Photosynthesis

Photosynthesis

1. a) Define the terms autotroph and heterotroph

   • Autotrophs: organisms that use light or chemical energy and inorganic molecules to synthesis complex organic molecules
   • Heterotrophs: organisms that ingest and digest complex organic molecules, releasing the potential energy stored in them

2. b) State that light energy is used during photosynthesis to produce complex organic molecules

3. c) Explain how respiration in plants and animals depends on the products of photosynthesis

   • One product of photosynthesis is TP, which can be used to make glucose and carbohydrates, that are used in respiration

4. d) State that in plants photosynthesis is a two-stage process taking place in chloroplasts

5. e) Explain, with the aid of diagrams and electron micrographs, how the structure of chloroplasts enables them to carry out their function

   • Chloroplasts have a double membrane. The outer membrane is smooth and permeable. The inner membrane is less permeable and is folded into lamellae, which are stacked into grana, called thylakoids. The stroma is a fluid filled matrix inside.
   • The grana provide a large surface area for the light dependent reaction to take place.
   • Photosynthetic pigments are arranged into photosystems, which allow the maximum absorption of light energy
   • The grana are surrounded by the stroma so that products of the light dependent reaction can easily pass into the stroma for the light independent reaction

6. f) Define the term photosynthetic pigment

   • Photosynthetic pigments: molecules that absorb light energy. Each pigment absorbs a range of light wavelengths and reflects others

7. g) Explain the importance of photosynthetic pigments in photosynthesis

   • Different pigments can absorb different wavelengths of light, meaning that having a range of pigments allows a range of wavelengths to be absorbed, allowing the maximum possible absorption of light
   • Chlorophyll is the primary pigment and is found at the reaction centres of both photosystems
   • Examples of accessory pigments are carotene and xanthophyll

8. h) State that the light-dependent stage takes place in thylakoid membranes and that the light independent stage takes place in the stroma

9. i) Outline how light energy is converted to chemical energy (ATP and NADPH) in the light dependent stage (reference should be made to cyclic and non-cyclic phosphorylation)

   • 1) Light energy enters photosystem 2. This excites the photosynthetic pigments, which pass the light energy down the photosystem to the primary pigment at the reaction centre
   • 2) Chlorophyll becomes excited and loses two electrons. These electrons pass along an electron transport chain, which releases energy
   • 3) This energy is used to pump hydrogen ions from the photolysis of water across the thylakoid membranes. This builds up an electrochemical gradient
   • 4) The hydrogen ions rush down the concentration gradient through ATP synthase enzymes. This changes their tertiary structure and allows them to phosphorylate ADP to ATP
   • 5) Light energy enters photosytem 1, aggravates pigments, and chlorophyll loses two electrons. The electrons enter the electron transport chain and release energy
   • 6) This energy is used to combine hydrogen ions with NADP to give NADPH
   • In cyclic photophosphorylation, only photosystem 1 is used. The electrons pass through the electron transport chain and rejoin the primary pigment. There is no photolysis of water and no NADPH is formed

10. j) Explain the role of water in the light dependent stage

    • Light energy is used to split water into oxygen, which is given off as a gas, hydrogen ions, which are pumped across thylakoid membranes and electrons, which rejoin the primary pigment of photosystem 2

11. k) Outline how the products of the light-dependent stage are used in the light independent stage to produce triose phosphate (TP) (reference should be made to ribulose bisphosphate (RuBP), ribulose bisphosphate carboxylase (rubsico) and glycerate 3-phosphate (GP)

    • NADPH is used to convert GP to TP

12. l) Explain the role of carbon dioxide in the light independent stage

    • Carbon dioxide is bonded to RuBP using rubisco to give GP

13. m) State that TP can be used to made carbohydrates, lipids and amino acids

14. n) State that most TP is recycled to RuBP

15. o) Describe the effect on the rate of photosynthesis, and on levels of GP, RuBP and TP, of changing carbon dioxide concentration, light intensity and temperature

    • A higher carbon dioxide concentration will allow more fixation of RuBP to GP, so GP and TP will build up, but there will be less RuBP
    • A greater light intensity will increase the rate of reaction of the light dependent stage. This means there will be more NADPH so more GP can be converted to TP. This leads to a build up of TP and RuBP but reduced GP
    • Temperature will increase the rate of reaction of the Calvin cycle, allowing more RuBP to be fixed to GP as it allows rubisco more kinetic energy. However, above 25 degrees, rubisco's tertiary structure begins to alter, and this allows oxygen to enter the active site as well as carbon dioxide, causing photorespiration
    • Temperature may also damage proteins holding the photosystems in place, or cause stomatal closure to prevent water loss, thus reducing gaseous exchange

16. p) Discuss limiting factors in photosynthesis with reference to carbon dioxide, light intensity and temperature

    • The limiting factor for a metabolic process is the factor that is present at the lowest or least favourable value

17. q) Describe how to investigate experimentally the factors that affect the rate of photosynthesis

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