Carbohydrates, Lipids and Proteins: Structure and Functions

Carbohydrates

Carbohydrates: compounds composed of carbon, hydrogen and oxygen, comprising sugars and derived substances.

Types

Monosaccharides and soluble disaccharides: sweet-tasting, soluble in water, crystalline solids, etc.

Functions

1. Energetic: constitute the main source of glucose energy.
2. Structural: part of structural polysaccharides in the skeleton.

Lipids

Lipids: compounds of C, H and O, also P, K and S. Common characteristics: oily, insoluble in water, soluble in organic solvents.

Classification

Saponifiable: lipids that originate esters that can be hydrolyzed. They are classified as:

• Acylglycerols: glycerol esters with 2 or 3 fatty acid molecules.
• Ceride: esters of monoalcohols and fatty acids.
• Phosphoglycerides: composed of glycerol, fatty acids, phosphoric acid and an alcohol. Important examples: lecithin and cephalin.
• Sphingolipids: derived from sphingosine and ceramide.

Non-saponifiable: cellular lipids that do not contain fatty acids nor esters. They include:

• Carotenoids: hydrocarbon pigments such as carotene and xanthophyll.
• Steroids: cholesterol, bile acids, brain lipids and plasma membrane components.

Functions

• Energy source: their oxidation releases more energy than carbohydrates.
• Structural: part of the structure of organisms.
• Protective: waxes, ear wax and animal fats under the skin isolate and protect.
• Digestive: bile acids promote the emulsion of fats, facilitating the action of digestive enzymes that decompose fats.
• Regulatory: steroid hormones, vitamins and carotenes perform regulatory roles.

Proteins

Proteins: macromolecular compounds composed of C, H, O, N and often S. They are specific, and each organism synthesizes its own proteins. Their solubility varies: some are soluble in water, others in saline solvents, and some form colloids.

Classification

Holoproteins: constituted only by polypeptide chains. There are two types:

• Globular: spherical and soluble, such as albumin and globulins.
• Fibrous: insoluble, filamentous and structural, found in animals and with plastic functions, such as collagen, keratin, myosin and elastin.

Heteroproteins: proteins formed by a protein portion associated with a non-protein group (prosthetic group). They are classified as:

• Glycoproteins: molecules formed by a protein combined with carbohydrates. Example: mucins, lubricants of the oral mucosa.
• Lipoproteins: combinations with fatty acids. Some transport fats in plasma, while others are lipid components of membranes.
• Phosphoproteins: constituted by proteins combined with phosphoric acid.
• Chromoproteins: pigmented proteins with a prosthetic group derived from porphyrin. Examples: hemoglobin, which transports oxygen in the blood; hemocyanin, which performs the same function in invertebrates; and chlorophyll, found in plant chloroplasts.

Features

Proteins are structural components that give rigidity and fluidity to tissues. All enzymes are proteins, and some hormones also have transport functions. Proteins transport and protect certain lipids, oxygen, and other substances.