Cellular Respiration: Mechanisms and Pathways

Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells extract energy from organic molecules. It includes both aerobic (with oxygen) and anaerobic (without oxygen) respiration. The process is essential for generating ATP, the energy currency of the cell, and occurs in all living organisms.

• Aerobic respiration uses oxygen as the final electron acceptor.

• Anaerobic respiration and fermentation occur in the absence of oxygen.

• Carbohydrates, fats, and proteins can all serve as fuel for cellular respiration.

General equation for aerobic respiration (using glucose):

$\mathrm{C_6H_{12}O_6} + 6\ \mathrm{O_2} \rightarrow 6\ \mathrm{CO_2} + 6\ \mathrm{H_2O} + \text{Energy (ATP + heat)}$

Role in Ecosystems

Cellular respiration and photosynthesis are interconnected processes that drive the flow of energy in ecosystems.

• Photosynthesis in chloroplasts converts light energy into chemical energy, producing organic molecules and oxygen.

• Cellular respiration in mitochondria breaks down organic molecules, releasing energy for cellular work and producing carbon dioxide and water.

• ATP generated by respiration powers most cellular activities.

Example: Plants produce glucose and oxygen via photosynthesis; animals consume these and release CO2 and H2O via respiration.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Cellular respiration involves a series of redox reactions, where electrons are transferred between molecules, releasing energy.

• Oxidation: Loss of electrons or hydrogen atoms (the molecule becomes oxidized).

• Reduction: Gain of electrons or hydrogen atoms (the molecule becomes reduced).

• Redox reactions are coupled; one molecule is oxidized while another is reduced.

Example: In the reaction between sodium and chlorine: Na loses an electron (oxidized), Cl gains an electron (reduced).

Oxidation of Organic Fuel Molecules

During cellular respiration, organic molecules such as glucose are oxidized, and oxygen is reduced.

• Glucose is oxidized to carbon dioxide.

• Oxygen is reduced to water.

• Energy released is used to synthesize ATP.

Stages of Cellular Respiration

Major Stages

Cellular respiration occurs in three main stages:

1. Glycolysis: Breaks down glucose into two molecules of pyruvate.

2. Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose, generating electron carriers.

3. Oxidative Phosphorylation: Uses electron transport chain and chemiosmosis to produce most ATP.

Electron Carriers

Electrons from organic compounds are transferred to electron carriers such as NAD+ and FAD, which shuttle electrons to the electron transport chain.

• NAD+ is reduced to NADH by dehydrogenase enzymes.

• FAD is reduced to FADH2.

Glycolysis

Process and Steps

Glycolysis is the first stage of cellular respiration, occurring in the cytosol. It converts one molecule of glucose into two molecules of pyruvate, producing ATP and NADH.

• Consists of 10 enzyme-catalyzed steps.

• Divided into two phases: Energy Investment Phase and Energy Payoff Phase.

Energy Investment Phase

• 2 ATP are used to phosphorylate glucose and its intermediates.

Energy Payoff Phase

• 4 ATP are produced by substrate-level phosphorylation.

• 2 NADH are produced by reduction of NAD+.

• Net gain: 2 ATP and 2 NADH per glucose.

Overall glycolysis equation:

$\text{Glucose} + 2\ \text{NAD}^+ + 2\ \text{ADP} + 2\ \text{P}_i \rightarrow 2\ \text{Pyruvate} + 2\ \text{NADH} + 2\ \text{ATP} + 2\ \text{H}_2\text{O}$

Pyruvate Oxidation (Transition Reaction)

Conversion to Acetyl CoA

Before entering the citric acid cycle, pyruvate is transported into the mitochondrion and converted to acetyl coenzyme A (acetyl CoA).

• Pyruvate dehydrogenase catalyzes three reactions:

  • Oxidation of pyruvate's carbonyl group, releasing CO2.

  • Reduction of NAD+ to NADH.

  • Combination of the remaining two-carbon fragment with coenzyme A to form acetyl CoA.

Equation:

$\text{Pyruvate} + \text{NAD}^+ + \text{CoA} \rightarrow \text{Acetyl CoA} + \text{NADH} + \text{CO}_2$

Citric Acid Cycle (Krebs Cycle)

Cycle Overview

The citric acid cycle completes the oxidation of organic molecules, generating electron carriers and ATP.

• Occurs in the mitochondrial matrix.

• Each turn produces: 1 ATP, 3 NADH, 1 FADH2, and 2 CO2 per acetyl CoA.

• Since two pyruvate are produced per glucose, the cycle runs twice per glucose molecule.

• Eight steps, each catalyzed by a specific enzyme.

• Acetyl group of acetyl CoA combines with oxaloacetate to form citrate; subsequent steps regenerate oxaloacetate.

Electron carriers (NADH and FADH2) produced by the cycle carry electrons to the electron transport chain.

Oxidative Phosphorylation

Electron Transport Chain and Chemiosmosis

Oxidative phosphorylation is the final stage of cellular respiration, occurring in the inner mitochondrial membrane. It couples electron transport to ATP synthesis via chemiosmosis.

• NADH and FADH2 donate electrons to the electron transport chain.

• Energy released as electrons move down the chain is used to pump H+ ions into the intermembrane space, creating a proton gradient.

• H+ flows back into the mitochondrial matrix through ATP synthase, driving ATP production.

ATP yield per glucose: About 30-32 ATP (actual yield may vary).

Fermentation and Anaerobic Respiration

ATP Production Without Oxygen

When oxygen is absent, cells can produce ATP via fermentation or anaerobic respiration.

• Fermentation is an anaerobic process that does not involve the Krebs cycle or electron transport chain.

• NADH is oxidized back to NAD+ by transferring electrons to organic molecules, forming acids or alcohols.

• Two common types: Alcohol fermentation and Lactic acid fermentation.

Alcohol Fermentation

• Pyruvate is converted to ethanol and CO2.

• Occurs in yeast and some bacteria.

Equation:

$\text{Glucose} \rightarrow 2\ \text{Ethanol} + 2\ \text{CO}_2 + 2\ \text{ATP}$

Lactic Acid Fermentation

• Pyruvate is reduced to lactic acid.

• Occurs in muscle cells and some bacteria.

Equation:

$\text{Glucose} \rightarrow 2\ \text{Lactic Acid} + 2\ \text{ATP}$

Versatility of Catabolism

Other Fuels in Cellular Respiration

Cells can metabolize carbohydrates, fats, and proteins through various pathways to generate ATP.

• Carbohydrates are converted to glucose or other sugars and enter glycolysis.

• Proteins are digested to amino acids, deaminated, and their carbon skeletons enter glycolysis or the Krebs cycle.

• Fats are broken down into glycerol (enters glycolysis) and fatty acids (converted to acetyl CoA via beta oxidation).

• Oxidation of fat produces more than twice as much ATP per gram as carbohydrates.

Table: Comparison of Aerobic Respiration, Anaerobic Respiration, and Fermentation

Process	Oxygen Required?	Final Electron Acceptor	ATP Yield (per glucose)	Main Products
Aerobic Respiration	Yes	O2	~30-32	CO2, H2O, ATP
Anaerobic Respiration	No	Alternate (e.g., SO42-, NO3-)	Varies (less than aerobic)	CO2, reduced compounds, ATP
Fermentation	No	Organic molecule (e.g., pyruvate, acetaldehyde)	2	Lactic acid or ethanol, ATP

Additional info: The notes include diagrams of mitochondria and ecosystem energy flow, which reinforce the role of cellular respiration in energy transformation and ATP production.