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:

6H2O + 6CO2 6O2 + C6H12O6

Fig. 6.13, p.104

sunlight

Calvin-Benson cycle

Light-DependentReactions

end products (e.g., sucrose, starch, cellulose)

ATP

Light-IndependentReactions

phosphorylated glucose

H2O

H2O O2

NADPHNADP+

CO2

ADP + Pi

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

Sites of Photosynthesis

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

THYLAKOIDMEMBRANE

Fig. 6.8b, p.99

NADPH

THYLAKOIDCOMPARTMENT

STROMA

Photosystem IPhotosystem II

electron transfer chainlight energy 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 CO2 to RuBP to start the Calvin–Benson cycle

Calvin–Benson Cycle

Cyclic pathway makes phosphorylated glucose• Uses energy from ATP, carbon and oxygen from

CO2, 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 CO2

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 • O2 from photosynthesis cannot escape

A Burning Concern

Photoautotrophs remove CO2 from atmosphere; metabolic activity of organisms puts it back

Human activities disrupt the carbon cycle• Add more CO2 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:

C6H12O6 + 6O2 → 6CO2 + 6 H2O

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 CO2 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 CO2

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 FADH2

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