Photosynthesis 2018-19 (2) PDF

STT 1043 PLANT PHYSIOLOGY LU 6: PHOTOSYNTHESIS  Photosynthesis The growth of plants involves energy conversion where incident light is transformed into biomass through the process of photosynthesis. During photosynthesis, incident light energy between 400 to 700 nm w...

STT 1043 PLANT PHYSIOLOGY LU 6: PHOTOSYNTHESIS  Photosynthesis The growth of plants involves energy conversion where incident light is transformed into biomass through the process of photosynthesis. During photosynthesis, incident light energy between 400 to 700 nm wavelengths is photosynthetically active radiation (PAR). This PAR is assumed as 50% of the incoming solar radiation.  Radiation Radiation = energy from sunlight In full sunlight, PAR is approximately 2000 umole photons m2/s. Solar radiation: 50% is PAR, 47% is infra-red and 3% is ultraviolet. Wavelength: 400-700 nm is PAR, 700 nm is infra-red.  Chlorophyll The most active photosynthetic tissue in higher plants is the mesophyll of leaves. Mesophyll cells have many chloroplasts, which contain the specialized light absorbing pigments, the chlorophylls. Chlorophyll appears green to our eyes because it absorbs light mainly in the red and blue parts of the spectrum, thus reflecting green spectrum to our eyes. Leaf cross section Structure of a chloroplast  Organelles in a chloroplast Stroma Inner membrane that encloses the chloroplast. Contain enzymes that convert CO2 into carbohydrates. The location of Calvin cycle. Thylakoid Flat, saclike membrane structures immersed in the stroma. Often stacked like coins to form units called grana. Thylakoid membrane is the site of light reaction. Stroma lamellae Thylakoid extensions that connected the individual grana.  Photosynthetic pigments Most abundant pigment – chlorophyll a and chlorophyll b Accessory pigments – carotenoids and phycocyanin  Process of photosynthesis The process of photosynthesis involves a complex series of chemical reactions that result in CO2 and H2O being converted into carbohydrate (C6H12O6). 6 CO2 + 6 H2O → C6H12O6 + 6O2 The conversion of light energy into chemical energy results in H2O being split into O2 and H+ (Light Reaction). The products of Light Reaction are ATP, NADPH and O2.  Process of photosynthesis Carbon dioxide from the air is bound by enzymes within the chloroplasts, and then converted into carbohydrate through the further addition of carbon in the Calvin cycle (Dark Reaction). The Calvin Cycle uses the ATP and NADPH (from the light reaction) to fix CO2 into carbohydrate. The product of Calvin cycle is Glyceraldehyde-3- Phosphate.  Output of Calvin Cycle Glyceraldehyde-3-phosphate (G3P) is combined to form fructose. Fructose is then rearranged to create glucose. Glucose will undergo respiration process where energy (ATP) will be released Or Glucose will combine with fructose to form sucrose. Sucrose will be transported throughout the plant  Process of photosynthesis Lets look at the overall process of photosynthesis: https://www.youtube.com/watch?v=wJDlxp17rY4 All plants undergo Calvin cycle photosynthetic pathway. However, C4 plants have an additional photosynthetic pathway, i.e. Hatch-Slack pathway.  Photorespiration Photorespiration occurs under conditions of low CO 2 concentration. Photorespiration occurs in chloroplast, peroxisome and mitochondria. When CO concentration is low, RuBP will bind to O 2 2 instead of CO2.  Photorespiration Photorespiration appears to be a losing process because: RuBP is lost from the Calvin cycle The fixation of CO2 is reversed: O2 is consumed, CO2 is released Only a part of the carbon is returned to the chloroplast  C3 Plants Plants which fix CO (photosynthesize) only by Calvin 2 cycle are termed C3 plants. Eg. Rice, soybean, sunflower, legumes In general, C3 plants are shade loving and thrive in an environment of lower temperature.  C4 Plants The C plants contain 2 distinct types of chloroplast: 4 In mesophyll and bundle sheath cells. C plants fix CO by two different pathways which are Hatch- 4 2 Slack pathway in the mesophyll and Calvin cycle in bundle sheath cells. Adapted to grow in warmer and dry climates such as the tropics. Eg. Maize, sugarcane, tropical grasses. Structural differences: C3 vs. C4 leaf  C3 vs. C4 photosynthesis C3 plants fix CO2 via Calvin Cycle only. In this process, Ribulose-1,5-bisphosphate (RUBP) binds with CO2 to produce 3-phosphoglycerate. C4 plants fix CO2 via 2 pathways: Hatch-Slack and Calvin Cycle. In this process, Phosphoenol Pyruvate (PEP) binds with CO2 to produce oxaloacetate. In C3 plants, the CO2 compensation point is high to begin photosynthesis process. C3 vs. C4 photosynthesis  C3 vs. C4 photosynthesis The graph shows: C4 plants are much better at trapping carbon dioxide at lower concentrations due to PEP enzyme activity and therefore minimize photorespiration. Explanation: In C4 pathway, PEP has a higher affinity for CO2, and a series of carriers that delivers CO2 at high concentrations into the Calvin Cycle. In C3 plants, the activity of Rubisco enzyme in the Calvin Cycle depends on the relative concentrations of CO2 and O2. Hence, the occurrence of photorespiration is high.  C3 vs. C4 photosynthesis The graph shows (cont.): The C3 plant is better at higher concentrations of CO2 reflecting the extra ATP needed to drive the C4 cycle. C pathways occur in 2 tissues: the mesophyll and 4 bundle-sheath, therefore rich in chloroplasts. As temperature rises and carbon dioxide becomes less available, the efficiency of photosynthesis decreases in C3 plants. This problem is not shown in the C4 plant. C3 vs. C4 photosynthesis C4 plants tolerate high temperatures!! C3 vs. C4 photosynthesis C4 plants can photosynthe size under high light intensities!!  C3 vs. C4 photosynthesis Where water and carbon dioxide are plentiful, C3 plants are good competitors. Where carbon dioxide is limiting and temperatures are warmer, C4 plants are good competitors. C4 plants have a higher water use efficiency compared to C3 plants. Reason: PEP enzyme brings CO2 faster therefore does not need to keep stomata open as much (less water lost by transpiration) for the same amount of CO2 gain for photosynthesis.  CAM Plants Plants with crassulacean acid metabolism (CAM), including many cacti, orchids, pineapple and other succulents, have stomatal activity patterns that contrast with those found in C3 and C4 plants. Opens stomata at night and close their stomata during the day to prevent water loss. At night, CO2 diffuses into CAM plants where it is combined with PEP and fixed into malate.  Radiation Use Efficiency Under optimum growth conditions (adequate moisture & nutrients & good agronomic practice), the maximum yield of a crop is dependent on the availability and amount of PAR intercepted by the leaf canopy. The accumulation of crop biomass at a given time is proportional to the accumulation of intercepted PAR. Radiation Use Efficiency Crop biomass (g/m2) Crop biomass (Y) = b*(PAR,X) Therefore, the slope (b) represents the radiation use efficiency Total absorbed PAR (MJ/m2)  Radiation Use Efficiency Okay, not too worry about the graph.. Let me explain in the simplest form: Radiation use efficiency (RUE) = How much biomass is produced from each absorbed PAR. In other words, it is saying… The ability (efficiency) of a plant to convert the captured radiation into a biomass (organic matter).  Radiation Use Efficiency Of radiation absorbed by leaf, >95% is converted into heat,