Lipids, Membranes, and Transport: Structure and Function of the Plasma Membrane

Chapter 6b: Lipids, Membranes

Introduction

The plasma membrane is a fundamental barrier that defines the boundaries of all living cells. It is primarily composed of lipids and proteins, which together regulate the movement of substances into and out of the cell. This chapter explores the structure of membranes, the processes of diffusion and osmosis, and the roles of membrane proteins in transport.

Lipid Bilayer Structure

The plasma membrane consists of a phospholipid bilayer interspersed with proteins. The bilayer forms a selectively permeable barrier, allowing certain molecules to pass while restricting others.

• Phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, causing them to arrange in a bilayer.

• Selective permeability means that small, nonpolar molecules pass easily, while ions and large polar molecules are restricted.

• Fluid-mosaic model: Membranes are dynamic, with proteins and lipids able to move laterally within the layer.

Concentration Gradient

A concentration gradient is established when there is a difference in the concentration of a substance across a space or membrane.

• Substances naturally move from areas of high concentration to areas of low concentration.

• This movement is exergonic (spontaneous), requiring no input of energy.

Diffusion Across a Selectively Permeable Membrane

Diffusion is the net movement of molecules from regions of higher concentration to regions of lower concentration, driven by the concentration gradient.

• Passive transport: No energy is required; molecules move down their concentration gradient.

• Equilibrium: Achieved when concentrations are equal on both sides of the membrane, resulting in no net movement.

Process of Diffusion Across a Lipid Bilayer

1. Separation of solutes: Different concentrations of solute on either side of the membrane.

2. Diffusion: Solute molecules move across the membrane.

3. Equilibrium: Solute concentrations become equal.

Osmosis: Diffusion of Water

Osmosis is the diffusion of water across a selectively permeable membrane. Water moves from regions of low solute concentration (high water concentration) to regions of high solute concentration (low water concentration).

• Occurs when solute cannot cross the membrane, but water can.

• Water movement continues until equilibrium is reached.

Key Points of Osmosis

• High solute concentration = low water concentration.

• Low solute concentration = high water concentration.

• Water moves to balance solute concentrations.

Summary Table: Comparison of Diffusion and Osmosis

Process	Substance Moved	Direction	Energy Required
Diffusion	Solute molecules	High to low concentration	No
Osmosis	Water molecules	Low to high solute concentration (high to low water concentration)	No

Key Terms and Definitions

• Phospholipid bilayer: Double layer of phospholipids forming the basic structure of cell membranes.

• Selective permeability: Property of membranes that allows some substances to pass while blocking others.

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

• Osmosis: Movement of water across a membrane in response to solute concentration differences.

• Equilibrium: State where concentrations are equal and there is no net movement.

Relevant Equations

• Fick's Law of Diffusion: $J = -D \frac{dC}{dx}$ Where $J$ is the rate of diffusion, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Example Application

• When a cell is placed in a hypotonic solution (lower solute concentration outside), water enters the cell by osmosis, potentially causing it to swell.

• When a cell is placed in a hypertonic solution (higher solute concentration outside), water leaves the cell, causing it to shrink.

Additional info: Later sections of the chapter (not fully shown here) likely cover membrane proteins, facilitated diffusion, and active transport, which are essential for understanding how cells regulate their internal environments.