Cellular Respiration

Releasing Stored Energy

Harvesting Chemical Energy Autotrophs like plants and cyanobacteria

use photosynthesis to change the sun’s light energy into chemical energy contained in the bonds within glucose molecules.

Both autotrophs and heterotrophs then use this energy-rich molecule to supply the energy they need to power their cellular activities like growth, movement, or reproduction.

When glucose or other large molecules are broken down into smaller molecules, the energy from the broken chemical bonds is used to make ATP from ADP and phosphate.

ATP is the main energy currency of living things.

The complex many-step procedure used by living things to make ATP from the breakdown of large molecules such as glucose is called cellular respiration.

Cellular respiration can be divided into several smaller biochemical pathways:

glycolysis

fermentation

aerobic respiration Glycolysis evolved very early in the

Earth’s history. There was no free oxygen in the atmosphere, so the first organisms (bacteria) all used glycolysis to produce ATP.

It took more than one billion years for bacteria to evolve the process of photosynthesis.

Glycolysis provides enough energy for many current unicellular organisms that have limited energy needs.

Larger organisms have greater energy needs and use aerobic respiration to meet those needs.

Glycolysis A biochemical pathway is a series of chemical reactions where the products of one reaction become the reactants of the next reaction.

Glycolysis is a biochemical pathway. In glycolysis, one molecule of glucose is split in half to produce two molecules of pyruvic acid (pyruvate) and two molecules of ATP.

Glycolysis happens in the cell’s cytoplasm. All living things can perform glycolysis.

Glycolysis has 10 chemical reactions, but can be summarized in four steps.

Step 1 – Two phosphate groups are added to the glucose molecule from 2 ATPs, making a 6-C compound that can be split.

Step 2 – The 6-C compound is split into two 3-C molecules of PGAL (phosphoglyceraldehyde).

Step 3 – The two PGAL molecules are oxidized and receive another phosphate group. The oxidation of PGAL accompanies the reduction of NAD+ to NADH.

Step 4 – the phosphate groups are removed, producing 2 pyruvic acid molecules and 4 ATP, for a net gain of 2 ATP.

2 NAD+

2 NADH + 2H+

2 ATP

2 ADP

4 ADP

4 ATP

C C C C C C Glucose

6-C CompoundC C C C C CP- -P

C C CP- C C C -P 2 PGAL

2 molecules of a 3-C compound

C C CP- -P C C C -PP-

C C C C C C

2 molecules of pyruvic acid (pyruvate)

Lactic Acid Fermentation In the absence of oxygen, some cells

convert pyruvic acid into other compounds using one of several other biochemical pathways.

These new steps do not make any more ATP. They are still necessary because glycolysis in the absence of oxygen will use up a cell’s supply of NAD+.

If all of the cell’s NAD+ is gone, glycolysis will stop and the cell can no longer make ATP by splitting glucose.

The combination of glycolysis and these other NAD+ regenerating steps are called fermentation.

There are two main types of fermentation: lactic acid fermentation and alcoholic fermentation.

In lactic acid fermentation, pyruvic acid is rearranged to form another 3-C molecule, lactic acid.

As this occurs, NADH + H+ are converted back into NAD+, so glycolysis can continue.

Lactic acid fermentation by bacteria makes yogurt or cheese.

Lactic acid fermentation also happens in animal muscle tissue during strenuous exercise, and can result in muscle cramps.

C C C C C C Glucose

C C C C C C

2 molecules of pyruvic acid (pyruvate)

2 NAD+

2 NADH + 2H+

C C C C C C

2 molecules of lactic acid (lactate)

Glycolysis

Lactic Acid Fermentation2 NAD+

2 NADH + 2H+

Alcoholic Fermentation Some plant cells, bacteria, and the

unicellular fungus known as yeast use a biochemical pathway to regenerate NAD+ known as alcoholic fermentation.

In this pathway, pyruvic acid is converted into a 2-C compound called ethyl alcohol (ethanol), and carbon dioxide.

Like in lactic acid fermentation, these steps turn NADH and H+ back into NAD+, thus allowing glycolysis to continue.

Alcoholic fermentation is important economically. It is used in the production of beers and wines.

As the yeast ferment the sugars present in the mix, the ethanol content rises until it reaches a concentration high enough to kill the yeast. This is about 12%. The CO2 is released during fermentation. To make champagne, the CO2 is retained.

Bread also depends on alcoholic fermentation. The bubbles of CO2 produced cause the bread to rise. The alcohol evaporates during baking, producing the wonderful smell of baking bread.

C C C C C C Glucose

2 NAD+

2 NADH + 2H+

C C C C C C

2 molecules of pyruvic acid (pyruvate)

Glycolysis

2 NAD+

2 NADH + 2H+

C C C

2 molecules of ethyl alcohol (ethanol) and 2 CO2

C C C

Alcoholic Fermentation

Glycolysis is not an efficient way to make ATP. Its efficiency is only about 3.5%

Aerobic Respiration and Mitochondria

Aerobic respiration has two major stages: the Krebs cycle and the electron transport chain.

In the presence of O2, in the reactions of the Krebs cycle, glucose is completely oxidized into CO2 and H2O.

As the glucose is oxidized, NAD+ is reduced to NADH and H+.

In the presence of oxygen, the electron transport chain can operate. NADH is fed into the transport chain and produces large amounts of ATP as well as regenerating NAD+.

Although the Krebs cycle produces a small amount of ATP, most of the ATP produced during aerobic respiration is produced by the electron transport chain.

In prokaryotes, the reactions of the Krebs cycle and the electron transport chain occur in the cell’s cytosol.

In eukaryotes, these reactions, happen within mitochondria.

The pyruvic acid made during glycolysis diffuses through the double membrane and into the mitochondrial matrix. The mitochondrial matrix contains the enzymes needed for the Krebs cycle.

When pyruvic acid enters the mitochondrial matrix, it reacts with a molecule called coenzyme A to form acetyl coenzyme A (acetyl CoA) and a molecule of CO2. One molecule of NAD+ is reduced to NADH and H+. Acetyl CoA can enter the Krebs cycle.

Aerobic Respiration - The Krebs Cycle

The Krebs cycle is a biochemical pathway that breaks the remaining bonds in acetyl CoA, forming CO2, H atoms, and ATP.

In eukaryotic cells, the Krebs cycle takes place inside the mitochondrial matrix.

The Krebs Cycle has 5 main steps:

Step 1 - Acetyl CoA bonds with a 4-C molecule (oxaloacetic acid) to form a 6-C molecule (citric acid), releasing coenzyme A.

Step 2 - Citric acid is oxidized, releasing CO2, an H atom to NAD+ forming NADH, and a 5-C compound.

Step 3 – The 5-C compound becomes a 4-C compound, forming CO2, NADH, and ATP

Step 4 – The 4-C compound changes to a different 4-C compound, releasing an H atom to FAD, which forms FADH2.

Step 5 – The 4-C compound changes back into oxaloacetic acid and forming another NADH.

CC CC

CC CC

4-C compound

Oxaloacetic acid

NAD+

NADH + H+

CC CC

4-C compound

CC

Acetyl CoA

CoA

CC CC CC

Citric Acid

C CO2

NAD+

NADH + H+

C

CC CC C

5-C compound

CO2

NAD+

NADH + H+

ADP

ATP

FAD

FADH2

Aerobic Respiration – Electron Transport Chain

The electron transport chain is the second part of aerobic respiration.

In eukaryotic cells, the molecules needed for this are embedded in the inner mitochondrial membrane.

In prokaryotes, the molecules for the electron transport chain are embedded in the cell membrane.

The purpose of the electron transport chain is to make ATP from ADP. This happens when NADH and FADH2 are converted back into NAD+ and FAD.

The electrons in the H atoms of NADH and FADH2 are high energy electrons.

As these electrons are passed along the electron transport chain they lose some of their excess energy.

This energy is used to pump the protons of the H atoms from the mitochondrial matrix to the other side of the inner membrane, building up a concentration gradient.

As protons pass back through ATP synthase molecules located in the membrane, ATP is made from ADP.

This process can only continue if the last molecule in the electron transport chain can get rid of its excess electrons. It does so by passing them off to oxygen atoms. This is why O2 is needed for aerobic respiration.

The Electron Transport Chain

e-

Cytosol

Mitochondrial Matrix

Intermembrane Space

NADH NAD+

H+

2e-

e-

e-

e-

O2 + 4e- + 4H+ 2H2O

ADP

ATPFADH2 FAD

ATP synthase

Cellular Respiration - Energy Output

Aerobic respiration is a more efficient way to produce ATP than glycolysis, with an efficiency rate of about 66%.

Glucose

Pyruvic Acid

Acetyl CoA

Glycolysis

Krebs cycle

2 ATP2 NADH 6 ATP from

electron transport chain (ETC)

2 NADH 6 ATP from ETC

6 NADH

2 ATP produced directly

18 ATP from ETC

2 FADH2 4 ATP from ETC

38 ATP