• Amino acids are composed of a central carbon joined to

• an amino (-NH₂) group which often loses an electron.

• a carboxyl (-COOH) group which often gains an electron.

• a functional group (R) unique to each of 20 different kinds of amino acids.

• Amino acids can be covalently linked by peptide bonds (dehydration synthesis) to produce a polypeptide chain.
A protein comprises one or more polypeptide chains.

• Protein structure: primary and secondary.

The sequence of amino acids of a polypeptide, linked by covalent bonds, determines its primary structure.

Hydrogen bonds between parts of the polypeptide may cause the chain to fold into regions such as an α (alpha) helix or a β (beta) pleated sheet, forming its secondary structure.

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• Protein structure: tertiary and quaternary.

Further folding of the polypeptide exposes the polar (hydrophilic) regions towards the exterior, and pushes the nonpolar (hydrophobic) regions towards the interior, forming a three-dimensional shape, its tertiary structure.

Some proteins are made of multiple polypeptide chains arranged in a quaternary structure.

Hemoglobin quaternary structure:

• Changes in a protein's environment, such as variations in temperature or pH, can cause it to lose its secondary and tertiary structure.

This process is called denaturation; the protein usually becomes inactive when it loses its proper 3-dimensional shape (conformation).

• Nucleotide structure.

The monomers of nucleic acids (DNA and RNA) are nucleotides.

Nucleotides are composed of a five-carbon sugar, a phosphate group, and a nitrogenous base.

The sugar is deoxyribose in DNA (Deoxyribonucleic Acid) and ribose in RNA (Ribonucleic Acid).

• Four different nitrogenous base are found in the nucleotides of DNA: adenine (A), guanine (G), cytosine (C), and thymine (T).

In RNA, uracil (U) takes the place of thymine (T).

• A DNA molecule is composed of two nucleotide chains twisted to form a double helix.

The sugar and phosphate groups are linked by covalent bonds to form the backbone of each of the two chains.

The two chains are held together by hydrogen bonds between A-T and G-C base pairs.

Review:

• Carbohydrates (carbon and water, also called sugars) are made of C, H, and O.
Sugar monomers are called monosaccharides.
An example is the 6-carbon glucose that forms a ring in water.

• The disaccharide sucrose (table sugar) is formed from glucose and fructose monomers in a dehydration reaction, where a molecule of water (H₂O) is released.

• Lactose is a disaccharide commonly found in the milk of mammals.

• Complex carbohydrates (or polysaccharides) include:

• cellulose

• starch

• glycogen

• chitin

• The polysaccharide cellulose is found in plant cells and is composed of glucose monomers.
Many strands of cellulose chains are held together by hydrogen bonds, making these polymers strong to support tall trees.

• The polysaccharide starch serves as energy storage in plants.

• The polysaccharide glycogen is highly branched and serves as energy storage in animals.
Humans store these energy-rich molecules in the liver.

• Chitin is a polysaccharide cross-linked with proteins to provide a strong material found in the exoskeleton of crustaceans and insects.
The tough exoskeleton requires these animals to molt as they grow.

• Lipid (fat) molecules are triglycerides, each containing a glycerol attached to 3 fatty acid tails.

A fatty acid is a long chain of carbon and hydrogen atoms (hydrocarbon).

The nonpolar hydrocarbons are hydrophobic ("afraid" of water) and are insoluble in aqueous environments.

• The membranes of cells are made of a type of lipid called phospholipid.

• A phospholipid is similar to a fat molecule except one of the 3 hydrophobic fatty acids is replaced by a phosphate and a polar (hydrophilic) functional group.