7.1 Cellular Respiration

• Cellular respiration is the process by which cells produce ATP, the body's main energy source.

• ATP is produced by the breakdown of organic molecules, primarily glucose.

• Cellular respiration occurs in the mitochondria, the powerhouses of the cell.

• Cellular respiration is a redox reaction, which means that electrons are transferred from one molecule to another.

• In cellular respiration, glucose is oxidized to form carbon dioxide, and oxygen is reduced to form water.

• The energy released from the redox reaction is used to produce ATP.

Phases of Complete Glucose Breakdown

Cellular Respiration

• Cellular respiration is a process that cells use to break down glucose and release energy.

• The energy released from glucose is used to make ATP, which is the cell's main source of energy.

• There are four phases of cellular respiration: glycolysis, the preparatory reaction, the citric acid cycle, and the electron transport chain.

Glycolysis

• Glycolysis is the first phase of cellular respiration.

• It takes place in the cytoplasm of the cell.

• In glycolysis, glucose is broken down into two molecules of pyruvate.

• Two ATP molecules are produced during glycolysis.

• NADH is also produced during glycolysis.

The Preparatory Reaction

• The preparatory reaction is the second phase of cellular respiration.

• It takes place in the mitochondria.

• In the preparatory reaction, pyruvate is broken down into a 2-carbon acetyl group and carbon dioxide.

• NADH is also produced during the preparatory reaction.

The Citric Acid Cycle

• The citric acid cycle is the third phase of cellular respiration.

• It takes place in the mitochondria.

• In the citric acid cycle, the acetyl group from the preparatory reaction is combined with oxaloacetate to form citrate.

• Citrate is then broken down into a series of other molecules, and carbon dioxide is released.

• NADH, FADH2, and ATP are also produced during the citric acid cycle.

The Electron Transport Chain

• The electron transport chain is the fourth and final phase of cellular respiration.

• It takes place in the mitochondria.

• In the electron transport chain, electrons from NADH and FADH2 are passed along a series of proteins.

• As the electrons move down the chain, energy is released.

• This energy is used to pump hydrogen ions across the inner mitochondrial membrane.

• The hydrogen ions then flow back down the gradient, and this energy is used to make ATP.

7.2 Outside the Mitochondria: Glycolysis

• Glycolysis is a metabolic pathway that takes place in the cytoplasm of both eukaryotes and prokaryotes.

• During glycolysis, glucose, a 6-carbon molecule, is broken down to two molecules of pyruvate, a 3-carbon molecule.

• Glycolysis is divided into two phases: the energy-investment phase and the energy-harvesting phase.

• In the energy-investment phase, two ATP molecules are used to initiate the reactions of glycolysis.

• In the energy-harvesting phase, four ATP molecules are produced, along with two NADH molecules.

• The net yield of glycolysis is two ATP molecules and two NADH molecules.

Energy-Investment Step

• During the energy-investment step of glycolysis, two ATP molecules transfer their phosphate groups to two different substrates.

• This results in the formation of two ADP molecules and two phosphate groups.

• The transfer of phosphate groups activates the substrates, making them more reactive.

• This allows the substrates to undergo the energy-harvesting reactions of glycolysis.

Energy-Harvesting Steps

• During the energy-harvesting steps of glycolysis, electrons and hydrogen ions are removed from the substrates and captured by NAD+, producing two NADH.

• The energized phosphate groups on the intermediates are used to synthesize four ATP through a process called substrate-level ATP synthesis.

• The net gain of ATP from glycolysis is 2 ATP.

• If oxygen is available, the end product of glycolysis (pyruvate) enters the aerobic reactions within mitochondria, where it is used to generate more ATP.

• If oxygen is not available, pyruvate enters the anaerobic fermentation pathways.

• In humans, if oxygen is not available, pyruvate is reduced to lactate.

7.3 Outside the Mitochondria Fermentation

• The complete breakdown of glucose requires oxygen to keep the electron transport chain working.

• If oxygen is limited, pyruvate molecules accumulate in the cell.

• Intermediates, such as NAD+ and FAD, cannot be recycled.

• Cells may enter anaerobic pathways, such as fermentation, following glycolysis.

• There are two basic forms of fermentation: lactic acid and alcohol.

Lactic Acid Fermentation

• In animals and some bacteria, pyruvate formed by glycolysis can be reduced to lactate.

• This reaction is catalyzed by lactate dehydrogenase, which uses NADH as a source of electrons.

• The reduction of pyruvate to lactate regenerates NAD+, which can then be used in the earlier reactions of glycolysis.

• This regeneration of NAD+ allows glycolysis to continue, producing ATP by substrate-level phosphorylation.

• Anaerobic glycolysis and fermentation are two processes that produce ATP in the absence of oxygen.

• These processes only produce a small fraction of the potential energy stored in a glucose molecule.

• Despite the low yield, anaerobic pathways are essential for providing a rapid burst of ATP.

• Muscle cells are more apt than other cells to carry on fermentation.

• When muscles are working vigorously over a short period of time, fermentation is a way to produce ATP even though oxygen is temporarily in limited supply.

  When a person stops running, the body is in an oxygen deficit. This means that the body has used up more oxygen than it has taken in.

• The body's response to an oxygen deficit is to breathe very heavily. This is called hyperventilation. Hyperventilation helps to increase the amount of oxygen in the blood.

• Recovery from an oxygen deficit is complete when the body has taken in enough oxygen to completely break down glucose.

• Blood carries away the lactate formed in muscles and transports it to the liver.

• In the liver, lactate is converted to pyruvate.

• Some of the pyruvate is oxidized completely, and the rest is converted back to glucose.

Alcohol Fermentation

• In other organisms (bacteria and fungi), pyruvate is reduced to produce alcohol.

• The electrons needed to reduce the pyruvate are supplied by NADH molecules.

• NAD+ molecules are regenerated for use in glycolysis.

• Alcohol fermentation releases small amounts of CO2.

• Yeasts (a type of fungi) are good examples of microorganisms that generate ethyl alcohol and CO2 when they carry out fermentation.

• Ethyl alcohol is the desired product when yeasts are used to ferment grapes for wine production or to ferment wort—derived from barley—for beer production.

7.4 Inside the Mitochondria

• After glycolysis, if oxygen is present, the cell will enter the aerobic phases of cellular respiration.

• The aerobic phases of cellular respiration occur inside the mitochondria.

Preparatory Reaction

• The preparatory reaction occurs in the matrix of the mitochondria.

• It is so named because it prepares the outputs of glycolysis (pyruvate molecules) for use in the citric acid cycle.

• Glycolysis splits each glucose into two pyruvate molecules, so the preparatory reaction occurs twice per glucose molecule.

• During the preparatory reaction:

  • Pyruvate is oxidized, and a CO2 molecule is given off.

  • NAD+ accepts electrons and hydrogen ions, forming NADH.

  • The product, a 2-carbon acetyl group, is attached to coenzyme A (CoA), forming acetyl-CoA.

• Per glucose molecule, the outputs of the preparatory reaction are: 2 CO2, 2 NADH, 2 acetyl-CoA

The Citric Acid Cycle

• The citric acid cycle is a cyclical metabolic pathway located in the matrix of mitochondria.

• It was originally called the Krebs cycle to honor the scientist who first studied it.

• At the start of the citric acid cycle, the 2-carbon acetyl group carried by CoA joins with a 4-carbon molecule, producing a 6-carbon citrate molecule. The CoA is released and is used again in the preparatory reaction.

• During the citric acid cycle:

  • The acetyl group is oxidized, in the process forming CO2.

  • Both NAD+ and FAD accept electrons and hydrogen ions, resulting in NADH and FADH2.

  • Substrate-level ATP synthesis occurs, and an ATP results.

• Because the citric acid cycle turns twice for each original glucose, the inputs and outputs of the citric acid cycle per glucose are as follows:

The Electron Transport Chain

• The electron transport chain is a series of protein complexes that pass electrons from one to another.

• NADH and FADH2 deliver electrons to the chain.

• The carriers of the electron transport chain accept only electrons and not hydrogen ions.

• The electron transport chain is a series of redox reactions that remove the high-energy electrons from NADH and FADH2.

• As these reactions occur, energy is released and captured for ATP production.

• The final acceptor of electrons is oxygen (O2).

• Once NADH has delivered electrons to the electron transport chain, NAD+ is regenerated.

  • In the same manner, FAD is regenerated.

• The recycling of coenzymes increases cellular efficiency.

Generating ATP

• The electron transport chain is a series of protein complexes that transfer electrons from NADH and FADH2 to oxygen.

• The electron transport chain is located in the inner mitochondrial membrane.

• The energy released by the electron transport chain is used to pump hydrogen ions (H+) from the mitochondrial matrix into the intermembrane space.

• The pumping of H+ into the intermembrane space creates a proton gradient.

• The proton gradient drives ATP synthase, which uses the energy to produce ATP.

• NADH and FADH2 are both produced during cellular respiration, but they do not produce the same amount of ATP.

• NADH produces 3 ATP, while FADH2 produces only 2 ATP.

• This is because NADH delivers electrons to the first carrier of the electron transport chain, whereas FADH2 delivers its electrons later in the chain.

• The H+ gradient is a concentration gradient of hydrogen ions (H+) across the inner mitochondrial membrane.

• The H+ gradient contains a large amount of stored energy that can be used to drive forward ATP synthesis.

• The ATP synthase complex is an enzyme that uses the energy of the H+ gradient to synthesize ATP from ADP + P.

• ATP leaves the matrix by way of a channel protein.

• This ATP remains in the cell and is used for cellular work.

7.5 Metabolic Fate of Food

Energy Yield from Glucose Metabolism

• Glycolysis produces 2 ATP per glucose molecule.

• The citric acid cycle produces 2 ATP per glucose molecule.

• Oxidative phosphorylation produces 34 ATP per glucose molecule.

• The total ATP yield from the complete breakdown of glucose to CO2 and H2O is 38 ATP.

• Electron transport chain and ATP synthase complex: Most of the ATP produced in cells comes from the electron transport chain and the ATP synthase complex.

• NADH and FADH2: For each glucose molecule, 10 NADH and 2 FADH2 take electrons from glycolysis and the citric acid cycle to the electron transport chain.

• Maximum number of ATP: The maximum number of ATP produced by the electron transport chain is, therefore 34 ATP.

• Maximum yield: The maximum number of ATP produced by both the electron transport chain and substrate-level ATP synthesis is 38.

• Metabolic differences: Not all cells produce the maximum yield. Metabolic differences cause some cells to produce 36 or fewer ATP.

• The yield of 36–38 ATP: A yield of 36–38 ATP represents about 40% of the energy that was initially available in the glucose molecule.

• The rest of the energy is lost in the form of heat.

Alternative Metabolic Pathways

• Fat breaks down into glycerol and three fatty acid chains when used as an energy source.

• Glycerol enters the process of cellular respiration during glycolysis because it is a carbohydrate.

• Fatty acids can be metabolized to acetyl groups, which enter the citric acid cycle.

• A fatty acid with a chain of 18 carbons can produce three times the number of acetyl groups that a glucose molecule does.

• The complete breakdown of glycerol and fatty acids results in more ATP per fat molecule than the breakdown of a glucose molecule.

• Only the hydrocarbon backbone of amino acids can be used by the cellular respiration pathways.

• The amino group is removed and converted into ammonia (NH3).

• Ammonia is then converted into urea, which is the primary excretory product of humans.

• The hydrocarbon backbone from an amino acid can enter cellular respiration pathways at pyruvate, acetyl-CoA, or during the citric acid cycle.