Photosynthesis

• Chloroplasts are transducers that convert light energy into chemical energy.
• Chloroplasts are mainly located in the palisade mesophyll and are able to move and rotate in that layer in order to maximise light absorption.
• Chloroplasts have a large surface area for maximum light absorption.
• Photosystems are light-capturing complexes located in the thylakoid membranes that contain different pigments, each of which absorb different wavelengths of light.
• Chromatography is used to observe and identify the different pigments involved in photosynthesis.
• In the light-dependent reaction, non-cyclic photophosphorylation (the Z scheme) is followed by cyclic photophosphorylation.
• Plants obtain a good source of ATP for the light-independent stage by using light (photo) to create the high-energy electrons needed to power the electron transport chain to drive ATP synthetase (phosphorylation).
• The antennae complex contains chlorophyll a, chlorophyll b, and the carotenoids xanthophyll and beta carotene.
• Light energy is absorbed and passed to the reaction centre.
• A mixture of pigments is extracted from leaves and applied to the origin of the chromatogram.
• The chromatogram is placed into a solvent and left to run.
• Pigments travel different distances up the chromatography paper according to their solubilities.
• The distance moved by the solvent (the solvent front) and the Rf values can be calculated and compared to known data to identify the different pigments.
• Electrons in these molecules are excited and rise to a higher energy level.
• An electron acceptor reduces an electron from these excited electrons, leaving the oxidised chlorophyll a.
• Electrons are passed along an electron transport chain powering proton pumps to pump protons from the stroma into the thylakoid space, decreasing the pH.
• The increased concentration of protons causes them to flow down the gradient back out of the thylakoid space through ATP synthetase, driving the synthesis of ATP from ADP and Pi.
• The light independent reaction, also known as the Calvin cycle, involves the conversion of carbon dioxide into sugars such as ribulose bisphosphate and triose phosphate.
• Light splits molecules of water within thylakoid spaces into hydrogen ions, electrons and oxygen.
• The electrons replace those lost by the chlorophyll a of photosystem II.
• Ribulose bisphosphate, an unstable 6C compound, is taken up by the stroma and fixed carbon dioxide to form an unstable 6C compound.
• The protons pass into the stroma and are picked up by reduced NADP.
• The unstable 6C compound immediately breaks down into 2x 3C compounds called glycerate-3-phosphate (GP).
• A photon of light hits PSI and the same process occurs as in section 1.
• This time the electron acceptor passes the electron to NADP, forming reduced NADP.
• Glycerate-3-phosphate is reduced by reduced NADP and using energy from ATP (both received from the light dependent cycle) to form 2x 3C carbohydrates called triose phosphate (TP).
• 1C from the triose phosphates will be used to synthesise a hexose sugar; the other 5C form ribulose phosphate which is regenerated to ribulose bisphosphate using ATP, and the cycle begins again.
• Mineral nitrogen (as nitrates or ammonium) is required for the synthesis of proteins, nucleic acids and chlorophylls.
• Deficiency in mineral nitrogen results in reduced growth of all organs and yellowing of leaves (chlorosis).
• Yellowing of the leaves (chlorosis) is a limiting factor in photosynthesis.
• The rate of a physiological process will be limited by the factor in shortest supply.
• Light intensity is limiting the rate of photosynthesis here as when the light intensity increases, the rate of photosynthesis also increases.
• Carbon dioxide can be described as a limiting factor as when in short supply, the process of photosynthesis is limited.
• Any increase in this factor will increase the rate of photosynthesis until a different factor becomes limiting.
• Temperature is a factor limiting the reaction and increases in temperature above the optimum can cause denaturation of enzymes that similarly limits the rate.