Structure and Function of the Plasma Membrane

Plasma Membrane Structure and Function

Fluid-Mosaic Model of the Plasma Membrane

The plasma membrane forms a barrier between cells and their external environment. Its structure is best described by the fluid-mosaic model, which states that the membrane is composed of a mosaic of various molecules that can move fluidly within the layer.

• Fluidity is influenced by the types of lipids present:

  • More unsaturated (bent) fatty acids in phospholipids increase membrane fluidity.

  • More saturated (straight) fatty acids make the membrane more viscous (less fluid).

  • Cholesterol increases membrane stability by holding the membrane together at moderate and high temperatures.

• The membrane is a mosaic because it contains many different types of molecules: phospholipids, proteins, glycoproteins, glycolipids, and cholesterol.

Major Molecules in the Plasma Membrane

• Phospholipids: The most abundant molecules, forming a bilayer with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

• Cholesterol: Embedded within the bilayer, it stabilizes the membrane and affects fluidity.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids) on the extracellular surface, important for cell recognition and signaling.

• Proteins: Embedded in or attached to the membrane, with various functions (see below).

Membrane Proteins and Their Functions

Proteins in the plasma membrane serve a variety of roles:

• Transmembrane (integral) proteins: Span the entire membrane and are involved in transport and signaling.

• Peripheral proteins: Attached to the surface of the membrane, often involved in signaling or maintaining cell shape.

• Pump proteins: Require energy from ATP to move substances against their concentration gradient.

• Signal transduction proteins: Receive and transmit chemical messages across the membrane.

• Intercellular joining proteins: Help glue cells together to form tissues (e.g., tight junctions, desmosomes).

Selective Permeability of the Plasma Membrane

The phospholipid bilayer is selectively permeable, allowing some molecules to move freely while restricting others.

• Freely permeable molecules:

  • Very small nonpolar and polar molecules (e.g., O2, CO2, H2O, ethanol)

  • Medium-sized hydrophobic molecules (e.g., steroids, testosterone, estrogen, cortisol)

• Impermeable or less permeable molecules:

  • Large polar molecules (e.g., glucose, amino acids)

  • Ions (e.g., Na+, K+, Cl-)

Transport Across the Plasma Membrane

Transport can be passive (not requiring energy) or active (requiring energy).

Passive Transport

• Diffusion: Movement of molecules from high to low concentration due to random motion.

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

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

  • Osmosis can occur directly through the bilayer or via aquaporins (water channels).

• Facilitated diffusion: Movement of molecules down their concentration gradient with the help of membrane proteins (channels or carriers).

Active Transport

• Primary active transport: Uses ATP to move substances against their concentration gradient (e.g., sodium-potassium pump).

  • $3 \text{Na}^+$ ions are pumped out of the cell, $2 \text{K}^+$ ions are pumped into the cell$

  • $\text{ATP}$ provides the energy for the transport$

• Secondary active transport: Uses the energy from the movement of one substance down its gradient to move another substance against its gradient.

Bulk Transport

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

• Endocytosis: The plasma membrane folds inward to bring substances into the cell.

  • Phagocytosis: "Cell eating"; the cell engulfs large particles or cells.

  • Pinocytosis: "Cell drinking"; the cell engulfs extracellular fluid and dissolved solutes.

  • Receptor-mediated endocytosis: Specific molecules bind to receptors, triggering vesicle formation.

Osmosis and Tonicity

Osmosis affects animal and plant cells differently depending on the tonicity of the surrounding solution.

Solution Type	Effect on Animal Cells	Effect on Plant Cells
Isotonic	No net water movement; cell remains normal	No net water movement; cell is flaccid
Hypertonic	Water leaves cell; cell shrivels (crenation)	Water leaves cell; cell membrane pulls away from wall (plasmolysis)
Hypotonic	Water enters cell; cell swells and may burst (lysis)	Water enters cell; cell becomes turgid (normal for plant cells)

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	O2, CO2
Facilitated Diffusion	No	High to Low	Glucose via carrier protein
Osmosis	No	High to Low (water)	Water via aquaporins
Active Transport	Yes (ATP)	Low to High	Na+/K+ pump
Bulk Transport	Yes (ATP)	Varies	Endocytosis, Exocytosis

Key Terms and Definitions

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

• Selective permeability: Property of membranes that allows some substances to cross more easily than others.

• Concentration gradient: Difference in the concentration of a substance across a space.

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

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

• Facilitated diffusion: Passive movement of molecules across the membrane via transport proteins.

• Active transport: Movement of substances against their concentration gradient using energy (ATP).

• Endocytosis: Process by which cells take in substances by engulfing them in a vesicle.

• Exocytosis: Process by which cells expel substances using vesicles that fuse with the plasma membrane.

Example: The sodium-potassium pump is a classic example of active transport, maintaining the electrochemical gradient essential for nerve impulse transmission.