BIO 101 Chapters 4-6: Enzymes, Cellular Respiration, and Metabolic Pathways Study Guide

Enzymes and Environmental Factors

Effects of Environmental Factors on Enzyme Structure and Activity

Enzymes are biological catalysts that speed up chemical reactions in cells. Their activity and structure are influenced by environmental conditions.

• Temperature: Enzyme activity typically increases with temperature up to an optimal point, beyond which activity decreases due to denaturation.

• pH: Each enzyme has an optimal pH range. Deviations can alter the enzyme's shape and reduce its activity.

• Substrate Concentration: Increasing substrate concentration increases reaction rate until the enzyme becomes saturated.

Example: Human amylase works best at pH 7; pepsin in the stomach works best at pH 2.

Enzyme Inhibitors: Competitive vs. Non-Competitive

Enzyme inhibitors are molecules that decrease or stop enzyme activity. They are classified based on their interaction with the enzyme.

• Competitive Inhibitors: Bind to the active site, blocking substrate access. Can be overcome by increasing substrate concentration.

• Non-Competitive Inhibitors: Bind to a site other than the active site, changing the enzyme's shape and reducing activity regardless of substrate concentration.

Example: Methotrexate is a competitive inhibitor of dihydrofolate reductase.

Feedback Inhibition in Metabolic Pathways

Feedback inhibition is a regulatory mechanism in which the end product of a metabolic pathway inhibits an earlier step, preventing overproduction.

• Mechanism: The final product binds to an allosteric site on an enzyme, reducing its activity.

• Purpose: Maintains homeostasis and conserves resources.

Example: Isoleucine inhibits the first enzyme in its biosynthetic pathway.

Cellular Respiration and Glycolysis

Glycolysis: Inputs and Outputs

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm. It breaks down glucose to produce energy.

• Input Molecule: Glucose (C6H12O6)

• Output Molecules: 2 Pyruvate, 2 ATP (net), 2 NADH

Equation:

$\text{Glucose} + 2\,\text{NAD}^+ + 2\,\text{ADP} + 2\,\text{P}_i \rightarrow 2\,\text{Pyruvate} + 2\,\text{NADH} + 2\,\text{ATP} + 2\,\text{H}_2\text{O}$

Cellular Respiration: Inputs and Outputs

Cellular respiration is a multi-step process that converts biochemical energy from nutrients into ATP.

• Input Molecule: Pyruvate (from glycolysis)

• Output Molecules: CO2, H2O, up to 32 ATP per glucose (in eukaryotes)

Stages: Pyruvate oxidation, Krebs cycle, Electron Transport Chain (ETC)

Equation:

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

Catabolism of Glucose: Aerobic vs. Anaerobic Exercise

Glucose catabolism differs depending on oxygen availability.

Condition	Pathway	ATP Yield	End Products
Aerobic	Glycolysis, Krebs Cycle, ETC	~32 ATP per glucose	CO2, H2O
Anaerobic	Glycolysis, Fermentation	2 ATP per glucose	Lactic acid (animals), ethanol + CO2 (yeast)

Comparison: Aerobic respiration is more efficient, producing more ATP and complete oxidation of glucose. Anaerobic respiration is less efficient and occurs when oxygen is limited.

Example: Muscle cells perform lactic acid fermentation during intense exercise.

Summary Table: Key Processes in Cellular Metabolism

Process	Location	Inputs	Outputs	ATP Yield
Glycolysis	Cytoplasm	Glucose	2 Pyruvate, 2 NADH, 2 ATP	2 ATP (net)
Krebs Cycle	Mitochondrial matrix	Acetyl-CoA	CO2, NADH, FADH2, ATP	2 ATP
Electron Transport Chain	Inner mitochondrial membrane	NADH, FADH2, O2	H2O, ATP	~28 ATP
Fermentation	Cytoplasm	Pyruvate	Lactic acid or ethanol + CO2	0 ATP (beyond glycolysis)

Additional info: Some details about the number of ATP produced and the specific steps of cellular respiration were inferred for completeness.