Cellular Membranes and Transport Mechanisms

Cellular Membranes: Structure and Function

Fluid Mosaic Model of Membranes

The plasma membrane is a dynamic structure described by the fluid mosaic model, consisting of a bilayer of phospholipids with embedded proteins. This model explains both the flexibility and selective permeability of membranes.

• Phospholipids are amphipathic molecules, containing hydrophobic (water-repelling) tails and hydrophilic (water-attracting) heads.

• The bilayer arrangement places hydrophobic tails inward, away from water, and hydrophilic heads outward, facing the aqueous environment.

• Cholesterol molecules are interspersed within animal cell membranes, modulating fluidity and stability.

• Membrane proteins are distributed throughout the bilayer, contributing to various cellular functions.

Additional info: The image provided visually represents the fluid mosaic model, showing the arrangement of phospholipids, proteins, and other components.

Types of Membrane Proteins

Membrane proteins are essential for the diverse functions of the plasma membrane.

• Peripheral proteins are attached to the membrane surface.

• Integral proteins penetrate the hydrophobic core; transmembrane proteins span the entire membrane.

• Functions include transport, enzymatic activity, signal transduction, cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix (ECM).

Membrane Permeability and Transport

Selective Permeability

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

• Small, nonpolar (hydrophobic) molecules (e.g., O2, CO2) pass through rapidly.

• Large, polar (charged) molecules (e.g., sugars, ions) do not cross easily and require transport proteins.

Types of Transport Across Membranes

Substances move across membranes by several mechanisms, classified as passive or active transport.

• Passive Transport: Movement down a concentration gradient without energy input.

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

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

Passive Transport Mechanisms

Simple Diffusion

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration.

• Occurs until equilibrium is reached.

• Does not require energy.

• Rate depends on the concentration gradient.

Equation:

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

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

Facilitated Diffusion

Facilitated diffusion uses transport proteins to move substances across the membrane down their concentration gradient.

• Channel proteins provide hydrophilic tunnels for specific molecules or ions (e.g., aquaporins for water, ion channels for ions).

• Carrier proteins bind to molecules and change shape to shuttle them across the membrane.

• No energy required.

Osmosis

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

• Water moves from regions of higher free water concentration (lower solute concentration) to regions of lower free water concentration (higher solute concentration).

• Osmosis affects cell volume and water balance.

Effects of Osmosis on Cells

Cells respond differently to their environment based on the tonicity of the surrounding solution.

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

Additional info: Turgor pressure in plant cells is essential for structural support.

Active Transport Mechanisms

Active Transport

Active transport moves substances against their concentration gradients, requiring energy input, typically from ATP.

• Performed by carrier proteins (e.g., sodium-potassium pump).

• Maintains essential differences in ion concentrations across the membrane.

• Example: The sodium-potassium pump ($Na^+/K^+$ pump) moves $Na^+$ out and $K^+$ into animal cells, using ATP.

Equation:

$ATP \rightarrow ADP + P_i$ (ATP hydrolysis provides energy)

Electrogenic Pumps

Electrogenic pumps generate voltage across membranes, storing energy for cellular work.

• In animals: Sodium-potassium pump

• In plants, fungi, bacteria: Proton pump

Bulk Transport: Endocytosis and Exocytosis

Exocytosis

Exocytosis is the process by which cells export large molecules (e.g., proteins, polysaccharides) by vesicle fusion with the plasma membrane.

• Requires energy.

• Example: Secretion of insulin by pancreatic cells.

Endocytosis

Endocytosis is the process by which cells import large molecules by engulfing them in vesicles.

• Phagocytosis (“cellular eating”): Engulfment of large particles.

• Pinocytosis (“cellular drinking”): Uptake of extracellular fluid.

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

Example: Human cells use receptor-mediated endocytosis to take in cholesterol carried by low-density lipoproteins (LDLs).

Additional info: Defective LDL receptor proteins can lead to familial hypercholesterolemia, causing cholesterol accumulation and increased risk of heart disease.