Cellular Respiration in Humans

Cellular respiration is a complex biochemical process in which cells break down organic compounds, mainly glucose, to release energy in the form of ATP (adenosine triphosphate). In humans, this process occurs primarily in the mitochondria and is essential for all life activities, including muscle contraction, nerve impulse transmission, growth, and repair.

Definition

Cellular respiration is the oxidation of food substances within living cells to release energy, carbon dioxide, and water.

Overall Chemical Equation

C₆H₁₂O₆+6O₂→6CO₂+6H₂O+Energy(ATP)

• Glucose = Energy source
• Oxygen = Oxidizing agent
• Carbon dioxide and water = End products
• ATP = Usable cellular energy

Types of Respiration

1. Aerobic Respiration

• Occurs in the presence of oxygen.
• Complete oxidation of glucose.
• Produces a large amount of energy (about 36–38 ATP molecules per glucose molecule).

2. Anaerobic Respiration

• Occurs in the absence of oxygen.
• Partial breakdown of glucose.
• Produces only 2 ATP molecules.
• In humans, it occurs temporarily in muscle cells during vigorous exercise.

Example:

C6​H12​O6​→2C3​H6​O3​+2ATP

Glucose → Lactic acid + 2 ATP

Stages of Cellular Respiration

Cellular respiration occurs in three major stages:

1. Glycolysis
2. Krebs Cycle (Citric Acid Cycle/TCA Cycle)
3. Electron Transport Chain (ETC) and Oxidative Phosphorylation

Glycolysis

Location

• Cytoplasm of the cell.

Meaning

• “Glyco” = Sugar
• “Lysis” = Splitting

Process

1. One molecule of glucose (6-carbon compound) is broken down into two molecules of pyruvate (3-carbon compound).
2. A series of enzyme-catalyzed reactions occurs.
3. Oxygen is not required.

Products of Glycolysis

• 2 molecules of pyruvate
• Net gain of 2 ATP
• 2 NADH molecules

Significance

• First step of respiration.
• Common to both aerobic and anaerobic respiration.

Krebs Cycle (Citric Acid Cycle/TCA Cycle)

Discovery

Discovered by Hans Krebs.

Location

• Mitochondrial matrix.

Process

1. Pyruvate enters mitochondria.
2. Converted into Acetyl-CoA.
3. Acetyl-CoA combines with oxaloacetic acid to form citric acid.
4. Through a series of reactions, carbon dioxide is released, and energy-rich molecules are formed.

Products (per glucose molecule)

• 4 CO₂
• 2 ATP
• 6 NADH
• 2 FADH₂

Importance

• Produces electron carriers (NADH and FADH₂) for the next stage.

Electron Transport Chain (ETC) and Oxidative Phosphorylation

Location

• Inner mitochondrial membrane (cristae).

Process

1. NADH and FADH₂ donate electrons.
2. Electrons pass through a chain of protein complexes.
3. Energy released is used to pump protons (H⁺).
4. Oxygen acts as the final electron acceptor.
5. Water is formed.
6. ATP synthase produces ATP.

Products

• Water (H₂O)
• Approximately 32–34 ATP molecules

Importance

• Produces the majority of ATP during respiration.

Role of Mitochondria

Mitochondria are known as the “powerhouses of the cell” because:

• They carry out aerobic respiration.
• They generate most of the ATP needed by cells.
• They contain enzymes required for the Krebs cycle and ETC.

ATP: Energy Currency of the Cell

ATP stores and transfers energy for cellular activities.

Structure:

• Adenine
• Ribose sugar
• Three phosphate groups

Energy is released when ATP breaks down into ADP and inorganic phosphate.

Energy Yield from One Glucose Molecule

Significance of Cellular Respiration

1. Provides energy for all cellular activities.
2. Supports muscle contraction and movement.
3. Helps in the growth and repair of tissues.
4. Maintains body temperature.
5. Supplies ATP for active transport and biosynthesis.
6. Essential for the survival of human cells.

Flow Chart

Glucose
↓
Glycolysis (Cytoplasm)
↓
Pyruvate
↓
Acetyl-CoA
↓
Krebs Cycle (Mitochondria)
↓
NADH + FADH₂
↓
Electron Transport Chain
↓
ATP + H₂O + CO₂

Cellular respiration is the process by which glucose is oxidized inside cells in the presence of oxygen to release energy in the form of ATP, producing carbon dioxide and water as by-products.

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