Cell Membranes: Structure, Function, and Transport Mechanisms

Cell Membranes

Introduction to Cell Membranes

The cell membrane, also known as the plasma membrane, is a fundamental structure that defines the boundary of the cell and regulates the movement of substances into and out of the cell. It is essential for maintaining cellular homeostasis and enabling communication with the environment.

• Cell membranes are present in all living cells, providing structural integrity and selective permeability.

• They are composed primarily of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• The fluid mosaic model describes the dynamic and heterogeneous nature of the membrane.

Cell Size and Surface Area-to-Volume Ratio

Constraints on Cell Size

Cells are generally small due to physical and metabolic constraints. The surface area-to-volume ratio is a critical factor that limits cell size and influences cellular function.

• As a cell increases in size, its volume grows faster than its surface area.

• This ratio affects the ability of the cell membrane to efficiently exchange materials (e.g., oxygen, nutrients, waste) with the environment.

• Large cells face challenges in transporting substances to and from the center of the cell.

Formula for Surface Area and Volume of a Cube:

• Surface Area: $6a^2$

• Volume: $a^3$

• Surface Area-to-Volume Ratio: $\frac{6}{a}$

Example: A plant cell (100-150 μm) has a lower surface area-to-volume ratio than a red blood cell (7.5 μm), making transport less efficient in larger cells.

Structure of the Plasma Membrane

Fluid Mosaic Model

The fluid mosaic model explains the structure and properties of the plasma membrane.

• Phospholipids are amphipathic molecules with hydrophilic heads and hydrophobic tails, forming a bilayer.

• The membrane is held together by weak hydrophobic interactions, allowing lateral movement of components.

• Proteins are interspersed within the bilayer, serving various functions (e.g., transport, signaling).

• Carbohydrates are attached to proteins (glycoproteins) or lipids (glycolipids) on the extracellular surface, playing roles in cell recognition.

• Cholesterol is present in animal cell membranes, modulating fluidity and stability.

Factors Affecting Membrane Fluidity

• Saturated fatty acids make the membrane more viscous (less fluid).

• Unsaturated fatty acids introduce kinks, preventing tight packing and increasing fluidity.

• Cholesterol acts as a fluidity buffer, reducing fluidity at high temperatures and preventing solidification at low temperatures.

• Organisms adapt membrane composition based on environmental conditions (e.g., cold-adapted organisms have more unsaturated fatty acids).

Membrane Proteins

Types and Functions of Membrane Proteins

• Peripheral proteins: Loosely bound to the membrane surface.

• Integral (transmembrane) proteins: Span the membrane, often involved in transport.

• Functions include:

  • Transport of molecules across the membrane

  • Enzymatic activity

  • Signal transduction

  • Cell-cell recognition

  • Intercellular joining

  • Attachment to the cytoskeleton and extracellular matrix

Selective Permeability and Transport Mechanisms

Types of Transport Across the Membrane

The plasma membrane is selectively permeable, allowing some substances to cross more easily than others.

• Passive Transport: Movement of substances down their concentration gradient without energy input.

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).

• Bulk Transport: Movement of large molecules or particles via vesicles (endocytosis and exocytosis).

Passive Transport

• Simple Diffusion: Nonpolar molecules (e.g., O2, CO2) diffuse directly through the lipid bilayer.

• Facilitated Diffusion: Polar molecules and ions move through channel or carrier proteins.

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

Osmosis and Tonicity

Osmosis is essential for maintaining water balance in cells. Tonicity describes the ability of a surrounding solution to cause a cell to gain or lose water.

Solution Type	Animal Cell	Plant Cell
Hypotonic	Lysed (bursts)	Turgid (normal)
Isotonic	Normal	Flaccid
Hypertonic	Shriveled	Plasmolyzed

Active Transport

• Requires energy to move substances against their concentration gradients.

• Maintains differences in solute concentrations between the cell and its environment.

• Sodium-potassium pump (Na+/K+ pump) is a classic example in animal cells.

• Active transport can also involve electrochemical gradients (membrane potential).

Equation for Membrane Potential:

• Membrane potential ($V_m$) is the voltage across the membrane, created by differences in ion concentrations.

Bulk Transport: Endocytosis and Exocytosis

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell.

• Endocytosis: The cell takes in materials by forming vesicles from the plasma membrane.

• Types of endocytosis:

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

  • Pinocytosis: "Cell drinking"; uptake of extracellular fluid.

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

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2 diffusion
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Osmosis	No	Down water gradient	Water via aquaporins
Active Transport	Yes	Against gradient	Na+/K+ pump
Endocytosis/Exocytosis	Yes	Bulk movement	Phagocytosis, neurotransmitter release

Key Terms

• Phospholipid bilayer: Double layer of phospholipids forming the core of the membrane.

• Amphipathic: Molecule with both hydrophilic and hydrophobic regions.

• Selective permeability: Property allowing some substances to cross more easily than others.

• Channel protein: Membrane protein forming a hydrophilic channel for specific molecules or ions.

• Carrier protein: Membrane protein that changes shape to transport substances across the membrane.

• Electrochemical gradient: Combined effect of concentration and electrical gradients on ion movement.

Additional info: The notes also reference the importance of membrane composition in different environments (e.g., cold-adapted archaea), and the role of aquaporins in facilitating water transport.