5.6- Photosynthesis

Question 1

During the light-independent stage of photosynthesis, triose phosphate (TP) is synthesised. State possible uses for this molecule within the plant?

Answer 1

• Synthesis of carbohydrates
• Such as the hexose sugars – e.g. glucose
• Synthesis of amino acids
• Synthesis of lipids
• Regeneration of RuBP

Question 2

Outline how complex organic molecules such as lipids are produced from the light independent stage of photosynthesis? (The Calvin Cycle)

Answer 2

• Carbon dioxide is combined with RuBP (carbon fixation)
• By the enzyme ribulose bisphosphate carboxylase (rubisco)
• This produces two 3 carbon compounds
• Glycerate Phosphate (GP) which is converted into Triose Phosphate (TP)
• This uses NADPH and ATP
• Triose phosphate is converted into lipids

Question 3

State two uses of Triose Phosphate which is synthesised during the LIS of photosynthesis

Answer 3

• Carbohydrate (e.g. Glucose)
• Hexose sugars
• Amino Acids
• Lipids

Question 4

With reference to the biochemistry of photosynthesis, explain why the theoretical rate of photosynthesis is NOT achieved at higher light intensities.

Answer 4

• Another factor such as temperature or carbon dioxide could become a LIMITING FACTOR.
• Temperature could become a limiting factor as in the Calvin cycle enzymes rely on kinetic energy of molecules
• Carbon Dioxide concentration could become limiting which is required in the Calvin cycle during fixation

Question 5

The role of water during photosynthesis

Answer 5

• The splitting of water is called photolysis
• Water is the source of protons (hydrogen ions) that are used in photophosphorylation
• Water donates electrons to chlorophyll a to replace those lost when light strikes chlorophyll.
• Water is the source of the waste product oxygen, some of which may be used by plants for respiration.
• Water keeps plant cells turgid

Question 6

Describe how the light energy absorbed by these photosystems is converted into chemical energy in the light dependent stage of photosynthesis

Answer 6

• non-cyclic photophosphorylation:
• Photosystems 1 and 2 are involved. These absorb the energy from photons of light.
• Electrons (a pair) are excited from chlorophyll a in photosystem II.
• The electrons escape from chlorophyll a and are captured by an electron acceptor molecule
• The electrons pass along a chain of electron carriers some energy associated with the electrons is released.
• This energy is used to pump protons across the thylakoid membrane into the thylakoid space.
• This sets up a proton/H+/hydrogen ion/pH gradient
• Protons diffuse down their concentration gradient
  through channels associated with ATP synthase
• This drives the formation of ATP from ADP and inorganic phosphate.
• This is known as chemiosmosis.
• The electrons from PSII are eventually captured by chlorophyll in PSI.
• The electrons from PSI are accepted by NADP, along with protons to form NADPH.
• Photolysis
• electrons from water replace the electrons excited from PS2
• cyclic photophosphorylation
• This involves PS1 only
• Electrons (a pair) are excited from chlorophyll a in photosystem I.
• The electrons escape from chlorophyll a and are captured by an electron acceptor molecule
• The electrons pass along a chain of electron carriers some energy associated with the electrons is released.
• This energy is used to pump protons across the thylakoid membrane into the thylakoid space.
• This sets up a proton/H+/hydrogen ion/pH gradient
• Protons diffuse down their concentration gradient through channels associated with ATP synthase
• This drives the formation of ATP from ADP and inorganic phosphate.
• This is known as chemiosmosis.
• The electrons eventually return back to chlorophyll in PSI.
• Not photolysis takes place.
• No NADPH is produced.