Summary: Cell Membrane Overview

Introduction

Cell membrane is a double layer of lipids that surrounds the cell, keeping internal components together and protecting the cell from its environment. It controls what enters and leaves the cell, regulating nutrients, ions, and blocking harmful substances.

What's it made up of?

The membrane has three main components:

• Phospholipids: Molecules with a water-attracting head and water-repelling tails that self-assemble into a bilayer, with heads facing the watery surroundings and tails tucked inside.
• Cholesterol: A steroid distributed within the bilayer that helps stabilize membrane structure and regulate movement of molecules across the membrane.
• Proteins: Include integral proteins embedded across the bilayer, with polar and nonpolar regions that enable transport of larger molecules, and peripheral proteins attached to membrane surfaces that assist in transport and communication.

What makes the cell membrane fluid?

The fluid mosaic model describes the membrane as a dynamic mix of molecules that move within the bilayer. Fluidity depends on:

• Temperature: Higher temperature increases lipid movement; lower temperature brings lipids closer together.
• Cholesterol: Modulates packing of phospholipids, preventing excessive tight packing in cold and excessive separation in heat.
• Saturated versus unsaturated fatty acid tails: Saturated tails are straight and pack tightly; unsaturated tails have kinks that increase spacing and reduce tight packing.

What can go through the cell membrane?

The membrane is semi-permeable, allowing some substances to cross easily and blocking others. Molecules fall into these groups based on size, polarity, and charge:

• Small, nonpolar molecules: Cross the bilayer easily by diffusion without proteins.
• Small, polar molecules: Can cross but more slowly because of the hydrophobic core.
• Large, nonpolar molecules: Can cross slowly over time.
• Large, polar molecules: Generally cannot cross the nonpolar core without transport proteins.
• Ions: Charged particles require transport proteins to cross the membrane.