Biochemistry

Enzymes are biological catalysts, mostly proteins, that dramatically accelerate chemical reactions by lowering the activation energy required. They achieve this through an active site whose shape and chemical environment are complementary to the substrate, forming a transient enzyme-substrate complex.

Adenosine triphosphate (ATP) is the primary energy currency of cells. Energy released from the hydrolysis of its terminal phosphate bond drives endergonic reactions, muscle contraction, active transport, and biosynthetic processes throughout the cell.

The central dogma describes the flow of genetic information from DNA to RNA to protein. DNA is transcribed into messenger RNA by RNA polymerase, and the mRNA is then translated into a polypeptide chain by ribosomes using the genetic code.

Proteins fold into specific three-dimensional conformations determined by their amino acid sequence. The hierarchy of structure ranges from primary (sequence) to secondary (alpha-helices, beta-sheets), tertiary (overall 3D shape), and quaternary (multi-subunit assembly). Misfolding can lead to disease.

Glycolysis is a ten-step metabolic pathway occurring in the cytoplasm that converts one molecule of glucose into two molecules of pyruvate, yielding a net gain of two ATP and two NADH. It is nearly universal across life and does not require oxygen.

The citric acid cycle is a series of eight enzyme-catalyzed reactions in the mitochondrial matrix that oxidize acetyl-CoA to $\text{CO}_2$, generating NADH, $\text{FADH}_2$, and GTP. These reduced coenzymes then feed electrons into the electron transport chain for further ATP production.

Oxidative phosphorylation is the process by which electrons from NADH and $\text{FADH}_2$ pass through a series of protein complexes in the inner mitochondrial membrane, creating a proton gradient that drives ATP synthase to produce ATP. It is the most efficient energy-generating pathway in aerobic organisms.

DNA replication is the semiconservative process by which a cell duplicates its entire genome before division. Helicase unwinds the double helix, primase synthesizes RNA primers, and DNA polymerase extends new strands, with additional enzymes handling proofreading and ligation.

Signal transduction refers to the cascade of molecular events by which a cell converts an extracellular chemical signal into an intracellular response. Ligands bind to cell-surface receptors, triggering second messengers and kinase cascades that ultimately alter gene expression or cellular behavior.

Biological membranes are composed of a phospholipid bilayer that forms a selectively permeable barrier around cells and organelles. Integral and peripheral membrane proteins facilitate transport of ions and polar molecules across this hydrophobic barrier through channels, carriers, and pumps.