Chapter 6: Energy and Metabolism

Introduction to Energy in Biology

Energy is a fundamental concept in biology, essential for all cellular processes. Organisms use energy to grow, reproduce, and maintain homeostasis. Understanding the forms and transformations of energy is crucial for studying metabolism and cellular function.

• Energy: The capacity to do work or cause change.

• Kinetic Energy: The energy of motion. Example: A child sliding down a slide.

• Potential Energy: Stored energy due to position or structure. Example: A child at the top of a slide has potential energy due to gravity.

• Application: In cells, potential energy is stored in chemical bonds and can be converted to kinetic energy during reactions.

Electron Energy Levels

Electrons occupy specific energy levels around the nucleus of an atom. The arrangement and movement of electrons are directly related to energy changes in chemical reactions.

• Energy Levels: Electrons closer to the nucleus have lower energy; those farther away have higher energy.

• Energy Absorption: Electrons can absorb energy and move to higher energy levels (excited state).

• Relevance: Electron transfers are central to cellular energy processes, such as respiration and photosynthesis.

Redox Reactions

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

• Oxidation: Loss of electrons from a molecule, often resulting in lower energy.

• Reduction: Gain of electrons by a molecule, often resulting in higher energy.

• Redox Reaction Example: In cellular respiration, glucose is oxidized and oxygen is reduced, releasing energy.

Flow of Energy and Thermodynamics

Thermodynamics in Biology

Thermodynamics is the branch of chemistry concerned with energy changes. Biological systems obey the laws of physics and chemistry, including the laws of thermodynamics.

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed from one form to another.

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe.

• Entropy: A measure of disorder or randomness in a system.

• Application: Cells must constantly obtain energy to maintain order and perform work.

Metabolism

Definitions and Types of Metabolic Pathways

Metabolism encompasses all chemical reactions in living organisms, divided into two main types: anabolism and catabolism.

• Metabolism: The sum of all chemical reactions in a cell.

• Anabolism: The synthesis of complex molecules from simpler ones; requires energy (endergonic).

• Catabolism: The breakdown of complex molecules into simpler ones; releases energy (exergonic).

• Example: Protein synthesis is anabolic; cellular respiration is catabolic.

Energy in Chemical Reactions

Chemical reactions can either require energy input or release energy. These are classified as endergonic or exergonic reactions.

• Endergonic Reaction: Requires energy input; not spontaneous.

• Exergonic Reaction: Releases energy; spontaneous.

• Reactants and Products: In endergonic reactions, products have more energy than reactants; in exergonic reactions, products have less energy.

Comparison of Endergonic and Exergonic Reactions

Role of Catalysts and Activation Energy

Catalysts, such as enzymes, speed up chemical reactions by lowering the activation energy required for the reaction to proceed.

• Activation Energy: The minimum energy required to start a chemical reaction.

• Catalyst: A substance that increases the rate of a reaction without being consumed.

• Enzymes: Biological catalysts that are typically proteins; they are specific to substrates and reactions.

ATP and Energy Coupling

ATP: The Energy Currency of the Cell

Adenosine triphosphate (ATP) is the primary energy carrier in cells. ATP hydrolysis releases energy that can be used to power endergonic reactions.

• ATP Hydrolysis:

• Energy Coupling: The energy released from ATP hydrolysis is used to drive endergonic reactions, such as biosynthesis.

• Anabolic Reaction Example: (requires energy)

• Catabolic Reaction Example: (releases energy)

Enzymes: Biological Catalysts

Structure and Function of Enzymes

Enzymes are globular proteins that catalyze biochemical reactions. Their function depends on their three-dimensional shape, which forms an active site for substrate binding.

• Active Site: The region of the enzyme where the substrate binds and the reaction occurs.

• Enzyme-Substrate Complex: Temporary association between enzyme and substrate during the reaction.

• Enzyme Properties:

  • Not changed or consumed in reactions

  • Highly specific for their substrates

  • Function depends on proper folding (shape)

• Denaturation: Loss of enzyme structure due to extreme temperature or pH, resulting in loss of function.

• Homeostasis: Maintaining stable internal conditions helps preserve enzyme structure and function.

Factors Affecting Enzyme Activity

Enzyme activity is influenced by temperature, pH, and the presence of inhibitors.

• Optimum Temperature: Each enzyme has a temperature at which it works best.

• Optimum pH: Each enzyme has a pH range where activity is highest (e.g., pepsin in stomach acid, trypsin in intestine).

Enzyme Activity Table

Enzyme Inhibition

Enzyme activity can be regulated by inhibitors, which decrease or stop enzyme function.

• Competitive Inhibition: Inhibitor competes with substrate for the active site, blocking substrate binding.

• Noncompetitive (Allosteric) Inhibition: Inhibitor binds to a site other than the active site (allosteric site), changing the enzyme's shape so the substrate cannot bind.

Feedback Inhibition

Feedback inhibition is a regulatory mechanism where the end product of a metabolic pathway inhibits an earlier step, preventing overproduction.

• Example: The end product binds to an enzyme involved in the pathway, reducing its activity.

• Application: Helps maintain metabolic balance and homeostasis in cells.