Photosynthesis - Paper 2

What is photosynthesis?

Photosynthesis is a reaction in which light energy is used to produce glucose in plants.

Requires water and carbon dioxide, with the products being glucose and oxygen.

Light dependent stage and the light independent stage.

The rate of photosynthesis is determined by carbon dioxide concentration, light intensity and well as temperature.

Chloroplasts in photosynthesis

Contains stacks of thylakoid membranes called grana -provides a large surface area for the attachment of chlorophyll, electrons and enzymes.

A network of proteins in the grana hold the chlorophyll in a very specific manner - help absorb the maximum amount of light.

The granal membrane has ATP synthase channels embedded - allows ATP to be synthesised as well as being selectively permeable allowing the establishment of a proton gradient.

Chloroplasts contain DNA and ribosomes - allows them to synthesise proteins needed in the light dependent reaction.

Light dependent reaction

1. Photons of light hit chlorophyll molecules in PSII causing the electrons to become excited.

This is called photoionisation. The charge separation from this drives the process of photolysis.

2. Photolysis is the splitting of water with light. One molecule of water requires 4 photons of light to split.

When water is split it produces 1 molecule of oxygen, 4 protons and 4 electrons.

The oxygen either naturally diffuses out through the stomata or is used in respiration.

The 4 electrons replace those lost from the chlorophyll, whilst the protons move into the stroma, later creating a proton gradient.

3. The excited electron then moves down a series of protein complexes.

At one of the complexes the energy from the electron is used to pump 4 protons from the stroma to the thylakoid space.

4. The electron then moves down the chain further to PSI.

Here more photons of light are absorbed causing the electron to move back up to a high energy level.

5. The electron then moves along the chain to another complex where the electron combines with a proton to form a hydrogen atom.

This is then used to reduce NADP, forming reduced NADP.

6. The pumping of protons across the membrane means that there is now a greater concentration of protons in the thylakoid space than the stroma.

As a result a proton gradient forms with a high concentration in the thylakoid space and a low concentration in the stroma.

The protons move across the membrane by diffusion through a protein known as a stalked particle.

The movement of these protons drives the process of photophosphorylation.

The enzyme ATP synthase phosphorylates ATP from ADP and Pi.

Light independent reaction

1. CARBON DIOXIDE FIXATION - carbon dioxide that has diffused it through the stomata is fixed with ribulose bisphosphate (RuBP) in a process known as carboxylation. The enzyme Rubisco is needed in order to do this. A 6 carbon sugar is formed first, however this is very unstable and therefore forms 2 molecules of glycerate-3-phosphate.

2. REDUCTION PHASE - The 2 molecules of glycerate-3-phosphate contain a -COOH group and is therefore an acid. The reducing power of reduced NADP therefore reduces the glycerate-3-phosphate, with energy being provided by ATP. This therefore forms 2 molecule of triose phosphate. All of the NADP from the light dependent reaction has now been used with only some of the ATP being used.

3. 3. REGENERATION OF RuBP - 5 molecules of triose phosphate are used in order to regenerate 3 molecules of ribulose bisphosphate. The remaining amount of ATP from the light dependent stage is now used. 4. ORGANIC MOLECULE PRODUCTION - 2 molecules of triose phosphate can combine to form the intermediate hexose sugar fructose 1,6 bisphosphate where after it forms molecules of glucose.