Where It Starts: Photosynthesis

Introduction  Before photosynthesis evolved, Earth’s atmosphere had little free oxygen  Oxygen released during photosynthesis changed the atmosphere Favored evolution of new metabolic pathways, including aerobic respiration

Electromagnetic Spectrum

Overview of Photosynthesis  Photosynthesis proceeds in two stages Light-dependent reactions Light-independent reactions Summary equation: 6H 2 O + 6CO 2 6O 2 + C 6 H 12 O 6

Fig. 6.13, p.104 sunlight Calvin-Benson cycle Light- Dependent Reactions end products (e.g., sucrose, starch, cellulose) ATP Light- Independent Reactions phosphorylated glucose H2OH2O H2OH2OO2O2 NADPH NADP + CO 2 ADP + P i

Sites of Photosynthesis: Chloroplasts  Light-dependent reactions occur at a much- folded thylakoid membrane Forms a single, continuous compartment inside the stroma (chloroplast’s semifluid interior)  Light-independent reactions occur in the stroma

Sites of Photosynthesis

Products of Light-Dependent Reactions  Typically, sunlight energy drives the formation of ATP and NADPH  Oxygen is released from the chloroplast (and the cell)

electron transfer chain THYLAKOID MEMBRANE Fig. 6.8b, p.99 NADPH THYLAKOID COMPARTMENT STROMA Photosystem I Photosystem II electron transfer chain light energy oxygen (diffuses away)

ATP Formation  In both pathways, electron flow through electron transfer chains causes H + to accumulate in the thylakoid compartment A hydrogen ion gradient builds up across the thylakoid membrane  H + flows back across the membrane through ATP synthases Results in formation of ATP in the stroma

Light Independent Reactions: The Sugar Factory  Light-independent reactions proceed in the stroma  Carbon fixation: Enzyme rubisco attaches carbon from CO 2 to RuBP to start the Calvin– Benson cycle

Calvin–Benson Cycle  Cyclic pathway makes phosphorylated glucose Uses energy from ATP, carbon and oxygen from CO 2, and hydrogen and electrons from NADPH  Reactions use glucose to form photosynthetic products (sucrose, starch, cellulose)  Six turns of Calvin–Benson cycle fix six carbons required to build a glucose molecule from CO 2

Light-Independent Reactions

Adaptations: Different Carbon-Fixing Pathways  Environments differ Plants have different details of sugar production in light-independent reactions  On dry days, plants conserve water by closing their stomata O 2 from photosynthesis cannot escape

A Burning Concern  Photoautotrophs remove CO 2 from atmosphere; metabolic activity of organisms puts it back  Human activities disrupt the carbon cycle Add more CO 2 to the atmosphere than photoautotrophs can remove  Imbalance contributes to global warming

Fossil Fuel Emissions

How Cells Release Chemical Energy

Overview of Carbohydrate Breakdown Pathways  All organisms (including photoautotrophs) convert chemical energy of organic compounds to chemical energy of ATP  ATP is a common energy currency that drives metabolic reactions in cells

Pathways of Carbohydrate Breakdown  Start with glycolysis in the cytoplasm Convert glucose and other sugars to pyruvate  Fermentation pathways End in cytoplasm, do not use oxygen, yield 2 ATP per molecule of glucose  Aerobic respiration Ends in mitochondria, uses oxygen, yields up to 36 ATP per glucose molecule

Pathways of Carbohydrate Breakdown

Overview of Aerobic Respiration  Three main stages of aerobic respiration: 1. Glycolysis 2. Krebs cycle 3. Electron transfer chain Summary equation: C 6 H 12 O 6 + 6O 2 → 6CO H 2 O

Overview of Aerobic Respiration

Glycolysis – Glucose Breakdown Starts  Enzymes of glycolysis use two ATP to convert one molecule of glucose to two molecules of three-carbon pyruvate  Reactions transfer electrons and hydrogen atoms to two NAD + (reduces to NADH)  4 ATP form by substrate-level phosphorylation

Products of Glycolysis  Net yield of glycolysis: 2 pyruvate, 2 ATP, and 2 NADH per glucose  Pyruvate may: Enter fermentation pathways in cytoplasm Enter mitochondria and be broken down further in aerobic respiration

Second Stage of Aerobic Respiration  The second stage of aerobic respiration takes place in the inner compartment of mitochondria  It starts with acetyl-CoA formation and proceeds through the Krebs cycle

Acetyl-CoA Formation  Two pyruvates from glycolysis are converted to two acetyl-CoA  Two CO 2 leave the cell  Acetyl-CoA enters the Krebs cycle

Krebs Cycle  Each turn of the Krebs cycle, one acetyl-CoA is converted to two molecules of CO 2  After two cycles Two pyruvates are dismantled Glucose molecule that entered glycolysis is fully broken down

Energy Products  Reactions transfer electrons and hydrogen atoms to NAD + and FAD Reduced to NADH and FADH 2  ATP forms by substrate-level phosphorylation Direct transfer of a phosphate group from a reaction intermediate to ADP

Fig. 7.6a, p.113

Third Stage: Aerobic Respiration’s Big Energy Payoff  Coenzymes deliver electrons and hydrogen ions to electron transfer chains in the inner mitochondrial membrane  Energy released by electrons flowing through the transfer chains moves H + from the inner to the outer compartment

Hydrogen Ions and Phosphorylation  H + ions accumulate in the outer compartment, forming a gradient across the inner membrane  H + ions flow by concentration gradient back to the inner compartment through ATP synthases (transport proteins that drive ATP synthesis)

The Aerobic Part of Aerobic Respiration  Oxygen combines with electrons and H + at the end of the transfer chains, forming water  Overall, aerobic respiration yields up to 36 ATP for each glucose molecule

Electron Transfer Chain

Summary: Aerobic Respiration

Anaerobic Pathways  Lactic acid fermentation End product: Lactic acid (lactate)  Alcoholic fermentation End product: Ethyl alcohol (or ethanol)  Both pathways have a net yield of 2 ATP per glucose (from glycolysis)

Alcoholic Fermentation

Muscles and Lactate Fermentation

Life’s Unity  Photosynthesis and aerobic respiration are interconnected on a global scale  In its organization, diversity, and continuity through generations, life shows unity at the bioenergetic and molecular levels

Energy, Photosynthesis, and Aerobic Respiration