Chapter 4: Cellular Metabolism – Study Notes for Anatomy & Physiology

Cellular Metabolism

Overview of Metabolism

Cellular metabolism encompasses all chemical reactions that occur within the body to maintain life. These reactions are essential for energy production, growth, and cellular repair.

• Metabolism: The sum of all chemical reactions in the body.

• Two types of metabolic reactions:

  • Anabolism: Synthesis of larger molecules from smaller ones; requires energy input.

  • Catabolism: Breakdown of larger molecules into smaller ones; releases energy.

Anabolism

Anabolic reactions are vital for cellular growth and repair, involving the synthesis of complex molecules from simpler ones.

• Dehydration Synthesis: Removal of water to form bonds between molecules.

• Types of anabolic processes:

  • Formation of polysaccharides, triglycerides, and proteins.

  • Production of water as a byproduct.

Catabolism

Catabolic reactions break down complex molecules into simpler units, releasing energy that can be used by the cell.

• Hydrolysis: Addition of water to break chemical bonds.

• Breakdown of carbohydrates, lipids, and proteins into smaller subunits.

• Water is used to split the bonds.

• Enables energy extraction from nutrients.

Control of Metabolic Reactions

Enzymes

Enzymes are biological catalysts that regulate the rate of metabolic reactions, ensuring that necessary reactions occur efficiently and at the right time.

• Lower activation energy required for reactions.

• Each enzyme is specific to a particular substrate due to its unique shape.

• Enzyme activity can be regulated by various factors, including temperature and pH.

Metabolic Pathways

Metabolic pathways are sequences of enzyme-controlled reactions that lead to the formation of a specific product.

• Each step is catalyzed by a different enzyme.

• The product of one reaction serves as the substrate for the next.

Enzyme Action

• Enzymes have a specific active site for substrate binding.

• Enzyme-substrate complex forms, facilitating the reaction.

• Enzymes are not consumed in the reaction and can be reused.

Factors Affecting Enzyme Activity

• Heat, radiation, electricity, chemicals, and pH can alter enzyme structure and function.

• Coenzymes and cofactors (often vitamins or minerals) may be required for enzyme activity.

Energy for Metabolic Reactions

ATP (Adenosine Triphosphate)

ATP is the primary energy currency of the cell, providing energy for various cellular processes.

• Composed of adenine, ribose, and three phosphate groups.

• Energy is stored in the high-energy phosphate bonds.

• Hydrolysis of ATP releases energy for cellular work.

Cellular Respiration

Overview

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

• Occurs in three main stages:

  • Glycolysis

  • Citric Acid Cycle (Krebs Cycle)

  • Electron Transport Chain

• Produces carbon dioxide, water, and ATP.

• Includes both aerobic (with O2) and anaerobic (without O2) reactions.

Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm and breaking down glucose into pyruvic acid.

• Does not require oxygen (anaerobic).

• Glucose (6 carbons) is split into two molecules of pyruvic acid (3 carbons each).

• Produces a net gain of 2 ATP and 2 NADH per glucose molecule.

Anaerobic Reactions

When oxygen is not available, cells undergo anaerobic respiration.

• Pyruvic acid is converted to lactic acid.

• NADH cannot donate electrons to the electron transport chain.

• Less ATP is produced compared to aerobic respiration.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle occurs in the mitochondria and completes the breakdown of glucose derivatives, generating electron carriers for the electron transport chain.

• Acetyl CoA combines with oxaloacetic acid to form citric acid.

• Produces CO2, ATP, NADH, and FADH2.

• Cycle repeats for each acetyl CoA molecule.

Electron Transport Chain (ETC)

The ETC is the final stage of cellular respiration, located in the inner mitochondrial membrane, where most ATP is generated.

• NADH and FADH2 donate electrons to the chain.

• Electrons are passed through a series of proteins, releasing energy to pump protons and generate ATP via ATP synthase.

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

Carbohydrate, Protein, and Nucleic Acid Metabolism

Carbohydrate Storage

• Excess glucose is stored as glycogen (in liver and muscle) or converted to fat.

Regulation of Metabolic Pathways

• Controlled by regulatory enzymes, often through feedback inhibition.

Nucleic Acid and Protein Synthesis

• Genetic information in DNA is used to synthesize proteins.

• DNA is transcribed into RNA, which is then translated into protein.

• Each gene codes for a specific protein.

RNA Molecules

• Messenger RNA (mRNA): Carries genetic code from DNA to ribosomes.

• Transfer RNA (tRNA): Brings amino acids to the ribosome during protein synthesis.

• Ribosomal RNA (rRNA): Forms part of the ribosome structure.

DNA Replication

• Hydrogen bonds between bases break, and the double helix unwinds.

• New nucleotides pair with exposed bases, forming two identical DNA molecules.

• Controlled by the enzyme DNA polymerase.

Mutations

• Changes in the genetic information due to addition, deletion, or alteration of bases.

• May or may not affect protein function.

• Repair enzymes can correct some mutations.

Clinical Application: Phenylketonuria (PKU)

• PKU is a genetic disorder caused by the absence of the enzyme that breaks down phenylalanine.

• Leads to accumulation of phenylalanine, causing mental retardation if untreated.

• Treated with a diet very low in phenylalanine.

Summary Table: Key Steps of Cellular Respiration

Stage	Location	Oxygen Required?	Main Products
Glycolysis	Cytoplasm	No	2 ATP, 2 NADH, 2 Pyruvic Acid
Citric Acid Cycle	Mitochondrial Matrix	Yes	2 ATP, 6 NADH, 2 FADH2, CO2
Electron Transport Chain	Inner Mitochondrial Membrane	Yes	~32 ATP, H2O

Key Equations

• Overall equation for cellular respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}$

Additional info: Some explanations and context have been expanded for clarity and completeness, including the summary table and the overall equation for cellular respiration.