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.

• Life requires a constant supply of energy, and cells are continually making ATP because it is unstable and cannot be stored in large amounts.

Energy Transfer in Metabolism

• Cells obtain glucose to make ATP.

• Plants produce glucose during photosynthesis (see Chapter 10).

• Other organisms obtain glucose from food through respiration.

Types of Metabolic Pathways

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

• Anabolic pathways: Synthesis of larger molecules using energy, often in the form of ATP.

• Major metabolic pathways include carbohydrate metabolism, lipid metabolism, nucleotide metabolism, and amino acid metabolism.

Common Themes in Metabolic Pathways

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

• Enzymes catalyze these chemical reactions, working together in a manner resembling an assembly line (e.g., Enzyme 1 → Enzyme 2 → Enzyme 3).

• Enzymes are regulated to maintain a balance (homeostasis) between catabolic and anabolic pathways.

• Regulation occurs via non-covalent and covalent modifications.

Enzyme Regulation

Non-covalent Modification

• Regulatory molecules can control when and where an enzyme functions.

• 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, causing a conformational change that increases enzyme activity.

Covalent Modifications

• Involves the cleavage of peptide bonds (e.g., converting a zymogen to an active enzyme) or the addition of chemical groups (e.g., phosphorylation).

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

Feedback Inhibition

• A convenient way to regulate a metabolic pathway is by using the final product to inactivate one of the pathway's enzymes.

• This prevents the overproduction of the end product and helps maintain metabolic balance.

Cellular Respiration

Definition and Requirements

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

• Cells require:

  1. An energy source to generate ATP.

  2. A source of carbon to use as raw materials for synthesizing macromolecules.

Central Role of Glucose

• Glucose is a primary fuel for cellular respiration.

• Molecules from food feed into the central pathway of cellular respiration, often entering as glucose or intermediates.

Major 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 (removal of amino group); carbon skeletons enter as pyruvate, acetyl-CoA, or other intermediates.

Intermediates and Biosynthesis

• Intermediates from cellular respiration are used to synthesize other molecules:

  • Pyruvate: Can be stored as glycogen or starch.

  • Glycolysis: Provides precursors for nucleotides (DNA/RNA).

  • Acetyl-CoA: Used to make fatty acids, phospholipids, and steroids.

  • Citric acid cycle: Provides intermediates for amino acid synthesis.

Why is Cellular Respiration Important?

• When glucose is oxidized to carbon dioxide, energy is released as heat and light:

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

• The cell only keeps enough ATP to sustain activity for 30 seconds to a few minutes.

• ATP is not stable, so cells must continually synthesize it.

• Rather than releasing all energy at once, cells harvest it in small, usable amounts.

• Enzymes and stepwise reactions help cells accomplish this task efficiently.

• Controlling energy flow has been a major evolutionary innovation.

Summary Table: Major Metabolic Pathways

Pathway	Main Function	Key Intermediates	End Products
Glycolysis	Breakdown of glucose to pyruvate	Glucose, pyruvate	ATP, NADH, pyruvate
Citric Acid Cycle	Oxidation of acetyl-CoA	Acetyl-CoA, citric acid	CO2, NADH, FADH2, ATP
Electron Transport Chain	Transfer of electrons to produce ATP	NADH, FADH2	ATP, H2O
Photosynthesis	Production of glucose from CO2 and H2O	CO2, H2O	Glucose, O2

Additional info: The notes emphasize the interconnectedness of metabolic pathways and the importance of regulation for maintaining cellular homeostasis. Enzyme regulation is a key concept, with both non-covalent and covalent mechanisms ensuring that metabolic processes are responsive to the cell's needs.