Transport in Plants

Xylem and phloem: the plant's two transport tissues

Plants have no heart and no blood, but they still need a transport system. Two tissues do the work: xylem carries water and mineral ions up from the roots, and phloem carries sucrose and amino acids around the plant. Cambridge gives the xylem one extra job that students often forget - support.

The two tissues are always found together in a vascular bundle. In a root the bundles form a cross in the centre; in a stem they form a ring of separate bundles towards the outside; in a leaf they run through the veins. That difference in position is what identify-the-diagram questions test, so learn the three arrangements rather than one.

You do not need to know every detail of how the tissues develop. You do need one clear sentence for each: what it transports, which way it transports it, and whether it is alive.

How xylem vessels are built for their job

A xylem vessel is a tube built from cells that have died. Cambridge limits the structure you must relate to function to three features: thick walls containing lignin, no cell contents, and cells joined end to end with no cross walls.

Lignin makes the wall rigid and waterproof, so the tube does not collapse under the pull of the moving water and water cannot leak out sideways. Because the cell contents have gone, the inside of the tube is an empty space, so nothing blocks the flow. Because the end walls have broken down, the cells form one long continuous tube running from root to leaf.

That rigid, empty, lignified tube explains the xylem's second job as well. Lignin makes the vessel strong enough to help hold the plant up, so xylem provides support - which is why a plant short of water, whose cells have lost their turgor, can no longer stand upright.

Do not write about lignification being laid down in spirals or rings. The syllabus explicitly excludes the details of lignification, and no marks are awarded for it.

• Thick walls with lignin -> the tube is strong and waterproof, and helps to support the plant.
• No cell contents -> the tube is hollow, so water flows freely through it.
• No cross walls -> the cells form one long continuous tube.
• Dead at maturity -> xylem does not use energy, and it cannot repair itself if it is damaged.

Phloem: living tissue that moves food both ways

Phloem transports sucrose and amino acids - the soluble product of photosynthesis and the building blocks for new growth. The movement of these substances from where they are made to where they are used is called translocation.

Phloem is a living tissue. It is made of sieve tubes, which are columns of living cells whose end walls are perforated and are called sieve plates, and companion cells, which sit beside each sieve tube and keep it alive. A sieve tube cell has lost its nucleus, so it cannot survive on its own; the companion cell carries out the respiration and controls the loading of sucrose into the tube.

Translocation runs in both directions, because sucrose can be loaded into the phloem wherever it is produced and removed wherever it is needed. That is the key difference from the xylem, which only ever carries water upwards.

The parts of the plant that release sucrose or amino acids into the phloem are the sources; the parts that use or store them are the sinks. A mature leaf in bright sunlight is a source, while a growing bud, a root tip, a developing fruit or a storage organ such as a potato tuber is a sink.

The same part can be a source at one time and a sink at another. A young leaf cannot yet photosynthesise enough for its own needs, so it is a sink; once it is fully grown it produces a surplus and becomes a source. A potato tuber is a sink while it is being filled with starch in the summer, and a source in the spring when it sprouts and the stored sucrose is moved to the growing shoots.

Water uptake and the transpiration stream

Water enters the plant through root hair cells. A root hair is a long, thin extension of an epidermal cell that pushes out between the soil particles. It gives the root a very large surface area, which increases the rate at which water and mineral ions are taken in. Water enters the root hair cell by osmosis, because the cell contents are more concentrated than the dilute soil solution.

From the root hair cell, water passes across the root cortex cells to reach the xylem in the centre of the root. It then travels up the xylem through the stem and into the leaf, where it finally enters the mesophyll cells. Cambridge wants that pathway in that exact order: root hair cells, root cortex cells, xylem, mesophyll cells.

Inside the leaf, water evaporates from the moist surfaces of the mesophyll cells into the air spaces between them; the interconnecting air spaces give a very large internal surface area for evaporation. The water vapour then diffuses out of the leaf through the stomata, because the air inside the leaf is more humid than the air outside. This loss of water vapour from the leaves is transpiration.

The whole journey - water into the root, up the xylem, into the leaf and out through the stomata - is called the transpiration stream. It is not only a loss: it is the mechanism that pulls water and the mineral ions dissolved in it up to the leaves in the first place.

The transpiration pull, and what controls the rate

Water moves up the xylem as a continuous column, held together by the forces of attraction between water molecules. When water evaporates from the mesophyll cells it is replaced by water drawn from the xylem in the leaf, and this sets up a transpiration pull - a tension that draws the whole column of water upwards, all the way down to the roots. The column does not break because the water molecules stick to each other and the narrow, waterproof xylem walls keep the thread of water intact.

The rate of transpiration depends on how quickly water can evaporate from the leaf and how quickly the vapour can diffuse away. Four factors matter: light intensity, temperature, humidity and wind speed. Light intensity affects the rate because light controls the stomata: in bright light they open to let carbon dioxide in for photosynthesis, which also lets water vapour out, and in darkness they close and transpiration almost stops. The graph rises steeply and then levels off, because once the stomata are fully open, more light cannot open them any further.

Temperature works through evaporation. At a higher temperature the water molecules have more kinetic energy, so they evaporate from the mesophyll cells more quickly, and the warmer air can also hold more water vapour. The graph rises steadily as temperature increases.

Humidity is the amount of water vapour in the air. When the air is very humid the water vapour concentration outside the leaf is close to that inside, so the diffusion gradient is shallow and transpiration is slow; when the air is dry the gradient is steep and transpiration is fast. The graph therefore falls as humidity rises. Wind speed acts by removing the vapour that has just left the leaf: in still air a layer of moist air builds up around the leaf and reduces the gradient, while moving air sweeps that layer away and keeps the gradient steep, so transpiration speeds up. The graph rises steeply at first and then more gently as the air layer is already being removed.

Leaf adaptations that cut water loss, and why plants wilt

A leaf must open its stomata to take in carbon dioxide, but every open stoma also lets water vapour out. Plants that live in dry places have adaptations that reduce water loss while still allowing gas exchange.

The waxy cuticle is a waterproof layer on the leaf surface; it stops water evaporating straight through the epidermis. Having fewer stomata, or having them only on the lower surface where it is cooler, reduces the number of openings. Rolled leaves trap a layer of moist air inside the roll, so the gradient for diffusion out of the stomata is shallow. Hairs on the leaf surface do the same job - they hold a layer of still, moist air close to the leaf and reduce air movement across it.

The advantage of these adaptations is survival: a plant that loses less water can stay alive in dry conditions. The disadvantage is that gas exchange is slower, which limits the rate of photosynthesis, so every one of these adaptations is a compromise.

Wilting is the plant's protective response when water loss exceeds water uptake. The cells lose water by osmosis, become flaccid, and lose turgor pressure, so the leaves and stems droop. This is not simply damage: a drooping leaf presents less surface area to the sun and its stomata turn away from the light, so less water evaporates and the plant loses water more slowly. If the water loss continues, the cells become plasmolysed and die.

Investigating transpiration with a potometer

A potometer measures the rate at which a leafy shoot takes up water, which is taken as a measure of the rate of transpiration as long as the plant is not storing water. A shoot is sealed into the apparatus with its cut stem under water, and an air bubble is introduced into a horizontal capillary tube. As the shoot transpires, it draws water from the tube and the bubble moves along the scale; the distance the bubble moves in a set time gives the rate.

To compare two conditions, change only the factor you are testing. The same shoot is used throughout, and everything else - temperature, humidity, light intensity and wind speed - is kept the same. To test wind speed, take a reading in still air and then place a fan at a fixed distance from the shoot and take another reading over the same time.

The precautions are what keep the results valid. The shoot must be cut under water and the apparatus assembled under water, so no air enters the xylem and blocks it. All joints must be sealed and the leaves kept dry, so the only water lost is through the stomata and not through a leak. The shoot should be allowed time to acclimatise before readings begin, and each condition should be repeated so a mean can be calculated.

The results are reported as a rate: the distance the bubble moved divided by the time taken. A larger distance in the same time means a faster rate. The main limitation is that the potometer measures water uptake, not the water vapour actually leaving the leaves, so the reading is only a good estimate of transpiration if the shoot is not wilting or storing water.