Energy and Cellular Metabolism: Core Concepts in Human Physiology

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Energy and Cellular Metabolism

Chapter Overview

This chapter explores the fundamental principles of energy flow, chemical reactions, enzyme function, and metabolic pathways in biological systems. These concepts are essential for understanding how cells acquire, transform, and utilize energy to sustain life.

Core Concepts

Energy: Required for living organisms to perform work, reproduce, and maintain order.
Gradients: Energy can be stored in concentration, electrical, or electrochemical gradients and is released as substances move down these gradients.
Compartmentalization: Cellular compartments isolate and separate biochemical processes for efficiency.
Molecular Interactions: Enzymes facilitate biological reactions and help store or release energy in covalent bonds.

Table 4.1 Properties of Living Organisms

#	Property
1	Complex structure with cells as the basic unit
2	Acquire, transform, store, and use energy
3	Sense and respond to internal and external environments
4	Maintain homeostasis via internal control systems with feedback
5	Store, use, and transmit information
6	Reproduce, develop, grow, and die
7	Exhibit emergent properties not predictable from the sum of parts
8	Individuals adapt and species evolve

4.1 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.
Animals: Obtain energy by ingesting plants or other animals.

Energy is the capacity to do work:

Chemical work: Making and breaking chemical bonds.
Transport work: Moving ions/molecules, creating concentration gradients.
Mechanical work: Moving organelles, changing cell shape, muscle contraction.

Kinetic and Potential Energy

Kinetic energy: Energy of motion.
Potential energy: Stored energy (e.g., in gradients or 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).

4.2 Chemical Reactions

Bioenergetics and Reaction Types

Bioenergetics: Study of energy flow through biological systems.
Chemical reactions: Reactants become products; reaction rate is the speed of conversion.
Free energy: Energy available to do work.
Activation energy: Minimum energy required to start a reaction.

Energy Changes in Reactions

Exergonic reactions: Release energy ().
Endergonic reactions: Require energy input ().
Endergonic and exergonic reactions are often coupled in cells.
Reversible vs. Irreversible reactions: Determined by net free energy change.

Table 4.2 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

*X and Y represent atoms, ions, or chemical groups.

4.3 Enzymes

Enzyme Function and Regulation

Enzymes: Biological catalysts that speed up chemical reactions by lowering activation energy.
Substrates: Reactants in enzyme-catalyzed reactions.
Isozymes: Enzymes that catalyze the same reaction under different conditions or in different tissues.
Enzyme activity is affected by substrate/enzyme concentration, temperature, and pH.
Enzymes can be activated, inactivated, or modulated (e.g., by coenzymes, proenzymes, or chemical factors).

Table 4.3 Diagnostically Important 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 disease
Lactate dehydrogenase (LDH)	Tissue damage to heart, liver, skeletal muscle, red blood cells

Enzyme Reaction Types

Oxidation-reduction: Electron transfer (oxidized = loses electrons, reduced = gains electrons).
Hydrolysis-dehydration: Water is added (hydrolysis) or removed (dehydration).
Addition-subtraction-exchange: Functional groups are added, removed, or exchanged (e.g., kinases, deamination, transamination).
Ligation: Synthetases join two molecules together.

Table 4.4 Classification of Enzymatic Reactions

Reaction Type	What Happens	Representative Enzymes
Oxidation-reduction	Add/subtract electrons, transfer to oxygen	Oxidase, dehydrogenase, reductase
Hydrolysis-dehydration	Add/subtract water, split/merge molecules	Peptidases, saccharidases, lipases, dehydratases
Transfer chemical groups	Exchange groups, add/subtract phosphate/amino groups	Kinase, transaminase, phosphatase, deaminase
Ligation	Join two substrates using ATP energy	Synthetase, ligase

4.4 Metabolism

Metabolic Pathways and Regulation

Metabolism: All chemical reactions in an organism.
Catabolism: Energy-releasing breakdown of molecules.
Anabolism: Energy-utilizing synthesis of molecules.
Kilocalories: Measure of energy released or stored in chemical bonds.
Intermediates: Molecules in metabolic pathways.

Regulation of Metabolic Pathways

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

ATP and Energy Transfer

ATP: Main energy currency of the cell, transfers energy between reactions via high-energy phosphate bonds.
Aerobic metabolism: One glucose yields 30–32 ATP.
Anaerobic metabolism: One glucose yields 2 ATP.
Catabolic pathways: Glycolysis, citric acid cycle, electron transport system.

Example: Glycolysis and Citric Acid Cycle

Glycolysis: Glucose is broken down to pyruvate, producing ATP and NADH.
Citric Acid Cycle: Pyruvate is converted to Acetyl CoA, which enters the cycle to produce ATP, NADH, and FADH2.
Electron Transport Chain: NADH and FADH2 donate electrons to produce ATP.

Feedback Inhibition

End products of metabolic pathways can inhibit earlier steps, preventing overproduction and maintaining balance.

Summary Table: Aerobic vs. Anaerobic Metabolism

Pathway	Oxygen Required?	ATP Yield (per glucose)
Aerobic	Yes	30–32
Anaerobic	No	2

Additional info: These concepts are foundational for understanding cellular physiology, energy balance, and the biochemical basis of health and disease.