Cellular Respiration

1. Aerobic Cellular Respiration

1. Cellular respiration includes all metabolic pathways where carbohydrates and other metabolites are broken down to build up ATP.
2. Aerobic cellular respiration includes pathways that require oxygen.
3. Breaking glucose (a high-energy molecule) into CO₂ and H₂O (low-energy molecules) is an exergonic process.
4. Upon breakdown, electrons are removed from glucose and eventually received by O₂.
5. Glucose is oxidized and O₂ is reduced; glucose breakdown is therefore an oxidation-reduction reaction.
6. The buildup of ATP is an endergonic reaction, it requires energy.
7. The breakdown of one glucose results in 36 to 38 ATP molecules being formed; this is under 40% of the potential energy within a glucose molecule, over 60% is lost as heat.
8. The Steps of Aerobic Respiration
   • a. Aerobic cellular respiration is a gradual process that prevents energy loss as heat.
   • b. Glycolysis is the breakdown of glucose to two molecules of pyruvate; occurs outside the mitochondria.
   • c. During the transition reaction, pyruvate is oxidized to acetyl CoA and CO₂ is removed; the transition reaction occurs twice per glucose molecule.
   • d. The Krebs cycle is cyclical series of oxidation reactions that give off CO₂ and produce one ATP per cycle; it turns twice per glucose and produces two ATP.
   • e. The electron transport system is a series of carriers that accept electrons removed from glucose and eventually pass then to oxygen; release of energy along this electron transport chain results in ATP buildup.
9. NAD is a coenzyme of oxidation-reduction.
   • a. NAD⁺ picks up two electrons and one hydrogen ion; the substrate is oxidized and NAD⁺ is reduced.
   • b. The electrons received by NAD⁺ are used by the cell to produce ATP.
   • c. Like an enzyme, the coenzyme NAD⁺ is used over and over again; only a small amount is therefore present in a cell.
   • d. After NAD⁺ accepts electrons and is reduced to NADH, NADH passes the electrons to another carrier and becomes oxidized to NAD⁺ again.
   • e. FAD is sometimes used instead of NAD⁺ to oxidize substrates; FAD accepts two electrons and two hydrogen ions and becomes FADH₂.

2. Outside the Mitochondria: Glycolysis

1. Glycolysis breaks down glucose to two molecules of pyruvate outside the mitochondria.
2. Found in all organisms, glycolysis probably evolved before the Krebs cycle and electron transport system and probably is why it occurs in the cytoplasm and does not require oxygen.
3. The Energy Investment Steps:
   • a. Two ATP are used to activate glucose (a six-carbon molecule).
   • b. The resulting molecule is phosphorylated (phosphate groups are added).
   • c. The C₆ molecule splits into two C₃ molecules, each of which is phosphorylated.
4. The Energy Generation Steps:
   • a. Oxidation of substrates is carried out by NAD⁺ twice producing two NADH.
   • b. Energy released allows formation of four ATP by substrate-level phosphorylation.
   • c. During substrate-level phosphorylation, a substrate passes a high-energy phosphate to ADP forming ATP. (Table 7.3)
   • d. Subtracting two ATP used to get the reaction started, there is a net gain of two ATP.
5. Glycolysis is not just an aerobic process but also occurs in anaerobic fermentation.

3. Inside the Mitochondria

1. The transition reaction, the Krebs cycle and the electron transport system all take place inside the mitochondria.
2. Enzymes for the Krebs cycle are located in the fluid-filled matrix of the mitochondria.
3. Pathways: oxygen and glucose diffuse into cells from bloodstream, pyruvate (as an end product of glycolysis) diffuses into mitochondria; CO₂ and ATP diffuse back out of mitochondria into cytoplasm and CO₂ further diffuses back to bloodstream. Water can remain in mitochondria, in cytoplasm, or enter bloodstream for excretion. ATP remains as a source of energy for the cell to do work.
4. Since most of ATP is produced in mitochondria, mitochondria are often called the powerhouses of the cell.
5. The transition reaction:
   • a. Connects glycolysis to Krebs cycle.
   • b. Occurs within matrix of mitochondria.
   • c. From glucose, two molecules of pyruvate are converted to a two-carbon acetyl group attached to coenzyme A (CoA).
   • d. CO₂ is given off.
6. The Krebs cycle:
   • a. Occurs in matrix of mitochondria.
   • b. Takes up acetyl group (acetyl CoA) from transition reaction and oxidizes it to two CO₂ molecules.
   • c. During the process, most of the electrons are accepted by NAD⁺ but in one instance they are taken by FAD.
7. The Electron Transport System:
   • a. Is located on cristae (projections of inner membrane).
   • b. Consists of a series of carriers that pass electrons; some of protein carriers are cytochrome molecules so system is also called the cytochrome system.
   • c. Accounts for most of the ATP produced.
   • d. When NADH gives up electrons, it becomes NAD⁺ while an electron carrier gains electrons and is reduced.
   • e. Each sequential carrier becomes reduced and then oxidized as electrons move down the system.
   • f. As electrons pass from carrier to carrier, energy is released and used to form ATP molecules.
   • g. When NADH delivers electrons to the first carrier, enough energy is released by time electrons reach the O₂ to produce three ATPs.
   • h. When FADH₂ delivers electrons to electron transport system, only two ATPs result.
8. The Cristae:
   • a. Contain carriers of electron transport system arranged in a functional manner.
   • b. The carriers use released energy to pump the surplus hydrogen ions carried by NADH and FADH2 into intermembrane space of mitochondria.
   • c. Cristae also contain ATP synthase complex:
     • i. Hydrogen ions flow from high to low concentration from intermembrane space across to matrix.
     • ii. Resulting H⁺ flow drive enzyme ATP synthase to synthesize ATP from ADP + (P) .
     • iii. Process is called chemiosmosis because ATP production is tied to an electrochemical gradient.
9. Calculating Energy Yield from Glucose Metabolism
   • a. Per glucose, four ATP are formed by substrate-level phosphorylation, two during glycolysis and two during two turns of Krebs cycle.
   • b. Per glucose, ten NADH and two FADH₂ take electrons to the electron transport system.
   • c. For each NADH formed inside the mitochondria by Krebs cycle, three ATP result; for each FADH₂, only two ATP are produced.
   • d. The glycolytic pathway outside the mitochondria produces only two ATP when the electrons are shuttled to the electron transport system inside the mitochondrion.
10. How Efficient is Aerobic Respiration?
    • a. Difference in energy content between glucose and O₂, and products CO₂ and H₂O is 686 kilocalories.
    • b. The ATP third phosphate bond has energy content of 7.3 kilocalories, 36 ATP are produced per glucose breakdown totaling 263 kilocalories.
    • c. Efficiency is 263/686 or 39%.
    • d. Sixty-one percent is lost as heat; in birds and mammals, this heat assists in maintaining body temperature.

4. Metabolic Pool and Biosynthesis

1. Catabolic Reactions
   • a. Catabolic reactions break down molecules, driving anabolic reactions which synthesize molecules.
   • b. Aerobic cellular respiration has already shown the catabolism of glucose.
   • c. Fats are broken down:
     • i. Glycerol is converted to PGAL in glycolytic pathway.
     • ii. Fatty acids are converted to acetyl-CoA, which enters the Krebs cycle.
     • iii. Fats are an efficient form of stored energy, an 18-carbon fatty acid results in nine acetyl-CoA molecules that produce 216 ATP molecules via respiration.
   • d. Amino acids are broken down:
     • i. Amino acids must undergo deamination (removal of amino group; this occurs in the liver).
     • ii. The amino group becomes ammonia (NH₃) which becomes urea via the urea cycle.
     • iii. The carbon skeleton produced by deamination can then enter Krebs cycle depending on the number of carbons left after deamination.
2. Anabolic Reactions
   • a. Anabolic reactions use ATP produced during catabolism to synthesize molecules.
   • b. The cell's metabolic pool consists of large molecules that can be converted to other molecules without buildup from small molecular units; for example, PGAL can be converted to glycerol and acetyl groups can be joined to form fatty acids.
   • c. Some metabolites are converted to amino acids through transamination, transfer of an amino group from one amino acid to another.
   • d. Plants can synthesize all the amino acids they need.
   • e. Animals lack some enzymes to synthesize some amino acids; therefore must secure these from diet; lack of sufficient amino acids in diet results in protein deficiency disease.
   • f. Humans can synthesize 11 of 20 common, necessary amino acids; therefore must secure other nine from diet.
   • g. All the reactions involved in cellular respiration are part of a metabolic pool in which one type of molecule can be converted to another in catabolism or anabolism.

5. Fermentation

1. Cellular respiration includes both aerobic cellular respiration and fermentation.
2. Fermentation is a series of enzymatic reactions where glucose is incompletely metabolized into lactate or CO₂ and alcohol.
3. Fermentation is anaerobic; it does not require O₂.
4. During fermentation, there is a net gain of only two ATPs..
5. Fermentation consists of glycolysis plus the reduction of pyruvate.
   • a. Pathway is anaerobic because after NADH transfers its electrons to pyruvate, it is free to return and pick up more electrons during earlier glycolysis reactions.
   • b. Lactic acid bacteria reduce pyruvate to lactic acid; other bacteria produce important industrial chemicals: isopropanol, butyric acid, propionic acid, and acetic acid.
   • c. Yeasts represent organisms that reduce pyruvate to alcohol and carbon dioxide.
   • d. Animals reduce pyruvate to lactate when pyruvate is produced faster than it can be oxidized.
6. Advantages and Disadvantages of Fermentation
   • a. In spite of low yield and toxicity, fermentation provides rapid burst of ATP.
   • b. In muscle cells of animals, fermentation provides two ATP molecules while oxygen is temporarily in limited supply while exercising.
   • c. When blood cannot carry away lactate fast enough, lactate builds up in muscles, changes the pH, and causes muscle fatigue and preventing contraction.
   • d. When we stop exercising, we are in oxygen debt and continue breathing heavily until lactate is transported to the liver to be reconverted to pyruvate, or pyruvate is respired completely.
   • e. Efficiency of Fermentation
     • i. Energy content of last ATP bond is 7.3 kilocalories and two are produced per glucose fermentation for a total of 14.6 kilocalories.
     • ii. Complete glucose breakdown to CO₂ and H₂O yields 686 kilocalories.
     • iii. Efficiency of fermentation is therefore 14.6/686 or 2.1%. (Table 7.10)