An Introduction to Metabolism

Overview

Metabolism encompasses all the chemical reactions that occur within a living organism, enabling it to maintain life, grow, and respond to environmental changes. This chapter introduces the principles of metabolism, the laws of thermodynamics as they apply to biological systems, and the role of enzymes in metabolic pathways.

Concept 8.1: Metabolism and Thermodynamics

Definition of Metabolism

• Metabolism is the totality of an organism’s chemical reactions.

• These reactions are essential for converting matter and energy to sustain life.

• Chemical reactions occur when atoms, ions, or molecules collide in a way that produces new substances.

Metabolic Pathways

• A metabolic pathway begins with a specific molecule and ends with a product.

• Each step in the pathway is catalyzed by a specific enzyme.

• Pathways can be linear, branched, or cyclic, depending on the sequence of reactions.

• Example: Glycolysis is a metabolic pathway that breaks down glucose to pyruvate.

Types of Energy

• Energy is the capacity to cause change or do work.

• Forms of energy relevant to biology include:

  • Kinetic energy: Energy of motion (e.g., movement of molecules).

  • Thermal energy: Associated with random movement of atoms or molecules; heat.

  • Potential energy: Stored energy due to position or structure (e.g., chemical bonds).

  • Chemical energy: Potential energy available for release in a chemical reaction.

• Energy can be converted from one form to another (e.g., chemical energy to kinetic energy).

The Laws of Thermodynamics

• First Law of Thermodynamics (Law of Conservation of Energy):

  • Energy can be transferred and transformed, but it cannot be created or destroyed.

  • Total energy in the universe remains constant.

• Second Law of Thermodynamics:

  • Every energy transfer or transformation increases the entropy (disorder) of the universe.

  • Some energy is always lost as heat, making it less available to do work.

Application: Biological processes obey these laws. For example, cells transform energy from food into usable forms, but some energy is lost as heat, increasing entropy.

Concept 8.2: Free Energy and Spontaneity of Reactions

Free Energy (G)

• Free energy (G) is the portion of a system’s energy that can perform work when temperature and pressure are uniform.

• Changes in free energy () determine whether a process is spontaneous.

Spontaneous vs. Non-Spontaneous Reactions

• Spontaneous reactions release free energy () and can occur without energy input (exergonic).

• Non-spontaneous reactions require an input of energy () (endergonic).

Gibbs Free Energy Equation

• The change in free energy is calculated as:

  • = change in free energy

  • = change in enthalpy (total energy)

  • = temperature in Kelvin

  • = change in entropy (disorder)

Concept 8.3: ATP and Energy Coupling

ATP: The Cell’s Energy Currency

• Adenosine triphosphate (ATP) is the primary energy carrier in cells.

• ATP is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups.

ATP Hydrolysis

• Hydrolysis of ATP releases energy:

• This reaction is exergonic () and powers cellular work.

Types of Cellular Work Powered by ATP

• Chemical work: Driving endergonic reactions (e.g., synthesis of macromolecules).

• Transport work: Pumping substances across membranes against their concentration gradients.

• Mechanical work: Movement of structures within cells (e.g., muscle contraction, vesicle transport).

Energy Coupling

• Energy coupling is the use of an exergonic process to drive an endergonic one, often mediated by ATP.

Concept 8.4: Enzymes and Activation Energy

Role of Enzymes

• Enzymes are catalytic proteins that speed up metabolic reactions by lowering the activation energy barrier ().

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

How Enzymes Work

• Enzymes bind to specific reactant molecules called substrates at their active site.

• The enzyme-substrate complex stabilizes the transition state, lowering and increasing the reaction rate.

• Enzymes do not affect ; they only speed up reactions that would occur eventually.

Mechanisms of Enzyme Action

• Orienting substrates correctly

• Straining substrate bonds

• Providing a favorable microenvironment

• Covalently bonding to the substrate (in some cases)

Concept 8.5: Regulation of Enzyme Activity

Allosteric Regulation

• Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site.

• Can either inhibit or stimulate enzyme activity.

Types of Enzyme Inhibitors

• Competitive inhibitors bind to the active site, competing with the substrate.

• Noncompetitive inhibitors bind to another part of the enzyme, causing a shape change that reduces the effectiveness of the active site.

Feedback Inhibition

• Feedback inhibition occurs when the end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, thus regulating the pathway’s activity.

• This prevents the cell from wasting resources by synthesizing more product than needed.