Harvesting Chemical Energy: Cellular RespirationChapter 8

Cellular Respiration:THE BIG PICTURE

Cellular respiration is the process by which organisms can get energy (ATP) from their food (glucose)

Cellular respiration is CRITICAL for life Occurs in BOTH plants and animals Two main mechanisms

Aerobic cellular Respiration – Requires OxygenAnaerobic cellular Respiration – Does not

require Oxygen

Main Types of Cellular Respiration Pathways

Aerobic Respiration

Evolved later Require oxygen Start with glycolysis

in cytoplasm Completed in

mitochondria

Anaerobic Respiration

Evolved first Don’t require oxygen Start with glycolysis in

cytoplasm Completed in cytoplasm

Aerobic Respiration

Overall Equation:

C6H1206 + 6O2 6CO2 + 6H20 glucose oxygen carbon water

dioxide

Several steps occur in the middle (intermediates) Each step (rxn) catalyzed by enzymes

Aerobic respiration Overview

Stage One: Glycolysis (cytoplasm)

Stage Two: Preparation for Krebs (mitochondrial matrix) Krebs Cycle (matrix)

Stage Three: Electron Transfer Chain (across inner

membrane of mitochondria)

The Role of Coenzymes Several oxidation-reduction rxns take place in

aerobic respiration (Glucose gets oxidized to carbon dioxide)

In order to aid in the redox rxns, enzymes use coenzymes NAD+ and FAD to carry electrons from glucose derivatives to the electron transfer chain

NAD+ and FAD accept electrons and hydrogen to become NADH and FADH2 during the first two stages of aerobic respiration (Glycolysis, Krebs) and deliver electrons and hydrogen to the Electron Transfer Chain to make ATP

Coenzymes: NAD+ & FAD

Nicotinamide adenine dinucleotide (NAD+)

Stage One: Glycolysis

Glucose (6-carbon) is broken down into 2 molecules of pyruvate (3-carbon)

Yields 2 ATP by substrate level phosphorylation

1 NADH

1 NADH

Glycolysis: Overall Reaction

Glucose(6C)

2 ATP 2 ADP

FructoseBisphosphate

(6C)

G3P(3C)

G3P(3C)

Pyruvate(3C)

Pyruvate(3C)

2 ADP 2 ATP

2 ADP 2 ATP

1 NAD+

1 NAD+

Aerobic Respiration:1. Glycolysis

Glycolysis: Net Yield Energy requiring steps:

2 ATP invested

Energy releasing steps:2 NADH formed 4 ATP formed

Net yield: 2 ATP + 2 NADH + 2 molecules of Pyruvate

What happens next?

Depends on the organism and the presence of oxygen

If oxygen is around: Aerobic respiration, proceed to Krebs cycle

If no oxygen: Anaerobic respiration, Proceed to Fermentation

Second Stage: Krebs cycle

Preparatory reactions: Oxidation of pyruvatePyruvate is oxidized into two-carbon acetyl units

and carbon dioxideNAD+ is reduced

Krebs cycleThe acetyl units are oxidized to carbon dioxideNAD+ and FAD are reduced

Oxidation of Pyruvate

Results of the Second Stage

All of the carbon atoms in pyruvate end up in carbon dioxide

Coenzymes are reduced (they pick up electrons and hydrogen)

One molecule of ATP forms Four-carbon oxaloacetate regenerates

Coenzyme Reductions during First Two Stages

Glycolysis 2 NADH Preparatory

reactions 2 NADH Krebs cycle 2 FADH2 + 6 NADH

Total 2 FADH2 + 10 NADH

Occurs in the mitochondria Coenzymes deliver electrons to electron

transfer chains Electron transfer sets up H+ ion gradients Flow of H+ down gradients powers ATP

formation

Electron Transfer Chain

Importance of Oxygen

Why does aerobic respiration require oxygen? Oxygen acts as the final electron (and

hydrogen ion) acceptor in ETCBinds to leftover e- and H+ to form water“Clean up crew”

ATP Actual Yield

Energy Harvest Varies

NADH formed in cytoplasm cannot enter mitochondrion

It delivers electrons to mitochondrial membrane

Membrane proteins shuttle electrons to NAD+ or FAD inside mitochondrion

Electrons given to FAD yield less ATP than those given to NAD+

686 kcal of energy are released

7.5 kcal are conserved in each ATP

When 32 ATP form, 240 kcal (32 X 7.5) are

captured in ATP

Efficiency is 240 / 686 X 100 = 35%

Most energy is lost as heat

Efficiency of Aerobic Respiration

Do not use oxygen

Produce less ATP than aerobic pathways

Two types

Fermentation pathways

Anaerobic electron transport

Anaerobic Pathways

Fermentation Pathways

Begin with glycolysis

Do not break glucose down completely to carbon

dioxide and water

Yield only the 2 ATP from glycolysis

Steps that follow glycolysis serve only to

regenerate NAD+

When life originated, atmosphere had little oxygen

Earliest organisms used anaerobic pathways

Later, noncyclic pathway of photosynthesis

increased atmospheric oxygen

Cells arose that used oxygen as final acceptor in

electron transport

Evolution of Metabolic Pathways