Cell Membrane Structure and Transport Processes – ANP1105 Study Notes

Cell Membrane Structure and Transport Processes

Fluid Mosaic Model of the Plasma Membrane

The plasma membrane is a dynamic structure that separates the cell from its external environment and regulates the movement of substances in and out of the cell. Its organization is best described by the fluid mosaic model, which highlights the flexible and varied nature of membrane components.

• Phospholipid Bilayer: The fundamental structure of the plasma membrane consists of a double layer of phospholipids. Each phospholipid molecule has a hydrophilic (water-loving) head and hydrophobic (water-fearing) tails. The hydrophilic heads face outward toward the aqueous environments inside and outside the cell, while the hydrophobic tails face inward, away from water.

• Fluid Mosaic Model: The term "fluid" refers to the lateral movement of lipids and proteins within the layer, while "mosaic" describes the patchwork of proteins embedded in or attached to the bilayer.

• Membrane Proteins: Proteins are interspersed throughout the bilayer and can be classified as integral (transmembrane) or peripheral. Integral proteins span the membrane and have both hydrophilic and hydrophobic regions, functioning as channels or carriers for transport.

Example: The sodium-potassium pump (Na+/K+ ATPase) is an integral membrane protein that actively transports ions across the membrane.

Phospholipids: Structure and Properties

Phospholipids are amphipathic molecules, meaning they contain both hydrophilic and hydrophobic regions. This property is essential for the formation of the bilayer.

• Hydrophilic Head: Composed of a phosphate group attached to glycerol.

• Hydrophobic Tails: Consist of two fatty acid chains.

• Arrangement: In the bilayer, hydrophobic tails face each other, creating a nonpolar interior, while hydrophilic heads interact with water.

Additional info: The bilayer's structure allows selective permeability, enabling the cell to control its internal environment.

Membrane Proteins: Types and Functions

Proteins embedded in the plasma membrane perform a variety of functions essential for cell survival and communication.

• Integral Proteins: Span the membrane; involved in transport (channels, carriers), signal transduction, and cell recognition.

• Peripheral Proteins: Attached to the surface of integral proteins; function as enzymes, provide structural support, and assist in cell shape.

• Cytoskeleton: Anchors to the plasma membrane, maintaining cell shape and facilitating intracellular transport.

• Glycocalyx: A carbohydrate-rich area on the cell surface, important for cell recognition and protection. Changes in the glycocalyx can affect immune recognition, especially in cancer cells.

• Cholesterol: Interspersed within the bilayer, cholesterol stabilizes membrane fluidity. Excess cholesterol can decrease membrane flexibility.

Functions of Plasma Membrane Proteins

Plasma membrane proteins are crucial for various cellular activities.

• Transport: Facilitate movement of substances across the membrane.

• Enzyme Activity: Catalyze reactions at the membrane surface.

• Cell-Cell Recognition: Allow cells to identify and interact with each other.

• Attachment: Anchor the membrane to the cytoskeleton and extracellular matrix.

• Signal Transduction: Transmit signals from the external environment to the cell's interior.

Cell Junctions: Types and Functions

Cell junctions are specialized structures that connect adjacent cells, providing structural integrity and communication.

• Tight Junctions: Seal adjacent cells to prevent passage of molecules between them. Important in tissues like the digestive tract.

• Desmosomes: Anchoring junctions that link cells together, allowing them to resist mechanical stress. Found in tissues subject to stretching, such as skin and heart muscle.

• Gap Junctions: Channels that allow direct communication between cells by permitting the passage of ions and small molecules. Essential in electrically excitable tissues like cardiac muscle.

Plasma Membrane Functions

The plasma membrane serves several vital roles in cellular physiology.

• Barrier: Separates intracellular and extracellular environments.

• Selective Permeability: Regulates entry and exit of substances.

• Communication: Responds to changes in the extracellular environment.

• Interaction and Recognition: Facilitates cell signaling and immune recognition.

Membrane Transport Mechanisms

Transport across the plasma membrane is essential for maintaining homeostasis. Mechanisms are classified as passive or active.

• Passive Transport: Does not require energy; substances move down their concentration gradient.

• Active Transport: Requires energy (usually ATP); substances move against their concentration gradient.

Passive Transport Types

• Simple Diffusion: Movement of nonpolar, lipid-soluble molecules (e.g., O2, CO2, fatty acids, alcohol) directly through the bilayer.

• Facilitated Diffusion: Movement of polar or large molecules via protein channels or carriers. Example: Glucose transport via carrier proteins.

• Channel-Mediated Facilitated Diffusion: Selective movement through protein channels, which may be gated or always open.

• Filtration: Movement of water and solutes through a membrane due to hydrostatic pressure. Example: Filtration in kidney capillaries.

Key Factors Affecting Diffusion:

• Concentration gradient

• Molecule size

• Temperature

Equation for Rate of Diffusion:

$J = -D \frac{dC}{dx}$

Where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Active Transport Types

• Primary Active Transport: Direct use of ATP to transport molecules against their gradient. Example: Na+/K+ ATPase pump.

• Secondary Active Transport: Uses the energy stored in gradients created by primary active transport. Example: Cotransport of glucose with Na+ as Na+ moves back into the cell.

• Symport: Two substances move in the same direction across the membrane.

• Antiport: Two substances move in opposite directions. Example: Na+/K+ ATPase.

Equation for Na+/K+ ATPase:

$3\,\text{Na}^+_{\text{out}} + 2\,\text{K}^+_{\text{in}} + \text{ATP} \rightarrow 3\,\text{Na}^+_{\text{in}} + 2\,\text{K}^+_{\text{out}} + \text{ADP} + \text{P}_i$

Vesicular Transport

• Exocytosis: Process by which cells expel materials in vesicles. Used for secretion of hormones, neurotransmitters, and waste.

• Endocytosis: Process by which cells take in large particles or fluids via vesicles. Includes phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Osmosis and Tonicity

Osmosis is the diffusion of water across a semipermeable membrane from an area of low solute concentration to high solute concentration.

• Water moves due to differences in solute concentration.

• Osmolarity: Total concentration of solute particles in a solution (mOsmol/L).

• Tonicity: Concentration of nonpenetrating solute particles; determines the effect of a solution on cell shape.

Types of Solutions:

Type	Description	Effect on Cell
Isotonic	Same solute concentration as cell	No net water movement; cell shape unchanged
Hypertonic	Higher solute concentration than cell	Water moves out; cell shrinks (crenation)
Hypotonic	Lower solute concentration than cell	Water moves in; cell swells and may lyse

Example: 0.9% NaCl is isotonic to human red blood cells. Placing RBCs in a hypertonic solution causes them to shrink, while a hypotonic solution causes them to swell and potentially burst (lysis).

Additional info: Hypertonic solutions are used to treat edema by drawing water out of tissues; hypotonic solutions are used to rehydrate patients.