Phospholipids and Cholesterol

Lipids are not only energy stores — they are essential components of every biological membrane. This lesson focuses on phospholipids and cholesterol, the two lipid classes that define the structure and properties of the cell surface membrane and internal membranes. It addresses OCR specification point 2.1.2 (c)(iii).

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1. Phospholipids — the Building Blocks of Membranes

A phospholipid is similar to a triglyceride, but with one key difference: one of the three fatty acids is replaced by a phosphate group. The result is a molecule with two hydrocarbon tails and a phosphate-containing head.

Key Definition — Phospholipid: A lipid molecule composed of glycerol, two fatty acid tails, and a phosphate group (often attached to a further polar group). It is amphipathic — having both hydrophilic and hydrophobic regions.

1.1 Structure

                     Phosphate
                       head  (hydrophilic)
                        |
                        O
                        |
                        P
                       / \
                     O     O
                     |     |
                     CH₂
                     |
      hydrophobic   CH
         tails      |
         (2)        CH₂
                    |
               (fatty acid 1)   (fatty acid 2)

• Glycerol backbone — three carbons
• Two fatty acid tails (can be saturated or unsaturated)
• Phosphate group on the third carbon of glycerol; typically bonded to an additional polar group such as choline (giving phosphatidylcholine, the commonest phospholipid in animal membranes)
• Three ester bonds in total: two between fatty acids and glycerol, one between phosphate and glycerol (also called a phosphoester bond)

1.2 Amphipathic Nature

The phosphate head is charged and polar, so it is hydrophilic — it forms hydrogen bonds with water.

The two fatty acid tails are non-polar hydrocarbons, so they are hydrophobic — they repel water but interact favourably with other non-polar molecules.

A molecule with both hydrophilic and hydrophobic regions is called amphipathic (or amphiphilic). This dual nature is the reason phospholipids spontaneously form membranes.

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2. Phospholipid Behaviour in Water

When phospholipids are placed in water, they spontaneously organise themselves to minimise unfavourable interactions between water and the hydrophobic tails. Several arrangements are possible:

2.1 Monolayer

At the air-water interface, phospholipids form a single layer with heads in the water and tails projecting into the air.

2.2 Micelle

In the bulk of water, phospholipids can form micelles — small spherical clusters with tails in the core and heads facing outward.

2.3 Bilayer (Membrane)

The most biologically important arrangement is the phospholipid bilayer. Two layers of phospholipids orient with their heads facing the watery environments on each side of the membrane and their tails buried in the centre of the bilayer, shielded from water.

Outside cell (water)

Heads  ●●●●●●●●●●●●●●●●●●  ← hydrophilic phosphate heads
Tails  ||||||||||||||||||
Tails  ||||||||||||||||||  ← hydrophobic fatty acid tails (core)
Heads  ●●●●●●●●●●●●●●●●●●  ← hydrophilic phosphate heads

Inside cell (water)

This bilayer structure is the fundamental architecture of all biological membranes, including the plasma membrane, nuclear envelope, endoplasmic reticulum, Golgi apparatus, mitochondrial membranes and thylakoid membranes.

2.4 Fluid Mosaic Model

Phospholipid molecules in a bilayer are not fixed — they can move laterally within each layer, giving the membrane a fluid quality (like a two-dimensional liquid). Proteins are embedded in or attached to the bilayer, creating the fluid mosaic model of membrane structure (Singer and Nicolson, 1972).

2.5 Role of Unsaturated Fatty Acids

• Unsaturated fatty acid tails have kinks (due to C=C double bonds) that prevent tight packing.
• Membranes with more unsaturated phospholipids are more fluid.
• Organisms in cold environments (e.g., Arctic fish) have more unsaturated fatty acids in their membranes to maintain fluidity at low temperatures.

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3. Biological Functions of Phospholipids

• Form membranes — bilayer structure is the backbone of every biological membrane.
• Barrier to water-soluble molecules — polar and charged molecules cannot cross the hydrophobic core without assistance.
• Allow selective transport — in conjunction with embedded transport proteins.
• Cell signalling — specific phospholipids (e.g., phosphatidylinositol) generate second messengers.
• Lung surfactant — phospholipids lining alveoli reduce surface tension, preventing collapse.

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4. Cholesterol — a Membrane Steroid

Cholesterol is a steroid lipid — it belongs to a class of lipids with a characteristic four-fused-ring structure. It is not built from glycerol and fatty acids, so it is structurally very different from triglycerides and phospholipids. Nevertheless, it plays crucial roles in animal cell membranes.

4.1 Structure