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AQA A Level Biology A* summary notes photosynthesis
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A LEVEL BIOLOGY NOTESPhotosynthesis
What are chloroplasts?
Small, flattened organelles which are 2-10 micrometres long
What do chloroplasts do?
Absorb light for photosynthesis
What is their double membrane evidence for?
The endosymbiotic theory
Describe the outer membrane
It is permeable to small molecules (water, CO2, oxygen and ions which diffuse easily, but it is not
permeable to larger proteins)
Describe the inner membrane
It regulates the passage of large substances (sugars and proteins) in and out of chloroplasts through
the membrane bound transport proteins
Describe the inside of chloroplasts
Contains a gel like fluid called the stroma which contains enzymes, starch granules, proteins,
chloroplast DNA and ribosomes (these starch granules are used to store the products of
photosynthesis)
What does the stroma contain?
Thylakoids
Describe the structure of thylakoids
Disc shaped fluid filled sacs made of thylakoid membrane, the inside of the sacs is hollow and known
as the thylakoid space, it contains the chlorophyll and other pigments required for absorbing lights
for photosynthesis as well as the enzymes required for the light dependent reactions
What are thylakoids stacked into?
Stacked like coins into structures called grana
Describe the grana
Linked by bits of thylakoid membrane called lamellae
What do thylakoid membranes contain?
The ATP synthase enzymes required to make ATP in the light dependent reactions
How does the structure of the chloroplast maximise the rate of the light dependent reaction?
ATP synthase channels within the granal membrane, the thylakoid membrane has a large surface
areas for the electron transport chain and photosystems position chlorophyll to enable the
maximum absorption of light
, How does the structure of the chloroplast maximise the rate of the light independent reaction?
Own DNA and ribosomes for synthesis of enzymes e.g., rubisco and the concentration of enzymes
and substrates in the stroma is high
Give an outline of photosynthesis
6CO2 + 6H20 = C6H12O6 + 6O2 – it is the capturing of light energy by chloroplast pigments such as
chlorophyll, in the LDR light energy absorbed is conserved in chemical bonds which creates and
electron flow and photolysis causing protons, electrons and oxygen to be reduced into reduced
NADP, ATP and oxygen
What is oxidation and reduction?
Oxidation= gain of oxygen/ loss of hydrogen
Reduction= loss of oxygen/ gain of hydrogen
What is the light energy used for?
To add an inorganic phosphate molecule to ADP making ATP and to split water into H+ ions (protons)
and OH- ions
What is this splitting known as?
As the splitting is caused by light, it is known as photolysis
What is stage one of the LDR?
The photoionization of chlorophyll
What happens to light energy?
It is absorbed by chlorophyll
What happens to the electrons?
They become excited and raise up an energy level to leave chlorophyll
Therefore…
Chlorophyll has been ionised (lost an electron) by light
What is some energy used for?
To make ATP (photophosphorylation) and reduced NADP in chemiosmosis and photolysis
What is stage two of the LDR?
Chemiosmosis
What happens to the electrons which gained energy and left chlorophyll?
They move along a series of proteins embedded within the thylakoid membrane (electron transfer
chain in a series of redox reactions)
What happens as they move along?
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