Photosynthesis

• Equation for ATP formation:

  ADP + Pi + Energy + ATP synthase -> ATP + H2O
• Equation for ATP hydrolysis:

  ATP + H2O + ATP hydrolase -> ADP + Pi + Energy
• What are some uses of the energy created by the hydrolysis of ATP?

  Muscle contraction, active transport and phosphorylation of other molecules.
• What are the components of ATP?

  Adenine, ribose and 3 phosphate groups.
• Distinguish between oxidation and reduction in terms of electrons and in terms of hydrogen:

  In oxidation, electrons are lost and hydrogen is lost. Whereas in reduction electrons are gained and hydrogen is gained.
• Define coenzyme and give 3 examples:

  A molecule that carries H atoms to ETCs.

  e.g. NAD, FAD and NADP
• Describe the role of coenzymes:

  They donate the electron of their H atom to the electron transport chain. The energy that is released as the electron passes down the ETC is used to combine ADP and Pi to form ATP (phosphorylate).
• How are coenzymes reduced?

  • During respiration/photosynthesis molecules are oxidised
  • This oxidation results in loss of hydrogen
  • Dehydrogenase enzymes catalyse the removal of hydrogen from these molecules
  • The hydrogen atoms are transferred to coenzymes, reducing them
• ATP synthase has 2 functions: it catalyses the synthesis of ATP and it allows the movement of H+ ions. Suggest how its shape allows this (4):

  The active site of ATP synthase is complementary to both ADP and Pi so enzyme-substrate complexes can form. There is a channel in the ATP synthase enzyme. This allows facilitated diffusion of H+ ions down a concentration gradient.
• Describe the process of photoionisation:

  Light energy is absorbed by chlorophyll molecules. The chlorophyll is oxidised so it loses 2 electrons and becomes positively charged (ionised).
• Give the equation for photolysis:

  H2O + light energy -> 2H+ + 2e- + 1/2O2
• What is the purpose of photolysis?

  The 2 electrons are used to replenish those lost from the chlorophyll. Also, the energy from the 2 electrons is used to pump H+ ions from the stroma into the thylakoid space. This creates a proton gradient across the thylakoid membrane.
• Light dependent reaction: products, processes involved and where it takes place:

  • ATP and NADPH
  • Photoionisation and photolysis- these happen simultaneously
  • In thylakoid discs containing chlorophyll molecules
• 3 key features of a chloroplast:

  1. Thylakoid- disc containing chlorophyll molecules
  2. Granum- stack of thylakoids; increases surface area for light absorption
  3. Stroma- site of the Calvin Cycle, contains the enzyme RUBISCO
• Describe chemiosmosis and the uses of the products:

  The proton gradient created as a result of photolysis means that protons move back across the thylakoid membrane. They do this through ATP synthase channel proteins. The energy released during the movement of protons is used to phosphorylate ADP forming ATP. The protons and electrons emitted from chlorophyll are used to reduce the co-enzyme NADP.
• Explain how the energy of light is converted into chemical energy in the light-dependent reaction (2)

  Light energy is absorbed by chlorophyll. The 2 electrons emitted are passed down an electron transport chain in the thylakoid membrane. The energy released is used to phosphorylate ADP, forming ATP.
• Steps for the Calvin Cycle:

  1. CO2 combines with RuBP (ribulose bisphosphate)
  2. This produces 2 molecules of GP (glycerate 3-phosphate). Reaction is catalysed by RUBISCO.
  3. GP is then reduced to 2 molecules of TP (triose phosphate). ATP hydrolysis provides the energy for reduction, and NADPH provides hydrogen for reduction.
  4. Some of the TP is converted to useful organic compounds e.g. glucose.
  5. 83% of the TP is used to recycle RuBP. This ensures that GP levels remain high.
• Equation for photorespiration and why it is not useful for plants:

  RuBP + O2 + RUBISCO -> GP + phosphoglycolate

  • O2 is a competitive inhibitor, and phosphoglycolate is not useful
• Where does the Calvin Cycle occur?

  Stroma.
• Light is not required for the Calvin Cycle to take place. Explain why it cannot take place for long in the absence of light. (2)

  It uses ATP and NADPH which are products of the light dependent reaction which requires light energy. Therefore in the absence of light, the rate of the Calvin Cycle is limited by the products of the light dependent reaction.
• Explain why concentration of GP rises in the absence of light and then falls:

  • No light means that no ATP and NADPH are made in the LDR, so less GP is reduced to TP.
  • It falls later as not enough TP is used to recycle RuBP and so less GP is produced.
• Concentrations of GP remain the same even though the plants were supplied with excess CO2. Use your knowledge of the Calvin Cycle to explain why. (2)

  The rate at which GP is produced from RuBP and CO2 is equal to the rate at which GP is reduced to TP.
• What are 3 factors affecting the rate of the light independent reaction?

  1. Products of the light dependent reaction.
  2. Temperature.
  3. Enzyme concentration.
• Heat stress decreases the light dependent reaction of photosynthesis. Explain why this leads to a decrease in the light independent reaction. (2)

  Less ATP and NADPH is made from the light dependent reaction, which are needed for the light independent reaction.
• A decrease in the activity of the enzyme rubisco would limit the rate of photosynthesis. Explain why. (2)

  Less RuBP and CO2 react with rubisco, so less GP is made. This causes less TP to be recycled to RuBP.
• Define limiting factors:

  Factors in short supply limiting the rate of photosynthesis.
• What are the 4 limiting factors for plants?

  1. Light intensity.
  2. Light quality- wavelength or colour.
  3. CO2 concentration.
  4. Temperature.
• What are the 2 ways we can measure the rate of photosynthesis?

  1. Oxygen (volume or concentration) produced in a given time.
  2. CO2 absorbed in a given time.
• Net CO2 versus Gross CO2:

  Net= atmospheric CO2 used in photosynthesis.

  Gross= atmospheric CO2 used in photosynthesis PLUS CO2 produced in respiration that is used in photosynthesis.
• Scientists predict that in the area where this plant grows the temperature will rise from 20 to 23 degrees. It is also likely to become much cloudier. Explain how these changes will affect the growth of the plant. (3)

  Increase in temperature will increase the rate of respiration so more glucose is used. In cloudier conditions, the rate of the light dependent reaction decreases so limits the rate of photosynthesis. The plant will grow more slowly.