Cells and organisation, Year 10: plant tissues and transport

They absorb as much light as possible for photosynthesis

HintAsk which side of a leaf faces the Sun.

The whyThe palisade layer is the main site of photosynthesis. Its tall cells stand upright in a tight row just under the upper skin of the leaf, where the light is brightest, and each holds many chloroplasts.

To let gases diffuse quickly to and from the cells

HintPhotosynthesis needs one gas brought in and produces another that has to get out.

The whyCarbon dioxide enters the leaf and has to reach the photosynthesising cells; oxygen has to leave. Gases diffuse much faster through air than through cells, so the spaces give a fast route and a large surface for exchange.

The epidermis

HintYour own skin has an outer layer with the same name.

The whyEpidermal tissue covers the whole plant. On a leaf it protects the tissues inside, and the upper layer is transparent, so light passes through to the cells beneath. The lower layer has small pores for gas exchange.

guard cells

HintThe first word is what a sentry does at a gate.

The whyGuard cells change shape to open or close the pore between them. In this way they control how much gas is exchanged and how much water is lost. Most stomata are on the underside of the leaf.

palisade; spongy

HintThe upper one is named after a fence of upright stakes; the lower one after something full of holes.

The whyThe tightly packed layer comes first because it needs the most light. Beneath it is the loosely packed layer with air spaces, next to the pores in the lower surface. Veins of xylem and phloem run through the middle of the leaf.

Transpiration

HintThe word begins like "transport" and ends like "perspiration".

The whyWater evaporates from the cells inside a leaf and diffuses out through the stomata. This loss pulls more water up through the xylem from the roots, in a continuous flow called the transpiration stream.

Moving air carries water vapour away from the leaf, keeping the concentration gradient steep

HintAsk what builds up just outside the pores when nothing stirs.

The whyIn still air, the water vapour that has left the leaf stays near the stomata, so the difference in concentration between inside and outside is small. Wind removes it, so diffusion out of the leaf stays fast.

Water evaporates faster and its particles diffuse faster

HintThink why washing dries sooner on a hot day.

The whyWarmer water molecules have more energy, so more of them escape from the wet cell surfaces inside the leaf, and they move out through the stomata more quickly.

The stomata open wider, so more water vapour escapes

HintIn daylight the leaf needs to let a gas in for photosynthesis.

The whyIn the light, guard cells open the stomata to let in carbon dioxide for photosynthesis. Open stomata also let water vapour out. In the dark the stomata close and transpiration slows.

6

HintShare the distance moved over the time taken.

The whyRate = distance moved by the bubble ÷ time = 30 ÷ 5. The bubble moves because the shoot takes up water to replace what its leaves lose, so the rate of uptake is used as a measure of the rate of transpiration.

Translocation

HintThe word means "moving from one place to another".

The whySugars made in the leaves by photosynthesis are needed by every part of the plant. Phloem tissue carries them, dissolved in cell sap, to growing regions and to storage organs.

lignin

HintIt is the tough material that makes wood woody.

The whyLignin is laid down in the walls of xylem cells. It makes the tubes strong enough to stay open while water is pulled up through them, and it helps to support the plant.

osmosis; active transport

HintOne process is the diffusion of water; the other needs energy because it works against a concentration gradient.

The whySoil water is a more dilute solution than the contents of the root hair cell, so water moves in across the membrane. Mineral ions are scarcer in the soil than in the cell, so they have to be moved in using energy from respiration.

Upwards, from the roots to the stem and leaves

HintThink where a plant takes water in and where it loses it.

The whyWater enters at the root hairs and is lost from the leaves, so the flow in xylem is one way. Phloem is different: it carries sugars from the leaves to wherever they are needed, which can be up or down.

It is stored

HintThink what a potato or a carrot is for.

The whyGrowing regions use sugar at once, for respiration and for building new cells. Storage organs such as potato tubers and carrot roots keep it for later, often after turning it into starch.