Biochem - photosynthesis

What is the basic equation for photosynthesis?

CO2 + H2O —> (CH2O) + O2

• (CH2O) represents carbohydrates (usually sucrose or starch)

What is an overview of photosynthesis?

• Light photon is absorbed

• Energy used to drive an electron from water to generate NADPH

• Also drives protons across a membrane

• Protons drive ATP synthesis

• ATP and NADPH are used in the light independent reactions to fix CO2

How is light absorbed by chlorophyll?

• Chlorophyll A is the photoreceptor molecule

• Four N atoms coordinate an Mg ion

• Chlorophylls have strong absorption bands in the visible region of the EM spectrum

• Energy from light photons excite an electron from its ground state to an excited state

• Light energy absorbed by light harvesting chlorophylls causes an exciton to be transferred to the reaction centre of the chlorophyll - transferred to an acceptor

What are the photosynthetic units within a leaf?

• Photosynthetic units have a large number of chlorophylls

• Each unit has 2 types of chlorophylls

• Around 300 antenna chlorophylls harvest light and transfer excitation energy to the reaction centre (RC) chlorophyll

• ONLY the RC chlorophyll undergoes photochemistry

• RC turns over ~100x per second in normal conditions - would be directly excited 1x per second without added capacity from antenna chlorophylls

How are electrons transferred to an acceptor during the light-dependent reactions?

• Once the light energy is funnelled to the RC chlorophyll, the excited electron can move to the acceptor molecule

• Results in the formation of a +ve charge on the donor molecule and a -ve charge on the acceptor molecule - photoinduced charge separation

• Site where the separation occurs is the special pair of chlorophylls in the reaction centre

• Photosynthetic apparatus is arranged to make photoinduced charge separation extremely difficult

What are the two photosynthetic electron transport chains?

• Photosystem I

• Photosystem II

Which wavelengths do PSI and PSII respectively respond to?

• PSI - < 700nm

• PSII - < 680nm

How do electrons move between the two photosystems?

• Electrons flow from PSII to PSI

• Electrons are derived from H2O and used to reduce NADP+

• Electrons flow first through PSII, then cytochrome bf, then PSI

• Transported by plastoquinones

What does the Z scheme show?

Energy diagram of the light dependent reaction

What are the overall reaction equations for photosynthetic electron transport?

• PSII: 2H2O + 4hv —> 4H+ + 4e- + O2

• PSI: 4e- + 2H+ + 2NADP+ + 4hv —> 2NADPH

• Overall: 2H2O + 2NADP+ + 8hv —> 2H+ + O2 + 2NADPH

What was the thylakoid membrane experiment and what were the results?

• Completed by Andre Jagendorf in 1996

• Thylakoid membranes were soaked in pH 4 buffer for several hours

• Rapidly submerged in pH 8 buffer containing ADP and Pi

• pH inside the thylakoids initially remained at pH 4

• Burst of ATP production was noted that accompanied the loss of the pH gradient

How is ATP produced in the thylakoids?

• ATP synthase: CF1-CF0 complex

• Orientation is reversed

• Protons flow out of the thylakoid lumen

• Protons flow into the mitochondrial matrix

• Both ATP and NADPH are released into the stroma ready for the light-independent reaction

What is an overview of the light-independent reactions (the Calvin Cycle)?

• Occurs in the chloroplast stroma

• Uses the products of the light reactions (ATP and NADPH) to fix CO2

• Calvin cycle required 2 NADPH and 3 ATP per CO2 fixed

• Net production of 1 G3P molecule requiring 9 ATP and 6 NADPH

What are the three components of the Calvin cycle?

• CO2 is fixed by the enzyme Rubisco

• Reacts CO2 with ribulose bisphosphate to produce 2 molecules of 3-phosphoglycerate

• 3-phosphoglycerate is phosphorylated by ATP to 1,3-bisphosphoglycerate

• Reduced to glyceraldehyde-3-phosphate (G3P) by NADPH

• Most G3P is used to regenerate ribulose bisphosphate

• Remainder can be used to make sucrose or starch

How can the Calvin Cycle be regulated?

Light independent reactions do not occur in the dark as the ATP and NADPH would be produced by metabolising stored carbohydrates - inefficient cycling

What are the three key enzymes that are regulated?

• Rubisco

• Fructose 1,6-bisphosphatase (FBPase)

• Sedoheptulose 1,7-bisphosphatase (SBPase)

• All enzymes are found in the chloroplast stroma

How are the enzymes regulated?

• All three enzymes cycle between active forms in the light and inactive forms in the dark

• All three have an optimum pH of 8

• pH in the stroma in the dark is ~7

• When illuminated, H+ are pumped into the thylakoids from the stroma causing the pH of the stroma to rise to ~8

• Pumping of protons into the thylakoids causes Mg2+ to leave increasing the stromal [Mg2+]

• Mg2+ activates all three enzymes

Which enzymes undergo disulphide bridge to thiol transitions?

• Fructose 1,6-bisphosphatase (FBPase)

• Sedoheptulose bisphosphatase (SBPase)

• The e- for reducing thioredoxin are supplied by PSI via ferredoxin