Cell Membranes and Transport: Study Notes for General Biology

Chapter 7: Inside the Cell

Overview

This chapter focuses on the structure and function of cell membranes, the Fluid Mosaic Model, membrane proteins, and mechanisms of transport across membranes. Understanding these concepts is essential for grasping how cells interact with their environment and maintain homeostasis.

Fluid Mosaic Model of Membranes

Definition and Explanation

• Fluid Mosaic Model: Describes the cell membrane as a dynamic structure composed of a phospholipid bilayer with embedded proteins, carbohydrates, and cholesterol.

• Phospholipids: Amphipathic molecules with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, forming a bilayer.

• Membrane Fluidity: Influenced by temperature, fatty acid composition (saturated vs. unsaturated), and cholesterol content.

Example: At lower temperatures, cholesterol helps maintain membrane fluidity by preventing tight packing of phospholipids.

Membrane Components

Major Components and Their Functions

• Phospholipids: Form the basic structure of the membrane.

• Proteins: Integral (span the membrane) and peripheral (attached to the surface); involved in transport, signaling, and structural support.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids); play roles in cell recognition and signaling.

• Cholesterol: Modulates fluidity and stability of animal cell membranes.

Membrane Proteins

Types and Functions

• Integral Proteins: Embedded within the lipid bilayer; often function as channels or transporters.

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

• Functions: Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton.

Membrane Carbohydrates

Role in Cell Recognition

• Glycoproteins and Glycolipids: Carbohydrate chains attached to proteins or lipids on the extracellular surface; crucial for cell-cell recognition and immune response.

Example: Blood type antigens are determined by specific carbohydrate patterns on red blood cell membranes.

Transport Across Membranes

Mechanisms of Transport

• Passive Transport: Movement of substances down their concentration gradient without energy input.

• Types of Passive Transport:

  • Simple Diffusion: Direct movement of small, nonpolar molecules (e.g., O2, CO2).

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

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

• Example: Sodium-potassium pump ($Na^+/K^+$-ATPase) maintains electrochemical gradients in animal cells.

Osmosis and Water Balance

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

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

Equation:

$ext{Osmotic Pressure} = iMRT$

where $i$ is the van 't Hoff factor, $M$ is molarity, $R$ is the gas constant, and $T$ is temperature.

Membrane Permeability

Factors Affecting Permeability

• Structure of the Lipid Bilayer: Nonpolar molecules pass easily; polar and charged molecules require transport proteins.

• Presence of Transport Proteins: Channels and carriers facilitate movement of specific substances.

Types of Membrane Transport

Comparison Table

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	Na+/K+ pump
Osmosis	No	Down water potential gradient	Water via aquaporins

Bulk Transport Mechanisms

Endocytosis and Exocytosis

• Endocytosis: Process by which cells engulf large particles or fluids.

• Types of Endocytosis:

  • Phagocytosis: "Cell eating"; uptake of large particles.

  • Pinocytosis: "Cell drinking"; uptake of fluids and small molecules.

  • Receptor-mediated Endocytosis: Specific uptake of molecules via receptor binding.

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

Summary of Key Concepts

• The cell membrane is a selectively permeable barrier, described by the Fluid Mosaic Model.

• Membrane proteins and carbohydrates play crucial roles in transport, signaling, and recognition.

• Transport across membranes occurs via passive (diffusion, osmosis) and active (pumps, bulk transport) mechanisms.

• Understanding membrane structure and function is essential for studying cell biology and physiology.