Discuss how photosynthesis responds to the environment Photosynthesis usually takes place in leaves of plants which contain chloroplasts and the pigments chlorophyll a and b organised into Light Harvesting Complexes in Photosystem I and Photosystem II. Although plants do not have locomotion like animals, they are very plastic and can change their photosynthetic rates in response to the changing environmental conditions to maximise growth rate, lifespan and reproduction. Photosynthesis is affected by many factors including temperature, light and water availability and I will explore what the responses are and how they take place. Photosynthesis requires sunlight but when there is too much sunlight the photosynthetic apparatus can become damaged, for example chlorophyll bleaching can occur which means that chlorophyll can not function anymore which means photosynthesis decreases. Shade leaves are more prone to photodamage than sun leaves because they have a smaller capacity to prevent photodamage since they are less likely to be exposed to intense sunlight. Damage occurs when excess light means that excited chlorophyll is converted into triplet chlorophyll which catalyses the reduction of an oxygen molecule into a singlet oxygen which can bleach chlorophyll. Superoxides (O2-) also form which damages the D1 protein in photosystem II thus lowering the efficiency of PSII. Superoxides can be converted into H2O2 which then form hydroxide radicals which are damaging to DNA. When there is a higher intensity of sunlight than ideal, plants respond by activating photoprotection mechanisms by dissipating the excess energy absorbed. A high electron transport chain in the thylakoid membrane leads to a build-up of protons in the lumen and therefore acidification. This leads to protonation of certain amino acids of proteins in the light harvesting complexes to reduce the efficiency of energy transfer to the reaction centre. Carotenoids within the LHCs absorb excess energy from chlorophyll molecules by resonance energy transfer and then dissipate this as heat. Chlorophyll molecules themselves can dissipate energy through fluorescence. The xanthophyll cycle is activated due to the low pH is the lumen which increases the rate of conversion of violaxanthin pigments to antheraxanthin then to zeaxanthin by de-epoxidation. This can then pass on energy from chlorophyll molecules to the PsBS protein which dissipates the energy as heat. These are nonphotochemical quenching mechanisms as they do not use photochemistry to dissipate energy. These mechanisms mean that photosynthesis can continue at a high rate without being damaged by too high sunlight. Photosynthetic rate is affected by temperature. As temperature increases photosynthesis increases then peaks when optimum is reached and decreases. The optimum temperature is different for different plants adapted to different climates. For example plants native to Death Valley will be able to photosynthesise at a higher temperature than coastal plants at Bodega Head in California and they will not survive if their locations were swapped around with each other. Photosynthesis first increases because molecules have more energy so the enzymes involved such as rubisco are more likely to collide with their substrates. However, as temperature becomes too high, the enzymes denature because the bonds holding the polypeptide shape breaks or weakens so the active site shape changes. At slightly higher than optimal temperatures before enzymes are damaged, the reduction in photosynthesis rate is due to rubisco activase which aggregates together so that it can no longer help in rubisco assembly. This means that rubisco activity decreases. Also, rate of photorespiration and respiration increases and when these reactions release more carbon dioxide than how much photosynthesis absorbs, net photosynthesis decreases. This means that net photosynthesis has an optimum temperature and decreases at a temperature higher than this. Some plants can acclimate to different temperatures, for example in Pine as daytime temperatures decrease from summer to winter, there is a loss of D1 protein so that photosynthetic apparatus is not damaged by the dehydrating conditions and an increase in PsBS protein so that photoprotection can occur more efficiently. At a higher atmospheric carbon dioxide concentration, photosynthesis rate is expected to increase. This is because carbon dioxide is usually a limiting factor so a higher concentration would increase rate as carbon dioxide is required in the Calvin cycle for rubisco to fix it with RuBP. An experiment was done which showed that beech grew more when carbon dioxide concentrations was increased when grown in alkaline soil. Because a higher atmospheric concentration would mean that there would be a steeper gradient between the atmosphere and the internal of the leaf via the stomata, many plants will be able to reduce the openings of their stomata to limit water loss by transpiration while still be able to allow carbon dioxide to enter so that photosynthetic rate is not affected, leading to an increase in water use efficiency so that fewer water molecules are lost per unit of carbon assimilated. However, many plants are able to downregulate photosynthetic rate by acclimation as carbon dioxide levels increase, especially if nitrogen levels in the soil does not increase along with carbon dioxide. Nitrates are required by plants to produce enzymes and photosynthetic apparatus including Rubisco therefore photosynthesis will not increase if nitrate levels are not increased simultaneously. Stomata opening is controlled by guard cells which respond to the environment to control water loss. This in turn affects photosynthesis because the amount of carbon dioxide diffusing into the leaf is affected. Guard cells respond to water content: when the soil is drier than ideal the guard cells become flaccid due to a loss of cell turgor which closes the stomatal pore. When the plant has a plentiful supply of water a P-type proton ATPase is activated in the guard cell membrane and this causes protons to be actively pumped out. The change in voltage activates voltage-gated potassium ion channels so that 350mM of potassium ions diffuse in. Around 90MM of chloride ions enter too via symport with protons. To balance out the positive charges of potassium ions, 120mM of malate2is made by PEP carboxylase and Malate dehydrogenase from phosphoenolpyruvate. Because of the increase in solutes inside the guard cells, water enters by osmosis so the guard cells become turgid and its expansion leads to the opening of the stomata and more carbon dioxide can diffuse in to increase the rate of photosynthesis. When there is a water deficit, abscisic acid is synthesised in the roots which travels via the phloem and activates calcium ion channels in the guard cell so that calcium ions diffuse in. this inhibits potassium ion influx and chloride ion influx channels and activates potassium and chloride efflux channels. This causes water to leave by osmosis so that guard cells become more flaccid to close the stomata. An increase in iron in oceans can increase photosynthesis rate of ocean plants because iron is required to synthesis many enzymes. Ocean Iron Fertilisation is currently a potential technique to combat global warming by adding more iron to waters to increase photosynthetic rate and carbon dioxide absorption. Fertilising soils with nitrogen can increase photosynthetic rate of some plants, especially crop plants. This is because nitrogen is commonly a limiting factor and is required for enzyme biosynthesis such as rubisco. Addition of more nitrates into the soil can increase relative growth rate. Photosynthesis can respond to a wide range of environmental changes which is important for plant survival as the environment is constantly changing so plants need to be able to increase survival and growth rates in different conditions. However, plants are not able to tolerate all environmental conditions because photosynthesis depends on enzymes and proteins which require a certain range for environmental conditions such as sunlight so there is a limit as to how much is can respond to the environment.
