Cells and organisation, Year 7: microscopes and photosynthesis

The lowest-power lens

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

为什么Low 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

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

为什么The 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

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

为什么Light 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

提示Onion bulbs grow underground, so their cells are not green.

为什么A 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

提示Think about which kind has a rigid wall and a permanent vacuole.

为什么The 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

提示The name of one particular sugar, beginning with 'gl'.

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

Carbon dioxide

提示You breathe out more of this gas than you breathe in.

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

Oxygen

提示Animals need this gas for respiration.

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

Light

提示A plant kept in a dark cupboard cannot do it.

为什么Light 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

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

为什么The 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.