5-1 Photosynthesis

Question 1

What is photosynthesis?

Answer 1

• Photosynthesis is a reaction in which light energy is used to produce glucose in plants.
• The process requires water and carbon dioxide with the products being glucose and oxygen.

Question 2

What are the two stages of photosynthesis?

Answer 2

• Light dependent stage
• Light independent stage

Question 3

What determines the rate of photosynthesis?

Answer 3

• Carbon dioxide concentration
• Light intensity
• Temperature

Question 4

Describe the structure and function of the chloroplasts?

Answer 4

• Site of photosynthesis
• Contains stacks of thylakoid membranes called grana which 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 to absorb the maximum amount of light.
• The granal membrane has ATP synthase channels embedded allowing ATP to be synthesised as well as being selectively permeable allowing the establishment of a proton gradient.
• Chloroplasts contain DNA and ribosomes allowing them to synthesise proteins needed in the light dependent reaction.

Question 5

What are the stages of the light dependent reaction?

Answer 5

1. Photons of light hit chlorophyll molecules in PSII causing the electrons to become excited. This is called photoionization. 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 the n 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 PS1. 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 derives the process of photophosphorylation. The enzyme ATP synthase phosphorylates ATP from ADP and Pi.

Question 6

What are the stages of the light independent reaction?

Answer 6

1. Carbon dioxide fixation.
   a. Carbon dioxide that has diffused through the stomata is fixed with RuBP in a process known as carboxylation.
   b. The enzyme Rubisco is needed to do this.
   c. A 6-carbon sugar is formed first; however, this is unstable and therefore forms 2 molecules of GP.
2. Reduction phase.
   a. The 2 molecules of GP contain a -COOH group and are therefore an acid.
   b. The reducing power or reduced NADP therefore reduces the GP, with energy from ATP.
   c. This forms 2 molecules of TP.
   d. All of the NADP from the light dependent reaction has now been used and only some ATP.
3. Regeneration of RuBP.
   a. 5 molecules of TP are used in order to regenerate 3 molecules of RuBP.
   b. The remaining ATP from the light dependent reaction is now used.
4. Organic molecule production.
   a. 2 molecules of TP can combine to form the intermediate hexose sugar fructose 1,6 bisphosphate which then forms molecules of glucose.
   * 6 turns of the Calvin Cycle are required in order to produce 1 molecule of glucose per molecule of CO2.