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- Lipids are hydrophobic molecules composed of carbon, hydrogen, and oxygen. - They are insoluble in water but soluble in other nonpolar substances.- Organisms use lipids for storing energy, building membranes and other cell parts, and as chemical signalling molecules. - Lipids can be divided into four families: fats, phospholipids, steroids, and waxes.

- Animals convert excess carbohydrate into fat and store the fat molecules as droplets in the cells of adipose tissue. - A layer of fat under the skin acts as thermal insulation in many animals, such as penguins.

- Plants also store energy in the form of fat. The most common fats in plants and animals are the triacylglycerols. These contain three fatty acids attached to a single molecule of glycerol.

- Animal fats, such as butter, are mostly composed of triacylglycerols containing only saturated fatty acids.

- Plant fats, such as olive oil, are mostly composed of triacylglycerols containing unsaturated and polyunsaturated fatty acids.

Phospholipids- The membranes of cells are mostly composed of fats called phospholipids.- Phospholipids are composed of a glycerol molecule to which is attached two fatty acids and a highly polar phosphate group.

Sterols (Steroids)- Membranes often contain another class of lipid called sterols. - These are compact hydrophobic molecules containing four fused hydrocarbon rings and several different functional groups.- Cholesterol is an important steroid component of cell membranes, but high concentrations of cholesterol in the bloodstream may lead to a heart attack. - Cells convert cholesterol into a number of compounds, such as vitaminD which is needed for healthy bones and teeth. - Other steroids include the sex hormones that control the development of sex traits and gametes.

Waxes- Waxes are lipids containing long-chain fatty acids linked to alcohols or carbon rings. - Waxes, such as cutin, are ideal for forming waterproof coatings on various plant parts. This helps the plant conserve water and also acts as a barrier to infection.- Birds secrete a waxy material that helps keep their feathers dry, and bees produce beeswax to construct honeycombs.


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