Ch 4 Energy and Cellular Metabolism: Foundations of Human Physiology

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Energy and Cellular Metabolism

Introduction

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

Emergent Properties of Living Organisms

Defining Emergent Properties

Emergent properties are characteristics of a system that arise from the interaction of its parts, and cannot be predicted by examining the parts in isolation.
Living organisms exhibit complex behaviors and functions that result from the organization and interaction of molecules, cells, and tissues.

Properties of Living Organisms

Cellular Organization: All living things have a complex structure, with the cell as the basic unit of organization.
Energy Utilization: Organisms acquire, transform, store, and use energy to sustain life.
Responsiveness: Ability to sense and respond to internal and external environments.
Homeostasis: Maintenance of stable internal conditions through feedback and control systems.
Information Transmission: Storage, use, and transmission of genetic and biochemical information.
Reproduction and Development: Capacity to reproduce, grow, develop, and eventually die.
Emergence: Exhibit properties not predictable from the sum of their parts.
Adaptation and Evolution: Individuals adapt, and species evolve over time.

Energy in Biological Systems

Sources and Flow of Energy

All living organisms require a continuous source of energy.
Plants capture energy from sunlight through photosynthesis.
Animals obtain energy by consuming chemical bonds in food molecules.

Example: Energy from the sun is stored in plant biomolecules via photosynthesis. Animals eat plants (or other animals), using the stored energy for cellular work or storing it for later use. Some energy is always lost as heat to the environment.

Energy: Capacity to Do Work

Types of Biological Work

Chemical Work: Making and breaking of chemical bonds (e.g., synthesis of macromolecules).
Transport Work: Moving ions, molecules, and larger particles across membranes, often creating concentration gradients.
Mechanical Work: Movement of organelles, changes in cell shape, beating of flagella and cilia, and muscle contraction.

Forms of Energy

Kinetic Energy: The energy of motion. All forms of work that involve movement utilize kinetic energy.
Potential Energy: Stored energy, found in concentration gradients and chemical bonds. To perform work, potential energy must be converted to kinetic energy.
Transformation Efficiency: Not all stored energy is converted to useful work; some is lost as heat.

Thermodynamics in Biological Systems

Key Laws

First Law of Thermodynamics: The total amount of energy in the universe is constant; energy can neither be created nor destroyed, only transformed.
Second Law of Thermodynamics: Natural processes tend to move toward a state of greater disorder (entropy).

Chemical Reactions in Physiology

Basic Concepts

Chemical Reaction: A process in which reactants are transformed into products.
Energetics: The study of energy flow through biological systems.
Activation Energy: The minimum energy required to initiate a chemical reaction.
Reaction Rate: The speed at which reactants are converted to products.

Types of Chemical Reactions

Reaction Type	General Equation	Description
Combination	A + B → C	Two or more reactants combine to form a single product.
Decomposition	C → A + B	A single reactant breaks down into two or more products.
Single Displacement	L + MX → X + ML	An element or group replaces another in a compound.
Double Displacement	LX + MY → LY + MX	Exchange of elements or groups between two compounds.

Free Energy Change

Exergonic Reactions: Release energy; products have less free energy than reactants.
Endergonic Reactions: Absorb energy; products have more free energy than reactants.

Equation:

If , the reaction is exergonic (spontaneous).
If , the reaction is endergonic (requires energy input).

Enzymes: Biological Catalysts

Enzyme Function and Regulation

Enzymes are proteins that speed up chemical reactions by lowering activation energy.
Substrates: The reactants upon which enzymes act.
Enzymes are highly specific and can be regulated by various factors (e.g., temperature, pH, modulators).
Some enzymes require cofactors (inorganic ions) or coenzymes (organic molecules, often vitamins) for activity.

Diagnostic Enzymes

Elevated levels of certain enzymes in the blood can indicate specific diseases.

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 disease
Lactate dehydrogenase (LDH)	Tissue damage to heart, liver, skeletal muscle, red blood cells

Enzyme Activity and pH

Most human enzymes function optimally near pH 7.4.
Changes in pH can alter enzyme activity, often reducing it if the pH deviates from the optimum.

Categories of Enzymatic Reactions

Oxidation-Reduction Reactions: Transfer of electrons between molecules.
Hydrolysis-Dehydration Reactions: Addition or removal of water to break or form bonds.
Addition-Subtraction-Exchange Reactions: Transfer or rearrangement of chemical groups.

Metabolism

Overview

Metabolism: The sum of all chemical reactions in an organism.
Anabolism: Synthesis of complex molecules from simpler ones (requires energy).
Catabolism: Breakdown of complex molecules into simpler ones (releases energy).
Energy in food is measured in calories, representing the energy released from chemical bonds.
Metabolic pathways consist of a series of enzyme-catalyzed reactions, with intermediates as products of one reaction and substrates for the next.

Regulation of Metabolic Pathways

Cells regulate metabolism by controlling enzyme concentrations and activity.
Feedback inhibition is a common mechanism, where the end product inhibits an earlier step.
Compartmentalization within organelles allows for regulation and efficiency.
Maintaining the ATP/ADP ratio is crucial for energy balance.

ATP: The Energy Currency

ATP Production and Use

ATP (Adenosine Triphosphate): The primary molecule for storing and transferring energy in cells.
ATP is produced via metabolic pathways such as glycolysis, the citric acid cycle, and the electron transport chain.
ATP transfers energy to cellular processes by donating a phosphate group (phosphorylation).

Aerobic vs. Anaerobic Metabolism

Aerobic metabolism (requires oxygen) yields more ATP per glucose molecule than anaerobic metabolism (does not require oxygen).
Compartmentalization of metabolic pathways (e.g., glycolysis in the cytosol, citric acid cycle in mitochondria) increases efficiency.

Proteins and Genetic Information

Genetic Code and Protein Synthesis

The genetic code is based on combinations of four nitrogenous bases (A, U, G, C in RNA) in triplets (codons), each coding for one of 20 amino acids.
Transcription: Synthesis of mRNA from DNA template.
Translation: Synthesis of protein from mRNA, involving ribosomes, tRNAs, and amino acids.
Alternative splicing allows a single gene to code for multiple proteins.

Post-Translational Modification

Proteins undergo folding, cleavage, cross-linkage, and addition of other molecules after translation.
These modifications are essential for proper protein function and cellular localization.

Summary Table: Key Concepts

Topic	Key Points
Energy in Biological Systems	Energy sources, transformation, and loss
Chemical Reactions	Reactants, products, reaction rates, activation energy, free energy change
Enzymes	Function, regulation, diagnostic use, categories of reactions
Metabolism	Anabolic vs. catabolic reactions, regulation, ATP production
Protein Synthesis	Genetic code, transcription, translation, post-translational modification