(5) Photosynthesis Flashcards
Photosynthesis and respiration
Photosynthesis supplies the o2 needed for aerobic respiration. Also supplies the complex molecules. Aerobic respiration is the reverse of photosynthesis.
C6H12O6 + 6O2 into 6CO2 + 6H2O
Structure of a chloroplast
Lamellae - strands between grana.
Grana - stacks of thylakoids.
Thylakoid (granum) - light dependent stage.
Stroma - light independent stage.
Photosynthetic pigments
Absorb light energy, each one is the colour of the wave length of light it reflects. By acting together they capture as much light energy as possible by absorbing as much of the visible spectrum as possible. Used in the might dependent stage.
Primary - Chlorophyll a - reaction centre.
Accessory - Chlorophyll b, carotene, xanthophyll - light harvesting system
Photosystem
Embedded in lamellae and thylakoid membranes.
Light harvesting system contains accessory pigments.
Reaction centre contains primary pigment - P680 or P700
Light dependent stage - cyclic
Occurs in the grana
Cyclic photophosphorylation - photon of light is absorbed by photosystem 1, electrons in P700 become excited. Those emitted are captured by electron acceptors, and return by a chain of electron carriers, releasing energy for ADP+Pi.
Does not produce NADP or o2, so only produces a small amount of ATP.
Light dependent stage - non cyclic
Light energy is absorbed by PSII, this excites electrons in the chlorophyll (P680), so high energy electrons move along the electron transport chain to PSI.
Excited electrons from chlorophyll leave PSII to move along the electron transport chain, so they must be replaced. Light energy splits water into protons and electrons and oxygen.
Excited electrons release energy as they move along the electron transport chain, this is used to pump protons into the thylakoid, so there is a higher concentration of protons in the thylakoid than the stroma. This forms a proton gradient.
Protons then move down the proton gradient into the stroma via ATP synthase.
Energy from this combines ADP and Pi to make ATP.
light energy is absorbed by PSI which excites the electrons to an even higher energy level. The electrons are transferred to NADP along with a proton from the stroma to form reduced NADP.
Light independent stage
Occurs in the stroma.
co2 enters the leaf through the stomata and diffuses into the stroma. It combines with RuBP (carbon fixation), forming an unstable 6C compound which quickly splits, forming 2x GP (3C) RuBisCo catalyses this reaction.
ATP, from LDS provides energy to turn GP into TP. Also requires H+ ions which comes from reduced NADP (from LDS). Reduced NADP is recycled into NADP.
1/6 of TP is used to make glucose, lipids, amino acids and other molecules. 5/6 of it goes into the cycle. Recycling of TP is used to regenerate RuBP - uses ATP.
Factors affecting photosynthesis.
Limiting factors - rate of photosynthesis is limited by the factor which is in shortest supply :
Light intensity - higher light intensity = more energy it provides for the light dependent stage. Only certain wavelengths of light are used for photosynthesis.
High light intensity = GP is low as it has been used up, TP and RuBP are both high because they accumulate.
Temperature - increased temperature causes increased kinetic energy of enzyme and substrate, so more collisions and faster photosynthesis. optimum temperature required for enzymes in photosynthesis (RuBisCo and ATP synthase) - too high = denatured. Too high temperatures cause stomatal closure to avoid loosing water, this causes photosynthesis to slow because less co2 enters the leaf.
Carbon dioxide - increasing = higher rate of photosynthesis, but too high causes stomata to close.
High co2 concentration = low RuBP as it is used up quickly, and GP and TP are high because the accumulate.
Practical investigation into limiting factors
Using pondweed to measure the effects of light intensity, temperatures and co2 concentration. Rate at which o2 is produced can be easily measured.
Test tube containing pondweed and water is connected to a capillary tube with water using a syringe.
Source of white light is placed at a specific distance away from the pondweed, then left to photosynthesise for a set time. It will release o2 which is collected in the capillary tube.
Syringe is used to draw up gas bugle I’m the tube, using a ruler to measure the length of the gas bubble.
Any other variables should be controlled.
Experiment repeated multiple times, also repeated at different distances.
Can also measure temperature using a beaker of water.
Thin layer chromatography and photosynthetic pigments
Photosynthetic pigments can be separated with thin later chromatography.
Involves a mobile phase (liquid solvent) and a stationary phase (solid).
Grid up several leaves with anhydrous sodium sulphate and propanone. Transfer liquid to test tube and add some petroleum ether and gently shake tube.
2 distinct layers with form in the liquid - top layer is pigments mixed with petroleum ether.
Transfer some liquid from the top layer into a second tube with anhydrous sodium sulphate.
Draw horizontal pencil line near the bottom of the chromatography plate, build up a single concentrated spot of the liquid, from the top layer mixed with anhydrous sodium sulphate,by applying several spots, ensuring each one is dry before the next one is added. This is the point of origin.
Once dry, put plate into glass beaker with prepared solvent just below point of origin. Put a lid on the beaker and leave plate to develop. As the solvent spreads up the plate, the different pigments move with it at different rates, so they separate.
When the solvent has nearly reached the top, take the plate out and make the furthest point the solvent has reached. Then leave to dry in a well ventilated place.
The calculate Rf values of separated pigments - allows to identify which pigments they are (Rf = distance travelled by solute/distance travelled by solvent).
