Membrane Structure and Function: Study Notes

Membrane Structure and Function

Overview

The plasma membrane is a fundamental structure in all living cells, responsible for regulating the movement of substances into and out of the cell. Its unique composition and properties allow it to maintain homeostasis, communicate with the environment, and support various cellular processes.

Major Ways the Plasma Membrane Regulates Traffic

Types of Transport Across the Membrane

• Passive Transport: Movement of small molecules (such as O2 and CO2) across the membrane without the use of cellular energy. This can occur via simple diffusion or with the help of transport proteins (facilitated diffusion).

• Active Transport: Movement of small molecules against their concentration gradient, requiring energy (typically from ATP) and a transport protein.

• Bulk Transport: Movement of large molecules (such as proteins and polysaccharides) via vesicles. Includes exocytosis (out of the cell) and endocytosis (into the cell).

Example: The uptake of glucose by intestinal cells involves both facilitated diffusion and active transport mechanisms.

Structure of Cellular Membranes

Components of the Membrane

• Phospholipids: Amphipathic molecules with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. They form a bilayer, with tails facing inward and heads facing the aqueous environment.

• Proteins: Embedded within or attached to the membrane, proteins perform various functions including transport, signaling, and structural support.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids), carbohydrates play a key role in cell recognition and signaling.

Amphipathic Nature of Phospholipids

• Definition: Amphipathic molecules contain both hydrophilic and hydrophobic regions.

• Phospholipids arrange themselves in a bilayer to shield their hydrophobic tails from water while exposing hydrophilic heads.

Fluid Mosaic Model

• The membrane is described as a "fluid mosaic" because it is a dynamic structure with proteins floating in or on the fluid lipid bilayer.

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

Membrane Fluidity

Factors Affecting Fluidity

• Saturated vs. Unsaturated Fatty Acids: Unsaturated fatty acids (with double bonds) prevent tight packing, increasing fluidity. Saturated fatty acids (no double bonds) allow tight packing, decreasing fluidity.

• Temperature: Lower temperatures can cause membranes to solidify; unsaturated fatty acids help maintain fluidity in cold conditions.

• Cholesterol: Acts as a "fluidity buffer" in animal cells. At moderate temperatures, it reduces membrane fluidity by restricting phospholipid movement; at low temperatures, it prevents tight packing and solidification.

Membrane Proteins and Their Functions

Types of Membrane Proteins

• Peripheral Proteins: Loosely bound to the membrane surface.

• Integral Proteins: Penetrate the hydrophobic core; those that span the membrane are called transmembrane proteins.

Functions of Membrane Proteins

• Transport

• Enzymatic activity

• Signal transduction

• Cell-cell recognition

• Intercellular joining

• Attachment to the cytoskeleton and extracellular matrix

Role of Membrane Carbohydrates

Cell Recognition and Signaling

• Carbohydrates on the cell surface function as identification markers, allowing cells to recognize each other.

• Glycolipids: Carbohydrates attached to lipids.

• Glycoproteins: Carbohydrates attached to proteins.

Selective Permeability of the Membrane

What Can and Cannot Permeate the Membrane

• Can Permeate: Small, nonpolar molecules (e.g., O2, CO2), and some small polar molecules (e.g., water, via aquaporins).

• Cannot Permeate Easily: Large polar molecules (e.g., glucose), ions (e.g., Na+, Cl-).

Transport Proteins

• Channel Proteins: Provide hydrophilic tunnels for specific 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; can be involved in passive or active transport.

Passive Transport

Diffusion and Facilitated Diffusion

• Diffusion: Movement of molecules from an area of higher concentration to lower concentration (down the concentration gradient).

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

Equation:

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

Osmosis and Water Balance

Osmosis

• Diffusion of water across a selectively permeable membrane.

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

Tonicity and Its Effects on Cells

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

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

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

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

Active Transport

Mechanisms and Examples

• Requires energy (usually ATP) to move substances against their concentration gradients.

• Sodium-Potassium Pump: Maintains high K+ and low Na+ inside animal cells.

Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Membrane Potential and Electrochemical Gradients

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

• Electrochemical gradients drive the movement of ions across membranes.

• Electrogenic Pumps: Transport proteins that generate voltage across a membrane (e.g., sodium-potassium pump in animals, proton pump in plants).

Bulk Transport: Exocytosis and Endocytosis

Exocytosis

• Vesicles fuse with the plasma membrane to release large molecules outside the cell.

• Example: Secretion of insulin by pancreatic cells.

Endocytosis

• Cell takes in macromolecules by forming vesicles from the plasma membrane.

• Types of endocytosis:

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

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

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

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2 across membrane
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Bulk Transport	Yes (ATP)	Both	Exocytosis, Endocytosis

Additional info: These notes are based on the Campbell Biology textbook and are suitable for college-level General Biology students studying membrane structure and function.