How Cells Release Stored Energy

Chapter 7

• Descendents of African honeybees that

were imported to Brazil in the 1950s

• More aggressive, wider-ranging than

other honeybees

• Africanized bee’s muscle cells have

enlarged mitochondria

“Killer” Bees

Producing the Universal Currency of Life

All energy-releasing pathways

– require characteristic starting materials

– yield predictable products and by-products

– produce ATP

• Photosynthesizers get energy from the sun

• Animals get energy second- or third-hand from plants or other organisms

• Regardless, the energy is converted to the chemical bond energy of ATP

ATP Is Universal Energy Source

Making ATP

• Plants make ATP during photosynthesis

• Cells of all organisms make ATP by

breaking down carbohydrates, fats, and

protein

Main Types of Energy-Releasing Pathways

Aerobic pathways

• Evolved later• Require oxygen• Start with glycolysis

in cytoplasm• Completed in

mitochondria

Anaerobic pathways

• Evolved first• Don’t require oxygen• Start with glycolysis in

cytoplasm• Completed in

cytoplasm

Main Pathways Start with Glycolysis

• Glycolysis occurs in cytoplasm

• Reactions are catalyzed by enzymes

Glucose 2 Pyruvate

(six carbons) (three carbons)

Overview of Aerobic Respiration

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

dioxide

Overview of Aerobic RespirationCYTOPLASM

MITOCHONDRION

GLYCOLYSIS

ELECTRON TRANSPORT

PHOSPHORYLATION

KREBS CYCLE ATP

ATP

energy input to start reactions

2 CO2

4 CO2

2

32

water

2 NADH

8 NADH

2 FADH2

2 NADH 2 pyruvate

e- + H+

e- + oxygen

(2 ATP net)

glucose

TYPICAL ENERGY YIELD: 36 ATP

e-

e- + H+

e- + H+

ATP

H+

e- + H+

The Role of Coenzymes

• NAD+ and FAD accept electrons and hydrogen from intermediates during the first two stages

• When reduced, they are NADH and FADH2

• In the third stage, these coenzymes deliver the electrons and hydrogen to the transport system

• A simple sugar

(C6H12O6)

• Atoms held together by covalent bonds

Glucose

Glycolysis Occurs in Two Stages

• Energy-requiring steps

– ATP energy activates glucose and its six-carbon

derivatives

• Energy-releasing steps

– The products of the first part are split into three-

carbon pyruvate molecules

– ATP and NADH form

Energy-Requiring Steps

ATP

ATP

glucose

glucose-6-phosphate

fructose-6-phosphate

fructose-1,6-bisphosphate

2 ATP investedADP

ADP

P

P

P

Energy-Releasing

StepsATP

PGAL PGAL

ATP

NADH NADH

ATP ATP

2 ATP invested

2 ATP invested

NAD+

Pi

NAD+

Pi

3-phosphoglycerate 3-phosphoglycerate

2-phosphoglycerate 2-phosphoglycerate

PEP PEP

ADP ADP

1,3-bisphosphoglycerate 1,3-bisphosphoglycerateP P P P

P P

P P

P P

pyruvate pyruvate

substrate-level phosphorylation

substrate-level phosphorylation

H2O H2O

ADP ADP

Net Energy Yield from Glycolysis

Energy requiring steps: 2 ATP invested

Energy releasing steps:2 NADH formed 4 ATP formed

Net yield is 2 ATP and 2 NADH

• Occur in the mitochondria

• Pyruvate is broken down to carbon dioxide

• More ATP is formed

• More coenzymes are reduced

Second-Stage Reactions

oxaloacetate

malate

citrate

isocitrate

-ketogluterate

fumarate

succinate

CoA

succinyl–CoA

ATP

NADH

NADH

NADH

NADH

FADH2

NAD+

NAD+

FAD NAD+ CoA

CoA

H2O

H2O

H2O

ADP + phosphate group (from GTP)

KREBS CYCLE

PREPARATORY STEPS

pyruvate

NAD+

CoAAcetyl–CoA

coenzyme A (CoA)

(CO2)

Two Parts of Second Stage

• Preparatory reactions– Pyruvate is oxidized into two-carbon acetyl

units and carbon dioxide– NAD+ is reduced

• Krebs cycle– The acetyl units are oxidized to carbon

dioxide– NAD+ and FAD are reduced

pyruvate + coenzyme A + NAD+

acetyl-CoA + NADH + CO2

• One of the carbons from pyruvate is released in CO2

• Two carbons are attached to coenzyme A and continue on to the Krebs cycle

Preparatory Reactions

What is Acetyl-CoA?

• A two-carbon acetyl group linked to coenzyme A

CH3

C=O

Coenzyme A

Acetyl group

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 molecules in pyruvate end up in carbon dioxide

• Coenzymes are reduced (they pick up electrons and hydrogen)

• One molecule of ATP is formed

• Four-carbon oxaloacetate is regenerated

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 transport systems

• Electron transport sets up H+ ion gradients

• Flow of H+ down gradients powers ATP formation

Electron Transport Phosphorylation

Electron Transport

• 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

Creating an H+ Gradient

NADH

OUTER COMPARTMENT

INNER COMPARTMENT

Making ATP: Chemiosmotic Model

ATP

ADP+Pi

INNER COMPARTMENT

Importance of Oxygen

• Electron transport phosphorylation requires the presence of oxygen

• Oxygen withdraws spent electrons from the electron transport system, then combines with H+ to form water

Summary of Energy Harvest(per molecule of glucose)

• Glycolysis– 2 ATP formed by substrate-level

phosphorylation

• Krebs cycle and preparatory reactions– 2 ATP formed by substrate-level

phosphorylation

• Electron transport phosphorylation– 32 ATP formed

• What are the sources of electrons used to generate the 32 ATP in the final stage?– 4 ATP - generated using electrons

released during glycolysis and carried by NADH

– 28 ATP - generated using electrons formed during second-stage reactions and carried by NADH and FADH2

Energy Harvest from Coenzyme Reductions

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+

Energy Harvest Varies

Liver, kidney, heart cells

– Electrons from first-stage reactions are delivered to NAD+ in mitochondria

– Total energy harvest is 38 ATP

Skeletal muscle and brain cells– Electrons from first-stage reactions are

delivered to FAD in mitochondria

– Total energy harvest is 36 ATP

• 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

• 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+

Lactate Fermentation

C6H12O6

ATP

ATPNADH

2 lactate

electrons, hydrogen from NADH

2 NAD+

2

2 ADP

2 pyruvate

2

4

energy output

energy input

GLYCOLYSIS

LACTATE FORMATION

2 ATP net

Alcoholic Fermentation

C6H12O6

ATP

ATPNADH

2 acetaldehyde

electrons, hydrogen from NADH

2 NAD+

2

2 ADP

2 pyruvate

2

4

energy output

energy input

GLYCOLYSIS

ETHANOL FORMATION

2 ATP net

2 ethanol

2 H2O

2 CO2

Yeasts

• Single-celled fungi

• Carry out alcoholic fermentation

• Saccharomyces cerevisiae– Baker’s yeast– Carbon dioxide makes bread dough rise

• Saccharomyces ellipsoideus– Used to make beer and wine

Anaerobic Electron Transport

• Carried out by certain bacteria

• Electron transport system is in bacterial plasma membrane

• Final electron acceptor is compound from environment (such as nitrate), NOT oxygen

• ATP yield is low

• Glucose is absorbed into blood

• Pancreas releases insulin

• Insulin stimulates glucose uptake by cells

• Cells convert glucose to glucose-6-phosphate

• This traps glucose in cytoplasm where it can

be used for glycolysis

Carbohydrate Breakdown and Storage

Making Glycogen

• If glucose intake is high, ATP-making

machinery goes into high gear

• When ATP levels rise high enough, glucose-6-

phosphate is diverted into glycogen synthesis

(mainly in liver and muscle)

• Glycogen is the main storage polysaccharide in

animals

Using Glycogen

• When blood levels of glucose decline,

pancreas releases glucagon

• Glucagon stimulates liver cells to convert

glycogen back to glucose and to release it to

the blood

• (Muscle cells do not release their stored

glycogen)

Energy Reserves

• Glycogen makes up only about 1 percent of the

body’s energy reserves

• Proteins make up 21 percent of energy reserves

• Fat makes up the bulk of reserves (78 percent)

Energy from Proteins

• Proteins are broken down to amino acids

• Amino acids are broken apart

• Amino group is removed, ammonia forms, is

converted to urea and excreted

• Carbon backbones can enter the Krebs cycle

or its preparatory reactions

Energy from Fats

• Most stored fats are triglycerides

• Triglycerides are broken down to glycerol and

fatty acids

• Glycerol is converted to PGAL, an intermediate

of glycolysis

• Fatty acids are broken down and converted to

acetyl-CoA, which enters Krebs cycle

• 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

Processes Are Linked

Aerobic Respiration

• Reactants

– Sugar

– Oxygen

• Products

– Carbon dioxide

– Water

Photosynthesis

• Reactants

– Carbon dioxide

– Water

• Products

– Sugar

– Oxygen

Life Is System of Prolonging Order

• Powered by energy inputs from sun, life

continues onward through reproduction

• Following instructions in DNA, energy and

materials can be organized, generation after

generation

• With death, molecules are released and may

be cycled as raw material for next generation