Chapter 9: Cellular Respiration: Harvesting Chemical Energy

Front 1

Acetyl CoA

Back 1

Acetyl coenzyme A

Front 2

Aerobic respiration

Back 2

A catabolic pathway that consumes oxygen (O2) and organic molecules, producing ATP. This is the most efficient catabolic pathway and is carried out in most eukaryotic cells and many prokaryotic organisms.

Front 3

Alcohol fermentation

Back 3

Glycolysis followed by the conversion of pyruvate to carbon dioxide and ethyl alcohol.

Front 4

Anabolic pathway

Back 4

A metabolic pathway that consumes energy to synthesize a complex molecule from simpler compounds.

Front 5

Anaerobic respiration

Back 5

The use of inorganic molecules other than oxygen to accept electrons at the “downhill” end of electron transport chains.

Front 6

ATP synthase

Back 6

A complex of several membrane proteins that provide a port through which protons diffuse. This complex functions in chemiosmosis with adjacent electron transport chains, using the energy of a hydrogen ion (proton) concentration gradient to make ATP. ATP synthases are found in the inner mitochondrial membrane of eukaryotic cells and in the plasma membrane of prokaryotes.

Front 7

Beta oxidation

Back 7

A metabolic sequence that breaks fatty acids down to two-carbon fragments that enter the citric acid cycle as acetyl CoA.

Front 8

Catabolic pathway

Back 8

A metabolic pathway that releases energy by breaking down complex molecules to simpler compounds.

Front 9

Cellular respiration

Back 9

The catabolic pathways of aerobic and anaerobic respiration, which break down organic molecules for the production of ATP.

Front 10

Chemiosmosis

Back 10

An energy-coupling mechanism that uses energy stored in the form of a hydrogen ion gradient across a membrane to drive cellular work, such as the synthesis of ATP. Most ATP synthesis in cells occurs by chemiosmosis.

Front 11

citric acid cycle

Back 11

A chemical cycle involving eight steps that completes the metabolic breakdown of glucose molecules begun in glycolysis by oxidizing pyruvate to carbon dioxide

Front 12

Cytochrome

Back 12

An iron-containing protein that is a component of electron transport chains in the mitochondria and chloroplasts of eukaryotic cells and the plasma membranes of prokaryotic cells.

Front 13

Electron transport chain

Back 13

A sequence of electron carrier molecules (membrane proteins) that shuttle electrons during the redox reactions that release energy used to make ATP.

Front 14

Facultative anaerobe

Back 14

An organism that makes ATP by aerobic respiration if oxygen is present but that switches to anaerobic respiration or fermentation if oxygen is not present.

Front 15

Fermentation

Back 15

A catabolic process that makes a limited amount of ATP from glucose without an electron transport chain and that produces a characteristic end product, such as ethyl alcohol or lactic acid.

Front 16

Glycolysis

Back 16

The splitting of glucose into pyruvate. Glycolysis occurs in almost all living cells, serving as the starting point for fermentation or cellular respiration.

Front 17

Lactic acid fermentation

Back 17

Glycolysis followed by the conversion of pyruvate to lactate, with no release of carbon dioxide.

Front 18

Nicotinamide adenine dinucleotide (NAD+/NADH)

Back 18

Nicotinamide adenine dinucleotide, A coenzyme that can accept an electron and acts as an electron carrier in the electron transport chain.

Front 19

Obligate anaerobe

Back 19

An organism that only carries out fermentation or anaerobic respiration. Such organisms cannot use oxygen and in fact may be poisoned by it.

Front 20

Oxidation

Back 20

The loss of electrons from a substance involved in a redox reaction.

Front 21

Oxidative phosphorylation

Back 21

The production of ATP using energy derived from the redox reactions of an electron transport chain

Front 22

Oxidizing agent

Back 22

The electron acceptor in a redox reaction.

Front 23

Proton-motive force

Back 23

The potential energy stored in the form of an electrochemical gradient, generated by the pumping of hydrogen ions across a biological membrane during chemiosmosis.

Front 24

Reducing agent

Back 24

The electron donor in a redox reaction.

Front 25

Reduction

Back 25

The addition of electrons to a substance involved in a redox reaction.

Front 26

Substrate-level phosphorylation

Back 26

The formation of ATP by an enzyme directly transferring a phosphate group to ADP from an intermediate substrate in catabolism.

Front 27

Overview of Cellular Respiration

Back 27

• Organic compounds such as glucose store energy in their arrangements of atoms. These molecules are broken down and their energy extracted in cellular respiration. The first stage of cellular respiration occurs in the cytosol, while the second and third stages occur in mitochondria. In cellular respiration, electrons are transferred from glucose to coenzymes such as NAD+ and finally to oxygen; the energy released by this relocation of electrons is used to make ATP. Carbon dioxide and water are given off as byproducts. Click on each stage of cellular respiration for a brief description.

o Glycolysis is a series of steps in which a glucose molecule is broken down into two molecules of pyruvate. As the chemical bonds in glucose are broken, electrons (and hydrogen ions) are picked up by NAD+, forming NADH. Glucose is oxidized and NAD+ is reduced. A net output of two ATP molecules are also produced in glycolysis for every glucose molecule processed. But most of the energy released by the breakdown of glucose is carried by the electrons attached to NADH.

o The pyruvate molecules are modified as they enter the mitochondrion, releasing carbon dioxide. The altered molecules enter a series of reactions called the citric acid cycle. More carbon dioxide is released as the citric acid cycle completes the oxidation of glucose. Two ATPs are formed per glucose, but most of the energy released by the oxidation of glucose is carried by NADH and FADH2.

o Almost all of the ATP produced by cellular respiration is banked in the final phase— oxidative phosphorylation. The NADH and FADH2 molecules produced in glycolysis and the citric acid cycle donate their electrons to the electron transport chain. At the end of the chain, oxygen exerts a strong pull on the electrons, and combines with them and hydrogen ions (protons) to form water. The electron transport chain converts chemical energy of moving electrons to a form that can be used to drive oxidative phosphorylation, which produces about 34 ATP molecules for each glucose molecule consumed.

Front 28

Glycolysis

Back 28

• Glycolysis, which begins the breakdown of glucose, is a series of ten enzyme-catalyzed chemical reactions that can be divided into two main phases.

• In the energy-investment phase, some ATP energy is used to start the process of glucose oxidation. By the end of this phase, a 6-carbon molecule (glucose) has been split into two 3-carbon molecules of glyceraldehyde-3-phosphate.

• The 3-carbon glyceraldehyde-3-phosphate molecules now enter the energy-payoff phase. Chemical bonds are broken, and NAD+ picks up electrons and hydrogen ions, forming NADH. The energy released is used to attach phosphate groups. The phosphates are transferred to ADP, finally making some ATP. This way of making ATP is called substrate-level phosphorylation. A couple more reactions rearrange the atoms in the 3-carbon molecules. More ATP is generated in the final reaction that yields pyruvate. For each glucose molecule broken down during glycolysis, a net of two ATPs are formed, along with two NADH molecules.

Front 29

The Citric Acid Cycle

Back 29

• The oxidation of glucose continues in the citric acid cycle. Pyruvate molecules formed during glycolysis move from the cytosol into the mitochondrion, but pyruvate itself does not enter the citric acid cycle. A reaction occurs that removes a carbon atom, releasing it in carbon dioxide. Electrons are transferred to an NADH molecule, storing energy. Coenzyme A, or CoA, joins with the 2-carbon fragment, forming acetyl CoA.

• One molecule of acetyl CoA enters the citric acid cycle. The 2-carbon fragment of acetyl CoA attaches to the 4-carbon molecule oxaloacetate in the first reaction of the cycle. This forms citrate. In a series of steps, bonds break and reform. Two carbon atoms are released, one at a time, in molecules of carbon dioxide. Electrons are carried off by molecules of NADH and FADH2. One step produces an ATP molecule by substrate-level phosphorylation. A 4-carbon oxaloacetate molecule is regenerated.

• Since two acetyl CoA molecules are produced for each glucose molecule broken down, a second acetyl CoA enters the citric acid cycle. The same series of reactions occurs, releasing carbon dioxide and producing more NADH, FADH2, and ATP.

• The cell has gained two ATPs that can be used directly. However, most of the energy originally contained in the bonds of glucose is now carried by the NADH and FADH2 molecules.

Front 30

Fermentation

Back 30

• All cells are able to synthesize ATP via the process of glycolysis. In many cells, if oxygen is not present, pyruvate (pyruvic acid) is metabolized in a process called fermentation. By oxidizing the NADH produced in glycolysis, fermentation regenerates NAD+, which can take part in glycolysis once again to produce more ATP. The net energy gain in fermentation is 2 ATP molecules per molecule of glucose. Fermentation complements glycolysis and makes it possible for ATP to be continually produced in the absence of oxygen.

• There are two types of fermentation. Alcohol fermentation, which occurs in yeast, results in the production of ethanol and carbon dioxide. Lactic acid fermentation, which occurs in muscle, results in the production of lactate (lactic acid).

1) Glycolysis produces NADH, ATP, and pyruvate (pyruvic acid). If oxygen is not present, NADH cannot be oxidized in the electron transport chain. Without fermentation, the cell would run out of NAD+, bringing glycolysis to a halt.

2) In alcohol fermentation, the pyruvate (pyruvic acid) from glycolysis loses one carbon in the form of carbon dioxide and the product is then reduced to ethanol by NADH. With the formation of ethanol, NADH is oxidized and becomes NAD+

3) With a continuous supply of NAD+, glycolysis can continue, producing more ATP.

4) During fermentation, the NADH produced by glycolysis is oxidized, ensuring a continuous supply of NAD+ for glycolysis. Alcohol fermentation occurs in yeast cells.