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 up molecules using energy) 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: Each enzyme catalyzes a particular reaction, determined by its unique shape and the shape of its active site.

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

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

• Naming: Enzymes often end with the suffix -ase (e.g., sucrase breaks down sucrose).

How Enzymes Affect Metabolic Reactions

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

• Activation energy: The minimum energy required 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.

Enzyme Specificity and Genetic Control

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

• Different enzymes have different shapes due to their unique amino acid sequences.

• Genes control the production and structure of enzymes.

Metabolic Rate

Metabolic rate is a measure of a person's energy use, influenced by the speed and efficiency of various enzymes.

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

• Metabolic rate changes with activity, exercise, sex, and genetics.

Factors Influencing Metabolic Rate

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

• Biological sex: Males generally require more calories due to higher muscle mass and testosterone, which increases fat breakdown.

• Genetics: Some individuals have lower BMRs and differences in fat storage/utilization.

Cellular Respiration

Overview and Purpose

Cellular respiration is a series of metabolic reactions that convert food energy into a usable form (ATP) and release waste products.

• Energy is stored in chemical bonds, especially in electrons.

• Breaking these bonds produces adenosine triphosphate (ATP).

Structure and Function of ATP

ATP is the primary energy carrier in cells, composed of adenine (a nitrogenous base), ribose (a sugar), and three phosphate groups.

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

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

ATP and Cellular Work

• ATP powers movement, active transport across membranes, and synthesis of complex molecules.

Overall Equation for Cellular Respiration

The process of aerobic cellular respiration can be summarized by the following equation:

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

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

Stages of Cellular Respiration

Cellular respiration occurs in three main stages:

1. Glycolysis: Occurs in the cytosol; breaks down glucose into two pyruvic acid molecules; produces 2 ATP; does not require oxygen.

2. Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; further breaks down glucose fragments; produces 2 ATP; releases carbon dioxide; harvests electrons.

3. Electron Transport Chain and ATP Synthesis: Occurs in the mitochondria; electrons from NADH are passed through a series of proteins, creating a proton gradient that drives ATP synthesis via ATP synthase; produces about 26 ATP.

Role of NAD+ in Cellular Respiration

Nicotinamide adenine dinucleotide (NAD+) acts as an electron carrier ('taxicab').

• NAD+ picks up electrons and hydrogen ions, becoming NADH.

• NADH carries electrons to the electron transport chain, then returns for more electrons.

Aerobic vs. Anaerobic Respiration

• Aerobic respiration: Requires oxygen; occurs mostly in mitochondria; produces large amounts of ATP.

• Anaerobic respiration: Does not require oxygen; occurs in certain cells (e.g., muscle cells during intense exercise); produces less ATP and leads to lactic acid buildup.

Fermentation

When oxygen is limited, cells regenerate NAD+ through fermentation.

• Lactic acid fermentation: Occurs in muscle cells; pyruvate is converted to lactic acid.

• Lactic acid is transported to the liver for conversion back to pyruvic acid.

• Alcoholic fermentation: Yeast cells convert glucose to ethyl alcohol (used in beer and wine production).

Oxygen Debt

• After intense exercise, heavy breathing continues to supply oxygen needed to convert lactic acid back to pyruvic acid.

• If oxygen debt is too large, muscles may shut down.

Metabolism of Other Nutrients

• When carbohydrates and fats are unavailable, proteins can be metabolized; amino groups are removed and other components enter the citric acid cycle.

• Fats can be broken down into glycerol and fatty acids, which also 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 Table

BMI Value	Classification
< 19	Underweight
19–24	Normal weight
25–29	Overweight
30–34.9	Grade 1 Obesity
35–39.9	Grade 2 Obesity (Moderate)
> 40	Grade 3 Obesity (Severe)

Health Risks Associated with Body Weight

• Underweight: Increased risk of anorexia, osteoporosis, and amenorrhea (loss of menstruation).

• Overweight: Associated with less mortality than normal weight, but may still pose health risks.

• Obesity (Grade 2 and 3): Increased risk of heart disease, stroke, type 2 diabetes.

• Obesity prevention is preferable to yo-yo dieting; healthy choices in food and exercise are recommended.

Eating Disorders

• Anorexia nervosa: Self-starvation; prevalent among college students; leads to amenorrhea, osteoporosis, and heart issues.

• Bulimia nervosa: Binge eating followed by purging; also associated with amenorrhea and osteoporosis.

Effects of Extreme Dieting and Exercise

• Extreme dieting does not prevent obesity; may cause the body to use calories more efficiently and lower metabolic rate.

• Leptin (hunger-controlling hormone) levels decrease after extreme dieting, leading to constant hunger; levels do not return to normal quickly.

• Participants in extreme weight loss programs often do not maintain weight loss long-term.

Summary Table: Key Concepts

Concept	Definition	Example/Application
Metabolism	Sum of all chemical reactions in the body	Digestion, protein synthesis
Enzyme	Protein catalyst for reactions	Sucrase breaks down sucrose
ATP	Main energy carrier	Muscle contraction
Glycolysis	Breakdown of glucose to pyruvate	Occurs in cytosol, produces 2 ATP
BMI	Body mass index	Health risk assessment

Additional info: Academic context and definitions have been expanded for clarity and completeness. Equations and tables have been reconstructed for study purposes.