Membrane Function and Cellular Transport: Study Notes

Membrane Function and Cellular Transport

Overview of Cell Membrane Structure

The cell membrane, also known as the plasma membrane, is a dynamic structure that separates the interior of the cell from its external environment. It is essential for maintaining cellular integrity, communication, and transport of substances.

• Phospholipid Bilayer: The fundamental structure of the membrane consists of a double layer of phospholipids, with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

• Proteins: Membrane proteins are classified as integral (embedded within the bilayer, often spanning the membrane) and peripheral (attached loosely to the surface).

• Cholesterol: A steroid molecule interspersed within the bilayer, modulating membrane fluidity and stability.

• Carbohydrates: Often attached to proteins (glycoproteins) or lipids (glycolipids), these molecules play roles in cell recognition and signaling.

• Extracellular Matrix (ECM) Fibers: Provide structural support and facilitate cell communication.

Example: The fluid mosaic model describes the membrane as a mosaic of components that move fluidly within the lipid bilayer.

Components of the Cell Membrane

Each component of the cell membrane contributes to its function and properties.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails, forming the bilayer.

• Integral Proteins: Span the membrane and are involved in transport, signaling, and structural support.

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

• Cholesterol: Regulates membrane fluidity, preventing extremes of rigidity or permeability.

• Glycoproteins and Glycolipids: Involved in cell-cell recognition and communication.

Additional info: Integral proteins can be transmembrane (spanning the entire membrane) or partially embedded.

Membrane Fluidity

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

• Factors Affecting Fluidity:

  • Temperature: Higher temperatures increase fluidity; lower temperatures decrease it.

  • Saturation of Fatty Acid Tails: Unsaturated (kinked) tails prevent tight packing, increasing fluidity. Saturated tails pack closely, decreasing fluidity.

  • Cholesterol: Acts as a buffer, restraining movement at high temperatures and preventing close packing at low temperatures.

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

Factor	Effect on Fluidity
High Temperature	Increases fluidity
Low Temperature	Decreases fluidity
Unsaturated Fatty Acids	Increase fluidity
Saturated Fatty Acids	Decrease fluidity
Cholesterol	Buffers fluidity (prevents extremes)

Functions of the Cell Membrane

The cell membrane performs several vital functions necessary for cell survival and communication.

• Selective Permeability: Controls the entry and exit of substances.

• Cell Signaling: Membrane proteins and carbohydrates act as receptors and signals for communication.

• Enzymatic Activity: Some membrane proteins catalyze reactions at the membrane surface.

• Intercellular Joining: Proteins facilitate cell-cell adhesion.

• Attachment: Links to the cytoskeleton and extracellular matrix for structural support.

Example: Glycoproteins on the surface of immune cells help recognize pathogens.

Types of Membrane Transport

Transport across the cell membrane is essential for maintaining homeostasis. It occurs via passive and active mechanisms.

Passive Transport

Passive transport does not require energy and moves substances down their concentration gradients.

• Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) directly through the lipid bilayer.

• Facilitated Diffusion: Movement of larger or polar molecules via membrane proteins (channels or carriers).

• Osmosis: Diffusion of water across a selectively permeable membrane, often through aquaporins.

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

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradients.

• Primary Active Transport: Direct use of ATP to transport molecules (e.g., sodium-potassium pump).

• Secondary Active Transport (Cotransport): Uses the energy from one molecule moving down its gradient to transport another molecule against its gradient.

Equation: $\text{Na}^+/\text{K}^+ \text{ATPase}: 3\text{Na}^+_{\text{out}} + 2\text{K}^+_{\text{in}} + \text{ATP} \rightarrow 3\text{Na}^+_{\text{in}} + 2\text{K}^+_{\text{out}} + \text{ADP} + \text{P}_i$

Bulk Transport

Large particles and macromolecules are transported via vesicles.

• Endocytosis: Uptake of materials into the cell via vesicle formation (includes phagocytosis and pinocytosis).

• Exocytosis: Release of materials from the cell via vesicle fusion with the membrane.

Example: An amoeba engulfs a food particle via phagocytosis.

Selective Permeability and Transport Proteins

The cell membrane is selectively permeable, allowing only certain substances to pass through.

• Channel Proteins: Provide hydrophilic pathways for ions and water (e.g., aquaporins).

• Carrier Proteins: Bind and transport specific molecules across the membrane.

• Transport Specificity: Each protein is specific for certain molecules or ions.

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose, ions
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Bulk Transport	Yes (ATP)	Variable	Endocytosis, exocytosis

Osmosis and Tonicity

Osmosis is the movement of water across a membrane, influenced by the concentration of solutes.

• Tonicity: Describes the ability of a solution to cause a cell to gain or lose water.

• Isotonic: Equal solute concentration inside and outside the cell; no net water movement.

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

• Hypotonic: Lower solute concentration outside the cell; cell gains water and may burst.

Example: Plant cells become turgid (firm) in hypotonic solutions and flaccid in isotonic solutions.

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

Summary of Key Concepts

• The cell membrane is a selectively permeable barrier composed of a phospholipid bilayer, proteins, cholesterol, and carbohydrates.

• Membrane fluidity is influenced by temperature, fatty acid saturation, and cholesterol content.

• Transport across the membrane occurs via passive (diffusion, osmosis, facilitated diffusion) and active (primary, secondary, bulk) mechanisms.

• Osmosis and tonicity affect cell volume and function.

• Membrane proteins are essential for transport, signaling, and structural support.