Enzymes, Metabolism, and Cellular Respiration: Study Notes

Enzymes, Metabolism, and Cellular Respiration

Learning Outcomes

• Describe the structure and function of enzymes.

• Explain how enzymes decrease a reaction’s activation energy barrier.

• Define metabolism.

• Describe the structure and function of ATP.

• Describe the process of cellular respiration from the breakdown of glucose through the production of ATP.

• Explain how proteins and fats are broken down during cellular respiration.

• Describe how anaerobic respiration differs from aerobic respiration.

• Discuss the relevance of body weight as a predictor of healthfulness.

Metabolism

Definition and Overview

Metabolism refers to the sum of all chemical reactions occurring in the body. These reactions are essential for maintaining life, enabling growth, reproduction, and response to the environment.

• Anabolism: Building up complex molecules from simpler ones (e.g., protein synthesis).

• Catabolism: Breaking down complex molecules into simpler ones (e.g., digestion, cellular respiration).

• Metabolic reactions are regulated by enzymes to ensure efficiency and control.

Enzymes

Structure and Function

Enzymes are proteins that act as biological catalysts, speeding up chemical reactions without being consumed in the process.

• Each enzyme is specific to a particular reaction, determined by its unique shape and the shape of its active site.

• Enzyme names typically end in -ase (e.g., sucrase breaks down sucrose).

• Enzymes are formed for the reaction they catalyze.

How Enzymes Affect Metabolic Reactions

Enzymes decrease the activation energy required for a reaction, making it easier for reactants to be converted into products.

• Activation energy: The energy required to start a metabolic reaction.

• Enzymes lower the activation energy barrier, increasing the rate of reaction.

• Without enzymes, reactions would proceed too slowly to sustain life.

Induced Fit Model

The induced fit model describes how enzymes interact with substrates:

• Substrate: The chemical being metabolized.

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

• Enzyme changes shape after substrate binds, stressing substrate bonds and facilitating the reaction.

Enzyme Specificity

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

• Unique enzyme shapes result from varying sequences of amino acids, which are determined by genes.

Cellular Respiration

Overview

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

• Energy stored in chemical bonds is released when bonds are broken.

• ATP (adenosine triphosphate) is the main energy currency of the cell.

Structure and Function of ATP

• ATP consists of a nitrogenous base (adenine), a sugar (ribose), and three phosphate groups.

• Energy is stored in the bonds between phosphate groups; removal of the last phosphate releases energy.

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

ATP Cycle

• Phosphorylation: A phosphate group is transferred from ATP to another molecule, transferring energy and producing ADP (adenosine diphosphate).

• Cellular respiration adds a phosphate group to ADP to regenerate 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; continues breakdown of glucose fragments; produces 2 ATP; releases carbon dioxide; harvests electrons.

3. Electron Transport Chain and ATP Synthesis: Series of proteins in the mitochondrial membrane; electrons from NADH are transferred, combine with oxygen to form water; ATP synthase produces up to 26 ATP.

Overall reaction:

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

Electron Carriers

• Nicotinamide adenine dinucleotide (NAD+): Acts as an electron carrier ("taxicab"); picks up electrons and hydrogen ions to become NADH; delivers electrons to the electron transport chain.

Aerobic vs. Anaerobic Respiration

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

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

Metabolism of Other Nutrients

• When carbohydrates and fats are unavailable, proteins may be used for energy; amino group is removed, and other components enter the citric acid cycle.

• Fats can be metabolized; glycerol and fatty acids enter the citric acid cycle.

Body Weight and Health

Body Mass Index (BMI)

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

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

Health Risks Associated with BMI

• Underweight: Risk of anorexia, amenorrhea, osteoporosis, and sterility.

• Overweight: Associated with less mortality than normal weight.

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

• Obesity prevention is better than yo-yo dieting; healthy choices for food and exercise are recommended.

Eating Disorders

• Anorexia: Self-starvation disease; prevalent on college campuses; leads to managed heart muscles, altered rhythms, amenorrhea, sterility, and osteoporosis.

• Bulimia: Binge eating followed by purging; associated with amenorrhea and decreased bone density.

• Eating disorders are the most fatal mental health condition but are treatable with proper help.

Effects of Extreme Dieting and Exercise

• Does not prevent obesity; may cause the body to use calories more efficiently.

• May alter levels of leptin (hunger-controlling hormone), leading to constant hunger and slowed metabolic rate.

• Weight loss maintenance is challenging after extreme dieting.

Additional info:

• Metabolic rate is influenced by exercise, biological sex, and genetics.

• Males generally require more calories due to higher muscle mass and testosterone levels.

• Genetic differences affect basal metabolic rate and fat storage/utilization.