Cell membranes are mainly made of lipids and proteins.
The lipid component consists of:
- Phospholipids
- Cholesterol
- Glycolipids
The lipid component allows lipid-soluble substances to pass easily through the cell membrane.
Examples of lipid-soluble substances include:
- Carbon dioxide (CO₂)
- Oxygen (O₂)
- Fatty acids
- Steroid hormones
The lipid component does not allow water-soluble substances to pass easily.
Examples of water-soluble substances include:
- Ions
- Glucose
- Amino acids
The protein component of the cell membrane contains:
- Transporters
- Enzymes
- Hormone receptors
- Cell-surface antigens
- Ion channels
- Water channels

KEY CONCEPT

Cell membranes are made mainly of lipids and proteins.
Lipids make the membrane highly permeable to lipid-soluble substances (CO₂, O₂, fatty acids, steroid hormones).
Lipids make the membrane poorly permeable to water-soluble substances (ions, glucose, amino acids).
Membrane proteins function as transporters, enzymes, receptors, antigens, and ion/water channels.

Phospholipid Component of Cell Membranes

Phospholipids are made of two main parts:
- A phosphorylated glycerol backbone (head).
- Two fatty acid tails.
The glycerol head is hydrophilic (water-soluble).
The fatty acid tails are hydrophobic (water-insoluble).
Because phospholipids have both hydrophilic and hydrophobic properties, they are called amphipathic molecules.
At an oil–water interface, phospholipids form a single layer (monolayer) and arrange themselves so that:
- The hydrophilic heads face the water.
- The hydrophobic tails face the oil. (Fig. 1.2A)
In cell membranes, phospholipids arrange into a lipid bilayer.
In this bilayer:
- The fatty acid tails face each other in the center of the membrane.
- The hydrophilic glycerol heads face the watery environments of the ICF and ECF.
This arrangement forms the lipid bilayer of the cell membrane. (Fig. 1.2B)

KEY CONCEPT

Phospholipids have a water-loving (hydrophilic) head and water-fearing (hydrophobic) tails.
Because they have both properties, they are called amphipathic molecules.
At an oil–water interface, they form a monolayer. (Fig. 1.2A)
In cell membranes, they form a lipid bilayer with tails facing inward and heads facing the ICF and ECF. (Fig. 1.2B)

Protein Component of Cell Membranes

Cell membrane proteins are of two types:
- Integral proteins
- Peripheral proteins
The arrangement of these proteins in the phospholipid bilayer is shown in the fluid mosaic model. (Fig. 1.3)
Integral membrane proteins are embedded in the cell membrane and are held in place by hydrophobic interactions.
Integral membrane proteins perform many functions, including:
- Receptors
- Cell adhesion molecules
- Transport of solutes and water
- Enzymes
- Cell signaling proteins
To remove an integral protein, its attachment to the lipid bilayer must be disrupted (for example, by detergents).
Some integral proteins are transmembrane proteins, which pass through the entire lipid bilayer.
These proteins are in contact with both the extracellular fluid (ECF) and intracellular fluid (ICF).
Examples of transmembrane proteins include:
- Ligand-binding receptors (for hormones and neurotransmitters)
- Transport proteins (such as the Na⁺-K⁺ ATPase)
- Water pores and ion channels
- Cell adhesion molecules
- GTP-binding proteins (G proteins)
Some integral proteins are embedded in the membrane but do not pass completely through it.
Other integral proteins are attached to membrane lipids by covalent bonds instead of being embedded in the lipid bilayer.
Peripheral membrane proteins are not embedded in the lipid bilayer.
They are not covalently attached to membrane components.
They are loosely attached to either the inner or outer surface of the cell membrane by:
- Ionic interactions
- Attachment to integral membrane proteins
Peripheral proteins can be removed by mild treatments that break ionic or hydrogen bonds.
An example of a peripheral protein is ankyrin.
Ankyrin connects the red blood cell cytoskeleton to the integral membrane Cl⁻-HCO₃⁻ exchanger (band 3 protein).

KEY CONCEPT

Cell membrane proteins are of two types: integral and peripheral.
Integral proteins are embedded in the membrane and perform transport, receptor, enzyme, adhesion, and signaling functions.
Some integral proteins span the entire membrane (transmembrane proteins).
Peripheral proteins are loosely attached to the membrane surface and can be removed easily.
The fluid mosaic model shows the arrangement of membrane proteins in the lipid bilayer. (Fig. 1.3)