The Working Cell

Introduction

This study guide covers the structure and function of biological membranes, mechanisms of molecular transport, and the role of enzymes and cellular respiration in energy transformation. These concepts are foundational for understanding how cells maintain homeostasis and interact with their environment.

Membrane Structure and the Fluid Mosaic Model

Overview of the Fluid Mosaic Model

• Definition: The fluid mosaic model describes the structure of cell membranes as a mosaic of diverse protein molecules embedded in a fluid bilayer of phospholipids.

• Phospholipid Bilayer: The membrane consists of two layers of phospholipids with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

• Proteins: Membrane proteins serve various functions, including transport, signal transduction, and cell recognition.

• Cholesterol: Cholesterol molecules interspersed within the bilayer help regulate membrane fluidity, especially in animal cells.

Example: The plasma membrane of animal cells contains cholesterol, which prevents the membrane from becoming too rigid or too fluid under different temperature conditions.

Membrane Fluidity

• Factors Affecting Fluidity: Temperature, fatty acid composition (saturated vs. unsaturated), and cholesterol content influence membrane fluidity.

• Rotational and Lateral Movement: Phospholipids and some proteins can move laterally within the layer, contributing to the dynamic nature of the membrane.

Additional info: Unsaturated fatty acids increase fluidity, while saturated fatty acids decrease it.

Membrane Transport Mechanisms

Passive Transport

Passive transport is the movement of substances across the membrane without the use of cellular energy (ATP).

• Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) down their concentration gradient.

• Facilitated Diffusion: Movement of larger or polar molecules (e.g., glucose, ions) via specific transport proteins (channels or carriers).

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

Equation:

where is the flux, is the permeability coefficient, and is the concentration difference across the membrane.

Active Transport

Active transport requires energy (usually from ATP) to move substances against their concentration gradient.

• Primary Active Transport: Direct use of ATP to transport molecules (e.g., sodium-potassium pump).

• Secondary Active Transport: Uses the energy from the movement of one substance down its gradient to move another substance against its gradient.

Example: The Na+/K+ ATPase pump moves 3 Na+ ions out and 2 K+ ions into the cell per ATP hydrolyzed.

Comparison of Transport Mechanisms

Osmosis and Water Balance

• Osmosis: The diffusion of water across a selectively permeable membrane from a region of lower solute concentration to higher solute concentration.

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

• Types of Solutions:

  • Isotonic: Equal solute concentration; no net water movement.

  • Hypotonic: Lower solute concentration outside; cell gains water (may burst in animal cells, becomes turgid in plant cells).

  • Hypertonic: Higher solute concentration outside; cell loses water (shrivels in animal cells, becomes plasmolyzed in plant cells).

Bulk Transport

• Endocytosis: Uptake of large molecules or particles by engulfing them in vesicles.

• Phagocytosis: "Cell eating"; engulfment of large particles.

• Pinocytosis: "Cell drinking"; uptake of fluids and dissolved solutes.

• Receptor-mediated Endocytosis: Specific uptake of molecules via receptor proteins.

• Exocytosis: Export of materials from the cell by fusion of vesicles with the plasma membrane.

Membrane Proteins: Channels vs. Active Transporters

Channels

• Function: Provide hydrophilic pathways for specific molecules or ions to cross the membrane.

• Example: Aquaporins facilitate rapid water movement.

• Passive: Do not require energy; allow movement down concentration gradients.

Active Transporters

• Function: Bind specific solutes and undergo conformational changes to transport them across the membrane, often against their gradient.

• Energy Use: Require ATP or another energy source.

Intercellular Channels: Gap Junctions and Plasmodesmata

Gap Junctions (Animal Cells)

• Definition: Specialized intercellular connections that directly connect the cytoplasm of two animal cells, allowing the passage of ions and small molecules.

• Function: Facilitate communication and coordination between adjacent cells.

Plasmodesmata (Plant Cells)

• Definition: Channels that traverse the cell walls of plant cells, enabling transport and communication between them.

• Function: Allow the movement of water, nutrients, and signaling molecules.

Energy and the Cell: Cellular Respiration

Overview

• Cellular Respiration: The process by which cells convert glucose and oxygen into ATP, the main energy currency of the cell, with carbon dioxide and water as byproducts.

• Equation:

Example: Muscle cells use ATP generated by cellular respiration to contract during movement.

How Enzymes Function

Enzyme Structure and Function

• Definition: Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required.

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

• Specificity: Each enzyme is specific to its substrate due to the unique shape of its active site.

Example: The enzyme sucrase catalyzes the breakdown of sucrose into glucose and fructose.