Cells and organisation, Year 7: microscopes and photosynthesis

The lowest-power lens

HintPick the one that gives the widest view, so the specimen is easy to find.

The whyLow magnification shows a wide area and is easy to focus. Once the specimen is centred and sharp, you switch up to a higher magnification to see detail.

The coarse focus knob

HintIt moves the stage in big steps; the other one is only for sharpening.

The whyThe coarse knob makes large movements to find the image on low power. The fine knob then makes tiny adjustments to sharpen it.

It makes the cell parts easier to see

HintThink about what an unstained specimen looks like against a bright light.

The whyLight passes straight through unstained cells, so there is little contrast. A stain is taken up more strongly by some parts, such as the nucleus, which then stand out.

Draw only what you actually see

HintOnion bulbs grow underground, so their cells are not green.

The whyA scientific drawing is a record of an observation, not a copy of a textbook diagram. Onion bulb cells have no chloroplasts, so none should appear.

A plant cell

HintThink about which kind has a rigid wall and a permanent vacuole.

The whyThe box-like outline is the cell wall and the central space is the vacuole. Animal cells have neither, so they look rounder and less regular.

glucose

HintThe name of one particular sugar, beginning with 'gl'.

The whyThe plant builds this sugar from two simple raw materials, using energy from light. Oxygen is the other product.

Carbon dioxide

HintYou breathe out more of this gas than you breathe in.

The whyIt enters the leaf through tiny pores and is one of the two raw materials. The other, water, comes up from the roots.

Oxygen

HintAnimals need this gas for respiration.

The whyIt is made alongside glucose and leaves through the pores of the leaf. Almost all the oxygen in the air was made this way.

Light

HintA plant kept in a dark cupboard cannot do it.

The whyLight is absorbed by the green chlorophyll in the leaf and is used to join carbon dioxide and water into glucose.

From the air and water

HintLook at the left-hand side of the word equation for photosynthesis.

The whyThe glucose a plant builds its body from is made out of carbon dioxide and water. Only a small amount of minerals comes from the soil.