Membrane Structure and Function: Study Guide

Chapter 7: Membrane Structure and Function

Plasma (Cell) Membrane

The plasma membrane is a fundamental structure in all living cells, composed primarily of phospholipids and proteins. It serves as a boundary that holds the cell together and regulates the movement of substances in and out of the cell.

• Phospholipids: Amphipathic molecules with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. They form a bilayer, creating a semi-permeable barrier.

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

• Functions:

  • Maintains cell integrity by holding the cell together.

  • Exhibits selective permeability, allowing certain substances to cross more easily than others.

Fluid Mosaic Model

The most widely accepted model for membrane structure is the fluid mosaic model. This model describes the membrane as a dynamic and flexible structure.

• "Fluid": Phospholipids and some proteins can move laterally within the layer, allowing the membrane to be flexible and self-healing. Phospholipids are not covalently bonded, so they are free to shift positions.

• "Mosaic": The membrane is a patchwork of proteins embedded or attached to the phospholipid bilayer, creating a mosaic-like appearance.

Maintaining Membrane Fluidity

Membrane fluidity is essential for proper function and is influenced by several factors:

• Phospholipid Movement: Lateral movement is common, while "flip-flop" (movement from one leaflet to the other) is rare.

• Fatty Acid Composition:

  • Unsaturated hydrocarbon tails (with kinks) increase fluidity.

  • Saturated hydrocarbon tails make the membrane more viscous (less fluid).

• Cholesterol: Acts as a "fluidity buffer," stabilizing membrane fluidity across temperature changes.

• Adaptations:

  • Plants may increase unsaturated fats in cold conditions (cold hardening).

  • Hibernating animals incorporate more cholesterol to maintain fluidity at low temperatures.

Types of Membrane Proteins

Membrane proteins are crucial for the diverse functions of the plasma membrane. They are classified based on their association with the lipid bilayer:

• Integral Proteins: Embedded within the phospholipid bilayer, often spanning the entire membrane (transmembrane proteins). They are involved in transport, signal transduction, and cell adhesion.

• Peripheral Proteins: Loosely attached to the membrane surface or to integral proteins. They play roles in signaling, maintaining cell shape, and anchoring the cytoskeleton.

Functions of Membrane Proteins

• Transport: Facilitate the movement of substances across the membrane (channels, carriers).

• Enzymatic Activity: Catalyze specific reactions at the membrane surface.

• Signal Transduction: Act as receptors for signaling molecules.

• Cell-Cell Recognition: Allow cells to identify each other (important in immune response).

• Intercellular Joining: Help cells adhere to each other.

• Attachment to Cytoskeleton and ECM: Maintain cell shape and stabilize membrane proteins.

Selective Permeability of Membranes

The plasma membrane allows some substances to cross more easily than others, a property known as selective permeability.

• Small, nonpolar molecules (e.g., O2, CO2) can diffuse freely.

• Polar molecules and ions (e.g., glucose, Na+) require transport proteins.

Transport Across Membranes

Passive Transport

Passive transport is the movement of substances across the membrane without the use of cellular energy (ATP). It relies on the concentration gradient.

• Diffusion: Net movement of molecules from an area of high concentration to low concentration until equilibrium is reached.

  • No energy required.

  • Example: Oxygen entering cells, carbon dioxide leaving cells.

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

  • Water moves from an area of high water concentration (low solute) to low water concentration (high solute).

  • No energy required.

• Facilitated Diffusion: Movement of molecules down their concentration gradient with the help of transport proteins (channels or carriers).

  • No energy required.

  • Example: Glucose transport into cells via GLUT proteins.

Tonicity and Its Effects on Cells

Tonicity refers to the relative concentration of solutes in the solution outside the cell compared to inside the cell.

Solution Type	Solute Concentration	Effect on Animal Cell	Effect on Plant Cell
Isotonic	Equal inside and outside	Normal	Flaccid
Hypotonic	Lower outside than inside	Lysed (bursts)	Turgid (normal)
Hypertonic	Higher outside than inside	Shriveled	Plasmolyzed

Aquaporins

Aquaporins are specialized channel proteins that facilitate the rapid transport of water molecules across the membrane, solving the problem of water's low permeability through the lipid bilayer.

Active Transport

Active transport moves substances against their concentration gradient (from low to high concentration) and requires energy, usually in the form of ATP.

• Carrier-Mediated Transport: Uses specific membrane proteins to move ions or molecules against their gradient.

  • Example: Sodium-Potassium Pump (Na+/K+ pump) – moves 3 Na+ ions out and 2 K+ ions into the cell, generating a membrane potential.

• Bulk Transport: Involves the movement of large particles or volumes via vesicles.

  • Endocytosis: Cell engulfs material by wrapping membrane around it, forming a vesicle.

  • Exocytosis: Vesicle fuses with the membrane to release contents outside the cell.

Comparison of Transport Mechanisms

Transport Type	Energy Required?	Direction (relative to gradient)	Examples
Simple Diffusion	No	High to Low	O2, CO2
Facilitated Diffusion	No	High to Low	Glucose, ions via channels
Active Transport	Yes	Low to High	Na+/K+ pump
Bulk Transport	Yes	Varies	Endocytosis, Exocytosis

Key Terms and Definitions

• Integral Protein: Protein embedded within the membrane.

• Peripheral Protein: Protein attached to the membrane surface.

• Selective Permeability: Property allowing some substances to cross more easily than others.

• Concentration Gradient: Difference in concentration of a substance across a space.

• Diffusion: Movement of particles from high to low concentration.

• Facilitated Diffusion: Passive transport aided by proteins.

• Equilibrium: State where concentrations are equal on both sides.

• Osmosis: Diffusion of water across a membrane.

• Hypertonic: Solution with higher solute concentration than the cell.

• Hypotonic: Solution with lower solute concentration than the cell.

• Isotonic: Solution with equal solute concentration as the cell.

• Endocytosis: Uptake of material by the cell via vesicles.

• Exocytosis: Release of material from the cell via vesicles.

Important Equations

• Osmotic Pressure: $\Pi = iMRT$ Where $\Pi$ = osmotic pressure, $i$ = van 't Hoff factor, $M$ = molarity, $R$ = gas constant, $T$ = temperature (K).

Summary and Study Tips

• Understand the structure and function of the plasma membrane.

• Be able to explain and differentiate between passive and active transport mechanisms.

• Know the effects of different tonicities on animal and plant cells.

• Be familiar with the roles of various membrane proteins.

• Practice applying concepts to predict outcomes (e.g., what happens to cells in different solutions).

Additional info: Some explanations and definitions have been expanded for clarity and completeness based on standard biology textbooks.