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Photosynthesis

THE LIGHT-DEPENDENT STAGE OF PHOTOSYNTHESIS

  1. The two photosystems are involved in non-cyclic photophosphorylation, PSII comes before PSI. The photosystems are found in the thylakoid membranes and are linked by electron carriers (proteins that transfer electrons). The photosystems and electron carriers form an electron transport chain.
  2. Light energy is absorbed by PSII and this excites electrons/reaction centres in the photosystem. The electrons move to a higher energy level (have more energy) and the high energy electrons are passed along the electron transport chain to PSI.
  3. When excited electrons move from PSII to PSI they need to be replaced – this is done by photolysis. Here, light energy splits water into protons (h+), electrons and oxygen. (the o2 in photosynthesis comes from water).
  4. Excited electrons lose energy as they move along the electron transport chain, and the energy is used to transport protons into the thylakoid through proton pumps. This gives the thylakoid a higher proton concentration than the stroma (forming a proton gradient). The protons will move down their concentration gradient (into the stroma) through ATP synthase. The energy from the movement combines ADP and Pi to form ATP. The process of a proton gradient driving ATP synthesis is called chemiosmosis.
  5. Light energy is also absorbed by PSI, exciting the electrons again to a higher energy level.
  6. The electrons are transferred to NADP with a proton (H+) from the stroma, forming reduced NADP (NADPH).
  7. Cyclic photophosphorylation only produces ATP. It only uses PSI. The electrons aren’t passed onto NADP, they’re passed back onto PSI by electron carriers. The electrons are basically recycled by PSI. No NADPH or O2 is produced.

 

THE LIGHT-INDEPENDENT STAGE OF PHOTOSYNTHESIS

  • Also called the Calvin cycle. It takes place in the stroma of the chloroplasts. The reactions are linked in a cycle, so the starting compound, ribulose bisphosphate, is regenerated. Sometimes called carbon dioxide fixation (carbon from CO2 is fixed into an organic molecule)
  1. Carbon dioxide is combined with ribulose bisphosphate to form 2 molecules of glycerate 3-phosphate. The CO2 enters the leaf through the stomata and diffuses into the stroma of the chloroplast. When it combines with RuBp it gives an unstable 6 carbon compound, which breaks down into 2 molecules of the 3 carbon compound glycerate 3-phosphate (GP). RuBisCO (ribulose bisphosphate carboxylase) catalyses the reaction between CO2 and ribulose bisphosphate.
  2. ATP and NADPH (reduced NADP) are required to reduce GP into triose phosphate (TP). 2 lots of ATP and 2 lots of NADPH are used from the light independent reaction. The NADP can then go back to the light-dependent reaction. The triose phosphate can then be converted into many useful organic compounds. (e.g. glucose)
  3. 5 out of every 6 TP molecules produced in the cycle are used to regenerate RuBP. This uses the rest of the ATP produced by the light-dependent reaction
  • TP and glycerate 3-phosphate (GP) are used to make:
    1. Carbohydrates – hexose sugars (like glucose) are made by joining 2 triose phosphate molecules together.
    2. Lipids – made using glycerol, which is synthesised from triose phosphate and fatty acids, which are synthesised from GP.
    3. Amino acids – some are made from GP.
  • 3 turns of the Calvin cycle produce 6 molecules of TP (2 molecules of TP are made for every CO2 molecule used. 5/6 of the molecules are used to regenerate RuBP in ach cycle, so in 3 turns of the cycle, only 1 TP is produced to make a hexose sugar. Hexose sugars have 6 carbons, and 2 TP molecules are used to make 1 hexose sugar. Six turns of the cycle require 18 ATP (3 are used in each cycle) and 12 NADPH (2 used in each cycle)

 

FACTORS AFFECTING PHOTOSYNTHESIS

Light intensity·         Lights needed to give energy for the light-dependent reaction. The higher the intensity, the more energy provided.

·         Only certain wavelengths of light are used for photosynthesis.

·         The photosynthetic pigments (chlorophyll a, chlorophyll b and carotene) only absorb red and blue light (green is reflected, which is why plants look green)

Temperature·         Photosynthesis involves enzymes (e.g. ATP synthase, RuBisCO). If temp. Falls below 10°C they become inactive, and they denature at temps above 45°C.

·         High temps have an effect on :

o   Stomata -They close at high temps to avoid losing too much water. This slows down photosynthesis because less CO2 enters the leaf when the stomata are closed.

o   Thylakoid membranes – may be damaged, reducing the rate of the light-dependent stage by reducing the number of sites available for electron transfer.

o   Chloroplasts – membranes around them could be damaged, possibly causing enzymes important in the Cycle to be released into the cell. This would lessen the rate of the light-independent stage.

o   Chlorophyll – could be damaged, reducing the amount of pigment that can absorb light energy, reducing the rate of the light-dependent reactions.

Carbon dioxide·         Makes up 0.04% of the gasses in the atmosphere.

·         Increasing this to 0.4% gives a higher rate of photosynthesis. Any higher than this and the stomata will close.

Water stress·         When plants don’t have enough water, their stomata will close to save the little water that they have.

·         Less CO2 will enter the leaf for the Calvin cycle

·         Slows photosynthesis down.

 

Saturation point = where increasing the factor after this point makes no difference because something else has become the limiting factor. A graph levels off here.

Light intensity, temperature and CO2 concentration all affect the rate of photosynthesis, so all affect the levels of GP, RuBP, and TP in the Calvin cycle.

Light intensity:

  • When its low the products of the light-dependent reaction will be in short supply (ATP and NADPH)
  • So the conversion of GP into TP and RuBP will be slow.
  • The level of GP will rise (still being made) and levels of TP and RuBP will fall (as they’re being used to make GP)

Temperature:

  • The reactions in the Calvin cycle are catalysed by enzymes (e.g. RuBisCO)
  • At low temperatures, all the reactions will be slower as the enzymes work more slowly.
  • So levels of RuBP, GP, and TP will fall.
  • They’re all affected in the same way at high temperatures because the enzymes will start to denature.

Carbon dioxide concentration:

  • When its low, conversion of RuBP to GP is slow (there’s less CO2 to combine with RuBP to make GP)
  • The level of RuBP will rise (still being made)and levels of GP and TP will fall (used to make RuBP)