Transport in plants

Plant tissues include epidermal tissues, palisade mesophyll, spongy mesophyll, xylem and phloem, and meristem tissue found at the growing tips of shoots and roots.

The leaf is a plant organ, and you are expected to explain how the structures of plant tissues are related to their functions. Knowledge of the leaf is limited to the epidermis, palisade and spongy mesophyll, xylem and phloem, and the guard cells surrounding stomata — so there is a fixed list to learn, and nothing beyond it.

The roots, stem and leaves form a plant organ system for the transport of substances around the plant. Three cell types do the work, and each one is adapted to it.

Root hair cells are adapted for the efficient uptake of water by osmosis, and of mineral ions by active transport — the long thin extension gives a large surface area, and the many mitochondria supply the energy that active transport needs.

Xylem tissue transports water and mineral ions from the roots to the stems and leaves. It is composed of hollow tubes strengthened by lignin, adapted for the transport of water in the transpiration stream.

Phloem tissue transports dissolved sugars from the leaves to the rest of the plant for immediate use or storage. It is composed of tubes of elongated cells, and cell sap moves from one phloem cell to the next through pores in the end walls. The movement of food molecules through phloem tissue is called translocation.

The role of stomata and guard cells is to control gas exchange and water loss. When the plant has plenty of water the guard cells swell and the stoma opens, letting carbon dioxide in for photosynthesis — and letting water vapour out. When water is short the guard cells become limp and the stoma closes, which protects the plant from wilting at the cost of stopping photosynthesis. That trade-off is the answer to most guard cell questions.

Four factors change the rate of transpiration, and each one works by changing how quickly water vapour leaves the stomata:

• Temperature. Warmer air means faster evaporation and faster diffusion, so a higher rate. • Humidity. Humid air already holds water vapour, so the concentration gradient out of the leaf is shallower and the rate falls. • Air movement. Wind blows away the water vapour gathered outside the stomata, keeping the gradient steep, so the rate rises. • Light intensity. Brighter light opens the stomata for photosynthesis, and open stomata lose more water.

Data from these investigations has to be processed as well as collected. Translating information between numerical and graphical form is the commonest version of it: put the factor you chose on the x-axis and the rate on the y-axis, select scales for the axes that fill at least half the grid, plot each point with a small cross, and draw a line of best fit rather than joining the dots. Reading in the other direction — extract and interpret information from a graph, chart or table someone else made — means quoting actual figures with their units, not just saying the line goes up.

Five fields of view is a sample of the data, not a census, and the principles of sampling apply. The sample has to be large enough that one unusual patch of leaf does not decide the answer, and representative — taken from different parts of the leaf, and from different leaves if you are describing the plant rather than that one leaf. If a question asks why a result is unreliable, sample size and how the sample was chosen are the two places to look.

A leaf has two jobs that pull in opposite directions. It has to absorb as much light and carbon dioxide as possible, which means being wide, thin and open to the air; and it has to avoid drying out, which means being the opposite. Every adaptation of a leaf is a compromise between the two, and saying so is often what turns a description into an explanation.

In a desert the compromise shifts decisively towards saving water. A cactus has leaves reduced to spines, so almost no surface is exposed and grazing animals are deterred; photosynthesis happens in the swollen green stem instead, which also stores water. The cuticle is thick, the stomata are few, sunken into pits and open only at night when the air is cooler, and the roots are shallow and spread widely to catch what little rain falls.

Marram grass on a sand dune faces the same problem for a different reason — constant wind and free-draining sand. Its leaves roll into a tube so that the stomata face inwards into a pocket of still, humid air; hairs around the stomata trap moisture, and the stomata themselves are sunken.