Chapter 7: Respiration in Plants
In the summer of 1779, a Dutch physician named Jan Ingenhousz shut himself away in a house near Vienna with jars, glass tubes and a great many plants. Joseph Priestley had already shown that a sprig of mint could somehow restore air that a candle had spoiled. Ingenhousz wanted to know exactly when a plant did this, and which parts of it were responsible.
He pushed leafy shoots under water in glass vessels and watched. In bright sunlight, tiny bubbles collected on the green surfaces, and when he tested the gas he had gathered, it turned out to be excellent for burning and for breathing. So far, this matched Priestley’s result.
Then he moved the same vessels into the shade, and later into the dark. The bubbling stopped. Worse, the air around the plant slowly became bad — the sort of air in which a flame could not keep going. Roots, flowers and ripening fruits spoiled the air in the same way even in daylight.
Ingenhousz drew a conclusion that surprised many people at the time. A plant does not only give out oxygen. Every part of it, all through the day and all through the night, is quietly using oxygen up and returning carbon dioxide — exactly as a breathing animal does.
Diagram
Fig. 7.0-1 — Ingenhousz’s Comparison Of A Plant In Light And In Darkness Two identical inverted glass vessels standing in water, each holding a leafy shoot; the left vessel in sunlight with bubbles rising from the leaf surfaces and collecting at the top, the right vessel covered by a dark cloth with no bubbles; label sunlight, dark cover, leafy shoot, water, gas collected.
7.1 What Is Respiration?
Plants look still. They do not run, hunt or call out. Yet inside every one of their cells, work is going on without a pause, and all work costs energy.
That energy is not taken in ready-made. It has to be released, inside the cell, from food. The process that releases it is called respiration — the controlled breakdown of food inside living cells to set free the energy stored in it.
7.1.1 Why Every Living Cell Needs Energy
Think of what a plant cell is doing on an ordinary afternoon. It is joining amino acids together to build proteins, and joining glucose molecules together to make starch.
At the same time, its root cells are pulling in minerals from the soil water, often against the natural direction of movement. Elsewhere, new cells are laying down fresh cell walls around themselves.
Not one of these jobs happens on its own. Each has to be pushed along, and the push comes from energy released inside the cell.
The fuel used for this is glucose, a simple sugar with the formula \(C_6H_{12}O_6\). A leaf makes glucose in photosynthesis; the rest of the plant receives it and breaks it down again to get the energy back out.
7.1.2 The Overall Reaction Of Respiration
In most cells, most of the time, glucose is broken down using oxygen. Written as a word equation, the whole change is:
glucose + oxygen → carbon dioxide + water + energy
The same change, written as a balanced chemical equation, is:
\[C_6H_{12}O_6 + 6O_2 \xrightarrow{\text{enzymes}} 6CO_2 + 6H_2O + \text{energy}\]
Read it as a summary, not as a single event. It tells you what goes in and what comes out, and nothing at all about what happens in between.
The energy released does not all appear in one form. Part of it leaves the cell as heat, and a large part is captured in a chemical form the cell can spend later.
7.1.3 Three Features Of This Reaction
Three things about that equation are worth pausing over, because each of them is what keeps respiration different from an ordinary chemical burn.
7.1.3.1 Breakdown In Steps: Glycolysis And The Krebs Cycle
Glucose is not smashed apart in one go. It is taken to pieces in a long series of small chemical steps, and only a little energy is released at each step.
These steps fall into two major phases. The first is glycolysis, in which one glucose molecule is broken into two molecules of pyruvate. Glycolysis takes place in the cytoplasm of the cell, and it does not need oxygen.
The second phase is the Krebs cycle, in which pyruvate is broken down completely to carbon dioxide and water. This phase runs inside the mitochondria — the tiny rod-shaped bodies in the cytoplasm where most of the cell’s energy is finally released. It needs oxygen, and it yields far more energy than glycolysis does. The two phases in order are set out in FC 7.1.
Flowchart
FC 7.1 — The Two Phases Of Glucose Breakdown Glucose (in cytoplasm) → glycolysis → 2 molecules of pyruvate → pyruvate enters a mitochondrion → Krebs cycle (oxygen used) → carbon dioxide + water + a large yield of energy
Releasing the energy in small instalments matters. A cell that let all of it go at once would simply cook itself; released a step at a time, most of it can be caught and stored instead.
7.1.3.2 Each Step Needs Its Own Enzyme
Every one of those small steps is brought about by a particular enzyme — a protein made by the cell that speeds up one specific reaction without being used up itself.
One enzyme handles one step and no other. So the whole of respiration depends on a full set of enzymes, working in a fixed order, like a line of workers each doing a single job.
This is also why respiration runs perfectly well at the ordinary temperature of a living body. The enzymes do the work that heat would otherwise have to do.
7.1.3.3 Energy Released As ATP
The energy set free in these steps is not all lost as heat. A large part of it is trapped in a chemical substance called ATP, short for adenosine triphosphate.
ATP is a small molecule that carries energy in the bonds holding its phosphate groups on. Whenever the cell needs energy anywhere — for building a protein, for pulling in a mineral — it breaks one of those bonds and spends the energy released.
Because every activity in the cell is paid for in this one substance, ATP is described as the energy currency of the cell. Glucose is the cell’s savings; ATP is the change it spends.
When one molecule of glucose is completely oxidised, a total of 38 molecules of ATP is formed.
7.1.4 The ATP–ADP Cycle
When ATP is spent, it does not vanish. It loses one phosphate group and becomes ADP, or adenosine diphosphate, and the energy held in that bond is handed over to the job in hand.
The ADP then waits. As soon as more glucose is broken down, the energy released is used to attach a phosphate group back on, and ADP becomes ATP once again.
So the cell runs a small, endless cycle, shown in Fig. 7.1-1. The same molecules are charged, spent, and charged again, hundreds of times over.
Diagram
Fig. 7.1-1 — The ATP–ADP Cycle In A Cell Circular diagram: at the top, breakdown of glucose in respiration releasing energy, with arrows branching to heat and to the cycle; the cycle itself shows ADP + phosphate taking in energy to form ATP on one side, and ATP breaking to ADP + phosphate on the other side with an arrow leading out to ‘energy used in work’; label glucose, oxygen, carbon dioxide, water, heat, ADP, phosphate, ATP, energy used in work.
7.1.5 Anabolic And Catabolic Activities
All the chemical activity of a living body falls into two broad kinds, and respiration belongs firmly to one of them.
Some processes build large molecules from small ones — proteins from amino acids, starch from glucose. These are anabolic processes, and they consume energy.
Others break large molecules down into smaller ones and set energy free as they do so. These are catabolic processes, and the energy they give out is what pays for the anabolic ones.
Respiration breaks glucose down, releases carbon dioxide and water, and gives out energy. It is therefore a catabolic process — in fact the central catabolic process of every living cell.
7.2 Respiration Compared With Burning
Burn a lump of coal and three things happen. It takes in oxygen, it gives out carbon dioxide and water vapour, and it releases a great deal of energy.
Respiration does all three as well. So people often say that a cell ‘burns’ its food, and at first the comparison looks exact.
It is not. Combustion, the ordinary burning of a fuel, and respiration are alike only in what goes in and what comes out. Everything about the way they get there is different, as Table 7.1 sets out.
Where The Comparison Breaks Down
Start with temperature. Coal will not burn until it is heated to its ignition point, and once it catches, the flame is far hotter still. A cell releases energy from glucose at the ordinary temperature of the plant, on a mild morning as readily as on a hot one.
The reason lies in the previous section. Burning is driven by heat, so it needs heat to begin. Respiration is driven by enzymes, so it needs no flame at all.
Then look at the pace. A fire releases the whole of the fuel’s energy in one uncontrolled rush, as heat and light together. A cell takes glucose apart in many small enzyme-controlled steps and saves most of the energy as ATP, letting only a part escape as heat. Nothing glows, because no step is violent enough to give out light.
7.3 The Entire Plant Respires
There is no organ in a plant that does the breathing for the rest of it. Every living part respires for itself — the leaves, the stem, the flowers, the roots buried in the soil.
That includes cells lying deep inside a thick stem or a swollen root, far from any surface. They too need a steady supply of oxygen, and they too must get rid of the carbon dioxide they make.
7.3.1 The Three Inlets Of Oxygen
Oxygen reaches the plant from the atmosphere through three openings, shown in Fig. 7.3-1.
The first is the stomata, tiny pores scattered over the surface of a leaf, mostly on its lower side. Each pore is bounded by two curved guard cells that open and close it.
The second is the lenticels, small raised slits in the bark of an older stem. They stay permanently open, and through them air passes into the loose tissue beneath.
The third is not a special opening at all. In a young root, the whole outer surface is thin and in direct contact with the film of air trapped between the soil particles, so oxygen simply passes straight in.
Diagram
Fig. 7.3-1 — The Three Inlets Of Oxygen In A Plant Whole plant drawing with three enlarged call-out circles: a leaf surface showing stomata, a portion of woody stem showing lenticels, and a young root showing gas exchange over its general surface; label stomata, lenticels, general surface of the root, and mark the direction of oxygen entry and carbon dioxide exit with arrows.
Once oxygen is inside, no vessel carries it anywhere. It moves by diffusion — the spreading of a substance from where it is more concentrated to where it is less concentrated — passing from cell to cell through the air spaces between them until it reaches the cells that are using it. Carbon dioxide travels out along the same route, in the opposite direction.
Diagram
Fig. 7.3-2 — Surface View Of A Stoma Enlarged surface view of a single stoma in the epidermis of a leaf; label guard cell, chloroplast in guard cell, stomatal pore, and neighbouring epidermal cells; show arrows for oxygen entering and carbon dioxide leaving.
7.3.2 Soil Air And The Respiration Of Roots
A root does not sit in solid earth. Between the soil particles lie narrow spaces holding air, and it is from this soil air that root cells take their oxygen.
Anything that squeezes those spaces shut starves the root. In compact soil the particles are pressed tightly together, and in waterlogged soil the spaces are filled with water instead of air. In both cases oxygen cannot reach the root, respiration slows, and the plant wilts even though the soil around it may be wet.
This is why fields are ploughed. Turning the soil over breaks up the packed lumps and opens fresh air spaces around the particles, so that oxygen can once again reach the roots.
7.3.3 Gas Exchange By Day And By Night
In daylight a leaf is doing two things at once, and this is where students most often get confused.
Photosynthesis is running, and it produces oxygen. Respiration is running too, and it uses oxygen up. During the day photosynthesis is much the faster of the two, so the leaf uses some of the oxygen it has just made and lets the surplus diffuse out.
The carbon dioxide made in respiration is dealt with in the same tidy way. Much of it never leaves the leaf at all, because photosynthesis takes it straight up as a raw material.
At night everything changes. There is no light, so photosynthesis stops; respiration does not. The leaf now behaves like any other living part — it takes oxygen in from the atmosphere and gives carbon dioxide out.
7.4 Two Kinds Of Respiration
Everything said so far has assumed that oxygen is available. Usually it is. But a seed buried in wet soil, or a fruit packed tightly in a basket, may find itself cut off from air for a while.
Such cells do not stop releasing energy. They switch to a second, poorer way of doing it. Respiration therefore comes in two kinds, named by whether or not free oxygen is used.
7.4.1 Aerobic Respiration
When free oxygen is available, glucose is oxidised completely. Nothing usable is left over: the carbon leaves as carbon dioxide, the hydrogen leaves combined with oxygen as water.
This is aerobic respiration, also called oxybiotic respiration, and it is the normal method in almost all plants and animals. Written as an equation, and remembering that many steps lie hidden inside it:
\[C_6H_{12}O_6 + 6O_2 \xrightarrow{\text{enzymes}} 6CO_2 + 6H_2O + 38\ \text{ATP}\]
The important figure in that line is 38. Because the glucose is taken all the way apart, almost all the energy stored in it is recovered.
7.4.2 Anaerobic Respiration
Without oxygen, glycolysis can still run and pyruvate can still be made — but the Krebs cycle cannot, because that phase needs oxygen. The breakdown therefore stops half-way.
Pyruvate is converted instead into ethanol, also called ethyl alcohol, and carbon dioxide is released alongside it. This is anaerobic or anoxybiotic respiration:
\[C_6H_{12}O_6 \xrightarrow{\text{enzymes}} 2C_2H_5OH + 2CO_2 + 2\ \text{ATP}\]
Only 2 molecules of ATP are gained, against 38. The reason is written into the products: ethanol is itself a fuel, and the energy still locked inside it is simply thrown away. The choice facing the cell is set out in FC 7.2.
Flowchart
FC 7.2 — What Happens To Glucose With And Without Oxygen Glucose → glycolysis in cytoplasm → pyruvate → branches into two paths: (a) oxygen available → Krebs cycle in mitochondrion → carbon dioxide + water + 38 ATP; (b) oxygen absent → ethanol + carbon dioxide + 2 ATP
For a plant, this arrangement is only a stopgap. No part of a plant can go on respiring anaerobically for more than a few days; the ethanol collects, the energy yield stays too low, and the part eventually dies.
Some organisms, however, live this way all the time. Certain bacteria and certain fungi respire only anaerobically, and get through life on the small yield that this gives.
Germinating seeds show the same behaviour when air is kept away from them, and Experiment 5 in section 7.5.6 is built on exactly that.
7.4.3 Aerobic And Anaerobic Respiration Compared
The two kinds differ in what they need, how far they go, what they leave behind, how much they yield and how long they can last. Table 7.2 places them side by side.
7.5 Experiments On Respiration In Plants
Respiration cannot be seen. What can be seen is its consequences — oxygen disappearing, carbon dioxide appearing, warmth building up in a flask.
Each of the experiments that follow isolates one of those consequences and makes it visible. Germinating seeds are used again and again, for a simple reason: a seed that is sprouting is building a new plant at speed, so it respires far faster than a resting seed does.
7.5.1 Every Experiment Needs A Control
Suppose limewater turns milky beside a jar of seeds. Does that prove the seeds produced the carbon dioxide? Not by itself. The gas might have come from the air already in the jar, or from bacteria growing on the seeds.
The only way to settle it is to run the same experiment twice over, side by side, with everything identical except the one thing being tested. The set-up in which that one condition is missing is called the control.
If the two set-ups then behave differently, the difference can only be due to the one condition that differed. In these experiments the control usually holds dead seeds — killed by boiling, and washed with an antiseptic, a chemical that prevents the growth of bacteria and fungi on them.
7.5.2 Showing That Oxygen Is Used Up
The apparatus is shown in Fig. 7.5-1. Two flasks are set up. Flask A holds germinating bean seeds; flask B, the control, holds an equal quantity of dead seeds that have been boiled and treated with an antiseptic such as carbolic acid.
Wet cotton wool at the base supplies water to both. A small tube of soda lime — a mixture of sodium hydroxide and calcium oxide, which absorbs carbon dioxide — hangs inside each flask. Each flask is corked, and a delivery tube leads from it down into a beaker of water.
The soda lime is the key to the design. Respiring seeds take oxygen out of the air in the flask and put carbon dioxide back in; if that carbon dioxide were allowed to stay, the volume of gas in the flask would hardly change at all. With the soda lime mopping it up, every molecule of oxygen used simply disappears from the flask.
Gas in the flask now occupies less space than before, so the pressure inside falls, and water is pushed up the delivery tube by the atmosphere outside. After a few days, the water has risen much higher in the tube of flask A than in that of flask B.
The corks are then removed and a burning splint of paper is lowered into each flask. In flask A the flame goes out at once. In flask B it burns on for a short while.
Both results say the same thing. The air of flask A has lost its oxygen, and the height of the water column measures how much was taken.
Diagram
Fig. 7.5-1 — Experiment To Show That Oxygen Is Used Up In Respiration Two identical corked flasks side by side on a stand, each with a small suspended tube of soda lime and a delivery tube dipping into a common trough of water; flask A contains germinating beans on wet cotton wool, flask B contains dead beans on wet cotton wool with antiseptic; show water risen high in the tube from flask A and only slightly in the tube from flask B; label flask A, flask B, soda lime, germinating beans, dead beans, wet cotton wool with antiseptic, water level.
There is still a small rise in flask B, and it is worth asking why. The dead seeds respire not at all, but the soda lime goes on absorbing the carbon dioxide that was already present in the air sealed inside the flask. That slight loss of gas lifts the water a little.
7.5.3 Showing That Germinating Seeds Give Out Carbon Dioxide
This experiment tests the other half of the exchange, and its arrangement is shown in Fig. 7.5-2.
Wet cotton wool is placed at the bottom of two flasks. Soaked pea or bean seeds go into flask A, and an equal number of boiled, dead seeds — with a little carbolic acid added — go into flask B. Both are corked securely and left together in the same light and the same temperature.
A few days later the seeds in flask A have clearly germinated, while those in flask B, being dead, show no change. The gases inside are now tested.
The cork is removed and the flask tilted over a test tube containing limewater, a dilute solution of calcium hydroxide, and the tube is shaken. Carbon dioxide is heavier than air, so it flows downward out of the tilted flask into the test tube.
The limewater from flask A turns milky. That from flask B stays clear. Since the two flasks differed only in whether their seeds were alive, the conclusion is fixed: it is the respiring seeds that give out carbon dioxide.
Diagram
Fig. 7.5-2 — Experiment To Show The Production Of Carbon Dioxide By Germinating Seeds Two corked flasks, A containing soaked germinating beans on wet cotton wool and B containing boiled beans, drawn beside a third view of flask A tilted over a test tube of limewater; label soaked beans, boiled beans, wet cotton wool, testing for carbon dioxide, limewater turning milky.
7.5.3.1 An Alternative Set-Up For The Same Test
The tilting method works, but the result arrives only at the end. A more directly observable version, shown in Fig. 7.5-3, lets air travel through a train of four flasks while a suction pump at the far end keeps it moving.
Air first passes through flask A, containing potassium hydroxide solution, which dissolves any carbon dioxide already present in it. It then bubbles through the limewater in flask B, which stays clear — proof that the air now entering the apparatus is free of carbon dioxide.
This carbon dioxide-free air passes over germinating seeds in flask C, and on into the limewater of flask D. That limewater turns milky.
Because the air arriving at flask C had been stripped of carbon dioxide, the gas detected in flask D can have only one source: the germinating seeds.
Diagram
Fig. 7.5-3 — Air Train To Show Carbon Dioxide Given Out By Germinating Seeds Four conical flasks A to D in a row, each with a two-hole rubber stopper and bent glass tubes joining one to the next, with an arrow marked ‘air’ entering at A and a tube to an air suction pump leaving D; flask A holds potassium hydroxide solution, flask B clear limewater, flask C germinating seeds, flask D limewater turning milky; label all four flasks, the solutions, the bent glass tubes, the two-hole rubber stoppers and the direction of air flow.
7.5.4 Showing That A Green Plant Gives Out Carbon Dioxide
Seeds are convenient because they are not green. A green plant is harder to test, and Fig. 7.5-4 shows how the difficulty is met.
A small potted plant, such as a geranium, is covered with a bell jar standing on a glass sheet. The rim of the jar and every joint are smeared with vaseline so that no outside air can leak in.
An air pump draws outside air through the apparatus. On its way in, the air passes through soda lime, which removes its carbon dioxide, and then through limewater in flask A. That limewater stays clear, confirming that the air reaching the plant carries no carbon dioxide.
The air then passes over the plant, leaves the bell jar, and bubbles through a second sample of limewater in flask B. This limewater turns milky, so the plant has added carbon dioxide to the air.
One precaution decides whether the experiment works at all. The whole apparatus must be kept in the dark, or the bell jar wrapped in black cloth. In light the plant would photosynthesise, and the carbon dioxide released in respiration would be taken up again for making starch — leaving flask B clear and the experiment apparently a failure.
Diagram
Fig. 7.5-4 — Experiment To Show That A Green Plant Gives Out Carbon Dioxide A vaselined bell jar on a glass sheet enclosing a small potted plant, connected on one side through a tube of soda lime and a flask of clear limewater (flask A), and on the other side to a flask of limewater turning milky (flask B) leading to an air suction pump; the bell jar shown covered with black cloth; label soda lime, carbon dioxide-free air, flask A with clear limewater, potted plant, black cloth, flask B with limewater turning milky, air suction pump, direction of air flow.
7.5.5 Showing That Heat Is Evolved
Energy released in respiration is partly saved as ATP and partly lost as heat. In a single cell that heat is far too little to notice, so this experiment collects it from a great many cells at once and keeps it from escaping. The arrangement is shown in Fig. 7.5-5.
About 30 grams (g) of pea or bean seeds are soaked in water for more than 24 hours, then divided into two equal groups. One group is killed by boiling, and washed with dilute formalin or carbolic acid to stop bacterial decay.
The live germinating seeds go into thermos flask A and the killed seeds into thermos flask B. A thermometer is inserted into each, and the mouths are plugged with cotton wool.
The vacuum walls of a thermos flask stop heat from leaking away, and the cotton wool plug lets air in while keeping warmth from drifting out. Any heat produced inside therefore stays inside.
After a few hours the thermometer in flask A reads distinctly higher. The one in flask B shows no rise at all. Since only flask A holds respiring seeds, respiration must be giving out heat.
Diagram
Fig. 7.5-5 — Experiment To Show That Heat Is Evolved During Respiration Two vertical thermos flasks in section, A labelled experiment and B labelled control, each with a thermometer inserted through a cotton wool plug at the mouth; flask A packed with germinating seeds, flask B with dead seeds treated with antiseptic; show a higher reading on thermometer A; label germinating seeds, dead seeds with antiseptic, thermometer, cotton wool plug, vacuum wall.
7.5.6 Demonstrating Anaerobic Respiration
The last experiment, shown in Fig. 7.5-6, must keep air away from the seeds altogether — and water will not do that, because air dissolves in it.
Eight to ten soaked pea seeds are peeled and placed in a test tube. The tube is filled to the brim with mercury and inverted in a beaker of mercury, so that the seeds are completely surrounded by it and no air can reach them.
After about two days the level of mercury in the tube has fallen, which means a gas has collected above it. The seeds have been respiring anaerobically, and the gas released is carbon dioxide.
The gas can be identified without dismantling the apparatus. A small stick of potassium hydroxide is introduced at the mouth of the tube; being lighter than mercury, it floats up through it, meets the gas and absorbs the carbon dioxide. As the gas is absorbed, the mercury rises again to fill the space.
A control is set up in the same way, with seeds that have been killed by heating and sterilised with an antiseptic. No gas is liberated in it.
Peeling the seed coat before the seeds go in is not fussiness. The coat slows the escape of carbon dioxide from the seed, and removing it lets the gas diffuse out quickly enough to be seen within two days.
Diagram
Fig. 7.5-6 — Experiment To Demonstrate Anaerobic Respiration Two beakers of mercury, each with an inverted test tube standing in it; the left tube at the start with peeled germinating seeds completely surrounded by mercury, the right tube after two to three days with the mercury level fallen and a space of collected carbon dioxide above it; label germinating seeds, mercury, carbon dioxide, after 2–3 days.
7.6 Respiration Contrasted With Photosynthesis
Set the two processes down next to each other and something striking appears. What photosynthesis needs, respiration produces; what photosynthesis produces, respiration needs.
Photosynthesis takes in carbon dioxide and water and makes glucose and oxygen. Respiration takes in glucose and oxygen and gives back carbon dioxide and water. The two are not merely different — they run in opposite directions.
This is why they are called complementary processes, and why the experiment in section 7.5.4 had to be carried out in the dark. In light, one process quietly undoes the evidence of the other.
Why One Gains Weight And The Other Loses It
The weight of a plant is largely the weight of the food it has built. Photosynthesis adds glucose that was not there before, so the plant gains dry weight as it works.
Respiration does the reverse. It breaks that stored food down and releases its carbon as a gas that drifts away, so the plant loses dry weight. A potato kept for months in a dark store is lighter at the end than at the start, and this is where the missing matter has gone.
The fundamental differences between the two are set out in Table 7.3.
7.7 Respiration In Plants Compared With Respiration In Animals
At the level of the cell, respiration is the same everywhere. Glucose is broken down step by step, enzymes control every step, and ATP is the energy released.
The differences lie outside the cell — in how the gases get about, in what is made when oxygen runs short, and in how much heat is given off. Table 7.4 sets the three out.
A plant manages without a transport system for gases because it does not need one. Its body is thin, branching and full of air spaces, so no living cell is ever far from the outside air, and diffusion alone is enough to serve it.
An animal is compact, and its inner cells lie deep. Diffusion could never reach them in time, so blood is pressed into service to carry oxygen in and carbon dioxide out.
The second difference matters in the muscles. When an animal’s muscle cells run short of oxygen, they do not produce ethanol; they convert pyruvate into lactic acid instead.
The third follows from the pace of life. An animal moves, hunts and, in many cases, holds its body at a steady warm temperature, so it respires far faster and gives out much more heat than a plant, which does none of these things.
7.8 Summary
Respiration is a catabolic process in which a living cell oxidises glucose in a series of enzyme-controlled steps, releasing carbon dioxide, water and energy for carrying out life processes.
The overall reaction is \(C_6H_{12}O_6 + 6O_2 \xrightarrow{\text{enzymes}} 6CO_2 + 6H_2O + \text{energy}\), and it is only a summary of many small steps.
The breakdown occurs in two major phases — glycolysis, in which glucose becomes pyruvate in the cytoplasm, and the Krebs cycle, in which pyruvate is broken down completely inside the mitochondria.
Every step is brought about by a particular enzyme, which is why respiration proceeds at ordinary body temperature.
Much of the energy released is stored as ATP, adenosine triphosphate, the energy currency of the cell; the rest is lost as heat.
Spent ATP becomes ADP, and is recharged to ATP when more glucose is broken down. Complete oxidation of one molecule of glucose yields 38 molecules of ATP.
Anabolic processes build large molecules and consume energy; catabolic processes break them down and release it. Respiration is catabolic.
Respiration differs from burning in being cellular, occurring at body temperature, running in many steps, being driven by enzymes, storing energy as ATP and producing no light.
Every living part of a plant respires. Oxygen enters through three inlets — stomata in leaves, lenticels in stems and the general surface of roots — and then moves from cell to cell by diffusion.
Roots take oxygen from the air spaces between soil particles. Compacted or waterlogged soil closes these spaces, while ploughing opens them again.
By day, a leaf uses some of the oxygen made in photosynthesis and its respiratory carbon dioxide is taken up as a raw material. At night, photosynthesis stops and the leaf takes in oxygen and gives out carbon dioxide.
Aerobic respiration uses free oxygen, breaks glucose down completely to carbon dioxide and water, and yields 38 ATP.
Anaerobic respiration proceeds without oxygen, breaks glucose down incompletely to ethanol and carbon dioxide, and yields only 2 ATP.
Anaerobic respiration in a plant is temporary; if it continues beyond a few days the part dies. Certain bacteria and fungi respire only anaerobically throughout life.
Every experiment on respiration needs a control — an identical set-up lacking only the condition under study, usually holding dead seeds treated with an antiseptic.
Germinating seeds in a flask with soda lime use up the oxygen, so water rises in the delivery tube and a flame is put out in that flask.
Gas from a flask of germinating seeds turns limewater milky, showing that respiring seeds give out carbon dioxide.
A potted green plant under a bell jar also turns limewater milky, provided the apparatus is kept in the dark so that photosynthesis cannot use the carbon dioxide up.
Germinating seeds in a thermos flask raise the thermometer reading, showing that heat is evolved in respiration.
Peeled soaked seeds surrounded by mercury release carbon dioxide within about two days, demonstrating anaerobic respiration.
Photosynthesis and respiration are complementary and opposite: one needs light and chlorophyll and builds food, the other occurs in all living cells at all times and breaks food down.
Plants have no transport of respiratory gases, form ethanol in anaerobic respiration and produce little heat; animals transport gases in the blood, form lactic acid and produce much more heat.
Flowchart
FC 7.3 — The Chapter In One Chain Glucose in a living cell → glycolysis in cytoplasm → pyruvate → branches: (a) oxygen available → Krebs cycle in mitochondrion → carbon dioxide + water + 38 ATP + some heat; (b) oxygen absent → ethanol + carbon dioxide + 2 ATP → ATP spent in cell work → ADP + phosphate → recharged to ATP by further breakdown of glucose
7.9 Key Terms
ADP (adenosine diphosphate) — the spent form of ATP, carrying two phosphate groups, which is recharged to ATP when more energy becomes available.
Aerobic respiration — respiration in which free oxygen is used and glucose is broken down completely to carbon dioxide and water, yielding 38 ATP.
Anabolic — describing a chemical process that builds large molecules from smaller ones and consumes energy.
Anaerobic respiration — respiration that proceeds without oxygen, in which glucose is broken down incompletely to ethanol and carbon dioxide, yielding 2 ATP.
Anoxybiotic — another name for anaerobic, meaning taking place without oxygen.
Antiseptic — a chemical used to prevent the growth of bacteria and fungi, added to dead seeds in a control set-up.
ATP (adenosine triphosphate) — the chemical substance in which the cell stores and spends energy; the energy currency of the cell.
Catabolic — describing a chemical process that breaks large molecules into smaller ones and releases energy.
Combustion — ordinary burning; the rapid oxidation of a fuel at a high temperature, releasing all its energy as heat and light.
Control — the set-up in an experiment identical to the experimental one in every condition except the one being studied, which is left out.
Diffusion — the spreading of a substance from a region where it is more concentrated to one where it is less concentrated.
Enzyme — a protein made by a cell that speeds up one particular chemical reaction without itself being used up.
Ethanol — ethyl alcohol; an end product, along with carbon dioxide, of anaerobic respiration in plants.
Glucose — the simple sugar, \(C_6H_{12}O_6\), that is the fuel broken down in respiration.
Glycolysis — the first phase of respiration, occurring in the cytoplasm, in which glucose is broken into two molecules of pyruvate.
Krebs cycle — the second phase of respiration, occurring in the mitochondria, in which pyruvate is broken down completely using oxygen.
Lactic acid — the product formed in animal muscle cells when they respire without enough oxygen.
Lenticels — permanently open slits in the bark of an older stem through which gases pass in and out.
Limewater — a dilute solution of calcium hydroxide that turns milky in the presence of carbon dioxide, used to detect the gas.
Mitochondria — the small bodies in the cytoplasm in which the Krebs cycle runs and most of the cell’s ATP is formed.
Oxybiotic — another name for aerobic, meaning taking place in the presence of oxygen.
Respiration — the enzyme-controlled breakdown of glucose inside living cells, releasing carbon dioxide, water and energy as ATP and heat.
Soda lime — a mixture of sodium hydroxide and calcium oxide that absorbs carbon dioxide, used to remove the gas from air.
Stomata — tiny pores in the surface of a leaf, each bounded by two guard cells, through which gases enter and leave.
7.10 Points To Ponder
Anaerobic respiration returns only 2 ATP from a glucose molecule, against 38 from aerobic respiration. If it is so wasteful, why has it not been lost from living things altogether?
Respiration releases energy in many small steps rather than one large one. What would happen inside a cell if the whole of the energy in a glucose molecule were set free at a single step?
A plant manages without any system for transporting respiratory gases, while an animal of similar mass cannot. What is it about the shape and the way of life of a plant that makes this possible?
Seeds are stored for months in a dry, cool godown rather than a warm, damp one. Using what you know about respiration, work out what each of those two conditions is doing to the seed.
Every experiment in this chapter compares a living set-up with a dead one. Would the results be equally convincing if a single set-up were used and simply observed over a longer time? Explain your reasoning.
Photosynthesis and respiration run in opposite directions inside the same green cell during daylight. What would you need to measure to find out which of the two is going faster at a given moment?
7.11 Objective Questions
Multiple Choice
- Glycolysis takes place in the (1 mark)
- mitochondrion
- cytoplasm
- chloroplast
- cell wall
- The number of ATP molecules obtained from one molecule of glucose in anaerobic respiration is (1 mark)
- 2
- 4
- 18
- 38
- The substance used to remove carbon dioxide from air before it enters an apparatus is (1 mark)
- limewater
- mercury
- soda lime
- carbolic acid
- Gases enter the woody stem of an older plant through (1 mark)
- stomata
- root hairs
- guard cells
- lenticels
- Which one of the following is not true of respiration? (1 mark)
- It occurs at ordinary body temperature.
- It is carried out by enzymes.
- It produces light energy.
- It occurs in a series of chemical steps.
Assertion-Reason
- In each of the following, choose from:
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
- Assertion (A): An experiment to show that a green plant gives out carbon dioxide is carried out in the dark. Reason (R): In light, the carbon dioxide released in respiration would be used up in photosynthesis. (1 mark)
- Assertion (A): A plant needs no system for transporting respiratory gases. Reason (R): Every living cell of a plant lies close to an air space, so diffusion alone can supply it. (1 mark)
Match The Following
- Match each item in Column A with the correct item in Column B. (5 marks)
| Column A | Column B |
|---|---|
| a. Glycolysis | i. Turns milky in the presence of carbon dioxide |
| b. Krebs cycle | ii. Energy currency of the cell |
| c. ATP | iii. Occurs in the cytoplasm |
| d. Limewater | iv. Openings in the bark of a stem |
| e. Lenticels | v. Occurs in the mitochondria |
Fill In The Blanks
- Complete the following. (4 marks)
- The end products of anaerobic respiration in plants are ______ and ______.
- Complete oxidation of one molecule of glucose yields ______ molecules of ATP.
- When ATP loses one phosphate group it becomes ______.
- Roots obtain oxygen from the air present in the ______ between soil particles.
True Or False
- State whether each statement is true or false. (4 marks)
- Respiration occurs in plants only during the night.
- Each step in the breakdown of glucose is brought about by a particular enzyme.
- Anaerobic respiration releases more energy than aerobic respiration.
- Respiration results in a loss of dry weight in a plant.
Name The Following
- Name the following. (5 marks)
- The phase of respiration in which glucose is converted into pyruvate.
- The organelle in which the Krebs cycle occurs.
- The end product of anaerobic respiration in animal muscle.
- The liquid used in place of water to keep air away from seeds in an experiment.
- The kind of respiration also described as oxybiotic.
Odd One Out
Odd one out, and give a reason: stomata, lenticels, general surface of the root, chlorophyll. (1 mark)
Odd one out, and give a reason: boiling, carbolic acid, formalin, soda lime. (1 mark)
7.12 Check Your Concepts
Define respiration. (2 marks)
Name the three inlets through which oxygen enters a plant, stating where each is found. (2 marks)
Why is ATP described as the energy currency of the cell? (2 marks)
Give a reason: ploughing a field improves the respiration of roots. (3 marks)
Give a reason: the seeds used in the control of a respiration experiment are boiled and then treated with an antiseptic. (3 marks)
Differentiate between glycolysis and the Krebs cycle, stating the site of each within the cell. (3 marks)
Write balanced chemical equations for aerobic and for anaerobic respiration in plants. (3 marks)
Differentiate between aerobic and anaerobic respiration on the basis of the requirement of oxygen, the extent of breakdown of glucose, the end products formed and the energy released. (4 marks)
Respiration is often compared with burning. Explain, with four differences, why this comparison is only superficial. (4 marks)
What would happen to a potted plant whose soil was kept waterlogged for a week? Explain the biology behind your answer. (4 marks)
Describe the exchange of gases in a leaf by day and by night, and explain why a plant must still take oxygen from the atmosphere although its leaves release oxygen. (5 marks)
Explain how energy released from glucose is stored and later spent in a cell, and state why the breakdown of glucose occurs in a series of small steps rather than in one step. (5 marks)
7.13 Figure-Based Questions
Study Fig. 7.5-1. (3 marks)
- Name the substance held in the small suspended tube and state its purpose.
- Which of the two flasks is the control?
- Explain why the water rises in the delivery tube of flask A.
Study Fig. 7.5-6. (3 marks)
- Name the liquid surrounding the seeds and give one reason for using it instead of water.
- Name the gas that collects in the tube after about two days.
- How is the identity of this gas confirmed without dismantling the apparatus?
Study Fig. 7.3-2. (3 marks)
- Identify the structure shown.
- Name the two cells that bound the pore.
- State one function of this structure in respiration.
With reference to Fig. 7.1-1, name the substance formed when ATP loses one phosphate group, and state what happens to the energy released at that moment. (2 marks)
Study Fig. 7.5-4. (3 marks)
- Why is the bell jar covered with black cloth?
- State the purpose of the soda lime placed before flask A.
- Name the change seen in the limewater of flask B and give its cause.
Redraw the apparatus shown in Fig. 7.5-5 as a neat labelled diagram, labelling the germinating seeds, the dead seeds, the thermometer and the cotton wool plug. Use straight, non-crossing lead lines. (4 marks)