Lesson 12: Mechanisms of Cellular Respiration and Anaerobic Respiration

1.Video Lesson

2.Objective

At the end of this lesson, you will be able to:-

Compare and contrast aerobic and anaerobic respiration.

State the products of alcoholic fermentation by yeast.

Show the mechanism electron transport system in mitochondria.

Explain the difference between substrate-level phosphorylation and oxidative level phosphorylation

Brainstorming questions

How does the electrochemical gradient generated by the electron transport chain influence ATP production efficiency, and what strategies could enhance this process in various organisms?
In what ways can the ability to utilize non-carbohydrate sources (such as fats and proteins) for cellular respiration impact metabolic flexibility and overall energy homeostasis in different species?

Key words

Electron Transport Chain (ETC): A series of protein complexes and other molecules embedded in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, creating an electrochemical gradient.
Proton Pump: Protein complexes within the ETC that actively transport protons from the mitochondrial matrix to the intermembrane space, creating an electrochemical gradient across the inner mitochondrial membrane.
Electrochemical Gradient: The difference in charge and pH across the inner mitochondrial membrane, created by the accumulation of protons in the intermembrane space.
ATP Synthase: An enzyme complex that uses the energy from the proton gradient to synthesize ATP from ADP and inorganic phosphate.
ATP Production Sites: Locations in the electron transport chain where energy from electron transfer is used to pump protons and drive ATP synthesis.
Glucose: A six-carbon sugar that is broken down through glycolysis, leading to the production of two pyruvate molecules.
Pyruvate: A three-carbon molecule produced from glucose during glycolysis. It is further processed in the mitochondria during aerobic respiration.
NADH and FADH2: Electron carriers produced during glycolysis, the Krebs cycle, and beta-oxidation. They donate electrons to the ETC to drive ATP production.
Krebs Cycle (Citric Acid Cycle): A series of reactions in the mitochondrial matrix that oxidizes acetyl-CoA to produce NADH, FADH2, ATP, and CO2.
Total Yield: From one glucose molecule, cellular respiration yields 6 NADH, 2 FADH2, and 2 ATP.
Beta-Oxidation: The process of breaking down fatty acids into two-carbon fragments that enter the Krebs cycle as acetyl-CoA.
Fermentation: An anaerobic process that regenerates NAD+ from NADH + H+ to allow glycolysis to continue when oxygen is unavailable.
Alcohol Fermentation: A type of fermentation where pyruvate is converted to ethanol and CO2, with NADH being oxidized to NAD+.
Lactic Acid Fermentation: A type of fermentation where pyruvate is converted to lactic acid (lactate), regenerating NAD+ in the process.
Facultative Anaerobes: Organisms that can perform fermentation in the absence of oxygen. Yeast is an example of a facultative anaerobe.
Lactic Acid: Produced during lactic acid fermentation, particularly in muscle cells under low oxygen conditions. It is used to regenerate NAD+ and is a byproduct of the process.

Mechanisms

The mechanism involves electron transfer through the electron transport chain, which causes protons to be pumped from the mitochondrial matrix to the intermembrane space at three ATP production sites, acting as a proton pump. This results in an electrochemical potential difference across the inner mitochondrial membrane. The electrical potential difference arises from the accumulation of positively charged hydrogen ions outside the membrane, while the chemical potential difference is due to the lower pH (more acidic conditions) outside the membrane. This electrochemical gradient drives ATP synthase to produce ATP from ADP and inorganic phosphate. From one glucose molecule, two pyruvate molecules are formed, leading to two cycles for complete breakdown. The total yield from this process is 6 NADH, 2 FADH2, and 2 ATP.

Energy can be derived from non-carbohydrate sources such as fats, proteins, sucrose, other disaccharides, and starch. All these organic molecules in food can be utilized by cellular respiration to produce ATP. Glycolysis can metabolize a variety of carbohydrates. In the digestive tract, starch is broken down into glucose, which is then processed in cells through glycolysis and the citric acid cycle. Similarly, glycogen, the polysaccharide stored in the liver and muscle cells of humans and many animals, can be converted to glucose between meals to fuel respiration. The digestion of disaccharides, like sucrose, also provides glucose and other sugars.

Proteins can be used for energy after being digested into amino acids. While many amino acids are used to build new proteins, excess amino acids are converted by enzymes into intermediates of glycolysis and the citric acid cycle. Before amino acids can enter these pathways, their amino groups must be removed through deamination, with the nitrogenous waste excreted as ammonia (NH3), urea, or other waste products.

Energy can also be harvested from fats obtained from food or body storage cells. Once fats are digested into glycerol and fatty acids, glycerol is converted into glyceraldehyde 3-phosphate, an intermediate of glycolysis. Most of the energy in fats is stored in the fatty acids, which are broken down through beta-oxidation into two-carbon fragments that enter the citric acid cycle as acetyl CoA. Beta-oxidation also generates NADH and FADH2, which feed into the electron transport chain to produce more ATP. Fats are excellent fuels because of their chemical structure and the high energy level of their electrons. A gram of fat oxidized by respiration yields more than twice the ATP as a gram of carbohydrate. Consequently, losing weight can be challenging since each gram of fat stores a large amount of energy.

The idea of net gain ATP like the profit of a business person makes. It invests in money materials, advertising and building the capacity of staff. He/ She sell his/her product where the extra money is profit – net gain. In a similar vein, glycolysis ‘invests’ in two molecules of ATP to make the glucose reactive, then, later, produces four molecules of ATP – a net gain of two molecules of ATP.

2. There are two molecules of pyruvate made from each molecule of glucose. So, all the gains of ATP and the reduced NAD and reduced FAD that accrue from each pyruvate must be doubled to give the gain from each molecule of glucose.

Anaerobic respiration

Fermentation

When oxygen is absent, pyruvate undergoes fermentation. During fermentation, the NADH + H+ produced in glycolysis is recycled back to NAD+ to allow glycolysis to continue. In glycolysis, NAD+ is reduced to form NADH + H+. Without NAD+, glycolysis cannot proceed. In aerobic respiration, NADH formed during glycolysis is oxidized to regenerate NAD+ for reuse in glycolysis. However, in the absence of oxygen or when an organism cannot perform aerobic respiration, pyruvate undergoes fermentation, which is anaerobic as it does not require oxygen. Fermentation replenishes NAD+ from NADH + H+ produced in glycolysis. One type of fermentation is alcohol fermentation. In this process, pyruvate is first decarboxylated (releasing CO2) to form acetaldehyde. Hydrogen atoms from NADH + H+ then convert acetaldehyde to ethanol, regenerating NAD+. Facultative anaerobes are organisms that can perform fermentation when oxygen is unavailable. Yeast is an example of a facultative anaerobe that undergoes alcohol fermentation.

Lactic acid fermentation

Lactic Acid Fermentation is the process in which pyruvate molecules are converted to lactic acid in the muscle cells of humans and in bacterial cells.

In lactic acid fermentation, pyruvate from glycolysis is used to oxidize NADH back to NAD+. This process produces lactic acid, or lactate, as a byproduct. Many animals and some bacteria can perform lactic acid fermentation. Animals use this process to regenerate NAD+ when oxygen is scarce. While anaerobic respiration does not produce enough ATP to sustain the entire organism, it can supplement ATP levels in tissues, such as muscles, where oxygen may deplete rapidly. Bacterial lactic acid fermentation products have been utilized by humans to make foods like yogurt, sour cream, and buttermilk.