Membranes: Structure, Function, and Permeability

Membranes: Structure, Function, and Permeability

Lipids, Membranes, & the First Cells

The plasma membrane, also known as the cell membrane, is a fundamental structure that separates the interior of the cell from the external environment. This boundary is essential for maintaining cellular integrity and function.

• Key Functions of Membranes:

  • Keep damaging materials out of the cell

  • Allow entry of materials needed by the cell

  • Facilitate the chemical reactions necessary for life

Phospholipid Structure

Phospholipids are the primary components of cell membranes. They are amphipathic molecules, meaning they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.

• Hydrophilic head: Polar, interacts with water

• Hydrophobic tails: Non-polar, avoid water and interact with each other

Example: In a phospholipid, the phosphate-containing head is hydrophilic, while the fatty acid tails are hydrophobic.

Phospholipid Bilayers and Micelles

When amphipathic lipids are placed in water, they spontaneously arrange themselves to minimize free energy, forming structures such as micelles and lipid bilayers.

• Micelles: Spherical structures with hydrophilic heads facing outward and hydrophobic tails tucked inside.

• Lipid bilayers: Double-layered sheets with hydrophilic heads facing the aqueous environment on both sides and hydrophobic tails sandwiched in the middle.

Diagram Description: In both micelles and bilayers, hydrophilic heads interact with water, while hydrophobic tails interact with one another, away from water.

Fluid Mosaic Model of Biological Membranes

The fluid mosaic model describes the structure of cell membranes as a mosaic of various proteins floating in or on the fluid lipid bilayer. This model highlights the dynamic and flexible nature of membranes.

• Components: Phospholipids, proteins, cholesterol, carbohydrates (e.g., saccharides)

• Function: Allows for lateral movement of components, contributing to membrane fluidity and function

Selective Permeability of Lipid Bilayers

Phospholipid bilayers exhibit selective permeability, allowing some substances to cross more easily than others.

• High permeability: Small, nonpolar molecules (e.g., O2, CO2)

• Low permeability: Charged or large polar substances (e.g., ions, glucose)

Example: Oxygen and carbon dioxide diffuse rapidly across the membrane, while ions and large molecules require transport proteins.

Factors Affecting Membrane Permeability

Several factors influence the fluidity and permeability of membranes:

• Number of double bonds in hydrophobic tails: More double bonds (unsaturated) create kinks, increasing fluidity and permeability.

• Length of hydrocarbon tails: Longer tails increase hydrophobic interactions, decreasing permeability.

• Cholesterol content: Cholesterol stabilizes membranes, reducing fluidity at high temperatures and preventing solidification at low temperatures.

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

Table: Factors Affecting Membrane Permeability

Factor	Effect on Fluidity	Effect on Permeability
More double bonds (unsaturated tails)	Increases	Increases
Longer hydrocarbon tails	Decreases	Decreases
More cholesterol (at moderate temp)	Decreases	Decreases
Higher temperature	Increases	Increases

Transport Across Membranes

Substances move across membranes by several mechanisms, depending on their properties and the membrane's structure.

• Simple diffusion: Movement of small, nonpolar molecules down their concentration gradient without energy input.

• Facilitated diffusion: Movement of substances down their concentration gradient via specific membrane proteins (channels or carriers), no energy required.

• Active transport: Movement of substances against their concentration gradient, requiring energy (usually from ATP).

Diffusion and Osmosis

Diffusion is the spontaneous movement of molecules from regions of high concentration to low concentration. Osmosis is a special case of diffusion involving water movement across a selectively permeable membrane.

• Osmosis: Water moves from areas of low solute concentration to high solute concentration, equalizing solute concentrations on both sides of the membrane.

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

• Hypertonic solution: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink.

• Hypotonic solution: Lower solute concentration outside the cell; water enters the cell, causing it to swell.

Table: Effects of Tonicity on Cells

Solution Type	Relative Solute Concentration (Outside vs. Inside)	Water Movement	Effect on Cell
Isotonic	Equal	No net movement	Cell remains the same
Hypertonic	Higher outside	Out of cell	Cell shrinks
Hypotonic	Lower outside	Into cell	Cell swells

Membrane Proteins and Their Functions

Membranes contain a variety of proteins that contribute to their structure and function. These proteins can be integral (spanning the membrane) or peripheral (attached to the surface).

• Integral proteins: Span the membrane, often involved in transport or signaling.

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

• Amphipathic nature: Allows proteins to interact with both hydrophobic and hydrophilic regions of the membrane.

Transport Proteins: Channels, Carriers, and Pumps

Transport proteins facilitate the movement of substances across membranes.

• Channel proteins: Form pores for specific ions or molecules to diffuse through, often regulated by electrochemical gradients.

• Carrier proteins: Bind specific molecules and change shape to shuttle them across the membrane.

• Pumps: Use energy (usually ATP) to move substances against their concentration gradients (e.g., sodium-potassium pump).

Example: Sodium-Potassium Pump

• Uses ATP to transport Na+ out of the cell and K+ into the cell, both against their concentration gradients.

• Maintains essential electrochemical gradients for cellular function.

Key Equations

• Fick's Law of Diffusion:

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

• Osmotic Pressure:

$\Pi = iMRT$

Where $J$ is the rate of diffusion, $D$ is the diffusion coefficient, $\frac{dC}{dx}$ is the concentration gradient, $\Pi$ is osmotic pressure, $i$ is the van 't Hoff factor, $M$ is molarity, $R$ is the gas constant, and $T$ is temperature in Kelvin.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via GLUT transporter
Active Transport	Yes (ATP)	Against gradient	Sodium-potassium pump

Additional info: The notes also reference the role of aquaporins (water channel proteins) in speeding up osmosis, and the importance of membrane proteins in determining overall membrane permeability and function.