chapter 7 how cells release chemical energy

Albia Dugger • Miami Dade College

Chapter 7How Cells Release Chemical Energy

7.1 Mighty Mitochondria

• More than forty disorders related to defective mitochondria are known (such as Friedreich’s ataxia); many of those afflicted die young

A Mitochondrion

Two Main Metabolic Pathways

• Aerobic metabolic pathways (using oxygen) are used by most eukaryotic cells

• Anaerobic metabolic pathways (which occur in the absence of oxygen) are used by prokaryotes and protists in anaerobic habitats

Aerobic Respiration

• In modern eukaryotic cells, most of the aerobic respiration pathway takes place inside mitochondria

• Like chloroplasts, mitochondria have an internal folded membrane system that allows them to make ATP efficiently

• Electron transfer chains in this membrane set up hydrogen ion gradients that power ATP synthesis

• At the end of these chains, electrons are transferred to oxygen molecules

INTERACTION: Structure of a mitochondrion

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7.2 Overview of Carbohydrate Breakdown Pathways

• Photoautotrophs make ATP during photosynthesis and use it to synthesize glucose and other carbohydrates

• Most organisms, including photoautotrophs, make ATP by breaking down glucose and other organic compounds

Figure 7-2 p118

energy

Photosynthesis

CO2glucose

H2O O2

Aerobic Respiration

energy

Overview of Aerobic Respiration

• Three stages• Glycolysis• Acetyl-CoA formation and Krebs cycle• Electron transfer phosphorylation (ATP formation)

C6H12O6 (glucose) + O2 (oxygen) → CO2 (carbon dioxide) + H2O (water)

• Coenzymes NADH and FADH2 carry electrons and hydrogen

Figure 7-3 p119

Aerobic Respiration

glucose

Glycolysis

2 NADH 2 pyruvate

Krebs Cycle

8 NADH, 2 FADH2

Electron Transfer Phosphorylationoxygen

2 ATP4 ATP (2 net)

6 CO2 2 ATP

H2O32 ATP

In the Cytoplasm

In the Mitochondrion

ANIMATED FIGURE: Overview of aerobic respiration



Aerobic Respiration vs. Anaerobic Fermentation

• Aerobic respiration and fermentation both begin with glycolysis, which converts one molecule of glucose into two molecules of pyruvate

• After glycolysis, the two pathways diverge• Fermentation is completed in the cytoplasm, yielding 2

ATP per glucose molecule• Aerobic respiration is completed in mitochondria, yielding

36 ATP per glucose molecule

Figure 7-4 p119

Carbohydrate breakdown pathways start in the cytoplasm, with glycolysis.

Glycolysis

Fermentation concludes in cytoplasm.

In eukaryotes, aerobic respiration concludes inside mitochondria.

ANIMATED FIGURE: Where pathways start and finish



Take-Home Message: How do cells access the chemical energy in carbohydrates?

• Most cells convert the chemical energy of carbohydrates to chemical energy of ATP by aerobic respiration or fermentation

• Aerobic respiration and fermentation pathways start in

cytoplasm, with glycolysis

• Fermentation is anaerobic and ends in the cytoplasm

• Aerobic respiration requires oxygen. In eukaryotes, it ends in mitochondria

3D ANIMATION: Cellular Respiration

7.3 Glycolysis – Glucose Breakdown Starts

• The reactions of glycolysis convert one molecule of glucose to two molecules of pyruvate for a net yield of two ATP

• An energy investment of ATP is required to start glycolysis

Glycolysis

• Two ATP are used to split glucose and form 2 PGAL, each with one phosphate group

• Enzymes convert 2 PGAL to 2 PGA, forming 2 NADH

• Four ATP are formed by substrate-level phosphorylation (net 2 ATP)

• Glycolysis ends with the formation of two three-carbon pyruvate molecules

ANIMATED FIGURE: Glycolysis



ATP-Requiring Steps

An enzyme (hexokinase) transfers a phosphate group from ATP to glucose, forming glucose-6-phosphate.1

A phosphate group from a second ATP is transferred to the glucose-6phosphate. The resulting molecule is unstable, and it splits into two three carbon molecules. The molecules are interconvertible, so we will call them both PGAL (phosphoglyceraldehyde). Two ATP have now been invested in the reactions.

2

ATP-Generating Steps

Enzymes attach a phosphate to the two PGAL, and transfer two electrons and a hydrogen ion from each PGAL to NAD+. Two PGA (phosphoglycerate) and two NADH are the result.

3

Enzymes transfer a phosphate group from each PGA to ADP. Thus, two ATP have formed by substrate-levelphosphorylation. The original energy investment of two ATP has now been recovered.

4

Enzymes transfer a phosphate group from each of two intermediates to ADP. Two more ATP have formed by substrate-level phosphorylation. Two molecules of pyruvate form at this last reaction step.

Summing up, glycolysis yields two NADH, two ATP (net), and two pyruvate for each glucose molecule. Depending on the type of cell and environmental conditions, the pyruvate may enter the second stage of aerobic respiration or it may be used in other ways, such as in fermentation.

5

6

Stepped ArtFigure 7-5 p121

Take-Home Message:

What is glycolysis?

• Glycolysis is the first stage of carbohydrate breakdown in both aerobic respiration and fermentation

• The reactions of glycolysis occur in the cytoplasm

• Glycolysis converts one molecule of glucose to two molecules of pyruvate, with a net energy yield of two ATP; two NADH also form

3D ANIMATION: Cellular Respiration

ANIMATION: Energy inputs and release in glycolosis



7.4 Second Stage of Aerobic Respiration

• The second stage of aerobic respiration completes the breakdown of glucose that began in glycolysis

• Occurs in mitochondria

• Includes two sets of reactions: acetyl CoA formation and the Krebs cycle (each occurs twice in the breakdown of one glucose molecule)

Acetyl CoA Formation

• In the inner compartment of the mitochondrion, enzymes split pyruvate, forming acetyl CoA and CO2 (which diffuses out of the cell)

• NADH is formed

The Krebs Cycle

• Krebs cycle• A sequence of enzyme-mediated reactions that break

down 1 acetyl CoA into 2 CO2

• Oxaloacetate is used and regenerated• 3 NADH and 1 FADH2 are formed• 1 ATP is formed

Second Stage of Aerobic Respiration

cytoplasm

outer membrane

inner membrane

The breakdown of 2 pyruvate to 6 CO2 yields 2 ATP and 10 reduced coenzymes (8 NADH, 2 FADH2). The coenzymes will carry their cargo of electrons and hydrogen ions to the third stage of aerobic respiration.

matrix

ANIMATED FIGURE: The Krebs Cycle - details



Acetyl–CoA Formation and the Krebs Cycle

Krebs Cycle

The final steps of the Krebs cycle regenerateoxaloacetate.

8

NAD+ combines with hydrogen ions and electrons,forming NADH.

7

The coenzyme FAD combines with hydrogen ions and electrons, forming FADH2.

6

One ATP forms by substrate-level phosphorylation.

5

An enzyme splits a pyruvate coenzyme A NAD+ molecule into a two-carbon acetyl group and CO2. Coenzyme A binds the acetyl group (forming acetyl–CoA). NAD+ combines with released hydrogen ions and electrons, forming NADH.

1

The Krebs cycle starts as one carbon atom is transferred from acetyl–CoA tooxaloacetate. Citrate forms, and coenzyme A is regenerated.

2

A carbon atom is removed from an intermediate and leaves the cell as CO2. NAD+ combines with released hydrogen ions and electrons, forming NADH.

3

A carbon atom is removed from another intermediate and leaves the cell as CO2, and another NADH forms.

Pyruvate’s three carbon atoms have now exited the cell, in CO2.

4

Stepped Art

Figure 7-7 p123

Take-Home Message: What happens during the second stage of aerobic respiration?

• The second stage of aerobic respiration, acetyl–CoA formation and the Krebs cycle, occurs in the inner compartment (matrix) of mitochondria

• The pyruvate that formed in glycolysis is converted to acetyl–CoA and CO2; the acetyl–CoA enters the Krebs cycle, which breaks it down to CO2

• For two pyruvate molecules broken down in the second-stage reactions, two ATP form, and ten coenzymes (eight NAD+; two FAD) are reduced

7.5 Aerobic Respiration’s Big Energy Payoff

• Many ATP are formed during the third and final stage of aerobic respiration

• Electron transfer phosphorylation• Occurs in mitochondria• Results in attachment of phosphate to ADP to form ATP

Electron Transfer Phosphorylation

• Coenzymes NADH and FADH2 donate electrons and H+ to electron transfer chains

• Active transport forms a H+ concentration gradient in the outer mitochondrial compartment

• H+ follows its gradient through ATP synthase, which attaches a phosphate to ADP

• Finally, oxygen accepts electrons and combines with H+, forming water

Electron Transfer Phosphorylation

Summary: The Energy Harvest

• Typically, the breakdown of one glucose molecule yields 36 ATP• Glycolysis: 2 ATP• Acetyl CoA formation and Krebs cycle: 2 ATP• Electron transfer phosphorylation: 32 ATP

Figure 7-9 p125

ANIMATED FIGURE: Third-stage reactions



Take-Home Message: What happens during the third stage of aerobic respiration?

• In electron transfer phosphorylation, energy released by electrons flowing through electron transfer chains is captured in the attachment of phosphate to ADP; a typical net yield of aerobic respiration is thirty-six ATP per glucose

• The reactions begin when coenzymes that were reduced in the first and second stages of reactions deliver electrons and hydrogen ions to electron transfer chains in the inner mitochondrial membrane

Take-Home Message: (cont.)

• Energy released by electrons as they pass through electron transfer chains is used to pump H+ from the mitochondrial matrix to the intermembrane space

• The H+ gradient that forms across the inner mitochondrial membrane drives the flow of hydrogen ions through ATP synthases, which results in ATP formation

ANIMATION: Mitochondrial chemiosmosis



7.6 Fermentation

• Fermentation pathways break down carbohydrates without using oxygen

• The final steps in these pathways regenerate NAD+ but do not produce ATP

Fermentation

• Glycolysis is the first stage of fermentation• Forms 2 pyruvate, 2 NADH, and 2 ATP

• Pyruvate is converted to other molecules, but is not fully broken down to CO2 and water• Regenerates NAD+ but doesn’t produce ATP

• Provides enough energy for some single-celled anaerobic species

Two Fermentation Pathways

• Alcoholic fermentation• Pyruvate is split into acetaldehyde and CO2

• Acetaldehyde receives electrons and hydrogen from NADH, forming NAD+ and ethanol

• Lactate fermentation• Pyruvate receives electrons and hydrogen from NADH,

forming NAD+ and lactate

Figure 7-10a p127

Glycolysisglucose

2 2

4

pyruvate

Alcoholic Fermentation

acetaldehyde

2 CO2

2 NAD+

ethanol

2

2 NAD+

Figure 7-10b p127

Figure 7-11a p127

Glycolysisglucose

2

2

pyruvate

Lactate Fermentation

2

lactate

2 CO2

2 NAD+

2 NAD+

4

ANIMATED FIGURE: Fermentation pathways



Red and White Muscle Fibers

• Red muscle fibers make ATP by aerobic respiration• Have many mitochondria• Myoglobin stores oxygen• Sustain prolonged activity

• White muscle fibers make ATP by lactate fermentation• Have few mitochondria and no myoglobin• Sustain short bursts of activity

Figure 7-11b p127

Figure 7-11c p127

Take-Home Message:What is fermentation?

• ATP can form by carbohydrate breakdown in fermentation pathways, which are anaerobic

• The end product of lactate fermentation is lactate. The end product of alcoholic fermentation is ethanol

• Both pathways have a net yield of two ATP per glucose molecule; the ATP forms during glycolysis

• Fermentation reactions regenerate the coenzyme NAD+, without which glycolysis (and ATP production) would stop

7.7 Alternative Energy Sources in Food

• Aerobic respiration can produce ATP from the breakdown of complex carbohydrates, fats, and proteins

• As in glucose metabolism, many coenzymes are reduced, and the energy of the electrons they carry ultimately drives the synthesis of ATP in electron transfer phosphorylation

Energy From Complex Carbohydrates

• Enzymes break starch and other complex carbohydrates down to monosaccharide subunits

• Monosaccharides are taken up by cells and converted to glucose-6-phosphate, which continues in glycolysis

• A high concentration of ATP causes glucose-6-phosphate to be diverted away from glycolysis and into a pathway that forms glycogen

Energy From Fats

• Enzymes cleave fats into glycerol and fatty acids• Glycerol products enter glycolysis• Fatty acids are converted to acetyl Co-A and enter the

Krebs cycle

• Compared to carbohydrates, fatty acid breakdown yields more ATP per carbon atom

• When blood glucose level is high, acetyl CoA is diverted from the Krebs cycle and into a pathway that makes fatty acids

Energy from Proteins

• Enzymes split dietary proteins into amino acid subunits, which are used to build proteins or other molecules

• The amino group is removed and converted into ammonia (NH3), a waste product eliminated in urine

• Acetyl–CoA, pyruvate, or an intermediate of the Krebs cycle forms, depending on the amino acid

Figure 7-12a p128

starch (a complex carbohydrate) glucose

A Complex carbohydrates are broken down to their monosaccharide subunits, which can enter glycolysis. 1

Figure 7-12b p128

a triglyceride (fat)

glycerol head

fatty acid tails

Figure 7-12b p128

Food

Fats Complex Carbohydrates Proteins

fatty acids glycerol glucose, other simple sugars

amino acids

acetyl–CoA PGAL acetyl–CoA

Glycolysis

NADH pyruvate

intermediate of Krebs cycle

KrebsCycle

NADH, FADH2

Electron TransferPhosphorylation

2 3 1 4

Figure 7-12c p128

alanine (an amino acid) pyruvate

ANIMATED FIGURE: Alternative energy sources



Take-Home Message: Can organic molecules other than glucose be used for energy?

• Complex carbohydrates, fats, and proteins can be oxidized in aerobic respiration to yield ATP

• First the digestive system and then individual cells convert molecules in food into intermediates of glycolysis or the Krebs cycle

chapter 7 how cells release chemical energy

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