Photosynthesis (OCR)

OCR, Biology
Unit F214, Module 3

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  • Created on: 24-01-12 19:38
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Sameer Biology
Unit 4 Photosynthesis
Photosynthesis is the process whereby light energy from the Sun is transformed into chemical
energy and used to synthesise large organic molecules (glucose) from inorganic substances (CO2 +
H2O). Photosynthesis forms the basis of most food chains.
Plants and animals depend upon the products of photosynthesis as a source of chemical potential
energy. This is because:
Autotrophs are organisms that use light energy or chemical energy and inorganic molecules to
synthesis complex organic molecules.
Heterotrophs are organisms that ingest and digest complex organic molecules, to release the
chemical potential energy stored within them.
Chloroplast (organelle) is the site of photosynthesis, which is a 2 stage process (light dependant
and light independent). The structure of the chloroplast consists of:
Varied disc shaped organelle between 2-10um
An envelope made of double membrane
An intermembrane space (10-20 nm wide)
An outer membrane
o permeable to small ions
Stroma (site of light-independent stage)
o Packed with enzymes to catalyse the light in-independant reactions
o An inner membrane less preamble with transporter proteins embedded in it.
o It is able to control the entry and exit of substances between the cytoplasm and the Stroma
The lamellae (thylakoid- thin plates) are stacked up in a tower called granum (grana plural)
Increase surface area for photosynthetic pigments, electron carriers and ATP synthase (all
involved in the light dependant reactions)
The many compartment within the granum are thylakoids (site of light-dependant stage)
o The photosynthetic pigments are arranged in a structure called a photosystem held by
proteins to absorb maximum light energy

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Sameer Biology
Unit 4 Photosynthesis
The grana are connected with the interregnal lamellae
Loop DNA and ribosomes
o Can assemble proteins needed for photosynthesis
Photosynthetic pigments are molecules that absorb light energy of a range of
wavelengths, in the visible region with its own distinct peak of absorption. It
has great importance as the absorption "excites electrons" for the light
dependant stage.…read more

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Sameer Biology
Unit 4 Photosynthesis
Light In-dependant stage is the Calvin cycle which takes place in the Stroma, it uses Carbon dioxide and the
products of the light dependant stage to produce TP (triose phosphate), which can be used to produce
glucose.…read more

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Sameer Biology
Unit 4 Photosynthesis
o Some are used to make amino acids
o Some are used to fatty acids
Most GP is reduced and phosphorylated to form TP (triose phosphate- 3C), using ATP and oxidising
NADPH (from Light dependant reactions):
o Most(5/6) of TP are recycled by phosphorylation, using ATP to form (3) RUBP again
o Pairs of TP are combine to form a hexose sugar (carbohydrates)
o Some are used to form lipids
o Some are used to form amino acids…read more


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