Key Points

Living organisms have a higher relative abundance of carbon and hydrogen compared to the Earth's crust. All elements found in the Earth's crust are also present in living tissues.

Grinding a living tissue in trichloroacetic acid ($\text{Cl}_3\text{CCOOH}$) separates its components into two fractions: the acid-soluble pool (micromolecules) and the acid-insoluble fraction (macromolecules).

Biomolecules with molecular weights from 18 to 800 daltons (Da) are called micromolecules. Those in the acid-insoluble fraction with weights over 10,000 Da are biomacromolecules, including proteins, nucleic acids, and polysaccharides.

Primary metabolites have known roles in normal physiological processes. Secondary metabolites, like alkaloids, rubber, and drugs, are found in plants and microbes and often have ecological or human importance.

Amino acids are organic compounds with an amino group ($\text{-NH}_2$) and a carboxyl group ($\text{-COOH}$) on the same $\alpha$-carbon. Proteins are polymers of 20 different types of amino acids.

Lipids are water-insoluble and include fatty acids, which can be saturated or unsaturated, and glycerol. Triglycerides consist of three fatty acids esterified to a glycerol molecule.

Although lipids have low molecular weight (less than 800 Da), they are found in the acid-insoluble fraction because they form vesicles from disrupted cell membranes which are not water-soluble.

Polysaccharides are long chains of monosaccharides. Examples include cellulose (a structural homopolymer of glucose), starch, and glycogen (energy storage polymers of glucose).

Nucleic acids, DNA and RNA, are polymers of nucleotides. A nucleotide consists of a nitrogenous base (purine or pyrimidine), a pentose sugar (deoxyribose in DNA, ribose in RNA), and a phosphate group.

Proteins exhibit four levels of structure: primary (the sequence of amino acids), secondary (local folding into $\alpha$-helices or $\beta$-sheets), tertiary (overall 3-D shape), and quaternary (assembly of multiple polypeptide subunits).

Almost all enzymes are proteins that act as biocatalysts to speed up metabolic reactions. They possess an 'active site' where the substrate binds. Ribozymes are catalytic RNA molecules.

Enzymes lower the activation energy of a reaction by forming a transient enzyme-substrate complex. The reaction pathway is represented as $E + S \leftrightarrow ES \rightarrow EP \rightarrow E + P$.

Enzyme activity is highly sensitive to changes in temperature, pH, and substrate concentration. Each enzyme functions best at an optimal temperature and pH.

As substrate concentration increases, the reaction velocity increases until it reaches a maximum velocity ($V_{max}$). At this point, the enzyme active sites are saturated with substrate.

Enzyme activity can be inhibited by specific chemicals. A competitive inhibitor structurally resembles the substrate and competes for the enzyme's active site, thus reducing the rate of reaction.

Enzymes are classified into six major classes based on the reaction they catalyze: Oxidoreductases, Transferases, Hydrolases, Lyases, Isomerases, and Ligases.

Some enzymes require a non-protein component called a cofactor to be active. The protein part is the apoenzyme. Cofactors include prosthetic groups, co-enzymes (often derived from vitamins), and metal ions.