How Cells Release Stored Energy Cell respiration

How Cells Release Stored Energy

Cell respiration

When a molecule has hydrogen, it has more energy and removing them releases the energy. The energy produced from the "burning" of glucose is used to make ATP. In chemistry this process is called the oxidation of glucose.

The hydrogen carriers NAD and FAD are used to help release the energy in glucose by moving hydrogens and electrons around.

• A simple sugar

(C6H12O6)

• Atoms held together by covalent bonds

Glucose

In-text figurePage 136

Summary Equation for Aerobic Respiration

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

dioxide

•Glycolysis -in cytoplasm-does not use oxygen

Occurs in Two Stages

• Energy-requiring steps

– ATP energy activates glucose and its six-carbon

derivatives

– Actually uses 2 ATP’s

• Energy-releasing steps

– The products of the first part are split into 2 three

carbon pyruvate molecules

– 4 ATP and 2NADH form

– 4 ATP’s form – 2 ATP’s used = 2 net ATP’s made

Glycolysis is often called anaerobic respriation because it does not need oxygen. This process occurs in the cytoplasm. Two net molecules of ATP are made for cell use. It involves glucose being converted to two molecules of pyruvic acid (or pyruvate).

This process is not very efficient at converting the energy of glucose into ATP as only 2 ADP are phosphorylated instead of 32 as in Krebs and chemiosmosis.

Energy-Requiring Steps 2 ATP invested

Energy-Requiring Steps of Glycolysis

glucose

PGAL PGALPP

ADP

P

ATP

glucose-6-phosphate

Pfructose-6-phosphate

ATP

fructose1,6-bisphosphateP P

ADP

Figure 8.4(2)Page 137

Energy-Releasing

Steps

ADPATP

pyruvate

ADPATP

pyruvate

H2OP

PEP

H2OP

PEP

P

2-phosphoglycerate

P

2-phosphoglycerate

ADPATP

P3-phosphoglycerate

ADPATP

P3-phosphoglycerate

NAD+

NADHPi

1,3-bisphosphoglycerateP P

NAD+

NADHPi

1,3-bisphosphoglycerateP P

PGALP

PGALP

Figure 8.4 Page 137

Phosphorylations

Glycolysis: Net Energy Yield

Energy requiring steps: 2 ATP invested

Energy releasing steps:2 NADH formed 4 ATP formed

Net yield is 2 ATP, 2 pyruvic acid and 2 NADH

After glycolysis:

-If oxygen is present, the pyruvic acid will go into the mitochondria where the Kreb’s cycle (or citric acid cycle) is performed followed by the ETC. This allows the rest of the energy stored in the hydrogen to be extracted.

-If no there is no oxygen, then the Kreb's cycle can not be completed (or started in some organisms). A cell can continue doing glycolysis in the absence of oxygen to produce some ATP, BUT it must regenerate NAD to keep glycolysis going.

This is called anaerobic respiration (or fermentation). Pyruvate will form either lactic acid (muscles) or ethanol (bacteria, yeast or plants). In either case NAD is regenerated so that glycolysis can continue.

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

Anaerobic Electron Transport• Carried out by certain bacteria

• Electron transfer chain is in bacterial plasma membrane

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

• ATP yield is low

After glysolysis, if oxygen is present aerobic respiration proceeds in the mitochondria.

• Preparatory reactions– Pyruvate is decarboxylated

and oxidized (releasing carbon dioxide)

– NAD+ is reduced– Acetyl Co-A is formed– Ex. 1

PreparatorySteps

pyruvate

NAD+

NADH

coenzyme A (CoA)

O O carbon dioxide

acetyl-CoA

Ex. 2

The Krebs Cycle

Overall Products

• Coenzyme A

• 2 CO2

• 3 NADH

• FADH2

• ATP

Overall Reactants

• Acetyl-CoA• 3 NAD+

• FAD

• ADP and Pi

The enzymes for Kreb's is found in the inner compartment of the mitochondria.

Summary of Krebs- Occurs in Mitochondria

2X's

Pyruvate---> 3 CO2 6 CO2

1 ADP ---> 1 ATP 2 ATP

4 NAD ---> 4 NADH 8 NADH

1 FAD ---> 1 FADH2 2 FADH2

(also, oxaloacetate regenerates so cycle can continue)

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 (chemiosmotic)

Electron Transfer Phosphorylation

The purpose of chemiosmosis is to extract the energy found in NADH and FADH2 to make more ATP. This involves the cristae. There are electron transport chains that are used.

The electrons from the NADH and FADH2 are used to move on the electron transport chain. As the electrons move down the electron transport chain, H+ ions are pumped across the membrane.

The electrons from one NADH can pump 6 H+ across the membrane, but the electrons from FADH2 can only pump 4 H+ across the membrane.

Creating an H+ Gradient

NADH

OUTER COMPARTMENT

INNER COMPARTMENT

The outer compartment of the mitochondria becomes positive and the inside becomes negative like a battery. This "battery" can do work. The hydrogen ions can cross an F1 particle and make ATP.

It takes 2 H+ to cross the F1 particle to provide enough energy to make ATP.

Making ATP: Chemiosmotic Model

ATP

ADP+Pi

INNER COMPARTMENT

Summary

8 NADH2 x 6 H = 48 H+

2 FADH2(Krebs)x 4 H = 8 H+

2 FADH2(glyc.) X 4 H = 8 H+

64 H+

64 H+ --> 32 ATP

Importance of Oxygen

• Electron transport phosphorylation 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 substrate-level phosphorylation

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

• Electron transport phosphorylation– 32 ATP formed

Overview of Aerobic Respiration

CYTOPLASM

Glycolysis

Electron Transfer

Phosphorylation

KrebsCycle ATP

ATP

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+

ATP2 4

Figure 8.3Page 135

• 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 (at

best)

• Most energy is lost as heat

Efficiency of Aerobic Respiration

**Fats/Lipids = Glycerol and 3 fatty acids

Glycerol is converted to PGAL and respired in glycolysis.

The fatty acids are chopped into 2 carbon acetyl groups and used in the Krebs or citric acid cycle.

**Proteins = amino acids

The amino acids are sent to the liver where the liver removes the amine group. The left over acid is then used at some point in the Krebs cycle.

Alternative Energy Sources

FOOD

complex carbohydrates

simple sugars

pyruvate

acetyl-CoA

glycogenfats proteins

amino acids

carbon backbones

fatty acids

glycerol

NH3

PGAL

glucose-6-phosphate

GLYCOLYSIS

KREBS CYCLE

urea

Figure 8.11Page 145

Other organic molecules can be involved in respiration.

• When life originated, atmosphere had little

(none) free 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 and were more

efficient

Evolution of Metabolic Pathways

Processes Are Linked

sunlight energy

water+

carbondioxide

PHOTOSYNTHESIS

AEROBICRESPIRATION

sugarmolecules oxygen

In-text figurePage 146

Linked Processes

Photosynthesis

• Energy-storing pathway

• Releases oxygen

• Requires carbon dioxide

• Makes carbs

Aerobic Respiration

• Energy-releasing pathway

• Requires oxygen

• Releases carbon dioxide

• Breaks carbs