Chapter 10: Photosynthesis

Photosynthesis??: Captures light energy from the sun and converts it to chemical energy. What types of organisms??

Photosynthesis in nature Autotrophs: self feeders/ producers photoautotrophs/ chemoautotrophs obtains organic food without eating other organisms Heterotrophs: consumers obtains organic food by eating other organisms or their by-products (includes decomposers)

The chloroplast Double membrane organelle Conducts photosynthesis Plant and algal cells Plastid

Photosynthesis: an overview Redox process Compared to cellular respiration, it reverses the flow of electrons Photons split water, excite electrons Electrons are then transferred to CO2 Reducing it to a sugar Electrons increase in potential energy

Two stage process: 1. Light reactions Converts solar energy to potential energy Produces ATP (photophosphorylation) Reduces NADP+ to NADPH

Two Stage Process: 2. Calvin Cycle Incorporates CO2 from the air to organic molecules Requires ATP Occurs during the day time because needs light to reduce NADP+ Light independent reaction(dark reaction)

Stage One: Light Reactions Converts solar energy to chemical energy (ATP and NADPH) Uses photosynthetic pigments Chlorophyll a and b Carotenoids

Photosystems Light harvesting units of the thylakoid membrane Composed mainly of protein and pigment antenna complexes Antenna pigment molecules are struck by photons Energy is passed to reaction centers (redox location) Excited e- from chlorophyll is trapped by a primary e- acceptor

Noncyclic electron flow Photosystem II (P680): photons excite electrons of chlorophyll to an acceptor electrons travel to Photosystem I down an electron transport chain (PqcytochromesPc) as electrons fall, ADP  ATP (noncyclic photophosphorylation)

Photosystem I (P700): Fallen electrons are excited by a photon Electrons enter second electron transport chain Ferrodoxin (Fp) Electrons from Fp reduce NADP+ to NADPH

Cyclic electron flow Alternative cycle when ATP is deficient Photosystem I used but not II; produces ATP but no NADPH Why? The Calvin cycle consumes more ATP than NADPH……. Cyclic photophosphorylation

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The Calvin Cycle: Uses ATP and NADPH from light reactions to convert CO2 to sugar Anabolic Endergonic

The Calvin Cycle: Glucose is not produced directly 3 carbon sugar molecule (G3P) Glyceraldehyde 3- phosphate 3 molecules of CO2 enter the cycle one at a time Produce 6 molecules of G3P- one will exit the cycle to become a sugar

Three phases of the Calvin Cycle: 1. Carbon Fixation: Each molecule of CO2 is fixed to ribose bisphosphate(RuBP) 5 carbon sugar Catalyzed by RuBisCo

RuBisCo Ribulose- 1,5- Bisphosphate carboxylase/oxygenase Most abundant enzyme in the chloroplast Most abundant protein on the planet Very large protein- 560,000 Da Very important for carbon fixation

Three phases of the Calvin Cycle: Product of Carbon Fixation: 6 carbon intermediate molecule Short lived/ unstable Breaks down into two -3 carbon molecules For each molecule of CO2

Three phases of the Calvin Cycle: 2. Reduction: Each 3- carbon molecule is phosphorylated by ATP Produces 1,3 bisphophoglycerate Each 1,3 bisphosphoglycerate gets reduced to G3P by NADPH 1 of the 6 G3P produced will go on to produce glucose

Three phases of the Calvin Cycle: 3. Regeneration of RuBP: The other 5 G3P molecules will be rearranged into 3 molecules of RuBP Requires an additional 3 ATP’s

Input vs. Output 3 CO2 molecules enter one at a time 3 short lived 6 carbon molecules are produced 6, 3-phosphoglycerate molecules result

Input vs. Output 6, 3 –phosphoglycerate are phosphorylated into 6, 1,3 biphosphoglycerate 6 ATP

Input vs. Output 6, 1,3 biphosphoglycerate are reduced to 6 G3P by NADPH 1 of the 6 G3P will go onto form glucose The other 5 of the G3P will go onto regenerate RuBP Phosphorylated by 3 ATP

Input vs. Output 9 ATP consumed 6 NADPH oxidized

How do plants fix carbon in hot/arid climates? Plants close their stomata to prevent dehydration during hot and dry days. How does this affect photosynthesis?

Alternative carbon fixation methods: Photorespiration: Plant adds O2 to the Calvin cycle when CO2 is unavailable No benefit to the plant No ATP produced No sugar produced

C4 Pathway C4 plants: alternative mode of carbon fixation 2 photosynthetic cells, bundle-sheath & mesophyll PEP carboxylase (instead of rubisco) fixes CO2 in mesophyll new 4C molecule releases CO2 to Calvin cycle (grasses, corn, sugar cane)

Alternative carbon fixation methods, II CAM plants: (crassulacean acid metabolism) open stomata during night, close during day Cacti and other succulents(Jade plant) pineapples CO2 is taken in at night and fixed into different organic acids During the day, when stomata are close but light reaction is active the organic acids deliver CO2 to the Calvin cycle

A review of photosynthesis