Reviewer in Biology by Ms Perez

CELLULAR RESPIRATION
Prepared by: Ms. Kinsley Meg G. Perez

Figure 1. Energy flow and chemical recycling involved in photosynthesis and cellular respiration.

Cellular respiration * a process by which energy from food is converted into chemical energy of ATP. * two types of cellular respiration are aerobic and anaerobic respiration.
Table 1. Comparison between aerobic and anerobic respiration. Type of respirationCharacteristics | Aerobic | Anaerobic | Requires oxygen | YES | NO | Catabolic process | COMPLETE breakdown of glucose into 6 molecules of carbon dioxide | INCOMPLETE breakdown of food into organic molecules like ethanol or lactic acid | Final electron acceptor/s | Oxygen and water | AcetaldehydePyruvic acid | Product /s | Water | Ethyl alcohol (ethanol)Lactic acid | Processes involved | Krebs cycleElectron Transport Chain | Fermentation (occur in cytosol) |

Table 2. Comparison between alcoholic and lactic acid fermentation. Characteristics | Alcoholic | Lactic acid | Organisms involved | Yeasts | Bacteria, Humans (muscles) | Mechanism | Pyruvate (from glycolysis) is broken into ethanol and carbon dioxide | Pyruvate (from glycolysis) is fermented into lactic acid. It also occurs in animal muscles when the tissue requires energy at a faster rate than oxygen can be supplied | Net Chemical Equation | C6H12O6 (glucose) → 2 C2H5OH (ethanol) + 2 CO2 (carbon dioxide) | C6H12O6 (glucose) → 2 CH3CHOHCOOH (lactic acid) | Economic importance | Beer, wine and bread production | Yogurt production | Table 3. Metabolic processes involved in cellular respiration and the specific region of the cell where they occur. Metabolic processes in Cellular Respiration | Region of the cell where it occurs | Glycolysis | cytosol | Krebs cycle/ Citric acid cycle/Tricarboxylic acid cycle | matrix of the mitochondria | Oxidative phosphorylation (via the electon transport chain) | inner mitochondrial membrane |

Figure 1. An overview of cellular respiration.
Glycolysis
* means "splitting sugars" * Glucose (a six carbon sugar) is split into two molecules of a pyruvate (a three carbon sugar). * Net gain: 2 ATP molecules, 2 NADH molecules, 2 molecules of pyruvate.
*Pyruvate and pyruvic acid are the same in other books although according to Campbell (2008), pyruvate is the ionized form of pyruvic acid.
*NADH- "high energy" electron carrying molecules * Glycolysis generates 2 ATP whether oxygen is present (aerobic) or not (anaerobic). * In the presence of oxygen (aerobic), glycolysis is the first stage of cellular respiration. * Without oxygen (anerobic), glycolysis allows cells to make small amounts of ATP in the process known as fermentation.

Figure 2. Summary of glycolysis.
Krebs cycle * begins when 2 molecules of pyruvate from glycolysis is transported into the mitochondria and loses carbon dioxide to form acetyl-CoA, a 2-carbon molecule (Refer to Figure 3). * through a series of intermediate steps, several compounds capable of storing "high energy" electrons are produced along with two ATP molecules. These compounds are: Nicotinamide Adenine Dinucleotide (NAD) & Flavin Adenine Dinucleotide (FAD) which are reduced (gained electrons) in the process. * when acetyl-CoA is oxidized (loss electrons) to carbon dioxide in the Krebs cycle, chemical energy is released and captured in the form of NADH, FADH2, and ATP. * occurs in the mitochondrial matrix and generates a pool of chemical energy (ATP, NADH, and FADH2) from the oxidation of pyruvate, the end product of glycolysis. * it occurs only when oxygen is present (aerobic) but it doesn 't use oxygen directly.

Figure 3. Conversion of pyruvate to acetyl-CoA, the junction between glycolysis and Krebs cycle.

Figure 4. An overview of Krebs cycle.

Oxidative Phosphorylation via the Electron Transport Chain

Figure 5. ETC which takes place in the inner membrane of mitochondrion.

* the electron transport chain allows the release of the large amount of chemical energy stored in reduced NAD+ (NADH) and reduced FAD (FADH2). * the energy released is captured in the form of ATP (3 ATP per NADH and 2 ATP per FADH2).
NADH + H+ + 3 ADP + 3 Pi + 1/2 O2 → NAD+ + H2O + 3 ATP
FADH2 + 2 ADP + 2 Pi + 1/2 O2 → FAD+ + H2O + 2 ATP * the electron transport chain (ETC) consists of a series of molecules, mostly proteins, embedded in the inner mitochondrial membrane.

Figure 6. A closer look of the electron transport chain.
Maximum ATP Yields
Table 3.Maximum ATP Yields in a prokaryotic and a eukaryotic cell. Type of cell | Maximum number of ATP molecules produced | Prokaryotic | 38 | Eukaryotic | 36 |
Why 36 only in eukaryotic cells?
It is because the NADH molecules produced in glycolysis pass through the mitochondrial membrane, which "costs" two ATP molecules. 2 ATP molecules subtracted to 38 ATP molecules will yield to 36 ATP molecules only.

Figure. ATP yield per molecule of glucose at each stage of cellular respiration.

References:
Campbell, N.A. and Reece, J.B. 2008.Biology.8th ed. California:Benjamin Cummings Pub. pp. 162-183.
Joaquin, C.C., Lagunzad, C.G.B., and Rabago, L.M. 2005.Functional Biology:Modular Approach. Philippines: Vibal Publishing House Inc.pp.221-230
Bailey, R. (2013). Cellular Respiration. Retrieved August 15, 2013, from http://biology.about.com/od/cellularprocesses/a/cellrespiration.htm. Helmenstine Ph.D., A.M. (2013). What is Fermentation?.Retrieved August 15, 2013, from http://chemistry.about.com/od/lecturenoteslab1/f/What-Is-Fermentation.htm.
Pearson Education Inc. (n.d.) Cellular Respiration Overview. Retrieved August 15, 2013, from http://www.phschool.com/science/biology_place/biocoach/cellresp/overview.html.

References: Campbell, N.A. and Reece, J.B. 2008.Biology.8th ed. California:Benjamin Cummings Pub. pp. 162-183. Joaquin, C.C., Lagunzad, C.G.B., and Rabago, L.M. 2005.Functional Biology:Modular Approach. Philippines: Vibal Publishing House Inc.pp.221-230 Bailey, R. (2013). Cellular Respiration. Retrieved August 15, 2013, from http://biology.about.com/od/cellularprocesses/a/cellrespiration.htm. Helmenstine Ph.D., A.M. (2013). What is Fermentation?.Retrieved August 15, 2013, from http://chemistry.about.com/od/lecturenoteslab1/f/What-Is-Fermentation.htm. Pearson Education Inc. (n.d.) Cellular Respiration Overview. Retrieved August 15, 2013, from http://www.phschool.com/science/biology_place/biocoach/cellresp/overview.html.