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Biology Unit 4
Topic 5: On the wild side
2. Describe the structure of chloroplasts in
relation to their role in photosynthesis.
Chloroplasts are large organelles, usually in a biconvex shape, and about 4-10 µm long and 2-3µ
Within plants most chloroplasts are found in the mesophyll cells of the leaves.
The stages of photosynthesis take place within the chloroplast, stage 1 within the granum
and stage 2 within the stroma.
Photosynthesis takes place in the chloroplasts. These are disc-shaped organelles surrounded by an
outer envelope of 2 layers of membrane. Inside, there are further membranes which are arranged in
stacks called grana. Surrounding these is the stroma. Chlorophyll and other light-capturing pigments
are found on the membranes of the grana and it is here that the light-dependant reaction takes
place. This generates the ATP and reduced NADP which are used in the light-independent reaction in
This is the light-capturing, photosynthetic pigment made up of 5 closely related pigments. Each of the
pigments absorbs and captures light from particular areas of the spectrum. As a result, far more of
the energy from the light falling on the plant can be used than if only one pigment was involved.
Structure Function(s) Features
Text Book: p. 12-13
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Biology Unit 4
Thylakoid membrane Light dependent reactions Contains chlorophyll, other
a system of interconnected accessory pigments, electron
flattened fluid-filled sacs carriers
Thylakoid Space Photolysis of water Photolysis enzyme(s)
fluid within the thylakoid
Granum Site for light-dependent Large surface area to absorb
a stack of thylakoids joined to reactions more light for photosynthesis
Stroma Light-independent reactions Enzymes for all LIR stages
The fluid surrounding the including Rubisco
thylakoid membranes.…read more