Molecules & Energy: Enzymes, Metabolism, and Cellular Respiration

Molecules & Energy

Introduction

This section explores how living organisms convert food into usable energy, focusing on the biochemical processes of metabolism and cellular respiration. These processes are essential for sustaining life, as they provide the energy required for cellular activities.

Metabolism

Definition and Overview

• Metabolism refers to the set of chemical reactions that occur within living organisms to maintain life.

• These reactions convert biochemical energy from food into ATP (adenosine triphosphate), the main energy currency of the cell.

• Metabolic pathways are interconnected networks that allow for the conversion of molecules, energy transfer, and synthesis of cellular components.

Types of Metabolic Pathways

• Catabolic pathways: Breakdown of molecules to harvest energy and produce ATP.

• Anabolic pathways: Synthesis of larger molecules from smaller units, usually requiring energy input in the form of ATP.

Main Classes of Metabolism

• Carbohydrate metabolism

• Lipid metabolism

• Amino acid metabolism

• Nucleotide metabolism

Common Themes in Metabolic Pathways

• Pathways that break down molecules are connected to those that build larger molecules.

• Enzymes catalyze each step, working together in a manner similar to an assembly line.

• Regulation ensures a balance between catabolic and anabolic processes, maintaining homeostasis.

Enzymes and Their Regulation

Role of Enzymes in Metabolism

• Enzymes are biological catalysts that speed up chemical reactions without being consumed.

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

Enzyme Regulation

• Non-covalent modification: Regulatory molecules bind to enzymes, affecting their activity.

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

• Allosteric inhibition: Inhibitor binds to a site other than the active site, causing a conformational change that reduces enzyme activity.

• Allosteric activation: Activator binds to a site other than the active site, increasing enzyme activity.

Covalent Modifications

• Enzyme activity can be regulated by covalent modifications such as phosphorylation.

• Phosphorylation can cause a conformational change in the enzyme, altering its activity. This process is often reversible.

• Cleavage of peptide bonds (e.g., activation of zymogens) is another form of covalent modification.

Feedback Inhibition

• A regulatory mechanism where the end product of a metabolic pathway inhibits an earlier step, preventing overproduction.

• This is a form of negative feedback that helps maintain metabolic balance.

Energy Transfer and Cellular Respiration

Energy Transfer in Cells

• Cells obtain glucose to make ATP.

• Plants produce glucose during photosynthesis; other organisms obtain glucose from food.

• Respiration is the process by which cells extract energy from glucose.

Cellular Respiration

• Cellular respiration is a set of reactions that uses electrons from high-energy molecules to make ATP.

• Two fundamental requirements for cells:

  • Energy source to generate ATP

  • Source of carbon for synthesizing macromolecules

Central Role of Glucose

• Glucose is a primary fuel for cellular respiration.

• All major classes of biomolecules (carbohydrates, lipids, proteins) can be converted into intermediates that feed into the central pathway of cellular respiration.

Fuel Sources for Cellular Respiration

• Carbohydrates: Digested and used for ATP production.

• Fats: Glycerol enters glycolysis; fatty acids are converted to acetyl-CoA and enter the citric acid cycle.

• Proteins: Amino acids are deaminated; carbon skeletons enter as pyruvate, acetyl-CoA, or other intermediates.

Key Intermediates and Their Roles

• Pyruvate: Glucose is stored as glycogen/starch and broken down to pyruvate.

• Glycolysis: Produces intermediates for nucleotides (DNA/RNA).

• Acetyl-CoA: Used to make fatty acids, phospholipids, and feeds into the citric acid cycle.

• Citric acid cycle: Produces many amino acids and other biosynthetic precursors.

Overall Reaction of Cellular Respiration

The oxidation of glucose to carbon dioxide and water releases energy:

$\mathrm{C_6H_{12}O_6} + 6\,\mathrm{O_2} \rightarrow 6\,\mathrm{CO_2} + 6\,\mathrm{H_2O}$

• This reaction releases energy as heat and light, but in cells, the energy is captured in ATP.

• ATP is not stable and must be continually synthesized by the cell.

Importance of Energy Flow

• Cells require a constant supply of ATP to sustain activity.

• Controlling energy flow is essential for life and has been a major step in the evolution of living organisms.

Summary Table: Major Metabolic Pathways

Pathway	Main Function	Key Intermediates	End Products
Glycolysis	Breakdown of glucose to produce ATP and pyruvate	Glucose, pyruvate	ATP, NADH, pyruvate
Citric Acid Cycle	Oxidation of acetyl-CoA to CO2; energy extraction	Acetyl-CoA, intermediates	CO2, NADH, FADH2, ATP
Electron Transport Chain	Transfer of electrons to produce ATP	NADH, FADH2	ATP, H2O
Lipid Metabolism	Breakdown and synthesis of fats	Fatty acids, glycerol, acetyl-CoA	ATP, fatty acids, glycerol
Amino Acid Metabolism	Breakdown and synthesis of amino acids	Amino acids, intermediates	ATP, amino acids, urea

Example: Feedback Inhibition in Glycolysis

• The end product of glycolysis, ATP, can inhibit the enzyme phosphofructokinase, slowing down the pathway when ATP is abundant.

Additional info: The notes also reference the importance of metabolic regulation and the interconnectedness of metabolic pathways, which is a key concept in understanding how cells adapt to changing energy demands.