Unit 3 - Photosynthesis The Basis of Life

Overall Process 6CO H 2 O + Light Energy  C 6 H 12 O 6 + 6O 2 + 6H 2

Structure of a Leaf Stomata Mesophyll

Chloroplast Site of photosynthesis – Light Reaction – thylakoids – Calvin Cycle – stroma (fluid)

Redox in Biology OIL – Oxidation Is Loss RIG – Reduction Is Gain In chemistry, “loss” or “gain” refers to e - In biology, “loss” or “gain” refers to H atom – Ex: NADPH is reduced (because it gained an H) NADP+ is oxidized (because it lost the H)

Phosphorylation Accomplished by enzymes called kinases – Attach a phosphate group (PO 4 ) to a molecule Makes molecule less stable due to increase in free energy (more ordered)

ADP Phosphorylation

Two stages: Light Rxn: – Reactants – Light (photons) and H 2 O – Products – O 2, ATP, and NADPH Calvin Cycle: – Reactants – ATP, NADPH, and CO 2 – Products – CH 2 O (sugar!), ADP + P i, NADP +

Light Rxn is divided into 2 parts Photosystem II Photosystem I Photosystem – reaction center located on thylakoid membrane – Contain many light-harvesting complexes (contain chlorophyll)

Photosystem II (PSII) 1.Light, in the form of photons, hits leaf and is absorbed by chlorophyll (usually chlorophyll a) in PSII rxn center 2.e - with chlorophyll a gets excited and moves to higher energy state (gains PE) same time, enzymatic rxn splits H 2 O  2e -, 2H +, and ½ O 2 – e - ’s from splitting of water (photolysis) replace e - that was excited in chlorophyll a

4. Electron Transport Chain (ETC) – e - is passed from PSII to PSI via a series of molecules – Each “pass” results in e - falling to a lower energy state – Exergonic process provides energy to pump H + (protons) into the thylakoid space.

Chemiosmosis Light rxn creates a proton gradient due to ETC – Results in a pH, charge, and [] gradient ATP Synthase – couples the diffusion of H + ions back down their concentration gradient to stroma with phosphorylation of ADP to make ATP

Concentration gradient pH gradient Charge gradient

Photosystem I (PSI) 5.Light energy transfers from PSII  PSI which excites e - 6.e - travels through PSI and enters another ETC 7.As e - travels down ETC, the exergonic process produces energy to convert NADP + to NADPH (reduction!). – NADPH is the final electron acceptor!

Thus…. Light rxns convert light energy to chemical energy stored in NADPH and ATP (via chemiosmosis). Light rxns also produce O 2 as a by- product.

Calvin Cycle (aka dark rxns aka light-independent rxns) ATP and NADPH from light reactions provide energy to drive carbohydrate synthesis. – CO 2 (from air) is reduced and “fixed” into carbohydrate – NADPH and ATP provide reducing power Anabolic due to building up of sugar from smaller molecules and consumption of energy

Fates/uses of glucose Provides plant with chemical energy and carbon (for synthesis of all organic macromolecules) – Proteins, lipids, nucleic acids – STARCH and CELLULOSE Heterotrophs (us) consume autotrophs (plants)

Evolutionary background Photosynthesis first evolved in prokaryotes – Evidence supports that prokaryotic photosynthesis was responsible for production of oxygenated atmosphere – Big point – prokaryotic photosynthetic pathways were foundation of eukaryotic photosynthesis