Cells and organisation, Year 8: more practice

Carbon dioxide in; oxygen and water vapour out

提示The leaf takes in one raw material and loses two things.

为什么Carbon dioxide diffuses in for photosynthesis. Oxygen made by photosynthesis and water vapour diffuse out.

It is closest to the light, so more photosynthesis can happen

提示Think about which side of the leaf the Sun shines on.

为什么The palisade cells are long, tightly packed and full of chloroplasts, positioned just beneath the transparent upper surface.

The enzymes that control the reactions stop working

提示Think about what heat does to the proteins which speed up chemistry in cells.

为什么Photosynthesis is controlled by enzymes. Above a certain temperature their shape changes and they can no longer do their job.

Something else, such as carbon dioxide or temperature, is now limiting the rate

提示More light is no longer the shortage.

为什么Once light is plentiful another factor in short supply holds the rate back, so adding more light changes nothing.

Harvested crops carry the minerals away, so the soil runs short

提示Think about what leaves the field at harvest.

为什么Plants take minerals from the soil and store them. When the crop is taken away the minerals go with it, so they must be replaced.

In the leaves

提示Think about where the chloroplasts are.

为什么Photosynthesis happens mainly in leaf cells. The phloem then moves the sugars to growing tips, flowers, fruits and roots.

So it uses up its existing starch, and any starch found must have been made afterwards

提示Think about what you need to be sure of at the start.

为什么With no light the plant cannot photosynthesise, so it uses up its stored starch. A later positive result then shows new starch was made.

From a dilute solution to a more concentrated solution

提示Water goes towards the side with the greater amount of dissolved substance.

为什么Water molecules pass through the membrane both ways, but more go from the dilute side, where there is more water, to the concentrated side.

Water enters the cells by osmosis and makes them firm again

提示Think about what the roots take in from the soil.

为什么Cells lose water and go limp when the soil is dry. When watered, water moves into the cell vacuoles by osmosis and pushes the cells firm.

It is underground, where there is no light for photosynthesis

提示Think about the soil surrounding it.

为什么Root hair cells take in water and minerals from the soil. Without light they could not photosynthesise, so chloroplasts would be wasted.