8.2.2 State that photosynthesis consists of light-dependent and light-independent reactions
Photosynthesis is a two step process which consists of light dependent (converts light energy into chemical energy) and light independent variable (uses chemical energy to make organic molecules)
8.2.3 Explain the light-dependent reactions
The light dependent reaction occurs on the thylakoid membrane
Chlorophyll in both photosystem I and II absorbs light, which triggers the release of high energy electrons (photoactivation)
- The electrons from photosystem II pass along a series of carriers
- The electrons lost from photosystem II are replaced by electrons generated by the photolysis of water with oxygen as by-product
- Hydrogen pump carriers use the excited electrons to pump hydrogen ions over into the thylakoid compartment, the electrons is then passed on to photosystem I
- Photosystem I re-excites the electrons and then passed on to NADP+ reductase
- The electrons then binds with the NADPH + H+ and is transported somewhere else.
- ATP synthase uses the concentration gradient to produce ATP from ADP and Pi
8.2.4 Explain photophosphorylation in terms of chemiosmosis
Photophosphorylation is simply phosphorylation but required light energy.
As the electrons cycle through the electron transport chain located on the thylakoid membrane, they lose energy. The energy is used to pump H+ ions into the thylakoid compartment to create a concentration gradient. The H+ ions return via ATP synthase.
This process is called chemiomosis
8.2.5 Explain the light-independent reactions
Calvin cycle occurs in the stroma and uses ATP and NADPH + H+ produced by the light dependent reaction. There are three main steps which include carbon fixation, reduction and regeneration of RuBP.
Carbon fixation:
- Enzyme RuBisCo catalyses the attachment of carbon dioxide to the 5 carbon compound ribulose bisphosphate (RuBP)
- The unstable 6 carbon compound that is formed immediately breaks down into two 3 carbon molecules called glycerate-3-phosphate (GP)
Reduction:
- ATP is used to put phosphate onto the GP
- NADPH + H+ reduces the compound by giving a hydrogen.
- This forms G3P
Regeneration
- For every six molecules of G3P produced, only one could be used to form half a sugar
- Th remaining G3P is used to restock RuBP in a reaction that require ATP
- With RuBP regenerated, the plant will use the cycle multiple times and construct long chains of sugars.
8.2.6 Explain the relationship between the structure of the chloroplast and its function
8.2.7 Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants.
Pigments require light as a source of energy.
The absorption spectrum indicates the wavelength of light absorbed by each pigment. The action spectrum indicates the rate of photosynthesis for each wavelength.
There are strong relationships between the two as it shows both the peak and the valleys in the graph.
8.2.8 Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration of carbon dioxide
The law of limiting factor states that when a chemical process depends on more than one essential condition to become favorable, its rate will be limited by the factors that is nearest its minimum value.
Light intensity
- Light is required for the light dependent reactions (photoactivation of chlorophyll and photolysis of water molecules)
- Low light intensities results in insufficient production of ATP and NADPH + H+
Temperature
- Primarily affect light-independent reaction as it requires more collisions and enzymes
- High temperatures could damage the enzymes and cause the prohibition of enzymes from occuring
Concentration of Carbon Dioxide
- Carbon dioxide is required for the light independent reaction to occur (carbon fixation of RuBP by RuBisCo)
- At low levels, oxygen will over take the enzyme RuBisCo creating toxic gas instead.
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