Photosynthesis in Detail

Photosynthesis: An Overview Autotrophs are the producers of the biosphere. Most use photosynthesis to produce organic compounds. 12H 2 O + 6CO 2  6O 2 + C 6 H 12 O 6 + 6H 2 O

Leaf

Cell

Chloroplast

Chloroplasts Contains chlorophyll: green pigment found within mesophyll (tissue in interior of leaf) Contains two outer layers of membranes surrounding a fluid interior called the stroma. Inner membrane system: thylakoid membrane –Stacked sacs referred to thylakoids

Key Questions to Keep In Mind Why is oxygen released? What is the purpose of water? What is the purpose of carbon dioxide?

Two Main Phases of Photosynthesis 1.Light Dependent Reactions (Electron Transport) 2.Light Independent Reactions (Calvin Cycle)

Photosynthesis Phase 1: The Light Reactions Purpose: Converts solar energy to the chemical energy needed for carbon fixation to take place. Location: occurs in thylakoids. –Chlorophyll and carotenoids are clustered together in groups called photosystems. Photosystem I (named in order of discovery) Photosystem II

Photosynthesis Phase 1: The Light Reactions

Reactants: Water Light Energy NADP + Products: ATP Oxygen NADPH

Photosynthesis Phase 1: The Light Reactions Two possible routes for electron flow in light reactions. 1.Noncyclic (predominates) 2.Cyclic We will start with Noncyclic Electron Flow

Photosynthesis Phase 1: The Light Reactions Begin when pigments in the photosystem absorb light. Light energy excites electrons in PSII. –The electrons have enough energy to to leave the photosystem and travel along the electron transport chain The chlorophyll that provided the electrons is now oxidized – its “hole” must be filled.

Photosynthesis Phase 1: The Light Reactions (noncyclic) An enzyme extracts electrons from water and supplies them to PSII. –This splits the water, which releases O 2 (leaves cell by diffusion) –2H 2 O  4H + + 4e - + O 2 The protons (H + ) stay in the thylakoid membrane to build up a concentration gradient. Electrons from PSII move to PSI

Photosynthesis Phase 1: The Light Reactions (noncyclic) Once at PSI, the e - are passed through another electron transport chain. This delivers them to the side of the thylakoid membrane that faces the stroma. The e - combine with a proton and NADP + to form NADPH

Photosynthesis Phase 1: The Light Reactions (noncyclic) The next step utilizes chemiosmosis: mechanism that uses energy stored in the form of an H+ gradient to drive cellular work. The protons that built up inside the membrane due to the splitting of H 2 O are exported from the membrane through ATP synthase. –ATP is formed by ADP –Called noncyclic phosphorylation

Photosynthesis Phase 1: The Light Reactions (cyclic) As light excites electrons in PSI, they enter the electron transport chain. The e - cycle back to PSI via the same electron transport chain. PSII is not involved. Electron flow pumps H + across the membrane to generate concentration gradient. There is no production of NADPH and no release of O 2.

Photosynthesis Phase 1: The Light Reactions (cyclic) Generates ATP via cyclic phosphorylation. (still utilizes chemiosismosis) The Calvin cycle uses more ATP than NADPH, noncyclic electron flow produces them evenly – cyclic makes up the difference.

Photosynthesis Phase II—The Calvin Cycle or Light Independent Reactions Light Energy + 6H2O + 6CO2  C6H12O6 + 6O2 Electron Transport used the reactants _______ and _______ to release _____ as a waste product and _______ and_______ as products. Therefore, the Calvin Cycle must use _________, _______, and _______as reactants and produce _______ as a product.

Photosynthesis Phase II—The Calvin Cycle Purpose: to “fix” or bond carbon atoms from CO 2 into organic compounds. Location: in stroma of thylakoid where CO 2 has diffused in from the cytosol of the cell.

Photosynthesis Phase 2: The Light-Independent Reactions

Step 1— An enzyme combines CO 2 with a 5-carbon carbohydrate (RuBP). The product, a six-carbon molecule, splits into a two three carbon PGA molecules.

Step 2—Each PGA receives a phosphate from an ATP, then receives a proton from NADPH and releases a phosphate group. This produces one PGAL molecule (aka G3P).

Step 3—Most PGAL is converted back to RuBP to be reused in the Calvin Cycle. Some PGAL leaves the Calvin Cycle and is used to produce organic compounds such as glucose.

Photosynthesis Phase II—The Calvin Cycle Every 3 CO 2 make 6 PGAL (G3P), only one is a net gain. For net synthesis of 1 PGAL(G3P), the Calvin cycle uses –9 ATP –6 NADPH

Key Questions to Keep In Mind Why is oxygen released? What is the purpose of water? What is the purpose of carbon dioxide?