How Cells Release Chemical Energy Chapter 7

Impacts, Issues When Mitochondria Spin Their Wheels Mitochondria are the organelles responsible for releasing the energy stored in foods In Luft’s syndrome, the mitochondria are active in oxygen consumption, but with little ATP formation to show for it In Friedreich’s ataxia, too much iron in the mitochondria causes an accumulation of free radicals that attack valuable molecules of life

The Impact Proper, or improper, functioning of mitochondria is the difference between health and disease

Overview of Energy-Releasing Pathways Section 7.1 Overview of Energy-Releasing Pathways

Producing the Universal Currency of Life All energy-releasing pathways: require characteristic starting materials yield predictable products and by-products produce ATP

ATP Is Universal Energy Source Photosynthesizers get energy from the sun Animals get energy second- or third-hand from plants or other organisms Regardless, the energy is converted to the chemical bond energy of ATP

Making ATP Plants make ATP during photosynthesis Cells of all organisms make ATP by breaking down carbohydrates, fats, and protein

Main Types of Energy-Releasing Pathways Anaerobic pathways Evolved first Don’t require oxygen Start with glycolysis in cytoplasm Completed in cytoplasm Aerobic pathways Evolved later Require oxygen Start with glycolysis in cytoplasm Completed in mitochondria

Energy-Releasing Pathways

Overview of Aerobic Respiration C6H1206 + 6O2 6CO2 + 6H20 glucose oxygen carbon dioxide water

Overview of Aerobic Respiration

Main Pathways Start with Glycolysis Glycolysis occurs in cytoplasm Reactions are catalyzed by enzymes Glucose 2 Pyruvate (six carbons) (three carbons)

p.106a

Three Series of Reactions Are Required for Aerobic Respiration Glycolysis is the breakdown of glucose to pyruvate Small amounts of ATP are generated

Three Series of Reactions Are Required for Aerobic Respiration The Krebs cycle degrades pyruvate to CO2 and water; NAD and FAD accept H+ ions and electrons to be carried to the electron transfer chain Small amounts of ATP are generated

Three Series of Reactions Are Required for Aerobic Respiration Electron transfer phosphorylation processes the H+ ions and electrons to generate lots of ATP Oxygen is the final electron acceptor

The Role of Coenzymes NAD+ and FAD accept electrons and hydrogen from intermediates during the first two stages When reduced, they are NADH and FADH2 In the third stage, these coenzymes deliver the electrons and hydrogen to the transfer chain

Overview of Aerobic Respiration

The First Stage: Glycolysis Section 7.2 The First Stage: Glycolysis

Glucose A simple sugar (C6H12O6) Atoms held together by covalent bonds

Glycolysis Occurs in Two Stages Energy-requiring steps ATP energy activates glucose and its six-carbon derivatives

Glycolysis Occurs in Two Stages Energy-releasing steps The products of the first part are split into 3-carbon pyruvate molecules ATP and NADH form

Energy-Requiring Steps

Energy-Releasing Steps ATP PGAL NADH 2 ATP produced ENERGY - RELEASING STEPS OF GLYCOLYSIS NAD + P i 3 phosphoglycerate 2 PEP ADP 1,3 bisphosphoglycerate pyruvate to second set of reactions substrate level phosphorylation H O

Glycolysis in a Nutshell Glucose is first phosphorylated in energy-requiring steps, then split to form two molecules of PGAL Enzymes remove H+ and electrons from PGAL to change NAD+ to NADH (which is used later in electron transfer PGAL is converted eventually to pyruvate By substrate-level phosphorylation, four ATP are produced

Substrate-level What? Substrate-level phosphorylation means that there is a direct transfer of a phosphate group from the substrate of a reaction to some other molecule – in this case, ADP Substrate is a reactant in a reaction – the substance being acted upon, for example, by an enzyme

Net Energy Yield from Glycolysis Energy requiring steps: 2 ATP invested Energy releasing steps: 2 NADH formed 4 ATP formed Net yield is 2 ATP and 2 NADH

Second Stage of Aerobic Respiration Section 7.3 Second Stage of Aerobic Respiration

Second-Stage Reactions Occur in the mitochondria Pyruvate is broken down to carbon dioxide More ATP is formed More coenzymes are reduced

Fig. 7-5b, p.112

Second Stage of Aerobic Respiration

Two Parts of Second Stage Preparatory reactions Pyruvate is oxidized into two-carbon acetyl units and carbon dioxide NAD+ is reduced Krebs cycle The acetyl units are oxidized to carbon dioxide NAD+ and FAD are reduced

Preparatory Reactions pyruvate + coenzyme A + NAD+ acetyl-CoA + NADH + CO2 One of the carbons from pyruvate is released in CO2 Two carbons are attached to coenzyme A and continue on to the Krebs cycle

What Is Acetyl-CoA? A two-carbon acetyl group linked to coenzyme A CH3

Second Stage of Aerobic Respiration

=CoA KREBS CYCLE acetyl-CoA CoA oxaloacetate citrate NAD+ NADH H2O H2O malate isocitrate O NAD+ NADH FAD FADH2 fumarate a-ketoglutarate succinyl-CoA O CoA NAD+ NADH succinate ADP + phosphate group ATP Stepped Art Fig. 7-7a, p.113

The Krebs Cycle Overall Reactants Overall Products Acetyl-CoA 3 NAD+ FAD ADP and Pi Overall Products Coenzyme A 2 CO2 3 NADH FADH2 ATP

Results of the Second Stage All of the carbon molecules in pyruvate end up in carbon dioxide Coenzymes are reduced (they pick up electrons and hydrogen) One molecule of ATP is formed Four-carbon oxaloacetate is regenerated

Two pyruvates cross the inner mitochondrial membrane. outer mitochondrial compartment 2 NADH inner mitochondrial compartment 6 NADH Krebs Cycle Eight NADH, two FADH 2, and two ATP are the payoff from the complete break-down of two pyruvates in the second-stage reactions. 2 FADH2 ATP 2 The six carbon atoms from two pyruvates diffuse out of the mitochondrion, then out of the cell, in six CO 6 CO2 Fig. 7-6, p.112

Coenzyme Reductions during First Two Stages Glycolysis 2 NADH Preparatory 2 NADH reactions Krebs cycle 2 FADH2 + 6 NADH Total 2 FADH2 + 10 NADH

Third Stage of Aerobic Respiration – The Big Energy Payoff Section 7.4 Third Stage of Aerobic Respiration – The Big Energy Payoff

Electron Transfer Phosphorylation 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

Electron Transfer Phosphorylation Electron transfer chains are embedded in inner mitochondrial compartment NADH and FADH2 give up electrons that they picked up in earlier stages to electron transfer chain Electrons are transferred through the chain The final electron acceptor is oxygen

Creating an H+ Gradient OUTER COMPARTMENT NADH INNER COMPARTMENT

ATP Formation ATP INNER COMPARTMENT ADP + Pi

Summary of Transfers glycolysis e electron transfer phosphorylation glucose glycolysis e – electron transfer phosphorylation 2 PGAL 2 pyruvate 2 NADH 2 CO ATP 2 FADH H + 6 NADH 2 acetyl - CoA Krebs Cycle 4 CO 36 ADP + P i

Importance of Oxygen Electron transfer 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 Electron transfer phosphorylation 32 ATP formed

Energy Harvest from Coenzyme Reductions What are the sources of electrons used to generate the 32 ATP in the final stage? 4 ATP - generated using electrons released during glycolysis and carried by NADH 28 ATP - generated using electrons formed during second-stage reactions and carried by NADH and FADH2

Energy Harvest Varies NADH formed in cytoplasm cannot enter mitochondrion It delivers electrons to mitochondrial membrane Membrane proteins shuttle electrons to NAD+ or FAD inside mitochondrion Electrons given to FAD yield less ATP than those given to NAD+

Energy Harvest Varies Skeletal muscle and brain cells Liver, kidney, heart cells Electrons from first-stage reactions are delivered to NAD+ in mitochondria Total energy harvest is 38 ATP Skeletal muscle and brain cells Electrons from first-stage reactions are delivered to FAD in mitochondria Total energy harvest is 36 ATP

Fermentation Pathways Section 7.5 Fermentation Pathways

Anaerobic Pathways Do not use oxygen Produce less ATP than aerobic pathways Two types of fermentation pathways Alcoholic fermentation Lactate fermentation

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+

Alcoholic Fermentation

Yeasts Single-celled fungi Carry out alcoholic fermentation Saccharomyces cerevisiae Baker’s yeast Carbon dioxide makes bread dough rise Saccharomyces ellipsoideus Used to make beer and wine

Lactate Fermentation

Lactate Fermentation 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

Lactate Fermentation Lactobacillus and some other bacteria produce lactate This produces cheeses, yogurt, buttermilk and other dairy products Fermenters also are used to cure meats and in pickling Sauerkraut is an example Sour taste due to lactic acid (form of lactate)

Slow-twitch v. Fast-twitch muscles Slow-twitch muscles make ATP only by aerobic respiration (no fermentation) Slow-twitch muscles are for light, steady, prolonged activity Slow-twitch muscles are red because they have lots of myoglobin, a pigment used to store oxygen They also have many mitochondria

Fast-twitch Muscles These pale (lighter colored) muscles have few mitochondria and no myoglobin Fast-twitch muscles, which are used for immediate and intense energy demands, use lactate fermentation to produce ATP It works quickly, but not for long Chickens have fast-twitch breast muscles used for quick flights (white meat) Ducks fly long distances – what color is their breast meat?

Alcoholic Fermentation

Lactate Fermentation

Alternative Energy Sources in the Body Section 7.6 Alternative Energy Sources in the Body

The Fate of Glucose After eating, glucose is absorbed into the blood Insulin levels rise, causing greater uptake of glucose by cells Glycolysis will follow Excess glucose is converted into glycogen Glycogen is known as “animal starch,” and is the main storage polysaccharide in animals Stored in the muscles and the liver

Between Meals When blood levels of glucose decline, pancreas releases glucagon, a hormone Glucagon stimulates liver cells to convert glycogen back to glucose and to release it to the blood Glycogen levels are adequate, but can be depleted in 12 hours (Muscle cells do not release their stored glycogen)

Energy Reserves Glycogen makes up only about 1 percent of the body’s energy reserves Proteins make up 21 percent of energy reserves Fat makes up the bulk of reserves (78 percent)

Energy from Fats Most stored fats are triglycerides in adipose tissue Triglycerides are “three-tailed” fats Triglycerides are broken down to glycerol and fatty acids Glycerol is converted to PGAL, an intermediate of glycolysis Fatty acids are broken down and converted to acetyl-CoA, which enters Krebs cycle

glycolysis e electron transfer phosphorylation glucose 2 PGAL 2 – electron transfer phosphorylation 2 PGAL 2 pyruvate 2 NADH 2 CO ATP 2 FADH H + 6 NADH 2 acetyl - CoA Krebs Cycle 4 CO 36 ADP + P i

Energy from Proteins Proteins are broken down to amino acids Amino acids are broken apart Amino group is removed, ammonia forms, is converted to urea and excreted Carbon backbones can enter the Krebs cycle or its preparatory reactions

Reaction Sites

Section 7.7 Perspective on Life

Evolution of Metabolic Pathways When life originated, atmosphere had little oxygen Earliest organisms used anaerobic pathways Later, cyclic pathway (simple form) of photosynthesis increased atmospheric oxygen Much more efficient cells arose that used oxygen as final acceptor in electron transfer

Processes Are Linked Aerobic Respiration Photosynthesis Reactants Sugar Oxygen Products Carbon dioxide Water Photosynthesis Reactants Carbon dioxide Water Products Sugar Oxygen

Life Is System of Prolonging Order Powered by energy inputs from sun, life continues onward through reproduction Following instructions in DNA, energy and materials can be organized, generation after generation With death, molecules are released and may be cycled as raw material for next generation