located in the thylakoid membranes. They are arranged in clusters of chlorophyll and accessory pigments, along with special types of chlorophyll molecules \(P_{680}\) and \(P_{700}\).
The letter P stands for Pigment, and 680 and 700 stand for the wavelengths of light, in nanometres, at which these molecules show maximum absorbance. \(P_{680}\) and \(P_{700}\) molecules form the Reaction centers or Photocenters.
The accessory pigments and other chlorophyll molecules harvest solar energy and pass it on to the reaction centers. These are called Light harvesting or Antenna molecules. They function to absorb light energy, which they transmit at a very high rate to the reaction center where the photochemical act occurs.
Photosystems I and II
The thylakoid membranes of chloroplasts have two kinds of photosystems, each with its own set of light harvesting chlorophyll and carotenoid molecules. Chlorophyll and accessory pigments help to capture light energy and pass it on to the photocenters.
Thus, a photon absorbed anywhere in the harvesting zone of a \(P_{680}\) center can pass its energy to the \(P_{680}\) molecule. The cluster of pigment molecules which transfer their energy to \(P_{680}\) absorb at or below the wavelength 680 nm. Together with \(P_{680}\) they form Photosystem-II or PS-II.
Likewise, \(P_{700}\) forms Photosystem-I or PS-I along with pigment molecules which absorb light at or below 700 nm.
Photosystem II
This system brings about photolysis of water and release of oxygen. In this act, when PS-II absorbs light, electrons are released and the chlorophyll molecule is oxidized. The electrons emitted by \(P_{680}\) (PS-II) are ultimately trapped by \(P_{700}\) (PS-I).
The oxidized \(P_{680}\) is now electron-deficient and must be refilled, otherwise it can never absorb light again. It regains its electrons by the photolysis of water as follows:
\[4H_2O \rightarrow 4H^+ + 4OH^-\] \[4OH^- \rightarrow 4(OH) + 4e^-\] \[4OH \rightarrow 2H_2O + O_2\] \[4H_2O \rightarrow 2H_2O + O_2 \uparrow + 4H^+ + 4e^-\]
Read the four steps as one story. Water first dissociates into protons and hydroxyl ions; the hydroxyl ions surrender their electrons — and those are precisely the electrons that refill the oxidized \(P_{680}\); the remaining hydroxyl groups then rearrange to give water and oxygen. The last equation is simply the net summary of the first three.
Oxygen is given out as a byproduct by the photosynthesizing plants. It is worth appreciating the scale of this “waste product” — the entire oxygen content of our atmosphere is the accumulated by-product of this single reaction.
Protons (\(H^+\)) accumulate inside the thylakoid, resulting in a Proton gradient. Since the thylakoid is a closed sac, the protons released by photolysis are trapped in its lumen and pile up there, making the lumen strongly acidic compared with the stroma outside.
Flowchart
FC 12.4 – Photolysis and refilling of PS-II: light absorbed by antenna pigments → energy funnelled to \(P_{680}\) → excited electron released → \(P_{680}\) becomes oxidized → water split by photolysis → electrons refill \(P_{680}\); protons accumulate in thylakoid lumen; oxygen released as by-product
The Proton Gradient and ATP
The energy released by the protons when they diffuse across the thylakoid membrane into the stroma, down the \(H^+\) concentration gradient, is used to produce ATP.
This is the principle of chemiosmosis, shown in Fig. 12.6. The protons cannot leak through the lipid membrane on their own; they can cross only through a channel enzyme, ATP synthase. As they rush through it from the crowded lumen into the stroma, the enzyme uses the flow to join ADP and inorganic phosphate into ATP.
Diagram
Fig. 12.6 – ATP synthesis through chemiosmosis: thylakoid membrane with PS-II, plastoquinone (PQ), cytochrome b and f complex acting as proton pump, PS-I, NADP reductase, accumulation of \(H^+\) in the lumen (pH low) against the stroma (pH high), and the \(F_0\)–\(F_1\) ATP synthase converting ADP to ATP
Photosystem I
When light quanta are absorbed by photosystem I (\(P_{700}\)), energy rich electrons are emitted from the reaction center. These flow down a chain of electron carriers to NADP, along with the proton generated by splitting of water. This results in the formation of \(NADPH_2\).
So PS-I performs a second, separate act of light absorption. Its own excited electrons, together with protons from the photolysis of water, are used to reduce \(NADP^+\) — while the electrons lost by PS-I are replaced by those arriving from PS-II.
Hydrogen attached to \(NADPH_2\) is used for the reduction of \(CO_2\) in the dark reaction. This is also called the Reducing power of the cell.
12.5 Photophosphorylation
The formation of ATP in the chloroplasts in presence of light is called photophosphorylation. It takes place in two forms.
The difference between the two forms lies in one simple question: does the electron that leaves the reaction centre come back to where it started, or does it travel one way and end up somewhere else?
Cyclic Photophosphorylation
Illumination of photosystem-I causes electrons to move continuously out of the reaction center of photosystem-I and back to it. The electron therefore completes a closed loop, which is exactly why the process is called cyclic.
The cyclic electron-flow is accompanied by the photophosphorylation of ADP to yield ATP. As the electron travels down the chain of carriers — ferredoxin, cytochrome \(b_6\), cytochrome f and plastocyanin — it loses energy at each step, and part of that released energy is captured as ATP before the electron returns to \(P_{700}\). This path is shown in Fig. 12.7.
Diagram
Fig. 12.7 – Cyclic photophosphorylation: PS I receiving light energy, electron passing to FRS, then ferredoxin, cytochrome \(b_6\), cytochrome f and plastocyanin, and back to PS I, with two points where ADP + Pi is converted to ATP
This is termed as Cyclic photophosphorylation. Since this process involves only pigment system I, photolysis of water and the consequent evolution of oxygen does not take place.
Flowchart
FC 12.5 – Cyclic electron flow: light energy → \(P_{700}\) of PS-I excited → electron released → FRS → ferredoxin → cytochrome \(b_6\) (ATP formed) → cytochrome f (ATP formed) → plastocyanin → back to \(P_{700}\) of PS-I
Non-cyclic Photophosphorylation
It involves both PS-I and PS-II photosystems. In this case, the electron transport chain starts with the release of electrons from PS-II.
In this chain, high energy electrons released from PS-II do not return to PS-II but, after passing through an electron transport chain, reach PS-I, which in turn donates it to reduce \(NADP^+\) to NADPH. The reduced \(NADP^+\) (NADPH) is utilized for the reduction of \(CO_2\) in the dark reaction. The full pathway is drawn in Fig. 12.8.
Diagram
Fig. 12.8 – Non-cyclic photophosphorylation: electron energy on the y-axis, showing PS II, primary acceptor, electron transport chain with ATP formation, PS I, second primary acceptor, and final transfer to \(NADP^+\) forming NADPH, with water splitting at PS II
Electron-deficient PS-II brings about oxidation of the water molecule. Due to this, protons, electrons and oxygen atoms are released. Electrons are taken up by PS-II itself to return to the reduced state, protons are accepted by \(NADP^+\), whereas oxygen is released.
So the electron flow here is one-way, not a loop: water supplies electrons to PS-II, PS-II supplies them to PS-I, and PS-I finally hands them to \(NADP^+\). Water is the ultimate donor at one end and \(NADP^+\) the ultimate acceptor at the other.
As in this process high energy electrons released from PS-II do not return to PS-II, and it is accompanied with ATP formation, this is called Non-cyclic photophosphorylation.
Flowchart
FC 12.6 – Non-cyclic electron flow: \(H_2O\) → photolysis (\(O_2\) released) → electrons to \(P_{680}\) of PS-II → light excites \(P_{680}\) → primary acceptor → electron transport chain (ATP formed) → \(P_{700}\) of PS-I → light excites \(P_{700}\) → primary acceptor → ferredoxin → \(NADP^+\) reductase → \(NADPH_2\)
Products of the Light Reaction
Thus, during the photochemical reactions, photolysis of water takes place, \(O_2\) is released, and ATP and \(NADPH_2\) are synthesized.
ATP and \(NADPH_2\) molecules function as vehicles for transfer of the energy of sunlight into the dark reaction, leading to carbon fixation. In this reaction \(CO_2\) is reduced to carbohydrate.
The light reaction gives rise to two important products: i. a reducing agent, \(NADPH_2\), and ii. an energy rich compound, ATP. Both these are utilized in the dark phase of photosynthesis. Together they are often called the assimilatory power of the cell, because they carry exactly what is needed to assimilate carbon dioxide into sugar.
The link between the two phases of photosynthesis is summarised in Fig. 12.9 — light and water entering the light reactions in the grana, oxygen leaving, and ATP and NADPH passing across into the dark reactions of the stroma, which take in carbon dioxide and give out glucose while returning ADP and \(NADP^+\).
Diagram
Fig. 12.9 – Relation between light and dark reaction: chloroplast showing thylakoid and granum with light reactions taking in light and water and releasing oxygen; arrows of ATP and NADPH going to the dark reactions in the stroma and ADP and \(NADP^+\) returning; dark reactions taking in carbon dioxide and releasing glucose
12.6 Dark Reaction
Carbon fixation occurs in the stroma by a series of enzyme catalyzed steps. Molecules of ATP and \(NADPH_2\) produced in the thylakoids (light reaction) come into the stroma, where carbohydrates are synthesized.
Notice that no pigment and no light are needed here. What the stroma needs is a supply of \(CO_2\), a molecule to catch it, and the ATP and \(NADPH_2\) delivered from the grana.
Discovering the Path of Carbon
The path of carbon fixation in the dark reaction, through intermediate compounds leading to the formation of sugar and starch, was worked out by Calvin, Benson and their co-workers. For this, Calvin was awarded the Nobel Prize in 1961.
The path of carbon was studied with the help of the radioactive tracer technique using Chlorella, a unicellular green alga, and radioactive \(^{14}CO_2\). With the help of radioactive carbon, it becomes possible to trace the intermediate steps of fixation of \(^{14}CO_2\).
The idea behind a tracer is simple but powerful. A radioactive carbon atom behaves chemically just like an ordinary one, but it announces its position — so wherever the radioactivity turns up, that is where the carbon has travelled.
The various steps in the dark reactions (Calvin cycle / C-3 pathway, Fig. 12.10) are as follows.
Diagram
Fig. 12.10 – Calvin cycle and regeneration of RuBP (C₃ pathway/cycle): the three labelled arcs CARBOXYLATION, REDUCTION and REGENERATION, showing \(CO_2\) (6 M) + 6 RuBP (5 C) with RUBISCO → unstable compound (6 M, 6 C) → 3-phosphoglycerate (12 M, 3 C) → with ATP (12 M)/ADP (12 M) → 1,3-diphosphoglycerate (12 M, 3 C) → with NADPH (12 M)/\(NADP^+\) (12 M) → 3-phosphoglyceraldehyde (12 M, 3 C), of which 10 M pass through xylulose-5-phosphate, erythrose-4-phosphate, sedoheptulose-7-phosphate, ribose-5-phosphate and 6 RuMP (5 C, 6 M) back to 6 RuBP using ATP (6 M), while 2 M enter the SYNTHESIS arm: dihydroxy acetone phosphate (3 C, 2 M) → fructose-1,6-diphosphate (1 M, 6 C) → fructose-6-phosphate (1 M, 6 C) → glucose-6-phosphate (1 M, 6 C) → glucose (1 M, 6 C)
Carboxylation
\(CO_2\) reduction starts with a 5-carbon sugar, ribulose-1,5-bisphosphate (RuBP). It is a 5-carbon sugar (pentose) with two phosphate groups attached to it. RuBP is the acceptor molecule — the “catcher” that grabs the incoming carbon dioxide.
RuBP reacts with \(CO_2\) to produce an unstable 6-carbon intermediate in the presence of an enzyme RuBP carboxylase or Rubisco, and immediately splits into a 3-carbon compound, 3-phosphoglyceric acid (3-PGA).
This is why the pathway is called the \(C_3\) cycle: the very first stable product of \(CO_2\) fixation contains three carbon atoms. The 6-carbon intermediate does exist, but it breaks apart so fast that it is never isolated.
Rubisco is a large protein molecule and comprises 16% of the chloroplast proteins.
Glycolytic Reversal
Molecules of 3-PGA form 1,3-diphosphoglyceric acid utilizing ATP molecules. These are reduced to glyceraldehyde-3-phosphate (3-PGAL) by \(NADPH_2\) supplied by the light reactions of photosynthesis.
The name “glycolytic reversal” is a useful clue. These same compounds appear in glycolysis during respiration, but there they are broken down and oxidised; here the identical steps are run backwards, with energy spent instead of released. ATP supplies the energy and \(NADPH_2\) supplies the hydrogen, which is exactly why the light reaction had to happen first.
This step is also called the reduction phase of the cycle, because \(CO_2\), which arrived in a fully oxidised form, now gains hydrogen and becomes part of a sugar.
For the Calvin cycle to run continuously, there must be a sufficient amount of RuBP which accepts \(CO_2\), and a regular supply of ATP and \(NADPH_2\). Out of each of 12 molecules of 3-phosphoglyceraldehyde (3-PGAL), 2 molecules are used for synthesis of one glucose molecule, and the remaining 10 molecules are used for regeneration of 6 molecules of RuBP.
Regeneration of RuBP
Through a series of complex reactions, 10 molecules of 3-PGAL are used for regeneration of six molecules of RuBP at the cost of 6 ATP. For this purpose, six turns of the Calvin cycle are needed to be operated so that a molecule of glucose can be synthesized.
This phase is what makes the pathway a cycle rather than a straight line. If the acceptor RuBP were not rebuilt, the leaf would fix carbon dioxide only once and then stop for want of a catcher.
Plants form a variety of organic compounds required for their structure and function through these complex reactions. The 3-PGAL leaving the cycle is not committed to glucose alone; it is the branch point from which starch, sucrose, cellulose, amino acids and fats can all be built.
Flowchart
FC 12.7 – Calvin cycle (C₃ pathway): \(CO_2\) + RuBP (5 C) → Rubisco → unstable 6-carbon intermediate → 2 × 3-PGA (3 C) → 1,3-diphosphoglyceric acid (ATP used) → 3-PGAL (NADPH₂ used) → branch point: 2 of every 12 molecules of 3-PGAL → glucose; remaining 10 of every 12 → regeneration of RuBP (6 ATP used) → back to RuBP
The Balance Sheet of the Cycle
Thus, for every 6 molecules of \(CO_2\) and Ribulose-1,5-biphosphate used, 12 molecules of 3-phosphoglyceraldehyde are produced. Out of these 12 molecules, only two are utilized for the formation of a molecule of glucose; the other 10 molecules are converted into ribulose-1,5-biphosphate, which combines with fresh \(CO_2\).
Thus, the Calvin cycle regenerates ADP and NADP required for the light reaction. The two phases of photosynthesis therefore feed each other continuously — the grana supply ATP and \(NADPH_2\) to the stroma, and the stroma returns ADP and \(NADP^+\) to the grana.
Overall Equations of Photosynthesis
The whole process can now be written as a set of balanced equations. Taking the two phases separately:
- Light Reaction (in granum):
\[24H_2O \rightarrow 24OH^- + 24H^+\] \[24OH^- \rightarrow 24OH + 24e^-\] \[24e^- + 24H^+ + 12NADP^+ \rightarrow 12NADPH_2\] \[18ADP + 18Pi \rightarrow 18ATP\] \[24OH \rightarrow 12H_2O + 6O_2 \uparrow\]
- Dark reaction (in stroma):
\[6CO_2 + 18ATP + 12NADPH_2 \rightarrow C_6H_{12}O_6 + 6H_2O + 18ADP + 18Pi + 12NADP^+\]
Adding the two together (i + ii):
\[6CO_2 + 24H_2O \xrightarrow[\text{Chloroplast}]{\text{Light}} C_6H_{12}O_6 + 18H_2O + 6O_2 \uparrow\]
Two numbers in these equations are worth committing to memory, because they are asked repeatedly: the synthesis of one molecule of glucose in the \(C_3\) pathway requires 18 ATP and 12 \(NADPH_2\).
12.7 Photorespiration
Photorespiration occurs under conditions like high temperature, bright light, high oxygen and low \(CO_2\) concentration. It is a wasteful process linked with the \(C_3\)-Cycle, where instead of fixation of \(CO_2\), it is given out.
These four conditions usually arrive together. On a hot, bright afternoon a plant closes its stomata to save water; once the pores shut, the \(CO_2\) inside the leaf is used up while the \(O_2\) produced by the light reaction keeps accumulating. The internal gas balance is thus tipped exactly the wrong way.
It involves three organelles — chloroplast, peroxisomes and mitochondria — and occurs in a series of cyclic reactions, which is also called the PCO cycle. The pathway is spread across all three organelles, as shown in Fig. 12.11, so intermediates have to be shuttled from one to the next.
Diagram
Fig. 12.11 – Photorespiration: chloroplast showing RuBP (5C) oxygenation with \(O_2\) giving PGA (3C) + phosphoglycolate (2C) → glycolate (2C), with PGA re-entering the Calvin cycle and glycerate returning; peroxisome showing glycolate (2C) + \(O_2\) → glyoxylate (2C) + \(H_2O_2\) → glycine (2C), and serine (3C) → glycerate (3C); mitochondrion showing two molecules of glycine (2C) → serine (3C) + \(CO_2\) (1C) + \(NH_4\)
When Rubisco Makes a Mistake
The enzyme Rubisco acts as oxygenase at higher concentration of \(O_2\), and photorespiration begins. Its full name — RuBP carboxylase-oxygenase — records this double personality: the very same enzyme can attach either \(CO_2\) or \(O_2\) to RuBP, and the gas that is more plentiful at that moment wins.
When RuBP reacts with \(O_2\) rather than \(CO_2\), it forms a 3-carbon compound (PGA) and a 2-carbon compound, phosphoglycolate. The latter is converted to glycolate, which is shuttled out of the chloroplast into the peroxisomes.
Compare this with normal carboxylation, where RuBP plus \(CO_2\) gave two molecules of PGA that both stayed in the cycle. Here only one PGA is produced, and the other fragment is a 2-carbon compound that the Calvin cycle cannot use at all — so it must be salvaged elsewhere.
In the Peroxisome
In the peroxisomes, the enzyme glycolate oxidase converts glycolate into glyoxylate, which is converted into the amino acid glycine by transamination. Transamination simply means the transfer of an amino group (\(-NH_2\)) from another amino acid onto the carbon skeleton.
This oxidation also produces hydrogen peroxide (\(H_2O_2\)), a toxic by-product. The peroxisome is exactly the right compartment for such a step, because it also contains catalase, the enzyme that immediately breaks hydrogen peroxide down into water and oxygen.
In the Mitochondrion
In mitochondria, two molecules of glycine are converted into serine (an amino acid) and \(CO_2\) is given out. Thus, the plant loses 25% of photosynthetically fixed carbon.
Follow the arithmetic and the loss becomes obvious. Two glycine molecules, of two carbons each, make four carbons in all; three of them appear in serine and the fourth escapes as \(CO_2\) — one carbon lost out of four, which is exactly 25%.
Note also what is not produced. Ordinary respiration releases \(CO_2\) but yields ATP in return. Photorespiration releases \(CO_2\) and yields neither ATP nor sugar, and it actually consumes energy in the salvage steps — which is why it is called a wasteful process.
Return to the Chloroplast
Serine is transported back to peroxisomes and converted into glycerate. It is shuttled back to the chloroplast to undergo phosphorylation and is utilized in the formation of 3-PGA, which is then used in the \(C_3\) pathway.
So the cell does recover most of the carbon that was diverted — but only by spending ATP and moving intermediates through three organelles. The complete circuit is set out in FC 12.8.
Flowchart
FC 12.8 – PCO cycle across three organelles: (chloroplast) RuBP + \(O_2\) → Rubisco as oxygenase → PGA (3C) + phosphoglycolate (2C) → glycolate (2C) → (peroxisome) glycolate + \(O_2\) → glycolate oxidase → glyoxylate (2C) + \(H_2O_2\) → transamination → glycine (2C) → (mitochondrion) 2 glycine → serine (3C) + \(CO_2\) released → (peroxisome) serine → glycerate (3C) → (chloroplast) phosphorylation → 3-PGA → back into the \(C_3\) cycle
12.8 C₄ Pathway or Hatch–Slack Pathway
Photorespiration showed us the weakness of the \(C_3\) pathway in hot, bright places. Some plants have evolved a way around it — not by changing Rubisco, but by changing the conditions in which Rubisco is made to work.
Discovery and Distribution
M. D. Hatch and C. R. Slack, while working on sugarcane, found a four carbon compound (dicarboxylic acid) as the first stable product of photosynthesis. This was unexpected, because the Calvin cycle had established a 3-carbon compound as the first product.
It has been also found to occur in tropical and sub-tropical grasses and some dicotyledons. Some of the important plants are sugarcane, maize, Sorghum etc.
The geography is a clue in itself. These are plants of hot, bright, open habitats — exactly the conditions under which photorespiration is most damaging.
Why “C₃” and Why “C₄”
The plants in which \(CO_2\) fixation takes place by the Calvin cycle are called \(C_3\) plants, because the first product of \(CO_2\) fixation is a 3-carbon compound, phosphoglyceric acid.
But in the Hatch–Slack pathway, the first product of \(CO_2\) fixation is a 4-carbon compound, oxaloacetic acid. Hence such plants are called \(C_4\) plants.
Keep one point clear from the start: \(C_4\) plants do not abandon the Calvin cycle. They simply add an extra step in front of it. Sugar is still finally made by the \(C_3\) cycle — but in a different set of cells, under much better conditions.
Kranz Anatomy
The anatomy of leaves of \(C_4\) plants is different from the leaves of \(C_3\) plants. \(C_4\) plants show Kranz anatomy. In the leaves of such plants, there is a bundle sheath around the vascular bundles, as seen in Fig. 12.12.
Diagram
Fig. 12.12 – Kranz anatomy of a \(C_4\) plant: portion of a cross section of a leaf, labelling vascular bundle (vein), bundle sheath cell forming a ring around it, mesophyll cells arranged around the sheath, air space beneath the stoma, and the differing chloroplasts of the two cell types
The chloroplasts in the bundle-sheath cells are large and almost without, or with less developed, grana; whereas in the mesophyll cells, the chloroplasts are small but with well-developed grana.
This difference is not accidental. Grana are where the light reaction and its photolysis of water take place, so bundle sheath cells — with poorly developed grana — release very little oxygen internally. Keeping oxygen low around Rubisco is precisely the point of the whole arrangement.
The Pathway in Mesophyll Cells
\(CO_2\) taken from the atmosphere is accepted by a 3-carbon compound, phosphoenolpyruvic acid, in the chloroplasts of mesophyll cells, leading to the formation of the 4-C compound oxaloacetic acid with the help of the enzyme PEPCO.
PEPCO — phosphoenolpyruvate carboxylase — is the key to the whole strategy. Unlike Rubisco, it has no affinity for oxygen at all and it binds \(CO_2\) very strongly, so it keeps fixing carbon even when the gas is scarce inside a leaf with closed stomata.
Oxaloacetic acid is converted to another 4-C compound, the malic acid. It is transported to the chloroplasts of bundle sheath cells. Malic acid is the carrier — a stable 4-carbon molecule that ferries carbon dioxide inward in a fixed, non-gaseous form.
Diagram
Fig. 12.13 – \(C_4\) pathway: mesophyll cell showing atmospheric \(CO_2\) + phosphoenolpyruvate (PEP) → PEP carboxylase → oxaloacetate → malate dehydrogenase with \(NADP^+\)/NADPH → malate; malate passing into the bundle sheath cell → malic enzyme with \(NADP^+\)/NADPH → pyruvate + \(CO_2\) → Calvin cycle; pyruvate returning to the mesophyll cell → pyruvate-phosphate dikinase with ATP → AMP → regenerating PEP
Decarboxylation in Bundle Sheath Cells
Here, malic acid (4-C) is converted to pyruvic acid (3-C) with the release of \(CO_2\) in the cytoplasm. Thus, the concentration of \(CO_2\) increases in the bundle sheath cells.
The bundle sheath therefore acts as a \(CO_2\) pump. Carbon dioxide is picked up outside at low concentration and released inside at high concentration — a concentrating device built out of ordinary chemistry.
Chloroplasts of these cells contain the enzymes of the Calvin cycle. Because of the high concentration of \(CO_2\), RuBP carboxylase participates in the Calvin cycle and not in photorespiration. Sugar formed in the Calvin cycle is transported into the phloem.
Pyruvic acid generated in the bundle sheath cells re-enters mesophyll cells and regenerates phosphoenolpyruvic acid by consuming one ATP. The acceptor is thus rebuilt, exactly as RuBP is rebuilt in the Calvin cycle, so the shuttle can run continuously.
Flowchart
FC 12.9 – \(C_4\) (Hatch–Slack) pathway: atmospheric \(CO_2\) enters mesophyll cell → accepted by PEP (3C) with PEPCO → oxaloacetic acid (4C) → malic acid (4C) → transported to bundle sheath cell → decarboxylation → \(CO_2\) released (concentration rises) + pyruvic acid (3C) → \(CO_2\) enters Calvin cycle with Rubisco → sugar → phloem; pyruvic acid returns to mesophyll cell → ATP used → PEP regenerated
The Extra Energy Cost
Since this conversion results in the formation of AMP (not ADP), two ATP are required to regenerate ATP from AMP. Thus, the \(C_4\) pathway needs 12 additional ATP.
Recall from the previous section that AMP is adenosine monophosphate, one phosphate poorer than ADP, so restoring it to ATP takes two phosphorylation steps rather than one. Six turns of the shuttle are needed for one glucose molecule, and at two extra ATP each, this adds \(6 \times 2 = 12\) ATP.
The \(C_3\) pathway requires 18 ATP for the synthesis of one glucose molecule, whereas the \(C_4\) pathway requires 30 ATP.
Why C₄ Plants Are Better Photosynthesizers
Thus, \(C_4\) plants are better photosynthesizers as compared to \(C_3\) plants, as there is no photorespiration in these plants. Moreover, \(C_4\) plants can fix \(CO_2\) even if the concentration of \(CO_2\) in the atmosphere is very low.
This second point matters most on a hot day. When a \(C_4\) plant partly closes its stomata to conserve water, the internal \(CO_2\) level falls — but PEPCO can still capture it efficiently, so photosynthesis continues while a \(C_3\) neighbour is forced to slow down.
12.9 CAM – Crassulacean Acid Metabolism
It is one more alternative pathway of carbon fixation, found in desert plants. It was first reported in the family Crassulaceae, so it is called CAM (Crassulacean Acid Metabolism).
A desert plant faces a problem the \(C_4\) plants of the previous section do not. Opening the stomata in the heat of the day would let \(CO_2\) in, but it would also let precious water vapour out. CAM is the solution to that conflict.
Stomata That Open at Night
In CAM plants, the stomata are scotoactive, i.e. active during the night, so the initial \(CO_2\) fixation occurs at night. The word comes from scoto, meaning darkness — these pores work on exactly the opposite schedule to those of an ordinary leaf.
Night air is cooler and more humid than day air, so a stoma that opens after sunset loses far less water for every molecule of \(CO_2\) it admits. Water conservation, not carbon gain, is what sets the timetable.
Thus the \(C_4\) pathway fixes \(CO_2\) at night and reduces \(CO_2\) in the day time via the \(C_3\) pathway, by using NADPH formed during the day. PEP carboxylase and Rubisco are present in the mesophyll cell (no Kranz anatomy).
This is the crucial structural point. A CAM plant carries out both steps inside one and the same cell, so it needs no bundle sheath and no Kranz anatomy. What separates the two steps is not distance but time.
Phase I: Acidification
The formation of malic acid during the dark is called acidification (phase I). \(CO_2\) entering through the open stomata is accepted by phosphoenolpyruvic acid with the help of PEP carboxylase, giving oxaloacetic acid and then malic acid — exactly as in the mesophyll cells of a \(C_4\) plant.
Malate is stored in vacuoles during the night. The vacuole is a large, acid-tolerant storage compartment, and as malic acid accumulates in it hour after hour, the sap of the leaf actually becomes measurably sour by dawn.
Phase II: Deacidification
Malate releasing \(CO_2\) during the day for the \(C_3\) pathway within the same cell is called deacidification (phase II). As daylight begins, malic acid moves out of the vacuole and is decarboxylated, and the \(CO_2\) so released is handed to Rubisco.
Because the stomata are shut, this released \(CO_2\) cannot escape. Its concentration inside the cell rises sharply — and just as in the bundle sheath of a \(C_4\) plant, a high internal \(CO_2\) level keeps Rubisco working as a carboxylase and suppresses photorespiration. The two phases and their timing are shown in Fig. 12.14.
Diagram
Fig. 12.14 – Crassulacean acid metabolism: a single cell shown in two halves — Night (stomata open): \(CO_2\) → \(HCO_3^-\) → PEP → PEPC → primary carboxylation → \(C_4\) acid stored; Day (stomata closed): \(C_4\) acid → decarboxylation → \(CO_2\) → Calvin–Benson cycle with RUBISCO → secondary carboxylation → \(C_3\) acid → starch, with PEP regeneration using ATP and AMP
Flowchart
FC 12.10 – CAM pathway across a day: (Night, stomata open) \(CO_2\) + PEP → PEP carboxylase → oxaloacetic acid → malic acid → stored in vacuole (acidification, phase I) → (Day, stomata closed) malic acid leaves vacuole → decarboxylation → \(CO_2\) released inside the same cell (deacidification, phase II) → Rubisco → Calvin cycle using ATP and NADPH from the light reaction → sugar → back to night phase
Examples of CAM plants are Kalanchoe, Opuntia, Aloe etc. The chemical reactions of the carbon dioxide fixation and its assimilation are similar to that of \(C_4\) plants.
12.10 Factors Affecting Photosynthesis
Like all other physiological processes, photosynthesis is also influenced by a number of factors.
It is useful to sort these into two groups. External factors come from the environment and can change from hour to hour, while internal factors belong to the plant itself and change only slowly, as the leaf grows and ages.
A. External Factors
Light
Light is an essential factor, as it supplies the energy necessary for photosynthesis. Without it the light reaction cannot run, and without ATP and \(NADPH_2\) the dark reaction soon stops as well.
Both quality and intensity of light affect photosynthesis. Quality means the wavelength or colour of the light, while intensity means how much light falls on the leaf in a given time.
The highest rate of photosynthesis takes place in the red wave length, and then comes the blue wave length. This fits exactly with what the action spectrum showed earlier — the wavelengths chlorophyll absorbs best are the wavelengths that drive the process fastest.
In a forest canopy, the rate of photosynthesis decreases considerably in plants growing under it. The tall trees above intercept most of the red and blue light, leaving the undergrowth in a dim, largely green shade that its pigments cannot use efficiently.
In most of the plants, photosynthesis is maximum in bright diffused sunlight. It decreases in strong light and again slows down in the light of very low intensity.
Both extremes are unhelpful for different reasons. In very dim light there are simply too few photons to keep the reaction centres working, while in very strong light the pigment system receives more energy than it can pass on, and the excess can begin to damage chlorophyll by photo-oxidation. Bright but diffused light is the comfortable middle.
It has also been found that uninterrupted and continuous photosynthesis, for relatively long periods of time, may be sustained without any visible damage to the plant. The relationship between light intensity and the rate of the process is plotted in Graph 12.16.
Diagram
Graph 12.16 – Light intensity and photosynthesis: light intensity on the x-axis and rate of photosynthesis on the y-axis, showing a steep rise at low intensity that gradually flattens into a plateau at high intensity
Carbon Dioxide
The main source of \(CO_2\) in land plants is the atmosphere, which contains only 0.03% of the gas. Recall that this same small percentage was described at the start of the chapter, and it is the reason \(CO_2\) is so often the bottleneck.
Under normal conditions of temperature and light, carbon dioxide acts as a limiting factor in photosynthesis. An increase in the concentration of \(CO_2\) increases the photosynthesis.
The increase in \(CO_2\) to about 1% is generally advantageous to most of the plants. Higher concentration of the gas has an inhibitory effect on photosynthesis. The response is therefore not endless — beyond a point the extra gas begins to interfere with normal cell function, including stomatal opening. This response is shown in Graph 12.15.
Diagram
Graph 12.15 – Effect of \(CO_2\) concentration: carbon dioxide concentration on the x-axis and rate of photosynthesis on the y-axis, showing a steep initial rise that levels off into a plateau at higher concentration
Temperature
Like all other physiological processes, photosynthesis also needs a suitable temperature. The reason is that the dark reaction is a chain of enzyme-catalysed steps, and enzymes are proteins that work well only within a narrow range of temperature.
In the presence of plenty of light and carbon dioxide, photosynthesis increases with the rise of temperature till it becomes maximum. After that, there is a decrease or fall in the rate of the process, because at high temperatures the enzymes begin to denature and lose their shape.
The optimum temperature at which the photosynthesis is maximum is 25–30 °C, though in certain plants like Opuntia, photosynthesis takes place at as high as 55 °C. This is known as the maximum temperature. The temperature at which the process just starts is the minimum temperature.
Water
Being one of the raw materials, water is also necessary for the photosynthetic process. An increase in water content of the leaf results in a corresponding increase in the rate of photosynthesis.
Thus the limiting effect of water is not direct but indirect. Very little of the water absorbed by a plant is actually used as a reactant in photosynthesis; the great bulk of it is lost in transpiration or held in the tissues.
It is mainly due to the fact that it helps in maintaining the turgidity of the assimilatory cells and the proper hydration of their protoplasm. When a leaf begins to wilt, its guard cells lose turgor and the stomata close — and once the pores shut, the supply of \(CO_2\) is cut off and photosynthesis falls sharply.
B. Internal Factors
Though the presence of chlorophyll is essential for photosynthesis, the rate of photosynthesis is not raised in simple proportion to the quantity of chlorophyll present beyond a certain amount. It is because of the fact that chlorophyll merely acts as a biocatalyst, and hence a small quantity is quite enough to maintain the large bulk of the reacting substances.
A biocatalyst is a substance that speeds a reaction without being consumed by it, so the same molecules are used over and over again. Doubling the amount of a catalyst therefore does not double the output once the supply of raw materials has become the limiting step.
The final product in the photosynthesis reaction is sugar, and its accumulation in the cells slows down the process of photosynthesis. This is an example of feedback — if the sugar formed is not exported through the phloem to the roots, stem and fruits, the products pile up and the reaction is held back.
The thickness of cuticle and epidermis of the leaf, the size and distribution of intercellular spaces, and the distribution of the stomata, as well as the development of chlorenchyma and other tissues, also affect the rate of photosynthesis.
Each of these is really a question of how easily gas can reach the chloroplasts. A thick waxy cuticle slows diffusion, wide intercellular spaces let \(CO_2\) move freely to every mesophyll cell, and a well-developed chlorenchyma provides more chloroplast-bearing cells to receive it.
Blackman’s Law of Limiting Factors
Both graphs in this section flattened into a plateau, and the reason is the same in each case. It was stated as a general rule by Blackman.
Blackman’s law of limiting factors states that when a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is controlled by the pace of the “slowest factor”.
The slowest factor is that factor which is present in the lowest or minimum concentration in relation to the others. The law of limiting factors can be explained by taking two external factors, such as carbon dioxide and light.
Suppose a plant is photosynthesizing at a fixed light intensity sufficient to utilize 10 mg of \(CO_2\) per hour only. On increasing the \(CO_2\) concentration, the photosynthetic rate also goes on increasing.
Now, if the \(CO_2\) concentration is further increased, no increase in the rate of photosynthesis will be noted. Thus, in this case, light becomes the limiting factor. Under such circumstances, the rate of photosynthesis can be increased only by increasing the light intensity.
This evidently shows that the photosynthetic rate responds to one factor alone at a time, and there would be a sharp break in the curve and a plateau formed exactly at the point where another factor becomes limiting.
If any one of the other factors which is kept constant (say, light) is increased, the photosynthetic rate increases again, reaching an optimum where again another factor becomes limiting. In practice, therefore, raising a crop’s photosynthesis is a matter of finding and correcting whichever factor is currently in shortest supply — improving any other one has no effect at all.
Flowchart
FC 12.11 – Applying the law of limiting factors: measure the rate → increase one factor → if the rate rises, that factor was limiting → keep increasing until the curve plateaus → at the plateau a different factor has become limiting → identify and increase that factor → the rate rises again to a new plateau
Significance of Photosynthesis
This anabolic process uses inorganic substances and produces food for all life, directly or indirectly. Anabolic means building up — small, simple molecules are joined into larger, energy-rich ones, which is the opposite of the breaking-down reactions of respiration.
The process transforms solar energy into chemical energy. This is the single entry point through which energy enters the living world; every food chain in every ecosystem begins here.
The released byproduct \(O_2\) is necessary not only for aerobic respiration in living organisms, but is also used in forming the protective ozone layer around the earth. That layer, high in the atmosphere, absorbs much of the sun’s ultraviolet radiation and makes life on land possible.
This process is also helping us in providing fossil fuels — coals, petroleum and natural gas. These are the remains of organisms that photosynthesised millions of years ago, so burning them releases solar energy that a green plant trapped in a distant geological age.
12.11 Summary
Photosynthesis is the synthesis of carbohydrates (glucose) from the inorganic materials \(CO_2\) and \(H_2O\), using solar energy trapped by chlorophyll pigments. It is the only process that brings solar energy into the living world, and all life ultimately depends on it.
The overall reaction is \(6CO_2 + 12H_2O \xrightarrow[\text{Chloroplast}]{\text{Light}} C_6H_{12}O_6 + 6O_2 + 6H_2O\).
The atmosphere holds only about 0.03 percent \(CO_2\) by volume, which amounts to 2200 billion tons; the oceans hold over 50 times this amount as dissolved gas or carbonates. From these two sources, green plants fix about 70 billion tons of carbon annually.
Photosynthesis takes place in chloroplasts, found mainly in the mesophyll cells of leaves. \(CO_2\) enters through stomata and water arrives through the veins.
A chloroplast is bounded by a double membrane and encloses a ground substance, the stroma. Within the stroma lie double-membrane sacs or lamellae stacked into grana; the individual sacs are thylakoids.
All pigments — chlorophylls, carotenes and xanthophylls — lie in the thylakoid membranes. They are fat soluble and sit in the lipid part of the membrane, and they absorb light of specific wavelengths in the visible region.
Chlorophyll a carries a methyl group (\(-CH_3\)) and chlorophyll b an aldehyde group (\(-CHO\)). Each molecule has a tetrapyrrole porphyrin ring as its head and a long hydrocarbon phytol tail.
Carotenoids are lipid compounds of two kinds — carotenes (\(C_{40}H_{56}\)), which are orange red, and xanthophylls, which contain oxygen. They pass their absorbed energy to chlorophyll a and also protect chlorophyll from photo-oxidation.
Light is electromagnetic radiation. By quantum theory it travels as photons, each carrying a quantum of energy; by wave theory it travels as waves of different wavelengths. Visible light spans 390 nm to 730 nm.
An absorption spectrum plots how much light a pigment absorbs at each wavelength; an action spectrum plots the rate of photosynthesis at each wavelength. For chlorophyll a and b the two spectra overlap and coincide, showing that chlorophyll is the pigment that drives the process.
Van Niel showed in 1931 that bacteria use \(H_2S\) instead of water, following \(6CO_2 + 12H_2S \rightarrow C_6H_{12}O_6 + 6H_2O + 12S \downarrow\), and postulated that green plants split water and release oxygen in place of sulphur.
Ruben confirmed this in 1941 using Chlorella and water labelled with heavy oxygen (\(H_2{}^{18}O\)); the oxygen released carried the label, giving the accepted equation \(6CO_2 + 12H_2{}^{18}O \xrightarrow{\text{Light}} C_6H_{12}O_6 + 6H_2O + 6\ {}^{18}O_2\).
Hill, in 1937, illuminated isolated chloroplasts with a ferric compound and no \(CO_2\), and found the ferric compound reduced and oxygen released. The Hill reaction proves that oxygen comes from water and that the electrons used to reduce \(CO_2\) are obtained from water.
Photosynthesis is an oxidation–reduction process: water is oxidised to release \(O_2\), and \(CO_2\) is reduced to sugar. It occurs as two successive reactions — the light reaction, which needs light, and the dark or Blackman’s reaction, which does not.
In the light reaction, a photon boosts an electron of chlorophyll from the ground state to the excited state, and a photon must carry a certain minimum quantum energy to do so.
Pigments are grouped into clusters. Antenna or light harvesting molecules absorb energy and pass it to the reaction centres \(P_{680}\) and \(P_{700}\), where the photochemical act occurs. \(P_{680}\) with its cluster forms PS-II, and \(P_{700}\) with its cluster forms PS-I.
PS-II carries out photolysis of water: \(4H_2O \rightarrow 2H_2O + O_2 \uparrow + 4H^+ + 4e^-\). Oxygen is released as a by-product, protons accumulate in the thylakoid to create a proton gradient, and their diffusion into the stroma through ATP synthase produces ATP by chemiosmosis.
PS-I emits energy-rich electrons that travel down a chain of carriers to NADP, and with the protons from photolysis form \(NADPH_2\) — the reducing power of the cell.
Photophosphorylation is the formation of ATP in chloroplasts in the presence of light, and has two forms. Cyclic photophosphorylation uses only PS-I, returns the electron to the same reaction centre, yields only ATP, and involves no photolysis and no oxygen release.
Non-cyclic photophosphorylation uses both PS-II and PS-I, moves electrons one way from water to \(NADP^+\), and yields ATP, \(NADPH_2\) and oxygen. Both products are carried into the dark reaction.
The dark reaction occurs in the stroma through enzyme-catalysed steps. Calvin, Benson and co-workers traced the path of carbon using Chlorella and radioactive \(^{14}CO_2\); Calvin received the Nobel Prize in 1961.
The Calvin cycle has three phases. In carboxylation, RuBP (a 5-carbon sugar with two phosphate groups) accepts \(CO_2\) with the help of Rubisco, forming an unstable 6-carbon compound that splits into two molecules of 3-PGA. Rubisco makes up 16% of chloroplast proteins.
In glycolytic reversal, 3-PGA forms 1,3-diphosphoglyceric acid using ATP, and this is reduced to 3-PGAL by \(NADPH_2\).
In regeneration, of every 12 molecules of 3-PGAL, 2 go to form one molecule of glucose and 10 are used to regenerate 6 molecules of RuBP at the cost of 6 ATP. Six turns of the cycle are needed for one glucose molecule, and the cycle also regenerates the ADP and NADP required by the light reaction.
Overall, the light reaction in the granum yields 12 \(NADPH_2\) and 18 ATP while releasing 6 \(O_2\), and the dark reaction in the stroma uses these as \(6CO_2 + 18ATP + 12NADPH_2 \rightarrow C_6H_{12}O_6 + 6H_2O + 18ADP + 18Pi + 12NADP^+\), giving the combined equation \(6CO_2 + 24H_2O \xrightarrow[\text{Chloroplast}]{\text{Light}} C_6H_{12}O_6 + 18H_2O + 6O_2 \uparrow\).
Photorespiration occurs in high temperature, bright light, high oxygen and low \(CO_2\). Rubisco then acts as an oxygenase, RuBP reacts with \(O_2\) to give PGA and phosphoglycolate, and fixed carbon is lost as \(CO_2\) instead of being fixed.
This PCO cycle spans three organelles. Glycolate leaves the chloroplast for the peroxisome, where glycolate oxidase converts it to glyoxylate and then to glycine; in the mitochondrion two glycine molecules give serine and release \(CO_2\), losing 25% of the photosynthetically fixed carbon. Serine returns through the peroxisome as glycerate to the chloroplast, where it forms 3-PGA for the \(C_3\) pathway.
Hatch and Slack, working on sugarcane, found a 4-carbon dicarboxylic acid as the first stable product in some plants. Plants fixing \(CO_2\) first as 3-carbon phosphoglyceric acid are \(C_3\) plants; those forming 4-carbon oxaloacetic acid first are \(C_4\) plants, such as sugarcane, maize and Sorghum.
\(C_4\) leaves show Kranz anatomy, with a bundle sheath around the vascular bundles. Bundle sheath chloroplasts are large with poorly developed grana, while mesophyll chloroplasts are small with well-developed grana.
In the \(C_4\) pathway, PEPCO in mesophyll cells joins \(CO_2\) to phosphoenolpyruvic acid to give oxaloacetic acid and then malic acid. Malic acid moves to the bundle sheath, where it is decarboxylated to pyruvic acid, raising the \(CO_2\) concentration so that RuBP carboxylase runs the Calvin cycle instead of photorespiration. Pyruvic acid returns to the mesophyll and regenerates PEP using ATP.
Because this regeneration forms AMP rather than ADP, the \(C_4\) pathway needs 12 additional ATP: the \(C_3\) pathway requires 18 ATP per glucose molecule while the \(C_4\) pathway requires 30 ATP. \(C_4\) plants are nevertheless better photosynthesizers, as photorespiration is absent and they can fix \(CO_2\) even at very low concentrations.
CAM, first reported in the family Crassulaceae, is found in desert plants. Their stomata are scotoactive, so \(CO_2\) is fixed at night as malic acid — acidification, phase I — and stored in vacuoles; by day the malate releases \(CO_2\) for the \(C_3\) pathway in the same cell — deacidification, phase II. CAM plants have no Kranz anatomy, and both PEP carboxylase and Rubisco lie in the mesophyll cell. Examples are Kalanchoe, Opuntia and Aloe.
External factors affecting photosynthesis include light, carbon dioxide, temperature and water. The rate is highest in red light, followed by blue; it is maximum in bright diffused sunlight and falls in very strong or very weak light.
\(CO_2\) normally acts as a limiting factor; raising it to about 1% benefits most plants, while higher concentrations are inhibitory. The optimum temperature is 25–30 °C, although Opuntia can photosynthesise at as high as 55 °C. Water acts indirectly, by maintaining turgidity of the assimilatory cells and hydration of their protoplasm.
Internal factors include the quantity of chlorophyll, which acts merely as a biocatalyst so that a small amount suffices; the accumulation of sugar, which slows the process; and leaf structure — the thickness of cuticle and epidermis, the size and distribution of intercellular spaces, the distribution of stomata and the development of chlorenchyma.
Blackman’s law of limiting factors states that when a process is governed by a number of separate factors, its rate is controlled by the pace of the slowest factor — the one present in the lowest amount relative to the others. The curve breaks into a plateau exactly where another factor becomes limiting.
Photosynthesis is significant as an anabolic process that makes food for all life directly or indirectly, transforms solar energy into chemical energy, releases the \(O_2\) needed for aerobic respiration and for the ozone layer, and has given us the fossil fuels — coal, petroleum and natural gas.
Flowchart
FC 12.12 – The whole of photosynthesis in one sequence: sunlight → absorbed by antenna pigments in thylakoid membranes → energy funnelled to \(P_{680}\) (PS-II) → electron released → photolysis of water refills \(P_{680}\), releasing \(O_2\) and protons → electron transport chain, proton gradient across thylakoid membrane → ATP by chemiosmosis → electron reaches \(P_{700}\) (PS-I) → excited again → \(NADP^+\) reduced to \(NADPH_2\) → ATP and \(NADPH_2\) pass into stroma → Calvin cycle: carboxylation (\(CO_2\) + RuBP with Rubisco) → reduction (3-PGA → 3-PGAL) → regeneration of RuBP → glucose; ADP and \(NADP^+\) returned to the light reaction; branch: in \(C_4\) plants \(CO_2\) first fixed by PEPCO in mesophyll cells and delivered to the bundle sheath; in CAM plants \(CO_2\) first fixed at night and stored as malic acid
12.12 Points to Ponder
All life on earth has been called “bottled solar energy”. Follow the energy in your last meal backwards, step by step, and decide whether that description is an exaggeration or a plain statement of fact.
Chlorophyll absorbs blue and red light strongly but reflects green — the very colour that is most abundant in sunlight at the earth’s surface. Was this a poor evolutionary choice, or might there be reasons why a pigment that absorbs everything would not actually suit a leaf?
Van Niel worked out something fundamental about green plants by studying bacteria that release no oxygen at all. What does this tell you about how scientific insight often travels between unrelated organisms?
Hill deliberately removed carbon dioxide from his experiment and added an artificial acceptor that no plant ever uses. Why is it sometimes necessary to study a process under conditions that never occur in nature?
The dark reaction is named for its independence from light, yet in an intact leaf it runs mainly during the day. How many other biological terms can you think of whose names mislead more than they describe?
Rubisco is slow, and it cannot reliably tell carbon dioxide apart from oxygen. Why has such an imperfect enzyme not been replaced over hundreds of millions of years of evolution?
Photorespiration wastes fixed carbon and yields no energy, and yet the pathway that salvages the wasted carbon is elaborate and costly. Under what circumstances might a wasteful process still be worth keeping?
A \(C_4\) plant spends 30 ATP to make a molecule of glucose where a \(C_3\) plant spends 18. Consider what this trade-off suggests about which resource is genuinely scarce for a plant growing in the tropics.
\(C_4\) plants separate the two carboxylation steps in space, while CAM plants separate them in time. Both solve the same underlying problem. What does the existence of two different solutions tell you about the way evolution works?
If the atmospheric concentration of carbon dioxide continues to rise, some plants may photosynthesise faster. Think about which of the world’s plants would gain the most from this, and whether faster photosynthesis alone would mean healthier ecosystems.
Blackman’s law says the rate is set by the slowest factor. Try applying the same reasoning outside biology — to a factory, a queue at a counter, or your own study routine — and judge how far the principle carries.
The oxygen we breathe and the ozone layer that shields us are both by-products of a reaction that plants carry out for their own benefit. What does this suggest about how habitable environments come to exist?
12.13 Check Your Concepts
Why is the “dark reaction” a misleading name? Under what conditions does it actually run fastest in an intact leaf, and why?
Explain why the thylakoid must be a closed sac for ATP synthesis by chemiosmosis to take place.
What happens to the light reaction if the enzymes of the Calvin cycle are suddenly inhibited? Explain your answer in terms of ADP and \(NADP^+\).
Arrange the following in the correct sequence of the non-cyclic pathway: \(NADP^+\) reductase, photolysis of water, \(P_{700}\), electron transport chain, \(P_{680}\), \(NADPH_2\).
Arrange the intermediates of the Calvin cycle in the correct order: 3-PGAL, unstable 6-carbon compound, RuBP, 1,3-diphosphoglyceric acid, 3-PGA, glucose.
Arrange the following steps of photorespiration in the correct order and name the organelle in which each occurs: glycine, glycolate, glycerate, phosphoglycolate, serine, glyoxylate.
Draw a labelled diagram of the ultrastructure of a chloroplast and mark the exact sites at which the light reaction and the dark reaction take place.
Draw a labelled sketch of Kranz anatomy and indicate the cell in which PEPCO acts and the cell in which Rubisco acts.
Sketch the absorption spectrum of chlorophyll a, chlorophyll b and carotenoids on the same axes, and mark the region of the spectrum that a leaf mostly reflects.
A student says that carotenoids are useless because they cannot carry out the photochemical act. Give two reasons why this statement is incorrect.
Why can an infra-red photon not excite an electron in a chlorophyll molecule, however many such photons strike the leaf?
Distinguish between the excited state and the ionized state of chlorophyll a.
Why does an extracted chlorophyll solution glow red when a torch is flashed on it, while chlorophyll inside an intact leaf does not behave this way?
What would be the consequence for a plant if its peroxisomes lacked the enzyme catalase, given the reactions that occur there during photorespiration?
Explain why photorespiration is described as wasteful, when respiration — which also releases \(CO_2\) — is described as essential.
A \(C_3\) plant and a \(C_4\) plant are grown side by side in an atmosphere with an unusually low \(CO_2\) concentration. Predict which one will continue to fix carbon, and explain why.
CAM plants have no Kranz anatomy, yet they achieve the same protection from photorespiration that \(C_4\) plants achieve. Explain how.
What would happen to a CAM plant that was forced, by some treatment, to keep its stomata open during the day and closed at night?
Explain why an increase in the water content of a leaf raises the rate of photosynthesis, even though very little of that water is used as a reactant.
A gardener doubles the light intensity falling on a plant and observes no change in its rate of photosynthesis. Using Blackman’s law of limiting factors, explain the observation and suggest what the gardener should do next.
The curves in the graph of \(CO_2\) concentration against rate of photosynthesis and in the graph of light intensity against rate of photosynthesis have the same general shape. Explain what this similarity reveals.
Why is raising the atmospheric \(CO_2\) concentration to about 1% helpful to most plants, while much higher concentrations are harmful?
Photosynthetic lamellae are removed from a chloroplast and suspended in a nutrient medium containing \(CO_2\), in the presence of light. Will sugar be synthesized? Justify your answer with reference to the site of each stage.
Explain, in terms of the reactions studied in this chapter, why the oxygen in the atmosphere and the ozone layer above it can both be traced back to photosynthesis.
Justify the statement: all life on earth is ‘bottled solar energy’.
Describe the light-dependent steps of photosynthesis. How are they linked to the dark reactions?
Distinguish between
- respiration and photorespiration
- absorption spectrum and action spectrum
- cyclic photophosphorylation and non-cyclic photophosphorylation
What are the steps that are common to \(C_3\) and \(C_4\) photosynthesis?
Are the enzymes that catalyse the dark reactions of carbon fixation located inside the thylakoids or outside the thylakoids?
Calvin cycle consists of three phases, what are they? Explain the significance of each of them.
Why are the plants that consume more than the usual 18 ATP to produce 1 molecule of glucose favoured in tropical regions?
What is the advantage of having more than one pigment molecule in a photocentre?
Why does chlorophyll appear green in reflected light and red in transmitted light? Explain the significance of these phenomena in terms of photosynthesis.
Explain why photosynthesis is considered the most important process in the biosphere.
Why is photolysis of water accompanied with non-cyclic photophosphorylation?
In \(C_4\) plants, why is the \(C_3\) pathway operated in bundle sheath cells only?
What would have happened if \(C_4\) plants did not have Kranz anatomy?
Why does RUBISCO carry out preferentially carboxylation rather than oxygenation in \(C_4\) plants?
What would have happened if plants did not have accessory pigments?
How can you identify whether a plant is \(C_3\) or \(C_4\)? Explain and justify your answer.
In \(C_4\) plants, bundle sheath cells carrying out the Calvin cycle are very few in number; even then, \(C_4\) plants are highly productive. Explain.
What is the functional significance of Kranz anatomy?
Study the given scheme, correct the pathway shown and name it.
Diagram
Fig. 12.17 – Scheme to be corrected: mesophyll cell showing \(CO_2\) → PEP → OAA → malic acid, with an arrow leading to the bundle sheath cell showing \(CO_2\) → \(C_3\) pathway; arrows deliberately misplaced for the student to correct
- Examine the given scheme of the light reaction, identify the error and redraw it correctly.
Diagram
Fig. 12.18 – Scheme to be corrected: \(H_2O\) + ½ \(O_2\) with 2H⁺ and 2e⁻ entering PS-II, primary acceptor, cytochrome complex, ADP → ATP, PS-I, primary acceptor, NADP reductase and \(NADP^+\) → NADPH, with the sequence deliberately shown incorrectly
12.14 Practice With Numericals
How many molecules of ATP and \(NADPH_2\) are required by a \(C_3\) plant to synthesize 5 molecules of glucose through the Calvin cycle?
A plant completes 36 turns of the Calvin cycle. Calculate the number of \(CO_2\) molecules fixed, the number of 3-PGA molecules formed, and the number of glucose molecules that can be synthesized.
In one round of the Calvin cycle beginning with 6 molecules of \(CO_2\), 12 molecules of 3-PGAL are produced. How many of these are used for glucose synthesis and how many for regeneration of RuBP? How many molecules of RuBP are regenerated, and at the cost of how many ATP?
Calculate the total number of ATP molecules required by a \(C_4\) plant to produce 6 molecules of glucose, and state how many of these are the additional ATP demanded by the \(C_4\) shuttle.
A \(C_3\) crop and a \(C_4\) crop each fix 30 molecules of \(CO_2\). Calculate the difference in the total ATP expenditure between the two.
During photorespiration, two molecules of glycine are converted into one molecule of serine with the release of \(CO_2\). If 40 molecules of glycine enter the mitochondria, how many carbon atoms are lost as \(CO_2\), and what percentage of the carbon entering as glycine does this represent?
A plant kept at a fixed light intensity can utilize a maximum of 10 mg of \(CO_2\) per hour. The \(CO_2\) supply is raised in steps from 4 mg per hour to 8 mg per hour and then to 16 mg per hour. State the rate of \(CO_2\) utilization at each step and name the limiting factor in each case.
Using the equation \(6CO_2 + 24H_2O \xrightarrow[\text{Chloroplast}]{\text{Light}} C_6H_{12}O_6 + 18H_2O + 6O_2 \uparrow\), calculate the number of water molecules that must be split and the number of oxygen molecules released when 4 molecules of glucose are formed.
12.15 Objective Questions
A cell that lacks chloroplast does not
- evolve carbon dioxide
- liberate oxygen
- require water
- utilize carbohydrates
Energy is transferred from the light reaction step to the dark reaction step by
- chlorophyll
- ADP
- ATP
- RuBP
Which one is wrong in photorespiration
- It occurs in chloroplasts
- It occurs in day time only
- It is characteristic of \(C_4\)-plants
- It is characteristic of \(C_3\)-plants
Non-cyclic photophosphorylation differs from cyclic photophosphorylation in that the former
- involves only PS I
- includes evolution of \(O_2\)
- involves formation of assimilatory power
- both ‘b’ and ‘c’
For fixation of 6 molecules of \(CO_2\) and formation of one molecule of glucose in Calvin cycle, requires
- 3 ATP and 2 \(NADPH_2\)
- 18 ATP and 12 \(NADPH_2\)
- 30 ATP and 18 \(NADPH_2\)
- 6 ATP and 6 \(NADPH_2\)
In maize and wheat the first stable products formed in bundle sheath cells respectively are
- OAA and PEPA
- OAA and OAA
- OAA and 3PGA
- 3PGA and OAA
The head and tail of chlorophyll molecule are made up of
- porphyrin and phytin respectively
- pyrrole and tetrapyrrole respectively
- prophyrin and phyrol respectively
- tetrapyrole and pyrrole respectively
The net result of photo-oxidation of water is release of ……………..
- electron and proton
- proton and oxygen
- proton, electron and oxygen
- electron and oxygen
For fixing one molecule of \(CO_2\) in Calvin cycle ………….. are required
- 3ATP + 1\(NADPH_2\)
- 3ATP + 2\(NADPH_2\)
- 2ATP + 3\(NADPH_2\)
- 3ATP + 3\(NADPH_2\)
In presence of high concentration of oxygen, RuBP carboxylase converts RuBP to …………….
- Malic acid and PEP
- PGA and PEP
- PGA and malic acid
- PGA and phosphoglycolate
The sequential order in electron transport from PSII to PSI of photosynthesis is
- FeS, PQ, PC and Cytochrome
- FeS, PQ, Cytochrome and PC
- PQ, Cytochrome, PC and FeS
- PC, Cytochrome, FeS, PQ
Assertion: Oxygen released during photosynthesis comes from water and not from carbon dioxide. Reason: Ruben supplied Chlorella* with \(H_2{}^{18}O\) and detected the heavy isotope in the oxygen evolved. Choose whether both statements are true and whether the reason correctly explains the assertion, and justify your choice.*
Assertion: Photosystem I can produce ATP without releasing any oxygen. Reason: Cyclic photophosphorylation does not involve photolysis of water. State whether both are correct and whether the reason explains the assertion.
Assertion: A \(C_4\) plant spends more ATP per molecule of glucose than a \(C_3\) plant, yet is described as a better photosynthesizer. Reason: Photorespiration is practically absent in \(C_4\) plants. Evaluate both statements and explain the connection between them.
Match the following:
| Column A | Column B |
|---|---|
| a. \(P_{680}\) | i. Regeneration of PEP |
| b. Rubisco | ii. Photolysis of water |
| c. PEPCO | iii. Carboxylation of RuBP |
| d. Pyruvate-phosphate dikinase | iv. Formation of oxaloacetic acid |
- Match the following:
| Column A | Column B |
|---|---|
| a. Xanthophyll | i. Storage of malic acid |
| b. Stroma | ii. Contains oxygen in the molecule |
| c. Vacuole of a CAM plant | iii. Site of carbon fixation |
| d. Granum | iv. Site of photolysis of water |