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Cyclic Photophosphorylation
ATP is produced in a cyclic process when the ratio of NADPH + H+ : NADP+ is high. Photosystem 1 does not generate any NADPH + H+, but acts to send electrons to the proton pump.
Photosystem 1 is oxidised, releasing an excited an electron to reduce the membrane proton pump. Protons, in the forms of H+ are pumped into the thylakoid space. This creates a concentration gradient necessary for the later production of ATP. The electrons are the cycled back to the first photosystem to reduce it.
Non-cyclic Photophosphorylation
The light energy from the sun is trapped in the chlorophyll, and ATP is produced. The co-enzyme NADP+ is reduced to form NADPH + H+. The first photosystem, photosystem two [PS2] is able to absorb light of the wavelength 680nm, which is why it is called P680. The second photosystem, photosystem one [PS1] is activated by wavelengths of 700nm, and called P700.
Light is first absorbed by the chlorophyll A in PS2. This energy is then converted into chemical energy by releasing electrons in oxidation.
The electrons from PS2 then pass along the thylakoid membrane in a series of redox reactions. The protein pumps are reduced to pump H+ ions into the thylakoid space.
In PS1, a different light frequency is absorbed. The photosystem is oxidised to release electrons.
The electrons pass from PS1 to ferrodoxins, which reduce NADP+ to NADPH + H+. The NADPH stays in the stroma to be used in the light independent reactions.
Photosystem 1 is reduced by the electrons from PS2.
Photosystem 2 is then reduced so that it can absorb more light. When water is split through photolysis, electrons are given to the photosystem. This is the source of H+ ions and the waste O2.
Since there is a high concentration of H+ ions in the thylakoid lumen, they can diffuse back into the stroma through the pore in ATP synthase. This process drives the phosphorylation of ADP to ATP.
During these redox reactions in the light dependent reactions, the energy levels of the electrons change. This is summarised below.
8.2.4 - Explain photophosphorylation in terms of chemiosmosis
A high concentration of H+ ions accumulates in the thylakoid space due to proton pumping. This results in a proton gradient, causing protons to be pumped across the membrane through the ATPase molecules. This drives the motor mechanism of the structure, reducing ADP into ATP. This is like the process used in respiration.
The excited electrons the move to fill the vacancies in the reaction centre of PS2, then in PS1. They are used to reduce NADP+, in non-cyclic photophosphorylation, in which the reaction pathway is linear.
8.2.5 - Explain the light-independent reactions
The CO2 in the air is fixed to form carbohydrates. This is done using the energy trapped from sunlight in the light dependent reactions in the form of ATP and NADPH. These reactions take place in the stroma.
During fixation of CO2, the carbon dioxide is trapped by RuBP to form two molecules of glycerate-3-phosphate.
The GP is then reduced to TP in the reduction step, using ATP and NADPH + H+ to provide the energy.
In the product synthesis step, TP is used to make organic molecules such as glucose phosphate, sugar, starch, lipids, amino acids, etc.
This is followed by the regeneration of the acceptor step, where some TP turns back into RuBP. This allows for more CO2 to be fixed from the atmosphere.
Ribulose biphosphate is a 5-carbon acceptor. The fixation of carbon itself is catalysed by the enzyme RuBisCo [ribulose biphosphate carboxylase], and is the most common protein in the leaves of green plants.
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
Chlorophyll best absorbs red and blue light. Green light, on the other hand, is reflected, causing plants to appear green.
The rate of photosynthesis is highest and blue and red, and lowest at yellow and green because of the optimum wavelength for chlorophyll.
8.2.8 - Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration of carbon dioxide
At any given time, only one of these factors will be the one limiting the rate of photosynthesis.
Light Intensity
Light is essential for photosynthesis, however it reaches a compensation point when the amount of oxygen being produced is the same as that being consumed in respiration. As a result, the relationship reaches a plateau at high intensities.
Light energy aids in the production of H+ ions from water and ATP. On the other hand, when there is no light, the plant can only respire. Too much light can damage the chlorophyll.
Temperature
As the surrounding temperature increases, the rate of photosynthesis increases, with each plant reaching an optimum temperature where the rate falls off steeply. The enzymes in the reactions a temperature-sensitive.
Carbon Dioxide Concentration
As the concentration of CO2 increases, the rate of photosynthesis increases, before it plateaus. Each plant has a different optimum concentration.
When the CO2 is the limiting factor, the NADPH simply accumulates in the stroma, stopping the photosystems from operating. ATP is formed through cyclic photophosphorylation.
