Cells and organisation, Year 10: plant tissues and transport

They absorb as much light as possible for photosynthesis

提示Ask which side of a leaf faces the Sun.

为什么The 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

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

为什么Carbon 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

提示Your own skin has an outer layer with the same name.

为什么Epidermal 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

提示The first word is what a sentry does at a gate.

为什么Guard 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

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

为什么The 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

提示The word begins like "transport" and ends like "perspiration".

为什么Water 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

提示Ask what builds up just outside the pores when nothing stirs.

为什么In 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

提示Think why washing dries sooner on a hot day.

为什么Warmer 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

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

为什么In 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

提示Share the distance moved over the time taken.

为什么Rate = 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

提示The word means "moving from one place to another".

为什么Sugars 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

提示It is the tough material that makes wood woody.

为什么Lignin 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

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

为什么Soil 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

提示Think where a plant takes water in and where it loses it.

为什么Water 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

提示Think what a potato or a carrot is for.

为什么Growing 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.