Membrane Structure and Function (Chapter 7) – Study Notes

Membrane Structure and Function

Introduction to the Plasma Membrane

The plasma membrane is a critical structure that regulates the movement of substances into and out of the cell. It maintains the internal environment of the cell and facilitates communication and transport between the cell and its surroundings.

• Selective Permeability: The plasma membrane allows some substances to cross more easily than others, maintaining homeostasis.

• Transport Mechanisms: Includes passive and active transport, as well as bulk transport via vesicles (exocytosis and endocytosis).

• Example: The release of neurotransmitters from nerve cells via exocytosis.

Cellular Membranes: Fluid Mosaics of Lipids and Proteins

Phospholipid Bilayer Structure

Cell membranes are primarily composed of a phospholipid bilayer, which forms the basic structural framework.

• Phospholipids: Amphipathic molecules with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Fluid Mosaic Model: Describes the membrane as a fluid structure with a "mosaic" of various proteins embedded in or attached to the bilayer.

• Membrane Fluidity: Influenced by the saturation of fatty acid tails (unsaturated tails increase fluidity; saturated tails decrease fluidity) and the presence of cholesterol.

• Example: Membranes in cold-adapted organisms often have more unsaturated fatty acids to maintain fluidity.

Membrane Proteins

Proteins are essential for the diverse functions of the membrane.

• Integral Proteins: Span the membrane (transmembrane proteins); have hydrophobic regions that interact with the membrane's interior.

• Peripheral Proteins: Loosely bound to the membrane surface; do not penetrate the hydrophobic core.

• Six Major Functions of Membrane Proteins:

  1. Transport

  2. Enzymatic activity

  3. Signal transduction

  4. Cell-cell recognition

  5. Intercellular joining

  6. Attachment to the cytoskeleton and extracellular matrix (ECM)

• Example: Channel proteins facilitate the passage of ions across the membrane.

Carbohydrates in Membranes

Carbohydrates are present on the external surface of the plasma membrane, often attached to proteins (glycoproteins) or lipids (glycolipids).

• Function: Important for cell-cell recognition and signaling.

• Example: ABO blood group antigens are determined by specific glycoproteins on red blood cell membranes.

Selective Permeability of Membranes

Permeability Properties

The plasma membrane's selective permeability is essential for cellular function.

• Nonpolar molecules (e.g., O2, CO2) pass through easily.

• Polar molecules (e.g., glucose) and ions (e.g., Na+, K+) require transport proteins.

• Large molecules and charged substances generally do not cross the membrane unaided.

Transport Proteins

• Channel Proteins: Provide corridors for specific molecules or ions to cross.

• Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane.

Passive Transport

Diffusion

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, down their concentration gradient.

• Simple Diffusion: Does not require energy or transport proteins.

• Equilibrium: Reached when the concentration of molecules is equal on both sides of the membrane.

• Example: Oxygen diffusing into cells from the bloodstream.

Osmosis

Osmosis is the passive diffusion of water across a selectively permeable membrane.

• Water moves from areas of low solute concentration to areas of high solute concentration.

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

Solution Type	Effect on Animal Cell
Isotonic	No net water movement; cell remains normal
Hypotonic	Water enters cell; cell may lyse (burst)
Hypertonic	Water leaves cell; cell shrivels

• Osmoregulation: The control of water balance; essential for organisms in hypotonic or hypertonic environments.

• Example: Paramecium uses a contractile vacuole to expel excess water.

Facilitated Diffusion

Facilitated diffusion is a type of passive transport that uses transport proteins to move substances across the membrane without energy input.

• Channel and Carrier Proteins: Enable the passage of ions and large polar molecules.

• Example: Glucose transporters in red blood cells.

Active Transport

Mechanism and Importance

Active transport moves substances against their concentration gradients, requiring energy (usually from ATP).

• Carrier Proteins: Involved in active transport; often called "pumps".

• Example: Sodium-potassium pump (Na+/K+ pump).

Sodium-Potassium Pump

The sodium-potassium pump is a key active transport mechanism in animal cells.

• Maintains high [Na+] outside and high [K+] inside the cell.

• Uses ATP to transport 3 Na+ ions out and 2 K+ ions in per cycle.

• Establishes the membrane potential (voltage difference across the membrane).

Membrane Potential Equation:

$V_m = V_{inside} - V_{outside}$

• Importance: Essential for nerve impulse transmission and muscle contraction.

Bulk Transport Across the Plasma Membrane

Exocytosis

Exocytosis is the process by which cells export large molecules (such as proteins and polysaccharides) by vesicle fusion with the plasma membrane.

• Example: Secretion of insulin by pancreatic cells.

Endocytosis

Endocytosis is the process by which cells take in macromolecules by forming vesicles from the plasma membrane.

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

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

• Receptor-mediated endocytosis: Uptake of specific molecules via receptor proteins.

Application: Osmosis and Tonicity Problems

Example Problem: U-tube Osmosis

Two solutions separated by a selectively permeable membrane (permeable to water and glucose, not sucrose):

Side	Sucrose (M)	Glucose (M)
A	2	1
B	1	2

• After osmosis, water will move to balance solute concentrations, but sucrose will remain uneven due to membrane impermeability.

• Key Concept: Only solutes that can cross the membrane will equilibrate; others contribute to osmotic pressure.

Example Problem: Glucose and NaCl Diffusion

Side	Glucose (M)	NaCl (M)
A	0.4	0.5
B	0.8	0.4

• Movement of water and solutes depends on membrane permeability to each substance.

Additional info: For all transport processes, the direction and rate depend on concentration gradients, membrane permeability, and the presence of specific transport proteins.