Photosynthesis
Photosynthesis and Respiration
Practical Investigations
The Light-Dependent Stage
The Light-Independent Stage
Factors affecting Photosynthesis
Chloroplast and Photosynthetic Pigments
Photosynthesis
Autotrophic Nutrition - Organic molecules are synthesised from non-organic material (water and carbon dioxide)
Plants are photoautotrophs, light energy is the source for autotrophic nutrition
6CO2 + 6H20 + energy from photons --(chlorophyll)--> C6H12O6 + H2O
Carbon Fixation - the process by which CO2 is converted into sugars, it is a reduction reaction and endothermic, requiring energy, it helps regulate CO2 levels
Respiration
Heterotrophs - obtain energy by digesting organic molecules
All living organisms respire, oxidising organic material, releasing chemical energy (exothermic)
C6H12O6 + 6O2 --> 6H2O + 6CO2 + energy
How they Interrelate
Photosynthesis and Respiration are opposites, allowing a balance of substances
Compensation Point
When photosynthesis and Respiration proceed at the same rate, with no net gain or loss of carbon dioxide
The time of day at which the plant is at compensation point is called the compensation period, this is different for each species
Chloroplasts
Grana
Stroma
Photosynthetic Pigments
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Photosystems
Water
Photophosphorylation
Non-Cyclic
Cyclic
Photosystem I (PSI) has a peak absorption of 700nm (P700)
Photosystem II (PSII) has a peak absorption of 680nm (P680)
An enzyme in PSII splits water in the presence of light, this is photolysis
2H20 --> 4H+ + 4e- + O2
Photolysis creates a source of protons for photophosphorylation and donates electrons to chlorophyll to replace the lost electrons
Electrons in the photosystem II are hit by light and become excited which therefore leave the chlorophyll(magnesium) which becomes oxidised
The electrons move through a series of electron carriers (redox reactions of iron) in the thykaloid membrane, losing energy each stage. The energy goes to help pump protons into the lumen
The electrons are replaced by electrons from photolysis
The electrons are captured by photosystem I, replacing those lost from PSI due to excitation by light energy
Ferredocxin accepts electrons from PSI and passes them to NADP in the stroma
Protons build up in the thykaloid, creating a proton gradient for chemiosmosis
Protons diffuse out of ATP Synthase, the flow of protons causing ADP and Pi to for ATP
The protons are accepted by NADP (along with the electrons) becoming reduced NADP, This is catalysed by NADP reductase
The Calvin Cycle
Light strikes PSI, releasing a pair of electrons into the electron carriers
ATP is generated and eventually the electrons are cycled back into PSI
Ribulose biphosphate (RuBP) accepts CO2, forming a 6C molecule before breaking down into 2 glycerate-3-phosphae (GP) (3C) molecules, catalysed by RuBisCO
2GP is reduced by 2 reduced NADP using the energy from 2ATP into 2 triose phosphate (TP)
It takes 6 turns of the cycle to create 12TP molecules, of which 10 are recycled and 2 are the product
ATP is used to convert 2TP into RuBP
It takes place in the stroma
TP Uses
TP can synthesis glucose
TP can synthesis amino acids, fatty acids and glycerol (lipds)
5 molecules of TP can create 3 RuBP molecules
RuBisCO has a optimum pH of 8, and is activated by large amounts of ATP and the cofactor magnesium
Light Intensity
Carbon Dioxide
Temperature
Water Stress
the conditions a plant will experience when water supply is limited
Cells become plasmolyed
Plant roots produce abscisic acid that, when transloacted to the leaves, cause stomata to close, reducing gas exchange
The rate of photosynthesis is reduced
Tissues become flaccid and leaves wilt
Roots are unable to replace water lost by transpiration
Increases in temperature increase rate of photosynthesis up to 30*C
Above 30*C, rates may reduce due to photorespiration: oxygen competing with carbon dioxide for RuBisCO
Above 45*C, enzymes may denature, reducing photosynthetic rates