Chapter 8: Energy and Metabolism – Study Notes

Energy and Metabolism

Introduction to Metabolism

Metabolism encompasses all chemical reactions occurring within an organism, enabling it to maintain life. These reactions are organized into metabolic pathways, each beginning with a specific molecule and ending with a product, catalyzed by enzymes.

• Metabolism: The totality of an organism’s chemical reactions.

• Emergent property: Metabolism arises from orderly interactions between molecules.

• Metabolic pathway: A series of chemical reactions, each catalyzed by a specific enzyme, transforming a starting molecule into a product.

• Enzyme: A molecule (usually a protein) that speeds up chemical reactions without being consumed.

• Example: The breakdown of glucose in cellular respiration involves multiple enzymes and steps.

Types of Metabolic Pathways

Metabolism is divided into two main types: anabolism and catabolism. These pathways either build up or break down molecules, managing the cell’s energy and material resources.

• Anabolism (anabolic reactions): Processes that build complex molecules from simpler ones. These reactions consume energy.

• Catabolism (catabolic reactions): Processes that break down complex molecules into simpler ones, releasing energy.

• Example: Synthesis of protein from amino acids (anabolism); breakdown of glucose during cellular respiration (catabolism).

Energy in Biological Systems

Forms of Energy

Energy is the capacity to cause change and exists in various forms, some of which can perform work in biological systems.

• Kinetic energy: Energy of motion.

• Heat energy (thermal energy): Kinetic energy associated with random movement of atoms or molecules.

• Potential energy: Energy stored due to an object’s position or structure, such as energy in 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).

• Example: A diver on a platform has potential energy, which is converted to kinetic energy when diving.

Thermodynamics in Biology

Thermodynamics is the study of energy transformations. Biological systems are open systems, exchanging energy and matter with their surroundings.

• First Law of Thermodynamics (Law of Conservation of Energy): Energy can be transferred and transformed, but not created or destroyed.

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe; energy conversions are less than 100% efficient, with some energy lost as heat.

• Organisms require a constant influx of energy to maintain order and function.

• Example: Cells use energy from food to maintain their structure and perform work.

Metabolic Reactions and Free Energy

Chemical Reactions and Free Energy

Chemical reactions involve changes in chemical bonds and concentrations of substances, resulting in changes in free energy (G), which is the energy available to do work.

• Free energy (G): Energy available to do work in a chemical system.

• Reactions reach dynamic equilibrium when forward and reverse rates are equal; living cells rarely reach equilibrium.

• Metabolism is never at equilibrium; pathways release free energy in a series of steps.

• Exergonic reactions: Release free energy; products have less free energy than reactants; spontaneous.

• Endergonic reactions: Require input of energy; products have more free energy than reactants; not spontaneous.

• Example: ATP hydrolysis is highly exergonic: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i$

ATP: The Energy Currency of the Cell

Structure and Function of ATP

Adenosine triphosphate (ATP) is the main energy currency in cells, used to power cellular work by transferring phosphate groups.

• ATP: A nucleotide composed of adenine, ribose, and three phosphate groups.

• Energy is released when ATP is hydrolyzed to ADP and inorganic phosphate ($\text{P}_i$).

• ATP powers three main types of cellular work: chemical (synthesis of molecules), transport (pumping substances across membranes), and mechanical (movement, e.g., muscle contraction).

• ATP is regenerated from ADP by addition of a phosphate group, using energy from catabolic reactions.

• Example: Muscle cells recycle their entire pool of ATP in less than a minute.

Redox Reactions and Energy Harvesting

Redox Reactions

Redox (reduction-oxidation) reactions transfer electrons between molecules, playing a key role in energy harvesting in cells.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Redox reactions often involve transfer of hydrogen atoms (proton + electron).

• Common electron acceptors: NAD+, NADP+, FAD, cytochromes.

• Example: $\text{XH}_2 + \text{NAD}^+ \rightarrow \text{X} + \text{NADH} + \text{H}^+$

Enzymes: Regulation of Metabolic Reactions

Role and Mechanism of Enzymes

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy, without being consumed in the process.

• Enzyme: Usually a protein; some RNA molecules (ribozymes) also act as enzymes.

• Enzymes are highly specific for their substrates.

• Enzyme names often end in –ase (e.g., sucrase).

• Enzyme-substrate complex: Temporary association between enzyme and substrate during catalysis.

• Enzymes lower activation energy by:

  • Orienting substrates correctly

  • Straining substrate bonds

  • Providing a favorable microenvironment

  • Participating directly in the reaction

• Example: Sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

Enzyme Classification and Cofactors

Enzymes are classified by the type of reaction they catalyze. Many require cofactors to function.

• Enzyme classes: Oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases.

• Cofactors: Non-protein helpers; can be organic (coenzymes like NAD+, FAD) or inorganic (metal ions like Mg2+, Fe2+).

• Most vitamins are coenzymes or precursors to coenzymes.

Enzyme Activity: Environmental Effects and Regulation

Enzyme activity is affected by temperature, pH, and regulatory molecules. Cells regulate enzyme activity to control metabolic pathways.

• Each enzyme has an optimal temperature and pH for activity.

• Extreme conditions can denature enzymes, reducing activity.

• Enzyme activity can be regulated by:

  • Substrate concentration

  • Enzyme concentration

  • Compartmentalization within the cell

  • Allosteric regulation (activators/inhibitors binding at sites other than the active site)

  • Feedback inhibition (end product inhibits an early step in the pathway)

• Enzyme inhibition can be reversible or irreversible:

  • Competitive inhibition: Inhibitor competes with substrate for active site.

  • Noncompetitive inhibition: Inhibitor binds elsewhere, changing enzyme shape and reducing activity.

  • Irreversible inhibition: Inhibitor permanently inactivates enzyme (e.g., toxins, antibiotics).

• Example: Feedback inhibition regulates amino acid synthesis; isoleucine inhibits the first enzyme in its synthesis pathway.

Table: Comparison of Anabolic and Catabolic Pathways

Pathway Type	Function	Energy Change	Example
Anabolic	Builds complex molecules from simpler ones	Consumes energy	Protein synthesis from amino acids
Catabolic	Breaks down complex molecules into simpler ones	Releases energy	Cellular respiration (glucose breakdown)

Table: Enzyme Classes and Functions

Enzyme Class	Function	Example
Oxidoreductases	Catalyze oxidation/reduction reactions	Dehydrogenases
Transferases	Transfer functional groups	Kinases
Hydrolases	Catalyze hydrolysis reactions	Sucrase
Lyases	Cleave bonds by means other than hydrolysis or oxidation	Decarboxylases
Isomerases	Catalyze isomerization changes	Phosphoglucoisomerase
Ligases	Join two molecules with covalent bonds	DNA ligase

Additional info: Some context and examples have been expanded for clarity and completeness, including enzyme classification and regulatory mechanisms.