Complete revision notes for Topic 6 of the CIE IGCSE Biology course: Plant nutrition. Explanations with diagrams for every specification point. These notes are written for candidates taking the Extended paper.
Define photosynthesis as the process by which plants manufacture carbohydrates from raw
materials using energy from light
If asked for the raw materials required for photosynthesis, the answer is carbon dioxide and
water. Although required for the reaction to take place, light energy is not a substance and
therefore cannot be a raw material.
The reaction requires energy which is obtained by the pigment chlorophyll trapping light
from the Sun.
State the word equation for photosynthesis: carbon dioxide + water → glucose + oxygen, in
the presence of light and chlorophyll
State the balanced chemical equation for photosynthesis:
Explain that chlorophyll transfers light energy into chemical energy in molecules, for the
synthesis of carbohydrates
The light energy is converted into chemical energy in the bonds holding the atoms in the
glucose molecules together.
Outline the subsequent use and storage of the carbohydrates made in photosynthesis
Plants use the glucose they make as a source of energy in respiration.
, They can also convert it into:
Starch for storage
Lipids for an energy source in seeds
Cellulose to make cell walls
Amino acids (used to make proteins) when combined with nitrogen and other
mineral ions absorbed by roots
Investigate the necessity for chlorophyll, light and carbon dioxide for photosynthesis, using
appropriate controls
How to prove a leaf is photosynthesising
Although plants make glucose in photosynthesis, leaves cannot be tested for its presence as
the glucose is quickly used, converted into other substances and transported or stored as
starch.
Starch is stored in chloroplasts where photosynthesis occurs so testing a leaf for starch is a
reliable indicator of which parts of the leaf are photosynthesising.
Leaves can be tested for starch using the following procedure:
A leaf is dropped in boiling water to kill and break down the cell walls
The leaf is left for 5-10 minutes in hot ethanol in a boiling tube. This removes the
chlorophyll so colour changes from iodine can be seen more clearly
The leaf is dipped in boiling water to soften it
The leaf is spread out on a white tile and covered with iodine solution
In a green leaf, the entire leaf will turn blue-black as photosynthesis is occurring in all
areas of the leaf
This method can also be used to test whether chlorophyll is needed for
photosynthesis by using a variegated leaf (one that is partially green and partially
white)
The white areas of the leaf contain no chlorophyll and when the leaf is tested only
the areas that contain chlorophyll stain blue-black
The areas that had no chlorophyll remain orange-brown as no photosynthesis is
occurring here and so no starch is stored
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