Energy, Enzymes, and Cellular Respiration: Study Notes for General Biology

Energy and Life

Definition and Importance of Energy

Energy is the capacity to perform work and is essential for all living organisms. Life depends on the ability to convert energy from one form to another.

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

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

• The most important potential energy for living things is chemical energy stored in molecules.

• Potential energy can be converted to kinetic energy.

Thermodynamics and Energy Transformations

The Laws of Thermodynamics

Thermodynamics is the study of energy transformations. Two fundamental laws govern these processes:

• First Law of Thermodynamics: Energy can be changed from one form to another but cannot be created or destroyed (also known as the law of energy conservation).

• Second Law of Thermodynamics: Energy transformations increase disorder, or entropy, and some energy is lost as heat.

Chemical Reactions and Energy

Exergonic and Endergonic Reactions

Chemical reactions either store or release energy:

• Exergonic reactions: Release energy to the surroundings (e.g., cellular respiration, burning fuel).

• Endergonic reactions: Require an input of energy and store energy in products (e.g., photosynthesis, synthesis of glycogen from monosaccharides).

Cells carry out thousands of chemical reactions, many of which are coupled so that energy released from exergonic reactions is used to drive endergonic reactions.

Energy Diagrams

• Exergonic reactions have products with less potential energy than reactants.

• Endergonic reactions have products with more potential energy than reactants.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

ATP (adenosine triphosphate) is the main energy carrier in cells. It consists of adenine, ribose, and three phosphate groups.

• The energy in ATP is stored in the bonds between its phosphate groups.

• ATP powers nearly all forms of cellular work by transferring a phosphate group to another molecule (phosphorylation).

Structure of ATP:

• Adenine (a nitrogenous base)

• Ribose (a five-carbon sugar)

• Three phosphate groups

ATP Cycle: ATP is continually regenerated from ADP and inorganic phosphate through cellular respiration.

Energy Coupling

Energy coupling is the use of energy released from exergonic reactions to drive endergonic reactions. ATP molecules are central to this process, enabling cells to perform work.

• Energy coupling is essential for all cellular activities.

Enzymes and Metabolic Pathways

Role of Enzymes

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required for the reaction to proceed.

• Activation energy: The amount of energy that must be input before a chemical reaction will proceed.

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

Enzyme Specificity and Function

• Each enzyme has a unique three-dimensional shape that determines which chemical reaction it catalyzes.

• The active site is the region on the enzyme where the substrate binds.

• The induced fit model describes how the enzyme changes shape to fit the substrate more closely.

• A single enzyme may act on thousands or millions of substrate molecules per second.

Factors Affecting Enzyme Activity

• Temperature, pH, and substrate concentration can affect enzyme activity.

• Some enzymes require nonprotein cofactors (e.g., metal ions or organic molecules called coenzymes) to function properly.

Enzyme Inhibition

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

• Noncompetitive inhibitors: Bind elsewhere on the enzyme, changing its shape and reducing activity.

• Feedback inhibition: The end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, regulating the pathway's activity.

Photosynthesis and Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is the process by which cells break down glucose and other organic molecules to produce ATP, releasing carbon dioxide and water as byproducts. It occurs mainly in the mitochondria of eukaryotic cells.

• Cellular respiration is an exergonic process that transfers energy from glucose to ATP.

• Approximately 34% of the energy in glucose is captured as ATP; the rest is lost as heat.

• Other organic molecules can also be used as energy sources.

Redox Reactions in Cellular Respiration

• Energy is extracted from organic molecules through the transfer of electrons in redox reactions.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance.

• When glucose is oxidized, electrons are transferred to oxygen, which is reduced to water.

Example equation for cellular respiration:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP} + \text{heat}$

Stages of Cellular Respiration

• Stage 1: Glycolysis

  • Occurs in the cytoplasm.

  • Breaks down glucose into two molecules of pyruvate.

• Stage 2: The Citric Acid Cycle (Krebs Cycle)

  • Occurs in the mitochondria.

  • Completes the breakdown of glucose and supplies electrons to the next stage.

• Stage 3: Oxidative Phosphorylation

  • Occurs in the inner mitochondrial membrane.

  • Uses electrons carried by NADH and FADH2 to generate ATP through the electron transport chain and chemiosmosis.

Summary Table: Stages of Cellular Respiration

Stage	Location	Main Events
Glycolysis	Cytoplasm	Glucose split into 2 pyruvate; small amount of ATP produced
Citric Acid Cycle	Mitochondrial matrix	Pyruvate broken down; CO2 released; electrons transferred to carriers
Oxidative Phosphorylation	Inner mitochondrial membrane	Electron transport chain and chemiosmosis generate most ATP

Additional info: These notes expand on the provided slides by including definitions, examples, and a summary table for clarity and completeness.