Transpiration

1. Water travels up a plant in a continuous stream, and almost all of it leaves again as vapour from the leaves.
2. Inside a leaf the cells are loosely packed with air spaces between them, and every cell surface facing those spaces is wet.
3. Water evaporates from those wet cell walls, so the air spaces fill with water vapour.
4. The air inside the leaf therefore holds far more water vapour than the air outside it.
5. Water vapour then diffuses out through the stomata down that concentration gradient, and this loss is transpiration.
6. Transpiration is therefore two steps: evaporation inside the leaf, then diffusion out of it.
7. As water leaves a mesophyll cell, that cell draws replacement water from the cell next to it, and so on back to the nearest xylem vessel.
8. That pull is passed all the way down the water in the xylem to the roots, producing the transpiration stream.
9. The column of water holds together because water molecules attract one another, an effect called cohesion, so pulling at the top drags the whole column upwards.
10. The plant spends no energy on this, because the energy that drives the evaporation comes from the sun.
11. Transpiration is useful as well as unavoidable: it delivers water for photosynthesis, carries dissolved mineral ions up to the leaves, cools the leaf as water evaporates, and keeps cells firm with water so soft stems stay upright.

Stomata and guard cells

1. A leaf is covered by a waxy cuticle that is waterproof, so gases and water vapour cannot pass freely through the surface.
2. The stomata are the controlled openings through it, and the singular of the word is stoma.
3. Each stoma is a pore held between two sausage-shaped guard cells.
4. Carbon dioxide diffuses in through the open pore for photosynthesis, oxygen diffuses out, and water vapour is lost at the same time.
5. Most stomata are on the lower surface of a broad leaf, which is shaded and cooler, so less water evaporates than would be lost from the hot upper surface.
6. Guard cells are the only epidermal cells of a leaf that contain chloroplasts, and they photosynthesise in the light.
7. The wall of a guard cell is thicker on the side facing the pore and thinner on the outer side, and that uneven thickness is what makes the pore open.

Opening and closing a stoma

1. In the light the guard cells take in water by osmosis and become turgid, which means swollen and firm.
2. The thin outer wall stretches more easily than the thick inner wall, so each cell curves away from the pore.
3. The two cells bow apart and the stoma opens, letting carbon dioxide in while the plant is photosynthesising.
4. In the dark the guard cells lose water, become flaccid and straighten, so the two cells meet and the stoma closes.
5. Closing at night costs the plant nothing, because there is no photosynthesis in the dark and so no need for carbon dioxide.
6. Stomata also close during the day when the plant is short of water, which saves the plant even though photosynthesis stops.

The cost of opening the stomata

1. A plant cannot let carbon dioxide in without letting water vapour out, because both use the same pore.
2. Transpiration is therefore an unavoidable consequence of gas exchange rather than a process the plant carries out on purpose.
3. Keeping the stomata open maximises photosynthesis but risks losing water faster than the roots can replace it.
4. Closing them saves water but starves the leaf of carbon dioxide, so photosynthesis and growth stop.
5. Guard cells settle that compromise minute by minute, opening the pores when water is plentiful and shutting them when it is not.

Translocation

1. Photosynthesis makes glucose in the leaves, but every other part of the plant needs it too.
2. Glucose is converted to sucrose for the journey, because sucrose is soluble but far less reactive, so it can be moved without being used up on the way.
3. The movement of sucrose and amino acids through the phloem is called translocation.
4. It runs from a source, where sugars are loaded in, to a sink, where they are taken out.
5. A photosynthesising leaf is the usual source, and growing root tips, flowers, developing fruits and storage organs are the usual sinks.
6. At a sink the sugar is respired for energy, built into cellulose for new cell walls, or stored as insoluble starch.
7. Translocation moves in either direction, upwards to a growing shoot and downwards to a root, depending on where the sinks are at the time.
8. The same part can switch role through the year, so a potato tuber is a sink while it is filling in summer and a source when it sprouts in spring.
9. Loading sugar into the phloem needs energy from respiration, so translocation is an active process, unlike transpiration.
10. That is why translocation slows or stops when a plant is starved of oxygen, while the transpiration stream keeps flowing.