Chapter 4: Energy and Cellular Metabolism – Study Notes

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Energy and Cellular Metabolism

Overview

This chapter explores the fundamental principles of energy use in biological systems, the nature of chemical reactions, the role of enzymes, and the processes of metabolism. Understanding these concepts is essential for comprehending how cells and organisms maintain life, grow, and adapt.

Properties of Living Organisms

Key Characteristics

Complex Structure: The cell is the basic unit of organization in all living organisms.
Energy Use: Organisms acquire, transform, store, and use energy.
Responsiveness: Ability to sense and respond to internal and external environments.
Homeostasis: Maintenance of stable internal conditions through control systems with feedback.
Information Storage: Store, use, and transmit genetic information.
Reproduction and Growth: Ability to reproduce, develop, grow, and die.
Emergent Properties: Properties that arise from the interaction of simpler parts.
Adaptation and Evolution: Individuals adapt and species evolve over time.

Energy in Biological Systems

Sources and Forms of Energy

All living organisms require energy.
Plants: Trap radiant energy from the sun and store it in chemical bonds (photosynthesis).
Animals: Obtain energy by ingesting plants or other animals.

Energy Transfer in the Environment

Energy flows from the sun to plants (producers) and then to animals (consumers).
Energy is stored in biomolecules and released through cellular respiration.

Types of Work Performed by Cells

Chemical Work: Making and breaking chemical bonds.
Transport Work: Moving ions, molecules, and larger particles; creating concentration gradients.
Mechanical Work: Moving organelles, changing cell shape, beating flagella/cilia, and contracting muscles.

Forms of Energy

Kinetic Energy: Energy of motion (e.g., moving molecules).
Potential Energy: Stored energy (e.g., in concentration gradients and chemical bonds).
Energy can be converted between forms, but some is lost as heat (transformation efficiency).

Thermodynamics

First Law (Conservation of Energy): Total energy in the universe is constant.
Second Law: Processes move from order to disorder (entropy increases).

Chemical Reactions

Bioenergetics and Reaction Types

Bioenergetics: Study of energy flow through biological systems.
Chemical Reactions: Reactants are transformed into products; reaction rate is the speed of this process.
Free Energy (G): Energy stored in chemical bonds.
Activation Energy: The minimum energy required to start a reaction.

Energy Changes in Reactions

Exergonic Reactions: Release energy (energy-producing).
Endergonic Reactions: Require input of energy (energy-utilizing).
Coupled Reactions: Endergonic and exergonic reactions are often linked in cells.
Reversibility: Net free energy change determines if a reaction is reversible or irreversible.

Types of Chemical Reactions

Reaction Type	Reactants (Substrates)	Products
Combination	A + B	C
Decomposition	C	A + B
Single Displacement	L + MX	LX + M
Double Displacement	LX + MY	LY + MX

Activation Energy Diagrams

Activation energy is the "push" needed to start a reaction.
Exergonic reactions have products with less energy than reactants; endergonic reactions have products with more energy.

Enzymes

Role and Function

Enzymes: Biological catalysts that speed up chemical reactions by lowering activation energy.
Reactants in enzyme-catalyzed reactions are called substrates.
Most enzymes are proteins; some are RNA molecules.
Isozymes: Different forms of an enzyme that catalyze the same reaction in different tissues or conditions (e.g., CK-MB vs. CK-MM).

Enzyme Activity Regulation

Reaction rates depend on substrate/enzyme concentration and temperature.
Enzymes can be activated, inactivated, or modulated (e.g., by cofactors, pH, or temperature).
Proenzymes/Zymogens: Inactive precursors requiring activation.
Coenzymes: Organic cofactors, often derived from vitamins.

Cofactors

Holoenzyme: Enzyme with its cofactor.
Apoenzyme: Enzyme without its cofactor.
Cofactors can be small organic molecules (coenzymes) or metals (e.g., Mg2+, Zn2+).

Enzyme Activity and pH

Most human enzymes have optimal activity near pH 7.4.

Diagnostic Enzymes

Enzyme	Related Diseases
Acid phosphatase	Cancer of the prostate
Alkaline phosphatase	Diseases of bone or liver
Amylase	Pancreatic disease
Creatine kinase (CK)	Myocardial infarction, muscle diseases
Lactate dehydrogenase (LDH)	Tissue damage to heart, liver, skeletal muscle, red blood cells

Categories of Enzymatic Reactions

Phosphorylation: Addition of a phosphate group.
Oxidation-Reduction: Transfer of electrons (oxidized = loses electrons, reduced = gains electrons).
Hydrolysis-Dehydration: Hydrolysis adds water to break bonds; dehydration removes water to form bonds.
Addition-Subtraction-Exchange: Addition or removal of functional groups, or exchange between molecules.
Ligation: Joining two molecules using energy from ATP.

Reaction Type	What Happens	Representative Enzymes
Oxidation-reduction	Add or subtract electrons	Oxidase, dehydrogenase, reductase
Hydrolysis-dehydration	Add or remove water	Peptidase, saccharidase, lipase, dehydratase
Transfer chemical groups	Exchange groups between molecules	Kinase, transaminase, deaminase
Ligation	Join two substrates using ATP	Synthase, ligase

Metabolism

Metabolic Pathways and Regulation

Metabolism: All chemical reactions in an organism.
Catabolism: Energy-releasing breakdown of molecules.
Anabolism: Energy-utilizing synthesis of molecules.
Energy is measured in kilocalories (kcal).
Pathways consist of intermediates and are regulated by enzymes.

Regulation of Metabolic Pathways

Control enzyme concentrations.
Produce modulators (e.g., feedback inhibition) to change reaction rates.
Use different enzymes for reversible reactions.
Compartmentalize enzymes within organelles.
Maintain optimum ATP/ADP ratio.

Feedback Inhibition

End product of a pathway inhibits an earlier enzyme, preventing overproduction.

Reversibility of Metabolic Reactions

Some reactions are reversible (use same or different enzymes for each direction).
Irreversible reactions require a different enzyme for the reverse process.

ATP and Energy Transfer

ATP Production and Use

ATP: Main energy currency of the cell, stores energy in high-energy phosphate bonds.
Aerobic Metabolism: Complete oxidation of one glucose yields 30-32 ATP.
Anaerobic Metabolism: Incomplete breakdown of glucose yields 2 ATP.
Catabolic Pathways: Glycolysis, citric acid cycle, and electron transport system produce ATP.

Stages of Glucose Catabolism

Stage and Site	Inputs	Outputs
Glycolysis (Cytoplasm)	1 Glucose, 2 ATP, 2 NAD+	2 Pyruvate, 2 ATP, 2 NADH
Pyruvate to Acetyl CoA (Mitochondria)	2 Pyruvate, 2 NAD+, 2 CoA	2 Acetyl CoA, 2 NADH, 2 CO2
Krebs Cycle (Mitochondria)	2 Acetyl CoA, 6 NAD+, 2 FAD, 2 ADP	4 CO2, 6 NADH, 2 FADH2, 2 ATP
Electron Transport Chain (Inner Mitochondrial Membrane)	10 NADH, 2 FADH2, 6 O2	~28 ATP, 6 H2O

Total ATP per glucose (aerobic): 30-32 ATP

Total ATP per glucose (anaerobic): 2 ATP

Proteins and Genetic Code

Protein Structure and Synthesis

Proteins are composed of 20 different amino acids.
The sequence and number of amino acids determine protein structure and function.
Codon: A sequence of three mRNA bases encoding one amino acid.

Genetic Code

The genetic code is universal and redundant; multiple codons can code for the same amino acid.

Gene Expression

Gene: Region of DNA that is transcribed into RNA.
Transcription: DNA is used as a template to synthesize RNA (mRNA, tRNA, rRNA).
Translation: mRNA is decoded by ribosomes to assemble amino acids into a protein.
Post-translational Modifications: Proteins may be folded, cleaved, or combined with other molecules after translation.

Alternative Splicing

Allows a single gene to code for multiple proteins by including or excluding certain exons.

Summary Table: Key Concepts

Concept	Definition/Explanation
Energy	Capacity to do work; exists as kinetic or potential energy
Enzyme	Biological catalyst that speeds up reactions by lowering activation energy
Metabolism	Sum of all chemical reactions in an organism
Catabolism	Breakdown of molecules, releasing energy
Anabolism	Synthesis of molecules, requiring energy
ATP	Main energy carrier in cells
Gene Expression	Process of transcribing DNA to RNA and translating RNA to protein

Key Equations

ATP Hydrolysis:
General Reaction Free Energy:

Example: During muscle contraction, ATP is hydrolyzed to provide the energy required for actin and myosin interaction.