Cellular Respiration

Introduction to Cellular Respiration

Cellular respiration is the process by which living cells obtain energy from organic molecules, primarily glucose. This energy is essential for various cellular activities, including transport, synthesis, and movement.

• Cellular respiration converts nutrients into chemical energy (ATP).

• ATP (Adenosine Tri-Phosphate) is the main energy currency of the cell.

• Energy is released when the bonds between phosphate groups in ATP are broken.

Equation for ATP hydrolysis:

Energy Flow in Biological Systems

Energy from food is transformed into ATP, which powers cellular functions. Photosynthesis in plants produces glucose and oxygen, which are then used in cellular respiration by animals and plants.

• Photosynthesis: Converts light energy to chemical energy (glucose).

• Cellular respiration: Converts glucose and oxygen into ATP, carbon dioxide, and water.

Overall equation for cellular respiration:

Overview of Metabolism and Cellular Respiration

Metabolic Pathways

Metabolic pathways are sequences of chemical reactions occurring within a cell, each step catalyzed by a specific enzyme. These pathways are classified as either catabolic or anabolic.

• Catabolic pathways: Break down cellular components, releasing energy (exergonic).

• Anabolic pathways: Synthesize cellular components, requiring energy input (endergonic).

Example of a metabolic pathway:

• Substrate → Intermediate 1 (via Enzyme 1) → Intermediate 2 (via Enzyme 2) → Final product (via Enzyme 3)

Catabolic reactions recycle intermediates like ATP and NADH, while anabolic reactions use these intermediates to build macromolecules.

Redox Reactions in Metabolism

Redox (reduction-oxidation) reactions are essential for energy transfer in cells. Electrons are removed from one molecule (oxidation) and added to another (reduction).

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• NAD+: An important electron carrier, reduced to NADH during cellular respiration.

Redox reaction example:

Regulation of Metabolic Pathways

Types of Regulation

Metabolic pathways are tightly regulated to maintain cellular homeostasis and prevent overaccumulation of products.

• Genetic regulation: Turning genes on or off to control enzyme production.

• Cell-signaling regulation: Pathways respond to signals such as hormones.

• Biochemical regulation: Feedback inhibition, where the end product inhibits an early step in the pathway.

Stages of Cellular Respiration

Major Pathways

Cellular respiration consists of four main metabolic pathways:

1. Glycolysis: Occurs in the cytoplasm; breaks down glucose into two pyruvate molecules, producing ATP and NADH.

2. Pyruvate oxidation: Pyruvate is converted to acetyl-CoA in the mitochondria, producing NADH and releasing CO2.

3. Krebs Cycle (Citric Acid Cycle): Acetyl-CoA enters the cycle, generating NADH, FADH2, ATP, and CO2.

4. Oxidative phosphorylation (Electron Transport Chain): NADH and FADH2 donate electrons to the electron transport chain, creating a proton gradient that drives ATP synthesis.

Location: Glycolysis occurs in the cytoplasm; other steps occur in the mitochondria (in eukaryotes).

Glycolysis

Glycolysis is the first step in cellular respiration, converting glucose to pyruvate and generating ATP and NADH.

• Input: Glucose, 2 NAD+, 2 ADP

• Output: 2 Pyruvate, 2 NADH, 2 ATP

Equation:

Fermentation

When oxygen is not available, cells undergo fermentation to regenerate NAD+ and allow glycolysis to continue.

• Alcoholic fermentation: Produces ethanol and CO2 (in yeast).

• Lactic acid fermentation: Produces lactate (in muscle cells).

Pyruvate Oxidation and Krebs Cycle

Pyruvate is transported into the mitochondria and converted to acetyl-CoA, which enters the Krebs cycle.

• Pyruvate oxidation: Produces acetyl-CoA, NADH, and CO2.

• Krebs cycle: Each acetyl-CoA produces 3 NADH, 1 FADH2, 1 ATP, and 2 CO2.

Electron Transport Chain and Chemiosmosis

The electron transport chain (ETC) is located in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through protein complexes, creating a proton gradient that drives ATP synthesis via chemiosmosis.

• Input: NADH, FADH2, O2, ADP

• Output: ATP (30-34 per glucose), NAD+, FAD, H2O

Equation for oxidative phosphorylation:

Comparisons Among Carbohydrate, Protein, and Fat Metabolism

Integration of Metabolic Pathways

Cells can metabolize carbohydrates, proteins, and fats through shared pathways, increasing efficiency and allowing for both energy production (catabolism) and biosynthesis (anabolism).

• Carbohydrates enter glycolysis or the citric acid cycle.

• Proteins are broken down into amino acids, which can enter the cycle at various points.

• Fats are broken down into fatty acids and glycerol, which are converted to acetyl-CoA.

Summary Table: Anaerobic vs. Aerobic Respiration

Measuring Cellular Respiration

Methods of Measurement

Cellular respiration rates are measured indirectly using calorimetry (heat production) or respirometry (CO2 production, O2 consumption).

• Calorimetry: Measures heat released during respiration.

• Respirometry: Measures changes in gas concentrations.

Human and Bird Metabolic Adaptations

Adaptations to Increased Energy Demand

Humans and birds have evolved metabolic adaptations to meet increased energy demands, such as during exercise or flight.

• Humans: Increased red blood cell production, higher breathing rate, more mitochondria in muscle cells.

• Birds: High mitochondrial density in flight muscles (15-22% of cell volume).

• Flight is the most energetically costly mode of locomotion, requiring rapid ATP generation.

Learning Objectives

• Distinguish catabolic from anabolic reactions.

• Define and understand redox reactions.

• Compare and contrast the ways metabolic pathways are regulated.

• Describe the four metabolic pathways needed to break down glucose.

• Explain how homeostasis is maintained in metabolic systems.