GRADE 8 BIOLOGY
Photosynthesis
Plants make their own food by photosynthesis.
In the chloroplasts plants use light energy to break carbon dioxide and water
molecules to create glucose.
Carbon dioxide and water react together in the presence of light and chlorophyll to
make glucose and oxygen.
The glucose is converted into starch, fats and oils for storage.
It is used to make cellulose for cell walls, and proteins for growth and repair.
It is also used by the plant to release energy by respiration.
chloroplasts
carbon + water
dioxide
+ light
energy
glucose +
oxygen + water
1

Show how light is needed to make starch.
Testing a leaf for starch.
If you want to see if starch has been produced in a plant, you have to start with a
plant that does not have starch.
1. Test the plant for starch.
2. Place the plant in the dark for 2 or 3 days.
3. Test again for starch.
Using iodine to test for the presence of starch.
Iodine solution ( an amber colored liquid ) turns dark ( blue/black ) in the presence
of starch.
If leaves are to be tested for the presence of starch, the cell walls in the leaf must
be broken, and the chlorophyll removed.
The cell walls must be broken because to make it easier for substances to leave
or enter the cell.
The chlorophyll must be removed so that the color change in the iodine can be
seen.
Showing how light is needed to make starch.
1. Two leaves of a destarched plant are to be tested for the presence of starch.
2. One is completely covered with aluminum foil, the other with clear plastic wrap.
3. The plant is put in the sun for 4 hours, and both leaves are tested for starch.
The leaf in the plastic contains starch.
The leaf in the foil does not.
This suggests that light is needed for the production of starch.
2

Provide evidence that only green parts of plants make starch.
A variegated plant is used to test for the relationship between the green pigment in
plants & starch production.
1. A destarched variegated plant was left in the sunlight for four hours.
2. A leaf is removed and tested for starch.
The green parts of the leaf contain starch.
The white do not.
3. This suggests that chlorophyll is needed for starch production.

Describe the effect of light, or lack of light, on
the growth of plants.
A plant needs these things to grow well:

air

light

warmth

water

nutrients
Without enough light, a plant cannot photosynthesise very quickly, even if there is
plenty of water and carbon dioxide.
Increasing the light intensity will boost the speed of photosynthesis.
3

Know that the green parts of plants are called chloroplasts.
4

Know that carbon dioxide can enter and oxygen escape
through stoma.
5

Explain how to measure rates of photosynthesis.
Photosynthesis can be measured by either:
 Depletion of substrate.
 Accumulation of products
The rate of photosynthesis can therefore be measured by:
 Depletion of substrate which includes measuring how much carbon dioxide
has been used or how much water is used.

Accumulation of product which might include measuring how much oxygen
is produced or organic molecules (biomass) produced.
6

Know that nutrients in the form of mineral ions are taken in by
roots.
The roots of a plant take up water and nutrients from the soil.
The roots also keep the plant upright in the soil; they "anchor" the plant.
the end
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