How Cells Release Stored Energy – Cellular Respiration

Aerobic Respiration: Whole Organism Aerobic respiration at the whole organism level

= process by which gases are exchanged with the environment.

O2

CO2

Aerobic Respiration: Whole Organism Respiratory Surface (= part of the organism

where O2 diffuses into and CO2 diffuses out of

the organism) must be moist, as gases must be dissolved in water before they can diffuse in or out.

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Aerobic Respiration: Whole Organism

In unicellular aquatic protozoans: O2 dissolved in water passes across the cell membrane by diffusion, and CO2 exits.

O2

CO2

Aerobic Respiration: Whole Organism

In multicellular aquatic plants and invertebrate animals: O2 dissolved in water enters cells by diffusion, and CO2 exits by diffusion.Elodea cell

O2 CO2

Planarian

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Aerobic Respiration: Whole Organism

In insects: O2 enters through small openings in the body wall (spiracles) and is carried through tracheal tubes to moist cell membranes, across which respiratory exchange occurs.

spiracle

Spiracles

SEM

Aerobic Respiration: Whole Organism

In fish: O2 (in H2O) diffuses across the surface of gills, into capillaries of the circulatory system and CO2 diffuses in the opposite direction.

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 Carrier Molecules 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 carrier molecules NAD+ and FAD to carry electrons from broken down glucose 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

Stage One: Glycolysis

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

Yields 2 ATP by substrate level phosphorylation

2 NADH

2 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

2 NAD+

2 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 Pyruvic

Acid

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 pyruvatePyruvic acid is oxidized into two-carbon acetyl

CoA units and carbon dioxideNAD+ is reduced into NADH2

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

NAHD2

Oxidation of Pyruvate

The Krebs Cycle

Overall Products

Coenzyme A 2 CO2

3 NADH FADH2

ATP

Overall Reactants

Acetyl-CoA 3 NAD+

FAD ADP and Pi

Results of the Second Stage

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

NAD and FAD are reduced (they pick up electrons and hydrogen)

One molecule of ATP forms Four-carbon oxaloacetate regenerates

NAD/FAD Reductions during First Two Stages

Glycolysis 2 NADH2

Preparatoryreactions 2 NADH2

Krebs cycle 2 FADH2 + 6 NADH2

Total 2 FADH2 + 10 NADH2

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

Oxygen is the finial Electron acceptor

Electron transport chain requires the presence of oxygen

Oxygen withdraws spent electrons from the electron transfer chain, then combines with H+ to form water

Summary of Energy Harvest(per molecule of glucose) Glycolysis

2 ATP formed by

Krebs cycle and preparatory reactions 2 ATP formed

Electron transport chain 32 ATP formed

686 kcal of energy are released

7.5 kcal are conserved in each ATP

When 36 ATP form, 270 kcal (36 X 7.5) are

captured in ATP

Efficiency is 270 / 686 X 100 = 39 percent

Most energy is lost as HEAT!

Efficiency of Aerobic Respiration

Do not use oxygen

Produce less ATP than aerobic pathways

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

Summary of Cellular Respiration

Respiration

Process

Where Process Occurs

Net Gain of ATP

Per Glucose

Anaerobic Glycolysis & Fermentation

Cytoplasm 2 ATP

Aerobic Krebs Cycle and Electron

Transport

Mitochondrion 36 ATP

ATP / ADP Cycle ATP is constantly being used and remade in

the cell. Energy is released or stored by breaking or

making a phosphate bond.

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