How Cells Harvest Chemical Energy Chapter 6 Cellular Respiration

Energy Flow and Chemical Cycling in the Biosphere Fuel molecules in food represent solar energy traced back to the sun Animals depend on plants: to convert solar energy to chemical energy In form of sugars and other organic molecules

Gas Exchange in the Body Cellular respiration and breathing are closely related Cellular respiration requires a cell to exchange gases with its surroundings Breathing exchanges these gases between blood and outside air

Cellular respiration Cellular respiration is an exergonic process that transfers energy from the bonds in glucose to ATP produces 38 ATP molecules from each glucose molecule Other foods (organic molecules) can be used as a source of energy as well Respiration only retrieves 40% of the energy in a glucose molecule. The other 60% of the energy is released as heat. We use this heat to maintain a relatively steady body temperature near 37°C (98–99°F). This is about the same amount of heat generated by a 75-watt incandescent light bulb. Organic compounds possess potential energy as a result of their arrangement of atoms. Compounds that can participate in exergonic reactions can act as food. Actually, cellular respiration includes both aerobic and anaerobic processes. However, it is generally used to refer to the aerobic process. It takes about 10 million ATP molecules per second to power one active muscle cell. Student Misconceptions and Concerns 1. Students should be cautioned against the assumption that energy is created when it is converted from one form to another. This might be a good time to review the principle of conservation of energy (the first law of thermodynamics addressed in Module 5.11). 2. Students often fail to realize that aerobic metabolism is a process generally similar to the burning of wood in a fireplace or campfire or the burning of gasoline in an automobile engine. Noting these general similarities can help students comprehend the overall reaction and heat generation associated with these processes. Teaching Tips 1. Energy coupling at the cellular level may be new to many students, but it is a familiar concept when related to the use of money in our society. Students might be discouraged if the only benefit of work was the ability to make purchases from the employer. (We all might soon tire of a fast-food job that only paid its employees in food!) Money permits the coupling of a generation of value (a paycheck, analogous to an energy-releasing reaction) to an energy-consuming reaction (money, which allows us to make purchases in distant locations). This idea of earning and spending is a common concept we all know well. 2. During cellular respiration, our cells convert about 40% of our food energy to useful work. The other 60% of the energy is released as heat. We use this heat to maintain a relatively steady body temperature near 37°C (98–99°F). This is about the same amount of heat generated by a 75-watt incandescent lightbulb. If you choose to include a discussion of heat generation from aerobic metabolism, consider the following. A. Ask your students why they feel warm when it is 30°C (86°F) outside, if their core body temperature is 37°C (98.6°F). Shouldn’t they feel cold? The answer is, our bodies are always producing heat. At these higher temperatures, we are producing more heat than we need to maintain a body temperature around 37°C. Thus, we sweat and behave in ways that helps us get rid of the extra heat from cellular respiration. B. Share this calculation with your students. Depending upon a person’s size and level of activity, a human might burn 2,000 dietary calories (kilocalories) a day. This is enough energy to raise the temperature of 20 liters of liquid water from 0 to 100°C. This is something to think about the next time you heat water on the stove! (Notes: Consider bringing a 2-liter bottle as a visual aid, or ten 2-liter bottles to make the point above. It takes 100 calories to raise 1 liter of water 100°C; it takes much more energy to melt ice or evaporate water as steam.)

Cellular Respiration Release of energy from molecules accompanied by the use of this energy to synthesize ATP molecules Metabolic pathway Main method that chemical energy is harvested from food and converted to ATP Aerobic Requires oxygen and gives off carbon dioxide

Where Is the Energy in Food? The process of aerobic respiration requires oxygen and carbohydrates C6H12O6 + 6 O2  6 CO2 + 6 H2O + energy The products are carbon dioxide, water, and energy (heat or ATP)

How do we get the energy?? Energy contained in the arrangement of electrons in chemical bonds in organic molecules Cells tap energy from electrons “falling” from organic fuels to oxygen When the carbon-hydrogen bonds of glucose are broken, electrons are transferred to oxygen Oxygen has a strong tendency to attract electrons

ATP Adenosine triphosphate (ATP) Nucleotide with the base adenine and the sugar ribose Main energy carrier in cells Formed during reactions that breakdown organic compounds to CO2 and water Requires ample oxygen Occurs within the mitochondrion Hydrolyzes phosphates to release energy form adenosine diphosphate (ADP)

Redox Reaction (O-R) Chemical reaction that transfers electrons from one substance to another electrons retain their potential energy Glucose loses its hydrogen atoms and is ultimately converted to CO2 O2 gains hydrogen atoms and is converted to H2O Oxidation Reduction

Redox Reaction Electrons pass from atoms or molecules to one another as part of many energy reactions Oxidation When an atom or molecule loses an electron Glucose is oxidized Reduction When an atom or molecule gains an elections Oxygen is reduced

Other important players…… Enzymes are necessary to oxidize glucose and other foods Dehydrogenase enzyme that removes hydrogen from an organic molecule requires a coenzyme called NAD+ (nicotinamide adenine dinucleotide) shuttle electrons NAD+ can become reduced when it accepts electrons and oxidized when it gives them up Reduced to NADH

The Finale…. First step is transfer of electrons from organic molecule to NAD+ Other electron carrier molecules represent the electron transport chain Undergoes series of redox reactions Release energy to make ATP

NADH ATP NAD+ + 2e– Controlled release of H+ energy for synthesis of ATP H+ Electron transport chain 2e– H+ 1  2 O2 H2O

Stages of Cellular Respiration: Glycolysis Citric Acid Cycle Oxidative Phosphorylation

High-energy electrons NADH High-energy electrons carried by NADH Mitochondrion NADH FADH2 and GLYCOLYSIS OXIDATIVE PHOSPHORYLATION (Electron Transport and Chemiosmosis) CITRIC ACID CYCLE Glucose Pyruvate Cytoplasm Figure 6.6 An overview of cellular respiration. Inner mitochondrial membrane CO2 CO2 ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation

1. Glycolysis Occurs in the cytoplasm Does not require oxygen to generate ATP Then, enters aerobic or anaerobic reactions

Glycolysis Glucose 2 ADP 6 Carbon oxidizes glucose into 2 molecules Pyruvate 3 Carbon breaking of the bond yields energy that is used to phosphorylate ADP to ATP in addition, electrons and hydrogen are donated to NAD+ to form NADH 2 NAD+ + 2 P 2 NADH 2 ATP + 2 H+ 2 Pyruvate

Enzyme Enzyme P ADP + ATP P P Substrate Product

Anaerobic verse aerobic? Absence of oxygen Fermentation Make lactate or ethanol Presence of oxygen Oxidative respiration Pyruvate transported to mitochondria Oxidize pyruvate to form acetyl-coA

When pyruvate is oxidized: A single carbon cleaved off by the enzyme pyruvate dehydrogenase This carbon leaves as part of a CO2 molecule hydrogen and electrons are removed from pyruvate donated to NAD+ to form NADH Remaining two-carbon fragment of pyruvate is joined to a cofactor called coenzyme A (CoA) Final compound called acetyl-CoA

Acetyl-CoA The fate of acetyl-CoA depends on the availability of ATP in the cell Insufficient ATP The acetyl-CoA heads to the Krebs cycle Plentiful ATP The acetyl-CoA is diverted to fat synthesis for energy storage

2. Citric Acid Cycle “Krebs Cycle” occurs within the mitochondrion Breaks down pyruvate into carbon dioxide electrons passed to an electron transport chain in order to power the production of ATP

Stages of Citric Acid Cycle acetyl (two-carbon) compound enters the citric acid cycle Acetyl-CoA enters the cycle and binds to a four-carbon molecule, forming a six-carbon molecule Two carbons are removed as CO2 and their electrons donated to NAD+ In addition, an ATP is produced The four-carbon molecule is recycled and more electrons are extracted, forming NADH and FADH2

Acetyl CoA CoA CoA The Krebs cycle 2 CO2 CITRIC ACID CYCLE Note: a single glucose molecule produces two turns of the cycle, one for each of the two pyruvate molecules generated by glycolysis 3 NAD+ FADH2 FAD 3 NADH  3 H+ ATP ADP + P

by oxidative phosphorylation Electron shuttle across membrane Cytoplasm Mitochondrion 2 NADH 2 NADH (or 2 FADH2) 2 NADH 6 NADH 2 FADH2 GLYCOLYSIS OXIDATIVE PHOSPHORYLATION (Electron Transport and Chemiosmosis) 2 Pyruvate 2 Acetyl CoA CITRIC ACID CYCLE Glucose  2 ATP  2 ATP  about 34 ATP Figure 6.12 An estimated tally of the ATP produced by substrate-level and oxidative phosphorylation in cellular respiration. by substrate-level phosphorylation by substrate-level phosphorylation by oxidative phosphorylation About 38 ATP Maximum per glucose:

3. Oxidative Phosphorylation Electron transport chain Shuttle molecules NADH and FADH take electrons to oxygen Final acceptor Forms H2O Carriers bind and release electrons in redox reactions Pass electrons down the “energy staircase” Use energy released from the transfers to transport H+

OXIDATIVE PHOSPHORYLATION Protein complex of electron carriers H+ H+ Electron carrier H+ H+ ATP synthase Intermembrane space Inner mitochondrial membrane FADH2 FAD Electron flow NADH NAD+ 2  1 O2 + 2 H+ H+ Mitochondrial matrix H+ Figure 6.10 Oxidative phosphorylation, using electron transport and chemiosmosis in the mitochondrion. ADP + P H+ ATP H2O H+ Electron Transport Chain Chemiosmosis OXIDATIVE PHOSPHORYLATION

3. Oxidative Phosphorylation Chemiosmosis Uses energy stored in a hydrogen ion gradient to drive ATP synthesis H+ concentration gradient stores potential energy ATP synthase drives hydrogen ions through Generates ATP

by oxidative phosphorylation Electron shuttle across membrane Cytoplasm Mitochondrion 2 NADH 2 NADH (or 2 FADH2) 2 NADH 6 NADH 2 FADH2 GLYCOLYSIS OXIDATIVE PHOSPHORYLATION (Electron Transport and Chemiosmosis) 2 Pyruvate 2 Acetyl CoA CITRIC ACID CYCLE Glucose  2 ATP  2 ATP  about 34 ATP Figure 6.12 An estimated tally of the ATP produced by substrate-level and oxidative phosphorylation in cellular respiration. by substrate-level phosphorylation by substrate-level phosphorylation by oxidative phosphorylation About 38 ATP Maximum per glucose:

Fermentation Occurs when O2 is not available Animal cells and bacteria convert pyruvate to lactate Other organisms convert pyruvate to alcohol and CO2

Glucose Is Not the Only Food Molecule Cells also get energy from foods other than sugars The other organic building blocks undergo chemical modifications that permit them to enter cellular respiration

Food, such as peanuts Carbohydrates Fats Proteins Sugars Glycerol Fatty acids Amino acids Amino groups Figure 6.15 Pathways that break down various food molecules. OXIDATIVE PHOSPHORYLATION (Electron Transport and Chemiosmosis) CITRIC ACID CYCLE Acetyl CoA Glucose G3P Pyruvate GLYCOLYSIS ATP

Do plants perform cellular respiration??