Photosynthesis
Autotroph= an organism that can produce its own food using light energy which can be converted into chemical energy for photosynthesis.
Heterotrophs= organisms that cannot photosynthesize. they rely on nutrients from plants or eating other animals.
Carotenoids are accessory pigments found in the photosystems:
Carotene absorbs blue light and reflects longer yellow, red wavelengths of light.
Xanthophylls absorb blue light and are yellow pigments.
The light dependent stage of photosynthesis occurs in the thylakoid membrane of the chloroplast. The thylakoid membrane contains carotenoids; carotenes, xanthophylls which are accessory pigments that absorb light energy. Contains primary pigments; chlorophyll a and chlorophyll b that absorb light energy.
Chlorophyll a is the most abundant pigment.
The Light dependant stage:
Light hits PSII in the thylakoid membrane where chlorophyll absorbs light energy from the sun. Photolysis of water occurs in the presence of light, splitting of water into protons (H+ ions), oxygen which is a waste product and electrons. Electrons become excited through photoionisation and move down an electron transport chain from one electron carrier to another, losing energy each time. This forms a proton gradient. The electrons reach PSI, non-cyclic photophosphorylation occurs where ATP is generated from ADP and Pi catalysed by ATP synthase and occurs through chemiosmosis. NADPH is formed as NADP ( a coenzyme ) accepts the electrons and H+ ions from photolysis which move down a concentration gradient. Both ATP and NADPH are used in the light independent reaction of photosynthesis.
The Calvin cycle:
The light independent reaction occurs in the stroma.
Rubisco is the most abundant enzyme.
Rubisco catalyses the fixation of carbon dioxide.
The Calvin cycle uses ATP, NADPH and carbon dioxide to form a hexose sugar.
ATP is hydrolysed to produce energy for this reaction and NADPH donates it H+ ions to reduce GP into TP ( a 3 carbon phosphorylated sugar ).
Carbon dioxide reacts with RuBP to form two molecules of GP (glycerate-3-phosphate) which is catalysed by the enzyme rubisco. GP is reduced to TP using energy from ATP and accepting a H+ ion from NADPH. Some of the carbon (1/6) from TP leaves the cycle to be converted into glucose, lipids or amino acids. The rest of the molecule is used to regenerate RuBP, with energy from ATP. Each turn of the Calvin cycle takes in or fixes 1 carbon atom from carbon dioxide. This means that the Calvin cycle has to turn 6 times to form glucose.
Lipids used to make cell membranes, amino acids for protein synthesis. Glucose can be converted into glycogen or starch for storage. Glucose can join molecules together through condensation reactions to form sucrose which is a disaccharide.
Limiting factors of photosynthesis:
Light intensity= Increases the rate of photosynthesis as light intensity increases. When light intensity is low, not enough light energy is accumulated for the light dependant stage of photosynthesis. The levels of ATP and NADPH will also be low, so the rate of the light independent reactions will be lower, less GP can be converted into TP, so the concentration of GP increases and the concentration of TP and RuBP decreases. As light intensity increases, the light dependant reactions increase in rate producing more ATP and NADPH for the light independent reactions. Light intensity becomes the limiting factor. As the graph plateaus, light intensity is no longer the limiting factor. Carbon dioxide concentration or temperature are the limiting factors.
Carbon dioxide concentration= If the concentration decreases, the rate of photosynthesis increases as the rate of the light dependant reaction decreases. Less ATP and NADPH produced. Less GP formed, less TP formed, concentration of RuBP increases as less of it is reacting with carbon dioxide.
Temperature= As temperature decreases, Rubisco will not be functioning at a fast rate. Decreases the rate of the light independent reaction and the overall rate of photosynthesis. However, if the light intensity is low, then the overall rate of photosynthesis is less affected by temperature. At low light intensity, the overall rate of photosynthesis is limited by the rate of the light dependant reaction. the light dependent reactions are less dependant on enzymes so are less affected by temperature.
Water= If water becomes scarce, stomata closes to reduce the rate of transpiration, however, this reduces the carbon dioxide concentration which becomes the limiting factor.
Compensation point:
The light intensity at which the rate of respiration equals the rate of photosynthesis. There is no net gas exchange. Carbon dioxide released from respiration is equal to the uptake of carbon dioxide during photosynthesis.
Absorption spectrum:
Shows the rate of photosynthesis with the absorption of different wavelengths of light by a particular pigment.
Chlorophylls absorb wavelengths in the blue and red regions of the light spectrum.
Carotenoids absorb wavelengths of light in the blue region, not in the red.
Action spectrum:
Shows the rate of photosynthesis at different wavelengths of light.
The rate of photosynthesis is highest in the blue and red regions of the spectrum.
Granum provides a large surface area for photosynthesis which increases the rate and allows for more sugar production. The granum consists of a stack of thylakoids. Attachment site for photosynthetic pigments to increase the absorption of light by chlorophyll.