Cellular Respiration: Harvesting Chemical Energy Figures 6.6 – 6.16 CHAPTER 6 Cellular Respiration: Harvesting Chemical Energy Figures 6.6 – 6.16

NADH and Electron Transport Chains The path that electrons take on their way down from glucose to oxygen involves many stops 1/2 (from food via NADH) Energy for synthesis of 2 H 2 e Electron transport chain 2 e 1/2 2 H Figure 6.6

The first stop is an electron acceptor called NAD+ The transfer of electrons from organic fuel to NAD+ reduces it to NADH The rest of the path consists of an electron transport chain This chain involves a series of redox reactions These lead ultimately to the production of large amounts of ATP

The Metabolic Pathway of Cellular Respiration Cellular respiration is an example of a metabolic pathway A series of chemical reactions in cells All of the reactions involved in cellular respiration can be grouped into three main stages Glycolysis The Krebs cycle Electron transport

A Road Map for Cellular Respiration Cytosol Mitochondrion High-energy electrons carried by NADH High-energy electrons carried mainly by NADH Glycolysis 2 Pyruvic acid Krebs Cycle Electron Transport Glucose Figure 6.7

Stage 1: Glycolysis A molecule of glucose is split into two molecules of pyruvic acid

Glycolysis breaks a six-carbon glucose into two three-carbon molecules These molecules then donate high energy electrons to NAD+, forming NADH

2 Pyruvic acid Glucose Figure 6.8

Glycolysis makes some ATP directly when enzymes transfer phosphate groups from fuel molecules to ADP Figure 6.9

Stage 2: The Krebs Cycle The Krebs cycle completes the breakdown of sugar

In the Krebs cycle, pyruvic acid from glycolysis is first “prepped” into a usable form, Acetyl-CoA 2 1 Acetic acid 3 Pyruvic acid Acetyl-CoA (acetyl-coenzyme A) CO2 Coenzyme A Figure 6.10

The Krebs cycle extracts the energy of sugar by breaking the acetic acid molecules all the way down to CO2 The cycle uses some of this energy to make ATP The cycle also forms NADH and FADH2

Krebs Cycle Input Output Acetic acid 2 CO2 ADP 3 NAD FAD 2 1 3 4 5 6 Figure 6.11

Stage 3: Electron Transport Electron transport releases the energy your cells need to make the most of their ATP

The molecules of electron transport chains are built into the inner membranes of mitochondria The chain functions as a chemical machine that uses energy released by the “fall” of electrons to pump hydrogen ions across the inner mitochondrial membrane These ions store potential energy

When the hydrogen ions flow back through the membrane, they release energy The ions flow through ATP synthase ATP synthase takes the energy from this flow and synthesizes ATP

Electron transport chain Protein complex Electron carrier Inner mitochondrial membrane Electron flow Electron transport chain ATP synthase Figure 6.12

The Versatility of Cellular Respiration Cellular respiration can “burn” other kinds of molecules besides glucose Diverse types of carbohydrates Fats Proteins

Food Polysaccharides Fats Proteins Sugars Glycerol Fatty acids Amino acids Amino groups Acetyl- CoA Krebs Cycle Glycolysis Electron Transport Figure 6.13

Adding Up the ATP from Cellular Respiration Cytosol Mitochondrion Glycolysis 2 Acetyl- CoA 2 Pyruvic acid Krebs Cycle Electron Transport Glucose Maximum per glucose: by direct synthesis by ATP synthase by direct synthesis Figure 6.14

FERMENTATION: ANAEROBIC HARVEST OF FOOD ENERGY Some of your cells can actually work for short periods without oxygen For example, muscle cells can produce ATP under anaerobic conditions Fermentation The anaerobic harvest of food energy

Fermentation in Human Muscle Cells Human muscle cells can make ATP with and without oxygen They have enough ATP to support activities such as quick sprinting for about 5 seconds A secondary supply of energy (creatine phosphate) can keep muscle cells going for another 10 seconds To keep running, your muscles must generate ATP by the anaerobic process of fermentation

Glycolysis is the metabolic pathway that provides ATP during fermentation Pyruvic acid is reduced by NADH, producing NAD+, which keeps glycolysis going In human muscle cells, lactic acid is a by-product

(a) Lactic acid fermentation 2 ADP+ 2 Glycolysis 2 NAD 2 NAD Glucose 2 Pyruvic acid + 2 H 2 Lactic acid (a) Lactic acid fermentation Figure 6.15a

Fermentation in Microorganisms Various types of microorganisms perform fermentation Yeast cells carry out a slightly different type of fermentation pathway This pathway produces CO2 and ethyl alcohol

(b) Alcoholic fermentation 2 ADP+ 2 2 CO2 released 2 ATP Glycolysis 2 NAD 2 NAD Glucose 2 Ethyl alcohol 2 Pyruvic acid + 2 H (b) Alcoholic fermentation Figure 6.15b

The food industry uses yeast to produce various food products Figure 6.16

EVOLUTION CONNECTION: LIFE ON AN ANAEROBIC EARTH Ancient bacteria probably used glycolysis to make ATP long before oxygen was present in Earth’s atmosphere Glycolysis is a metabolic heirloom from the earliest cells that continues to function today in the harvest of food energy

SUMMARY OF KEY CONCEPTS Chemical Cycling Between Photosynthesis and Cellular Respiration Heat Sunlight Cellular respiration Photosynthesis Visual Summary 6.1

The Overall Equation for Cellular Respiration Oxidation: Glucose loses electrons (and hydrogens) Glucose Carbon dioxide Electrons (and hydrogens) Energy Reduction: Oxygen gains electrons (and hydrogens) Oxygen Visual Summary 6.2

The Metabolic Pathway of Cellular Respiration Glucose Oxygen Water Energy Visual Summary 6.3