Enzymes, Metabolism, Cellular Respiration, and Body Weight: Study Notes

Enzymes and Metabolism

Definition and Role of Metabolism

Metabolism refers to the sum of all chemical reactions occurring within the body, which are necessary for maintaining life. These reactions allow organisms to grow, reproduce, maintain their structures, and respond to environmental changes.

• Metabolism includes both catabolic (breaking down molecules to release energy) and anabolic (building complex molecules from simpler ones) pathways.

• Example: Digestion of food is a catabolic process, while synthesis of proteins from amino acids is anabolic.

Enzymes: Structure and Function

Enzymes are biological catalysts, typically proteins, that speed up chemical reactions in the body without being consumed in the process.

• Enzyme specificity is determined by the unique shape of the enzyme, especially the active site where the substrate binds.

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

• Induced fit model: The enzyme changes shape slightly when the substrate binds, stressing substrate bonds and facilitating the reaction.

How Enzymes Affect Metabolic Reactions

Enzymes decrease the activation energy required for a reaction, making it easier and faster for the reaction to occur.

• Activation energy is the minimum energy needed to start a metabolic reaction.

• Enzymes do not increase activation energy or reduce the rate of reaction; they lower the energy barrier and speed up reactions.

• Example: Sucrase lowers the activation energy for the breakdown of sucrose into glucose and fructose.

Enzyme Specificity and Genetic Control

Each enzyme catalyzes a specific reaction due to its unique amino acid sequence and three-dimensional structure, which are determined by genes.

• Genes encode the sequence of amino acids in an enzyme, determining its shape and function.

Metabolic Rate and Influencing Factors

Metabolic rate is a measure of a person's energy use, influenced by enzyme efficiency, activity level, sex, and genetics.

• Basal Metabolic Rate (BMR): The energy use of a resting, fasting person. Average BMR is about 70 calories/hour or 1680 calories/day.

• Exercise: Increases metabolic rate during and after activity; intensity and duration affect total energy expenditure.

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

• Genetics: Influence BMR, fat storage, and utilization.

Cellular Respiration

Overview and Purpose

Cellular respiration is a series of metabolic reactions that convert food energy (mainly glucose) into a usable form (ATP), releasing waste products such as carbon dioxide and water.

• ATP (Adenosine Triphosphate): The main energy currency of the cell, composed of adenine (nitrogenous base), ribose (sugar), and three phosphate groups.

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

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

Equation for Cellular Respiration

The overall reaction for aerobic cellular respiration is:

• $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$

• Glucose and oxygen are converted to carbon dioxide, water, and ATP.

Stages of Cellular Respiration

Cellular respiration occurs in three main stages, each with distinct roles and locations within the cell.

• Stage 1: Glycolysis

  • Occurs in the cytosol (outside mitochondria).

  • Breaks down one 6-carbon glucose molecule into two 3-carbon pyruvic acid molecules.

  • Does not require oxygen (anaerobic).

  • Produces 2 ATP molecules.

• Stage 2: Citric Acid Cycle (Krebs Cycle)

  • Occurs in the mitochondrial matrix.

  • Continues breakdown of glucose fragments.

  • Harvests electrons and produces 2 ATP.

  • Releases carbon dioxide as a waste product.

• Stage 3: Electron Transport Chain and ATP Synthesis

  • Occurs in the inner mitochondrial membrane.

  • Electrons from NADH are passed through a series of proteins, creating a proton gradient.

  • ATP synthase uses this gradient to produce about 26 ATP molecules.

  • Oxygen acts as the final electron acceptor, forming water.

Electron Carriers: NAD+ and NADH

Nicotinamide adenine dinucleotide (NAD+) acts as an electron carrier, transporting electrons from glycolysis and the citric acid cycle to the electron transport chain.

• NAD+ picks up electrons and hydrogen ions to become NADH.

• NADH delivers electrons to the electron transport chain and returns as NAD+ for reuse.

Anaerobic Respiration and Fermentation

When oxygen is unavailable, cells can generate energy through anaerobic respiration or fermentation.

• Anaerobic respiration: Occurs in some cells (e.g., muscle cells during intense exercise).

• ATP is produced only by glycolysis, leading to lower NAD+ levels.

• Fermentation: Regenerates NAD+ by converting pyruvic acid to lactic acid (in animals) or ethanol (in yeast).

• Lactic acid buildup in muscles causes fatigue and is transported to the liver for conversion back to pyruvic acid.

• Oxygen debt: After exercise, heavy breathing continues to supply oxygen needed to convert lactic acid to pyruvic acid.

Metabolism of Other Nutrients

Cells can metabolize proteins and fats when carbohydrates are unavailable.

• Proteins: Amino group is removed; remaining components enter the citric acid cycle.

• Fats: Glycerol and fatty acids are broken down and enter the citric acid cycle.

Body Weight and Health

Body Mass Index (BMI)

BMI is a measure used to estimate weight-related health risk, calculated from height and weight.

• BMI formula: $\text{BMI} = \frac{\text{weight (kg)}}{\text{height (m)}^2}$

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

BMI Classification	BMI Range
Underweight	< 19
Normal weight	19–24
Overweight	25–29
Grade 1 Obesity	30–34.9
Grade 2 Obesity	35–39.9
Severe Obesity	> 40

Health Risks Associated with Body Weight

Both underweight and obesity are associated with health risks, while overweight may be associated with less mortality than normal weight.

• Underweight: Risk of anorexia, amenorrhea, osteoporosis.

• Obesity: Increased risk of heart disease, stroke, type 2 diabetes.

• Obesity prevention: Healthy choices for food and exercise are more effective than extreme dieting.

Eating Disorders: Anorexia and Bulimia

Anorexia and bulimia are serious eating disorders with significant health consequences.

• Anorexia: Self-starvation, common among college students, leads to damaged heart muscles, altered rhythms, low estrogen, amenorrhea, sterility, and osteoporosis.

• Bulimia: Binge eating followed by purging; associated with amenorrhea and increased risk of osteoporosis.

Effects of Extreme Dieting and Exercise

Extreme dieting and exercise do not prevent obesity and may cause the body to use calories more efficiently, lowering metabolic rate and altering hormone levels.

• Leptin: Hunger-controlling hormone; levels decrease after extreme dieting, causing constant hunger.

• Metabolic rate slows, making weight loss harder to maintain.

• Participants in extreme weight loss programs often regain lost weight.

Additional info:

• BMI tables do not account for muscle mass, so very fit athletes may be classified as overweight despite low body fat.

• Fermentation by microbes is used in food production (e.g., yogurt, cheese, beer, wine).