Page 17 - 新思维科学学生用书9 样章

1 Photosynthesis and the carbon cycle

Continued

Questions

1 Iodine solution cannot get through cell membranes. Boiling the leaf breaks the
cell membranes apart.
a In which part of a cell is starch stored?
b Explain why it was important to boil the leaf before the starch test could
work.

2 Suggest why it is a good idea to remove the green colour from the leaf before
you add iodine solution to it.
3 Did the leaf contain starch? If so, explain where the starch came from.

4 If your leaf had some green parts and some white parts, which parts contained
starch? Can you explain why?

Inside a leaf

Photosynthesis happens inside chloroplasts, which are inside some of the cells in a leaf.

The diagram opposite shows a magnified view of the inside of a leaf.

On the diagram, you can see that chloroplasts are mostly inside the cells in the middle layers of the
leaf. Leaves are very thin, so it is easy for sunlight to reach these cells.

Chloroplasts also need plenty of water and carbon dioxide, because these are used in photosynthesis.
Water is brought to the cells in the leaf along the veins. You can read more about this in Unit 4.

Carbon dioxide diffuses into the leaf from the air. If you look at the diagram, you can see that there are

tiny holes in the leaf, which allow gases to diffuse in and out. These holes are called stomata (singular:
stoma). The gases can easily diffuse through the air spaces between the cells inside the leaf.

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