Cellular Respiration

How Cells Harvest EnergyChapter 6

ATP Is Universal Energy Source

Photosynthetic organisms get energy from the sun

The light energy is converted to the chemical bond energy of ATP

Main Types of Energy-Releasing Pathways

Anaerobic pathways

Evolved first Don’t require oxygen Start with glycolysis

in cytoplasm Completed in

cytoplasm

Aerobic pathways

Evolved later Require oxygen Start with glycolysis

in cytoplasm Completed in

mitochondria

Redox reactions release energy when electrons “fall” from a hydrogen carrier to oxygen

NADH delivers electrons to a series of protein complexes in an electron transport chain As electrons move from carrier to carrier,

their energy is released in small quantities

In cellular respiration, electrons “fall” down an energy staircase and finally reduce O2

H2O

NAD

NADH

ATP

H

H

Controlled release of energy for synthesis

of ATPElectron transport

chain

2 O2

12

2e

2e

Main Pathways Start with Glycolysis

Glycolysis occurs in cytoplasm Reactions are catalyzed by

enzymes

Glucose 2 Pyruvate(six carbons) (three carbons)

Glycolysis harvests chemical energy by oxidizing glucose to pyruvate

Glucose Pyruvate

Net Energy Yield from Glycolysis

Energy investment phase: 2 ATP invested

Energy releasing phase:2 NADH formed

4 ATP formed

Net yield: 2 ATP and 2 NADH

Pyruvate is chemically groomed for the Krebs cycle

• Each molecule of pyruvate is broken down to form CO2 and acetyl co-A, which enters the Krebs cycle

Pyruvic

acid

Acetyl CoA(acetyl

coenzyme A)

CO2

The Krebs cycle completes the oxidation of glucose, generating many NADH and FADH2 molecules

• The Krebs cycle is a series of redox reactions in which enzymes strip away electrons and H+

Acetyl CoA

KREBSCYCLE

2

2 CO2

Oxidative phosphorylation powers most ATP production

The electrons from NADH and FADH2 travel down the electron transport chain to oxygen

Chemiosmosis powers most ATP production Energy released by the electrons is

used to pump H+ into the space between the mitochondrial membranes (intermembrane space) by active transport

In chemiosmosis, the H+ ions diffuse back through the inner membrane through ATP synthase , which capture the energy to synthesize ATP

Overview: Oxidative Phosphorylation Occurs in the inner mitochondrial

membrane Coenzymes (NADH, FADH2) deliver

electrons to electron transport systems

Electron transport sets up H+ ion gradients

Flow of H+ down concentration gradients powers ATP formation

Overview:Electron Transport System Electron transport systems are

embedded in inner mitochondrial compartment

NADH and FADH2 give up electrons that they picked up in earlier stages to electron transport system

Electrons are transported through the system

The final electron acceptor is oxygen

Summary of Aerobic Cellular Respiration Glycolysis

2 ATP formed by substrate-level phosphorylation Krebs cycle

2 ATP formed by substrate-level phosphorylation ETC & Chemiosmosis (oxidative

phosphorylation) 32 - 34 ATP formed

Total ATP molecules formed: 36 - 38

Anaerobic Pathways

Do not use oxygen Produce less ATP than aerobic

pathways Two types

Lactic acid fermentation Alcoholic fermentation

Lactic Acid Fermentation

2 Lactate

NAD NADH NADH NAD2 2 22

2 ATP2 ADP 22 Pyruvate

GLYCOLYSIS

P

Glucose

Alcoholic Fermentation

NAD NADH NADH NAD2 2 2 2

GLYCOLYSIS

2 ADP 2 P ATP

Glucose 2 Pyruvate

releasedCO2

2 Ethanol

22