8.3 Photosynthesis Detail (HL)

Question 1

Summary of light dependent phase

Answer 1

• takes place in thylakoid membrane
• may occur by either cyclic or non-cyclic phosphorylation
• in both proceses, light excites chlorophyll which releases electrons that pass through an ETC making ATP
• chemiosmosis

Question 2

What is non-cyclic phosphorylation?

Answer 2

• electrons in photosystem II (PS2) are replaced by electrons generated by the photolysis of water (here O2 is produced as a waste product)
• chlorophyll in both PS2 and PS1 absorb light which triggers the release of high energy electrons (photo activation)
• the electrons from PS2 pass through a series of carriers (ETC) producing ATP via chemiosmosis
• the electrons from PS1 reduce NADP+ to generate NADPH+H+
• the electrons lost from PS1 are replaced by electrons from PS2
• water is needed for this process to continue
• it is non-cyclic phos. which allows Calvin Cycle to continue

Question 3

What is cyclic phosphorylation?

Answer 3

• only PS1 is used in cyclic phosphorylation
• it is a cycle and can make ATP but cannot be used to make organic molecules
• the high energy electrons released by photoactivation pass along an ETC, producing ATP before returning to PS1
• cyclic phosphorylation does not produce NADPH+H+ which is needed for Calvin Cycle

Question 4

Phosphorylation in terms of chemiosmosis

Answer 4

• this occurs in both cyclic and non-cyclic
• the way that ATP is produced
• as the electrons cycle through the ETC located on the thylakoid membrane they lose energy
• this free energy is used to pump H+ ions from the stroma into the thylakoid
• this buildup of protons inside the thylakoid creates an electrochemical gradient (proton motive force)
• the H+ ions return to the stroma via the transmembrane enzyme ATP synthase which uses potential energy from the PMF to convert ADP and inorganic phosphate, Pi, into ATP

Question 5

Summary of light independent phase (Calvin Cycle)

Answer 5

• occurs in the stroma
• uses the ATP and NADPH+H+ produced in the light dependent phase (non-cyclic)
• occurs in three main steps: Carbon fixation, reduction and regeneration of RuBP

Question 6

Step 1: Carbon Fixation

Answer 6

• the enzyme rubisco (RuBP carboxylase) catalyses the attachment of CO2 to the 5C compound ribulose biphosphate (RuBP)
• the unstable 6C compound that is formed immediately breaks down into two 3C molecules called glycerate 3 phosphate (GP)
• a single cycle involves three molecules of RuBP combining with three molecules of CO2 to make six molecules of GP

Question 7

Step 2: Reduction

Answer 7

• each GP is then phosphorylated by ATP and reduced by NADPH+H+
• this converts each GP molecule into a triose phosphate (TP) called glyceraldehyde phosphate
• each GP requires one NADPH and one ATP to form a triose phosphate, so a single cycle requires six of each molecule