Membrane Structure and Function: Study Guide

Ch 7 Pearson Study Guide from Study Guide

Chapter 7: Membrane Structure and Function

Overview

This chapter explores the structure and function of biological membranes, focusing on the plasma membrane, its components, and the mechanisms by which substances move across it. Understanding these concepts is essential for grasping how cells interact with their environment and maintain homeostasis.

Fluid Mosaic Model

Plasma (Cell) Membrane Structure

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails. They form a bilayer, providing the fundamental structure of the membrane.

• Sterols (Cholesterol): Interspersed within the phospholipid bilayer, cholesterol modulates membrane fluidity and stability.

• Proteins: Embedded (integral) or attached (peripheral) proteins serve various functions, including transport, signaling, and cell recognition.

• Glycolipids and Glycoproteins: Lipids and proteins with attached carbohydrate chains, important for cell recognition and signaling.

• Extracellular Matrix (ECM): A network of proteins and carbohydrates outside the cell membrane, providing structural support and mediating cell interactions.

Key Terms:

• Hydrophobic: Repelled by water; nonpolar.

• Hydrophilic: Attracted to water; polar.

• Amphipathic: Molecules with both hydrophobic and hydrophilic regions (e.g., phospholipids).

Membrane Proteins

Types and Functions

• Integral Proteins: Span the membrane; involved in transport and signaling.

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

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

• Receptor Proteins: Bind signaling molecules and initiate cellular responses.

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

Membrane Fluidity

Factors Affecting Fluidity

• Cholesterol: Acts as a "fluidity buffer," stabilizing the membrane at high temperatures and preventing it from becoming too rigid at low temperatures.

• Fatty Acid Composition: Unsaturated fatty acids (with double bonds) increase fluidity by preventing tight packing; saturated fatty acids decrease fluidity.

Example: Increasing the proportion of unsaturated phospholipids or adding cholesterol at low temperatures increases membrane fluidity.

Transport Across Membranes

Passive Transport

• Simple Diffusion: Movement of molecules from high to low concentration without energy input.

• Facilitated Diffusion: Movement of molecules down their concentration gradient via transport proteins; no energy required.

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

Osmosis and Tonicity

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

• Hypotonic Solution: Lower solute concentration outside the cell; water enters the cell, which may swell or burst (lysis in animal cells, turgor in plant cells).

• Hypertonic Solution: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink (crenation in animal cells, plasmolysis in plant cells).

Active Transport

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

• Examples: Sodium-potassium pump, proton (hydrogen) pump.

• Bulk Transport: Endocytosis (phagocytosis, pinocytosis) and exocytosis move large molecules or particles into or out of the cell via vesicles.

Comparison of Transport Mechanisms

Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	O2, CO2
Facilitated Diffusion	No	High to Low	Glucose via carrier protein
Osmosis	No	High to Low (water)	Water movement
Active Transport	Yes	Low to High	Na+/K+ pump
Bulk Transport	Yes	Varies	Endocytosis, Exocytosis

Key Concepts and Applications

• Solute: Substance dissolved in a solvent.

• Solvent: The dissolving agent (usually water in biological systems).

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

• Electrochemical Gradient: Combined effect of concentration gradient and electrical charge difference across a membrane.

Formulas and Equations

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

• Water Potential (Ψ): $\Psi = \Psi_s + \Psi_p$ Where $\Psi$ = total water potential, $\Psi_s$ = solute potential, $\Psi_p$ = pressure potential.

Practice Questions and Applications

• Be able to identify the type of molecules found in the plasma membrane and their functions.

• Know how the structure and function of phospholipids and sterols (cholesterol) affect membrane properties.

• Understand the difference between passive and active transport, and be able to predict the direction of water and solute movement in different tonicities.

• Recognize the role of proteins in transport, signaling, and cell recognition.

• Apply knowledge of osmosis to predict cell behavior in hypotonic, hypertonic, and isotonic solutions.

Example Application

• Red blood cell in a hypotonic solution: Water enters the cell, causing it to swell and possibly burst (lysis).

• Plant cell in a hypertonic solution: Water leaves the cell, causing the plasma membrane to pull away from the cell wall (plasmolysis).

Summary Table: Tonicity Effects on Cells

Solution Type	Animal Cell Effect	Plant Cell Effect
Isotonic	No net change	Flaccid
Hypotonic	Lysis (bursting)	Turgid (normal)
Hypertonic	Crenation (shrinking)	Plasmolysis

Additional info:

• Bulk transport processes (endocytosis and exocytosis) are essential for moving large molecules or particles that cannot pass through the membrane by diffusion or transport proteins.

• Facilitated diffusion and active transport both require specific membrane proteins, but only active transport requires energy input.