Plasma Membrane Structure (College Board AP® Biology): Study Guide

Membrane components

• Membranes surround all living cells, as well as forming the structure of many internal cellular organelles

• Membranes contain many components, including:

  • phospholipids

  • proteins

  • glycoproteins

  • glycolipids

  • steroids e.g. cholesterol

Phospholipids

• Phospholipids consist of a phosphate head and two fatty acid tails

  • The phosphate head of a phospholipid is polar and can therefore interact with water; this region is said to be hydrophilic

  • The fatty acid tails of a phospholipid are nonpolar and therefore cannot interact with water; the tails are hydrophobic

• When placed in an aqueous environment, phospholipids arrange themselves so that the hydrophilic heads are oriented towards water and the hydrophobic tails are oriented away from water; this means that they can form phospholipid bilayers

• Phospholipid bilayers form the basic structure of membranes

Proteins

• Proteins can be embedded within phospholipid bilayers

• The roles of such proteins include:

  • cell recognition

  • cell signalling

  • membrane transport

  • membrane-bound enzymes

• The position of an embedded protein within a membrane will be determined by its structure; hydrophilic proteins will face the surrounding aqueous environment while hydrophobic proteins will face the interior of the bilayer

  • Hydrophilic proteins have charged and polar side groups

  • Hydrophobic proteins have nonpolar side groups

Glycoproteins

• Glycoproteins are proteins with attached carbohydrate chains

• They can be involved with:

  • cell recognition

  • cell signalling

Glycolipids

• Glycolipids are lipids with attached carbohydrate chains

• Their roles can include:

  • cell recognition

  • cell signalling

Steroids

• Cholesterol is a steroid lipid present within cell membranes

• The role of cholesterol is to regulate membrane fluidity

The fluid mosaic model

• The fluid mosaic model describes modern understanding of membrane structure; it states that:

  • cell membranes consist of a structural framework of phospholipids that contains embedded components such as proteins, steroids, glycoproteins, and glycolipids

  • the embedded components can flow around the surface of the cell within their membrane layer