Summary of the A-level photosynthesis topic, according to the AQA specification. Notes will still apply to there specifications but take caution with key words and processes.
-> in which photosynthesis is one of the most important processes
Occurs at the leaf of the plant, in organelles called
chloroplasts
This has some adaptations for efficiency:
The leaf has a large surface area to maximise sunlight
absorption
The leaf has a thin and transparent cuticle, allowing
necessary gases and sunlight through
Stomata allow for gas exchange to maintain a concentration
gradient
The chloroplasts themselves contain grana / thylakoids which
provide a large surface area Figure 1 – reproduced from [1]
-> as well as containing pigments such as chlorophyl (green) which absorb sunlight
6CO2 + 6H2O -> C6H12O6 + 6O2
The stages of photosynthesis:
The general equation for photosynthesis is highly simplified and is really a more complex metabolic
pathway
1. Capturing of light energy:
Occurs via pigments such as chlorophyll absorbing light in the chloroplast
2. The light-dependant reaction:
This reaction uses the light energy in the chemical bonds of the pigment
-> this energy creates an electron pathway through the chlorophyll
Due to this light energy, water splits in a process called photolysis
This produces protons, electrons and oxygen
2H2O -> 4H+ + 4e- + O2
The oxygen leaves as waste via diffusion
At the same time some light energy is used in ‘photophosphorylation’
ADP + P -> ATP
Overall – this process produces ATP and NADPH
3. The light independent reaction:
This involves the protons (H+) producing sugars and other organic molecules.
, AQA Biology A-Level Notes
Role of chloroplast:
The grana are stacks of 100 disc-like structures
-> these make up the bulk of the chloroplast
The discs are called thylakoids – where the light dependent reaction occurs
These contain chlorophyll and other photosynthetic pigments
The stroma makes up the rest of the chloroplast
-> this is a fluid filled matrix where the light independent stage of photosynthesis occurs
Also contains ions and enzymes for both stages
Light Dependant Reaction:
The light dependant reaction involves the formation of ATP and NADPH by a sequence of reactions
-> this is split up into two separate ‘sub reactions’
Photolysis – splitting of water with light
Photophosphorylation – formation of ATP using light energy
These processes use a series of reduction and oxidation reactions
-> where:
Oxidation is the loss of electrons or H+
Reduction is the opposite with the gain of H + or electrons
Photolysis + Photophosphorylation:
This reaction is caused by the action of light absorbed during photosynthesis
-> splitting up water into products:
2H2O -> 4H+ + 4e- + O2
These products are then used later in photosynthesis, apart from oxygen which diffuses away as
waste
Photophosphorylation also uses light energy, but instead to make ATP
-> this uses inorganic phosphate (Pi)
ADP + Pi -> ATP
These both occur in and around the thylakoid membrane:
Photolysis occurs within the thylakoid space
Photophosphorylation occurs on the membrane (but on the
stroma side mainly)
These contribute to the bulk of the processes involved in the light
dependant reaction, which follows a complex chain of events.
Figure 2 – Reproduced from [1]
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