<Tr> <Td> Nucleic acids </Td> <Td> Nucleotides </Td> <Td> Polynucleotides </Td> <Td> DNA and RNA </Td> </Tr> <P> Proteins are made of amino acids arranged in a linear chain joined together by peptide bonds . Many proteins are enzymes that catalyze the chemical reactions in metabolism . Other proteins have structural or mechanical functions, such as those that form the cytoskeleton, a system of scaffolding that maintains the cell shape . Proteins are also important in cell signaling, immune responses, cell adhesion, active transport across membranes, and the cell cycle . Amino acids also contribute to cellular energy metabolism by providing a carbon source for entry into the citric acid cycle (tricarboxylic acid cycle), especially when a primary source of energy, such as glucose, is scarce, or when cells undergo metabolic stress . </P> <P> Lipids are the most diverse group of biochemicals . Their main structural uses are as part of biological membranes both internal and external, such as the cell membrane, or as a source of energy . Lipids are usually defined as hydrophobic or amphipathic biological molecules but will dissolve in organic solvents such as benzene or chloroform . The fats are a large group of compounds that contain fatty acids and glycerol; a glycerol molecule attached to three fatty acid esters is called a triacylglyceride . Several variations on this basic structure exist, including alternate backbones such as sphingosine in the sphingolipids, and hydrophilic groups such as phosphate as in phospholipids . Steroids such as cholesterol are another major class of lipids . </P> <P> Carbohydrates are aldehydes or ketones, with many hydroxyl groups attached, that can exist as straight chains or rings . Carbohydrates are the most abundant biological molecules, and fill numerous roles, such as the storage and transport of energy (starch, glycogen) and structural components (cellulose in plants, chitin in animals). The basic carbohydrate units are called monosaccharides and include galactose, fructose, and most importantly glucose . Monosaccharides can be linked together to form polysaccharides in almost limitless ways . </P>

Where does the first stage of catabolic metabolism occur