Chapter 6: An Introduction to Metabolism – Study Notes

Chapter 6: An Introduction to Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within an organism, enabling the transformation of matter and energy. These reactions are organized into metabolic pathways, each catalyzed by specific enzymes, and are fundamental to life’s processes.

• Metabolism: The sum of all chemical reactions in an organism.

• Metabolic pathway: Begins with a specific molecule and ends with a product, with each step catalyzed by a specific enzyme.

• Catabolic pathways: Break down complex molecules, releasing energy (e.g., cellular respiration).

• Anabolic pathways: Build complex molecules from simpler ones, consuming energy (e.g., protein synthesis).

• Bioenergetics: The study of how energy flows through living organisms.

Forms of Energy

Energy is the capacity to cause change and exists in various forms, some of which can perform work. Biological systems utilize and transform energy to maintain order and drive cellular processes.

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

• Thermal energy: Kinetic energy associated with random movement of atoms or molecules; heat is thermal energy in transfer.

• Potential energy: Energy due to location or structure (e.g., chemical energy in bonds).

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

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

The Laws of Thermodynamics in Biology

The First Law of Thermodynamics

The first law, also known as the principle of conservation of energy, states that energy can be transferred and transformed, but not created or destroyed. In biological systems, energy from food is converted into usable forms for cellular work.

• Energy input (e.g., food, sunlight) is transformed but not lost.

• Example: Chemical energy in food is converted to kinetic energy and heat in animals.

The Second Law of Thermodynamics

The second law states that every energy transfer or transformation increases the entropy (disorder) of the universe. Some energy is always lost as heat, increasing disorder.

• Entropy (S): A measure of disorder or randomness.

• Spontaneous processes increase the entropy of the universe and do not require energy input.

• Nonspontaneous processes decrease entropy and require energy input.

Biological Order and Disorder

Living organisms create ordered structures from less organized materials, but overall, the entropy of the universe increases. Organisms are islands of low entropy in a universe tending toward disorder.

Free Energy and Spontaneity

Free Energy Change (ΔG), Stability, and Equilibrium

Free energy (G) is the portion of a system’s energy that can do work at constant temperature and pressure. The change in free energy (ΔG) determines whether a process is spontaneous.

• ΔG < 0: Spontaneous process; system becomes more stable.

• ΔG > 0: Nonspontaneous process; requires energy input.

• At equilibrium, ΔG is at its lowest value and no work can be done.

Equation:

$\Delta G = \Delta H - T\Delta S$

Where: $\Delta G$ = change in free energy $\Delta H$ = change in enthalpy (total energy) $T$ = temperature in Kelvin $\Delta S$ = change in entropy

Exergonic and Endergonic Reactions

Metabolic reactions can be classified based on their free energy changes:

• Exergonic reactions: Release free energy; spontaneous ($\Delta G < 0$).

• Endergonic reactions: Absorb free energy; nonspontaneous ($\Delta G > 0$).

• Cellular respiration is exergonic; photosynthesis is endergonic.

Equilibrium and Metabolism

Cells are open systems and never reach equilibrium, allowing continuous work. Catabolic pathways release free energy in a series of reactions, preventing equilibrium and enabling life.

ATP and Cellular Work

ATP: Structure and Function

ATP (adenosine triphosphate) is the cell’s main energy currency. It consists of ribose, adenine, and three phosphate groups. ATP hydrolysis releases energy used to power cellular work.

• ATP hydrolysis: $\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{energy}$

• Energy is released due to the repulsion of negatively charged phosphate groups.

ATP Powers Cellular Work

ATP drives three main types of cellular work by coupling exergonic and endergonic reactions:

• Chemical work: Pushing endergonic reactions.

• Transport work: Pumping substances across membranes.

• Mechanical work: Movement, such as muscle contraction.

The ATP Cycle

ATP is regenerated by the addition of a phosphate group to ADP, using energy from catabolic reactions. This cycle is essential for maintaining cellular energy balance.

Enzymes and Metabolic Reactions

Enzymes as Catalysts

Enzymes are biological catalysts, usually proteins, that speed up metabolic reactions by lowering activation energy barriers without being consumed.

• Catalyst: Substance that increases reaction rate without being consumed.

• Enzyme: Macromolecule that acts as a catalyst (e.g., sucrase).

Activation Energy

Activation energy (EA) is the energy required to start a reaction. Enzymes lower EA, allowing reactions to proceed at cellular temperatures.

How Enzymes Work

Enzymes lower activation energy by:

• Orienting substrates correctly

• Straining substrate bonds

• Providing a favorable microenvironment

• Covalently bonding to the substrate

Substrate Specificity and the Active Site

Enzymes are specific for their substrates, binding at the active site to form an enzyme-substrate complex. The induced fit model describes how enzymes change shape to fit substrates.

The Catalytic Cycle of an Enzyme

The enzyme’s active site binds substrates, catalyzes the reaction, releases products, and is ready for another cycle.

Factors Affecting Enzyme Activity

Enzyme activity is influenced by temperature, pH, and chemicals. Each enzyme has optimal conditions for activity.

• High temperatures can denature enzymes.

• Optimal pH varies (e.g., pepsin in the stomach, trypsin in the intestine).

Cofactors and Coenzymes

Cofactors are nonprotein helpers required for enzyme function. They can be inorganic (e.g., metal ions) or organic (coenzymes, often derived from vitamins).

Enzyme Inhibitors

Enzyme inhibitors regulate enzyme activity:

• Competitive inhibitors: Bind to the active site, blocking substrate binding.

• Noncompetitive inhibitors: Bind elsewhere, altering enzyme shape and function.

• Inhibitors can be reversible or irreversible (e.g., toxins, antibiotics).

Regulation of Enzyme Activity

Cells regulate metabolism by controlling enzyme activity through:

• Switching genes on/off

• Allosteric regulation: Regulatory molecules bind to sites other than the active site, stabilizing active or inactive forms.

• Cooperativity: Substrate binding increases enzyme activity.

• Feedback inhibition: End product of a pathway inhibits an early enzyme, preventing overproduction.

Organization of Enzymes Within the Cell

Cellular compartmentalization brings order to metabolic pathways. Some enzymes are structural components of membranes or are localized within specific organelles (e.g., mitochondria for cellular respiration).

Additional info: These notes provide a comprehensive overview of metabolism, energy transformations, enzyme function, and regulation, as covered in a typical introductory biology course.