How Cells Release Stored Energy Chapter 7

“Killer” Bees Descendents of African honeybees that were imported to Brazil in the 1950s More aggressive, wider-ranging than other honeybees Africanized bee’s muscle cells have enlarged mitochondria

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

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

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

Overview of Aerobic Respiration CYTOPLASM glucose ATP GLYCOLYSIS energy input to start reactions e- + H+ (2 ATP net) 2 NADH 2 pyruvate MITOCHONDRION e- + H+ 2 CO2 2 NADH e- + H+ 8 NADH 4 CO2 KREBS CYCLE e- + H+ 2 2 FADH2 ATP e- ELECTRON TRANSPORT PHOSPHORYLATION 32 ATP H+ water e- + oxygen TYPICAL ENERGY YIELD: 36 ATP

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 transport system

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 Energy-releasing steps The products of the first part are split into three-carbon pyruvate molecules ATP and NADH form

Energy-Requiring Steps glucose ATP ADP 2 ATP invested P glucose-6-phosphate P fructose-6-phosphate ATP ADP P fructose-1,6-bisphosphate

Energy-Releasing Steps PGAL PGAL Energy-Releasing Steps NAD+ NAD+ NADH NADH Pi Pi P P P P 1,3-bisphosphoglycerate 1,3-bisphosphoglycerate substrate-level phosphorylation ADP ADP ATP ATP 2 ATP invested P P 3-phosphoglycerate 3-phosphoglycerate P P 2-phosphoglycerate 2-phosphoglycerate H2O H2O P P PEP PEP substrate-level phosphorylation ADP ADP ATP ATP 2 ATP invested pyruvate pyruvate

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 Reactions PREPARATORY STEPS pyruvate Second-Stage Reactions coenzyme A (CoA) NAD+ NADH (CO2) CoA Acetyl–CoA KREBS CYCLE CoA Occur in the mitochondria Pyruvate is broken down to carbon dioxide More ATP is formed More coenzymes are reduced oxaloacetate citrate H2O NADH H2O NAD+ malate isocitrate NAD+ H2O NADH fumarate a-ketogluterate FADH2 CoA FAD NAD+ NADH succinate CoA succinyl–CoA ATP ADP + phosphate group (from GTP)

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 C=O Coenzyme A Acetyl group

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

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

Electron Transport Phosphorylation Occurs in the mitochondria Coenzymes deliver electrons to electron transport systems Electron transport sets up H+ ion gradients Flow of H+ down gradients powers ATP formation

Electron Transport Electron transport systems are embedded in inner mitochondrial compartment NADH and FADH2 give up electrons that they picked up in earlier stages to electron transport system Electrons are transported through the system The final electron acceptor is oxygen

Creating an H+ Gradient OUTER COMPARTMENT NADH INNER COMPARTMENT

Making ATP: Chemiosmotic Model INNER COMPARTMENT ADP + Pi

Importance of Oxygen Electron transport phosphorylation requires the presence of oxygen Oxygen withdraws spent electrons from the electron transport system, 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 transport 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

Efficiency of Aerobic Respiration 686 kcal of energy are released 7.5 kcal are conserved in each ATP When 36 ATP form, 270 kcal (36 X 7.5) are captured in ATP Efficiency is 270 / 686 X 100 = 39 percent Most energy is lost as heat

Anaerobic Pathways Do not use oxygen Produce less ATP than aerobic pathways Two types Fermentation pathways Anaerobic electron transport

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+

Lactate Fermentation GLYCOLYSIS C6H12O6 2 ATP energy input 2 ADP 2 NAD+ 2 NADH 4 ATP 2 pyruvate energy output 2 ATP net LACTATE FORMATION electrons, hydrogen from NADH 2 lactate

Alcoholic Fermentation GLYCOLYSIS Alcoholic Fermentation C6H12O6 2 ATP energy input 2 ADP 2 NAD+ 2 NADH 4 ATP energy output 2 pyruvate 2 ATP net ETHANOL FORMATION 2 H2O 2 CO2 2 acetaldehyde electrons, hydrogen from NADH 2 ethanol

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

Anaerobic Electron Transport Carried out by certain bacteria Electron transport system is in bacterial plasma membrane Final electron acceptor is compound from environment (such as nitrate), NOT oxygen ATP yield is low

Carbohydrate Breakdown and Storage Glucose is absorbed into blood Pancreas releases insulin Insulin stimulates glucose uptake by cells Cells convert glucose to glucose-6-phosphate This traps glucose in cytoplasm where it can be used for glycolysis

Making Glycogen If glucose intake is high, ATP-making machinery goes into high gear When ATP levels rise high enough, glucose-6-phosphate is diverted into glycogen synthesis (mainly in liver and muscle) Glycogen is the main storage polysaccharide in animals

Using Glycogen When blood levels of glucose decline, pancreas releases glucagon Glucagon stimulates liver cells to convert glycogen back to glucose and to release it to the blood (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 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

Energy from Fats Most stored fats are triglycerides 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

Evolution of Metabolic Pathways When life originated, atmosphere had little oxygen Earliest organisms used anaerobic pathways Later, noncyclic pathway of photosynthesis increased atmospheric oxygen Cells arose that used oxygen as final acceptor in electron transport

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