Photosynthesis

Photosynthesis - overview 1. The conversion of light energy (from the sun) into chemical energy (stored in sugar & organic molecules. 2. Plants, algae (protists), cyanobacteria, phytoplankton 3. Plants are primary producers – produce organic molecules from CO 2 & H 2 O They are the bottom of the terrestrial food chain All complex organisms on land depend on plants (ultimately) for food & O 2.

Photosynthesis occurs In the chloroplast Fig 10.3

Chloroplast anatomy 1. 2 membranes (inner & outer) 2. Thylakoid membranes – site of the light reactions 3. Stroma – site of the Calvin cycle, contains rubisco

Photosynthesis - overview 1. 6CO 2 + 6H 2 0 C 6 H 12 O 6 + 6O 2 2. O 2 is released as a by-product from the splitting of water 3. 2 interdependent pathways: A. Light reactions – chlorophyll absorbs light energy to create high energy molecules: ATP & NADPH + H +. These are used to drive: B. Calvin cycle – carbon fixation

Fig 10.5

Light 1. What is light?? = electromagnetic energy (radiation) 2. Consists of groups of particles called photons 3. Travels in waves 4. Wavelength = distance between peaks in wave 5. Only visible light drives PSN: 380 to 750nm

Fig 10.6

Pigments 1. Molecular substances that absorb visible light. 2. Chlorophyll a – main PSN pigment, absorbs red & blue light (reflects green) to initiate the light reactions 3. Accessory pigments – absorb light energy & transfer it to chlorophyll a Chlorophyll b, carotenoids

Fig 10.9

Photosystems 1. The functional units of chlorophyll & accessory pigments that work together to absorb a photon of light. “Kicks” electrons to an excited state (high energy) 2. 2 main components: A. Light-harvesting complex – accessory pigments absorb light, pass their excited electrons to B. Reaction center complex – a pair of chlorophyll a molecules. Passes the excited electrons to a primary electron acceptor.

Fig 10.12

Fig 10.11

Two types of photosystems in the thylakoid membranes: 1. Photosystem I (P700) – absorbs 700nm wavelengths best (far-red) 2. Photosystem II (P680) – absorbs 680nm wavelengths best (red)

Flow of electrons from the photosystems through various proteins in the thylakoid drives the synthesis of NADPH + H + & ATP The Light Reactions

1. Non-cyclic electron flow 1. PS II absorbs light, splitting water into H+, electrons, and O Further light absorption by PS II kicks the electrons up to an excited state. 3. Excited electrons are pass from PS II to PS I along an electron transport chain (PSII  Pq  cytochrome complex  Pc  PS I). Energy released at every step used to drive ATP synthesis 4. When electron reaches PS I, light absorption kicks it up to excited state again. 5. Excited electron is passed to Fd to NADP+ - the terminal electron acceptor. Reduced to NADPH

Fig 10.14

Fig 10.13

Figure 10.17

How is ATP produced? Two pathways for the electrons: 1. Non-cyclic electron flow A. produces ATP & NADPH + H + B. Energy released during electron transfer drives proton pump at cytochrome complex C. H + pumped from stroma into thylakoid lumen, creating electrochemical gradient D. H + diffusion back out into stroma drives ATP synthesis in stroma

2. Cyclic electron flow A. Produces ATP only B. electrons transferred from Fd back to the cytochrome complex (instead of to NADP+) C. thus more H + pumped into lumen thus more ATP produced

Why cyclic flow? 1. Non-cyclic flow produces equal amounts of ATP & NADPH + H + 2. But… Calvin cycle requires more ATP than NADPH + H + 3. So…. NADPH + H + builds up builds up in the stroma, triggering the shift to cyclic phosphorylation

Fig 10.15

Rubisco The Calvin Cycle 1. The use of ATP & NADPH to convert CO 2 to carbohydrates 2. 3 steps: 1.Carbon fixation: RuBP + CO 2 3PGA 2.Reduction: 3PGA + ATP + NADPH G3P 3.Regeneration of RuBP from G3P 3. G3P leaves the chloroplast to become sucrose

Fig 10.18

Factors decreasing the efficiency of PSN Overall efficiency < 35% 1. Wavelength (λ) of light – shorter λ have greater energy. PSN use of longer λ means more photons needed 2. Cyclic phosphorylation – need extra photons just for extra ATP production

Photorespiration 1. light-dependent inhibition of C fixation 2. Cause: increased O 2 concentration in leaf – How? Stomata are closed 3. Rubisco adds O 2 to RuBP to make a compound that is converted back to CO 2 in the mitochondria. 4. No carb’s produced, but energy required. 5. Wastes about 50% of C compounds in the chloroplast

Adaptations that increase PSN efficiency C4 plants (many grasses & tropical plants) 1. 4-C compound instead of a 3-C compound (3PGA) as first product of Calvin cycle 2. CO 2 + PEP Oxaloacetate Malate 3. Malate exported to bundle sheath cells 4. CO 2 released from malate to enter Calvin cycle PEP carboxylase

Fig 10.18

Why is the C4 pathway more efficient? 1. PEP carboxylase has much higher affinity for CO 2 than Rubisco 2. CO 2 is stored as malate in the bundle-sheath cells – thus plenty of CO 2 even when stomata closed 3. Benefits? –Increased WUE –Little photorespiration

Crassulacean Acid Metabolism (CAM) 1. Succulents, cacti 2. Stomata open only at night 3. CO 2 is stored in organic acids in mesophyll cells. 4. During day, CO 2 released from organic acids and enters Calvin cycle in same mesophyll cells 5. Same benefits as C4 pathway, but even greater WUE.

Fig 10.20

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