Transport in Flowering Plants

• Xylem is dead, hollow and upward. Phloem is living, loaded and two-way. Transpiration is a loss of water vapour from leaves — never “water moving up the stem”.
• Evaporation is a change of state. Diffusion is a movement of vapour. Osmosis is a movement of liquid water through a membrane. All three happen in a transpiring leaf, in that order, and each earns its own mark.
• Say the route out loud as seven words: soil, hair, cortex, xylem, mesophyll, air space, atmosphere. Then say the processes: osmosis, osmosis, osmosis, mass flow, osmosis, evaporation, diffusion.
• In the root, xylem is a star in the middle. In the stem, xylem is the inner half of each bundle in a ring. In both organs, xylem ends up nearer the centre than phloem — one rule, two pictures.
• If a sentence you have written could be true of a pump, a sponge or a drinking straw, it is probably not biology. Name the tissue, name the process, name the gradient.

• Relate the structure of a root hair cell to its function in the uptake of water and mineral ions.
• Explain that water enters root hair cells by osmosis, moving from a higher to a lower water potential through a partially permeable membrane.
• Explain that mineral ions enter by diffusion when they move down a concentration gradient and by active transport, using energy from respiration, when they move against one.
• Trace the pathway of water through the root hair cell, the root cortex cells, the xylem and the mesophyll cells, naming the process at every step.
• Describe an investigation, using a coloured stain, that shows the pathway of water through a plant, and separate the observation from the inference.
• Identify the positions of xylem, phloem and cortex in a transverse section of a non-woody dicotyledonous root and stem.
• Relate the structure of xylem vessels to their function in transport and in support.
• Define translocation as the movement of sucrose and amino acids in phloem from a source to a sink, and identify sources and sinks in a given situation.
• Define transpiration as the loss of water vapour from leaves, and describe the whole chain from evaporation in the mesophyll to the transpiration pull in the xylem.
• Explain the effects of wind speed, temperature, humidity and light intensity on the rate of transpiration, and state the limits of each explanation.
• Explain wilting as the consequence of water loss exceeding water uptake.
• Describe how a potometer is set up and used, explain why the shoot is cut under water, and explain why uptake is only an estimate of transpiration.
• Calculate a rate of water uptake from bubble movement, including the volume form when the capillary radius is known.

Why wilting is sometimes useful to the plant. A wilted leaf hangs down and often curls, which reduces the surface area exposed to the sun and the moving air. Together with closed stomata, that slows further water loss. Wilting is a plant in trouble, but it is not a plant that has given up.

Root Hair Cell

A specialised epidermal cell of a young plant root that bears a long, narrow, hair-like projection extending between the soil particles. The projection greatly increases the surface area in contact with the soil solution, the thin cell wall gives a short pathway for substances to cross, the partially permeable cell membrane allows water to enter by osmosis, the large vacuole holds cell sap that keeps the water potential of the cell below that of the soil solution, and numerous mitochondria release the energy from respiration that is needed for the active transport of mineral ions.

Transpiration

The loss of water vapour from the leaves of a plant. Water reaching the leaf in the xylem passes into the mesophyll cells, evaporates from their moist surfaces into the air spaces of the leaf, and the water vapour then diffuses out through the stomata down a water-vapour concentration gradient into the atmosphere. Transpiration is a loss of water vapour at the leaf, and is not the same as the transpiration stream, which is the movement of water up the plant that this loss causes.

Transpiration Rate

The volume or mass of water vapour lost from a plant's leaves in a given time. It rises when the water-vapour concentration gradient between the leaf air spaces and the atmosphere becomes steeper, as it does in moving air or in dry air; when evaporation from the mesophyll surfaces becomes faster, as it does at higher temperature; and when the stomata are open, as they usually are in the light. It falls in humid air, in still air and in darkness, and it can also fall in extreme heat or drought, when the stomata close to conserve water.

Vascular Bundle

A strand of transport tissue in a plant containing both xylem and phloem. In a transverse section of a young non-woody dicotyledonous stem the vascular bundles are arranged in a ring near the outside, with the xylem toward the centre of the stem and the phloem toward the outside of each bundle, and the cortex lies outside the ring. In a young dicotyledonous root the vascular tissue lies in the centre, with the xylem often forming a star shape and the phloem lying between the arms of the star, surrounded by the cortex.

Mineral Ion Uptake

The absorption of dissolved mineral ions, such as nitrate and magnesium ions, from the soil solution into a root hair cell. Ions enter by diffusion when they are more concentrated in the soil solution than in the cell, so that they move down a concentration gradient, and by active transport when they are already more concentrated inside the cell, so that they must be moved against a concentration gradient using energy released by respiration in the mitochondria.

Xylem Vessel

A long, hollow tube in the vascular tissue of a plant, formed from a file of cells joined end to end whose cross walls have broken down and whose cell contents have been lost, so that the mature vessel is dead and its lumen is continuous. Its walls are thick and strengthened with lignin, which both supports the plant and keeps the vessel open. Xylem vessels carry water and dissolved mineral ions in one direction, from the roots to the leaves, drawn upwards by the transpiration pull.

Transpiration Pull

The tension set up in the water column of the xylem when water is lost from the leaves. Evaporation from the mesophyll cell surfaces, followed by diffusion of water vapour out of the stomata, lowers the water potential of the leaf cells, so water is drawn out of the xylem into them by osmosis. This puts the water in the xylem under tension, and because water molecules are attracted to one another the column does not break, so the whole column is drawn upwards from the roots. No living process and no pumping is involved.

Wilting

The drooping of the leaves and soft stems of a plant that occurs when water is lost from the leaves faster than it is taken up by the roots. The cells lose water by osmosis, their vacuoles become smaller, the turgor pressure of the cell contents against the cell wall falls and the cells become flaccid. Because turgid cells are what hold soft plant tissue rigid, the tissue loses its support and the leaves and stem droop. Wilting is often reversible: if water is supplied, the cells regain turgor and the plant recovers.

Potometer

An apparatus used to measure the rate at which a cut leafy shoot takes up water. The shoot is sealed into a water-filled tube connected to a horizontal capillary tube containing a single air bubble, and the distance the bubble moves along a scale in a measured time gives the rate of water uptake. Because most of the water taken up is lost from the leaves as water vapour, this uptake is used as an estimate of the rate of transpiration; it is not a direct measurement of it, since some water is used in photosynthesis, retained in growth and held in the cells to maintain turgor.

Coloured-Water Investigation

A practical investigation in which a freshly cut leafy shoot, or a celery petiole, is stood in water containing a suitable stain such as eosin or red food colouring. After an hour or more the stem is cut into thin transverse sections and examined. The stain is found in the xylem, and the coloured strands can be followed into the leaf veins, which supports the conclusion that water travels through the xylem. The observation is the position of the colour; the conclusion that water moves in the xylem is an inference drawn from it.

Translocation

The movement of sucrose and amino acids in the phloem, from regions of production or release, called sources, to regions of use or storage, called sinks. Phloem is living transport tissue, and because the direction of movement depends on where the source and the sink lie, translocation may carry solutes upwards, downwards, or in both directions in the same plant at the same time. A single organ can be a sink at one stage of the life cycle and a source at another.

Transpiration Stream

The continuous movement of water through a plant, from the soil into the root hair cells by osmosis, across the root cortex cells, into the xylem, up the xylem vessels as an unbroken column, and into the mesophyll cells of the leaf, from whose moist surfaces it evaporates into the leaf air spaces before diffusing out through the stomata as water vapour. The stream is drawn upwards by the transpiration pull created when water is lost from the leaf, not pushed upwards by the root.

• 1. “Water is taken into the root by active transport.” Why wrongActive transport moves particles against a concentration gradient using energy. Water never needs that: it moves down a water-potential gradient by itself. WriteWater enters the root hair cell by osmosis, from the soil solution of higher water potential to the cell sap of lower water potential, through the partially permeable cell membrane.
• 2. “Osmosis is just water moving from high to low concentration — no membrane needed.” Why wrongWithout a partially permeable membrane the solute would simply diffuse too, and there would be no osmosis to describe. WriteOsmosis is the net movement of water molecules from a higher to a lower water potential through a partially permeable membrane.
• 3. “Xylem transports sucrose to the rest of the plant.” Why wrongXylem carries water and dissolved mineral ions only. Sugars are carried by the phloem — a different tissue, in the same bundle. WriteXylem transports water and mineral ions from the roots to the leaves; phloem transports sucrose and amino acids from source to sink.
• 4. “Phloem transports downward, xylem upward.” Why wrongDirection in the phloem is set by where the source and the sink are. A sprouting tuber sends sucrose up; a photosynthesising leaf may send it up to a shoot tip and down to a root at the same time. WritePhloem transports from source to sink, which may be upward or downward.
• 5. “Xylem vessels pump the water up.” Why wrongA mature xylem vessel is dead and empty. It has no membrane, no cytoplasm and no mitochondria, so it cannot do anything that requires energy. WriteWater is drawn up the xylem by the transpiration pull, a tension created when water is lost from the leaves.
• 6. “Transpiration is water moving up the stem.” Why wrongThat is the transpiration stream, which transpiration causes. Confusing the cause with its consequence loses the definition mark. WriteTranspiration is the loss of water vapour from leaves.
• 7. “The water evaporates out of the stomata.” Why wrongTwo separate processes are being merged. Evaporation happens at the moist mesophyll cell surfaces, inside the leaf. What leaves through the stoma is vapour, and it leaves by diffusion. WriteWater evaporates from the mesophyll cell surfaces into the air spaces, and the water vapour then diffuses out through the stomata.
• 8. “Wind blows the stomata shut, so transpiration stops.” Why wrongOrdinary wind increases transpiration by sweeping away the humid air just outside the leaf. Only extreme conditions, usually combined with water shortage, cause the stomata to close. WriteMoving air removes water vapour from around the leaf, keeping the concentration gradient steep, so transpiration is faster.
• 9. “Light increases transpiration because the lamp heats the plant.” Why wrongIf that is what happened in your experiment, you changed two variables at once and the investigation was invalid. The biological link being tested is stomatal opening. WriteIn the light the stomata open to allow carbon dioxide in for photosynthesis, so more water vapour can diffuse out. Temperature must be controlled separately, with a heat shield or a water bath.
• 10. “Humid air holds more water, so the plant transpires more.” Why wrongThe direction of diffusion depends on the difference between inside and outside. Humid air outside makes that difference smaller, not larger. WriteIn humid air the water-vapour concentration gradient between the leaf air spaces and the atmosphere is smaller, so transpiration is slower.
• 11. “A potometer measures transpiration.” Why wrongIt records the water drawn into the shoot. Water loss is not measured at all; it is inferred. WriteA potometer measures the rate of water uptake, which is used as an estimate of the rate of transpiration.
• 12. “The bubble moved 30 mm, so the plant took up 30 mm3 of water.” Why wrongA distance is not a volume. The volume depends on how wide the capillary is. WriteVolume = cross-sectional area × distance, where the area is \(\pi r^{2}\) and \(r\) is the internal radius of the capillary — half its diameter.
• 13. “It does not matter whether the shoot is cut in air or under water.” Why wrongCutting in air lets air into the open xylem vessels. The water column is broken, the transpiration pull cannot be transmitted past the air lock, and uptake falls or stops. WriteThe shoot is cut under water so that air does not enter the xylem and break the continuous water column.
• 14. “The plant has wilted, so it has died.” Why wrongWilting is a loss of turgor, and it is usually reversible. Water the plant and the cells take up water by osmosis and become turgid again. WriteWilting occurs when water loss exceeds water uptake, so cells become flaccid and the tissue loses support; prolonged wilting damages the plant, but drooping alone does not mean death.
• 15. “The cell wall becomes flaccid.” Why wrongThe wall is made of cellulose and does not change. What changes is the pressure of the cell contents against it. WriteThe vacuole becomes smaller and turgor pressure falls, so the cell becomes flaccid; the cell wall is still present.
• 16. “All the water a plant absorbs is lost by transpiration.” Why wrongMost of it is, but not all. Some is used as a raw material in photosynthesis, some is retained in new tissue as the plant grows, and some stays in the cells keeping them turgid. WriteMost of the water taken up is lost as water vapour, which is why uptake is a good estimate of transpiration; a small amount is used in photosynthesis, in growth, and in maintaining turgor.