Ch. 7 Membrane Structure and Function (Chapter 7) – Study Notes

Membrane Structure and Function

Introduction

The plasma membrane is a fundamental component of all cells, serving as a selective barrier that regulates the movement of substances into and out of the cell. Its structure and function are essential for maintaining cellular homeostasis and enabling communication with the environment.

Membrane Structure

Fluid Mosaic Model

The fluid mosaic model describes the structure of the plasma membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids. This model highlights the dynamic and heterogeneous nature of the membrane.

• Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Phospholipids arrange themselves into a bilayer with hydrophobic tails facing inward and hydrophilic heads facing outward toward the aqueous environment.

• Proteins are interspersed throughout the bilayer, contributing to the mosaic aspect of the model.

• Carbohydrates are attached to proteins (glycoproteins) or lipids (glycolipids) on the extracellular surface, playing roles in cell recognition.

Example: The plasma membrane of animal cells contains cholesterol, which modulates membrane fluidity.

Membrane Fluidity

Membrane fluidity is crucial for proper function, affecting the movement of proteins and lipids within the bilayer.

• Most lipids and some proteins can move laterally within the membrane; rarely, lipids may flip-flop between layers.

• Unsaturated fatty acids increase fluidity due to kinks in their tails, preventing tight packing.

• Saturated fatty acids decrease fluidity by allowing tighter packing of phospholipids.

• Cholesterol acts as a fluidity buffer: it restrains movement at high temperatures and prevents tight packing at low temperatures.

Additional info: In plant cells, related steroid lipids serve a similar function to cholesterol.

Membrane Proteins

Membrane proteins are essential for various cellular functions and are classified based on their association with the membrane.

• Integral proteins penetrate the hydrophobic core of the lipid bilayer; some span the entire membrane (transmembrane proteins).

• Peripheral proteins are loosely bound to the membrane surface.

• Functions of membrane proteins include:

  • Transport

  • Enzymatic activity

  • Signal transduction

  • Cell-cell recognition

  • Intercellular joining

  • Attachment to the cytoskeleton and extracellular matrix

Membrane Carbohydrates and Cell Recognition

Carbohydrates on the cell surface are involved in cell recognition and signaling.

• Glycolipids: Carbohydrates covalently bonded to lipids.

• Glycoproteins: Carbohydrates covalently bonded to proteins.

• These molecules serve as identification tags recognized by other cells.

Membrane Function: Selective Permeability

Selective Permeability

The plasma membrane allows some substances to cross more easily than others, maintaining the internal environment of the cell.

• Hydrophobic (nonpolar) molecules (e.g., O2, CO2) pass through the membrane easily.

• Hydrophilic (polar) molecules (e.g., ions, sugars, water) require transport proteins to cross the membrane.

Transport Proteins

Transport proteins facilitate the movement of specific substances across the membrane.

• Channel proteins provide hydrophilic tunnels for molecules or ions to pass through (e.g., aquaporins for water).

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

• Transport proteins are specific for the substances they move.

Types of Membrane Transport

Passive Transport

Passive transport is the movement of substances across the membrane without energy input, driven by concentration gradients.

• Simple diffusion: Movement of molecules from high to low concentration.

• Facilitated diffusion: Movement of molecules down their concentration gradient via transport proteins.

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

Equation:

$J = -P (C_2 - C_1)$ Where $J$ is the flux, $P$ is the permeability coefficient, and $C_2 - C_1$ is the concentration difference across the membrane.

Osmosis and Tonicity

Osmosis affects water balance in cells, depending on the relative concentrations of solutes inside and outside the cell.

• Isotonic solution: Solute concentration is equal inside and outside; no net water movement.

• Hypertonic solution: Higher solute concentration outside; cell loses water and shrivels.

• Hypotonic solution: Lower solute concentration outside; cell gains water and may burst (animal cells) or become turgid (plant cells).

Example: Paramecium uses a contractile vacuole to expel excess water in a hypotonic environment.

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

Facilitated Diffusion

Facilitated diffusion is a type of passive transport aided by proteins.

• Channel proteins can be gated, opening or closing in response to stimuli.

• Carrier proteins undergo conformational changes to move substances across the membrane.

Active Transport

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

• Performed by carrier proteins (pumps).

• Maintains concentration differences essential for cell function.

Example: The sodium-potassium pump in animal cells moves Na+ out and K+ in, using ATP.

Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i$ (hydrolysis of ATP provides energy for the pump)

Membrane Potential and Electrochemical Gradients

The membrane potential is the voltage difference across a membrane, created by the unequal distribution of ions.

• Inside of the cell is typically negative relative to the outside.

• Electrochemical gradient combines the chemical gradient (concentration) and electrical gradient (charge).

• Electrogenic pumps (e.g., sodium-potassium pump in animals, proton pump in plants) generate membrane potential.

Cotransport

Cotransport occurs when the active transport of one solute indirectly drives the transport of another.

• As one substance moves down its gradient, another is transported against its gradient.

• Example: In plants, the proton gradient created by the proton pump is used to drive the uptake of sucrose via a cotransporter.

Bulk Transport

Exocytosis and Endocytosis

Large molecules (e.g., proteins, polysaccharides) cross the membrane in bulk via vesicles, requiring energy.

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

• Endocytosis: The cell takes in macromolecules by forming vesicles from the plasma membrane.

Types of Endocytosis

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

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

• Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors on the cell surface.

Example: Human cells use receptor-mediated endocytosis to take in cholesterol via LDL particles.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose, ions via channels
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Bulk Transport	Yes	Varies	Exocytosis, endocytosis