Cellular Respiration and Metabolism

Introduction

Cellular respiration is a fundamental process in living organisms, converting food molecules into ATP, the universal energy currency of cells. Metabolism encompasses all chemical reactions in the body, including those that break down nutrients to release energy and those that build complex molecules.

Metabolism

Definition and Overview

• Metabolism refers to the sum of all chemical reactions occurring in the body.

• These reactions are essential for maintaining life, growth, and cellular functions.

• Metabolic reactions are catalyzed by enzymes, which increase the speed and efficiency of these processes.

Metabolic Rate

• Metabolic rate is a measure of a person's energy use over time.

• It depends on factors such as activity level, age, sex, genetics, and body composition.

• Basal Metabolic Rate (BMR) is the energy expended by a resting, wakeful person. Average BMR is about 70 calories/hour or 1680 calories/day.

ATP: Structure and Function

Adenosine Triphosphate (ATP)

• ATP consists of an adenine base, a ribose sugar, and three phosphate groups.

• Energy is stored in the chemical bonds between the phosphate groups.

• ATP is used for cellular work, including movement, active transport, and synthesis of complex molecules.

ATP Cycle and Phosphorylation

• Phosphorylation is the transfer of a phosphate group from ATP to another molecule, transferring energy.

• When ATP loses a phosphate, it becomes ADP (adenosine diphosphate).

• Cellular respiration replenishes ATP by adding a phosphate back to ADP.

Cellular Respiration

Overview

• Cellular respiration is a series of metabolic reactions converting food energy into ATP.

• It provides energy for cellular work, such as movement, transport, and synthesis.

• Occurs in both the cytosol and mitochondria of cells.

Stages of Cellular Respiration

1. Glycolysis: Occurs in the cytosol; breaks down glucose (6 carbons) into two pyruvic acid molecules (3 carbons each). Produces 2 ATP. Does not require oxygen.

2. Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; further breaks down pyruvic acid, releases carbon dioxide, produces NADH, and generates 2 ATP.

3. Electron Transport Chain (ETC) and ATP Synthesis: Occurs in the mitochondria; NADH delivers electrons, which move through a series of proteins, creating a proton gradient that drives ATP synthesis (about 26 ATP). Oxygen is the final electron acceptor, forming water.

Key Molecules

• NAD+ (Nicotinamide adenine dinucleotide): An electron carrier that picks up electrons and hydrogen ions, becoming NADH.

• NADH: Delivers electrons to the electron transport chain.

Overall Equation for Aerobic Cellular Respiration

Aerobic vs. Anaerobic Respiration

Aerobic Respiration

• Requires oxygen.

• Occurs mostly in mitochondria.

• Produces up to 30 ATP per glucose molecule.

Anaerobic Respiration (Fermentation)

• Occurs when oxygen is scarce (e.g., during intense exercise).

• Only glycolysis occurs, producing 2 ATP per glucose.

• In muscle cells, lactic acid is produced; in yeast, ethanol and carbon dioxide are produced.

Comparison Table

Energy from Fats, Carbohydrates, and Proteins

Alternative Fuels

• Cells can use fats and proteins for energy when glucose is unavailable.

• Fats are broken down into fatty acids and glycerol, which enter cellular respiration pathways.

• Proteins are broken down into amino acids, which are deaminated and enter the citric acid cycle.

Body Weight and Health

Body Mass Index (BMI)

• BMI uses height and weight to estimate risk of illness and death related to body weight.

• BMI Categories:

  • Less than 18.5: Underweight

  • 18.5–24.9: Normal weight

  • 25–29.9: Overweight

  • 30–34.9: Moderate obesity

  • 35 and above: Severe obesity

Health Risks Associated with Body Weight

• Underweight: Risk of anorexia, lower bone density, and other health issues.

• Overweight/Obesity: Increased risk of heart disease, stroke, type 2 diabetes, and other conditions.

• Obesity prevention is more effective than repeated dieting.

Eating Disorders

• Anorexia nervosa: Self-starvation, common among college students, especially women.

• Bulimia nervosa: Binge eating followed by purging; can damage heart muscles and reduce estrogen production, leading to amenorrhea and osteoporosis.

Summary Questions for Review

• Can you define metabolism?

• Can you describe the structure and function of ATP?

• Can you describe the process of cellular respiration from the breakdown of glucose through the production of ATP?

• Can you explain how proteins and fats are broken down during cellular respiration?

• Can you compare and contrast aerobic and anaerobic respiration?

• Can you discuss the relevance of body weight as a predictor of healthfulness?