Chemical composition of the cell. Carbohydrates. Lipids

By mass a cell is mostly water, 70 to 80 per cent. The rest is inorganic molecules and four classes of organic compound, and their proportions are worth memorising because they are so uneven: protein 12 per cent, nucleic acid 2 per cent, carbohydrate 1 per cent, lipid 0.5 per cent.

Carbohydrates come as monosaccharides, disaccharides and polysaccharides. They have three functions: energy production as the primary role, energy storage as glycogen, and building macromolecules, since glucose is converted to ribose and deoxyribose.

Lipids form half of the phospholipid bilayer, and fall into three classes: simple, compound and derived.

The essential elements divide into two groups:

• Macroelements, about 98 per cent: C, N, O, H.
• Microelements: Cu, Zn, Mg.

Water makes up 70 to 80 per cent of the cell. Its functions are:

• it is the matrix of life;
• it is the solvent for nutrients;
• it eliminates waste products of the body;
• it maintains a constant temperature.

Inorganic molecules are present as free ions and bound to organic molecules.

Protein is by far the largest fraction, and lipid the smallest. That is worth noticing, because lipid nonetheless makes up half of every membrane in the cell: a small share of the total mass doing a structurally enormous job.

Carbohydrates are classified by how many sugar units they contain:

Three, and the first is the primary one.

Energy production, the primary role. Glucose is broken down by glycolysis in the cytoplasm, and the pyruvate it gives is then oxidised in the mitochondria, which releases most of the energy.

Energy storage. Too much energy means glucose is stored as glycogen, in muscle and liver.

Building macromolecules. Some glucose is converted to ribose and deoxyribose, which are essential components of RNA, DNA and ATP.

• On the outer surface of the plasma membrane, as glycoprotein and glycolipid.
• In the cytoplasm, as glycogen granules, mainly in liver and muscle cells.

Lipids form half of the phospholipid bilayer.

Their functions are energy storage, signalling, and acting as a structural component of the plasma membrane.

Simple lipids consist of esters of fatty acids. They include triglycerides and waxes.

Compound lipids consist of esters of fatty acids with additionally other groups, such as sugars, phosphoric acid and proteins. They include phospholipids and glycolipids.

Derived lipids consist of hydrocarbon rings and a long hydrocarbon side chain. They do not contain fatty acid. They include steroids, cholesterol and sex hormones.

The dividing line to hold on to is fatty acid. Simple and compound lipids are both built on esters of fatty acids and differ only in what else is attached; derived lipids have no fatty acid at all.