Lipids & Cell Membranes

Lipids are a family of molecules that all share one key property: they don't dissolve in water. You've seen this — pour oil into water and it clumps together. This is because lipids are made mostly of hydrocarbons (carbon and hydrogen), which are non-polar and avoid water. This water-avoiding property isn't a flaw — it's exactly what the body exploits. Lipids serve four major roles: 1. Cell membranes — the lipid bilayer is the structural wall of every cell 2. Energy storage — fat stores more than twice the energy per gram compared to carbohydrates (9 kcal/g vs 4 kcal/g) 3. Hormones — steroid hormones (like oestrogen and testosterone) are made from lipids 4. Insulation and protection — fat under the skin keeps you warm; fat around organs cushions them The main types of lipids you need to know are: fatty acids, triglycerides, phospholipids, sterols (cholesterol), and sphingolipids.

A fatty acid is a long chain of carbons with hydrogen atoms attached, ending in a carboxyl group (–COOH). Think of it like a long tail with a head. Saturated fatty acids have no double bonds between carbons — every carbon is "saturated" with hydrogen atoms. They tend to be solid at room temperature (e.g. butter, animal fat). Their straight shape lets them pack tightly together. Unsaturated fatty acids have one or more double bonds. Each double bond creates a "kink" in the chain. This kink stops them packing tightly, so they're liquid at room temperature (e.g. olive oil, fish oil). - Monounsaturated = one double bond (e.g. oleic acid in olive oil) - Polyunsaturated = multiple double bonds (e.g. omega-3, omega-6 fatty acids) Trans fats are unsaturated fats that have been artificially straightened by adding hydrogen (hydrogenation). Their kinked bonds are straightened into a "trans" configuration. They behave like saturated fats in the body and are strongly linked to cardiovascular disease. Essential fatty acids — like omega-3 and omega-6 — cannot be made by the body and must come from diet. They're needed for brain development, inflammation control, and cell membrane function.

A triglyceride is the main form of fat stored in the body. It consists of one glycerol molecule (a 3-carbon backbone) with three fatty acid chains attached. When you eat fat, triglycerides are digested: lipase enzymes (from the pancreas) break the fatty acids off the glycerol backbone. The fatty acids are absorbed into the bloodstream. When you eat more calories than you need, the excess is converted to triglycerides and stored in adipose (fat) tissue. When you need energy (e.g. fasting, exercise), triglycerides are broken down — a process called lipolysis — to release fatty acids for energy. Clinical connection: High blood triglyceride levels (hypertriglyceridaemia) are a risk factor for cardiovascular disease and pancreatitis. They're usually caused by a high-sugar diet, obesity, alcohol, or genetic factors.

Phospholipids are the most important lipids in medicine because they form every cell membrane in your body. A phospholipid is similar to a triglyceride but with one key difference: instead of three fatty acids, it has two fatty acids AND a phosphate group attached to a small head molecule (like choline or serine). This gives a phospholipid a split personality: - The head (phosphate + small molecule) is hydrophilic — it loves water - The two fatty acid tails are hydrophobic — they avoid water When many phospholipids are placed in water, they spontaneously arrange themselves into a bilayer: two sheets facing each other, hydrophilic heads facing outward (towards water), hydrophobic tails sandwiched in the middle, hiding from water. This phospholipid bilayer is the foundation of every cell membrane. It's not a rigid wall — it's a fluid structure. Lipids can move sideways (lateral diffusion), which is why the membrane can flex, reseal after damage, and allow membrane proteins to relocate. What can cross the membrane freely? Small, uncharged, fat-soluble molecules (O₂, CO₂, alcohol, steroid hormones) slip through easily. Charged ions (Na⁺, K⁺, Cl⁻) and large molecules (glucose, proteins) cannot cross without help from transport proteins.

Cholesterol has a bad reputation, but it's actually essential for life. Every cell membrane in your body contains cholesterol, and it's the starting material for making steroid hormones, bile acids, and vitamin D. Cholesterol in the membrane: Cholesterol molecules sit between phospholipids in the membrane. Their function depends on temperature: - In warm conditions: cholesterol reduces fluidity by restricting phospholipid movement (stops the membrane becoming too liquid) - In cold conditions: cholesterol prevents phospholipids packing too tightly (stops the membrane becoming too rigid) The result: membrane fluidity stays in a "Goldilocks" zone — not too fluid, not too rigid. Cholesterol transport in the blood: Cholesterol and fats can't dissolve in blood (water-based), so they're packaged into lipoproteins — spherical particles with a hydrophilic shell and a lipid core. - LDL (Low Density Lipoprotein) — carries cholesterol FROM the liver TO tissues. High LDL is associated with atherosclerosis (plaques in arteries) — commonly called "bad cholesterol." - HDL (High Density Lipoprotein) — carries cholesterol FROM tissues BACK TO the liver for disposal — "good cholesterol." Higher HDL is protective. Statins work by inhibiting HMG-CoA reductase — the key enzyme in cholesterol synthesis in the liver. Less synthesis = less LDL in the blood = reduced cardiovascular risk.

Understanding lipids is essential across many areas of medicine: Cardiovascular disease: Excess LDL cholesterol deposits in artery walls forming atherosclerotic plaques — fatty buildups that narrow and harden arteries. This reduces blood flow and can lead to heart attacks and strokes. Risk is reduced by diet, exercise, and statins. Gallstones: Most gallstones (about 80%) are made of cholesterol that has precipitated out of bile. They form when bile contains too much cholesterol, not enough bile salts, or the gallbladder doesn't empty well. Non-alcoholic fatty liver disease (NAFLD): Excess fat accumulates in liver cells (hepatocytes) — often from high-calorie diets and obesity. The liver becomes enlarged and inflamed, potentially progressing to cirrhosis. Respiratory distress syndrome (RDS) in premature babies: The lungs are lined with a lipid substance called surfactant (made mostly of phospholipids) that prevents alveoli (air sacs) from collapsing with each breath. Premature babies haven't made enough surfactant yet — their lungs collapse after each breath. Treatment: synthetic surfactant given via breathing tube.