Photosynthesis Key Terms and Processes Flashcards
What’s the equation for photosynthesis?
6CO2 + 6H2O –> C6H12O6 + 6O2
Describe the Light Dependent Stage
- light energy is trapped in PS ll and boosts electrons to a higher energy level
- the electrons are received by an electron acceptor
- the electrons are passed from the e acceptor along a series of electron carriers to PS l which is at a lower energy level
- the energy lost by the electrons is captured by converting ADP to ATP. Light energy has thereby been converted to chemical energy
- light energy absorbed by PS l boosts the electron to an even higher energy level
- electrons are accepted by another e acceptor
- the electrons which have been removed from the chlorophyll are being replaced by pulling other electrons from a water molecule
- the loss of electrons from the water molecules causes it to dissociate into protons and O2 gas
- the protons from the water molecules combine with the electrons from the second acceptor and these reduce NADP
- the cyclic reaction occurs; this causes the electrons to be recycled and is used for forming more ATP
Describe the Light Independent Stage
- CO2 from the air diffuses into the leaf through open stomata, most of which are on the underside of the leaf.
- It then diffuses through the air spaces in the spongy mesophyll and reaches the palisade mesophyll layer.
- Here it diffuses through the thin cellulose walls, the cell surface membrane, the cytoplasm and the chloroplast envelope, into the stroma.
- In the stroma, CO2 combines with a 5-carbon compound, ribulose bisphosphate (RuBP) (a CO2 acceptor) .
- The reaction is catalysed by the enzyme, ribulose bisphosphate carboxylase-oxygenase (rubisco). RuBP becomes carboxylated.
- The product of this reaction is two molecules of a 3-carbon compound, glycerate 3-phosphate (GP). The carbon dioxide has now been fixed.
- GP is reduced and phosphorylated to another 3-carbon compound, triose phosphate (TP). ATP and reduced NADP from the light-dependent reaction are used in this process.
- Five out of every six molecules of TP (3C) are recycled by phosphorylation, using ATP, to three molecules of RuBP (5C).
Non-cyclic phosphorylation
Light strikes PS II, exciting a pair of electrons that leave the chlorophyll molecule from the primary pigment reaction centre.
The electrons pass along a chain of electron carriers and the energy released is used to synthesise ATP.
Light has also struck PS I and a pair of electrons has been lost.
These electrons, along with protons (produced at PS II by photolysis of water) join NADP, which becomes reduced NADP.
The electrons from the oxidised PS II replace the electrons lost from PS I.
Electrons from photolysed water replace those lost by the oxidised chlorophyll in PS II.
Protons from photolysed water take part in chemiosmosis to make ATP and are then captured by NADP, in the stroma. They will be used in the light -independent stage.
Cyclic phosphorylation
Light strikes PS I.
Exciting a pair of electrons that leave the chlorophyll molecule from the primary pigment reaction centre.
The electrons pass along a chain of electron carriers back to PS I and the energy released is used to synthesise ATP.
There is no photolysis of water and no generation of reduced NADP, but small amounts of ATP are made.
Difference between primary and accessory pigments
- Primary act as reaction centres/where electrons are excited.
- Accessory/other part of photo system surround reaction centre.
- Accessory absorb different wavelengths of light (not absorbed by primary).
- Accessory pigments transfer energy to primary pigments.
- Primary - chlorophyll a P680, P700.
Accessory - chlorophyll b/carotenoid.
How is light is harvested in chloroplast membranes
- Pigments arranged in photosystem.
- Light energy absorbed by pigment molecules.
- Electron excited and returned to pigment.
- Energy passed from one pigment to another.
- Energy passed to reaction centre/chlorophyll a/primary pigment.
- Range of accessory pigments allows a range of wavelengths to be absorbed.
How are the products of the Calvin cycle are used?
- Some GP can be used to make amino acids and fatty acids
- Condensation of 2 molecules of TP forms hexose sugars such as glucose.
- Some glucose molecules can be isomerised to make another hexose molecule, such as fructose.
- Glucose and fructose molecules may be combined to form the disaccharide sucrose - the sugar translocated in phloem to sieve tubes.
- Hexose sugars can be polymerised into other carbohydrates such as cellulose and starch
- TP can be converted to glycerol, this combines with fatty acids to form lipids.
Role of NAD, FAD and NADP in palisade mesophyll cell
- NAD and FAD respiration, NAD and NADP photosynthesis
- Transfer H to cristae.
- GP to TP.
- NADP from grana to stroma.
- In light dependent stage water split by light energy
- H split into electrons and protons at ETC.
- 2 x reduced NAD glycolysis, 1 x link and 3 x reduced NAD in krebs and 1 x reduced FAD.
Limiting factors and the Calvin cycle
- In bright light, levels of RuBP and TP are relatively high whilst levels of GP are relatively low, this is because in bright light the rate of the light dependent reactions will be high, more ATP and reduced NADP will be produced which are used to convert GP to TP and to regenerate RuBP.
- In dim light the light dependent reactions will cease, therefore GP cannot be changed to TP so levels of GP will increase and levels of TP will fall, levels of RuBP will fall as all available RuBP will be converted to GP and no - TP available to regenerate RuBP.
- In high CO2, levels of GP and TP are relatively high whilst levels of RuBP are relatively low, this is because there is more CO2 fixation, more GP and therefore more TP, RuBP which is regenerated is quickly carboxylated to GP.
- In low CO2, levels of RuBP increase as very little is carboxylated to GP, levels of GP and TP fall.
Effects of carbon dioxide concentration on rate of photosynthesis
- If light intensity isn’t limiting, CO2 concentration leads to faster C fixation in the Calvin cycle.
- More CO2 means more RuBP converted to GP so more TP and more glucose.
- Less CO2 means RuBP will build up but GP will decrease
- But in high CO2 more stomata open which can mean the rate of transpiration increases the water uptake from the soil, leading to a stress response so abscisic acid is produced, closing stomata and decreasing CO2 concentration.
Effects of light intensity factors on rate of photosynthesis
- More light intensity means stomata open so more CO2 can enter so faster C fixation, production of GP,TP and RuBP etc.
- More light intensity means faster light-dependent reactions so photolysis occurs more quickly as more electrons are excited and ATP and NADPH2 produced more quickly.
- Both used in light-independent stage as sources of H2 and energy, to reduce GP to TP. ATP is also used to phosphorylate 5/6 TP to regenerate RuBP.
Effects of temperature factors on rate of photosynthesis
- Increasing temperature will have little effect on light-dependent reaction because besides photolysis of water, it isn’t dependent on enzymes.
- Light independent will be affected as enzyme catalysed. Enzymes and electrons more kinetic energy.
- Increasing temperature will at first increase rate of photosynthesis
- As temperature rises above 25C, the oxygenase activity of rubisco increases more than the carboxylase activity so photorespiration exceeds photosynthesis. ATP and r.NADP wasted, reducing overall rate of photosynthesis. Photorespiration more likely to occur, rate p/s decreases
- Very high temperatures may damage proteins involved in photosynthesis.
- An increase in temperature can lead to more transpiration, stomata close, stress, abscisic acid etc so reduced photosynthesis.
Describe how the investigation into rate of photosynthesis would be carried out
- Plant gives off oxygen/gas/air in photosynthesis
- Bubbles trapped
- Drawn into capillary tube by syringe
- Length of bubble measured
- After certain length of time, repeat
- Move lamp towards/away/set distance from plant
- Use a light meter to measure light intensity
- Allow time for apparatus to equilibrate at each distance
- Control temperature with thermostat/heater/cooler
- Darkened room/lamp only light source
- Calculate volume oxygen using πr2d
- Vaseline to seal joints, same plant, same CO2 content of water
Suggest why the volume of oxygen released doesn’t give a true reflection of the rate of photosynthesis
Aerobic respiration uses oxygen
Oxygen dissolves in water
