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All living organisms require a continuous supply of energy to carry out the fundamental processes of life — growth, reproduction, movement, and homeostasis. In biological systems, this energy is stored and transferred primarily through the molecule adenosine triphosphate (ATP). ATP acts as the universal energy currency of the cell, coupling energy-releasing reactions to energy-requiring ones.
Energy flows through ecosystems in one direction — from sunlight to producers and then through consumers — while matter cycles continuously. The two central processes that govern this flow are photosynthesis, which captures light energy, and cellular respiration, which releases stored chemical energy.
Photosynthesis occurs primarily in the chloroplasts of plant cells and in the cells of algae and cyanobacteria. The overall equation can be summarized as:
The light-dependent reactions take place in the thylakoid membranes and produce ATP and NADPH. The Calvin cycle occurs in the stroma and uses those molecules to fix carbon dioxide into organic compounds. When light strikes Photosystem II, electrons energize and travel down the electron transport chain, driving ATP synthase via a proton gradient — a process called photophosphorylation. Water is split by photolysis, releasing O₂ as a by-product.
Cellular respiration breaks down glucose to release stored energy as ATP. The overall reaction is the inverse of photosynthesis:
The three stages — glycolysis (cytoplasm), the Krebs cycle (mitochondrial matrix), and oxidative phosphorylation (inner mitochondrial membrane) — together yield approximately 36–38 ATP per glucose molecule. Glycolysis produces only 2 net ATP but does not require oxygen; it is the shared gateway for both aerobic and anaerobic metabolism.
Deoxyribonucleic acid (DNA) is a double-stranded polymer composed of nucleotides, each containing a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C). The two strands are held together by hydrogen bonds following Chargaff's rules (A pairs with T; G pairs with C) and wind into a double helix.
The central dogma of molecular biology describes the directional flow of genetic information: DNA is transcribed into messenger RNA (mRNA) in the nucleus, and mRNA is then translated into protein at ribosomes in the cytoplasm. Each three-base sequence on mRNA, called a codon, specifies a particular amino acid or a stop signal.
Meiosis is a specialized cell division that produces four genetically unique haploid gametes from a single diploid parent cell. It consists of two sequential divisions: Meiosis I separates homologous chromosome pairs (a reductional division), while Meiosis II separates sister chromatids. Genetic diversity arises through crossing over during prophase I, when non-sister chromatids exchange segments at chiasmata, and through independent assortment of homologous pairs.
Gregor Mendel's experiments with pea plants established the foundational laws of heredity. The Law of Segregation states that allele pairs separate during gamete formation, so each gamete receives only one allele for each gene. The Law of Independent Assortment states that alleles for different genes segregate independently, provided those genes are on non-homologous chromosomes.
A monohybrid cross (Aa × Aa) yields a 3:1 phenotypic ratio in the F₂ generation. A dihybrid cross (AaBb × AaBb) yields a 9:3:3:1 ratio. Variations such as incomplete dominance, codominance, and polygenic inheritance extend Mendel's model to account for continuous and blended phenotypes.
The cell cycle is an ordered sequence of events by which a cell duplicates its contents and divides into two daughter cells. It consists of two broad phases: interphase, during which the cell grows and replicates its DNA, and the mitotic (M) phase, during which the duplicated chromosomes are segregated and the cell divides.
Interphase is subdivided into G₁ (first gap — cell growth, protein synthesis), S phase (DNA synthesis — each chromosome is duplicated into two sister chromatids joined at the centromere), and G₂ (second gap — continued growth and preparation for division). Cells that exit the cycle and enter a non-dividing state are said to be in G₀.
Mitosis is conventionally divided into four stages:
Cytokinesis — the division of cytoplasm — typically overlaps with telophase. In animal cells, a cleavage furrow pinches the cell in two. In plant cells, a cell plate forms along the former metaphase plate and grows outward to separate the daughter cells.
Progression through the cell cycle is tightly regulated at checkpoints — G₁/S, G₂/M, and the spindle assembly checkpoint — by proteins called cyclins and cyclin-dependent kinases (CDKs). Tumor suppressor proteins such as p53 and Rb halt division when DNA damage is detected, while proto-oncogenes stimulate growth.
Cancer arises when mutations disable checkpoints or constitutively activate growth signals, allowing cells to divide uncontrollably. The resulting mass of cells — a tumor — may be benign (localized) or malignant (capable of metastasis, spreading to other tissues).
Charles Darwin's On the Origin of Species (1859) proposed that species change over time through a process of natural selection. The core argument rests on four observations: (1) individuals in a population vary in heritable traits; (2) more offspring are produced than can survive; (3) individuals with advantageous traits are more likely to survive and reproduce (differential reproductive success); and (4) advantageous traits accumulate in the population over generations.
Darwin's contemporary Alfred Russel Wallace independently arrived at the same conclusion. Together, their work shifted biology from a static, species-fixed worldview to a dynamic one in which descent with modification from common ancestors explains the diversity of life.
Multiple independent lines of evidence support evolutionary theory:
Beyond natural selection, evolutionary change occurs through genetic drift (random changes in allele frequency, most pronounced in small populations — the bottleneck effect and founder effect), gene flow (movement of alleles between populations), and mutation (the ultimate source of new genetic variation).
Speciation — the formation of new species — occurs when populations become reproductively isolated. Allopatric speciation follows geographic separation; sympatric speciation occurs without physical barriers, often through polyploidy in plants or niche divergence in animals. Over evolutionary time, these processes generate the phylogenetic tree of life.
Carbohydrates are composed of carbon, hydrogen, and oxygen in approximately a 1:2:1 ratio. Monosaccharides (e.g., glucose, fructose, galactose) are the monomers; they link via glycosidic bonds (dehydration synthesis) to form disaccharides (sucrose, maltose, lactose) and polysaccharides. Polysaccharides serve diverse roles: starch and glycogen store energy in plants and animals respectively, while cellulose provides structural support in plant cell walls, and chitin forms the exoskeletons of arthropods and the cell walls of fungi.
Proteins are polymers of amino acids linked by peptide bonds. Each of the 20 standard amino acids has a unique R-group (side chain) that determines its chemical properties. Protein structure is described at four hierarchical levels: primary (amino acid sequence), secondary (α-helices and β-pleated sheets stabilized by hydrogen bonds), tertiary (overall 3D folding), and quaternary (association of multiple polypeptide subunits). Proteins function as enzymes, structural components, signal molecules, transport proteins, and immune antibodies.
Lipids are hydrophobic molecules including triglycerides (energy storage), phospholipids (bilayer membrane structure), and steroids (cholesterol, hormones). Phospholipids are amphipathic — their hydrophilic heads face aqueous environments while hydrophobic fatty-acid tails orient inward, forming the fluid mosaic model of the cell membrane.
Nucleic acids — DNA and RNA — are polymers of nucleotides, each consisting of a pentose sugar, a phosphate group, and a nitrogenous base. DNA stores genetic information in the sequence of its bases; RNA carries that information to ribosomes for translation. ATP is itself a modified nucleotide that serves as the primary energy currency of the cell.