Membrane Structure and Function

Plasma Membrane: Structure and Selective Permeability

The plasma membrane is a dynamic boundary that separates the interior of the cell from its external environment. It is primarily composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. The membrane exhibits selective permeability, allowing certain substances to cross more easily than others.

• Selective Permeability: The ability of the membrane to regulate the passage of materials, permitting some molecules to pass while restricting others.

• Phospholipid Bilayer: Consists of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, creating a semi-permeable barrier.

• Embedded Proteins: Serve as channels, carriers, receptors, and enzymes, facilitating communication and transport.

• Cholesterol: Modulates membrane fluidity and stability, especially in animal cells.

Example: Oxygen and carbon dioxide can diffuse directly through the lipid bilayer, while ions and large polar molecules require transport proteins.

Membrane Fluidity

Factors Affecting Membrane Fluidity

Membrane fluidity is crucial for proper cell function, affecting the movement of proteins and lipids within the bilayer and the ability of the membrane to self-heal.

• Fatty Acid Composition: Unsaturated fatty acids (with double bonds) increase fluidity, while saturated fatty acids (no double bonds) decrease fluidity.

• Fatty Acid Tail Length: Shorter tails increase fluidity; longer tails decrease it.

• Cholesterol Content: At moderate temperatures, cholesterol reduces fluidity; at low temperatures, it prevents solidification.

• Temperature: Higher temperatures increase fluidity; lower temperatures decrease it.

Example: In response to elevated temperatures, plant cells may alter their membrane lipid composition to maintain optimal fluidity.

Table: Strategies to Compensate for Excessive Membrane Fluidity

Additional info: Plant cells typically adjust fatty acid composition rather than cholesterol content, as cholesterol is less abundant in plant membranes.

Membrane Transport Mechanisms

Active vs. Passive Transport

Cells use various mechanisms to move substances across membranes, classified as active or passive transport.

• Passive Transport: Movement of substances down their concentration gradient without energy input. Includes simple diffusion, facilitated diffusion, and osmosis.

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

Example: The sodium-potassium pump uses ATP to move Na+ out of and K+ into the cell against their gradients.

Simple vs. Facilitated Diffusion

• Simple Diffusion: Direct movement of small, nonpolar molecules (e.g., O2, CO2) through the lipid bilayer.

• Facilitated Diffusion: Movement of larger or polar molecules (e.g., glucose, ions) via specific transport proteins, down their concentration gradient.

Example: Glucose enters most animal cells via facilitated diffusion through glucose transporters.

Types of Substances Crossing the Membrane

• Simple Diffusion: Small, nonpolar molecules (O2, CO2, N2), and some small polar molecules (water, to a limited extent).

• Facilitated Diffusion: Ions (Na+, K+, Cl-), glucose, amino acids.

Active Transport Example: Calcium Ion Movement

Calcium ions (Ca2+) are maintained at high concentrations in the endoplasmic reticulum (ER) compared to the cytosol. Two types of transport proteins regulate Ca2+ movement:

• ER to Cytosol: Ca2+ moves down its concentration gradient via a channel protein (passive transport, no ATP required).

• Cytosol to ER: Ca2+ is pumped against its gradient via a Ca2+-ATPase (active transport, requires ATP).

Example: Release of Ca2+ into the cytosol triggers muscle contraction and nerve cell communication.

Osmosis and Tonicity

Effects of Sodium Intake on Blood Tonicity

Osmosis is the movement of water across a selectively permeable membrane from areas of low solute concentration to high solute concentration. Tonicity describes the relative concentration of solutes in two solutions separated by a membrane.

• Hypertonic Solution: Higher solute concentration than the cell; water moves out of cells.

• Hypotonic Solution: Lower solute concentration than the cell; water moves into cells.

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

Example: Consuming a salty meal increases blood sodium, making blood temporarily hypertonic relative to body tissues, causing water to move from tissues into the bloodstream.

Specialized Membrane Transport

Receptor-Mediated Endocytosis

Receptor-mediated endocytosis is a form of active transport where cells internalize specific molecules by engulfing them in vesicles formed from the plasma membrane. This process is highly selective, relying on receptor proteins to bind target molecules (ligands).

• Function: Allows cells to acquire large quantities of specific substances (e.g., cholesterol, hormones) efficiently.

• Cellular Balance: Helps regulate the internal environment by controlling the uptake of essential molecules.

Example: Animal cells use receptor-mediated endocytosis to take up low-density lipoprotein (LDL) particles from the bloodstream.

Summary Table: Membrane Transport Mechanisms