Presentation on theme: "Photosynthesis. A. Background 1. The conversion of light energy (from the sun) into chemical energy (stored in sugar & organic molecules. 2. Plants, algae."— Presentation transcript:
A. Background 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. I. Overview
a. Chloroplast i. Location ii. Structure 2 membranes (inner & outer) Thylakoid membranes – site of the light reactions Stroma – site of the Calvin cycle, contains rubisco 3. Plants
1. Reaction a. 6CO 2 + 6H 2 0 C 6 H 12 O 6 + 6O 2 O 2 is released as a by-product from the splitting of water 2. 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 C. Photosynthesis - overview
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 D. Characteristics of Light
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 E. Plant Pigments
A. Light Dependent Reactions 1. Background a. The functional units of chlorophyll & accessory pigments that work together to absorb a photon of light. “Kicks” electrons to an excited state (high energy) b. 2 main components: i. Light-harvesting complex – accessory pigments absorb light, pass their excited electrons to ii. Reaction center complex – a pair of chlorophyll molecules. Passes the excited electrons to a primary electron acceptor. II. Photosystems
i. Photosystem I (P700) – absorbs 700nm wavelengths best (far-red) ii. Photosystem II (P680) – absorbs 680nm wavelengths best (red) c. Types of photosystems in the thylakoid membranes:
a. Structure and Location 2. Non-cyclic Photophophorylation
i. PS II absorbs light, splitting water into H +, electrons, and O 2. ii. Further light absorption by PS II kicks the electrons up to an excited state. iii. 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 iv. When electron reaches PS I, light absorption kicks it up to excited state again. v. Excited electron is passed to Fd to NADP + - the terminal electron acceptor. Reduced to NADPH b. Steps
i. Pathway Non-cyclic electron flow produces ATP & NADPH + H +. Energy released during electron transfer drives proton pump at cytochrome complex. H + pumped from stroma into thylakoid lumen, creating electrochemical gradient H + diffusion back out into stroma drives ATP synthesis in Stroma. c. ATP Production
a. Structure and Location b. Process or Steps i. Produces ATP only ii. electrons transferred from Fd back to the cytochrome complex (instead of to NADP + ) iii. thus more H + pumped into lumen thus more ATP produced 3. Cyclic Photophosphorylation
Non-cyclic flow produces equal amounts of ATP & NADPH + H +. But… Calvin cycle requires more ATP than NADPH + H +. So…. NADPH + H + builds up in the stroma, triggering the shift to cyclic phosphorylation iv. Why cyclic flow?
1. The Calvin Cycle a. Location The use of ATP & NADPH to convert CO 2 to carbohydrates 2. Process and Steps: a. Carbon fixation: RuBP + CO 2 3PGA b. Reduction: 3PGA + ATP + NADPH G3P c. Regeneration of RuBP from G 3 P d. G 3 P leaves the chloroplast to become ?!?! B. Light Independent Reactions Rubisco
A. Overall efficiency < 35% B. Wavelength (λ) of light – shorter λ have greater energy. PSN use of longer λ means more photons needed C. Cyclic phosphorylation – need extra photons just for extra ATP production III. Problems and Limitations
1. Steps a. light-dependent inhibition of C fixation b. Cause: increased O 2 concentration in leaf – How? Stomata are closed c. Rubisco adds O 2 to RuBP to make a compound (phosphoglycolate) that is converted back to CO 2 in the mitochondria. d. No carb’s produced, but energy required. e. Wastes about 50% of C compounds in the chloroplast D. Photorespiration
A. C 3 Plants senescent IV. Plant Adaptations 1. Strategy 2. Examples
B. C 4 plants 1. Strategy a. 4-C compound instead of a 3-C compound (3PGA) as first product of Calvin cycle b. CO 2 + PEP Oxaloacetate Malate c. Malate exported to bundle sheath cells d. CO 2 released from malate to enter Calvin cycle PEP carboxylase 2. Examples
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 Why is the C 4 pathway more efficient?
1. Strategy a. Stomata open only at night b. CO 2 is stored in organic acids in mesophyll cells. c. During day, CO 2 released from organic acids and enters Calvin cycle in same mesophyll cells d. Same benefits as C4 pathway, but even greater WUE. C. Crassulacean Acid Metabolism (CAM) 2. Examples
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