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Membrane Structure
Fluid Mosaic Model

Explore the dynamic architecture of biological membranes: phospholipid bilayers, embedded proteins, and the principles that keep cells alive.

Biochemistry

What Is a Biological Membrane?

Every living cell is enclosed by a plasma membrane: an ultrathin barrier only about \(7\) to \(8\) nm thick. It separates the internal environment of the cell from the external world, controlling what enters and exits.

The Phospholipid: Building Block

Hydrophilic
Head
Phosphate + Glycerol
(polar, water-loving)
Fatty Acid Tails
(nonpolar, water-fearing)
Hydrophobic
Tails

Assembling the Bilayer

Extracellular Fluid (aqueous) Hydrophobic Interior Cytoplasm (aqueous)

In water, phospholipids spontaneously arrange into a bilayer: polar heads face outward toward the aqueous environment while nonpolar tails face inward, shielded from water.

The Fluid Mosaic Model

Proposed by S. J. Singer and G. L. Nicolson in 1972, the Fluid Mosaic Model describes the membrane as a two-dimensional fluid in which proteins float like a mosaic of tiles in a sea of lipids.

Fluid
Lipids and proteins move laterally within the membrane plane, giving it liquid-like behavior.
Mosaic
The membrane surface is a patchwork of diverse proteins, cholesterol, and carbohydrates embedded among lipids.

Membrane Components at a Glance

Integral
Peripheral
Cholesterol
Glycoprotein
 Extracellular Side Intracellular Side
Phospholipid

Integral vs Peripheral Proteins

Integral Proteins
Embedded within the bilayer. Many span the entire membrane (transmembrane). They have hydrophobic regions that interact with lipid tails.
Peripheral Proteins
Attached to the membrane surface only. They bind via electrostatic interactions and do not penetrate the hydrophobic core.

Cholesterol: The Fluidity Buffer

Cholesterol is wedged between phospholipids in animal cell membranes. It plays a dual role depending on temperature:

At High Temp
Cholesterol restrains phospholipid movement, reducing fluidity and preventing the membrane from becoming too fluid.
At Low Temp
Cholesterol prevents tight packing of lipid tails, maintaining fluidity and stopping the membrane from solidifying.

Factors Affecting Fluidity

1
Temperature: Higher temperature increases kinetic energy and fluidity; lower temperature decreases it.
2
Saturated vs Unsaturated Tails: Unsaturated fatty acids have kinks (cis double bonds) that prevent tight packing, increasing fluidity.
3
Tail Length: Shorter tails have fewer van der Waals interactions, making the membrane more fluid.
4
Cholesterol Content: Buffers fluidity: prevents extremes at both high and low temperatures.

Saturated vs Unsaturated Tails

Saturated (straight, tightly packed) Less fluid Unsaturated (kinked, loosely packed) More fluid cis kink

Carbohydrates and the Glycocalyx

Short carbohydrate chains are attached to proteins (glycoproteins) or lipids (glycolipids) on the extracellular surface only. Together they form the glycocalyx: a sugar coat around the cell.

Cell Recognition
Glycocalyx carbohydrates act as molecular ID tags, letting immune cells distinguish "self" from "non-self."
Protection
The sugar coat cushions the cell surface and helps in cell-to-cell adhesion.
Blood Types
ABO blood groups are determined by specific carbohydrate chains on red blood cell glycolipids.

Selective Permeability

The membrane is selectively permeable: it allows some substances to cross freely while blocking others. This is essential for maintaining the cell's internal environment.

1
Small nonpolar molecules (\(\text{O}_2\), \(\text{CO}_2\), \(\text{N}_2\)) pass through the hydrophobic core easily.
2
Small polar molecules (\(\text{H}_2\text{O}\), ethanol) cross slowly via osmosis or aquaporins.
3
Large polar molecules (glucose, amino acids) require transport proteins.
4
Ions (\(\text{Na}^+\), \(\text{K}^+\), \(\text{Cl}^-\)) cannot cross the hydrophobic core and need ion channels or pumps.

How to Analyze a Membrane

1
Identify the bilayer: locate the two leaflets of phospholipid heads and the hydrophobic core between them.
2
Classify proteins: determine whether each protein is integral (embedded) or peripheral (surface-attached).
3
Check for cholesterol: look for small flat molecules wedged between phospholipid tails.
4
Locate carbohydrates: branching sugar chains attached to proteins or lipids on the extracellular face only.
5
Predict permeability: small and nonpolar crosses easily; large, charged, or polar requires transport proteins.

Example: Predicting Permeability

Question: Which of the following can cross the lipid bilayer most easily without a transport protein: glucose, \(\text{O}_2\), \(\text{Na}^+\), or a large protein?

Example: Temperature and Fluidity

Scenario: A cell membrane contains mostly saturated fatty acid tails and low cholesterol. The cell is moved from \(37^\circ\text{C}\) to \(10^\circ\text{C}\). What happens?

Common Mistakes

✕ Membrane is static
The membrane is not a rigid, fixed structure. Lipids and proteins constantly move laterally: it is a dynamic fluid.
✕ Cholesterol always decreases fluidity
Cholesterol buffers fluidity in both directions: it reduces fluidity at high temperatures and increases it at low temperatures.
✕ Carbs on both sides
Carbohydrate chains (glycocalyx) are found only on the extracellular surface, never on the cytoplasmic side.

Summary Comparison

Feature Details
Phospholipids Amphipathic, form bilayer Polar heads out, nonpolar tails in
Integral Proteins Embedded in bilayer Transport, signaling, enzymatic
Peripheral Proteins Surface-attached only Cytoskeleton links, enzymes
Cholesterol Fluidity buffer Prevents extremes in fluidity
Glycocalyx Extracellular carbs Cell recognition, protection
Unsaturated Tails Kinked, loose packing Increase membrane fluidity

Question 1 of 5

The Fluid Mosaic Model describes the cell membrane as:

Question 2 of 5

Which molecule would most easily cross the lipid bilayer without a transport protein?

Question 3 of 5

A cell membrane rich in unsaturated fatty acids compared to one rich in saturated fatty acids would be:

Question 4 of 5

Where are carbohydrate chains of glycoproteins and glycolipids located?

Question 5 of 5

What is the primary role of cholesterol in the cell membrane?