Cell Transport and Membrane Function

Cell Transport Overview

  • Study of how substances are moved across the plasma membrane.

Plasma Membrane Structure and Function

  • Also called the cell membrane.

  • Functions as a boundary between the outside and inside of the cell.

  • Composed of a phospholipid bilayer with attached or embedded proteins.

  • At body temperature, the bilayer is liquid, enabling the proteins to move within the membrane.

  • The fluid-mosaic model describes the structure and function of the plasma membrane.

Fluid Mosaic Model Components

  • Flexible: The phospholipids in each layer can move laterally, resembling a fluid.

  • Mosaic: Various proteins and molecules are embedded in the bilayer, contributing to a diversity of functions.

Components of the Cell Membrane

  • Proteins (blue)

  • Carbohydrates (green)

  • Cytoskeletal Proteins

  • Cholesterol (bright yellow)

  • Phospholipids (red and gold)

Membrane Functions

  • Allows cell-cell communication.

  • Attached carbohydrates help the body identify cells as “self.”

  • Maintains the integrity of the cell.

  • The overall structure is selectively permeable, allowing only certain molecules to enter and exit without assistance.

Types of Molecular Movement Across Plasma Membrane

  1. Passive Transport

    • No ATP is required.

    • Movement due to differences in concentration.

  2. Active Transport

    • Powered by cellular energy (ATP).

    • Molecules move against the concentration gradient.

Passive Transport Mechanisms

  • Diffusion: Movement from an area of higher concentration to an area of lower concentration until equilibrium is reached.

  • Osmosis: Diffusion of water across the cell membrane towards higher solute concentration.

  • Facilitated diffusion: Molecules move through specialized proteins embedded in the membrane.

Osmosis in Detail

  • Cells contain dilute solutions of ions, sugars, and amino acids.

  • The cell membrane is semi-permeable.

  • Water moves by osmosis, following the concentration gradient of solute:

    • Water will migrate towards a higher solute concentration, leading to opposing concentrations of solute and water within the cell and surrounding environment.

Diagram of Osmosis Model

  • Partially permeable membrane separating two solutions:

    • Solution 1: Lower sugar concentration (Higher water concentration).

    • Solution 2: Higher sugar concentration (Lower water concentration).

  • Water molecules diffuse from Solution 1 to Solution 2, demonstrating osmotic movement.

Significance of Osmosis in Cells

  • Animal cells do not have a rigid cell wall, making them susceptible to osmotic pressure.

  • Red blood cells (RBCs) can be destroyed in hypertonic or hypotonic solutions.

Tonicity Concept

  • Tonicity: Comparing solute concentration inside the cell to the solution outside.

  • Types of Tonicity:

    • Isotonic: Solute concentration equal inside and outside; water moves in and out equally.

    • Hypotonic: Lower solute concentration outside; water moves inside causing cells to swell and potentially lyse.

    • Hypertonic: Higher solute concentration outside; water moves out causing cells to crenate or shrink.

Isotonic Solution Example

  • Example: 0.9% NaCl solution: No net gain or loss of water; RBCs remain normal in shape.

Hypotonic Solution Example

  • Example: 100% water: RBCs swell due to higher water movement into the cell.

Hypertonic Solution Example

  • Example: 5% NaCl solution: Water exits RBCs, causing them to shrink.

Active Transport Mechanisms

  • Active transport: Requires energy (ATP) to move substances against their concentration gradient.

Types of Active Transport

  1. Molecular pumps: Move molecules from lower concentration to higher concentration.

    • Example: Sodium-Potassium Pump.

  2. Bulk Transport: Involves vesicles for moving larger quantities of materials.

    • Endocytosis: Material intake by surrounding with membrane and forming vesicles.

      • Phagocytosis: Engulfing particles or cells.

      • Pinocytosis: Intake of fluid and small molecules.

    • Exocytosis: Fusing vesicles with the plasma membrane to release substances outside the cell.

Why Exocytosis Matters

  • Essential for communication between nerve cells and muscle movements. Without exocytosis, muscle contractions would not occur.

Summary of Cell Transport Mechanisms

  • Passive Transport:

    • Diffusion, Osmosis, Facilitated transport

  • Active Transport:

    • Molecular pumps

    • Bulk transport (Endocytosis: phagocytosis/pinocytosis, Exocytosis)

  • Energy Requirement: Passive transport does not require energy; active transport requires energy (ATP).