Cells and organisation, Year 8: more practice

Carbon dioxide in; oxygen and water vapour out

HintThe leaf takes in one raw material and loses two things.

The whyCarbon 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

HintThink about which side of the leaf the Sun shines on.

The whyThe 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

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

The whyPhotosynthesis 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

HintMore light is no longer the shortage.

The whyOnce 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

HintThink about what leaves the field at harvest.

The whyPlants 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

HintThink about where the chloroplasts are.

The whyPhotosynthesis 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

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

The whyWith 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

HintWater goes towards the side with the greater amount of dissolved substance.

The whyWater 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

HintThink about what the roots take in from the soil.

The whyCells 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

HintThink about the soil surrounding it.

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