Classwork Series and Exercises {Biology- SS2}: Cellular Respiration

Definition of Cellular Respiration

Cellular respiration refers to the biochemical pathway by which cells release energy from the chemical bonds of food molecules and provide that energy for the essential processes of life. It can also be defined as the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy in the process. Cellular respiration allows organisms to use (release) energy stored in the chemical bonds of glucose (C₆H₁₂O₆). The energy in glucose is used to produce ATP. Cells use ATP to supply their energy needs. Cellular respiration is therefore a process in which the energy in glucose is transferred to ATP. In respiration, glucose is oxidized and thus releases energy. Oxygen is reduced to form water.The carbon atoms of the sugar molecule are released as carbon dioxide (CO₂).The complete breakdown of glucose to carbon dioxide and water requires two major steps: Glycolysis and Aerobic respiration. Glycolysis produces two ATP. Thirty-four more ATP are  produced by aerobic pathways if oxygen is present. In the absence of oxygen, fermentation reactions produce alcohol or lactic acid but no additional ATP. Cellular Respiration is divided into two series of biochemical reactions: anaerobic and aerobic reactions.

Process of an aerobic respiration

Aerobic respiration – the process by which a cell uses oxygen to “burn” molecules and release energy

This reaction takes place over the course of four major reaction pathways

• Glycolysis
• Conversion of Pyruvate to Acetyl CoA (Oxidation of Pyruvate or Pyruvate Processing)
• The Krebs Cycle
• Electron Transport Phosphorylation (chemiosmosis)

Glycolysis (glyco = sugar; lysis = breaking): is a metabolic pathway that takes place in the cytoplasm (cytosol) of cells in all living organisms through which glucose is broken to form two pyruvates, with or without oxygen. In aerobic conditions, the process converts one molecule of glucose into two molecules of pyruvate (pyruvic acid), generating energy in the form of two net molecules of ATP. Four molecules of ATP per glucose are actually produced, however, two are consumed as part of the preparatory phase. The initial phosphorylation of glucose is required to increase the reactivity in order for the molecule to be cleaved into two pyruvate molecules by the enzyme aldolase. The overall reaction can be expressed this way:

Glucose + 2 NAD⁺ + 2 P_i + 2 ADP → 2 pyruvate + 2 NADH + 2 ATP + 2 H⁺ + 2 H₂O + heat

Starting with glucose, 1 ATP is used to donate a phosphate to glucose to produce glucose 6-phosphate. During energy metabolism, glucose 6-phosphate becomes fructose 6-phosphate. An additional ATP is used to phosphorylate fructose 6-phosphate into fructose 1,6-disphosphate by the help of phosphofructokinase. Fructose 1,6-diphosphate then splits into two phosphorylated molecules with three carbon chains which later degrades into pyruvate.

 Conversion of Pyruvate to Acetyl CoA

Pyruvate produced by glycolysis enters the mitochondrion by active transport and is oxidized to acetyl- CoA (two-carbon compound)   and CO₂ by the pyruvate dehydrogenase complex (PDC). In the process  Coenzyme A is attached to each of the two-carbon compounds producing two acetyl CoA molecules. In the conversion of pyruvate to acetyl-CoA, one molecule of NADH and one molecule of CO₂ is formed.

The Krebs Cycle

This is also called the Citric acid cycle or the tricarboxylic acid cycle. When oxygen is present, acetyl-CoA is produced from the pyruvate molecules created from glycolysis. When oxygen is present, the mitochondria will undergo aerobic respiration which leads to the Krebs cycle. However, if oxygen is not present, fermentation of the pyruvate molecule will occur. In the presence of oxygen, when acetyl-CoA is produced, the molecule then enters the citric acid cycle (Krebs cycle) inside the mitochondrial matrix, and gets oxidized to CO₂ while at the same time reducing NAD to NADH. NADH can be used by the electron transport chain to create further ATP as part of oxidative phosphorylation. Two waste products, H₂O and CO₂, are created during this cycle.

The citric acid cycle is an 8-step process involving different enzymes and co-enzymes. During the cycle, acetyl-CoA (2 carbons) + oxaloacetate (4 carbons) yields citrate (6 carbons), which is rearranged to a more reactive form called isocitrate (6 carbons). Isocitrate is modified to become α-ketoglutarate (5 carbons), succinyl-CoA, succinate, fumarate, malate, and, finally, oxaloacetate. The net gain of high-energy compounds from one cycle is 3 NADH, 1 FADH₂, and 1 GTP; the GTP may subsequently be used to produce ATP. Thus, the total yield from 1 glucose molecule (2 pyruvate molecules) is 6 NADH, 2 FADH₂, and 2 ATP.

DIAGRAM OF CITRIC ACID CYCLE

Net Energy Production from Aerobic Respiration

• Glycolysis produces 2 ATP
• Krebs Cycle produces 2 ATP
• Electron Transport Phosphorylation produces 32 ATP
• Net Energy Production is 36 ATP (38 ATP for Plants).

Difference between Aerobic and Anaerobic Respiration

             Aerobic Respiration                                              Anaerobic Respiration                          

 Aerobic respiration uses oxygen                               Anaerobic respiration is respiration without oxygen

Aerobic respiration occurs in most cells.                 Occurs in many anaerobic bacteria and human muscle cells.

CO₂ and water is produced                                         Lactic Acid or Alcohol is produced

lot of energy is liberated (36-38 ATP)                       Relatively small energy is liberated (2ATP)

C₆H₁₂O₆ –> CO₂ + H₂O + ATP (Energy)                C₆H₁₂O₆ –> Lactic acid / C₂H₅OH + ATP

QUESTIONS

Lets see how much you’ve learnt, attach the following answers to the comment below:

1. The end product of glycolysis is _______________ (a) NADH (b) Acetyl-CoA (c) Lactate (d) Pyruvate
2. The final output of the Krebs cycle includes all of the following except (a) NADP (b) FADH₂ (c) ATP (d) CO₂
3. What substance is produced by the oxidation of Pyruvate and feeds into the citric acid cycle? (a) Pyruvate (b) Acety-CoA (c) O₂ (d) Glucose
4. What role does O₂ play in aerobic respiration? (a) it plays no role (b) it combines with acetyl-CoA at the start of the Krebs cycle (c) it combines with H₂O to help drive the formation of ATP (d) it is the final electron acceptor at the end of the electron transport chain
5. During what stage of cellular respiration is the most ATP synthesized? (a) Glycolysis (b) Krebs cycle (c) Fermentation (d) Chemiosmosis

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