Unit 3 - Photosynthesis

The Basis of Life

Overall Process6CO2 + 12H2O + Light Energy C6H12O6 + 6O2 + 6H2

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 (PO4) 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 H2O

– Products – O2, ATP, and NADPH

• Calvin Cycle: – Reactants – ATP, NADPH, and CO2

– Products – CH2O (sugar!), ADP + Pi, 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)

3. @ same time, enzymatic rxn splits H2O 2e-, 2H+, and ½ O2

– 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 ETC7. 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 (@PSI) and ATP (via chemiosmosis).

• Light rxns also produce O2 (@PSII) 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. – CO2 (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