Enzymes, Metabolism, and Cellular Respiration

Overview

This chapter explores the roles of enzymes in metabolism, the process of cellular respiration, and the relationship between body weight and health. It covers the structure and function of enzymes, how they catalyze metabolic reactions, the stages of cellular respiration, and the health implications of body weight.

Metabolism and Enzymes

Metabolism

• Metabolism refers to all of the chemical reactions occurring in the body that are necessary for life.

• These reactions include both the breakdown of molecules to release energy (catabolism) and the synthesis of new molecules (anabolism).

Enzymes: Structure and Function

• Enzymes are biological catalysts—proteins that speed up chemical reactions without being consumed in the process.

• Enzymes are named for the reactions they catalyze, often ending in -ase (e.g., sucrase breaks down sucrose).

• They work by lowering the activation energy required to start a reaction.

Activation Energy

• Activation energy is the minimum energy required to initiate a chemical reaction.

• Enzymes decrease the activation energy, making it easier for reactions to occur at body temperature.

Induced Fit Model

• Substrate: The specific molecule upon which an enzyme acts.

• Active site: The region of the enzyme where the substrate binds.

• When the substrate binds, the enzyme changes shape slightly to fit more snugly around the substrate, stressing substrate bonds and facilitating the reaction.

Enzyme Specificity

• Each enzyme catalyzes a particular reaction based on the shape of its active site.

• Enzyme shape is determined by the sequence of amino acids, which is encoded by genes.

Metabolic Rate and Influencing Factors

Metabolic Rate

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

• It depends on the speed and efficiency of different enzymes and changes according to activity levels.

Basal Metabolic Rate (BMR)

• BMR is the energy use of a resting, wakeful person.

• Average BMR: ~70 calories/hour or ~1680 calories/day.

Factors Affecting Metabolic Rate

• Exercise: Increases metabolic rate during and after activity.

• Biological sex: Males generally have higher BMR due to more muscle mass and the effects of testosterone.

• Genetics: Inherited traits can influence BMR and fat storage/utilization.

Cellular Respiration

ATP: The Energy Currency

• Adenosine triphosphate (ATP) is the primary energy carrier in cells.

• Structure: adenine base, ribose sugar, and three phosphate groups.

• Energy is released when the terminal (last) phosphate group is removed, forming ADP (adenosine diphosphate).

Phosphorylation

• Transfer of a phosphate group from ATP to another molecule, transferring energy.

Overview of Cellular Respiration

• Cellular respiration is the process by which cells extract energy from glucose to produce ATP.

• Overall equation:

• Occurs in three main stages: Glycolysis, Citric Acid Cycle, and Electron Transport Chain.

Stages of Cellular Respiration

1. Glycolysis: Occurs in the cytoplasm; breaks down glucose into pyruvate, producing a small amount of ATP and NADH. Does not require oxygen (anaerobic).

2. Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondria; completes the breakdown of glucose, releases CO2, and produces NADH and FADH2.

3. Electron Transport Chain and ATP Synthesis: Electrons from NADH and FADH2 are transferred through protein complexes in the mitochondrial membrane, creating a proton gradient that drives ATP synthesis. Oxygen is the final electron acceptor, forming water.

NAD+ and NADH

• Nicotinamide adenine dinucleotide (NAD+) is an electron carrier.

• It accepts electrons during glycolysis and the citric acid cycle, becoming NADH, which transports electrons to the electron transport chain.

ATP Yield

• Most ATP is produced during the electron transport chain stage, via ATP synthase.

• Hydrogen ions flow through ATP synthase, similar to water turning a turbine in a dam, generating ATP.

• Approximately 26-28 ATP molecules are produced per glucose molecule.

Metabolism of Other Nutrients

• When carbohydrates are unavailable, fats and proteins can be metabolized for energy.

• Proteins are deaminated (removal of amino groups) before entering the respiration pathway.

Aerobic vs. Anaerobic Respiration

• Aerobic respiration requires oxygen and produces more ATP.

• Anaerobic respiration (fermentation) occurs without oxygen, producing less ATP and resulting in byproducts such as lactic acid in muscle cells.

Body Weight, Health, and Metabolism

Body Mass Index (BMI)

• BMI is a measure using height and weight to estimate body fat and assess health risk.

Limitations of BMI

• BMI does not distinguish between lean muscle mass and body fat.

• Very fit athletes may have a high BMI due to muscle, not excess fat.

Health Risks Associated with Body Weight

• Both underweight and overweight classifications can be associated with health risks.

• Some studies suggest that being slightly overweight may be associated with lower mortality than being underweight or severely obese.

Eating Disorders

• Anorexia nervosa: Self-starvation, more common in college-aged women, can lead to heart damage, loss of estrogen, amenorrhea, and osteoporosis.

• Bulimia nervosa: Binge eating followed by purging; associated with similar health risks.

• Both conditions can cause amenorrhea (loss of menstrual cycle).

Effects of Extreme Dieting and Exercise

• Extreme dieting does not prevent obesity and may slow metabolic rate.

• Body adapts by using calories more efficiently and may alter hormone levels (e.g., leptin).

• Most participants in extreme weight loss programs struggle to maintain weight loss long-term.

Summary Table: Aerobic vs. Anaerobic Respiration

Key Terms

• Enzyme: Protein catalyst that speeds up chemical reactions.

• Metabolism: All chemical reactions in the body.

• ATP: Main energy carrier in cells.

• Glycolysis: First stage of cellular respiration; breaks down glucose.

• Citric Acid Cycle: Second stage; completes glucose breakdown.

• Electron Transport Chain: Final stage; produces most ATP.

• BMR: Basal metabolic rate.

• BMI: Body mass index.

• Aerobic/Anaerobic Respiration: With/without oxygen.

• Amenorrhea: Absence of menstruation.