Cellular Transport and Metabolism: Microbiology Study Notes

Chapter 4: Membrane Transport Mechanisms

Overview of Membrane Transport

Cells regulate the movement of substances across their membranes using various transport mechanisms. These processes are essential for maintaining homeostasis, acquiring nutrients, and removing waste products. Transport can be classified based on energy requirements and the involvement of carrier proteins.

• Two main forms of transport: Active (requires energy) and Passive (does not require energy)

Active Transport

• Definition: Movement of molecules against their concentration gradient (from low to high concentration), requiring energy (usually ATP) and carrier proteins.

• Examples:

  • Sodium-potassium pump in animal cells

  • Proton pumps in bacterial cells

• Energy Source: ATP is needed

Passive Transport

• Definition: Movement of substances down their concentration gradient (from high to low concentration) without the use of cellular energy.

• Types:

  • Simple Diffusion: Direct movement of molecules across the membrane; no carrier required. Example: O2, CO2.

  • Osmosis: Diffusion of water across a selectively permeable membrane. Water moves toward the side with higher solute concentration.

  • Facilitated Diffusion: Movement of molecules via specific carrier proteins; no energy required. Example: Glucose transporters.

Bulk Transport (Vesicular Transport)

Used for large particles or large numbers of molecules. Involves vesicles or vacuoles.

• Endocytosis: Uptake of substances into the cell via vesicle formation.

  • Phagocytosis: Ingestion of large particles ("cell eating")

  • Pinocytosis: Uptake of dissolved substances ("cell drinking")

• Exocytosis: Expulsion of materials out of the cell via vesicles

Tonicity and Osmotic Effects

Tonicity describes the effect of a solution on cell volume. It is determined by the relative concentrations of solutes inside and outside the cell.

Solution Type	Solute Concentration	Effect on Cell
Isotonic	Equal inside and outside	No net water movement; cell remains the same
Hypertonic	More solute outside	Water leaves cell; cell shrinks (crenation in animal cells, plasmolysis in plant/bacterial cells)
Hypotonic	Less solute outside	Water enters cell; cell swells (lysis in animal cells, turgor in plant cells)

Chapter 5: Microbial Metabolism

Introduction to Metabolism

Metabolism encompasses all chemical reactions in a cell, divided into two main types:

• Anabolism: Biosynthetic reactions that build complex molecules from simpler ones; require energy.

• Catabolism: Degradative reactions that break down complex molecules into simpler ones; release energy.

Enzymes and Their Function

• Definition: Enzymes are biological catalysts, usually proteins, that speed up chemical reactions by lowering activation energy.

• Properties:

  • Specific for their substrates

  • Not consumed in the reaction

  • Names often end in "-ase"

• Enzyme Structure:

  • Apoenzyme: Protein portion (inactive without cofactor)

  • Cofactor: Non-protein component (metal ion or coenzyme)

  • Holoenzyme: Complete, active enzyme (apoenzyme + cofactor)

Factors Affecting Enzyme Activity

• Temperature: High temperatures can denature enzymes (loss of shape and function)

• pH: Extreme pH can denature enzymes

• Substrate Concentration: Increased substrate increases reaction rate until saturation

• Inhibitors:

  • Competitive: Bind to active site, blocking substrate

  • Non-competitive: Bind elsewhere, changing enzyme shape

  • Feedback inhibition: End product inhibits an earlier step in the pathway

ATP: The Cell's Energy Currency

• ATP (Adenosine Triphosphate): Main energy carrier in cells

• Energy Release: Hydrolysis of ATP to ADP + Pi releases energy

• ATP Generation:

  • Substrate-level phosphorylation: Direct transfer of phosphate to ADP

  • Oxidative phosphorylation: Electron transport chain and chemiosmosis

  • Photophosphorylation: Light-driven ATP synthesis (photosynthetic organisms)

Cellular Respiration Pathways

Overview

Cellular respiration is the process by which cells extract energy from nutrients. It involves a series of enzyme-catalyzed reactions that convert glucose to ATP, CO2, and water.

Major Steps of Cellular Respiration

• Glycolysis: Glucose is split into two pyruvate molecules. Occurs in the cytoplasm. Net gain: 2 ATP, 2 NADH, 2 pyruvate.

• Transition Step: Pyruvate is converted to acetyl-CoA. Net gain: 2 NADH, 2 CO2.

• Krebs Cycle (Citric Acid Cycle): Acetyl-CoA is oxidized, producing CO2, NADH, FADH2, and ATP. Net gain: 2 ATP, 6 NADH, 2 FADH2, 4 CO2 (per glucose).

• Electron Transport Chain (ETC): NADH and FADH2 donate electrons, driving ATP synthesis via oxidative phosphorylation. Net gain: 34 ATP (theoretical maximum).

Total ATP yield per glucose: 36-38 ATP (in the presence of oxygen and complete oxidation).

Fermentation

• Definition: Anaerobic process that allows ATP production without oxygen.

• Products: Lactic acid, ethanol, or other organic molecules, depending on the organism.

• ATP Yield: Only 2 ATP per glucose (from glycolysis).

Summary Table: Cellular Respiration Steps and ATP Yield

Step	Main Events	ATP Produced
Glycolysis	Glucose → 2 Pyruvate	2
Transition Step	2 Pyruvate → 2 Acetyl-CoA	0
Krebs Cycle	2 Acetyl-CoA → 4 CO2	2
Electron Transport Chain	NADH, FADH2 → ATP	34
Total		36-38

Key Terms and Definitions

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Tonicity: The ability of a solution to cause a cell to gain or lose water.

• Enzyme: Biological catalyst that speeds up chemical reactions.

• ATP: Adenosine triphosphate, the main energy currency of the cell.

• Fermentation: Anaerobic process for ATP production.

Formulas and Equations

• ATP Hydrolysis: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Overall Cellular Respiration: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy (ATP)}$

Additional info:

• Some details, such as the exact number of ATP produced, can vary depending on the organism and conditions.

• Feedback inhibition is a common regulatory mechanism in metabolic pathways.