Biology

Cell theory states that all living organisms are composed of one or more cells, that the cell is the basic unit of structure and function in life, and that all cells arise from pre-existing cells. This foundational principle unifies all of biology and was established through the combined work of Schleiden, Schwann, and Virchow in the 19th century.

Natural selection is the process by which organisms with heritable traits better suited to their environment tend to survive and reproduce at higher rates than organisms without those traits. Over many generations, this differential reproductive success leads to changes in the genetic composition of populations, driving adaptation and evolution.

Deoxyribonucleic acid (DNA) is the molecule that carries the hereditary instructions for building and maintaining an organism. DNA is organized into genes, which are transcribed into RNA and translated into proteins according to the genetic code, a set of rules mapping three-nucleotide codons to specific amino acids.

Homeostasis is the ability of a biological system to maintain relatively stable internal conditions despite changes in the external environment. This dynamic equilibrium is achieved through feedback mechanisms, most commonly negative feedback loops, that detect deviations from a set point and trigger corrective responses.

Photosynthesis is the biochemical process by which plants, algae, and certain bacteria convert light energy into chemical energy stored in glucose, using carbon dioxide and water as raw materials and releasing oxygen as a byproduct. The process occurs in two main stages: the light-dependent reactions in the thylakoid membranes and the Calvin cycle in the stroma of chloroplasts.

Cellular respiration is the metabolic process by which cells break down glucose and other organic molecules to produce adenosine triphosphate (ATP), the primary energy currency of the cell. The process consists of glycolysis, the citric acid cycle, and oxidative phosphorylation, and in aerobic organisms it consumes oxygen and produces carbon dioxide and water.

Mendelian genetics describes the patterns of inheritance first characterized by Gregor Mendel through his experiments with pea plants in the 1860s. Mendel's laws of segregation and independent assortment explain how alleles of genes are transmitted from parents to offspring, producing predictable ratios of phenotypes in subsequent generations.

Ecosystem dynamics refers to the complex interactions among living organisms and their physical environment, including the flow of energy through trophic levels and the cycling of nutrients such as carbon, nitrogen, and phosphorus. These interactions determine the structure, biodiversity, and resilience of ecosystems.

Mitosis and meiosis are the two forms of cell division in eukaryotic organisms. Mitosis produces two genetically identical daughter cells for growth and tissue repair, while meiosis produces four genetically unique haploid gametes through two successive divisions, enabling sexual reproduction and genetic diversity.

Proteins are large, complex macromolecules made of amino acid chains that fold into specific three-dimensional shapes, and their structure directly determines their function. Proteins serve as enzymes, structural components, transport molecules, hormones, and immune defenders, making them essential to virtually every biological process.