Chapter 4: Absorption by Roots — The Processes Involved
In 1748, a French priest and physicist named Jean-Antoine Nollet set up a simple piece of apparatus on his bench. He filled a glass cylinder with spirit of wine — strong alcohol — and closed its open mouth with a piece of pig’s bladder, tied down tightly. He then stood the sealed cylinder in a vessel of ordinary water and left it alone.
When he looked again some hours later, the bladder was no longer flat. It had swelled outward into a taut dome, stretched as though something inside were pushing hard against it. Nothing had been poured in. Nothing had been taken out. Yet the liquid inside had plainly gained volume, and the bladder was under such strain that it eventually gave way.
Nollet worked out what must have happened. Water from the outer vessel had crossed the bladder and joined the alcohol inside, while the alcohol had not crossed outward to anything like the same extent. The bladder was not a simple sieve, letting everything through. It let some molecules pass and held others back, and that one-sided traffic alone was enough to build a real, measurable push.
That same one-sided traffic, running across a membrane far thinner than a bladder, is what draws water out of damp soil, into a root hair, and onward into a plant — with no pump anywhere in the system.
Diagram
Fig. 4.0-1 — Nollet’s Bladder Experiment Of 1748 Line drawing of a tall glass cylinder standing in a wide vessel of water; the cylinder is filled with spirit of wine and its mouth is closed by a piece of animal bladder tied down with cord, the bladder bulging outward into a dome; label glass cylinder, spirit of wine, animal bladder tied over the mouth, cord, outer vessel of water, and an inward arrow through the bladder labelled water moving in.
4.1 Absorption By The Roots
Every part of a plant that you can see is cut off from the soil. A leaf never touches soil water, and neither does a flower or a fruit. Whatever water and minerals they receive must first be taken in somewhere else and carried to them.
That somewhere else is the root. Roots do three things for a plant, and the first is the one you can see for yourself when you try to pull a weed out of the ground.
The Three Jobs Of A Root
The first job is fixing. The root grows down and outward through the soil and holds the plant in place, so that a tall stem does not topple in wind or rain.
The second job is absorption. Water and dissolved mineral nutrients are drawn in from the soil into the root.
The third job is conduction. What the root takes in does not stay there; it is passed on into the stem and carried upward to the leaves, flowers and fruits, as shown in Fig. 4.1-1.
Diagram
Fig. 4.1-1 — The Route Taken By Absorbed Water And Minerals Line drawing of a young flowering plant growing in soil, the soil shown in section; draw arrows tracing the movement of water and dissolved minerals from the soil into the root, upward through the stem, and outward into the leaves, flower and fruit; label soil water, root, root hair zone, stem, leaf, flower, fruit, and mark the arrows as direction of flow.
Of these three, absorption and conduction are the life-supporting ones. A plant with a damaged hold in the soil may still survive; a plant that cannot draw water from the soil cannot photosynthesise, cannot cool itself and cannot keep its soft tissues firm. It dries out and dies.
How A Plant Works
The study of these working processes has a name of its own. Plant physiology is the branch of biology that deals with the life functions, or metabolism, of a plant — how it works rather than merely what it is made of.
Plant physiology covers every level of organisation: the functioning of cells, of tissues, of organs, of organ systems, and of the whole organism together. This chapter stays with one of the most fundamental of those processes, osmosis, which matters not only to plants but to every living thing.
4.2 The Need For Water And Minerals In Plants
Water is not merely something a plant takes in and passes on. It is part of the plant’s own substance. Protoplasm, the living material inside every cell, is mostly water, so a cell that loses its water stops being a working cell at all.
Beyond that, water is needed inside the plant body for four separate purposes: photosynthesis, transpiration, transportation and mechanical stiffness. It is worth taking these one at a time, because they are four quite different kinds of demand.
4.2.1 The Four Purposes Water Serves
Photosynthesis. In the green leaves, water is used up as a raw material in the synthesis of glucose. Here the water molecule is not simply a carrier — it is taken apart and built into the food the plant makes.
Transpiration — the loss of water from a plant as water vapour, mainly through the leaves. A large quantity of water is evaporated in this way. The evaporation cools the plant in hot weather, and it also produces a suction force that helps to draw more water upward from below.
Transportation. Substances travel through a plant only in water solution. Mineral salts move upward from the roots into the shoot, and sugar made in the leaves moves out to the other parts. Water here is the solvent that carries everything else.
Mechanical stiffness. Water fills the cells and keeps them fully distended, a firm condition called turgidity. A stem that stands upright and a leaf that lies flat and open owe their shape to this, not to any woody skeleton. Section 4.4.5 examines how the condition is produced.
The four uses are drawn together in Fig. 4.2-1.
Diagram
Fig. 4.2-1 — The Four Uses Of Water In A Plant Four-panel line drawing sharing one caption: panel 1, a leaf in sunlight with an arrow marked water in and a glucose molecule symbol, labelled photosynthesis; panel 2, a leaf surface with water vapour escaping through a stoma, labelled transpiration; panel 3, a stem in section with an upward arrow marked mineral salts and a downward arrow marked sugar, labelled transportation; panel 4, an upright firm shoot beside a drooping shoot, labelled mechanical stiffness.
4.2.2 Mineral Nutrients And Their Use In The Plant
A plant can take in carbon dioxide through its leaves, but it cannot take in a single mineral that way. The mineral nutrients a plant needs are absorbed from the soil, and by the roots only. If they are not in the soil within reach of the roots, the plant does not get them.
These nutrients arrive in two forms. Some are absorbed as dissolved salts, such as nitrates, phosphates and sulphates. Others are absorbed simply as ions — the separate charged particles that a salt splits into when it dissolves in water — such as potassium, calcium, magnesium and chlorine.
Once inside, these elements are put to two kinds of use. Some become constituents of the cell itself and of its organelles, built into the structures that do the work. Others are used in the synthesis of a wide variety of compounds and enzymes within the cell.
4.3 Characteristics Of Roots For Absorbing Water
A root does not draw water out of the soil by wanting to. It does so because of three particular features it happens to possess, and if any one of them were missing, absorption would slow down or stop.
The three are: a huge surface area, provided by the rootlets and the root hairs; root hairs holding a solution more concentrated than the soil water around them; and root hairs with thin walls. Each is taken up below.
4.3.1 A Huge Surface Area
Water can only enter where root and soil are actually touching. So the more root surface there is in contact with the damp soil, the more water can be taken in at once.
Pull up a small garden plant such as balsam (Impatiens balsamina) gently from the soil and look at what comes out. The main root carries a thick bunch of rootlets, which are simply branch roots, and this is what a young seedling’s root looks like even a few days after germination (Fig. 4.3-1).
Diagram
Fig. 4.3-1 — A Germinated Seed Showing Root Hairs Line drawing of a freshly germinated seed with its cotyledons and young shoot above, and below it a single root bearing a dense covering of fine root hairs along its length; label cotyledon, young shoot, root, and root hairs.
Examine any one rootlet carefully and it shows hundreds of fine outgrowths called root hairs. Each is a delicate thread, far too fine to be picked out by the unaided eye at a glance. If all the root hairs of one small plant were laid end to end, they would cover a length of many kilometres.
That claim sounds unlikely until somebody counts them, which is exactly what happened.
4.3.2 Cell Sap Of Higher Concentration
Root hairs are not separate structures stuck onto the root. Each one is an extension of an outer, or epidermal, cell of the root — the cell simply pushes out a long finger of itself into the soil.
Inside, a root hair contains a large vacuole, the fluid-filled space that occupies much of a mature plant cell. The solution filling that vacuole is called cell sap.
Some salts are dissolved in the cell sap. Because of them, the cell sap is usually more concentrated than the soil water outside — that is, a given volume of it holds more dissolved matter and correspondingly less water than the same volume of soil water does.
Keep that difference in mind, because it is the reason water moves inward rather than outward. The process that turns a difference in concentration into a movement of water is osmosis, and it is described in section 4.4.3.
4.3.3 Thin Walls Of The Root Hair
Water entering a root hair has to cross whatever lies between the soil and the cell contents. In a root hair, as in every plant cell, that means two outer layers: a cell wall on the outside and a cell membrane just inside it (Fig. 4.3-2).
Diagram
Fig. 4.3-2 — A Fully Grown Root Hair And Two Younger Ones Line drawing of a row of root epidermal cells in section, one bearing a long fully grown root hair and two others bearing short outgrowths at earlier stages of growth; label epidermal cell, cell wall, cell membrane, cytoplasm, vacuole, nucleus, root hair beginning to grow out, and fully grown root hair; keep all labels on straight, non-crossing lead lines.
The cell wall is thin and permeable, which means it lets things through without picking and choosing. Water molecules and dissolved substances alike move freely in and out across it.
The cell membrane, lying just within, is very thin and semi-permeable. It allows water molecules to pass through, but holds back the larger molecules of the dissolved salts. It is selective, and the wall is not.
That single difference is the secret of the whole business. Because water can cross the membrane while dissolved salts largely cannot, the concentration difference described in section 4.3.2 is never simply evened out by salts moving outward. Instead it is settled by water moving inward.
4.4 Absorption And Conduction Of Water And Minerals
Getting water from the soil to a leaf is not one event but several. Water has to enter the root, cross the thickness of the root from cell to cell, and then be carried upward through the stem.
The whole mechanism — absorption from the soil by the roots, movement through the root, and upward conduction through the stem — is the result of five main phenomena.
- Imbibition
- Diffusion
- Osmosis
- Active transport
- Turgidity and flaccidity
They are taken up one by one in the sections that follow. The first two are simple physical processes that happen in non-living matter as readily as in a cell; the later ones need a membrane, and one of them needs a living cell.
4.4.1 Imbibition
Leave a dry wooden door out in the monsoon damp and it swells until it will no longer shut. Nothing dissolved in it; the wood simply took up water and expanded.
This taking up of water by a solid surface is called imbibition. It is a phenomenon by which living or dead plant cells absorb water by surface attraction — the water clings to the material and is held there.
Not every substance behaves this way. Substances made of cellulose or of proteins are hydrophilic, meaning they have a strong affinity for water, and it is these that imbibe water or moisture and swell up. Dry seeds and wooden doors both swell on contact with water, or even on exposure to moist air.
The swelling generates a real force, called imbibitional pressure. In a germinating seed, this pressure is what ruptures the tough seed coat and lets the young root out. Imbibition is also one of the forces involved in the ascent of sap, which section 4.8 takes up.
4.4.2 Diffusion
Open a bottle of scent at one end of a room and, after a while, someone at the other end can smell it. Nobody carried it across. The molecules spread out on their own, from where they were crowded to where they were not.
Diffusion is that spreading. It is the free movement of molecules of a substance — solute or solvent, gas or liquid — from the region of their higher concentration to the region of their lower concentration, when the two regions are in direct contact.
The phrase ‘in direct contact’ matters. In diffusion there is nothing in the way — no barrier, no membrane, no filter. The molecules simply move.
Watching Diffusion Happen
Diffusion can be shown with a simple activity, set out in Fig. 4.4-1.
Experiment 1. Place a crystal of potassium permanganate in one corner of a beaker of water and leave it undisturbed. The crystal slowly dissolves and the colour spreads through the liquid. Eventually the dissolved substance is distributed uniformly, and the whole beaker is evenly coloured.
Diagram
Fig. 4.4-1 — Diffusion Of A Coloured Solute In Water Three beakers of water drawn in a row and labelled A, B and C; in A a crystal sits in one corner with colour just beginning to spread from it, in B the colour has spread through much of the beaker, in C the colour is uniform throughout; label A diffusion started, B diffusion progressing, C diffusion completed, and mark the direction of spread with small arrows in A and B.
Read what happened in the language of the definition. At the start, the molecules of the coloured substance are crowded together in and near the crystal, so their concentration there is high. They then move away, farther and farther, into the regions where they are fewer or altogether absent.
The movement continues until the molecules are spread uniformly through the water. Stirring with a spoon or a glass rod does not change the direction of the process; it only hastens it, so that a homogeneous solution is obtained much sooner.
Notice that in this beaker there was no obstacle of any kind in the path of the dissolved molecules. They could move freely in all directions. Put a barrier in their way and something more interesting happens, which is the subject of the next section.
4.4.3 Osmosis And Osmotic Pressure
4.4.3.1 Osmosis
Suppose the two regions are not in direct contact but are separated by a membrane that lets water through and holds dissolved substances back. Now the solute cannot move to even things out. Only the water can move — and it does.
Osmosis is the movement of water molecules from their region of higher concentration to their region of lower concentration through a semi-permeable membrane. The region of higher water concentration is the dilute solution, the one with less solute in it; the region of lower water concentration is the concentrated solution, the one with more solute.
One thing in that definition is easy to misread. Only the water molecules move. They move from where water is more concentrated — whether that is a dilute solution or pure water — to where water is less concentrated, which is the stronger solution.
Water is the solvent that matters in living things, but it is not the only one. A solution made with some other solvent can also show osmosis, the solvent molecules moving from their higher to their lower concentration through a membrane that is semi-permeable to molecules of that size.
4.4.3.2 Endosmosis And Exosmosis
With reference to a cell, osmosis can run in either direction. Which direction depends on how concentrated the solution surrounding the cell is.
Endosmosis — from endo, meaning inward, and osmo, meaning push or thrust — is the inward diffusion of water through a semi-permeable membrane when the surrounding solution is less concentrated. It tends to swell up the cell.
Exosmosis — from exo, meaning outward — is the outward diffusion of water through a semi-permeable membrane when the surrounding solution is more concentrated. It tends to cause shrinkage of the cell.
4.4.3.3 Experiments To Demonstrate Osmosis
Osmosis is easiest to believe when you have watched a liquid climb uphill on its own.
Experiment 2. Take some concentrated sugar solution in a thistle funnel. Cover the mouth of the funnel with a piece of cellophane paper — an egg membrane or an animal bladder will do as well — and tie it securely. Invert the funnel in a beaker containing water and suspend it as shown in Fig. 4.4-2. Mark the level of the sugar solution on the stem of the funnel. This is the experimental set-up.
For comparison, set up a control: another thistle funnel filled with plain water, covered in the same way and suspended in another beaker of water, with its level marked on the stem.
Diagram
Fig. 4.4-2 — Demonstrating Osmosis With A Thistle Funnel Line drawing of a thistle funnel held mouth-downward in a clamp stand, its bulb containing sugar solution and its mouth tied over with cellophane paper, the bulb immersed in a beaker of water; mark two levels on the stem, an initial level and a higher final level, with an upward arrow between them; label thistle funnel, clamp stand, initial level, final level, sugar solution, cellophane paper, beaker, water.
Leave both set-ups for a few hours, then look at them.
- In the experimental set-up, the level of the sugar solution in the stem of the thistle funnel has risen.
- In the control, the level of water in the thistle funnel is unchanged.
- The water in the experimental beaker has dropped slightly, while the water in the control beaker is unchanged.
- If the water in the experimental beaker is tasted, it is not found sweet.
Three conclusions follow from these observations.
- Some water from the beaker has passed through the cellophane paper and entered the thistle funnel containing the sugar solution.
- Sugar from the thistle funnel has not passed out into the beaker.
- The cellophane paper has therefore acted as a selectively, or differentially, permeable membrane. It has allowed water molecules to pass through, but not the sugar molecules.
Changing One Thing At A Time
The set-up is worth altering, because each change tests a different part of the explanation.
Experiment 3. Suppose the thistle funnel holds a more concentrated sugar solution and the beaker holds a very dilute sugar solution instead of plain water. The result is the same: the level of the solution in the thistle funnel rises. Water from the less concentrated solution in the beaker moves across the cellophane paper into the more concentrated solution in the funnel. What drives osmosis is the difference between the two, not the presence of pure water on one side.
Now change the barrier instead. If a rubber sheet is used in place of cellophane, no change occurs in the level of the sugar solution at all. The rubber sheet is impermeable, and will not let the water molecules from the beaker cross to the other side.
If a muslin cloth is used, the opposite trouble arises. The meshes, or pores, of muslin are so large that they cannot hold back even the sugar molecules, so the entire sugar solution simply flows down to a common level under gravity. Muslin is freely permeable to sugar solution, and so it demonstrates nothing.
The same experiment can also be performed with a visking bag, which behaves as a semi-permeable membrane (Fig. 4.4-3). Place sugar solution in a knotted visking bag and insert a long glass capillary tube until some of the solution rises into the tube. Tie the mouth of the bag firmly round the tube, support it on a clamp stand, and immerse the bag in a beaker of water. After about an hour, the level of sugar solution in the capillary tube has risen, because water molecules have diffused through the wall of the bag.
Diagram
Fig. 4.4-3 — Demonstrating Osmosis With A Visking Bag Line drawing of a visking bag containing sugar solution, knotted at the base and tied at its mouth around a long vertical glass capillary tube held in a clamp stand, the bag immersed in a beaker of water; mark an initial level and a higher final level on the capillary tube with an upward arrow between them; label capillary tube, binding, visking bag, sugar solution, knot, beaker, water, initial level, final level.
4.4.3.4 What The Experiments Establish
Two points run through all these set-ups, and both are worth stating plainly.
The first is that there are two liquids of different concentrations. The sugar solution has a high concentration of sugar molecules, while the beaker has few or none. But concentration can equally well be counted from the side of the solvent: a unit volume of water in the beaker contains more water molecules than the same volume of sugar solution in the thistle funnel.
So there are two regions of different concentration, and this difference is called tonicity. It can be expressed in either of two ways — as two regions of different concentrations of water molecules, or as two regions of different concentrations of sugar molecules. The two statements describe the same situation from opposite ends.
The second is that the two liquids are separated by a membrane that selects. The cellophane paper behaves as a semi-permeable membrane, allowing the solvent through freely while preventing the passage of the solute, as Fig. 4.4-4 shows.
Diagram
Fig. 4.4-4 — How A Semi-Permeable Membrane Works Schematic line drawing of a vertical semi-permeable membrane with fine pores separating solution A on the left from solution B on the right; show small circles for water molecules and larger circles for sugar molecules, with more water molecules on the left and more sugar molecules on the right; draw arrows for water molecules crossing left to right through the pores, and show the larger sugar molecules stopped at the membrane; label semi-permeable membrane, more dilute solution (more water molecules), less dilute solution (fewer water molecules), water molecules move in this direction, sugar molecules cannot pass through the pores.
How Long Can Osmosis Continue
In theory, osmosis should carry on until the concentration of water molecules is equal — isotonic — on both sides of the membrane. In practice, the thistle funnel experiment stops sooner than that, and the reason is worth following.
As water enters, the column of sugar solution in the stem keeps rising. As it rises, both the height and the weight of that column increase. The weight of the column presses downward on the solution, and this pressure works against the entry of more water.
A stage is therefore reached when no further osmosis occurs, even though the concentration of water molecules is still not the same on the two sides. At this equilibrium the water molecules in the beaker are still tending to force their way upward through the membrane, but the pressure from the column above holds them back.
4.4.3.5 Osmotic Pressure
That last observation suggests a way of measuring the strength of osmosis: find out how much pressure is needed to stop it.
Take the same set-up, but this time fit an airtight piston bearing some weight into the thistle funnel from the very beginning, as in Fig. 4.4-5. Now the level of the solution does not rise at all. No water has entered. The weight has exactly cancelled the tendency of water to come in.
Diagram
Fig. 4.4-5 — An Experiment To Demonstrate Osmotic Pressure Line drawing of a thistle funnel held in a clamp stand, mouth downward and closed with cellophane paper, immersed in a beaker of water; the stem contains sugar solution with an airtight piston fitted into it and a weight resting on the piston; mark the level of the solution as unchanged; label weight, piston, initial level, sugar solution, cellophane paper, beaker, water.
The weight or pressure required to nullify osmosis in this way is a measure of the osmotic pressure of the solution.
4.4.3.6 Tonicity And Its Three Types
Tonicity was defined above as the difference in concentration between two solutions. It is the relative concentration of the solutions that decides both the direction and the extent of diffusion across a membrane.
On this basis, the solution surrounding a cell can be of three types.
- Isotonic — from iso, meaning same, and tonus, meaning tension or concentration. The relative concentration of water molecules and of solute on either side of the cell membrane is the same. There is no net movement of water molecules across the membrane, so there is no osmosis.
- Hypotonic — from hypo, meaning lower. The solution outside the cell has a lower solute concentration than the fluids inside the cell. Water molecules from outside therefore move into the cell, by endosmosis.
- Hypertonic — from hyper, meaning higher. The solution outside the cell has a higher solute concentration than the fluids inside the cell. Water molecules from inside the cell therefore move out, by exosmosis.
4.4.3.7 Cells Placed In Solutions Of Different Tonicity
To see what these three conditions do, imagine a human red blood cell and a plant cell each placed in the three kinds of solution, as in Fig. 4.4-6.
Diagram
Fig. 4.4-6 — Cells In Isotonic, Hypertonic And Hypotonic Solutions Two rows of line drawings sharing one caption. Upper row, a human red blood cell in three solutions: A, normal biconcave shape with equal arrows in and out; B, shrunken and irregular with the arrow pointing outward; C, swollen and rounded with the arrow pointing inward. Lower row, a plant cell in the same three solutions: A, unchanged with equal arrows; B, protoplasm shrunken away from the cell wall with the arrow outward; C, fully distended against the wall with the arrow inward. Label the three columns isotonic solution, hypertonic solution and hypotonic solution, and label cell wall on the plant cell.
In the isotonic solution, water entering the cell equals the water leaving it. The shape and size of the cell remain unchanged.
In the hypertonic solution, water entering the cell is less than the water leaving it. The cell loses water, shrinks in size and loses its shape.
In the hypotonic solution, water entering the cell exceeds the water leaving it, and the cell swells. Here the two kinds of cell part company. A red blood cell has only a membrane to contain it, so it enlarges and may finally burst. A plant cell has, in addition, a rigid cell wall, which resists bulging and protects the delicate parts within; the cell becomes fully distended instead, and in that state it is said to be turgid — it cannot withstand any further inflow of water.
This behaviour of the plant cell against its own wall has three phenomena attached to it — turgidity, plasmolysis and flaccidity — and section 4.4.5 deals with them.
4.4.4 Active Transport
Diffusion and osmosis both run downhill: molecules move from where they are more concentrated to where they are less. Minerals, however, present the root with an awkward problem.
Certain nutrients, such as ions of nitrates, sulphates, potassium, zinc and manganese, cannot pass through the cell membrane of the root cells easily. Their concentration is already higher inside the root cells than in the soil water outside. Moreover, that high internal concentration is deliberately maintained, because it is what develops the osmotic pressure needed for absorbing water in the first place.
So for these ions the concentration gradient runs the wrong way. They would diffuse outward, not inward. To obtain them, the cell has to carry them forcibly inward, from their lower concentration outside to their higher concentration inside — and that costs energy, supplied by the cell as ATP, the chemical form in which a cell stores and spends energy.
Active transport is this uphill carriage.
Two words in that definition carry weight. ‘Living’ — because a dead membrane cannot supply energy, active transport stops the moment the cell dies. And ‘opposite’ — this is the one process in the chapter that runs against the concentration gradient.
4.4.4.1 Passive Transport
Passive transport is nothing different from diffusion. The word ‘passive’ refers to requiring no input of energy: there is simply a free movement of molecules from their higher concentration to their lower concentration.
So the two names describe the same distinction from the energy side. Diffusion and osmosis are passive; only active transport spends the cell’s energy.
4.4.5 Turgidity And Flaccidity
Every living plant cell is in contact with fluids, directly or indirectly. The root hairs, in particular, are surrounded by soil water. All such cells are therefore subject to osmosis, and water keeps entering them for as long as the cell sap is more concentrated than the fluid outside.
But water cannot keep entering for ever, and what stops it is the subject of this section.
4.4.5.1 Turgor Pressure And Wall Pressure
When a cell reaches the state where it cannot accommodate any more water — when it is fully distended — it is said to be turgid, and the condition is called turgidity.
A turgid cell is under strain. Its wall is stretched by the pressure from inside, and the stretched wall in turn presses the cell contents back towards the centre of the cell. There are thus two opposing pressures at work, shown together in Fig. 4.4-7.
The pressure of the cell contents on the cell wall is called turgor pressure. The pressure exerted by the cell wall on the cell contents is called wall pressure.
Diagram
Fig. 4.4-7 — Osmotic, Turgor And Wall Pressures In A Plant Cell Line drawing of a single rectangular plant cell with a large central vacuole; show inward arrows crossing the wall marked osmotic pressure, outward arrows from the vacuole pressing on the wall marked turgor pressure, and inward arrows from the wall marked wall pressure; label cell wall, plasma membrane, vacuolar sap, and add the notes water moves into the cell due to the osmotic concentration of the cell contents, cell contents press on the wall, and the cell wall restrains expansion of the cell with wall pressure.
If at any time the cell wall is unable to bear the turgor pressure, it ruptures and the cell contents burst out. This is exactly what happens when fruits and vegetables sometimes split open.
Normally, though, the turgid cell settles into a somewhat balanced state. No more water is entering or leaving it, because its turgor pressure counter-balances the wall pressure. There is no further absorption of water even though the concentration of solutes inside the cell may still be greater than that outside.
That last sentence is worth reading twice. A concentration difference alone does not guarantee that water will keep entering. Once the wall pushes back hard enough, the movement stops.
4.4.5.2 Plasmolysis, Flaccidity And Deplasmolysis
Take a living cell from the leaf of an aquatic plant and keep it in fresh water. It stays fully distended, its plasma membrane in close contact with the cell wall and pressing against it, much as the rubber bladder of a football presses against the leather casing.
Now keep the same cell in a 5% salt solution for a few minutes. It loses its distended appearance. The cytoplasm shrinks, and the plasma membrane withdraws from the cell wall (Fig. 4.4-8).
This shrinkage of the protoplasm away from the cell wall is called plasmolysis. Cells in this state are limp, or flaccid, and the condition is called flaccidity. Flaccidity is the reverse of turgidity.
Diagram
Fig. 4.4-8 — Successive Stages Of Plasmolysis Row of four line drawings of the same plant cell, labelled C, D, E and F, placed under the heading hypertonic solution: C, a turgid cell with the protoplasm pressed against the wall; D and E, successive stages with the vacuole shrinking and the plasma membrane pulling away from the wall; F, a fully plasmolysed cell with the protoplasm gathered in the centre and a clear space between it and the wall; label cell wall, plasma membrane, vacuolar membrane, vacuole, nucleus, turgid cell and plasmolysed cell.
The change can be reversed. If a plasmolysed, flaccid cell is returned to water before it is dead, its protoplasm swells again and presses tight against the cell wall once more. This recovery, or reversal, of plasmolysis is called deplasmolysis.
Put the two together and they are easy to hold on to. Plasmolysis is the result of the outflow of water from the cell; deplasmolysis is the result of its re-entry. The whole sequence is traced in FC 4.1.
Flowchart
FC 4.1 — Plasmolysis And Deplasmolysis Turgid cell in water → cell placed in hypertonic solution → exosmosis, water leaves the vacuole → cytoplasm shrinks → plasma membrane withdraws from the cell wall → cell is flaccid (plasmolysed) → cell returned to water → endosmosis, water re-enters the vacuole → protoplasm swells and presses on the wall → cell turgid again (deplasmolysed)
The Three Terms Defined
4.4.5.3 Uses Of Turgidity To Plants
Turgidity is not merely a state a cell happens to be in. Plants put it to work, in at least five ways.
1. Turgidity provides rigidity to soft tissues such as leaves. A leaf has no wood in it; it is held out flat by the turgor of its cells. When there is not enough water in a leaf, it wilts — the petiole and the lamina become loose and the leaf droops down.
Wilting is usually seen when a plant stands in the hot afternoon sun, and the water lost during transpiration is more than the water absorbed by the roots. In the evening transpiration falls, the water absorbed then exceeds the water lost, the turgidity of the leaf cells is restored, and the leaves stand out again.
2. Turgor pressure helps a plant push through hard ground. A mushroom or a young seedling forces its way up through packed soil on turgor pressure alone (Fig. 4.4-9). The roots of certain trees have even been seen to crack the wall or the concrete floor of an adjoining building, and this too is turgor pressure at work.
Diagram
Fig. 4.4-9 — A Germinating Seed Pushing Through The Soil Line drawing in section of a germinating seed just below the soil surface, its swollen young shoot arching upward and lifting the crust of soil above it, with the root growing downward; label seed, young shoot, root, soil surface, and mark the upward force with an arrow.
3. Turgor in root cells builds up root pressure. As one turgid cell presses on the next, a pressure develops in the root that helps to drive water upward into the stem. This pressure is important enough to have a section of its own, and section 4.5 takes it up.
4. Turgor opens and closes the stomata. You have already met stomata, the tiny pores in a leaf surface, in Class IX. Whether a stoma is open or shut depends on the turgidity of its two guard cells.
Each guard cell has a thicker wall on the side facing the stoma and a thin wall on the side away from it. Guard cells also contain chloroplasts. When glucose is made there during photosynthesis, and other chemical changes follow, the osmotic pressure of the guard cell contents increases.
The guard cells then absorb more water from the neighbouring cells and become turgid. Because the two walls are unequal, the turgid cells arch outwards rather than swelling evenly, the aperture between them widens, and the stoma opens.
At night, or when the leaf is short of water, the guard cells turn flaccid. Their inner rigid walls become straight again, and the stomatal aperture closes. FC 4.2 sets out both halves of the cycle.
Flowchart
FC 4.2 — Opening And Closing Of A Stoma Glucose made in guard cells during photosynthesis → osmotic pressure of guard cell contents rises → water absorbed from neighbouring cells → guard cells become turgid → thin outer walls stretch more than thick inner walls → guard cells arch outwards → stoma opens. At night or when water is short: guard cells lose water → guard cells become flaccid → inner rigid walls straighten → stoma closes → cycle returns to the start next morning
5. Turgor movement. The rapid drooping of the leaves of the sensitive plant, Mimosa pudica, is an outstanding example. Touch one leaf, even lightly, and the leaflets fold up; within 2 to 3 seconds the entire leaf droops. Touch it somewhat strongly and the wave of folding and drooping spreads from the stimulated leaf to all the neighbouring leaves. The leaves then recover slowly and stand erect again.
In this plant, the stimulus of touch leads to a loss of turgor at the base of the leaflets and at the base of the petioles. That swollen base is called the pulvinus (Fig. 4.4-10). While its cells are turgid the petiole is held up; when they lose water the petiole collapses.
Diagram
Fig. 4.4-10 — Turgor Movement In The Sensitive Plant Two line drawings side by side of the same shoot of Mimosa pudica, labelled A and B. In A the petiole is held out horizontally with its swollen base intact; in B the petiole has collapsed downward. Label stem, petiole, pulvinus, cells remaining turgid, cells losing turgor, and vascular tissue.
Somewhat similar turgor movements are found in insectivorous plants, whose leaves close up to entrap a living prey. The bending of certain flowers towards the sun, and the sleep movements of the leaves of certain plants at night, are also due to turgor movements.
4.4.5.4 Guttation
In certain plants, such as tomato, grass, banana and ferns, the root pressure becomes high enough to force water all the way up through the stem and out through the ends of the leaf veins.
The water appears as tiny drops along the margins or at the tips of the leaves, most often in the early morning. This loss of excess water in liquid form is called guttation.
4.4.6 Diffusion, Osmosis And Active Transport Compared
Three of the five phenomena move substances into and through a plant, and students routinely confuse them. Table 4.1 sets them side by side.
4.5 Root Pressure
Cut the shoot off a well-watered plant close to the ground and watch the stump. Within a short while, water wells up out of the cut surface and pushes its way out.
Nothing is squeezing it. The plant has lost the leaves that might have pulled water upward, and yet water is still being driven up the stump from below. Something in the root is pushing, and that something is called root pressure.
How The Pressure Builds Up
Root pressure is built up by cell-to-cell osmosis in the root tissue. Water entering a root hair makes that cell turgid; the turgid cell presses on the next cell inward, and water passes on to it.
Repeat this across the whole thickness of the root and the force of the flow increases inward, cell after cell. There is no pump anywhere — only a chain of cells each pressing on the next.
When the water finally reaches the xylem vessels, which are the centrally placed vertical channels of the root, it enters the pores of their thick walls with considerable force. That force, transmitted into a continuous column of water, is what pushes sap upward through the stem.
Measuring Root Pressure
The push can be measured, and the set-up in Fig. 4.5-1 does it.
Take a well-watered potted plant and cut the stem a few centimetres above the soil. Immediately fix a glass tube to the cut stump using a rubber connection, so that nothing leaks. Water at once begins to come out of the cut end of the stem and rise up in the glass tube.
Connect the tube to a manometer — a U-shaped glass tube containing mercury, used for measuring pressure. The rising water presses on the mercury and pushes its level up in the far limb. The height the mercury rises is a measure of how hard the roots are pushing.
Diagram
Fig. 4.5-1 — An Experiment To Demonstrate Root Pressure Two line drawings side by side of the same potted plant with its shoot cut off a few centimetres above the soil and a glass tube joined to the stump by a rubber connection, the tube leading to a U-shaped manometer containing mercury; in the first drawing the mercury stands level in both limbs, in the second it has risen in the far limb; label cell sap, rubber connection, manometer, mercury, and mercury level rises, and mark the rise with an upward arrow.
This upward flow of water is due to a heavy pressure from the roots, and that pressure is what the term names.
Bleeding
Root pressure can raise cell sap in the glass tube — or in an intact stem, under natural conditions — only up to a certain height. It is a real force, but a limited one.
Where a stem has been cut, that force shows itself as a steady loss of cell sap from the cut surface. This loss of water and dissolved substances through a cut stem is called bleeding.
Root pressure is thus one of the forces that raise water up through the stem and into the leaves. It is not, however, the whole story. This force by itself cannot push water to the top of a tall tree, and section 4.8 takes up the other forces that finish the job.
4.6 Importance Of Root Hairs And The Upward Movement Of Water
Absorption of water by a root happens at the root hairs, and almost nowhere else. Everything the earlier sections established now comes together at that one point.
A root hair holds cell sap in which salts are dissolved, so the sap is more concentrated than the soil water outside it. That difference is what sets osmosis going, and the soil water passes inward into the root hair.
Look closely at where a root hair actually sits (Fig. 4.6-1). It does not lie in a pool of water. It threads its way between the soil particles, and the water it takes in is the thin film clinging to the surface of those particles.
Diagram
Fig. 4.6-1 — A Root Hair Among The Soil Particles Line drawing of a single root hair growing out from an epidermal cell of a root and pushing between rounded soil particles, each particle surrounded by a thin film of water; show the epidermal and cortical cells behind it and a xylem vessel at the right with an upward arrow; draw arrows tracing water from the soil film into the root hair and inward from cell to cell; label film of soil water around particles, soil particles, root hair, vacuole, epidermis, xylem vessel, and to stem.
From One Cell To The Next
Water does not stop at the root hair. From the cell bearing the root hair it passes to the adjoining cell, and from that one to the next, all the way inward across the thickness of the root until it reaches the xylem vessels (Fig. 4.6-2).
Diagram
Fig. 4.6-2 — Cell-To-Cell Conduction Of Water From Root Hair To Xylem Diagrammatic cross-section of part of a root, drawn as a wedge from the surface inward; show a root hair projecting from the surface and a series of arrows passing inward from cell to cell until they reach the xylem at the centre; label root hair, water entering root hair, epidermis, cortex, endodermis, pericycle, xylem and phloem, with all labels on straight, non-crossing lead lines.
The turgidity the cells acquire along the way is not wasted either. Each cell that fills with water presses on its neighbour, and this is the pressure that helps push water onward and upward into the xylem vessels — the root pressure of section 4.5.
Flowchart
FC 4.3 — Path Of Water From Soil To Xylem Film of water on soil particles → cell wall of root hair → cell membrane of root hair → cell sap in the vacuole → adjoining cortical cells, one after another → endodermis → pericycle → xylem vessel → upward through the stem
Taking In The Minerals
The mineral elements travel the same road but not by the same means. Absorption of mineral elements from the soil involves active transport by the cells, which spend energy to carry them in against the concentration gradient.
Minerals may also be absorbed as ions rather than as whole salts. Either way, once inside the root they are carried inward and upward dissolved in the same water.
And Then Upward
What the root has now collected is a dilute solution of water and mineral salts. In the root it is of no use to the plant at all.
That solution can be used for manufacturing food only if it reaches the leaves, and in a tall plant that means travelling to the highest points of the shoot. This upward flow occurs through the xylem — and how a column of water is raised that far is the question section 4.8 answers.
4.7 Experiments On Absorption And Conduction Of Water
So far the chapter has argued that roots absorb water and that the xylem carries it upward. Both claims can be tested on a bench, and this section sets out four experiments that do it.
Each one is built the same way: change one thing, keep everything else the same, and see what difference it makes.
4.7.1 To Show That Roots Absorb Water
Experiment 1. Fill a test tube A with water and mark the level on the glass. Put a few drops of oil on the surface of the water. The oil floats and seals the surface, so that no water can escape by evaporation. This is the control set-up (Fig. 4.7-1).
For the experimental set-up, prepare a similar test tube B with an equal level of water. Pull a young leafy plant out of the soil with its roots intact and insert the roots into test tube B. Put a few drops of oil on this water surface too, again to prevent any loss by evaporation.
Diagram
Fig. 4.7-1 — Experiment To Show That Roots Absorb Water Three test tubes in a row, each held upright and each with a layer of oil floating on the water. Tube A holds water alone; tube B holds water with a young leafy plant standing in it, roots submerged, drawn twice — once at the start and once after one to two days with a clearly lower water level. Label oil, test tube, water, control set-up (no change), experimental set-up (initial stage), experimental set-up (final stage), and mark the fall in level with a line and an arrow.
Leave the two test tubes undisturbed. After a day or two, the level of water in test tube B has fallen, while the level in test tube A has not changed at all.
Both tubes were sealed against evaporation, so the water that left tube B did not go into the air. The only route out of the tube was through the roots standing in it. The water lost from B was therefore absorbed by the roots.
4.7.2 To Show That Water Is Conducted Upwards Through The Xylem
Experiment 2. Uproot a medium-sized young herbaceous plant, wash the soil off its roots, and place it in a beaker containing eosin solution — a pink stain — dissolved in water (Fig. 4.7-2). The roots should be completely submerged in the solution. Keep the set-up aside for three to four hours.
At the end of that period, take the plant out of the solution and wash it in tap water. Then cut transverse sections of the root, the stem and the leaf, and examine each of them under a microscope.
Diagram
Fig. 4.7-2 — Experiment To Show That Water Is Conducted Upwards Through The Xylem Line drawing of a leafy plant standing with its roots submerged in a beaker of eosin solution, with three circular insets joined to it by lead lines showing transverse sections of the leaf, the stem and the root; in each section, shade only the xylem tissue to show it stained; label the leaf (transverse section), the stem (transverse section), the root (transverse section), xylem tissue stained pink, and beaker containing eosin solution.
In every section, the xylem vessels stand out distinctly from the rest of the tissue, because they alone have been stained by the dye. The dye could only have reached the leaf by the route the water took, so that route was the xylem.
This works because the conducting tissue is continuous. The vascular bundles of the root, the stem, the leaf stalks and the veins of the leaf are all joined into one unbroken system of tubes, forming a transport system that runs through the entire plant. Water and mineral salts travel upward mainly in the xylem, while food substances travel both up and down in the phloem.
4.7.3 The Ringing Experiment On Water Conduction
Experiment 3. The same conclusion can be reached by a more delicate experiment (Fig. 4.7-3), which removes one tissue at a time and sees which loss matters.
Take two leafy shoots and cut both under water, so that no air bubble gets into the cut ends. An air bubble would break the continuous column of water inside and stop the flow for reasons that have nothing to do with the experiment. Keep the lower ends of both shoots dipping in water.
From the first shoot, in beaker A, remove a ring of the outer layer of the stem — the phloem — about 3 cm long, keeping the central part intact. Removing a ring of tissue in this way is called ringing, or girdling.
From the second shoot, in beaker B, remove an equal length of the central part, the xylem, by incising the stem through its full thickness and keeping the peripheral part intact.
Diagram
Fig. 4.7-3 — An Experiment To Show That Water Is Conducted Through The Xylem Two leafy shoots standing in separate beakers of water, each held upright in a clamp stand. Shoot A has a ring of the outer tissue removed from a short length of the stem, the central core left in place, and its leaves stand out firm; shoot B has the central core removed through a window in the stem, the outer tissue left in place, and its leaves droop and wilt. Label xylem, phloem, beaker A, beaker B, and mark A as phloem removed and xylem intact and B as xylem removed and phloem intact.
Fix both shoots to stands and let them stand for about two days with their lower ends immersed in water.
The leaves of the first shoot remain turgid and stand out almost normally. The leaves of the second shoot wilt and droop down.
The shoot that kept its xylem kept its water supply; the shoot that lost its xylem could not draw water up at all, even though its phloem was untouched. Water is therefore conducted upward through the deeper part of the stem, the xylem.
4.7.4 To Show That Food Is Conducted Downwards Through The Phloem
Experiment 4. Girdling can also be used the other way round, to find out what the phloem carries.
Cut a ring right round the stem of a healthy potted plant, or around a thin twig of guava or of any other tree. The cut must be deep enough to penetrate the phloem and the cambium, but not deep enough to reach the xylem.
Sap soon starts oozing out of the upper cut margin — the one on the far side of the ring from the root. Sap in the peripheral part of the stem was therefore travelling downward and has been stopped by the cut.
Leave the plant and look at it again after some weeks (Fig. 4.7-4). The part of the stem just above the ring has grown thicker, swollen with the food that can no longer get past. The stem below the girdle has stopped growing, and may even die once the organic contents already stored in the lower part are used up.
Diagram
Fig. 4.7-4 — A Girdled Woody Stem, At The Start And After Some Weeks Line drawing in two parts. Part A, a potted plant with a ring of bark removed from the stem and a swelling developing just above the ring; label end of experiment and stem swollen with accumulated food. Part B, two lengths of woody stem side by side showing the girdle, one marked start with the cut margins level and one marked end with a distinct bulge above the girdle.
One more thing is worth noticing in this experiment. Throughout it, the leaves above the ring stay fresh and healthy. They are still receiving water, which proves that their water supply travels in the more deeply placed xylem and was never touched by a cut that removed only the bark.
Taken together, the last two experiments separate the two tissues cleanly, as Table 4.2 sets out.
4.8 Forces Contributing To The Ascent Of Sap
The water and dissolved mineral salts absorbed by the roots are together called sap. Its upward movement through the plant is called the ascent of sap.
The problem is a real one. A column of water standing in a stem has weight, and something must lift it — in a tall tree, tens of metres. Four main forces contribute to the ascent of sap, and no single one of them accounts for the whole climb. They are shown acting together in Fig. 4.8-1.
Diagram
Fig. 4.8-1 — The Forces That Raise Sap Through A Plant Tall line drawing of a whole plant with the stem shown in section as a narrow xylem channel running from root to leaf, the water column drawn continuous inside it. Mark four forces on the drawing: an upward arrow at the root labelled root pressure; the narrow bore of the vessel labelled capillarity; small linking arrows between water molecules along the column labelled cohesion, and short arrows from the column to the vessel wall labelled adhesion; and at the leaf, vapour escaping from a stoma with a strong upward arrow labelled transpirational pull.
4.8.1 Root Pressure And Capillarity
The first force is one you have already met. Root pressure builds up enough force to push the sap in the xylem vessels up to a certain height, and for herbaceous plants, which are not tall, that may be enough by itself.
The second force belongs to the xylem vessels themselves. Capillarity is the rise of a liquid in a tube of narrow bore. Water rises in such a tube on its own, and the narrower the tube, the higher it rises, exerting what is called capillary force.
Xylem vessels are tubes of very narrow diameter, so capillarity operates in them all the time. As water is lost from the leaves by transpiration, water from a lower level rises to fill the space it leaves behind.
4.8.2 Transpirational Pull, Cohesion And Adhesion
The third force does not push from below at all. It pulls from above.
As water is lost from the leaf surface by transpiration, more water molecules are pulled up behind it. This works only because water molecules have a strong tendency to remain joined to one another, an attraction called cohesion, which keeps the water in the stem as one continuous, unbroken column.
Pull on the top of that column and the whole of it moves. This pulling force from the leaves is the transpirational pull, and FC 4.4 follows it down the stem.
Flowchart
FC 4.4 — How Transpirational Pull Raises Water Water vapour escapes from the leaf through the stomata → water leaves the moist leaf cells → those cells draw water from the leaf xylem → a pull is set up at the top of the water column → cohesion holds the column together so the pull is passed all the way down → the whole column moves upward → water is drawn from the root xylem → more water is absorbed by the root hairs from the soil
The fourth force works sideways rather than upward. Adhesion causes water to stick to the surface of the cells lining the vessel, and this attachment helps draw more water molecules up from below when the leaf cells lose water during transpiration.
The pulling force, or suction force, provided by the leaves is especially important in tall trees such as pines, in which root pressure alone is nowhere near enough to lift water to the topmost branches.
4.8.3 Downward Movement Of Sap
The downward movement is much the simpler of the two. Food manufactured in the leaves is dissolved in water, and this solution flows down mainly on account of the force of gravity.
Nothing has to be lifted, so no comparable set of forces is needed. That is the whole difference between the two journeys: going up is work, and coming down is not.
4.9 Summary
Roots do three things: they fix the plant in the soil, absorb water and mineral nutrients from it, and conduct them into the stem for supply to the rest of the plant.
Besides being a constituent of protoplasm, water is needed for four purposes — photosynthesis, transpiration, transportation and mechanical stiffness.
Mineral nutrients are absorbed from the soil by the roots only, some as dissolved salts such as nitrates, phosphates and sulphates, and some simply as ions such as potassium, calcium, magnesium and chlorine.
These elements become constituents of the cell and its organelles, and are used in the synthesis of compounds and enzymes within the cell.
Three characteristics fit a root for absorbing water: an enormous surface area provided by rootlets and root hairs; root hairs holding cell sap more concentrated than the surrounding soil water; and root hairs with thin walls.
The cell wall of a root hair is thin and permeable, letting water and dissolved substances pass freely. The cell membrane within it is thin and semi-permeable, letting water through but holding back the larger molecules of dissolved salts.
Absorption and conduction result from five phenomena: imbibition, diffusion, osmosis, active transport, and turgidity and flaccidity.
Imbibition is the absorption of water by surface attraction by living or dead plant cells. Hydrophilic substances such as cellulose and proteins swell up, and the imbibitional pressure produced ruptures the seed coat of a germinating seed.
Diffusion is the free movement of molecules of a substance from their higher to their lower concentration when the two regions are in direct contact, with no membrane in the way.
Osmosis is the movement of water molecules from their higher concentration, in a dilute solution, to their lower concentration, in a concentrated solution, through a semi-permeable membrane. Only the water molecules move.
Endosmosis is the inward movement of water when the surrounding solution is less concentrated, and it swells the cell. Exosmosis is the outward movement when the surrounding solution is more concentrated, and it shrinks the cell.
A semi-permeable membrane passes molecules selectively — the solvent freely, the solute not at all. A rubber sheet in its place is impermeable and nothing crosses; a muslin cloth has pores so large that the solute crosses too.
Osmosis stops when the weight of the rising column of solution balances the tendency of water to enter, even though the concentrations on the two sides are still unequal.
Osmotic pressure is the minimum pressure that must be exerted to prevent pure solvent from passing into a solution across a semi-permeable membrane. It measures the solution’s tendency to take in water by osmosis.
Tonicity is the relative concentration that decides the direction and extent of diffusion. A solution is isotonic (no net movement), hypotonic (water enters the cell by endosmosis) or hypertonic (water leaves the cell by exosmosis).
In a hypotonic solution an animal cell swells and may burst, while a plant cell is held by its rigid cell wall and becomes turgid instead.
Active transport is the passage of a salt or an ion from its lower to its higher concentration through a living cell membrane, using energy from the cell as ATP. It runs opposite to diffusion. Passive transport needs no energy and is simply diffusion.
A fully distended cell is turgid. Turgor pressure is the pressure of the cell contents on the cell wall; wall pressure is the pressure of the wall on the contents. In a turgid cell the two balance and no further water is absorbed.
Plasmolysis is the shrinkage of the protoplasm away from the cell wall when a cell is placed in a hypertonic solution, leaving the cell flaccid. Returning it to water before it dies reverses the change, which is deplasmolysis.
Turgidity is put to work in five ways: it keeps soft tissues such as leaves rigid, so that loss of it causes wilting; it pushes seedlings and mushrooms through hard ground; it builds up root pressure; it opens and closes the stomata through the guard cells; and it produces turgor movements, as in the sensitive plant.
Guttation is the loss of excess water as drops at the tips and margins of leaves, driven by high root pressure, usually seen in the early morning.
Root pressure is the pressure developed in the roots by continued cell-to-cell osmosis, which helps in the ascent of cell sap through the stem. Loss of cell sap through a cut stem is called bleeding.
Water is absorbed by the root hairs by osmosis and passes from cell to cell across the root into the xylem vessels, while mineral elements are taken in by active transport.
Four experiments establish the pathway: sealed test tubes show that roots absorb water; eosin solution stains only the xylem; ringing shows that the xylem conducts water upward; and girdling shows that the phloem conducts food downward, swelling the stem above the ring.
Four forces contribute to the ascent of sap — root pressure, capillarity, transpirational pull acting through the cohesion of water molecules, and adhesion. The downward movement of sap is mainly due to gravity.
Flowchart
FC 4.5 — The Chapter In One Line Film of water on soil particles → root hair, entered by osmosis → cortical cells, crossed by cell-to-cell osmosis → turgid cells press inward and build root pressure → xylem vessel → column of water raised by capillarity, cohesion, adhesion and transpirational pull → leaf → water used in photosynthesis, kept as turgidity, or lost as vapour by transpiration
4.10 Key Terms
- Active transport — the movement of a salt or an ion from its lower to its higher concentration through a living cell membrane, using energy from the cell.
- Adhesion — the attraction between water molecules and the surface of the cells or vessel walls they touch.
- Ascent of sap — the upward movement of water and dissolved mineral salts from the roots to the topmost parts of a plant.
- Bleeding — the loss of cell sap from a cut or injured stem, driven by root pressure.
- Capillarity — the rise of a liquid in a tube of narrow bore, greater the narrower the tube.
- Cell sap — the solution of water and dissolved substances filling the vacuole of a plant cell.
- Cohesion — the attraction by which water molecules remain joined to one another, keeping the water column unbroken.
- Deplasmolysis — the recovery of a plasmolysed cell when it is returned to water and its protoplasm swells against the wall again.
- Diffusion — the free movement of molecules of a substance from their higher to their lower concentration, the two regions being in direct contact.
- Endosmosis — the inward movement of water into a cell across a semi-permeable membrane, when the surrounding solution is less concentrated.
- Exosmosis — the outward movement of water from a cell across a semi-permeable membrane, when the surrounding solution is more concentrated.
- Flaccidity — the condition in which the cell contents are shrunken and the cell is no longer tight.
- Girdling — the removal of a complete ring of tissue from around a stem, also called ringing.
- Guttation — the loss of excess water as drops from the tips and margins of intact leaves, caused by high root pressure.
- Hypertonic — describing a surrounding solution with a higher solute concentration than the cell, so that water leaves the cell.
- Hypotonic — describing a surrounding solution with a lower solute concentration than the cell, so that water enters the cell.
- Imbibition — the absorption of water by surface attraction by living or dead plant cells.
- Isotonic — describing a surrounding solution of the same concentration as the cell, so that there is no net movement of water.
- Osmosis — the movement of water molecules from their higher to their lower concentration through a semi-permeable membrane.
- Osmotic pressure — the minimum pressure needed to prevent pure solvent from passing into a solution across a semi-permeable membrane; a measure of the solution’s tendency to take in water.
- Passive transport — the movement of molecules from their higher to their lower concentration with no input of energy; another name for diffusion.
- Permeable — describing a layer that lets substances pass through without selecting between them.
- Plant physiology — the branch of biology dealing with the life functions, or metabolism, of a plant.
- Plasmolysis — the shrinkage of the protoplasm away from the cell wall when a cell is placed in a hypertonic solution.
- Pulvinus — the swollen base of a leaflet or petiole whose cells lose turgor and cause the leaf to droop.
- Root hair — a fine outgrowth of an epidermal cell of a root, through which water is absorbed from the soil.
- Root pressure — the pressure developed in the roots by continued cell-to-cell osmosis, which helps push cell sap up the stem.
- Sap — water together with the mineral salts absorbed by the roots.
- Semi-permeable — describing a membrane that lets the solvent pass through but holds back the solute.
- Tonicity — the relative concentration of two solutions, which decides the direction and extent of diffusion between them.
- Transpiration — the loss of water from a plant as water vapour, mainly through the leaves.
- Transpirational pull — the pulling force on the water column in the stem, set up by the loss of water from the leaves.
- Turgidity — the state of a cell whose wall is rigid and stretched by an increase in vacuole volume, due to the absorption of water.
- Turgor pressure — the pressure exerted by the cell contents on the cell wall.
- Wall pressure — the pressure exerted by the cell wall on the cell contents.
4.11 Points To Ponder
A root hair has a permeable wall on the outside and a semi-permeable membrane within. Suppose the two were swapped, so that the wall selected and the membrane let everything through. Would the root hair still be able to absorb water from the soil, and what exactly would go wrong?
Root pressure pushes from below and transpirational pull draws from above. If a tall tree could keep only one of the two, which would serve it better, and what would it lose by giving up the other?
Osmosis costs a cell nothing, while active transport spends ATP on every single ion carried in. Why might it be worth a plant’s while to pay for something it could otherwise get free, simply by waiting for the gradient to change?
A plant rooted in salty soil is surrounded by water it cannot take in. What would have to be true of its cell sap for absorption to become possible again, and what fresh difficulty might that create for the plant?
Turgidity does two jobs at once on a hot afternoon: it holds the leaf out flat, and it keeps the stomata open. These two uses of the same water pull against each other. How might a plant settle the conflict, and what would it sacrifice either way?
A girdled tree keeps its green, healthy leaves for weeks while the stem below the ring slowly starves. What does that long delay tell you about where a plant keeps what its leaves have made, and how much of it is held in reserve?
4.12 Objective Questions
Select the most appropriate option in each case.
- The layer of a root hair that decides which substances get into the cell is: (1 mark)
- the cell wall
- the cell membrane
- the vacuolar sap
- the cytoplasm
- A plant cell is kept in a solution for a few minutes and its protoplasm shrinks away from the cell wall. The solution was: (1 mark)
- isotonic
- hypotonic
- hypertonic
- distilled water
- The only process in this chapter that carries a substance against its concentration gradient is: (1 mark)
- diffusion
- osmosis
- imbibition
- active transport
- The force that keeps the column of water in a tall stem unbroken is: (1 mark)
- adhesion
- cohesion
- capillarity
- root pressure
- Tiny drops of water seen at the tips of grass leaves at daybreak are produced by: (1 mark)
- transpiration
- bleeding
- guttation
- plasmolysis
- A dry seed placed in water swells and its seed coat splits. The process responsible is: (1 mark)
- imbibition
- exosmosis
- plasmolysis
- active transport
The following questions are assertion–reason based. Choose the answer using the codes given below.
- A is true and R is false
- A is false and R is true
- Both A and R are true
- Both A and R are false
Assertion (A): A plant cell placed in distilled water becomes turgid but does not burst. Reason (R): The cell membrane of a plant cell is impermeable to water. (1 mark)
Assertion (A): Active transport comes to a stop the moment a root cell dies. Reason (R): Active transport requires energy supplied by the living cell in the form of ATP. (1 mark)
Match the items in Column I with those in Column II. (5 marks)
| Column I | Column II |
|---|---|
| a. Wall pressure | i. used to stain the xylem |
| b. Muslin cloth | ii. swollen base of a leaflet |
| c. Pulvinus | iii. pressure of the cell wall on the cell contents |
| d. Manometer | iv. a freely permeable barrier |
| e. Eosin | v. used to measure root pressure |
Fill in the blanks. (3 marks)
- The pressure of the cell contents on the cell wall is called ____________.
- A membrane that allows the solvent to pass but holds back the solute is described as ____________.
- The absorption of water by living or dead plant cells by surface attraction is called ____________.
State whether the following are true or false. (4 marks)
- Diffusion requires a semi-permeable membrane.
- Exosmosis occurs when the solution surrounding a cell is hypertonic.
- The cell wall of a root hair is permeable.
- Water rises higher in a tube of wider bore than in a narrow one.
Name the following. (4 marks)
- The fine outgrowth of a root epidermal cell through which water is absorbed.
- The condition of a cell whose contents are shrunken and which is no longer tight.
- The recovery of a plasmolysed cell that is returned to water in time.
- The tissue through which food travels down a stem.
Pick the odd one out and give a reason for your choice: imbibition, diffusion, osmosis, active transport. (2 marks)
Pick the odd one out and give a reason for your choice: root pressure, capillarity, transpirational pull, plasmolysis. (2 marks)
4.13 Check Your Concepts
Define osmotic pressure. (2 marks)
State two characteristics of a root hair that suit it for absorbing water from the soil. (2 marks)
Give a reason why a wooden door becomes difficult to shut during the monsoon. (2 marks)
Differentiate between turgor pressure and wall pressure, stating the basis of the difference. (2 marks)
Differentiate between diffusion and osmosis on the basis of what moves and of the barrier required. (3 marks)
In the thistle funnel experiment, the level of sugar solution stops rising after some hours, although the water in the beaker is still more dilute than the solution. Explain why. (3 marks)
Explain with an example why certain mineral ions cannot be taken into a root cell by diffusion. (3 marks)
What would happen to a potted plant that was watered with a strong salt solution? Name and explain the process involved. (3 marks)
Describe how a stoma is opened, beginning with the glucose made in the guard cells and ending with the widening of the aperture. (4 marks)
Explain how root pressure is built up in a root, and state one limitation of this force. (4 marks)
Why does a leaf droop on a hot afternoon and stand out again by evening, without any water being given to the plant? (4 marks)
Describe an experiment to show that water is conducted upwards through the xylem. State the material used, the procedure followed and the conclusion drawn. (5 marks)
Trace the path of a water molecule from the film of water on a soil particle to a xylem vessel of the root, naming the process involved at each step. (5 marks)
4.14 Figure-Based Questions
- Study Fig. 4.4-2, which shows a thistle funnel containing sugar solution suspended in a beaker of water. (4 marks)
- Name the process being demonstrated.
- State what happens to the level of the solution in the stem after a few hours, and explain why.
- What would you observe if the cellophane paper were replaced by a rubber sheet?
- Describe the control set-up that should be prepared alongside it.
- Fig. 4.4-8 shows a plant cell at successive stages C to F. (4 marks)
- Name the process represented by the change from C to F.
- Name the kind of solution in which the cell has been placed.
- Redraw the cell as it would appear if it were returned to water before it died, and label the cell wall and the plasma membrane.
- Fig. 4.4-7 shows the pressures acting on a plant cell. (3 marks)
- Name the pressure represented by the arrows directed from the cell contents towards the wall.
- Name the pressure acting in the opposite direction.
- State what happens to the absorption of water when these two pressures balance each other.
- Fig. 4.7-4 shows a woody stem some weeks after a ring of bark was removed from it. (3 marks)
- Name the operation that was performed on the stem.
- Give a reason for the bulge that has developed above the ring.
- State why the leaves above the ring have remained fresh.
- Redraw the cross-section of the root shown in Fig. 4.6-2. Label the root hair, the cortex, the endodermis and the xylem, and mark with an arrow the direction in which water travels through it. (4 marks)