Cell Biology Terms Starting With A

Actin is a globular protein that polymerizes into filaments called microfilaments, forming part of the cytoskeleton and contributing to cell shape, movement, and division.

Actin monomers, each roughly 42 kilodaltons in mass, polymerize into helical microfilaments that assemble and disassemble within seconds to minutes, allowing cells to remodel their cytoskeleton rapidly. In muscle cells, actin filaments interact with myosin to generate contractile force. Non-muscle cells rely on actin networks to drive cell migration, enable cytokinesis during cell division, and support vesicle transport.

Actin can reach 5 to 10 percent of total cellular protein in some cell types, making it one of the most abundant proteins in eukaryotes.

Active transport is the movement of molecules or ions across a membrane against their concentration or electrochemical gradient, requiring an input of cellular energy.

The sodium-potassium pump uses ATP hydrolysis to export three sodium ions while importing two potassium ions per cycle, maintaining steep ion gradients that underlie nerve signals and muscle contraction. Primary active transport directly couples ATP hydrolysis to ion movement, as seen in the calcium pump of muscle sarcoplasmic reticulum, which can reduce cytoplasmic calcium to below 0.1 micromolar. Secondary active transport harnesses energy stored in preexisting ion gradients; sodium-glucose cotransporters in intestinal epithelial cells exploit the inward sodium gradient to pull glucose into the cell against its own gradient.

These mechanisms let cells accumulate nutrients, maintain osmotic balance, and generate electrical potentials.

Adherens junction is a cell-to-cell contact site that uses transmembrane cadherin proteins and intracellular catenin proteins to mechanically link the actin cytoskeletons of neighboring cells.

Adherens junctions form when the extracellular domains of cadherin proteins on adjacent cells bind each other in a calcium-dependent manner, while their cytoplasmic tails connect through beta-catenin and alpha-catenin to actin microfilaments. This linkage creates a continuous mechanical network across an entire tissue, distributing tensile forces among many cells rather than concentrating stress at single points. Adherens junctions are especially dense in cardiac muscle, where they help cells withstand the repeated mechanical stress of contraction.

In epithelial sheets, they organize cell contacts into coherent layers and coordinate tissue-level shape changes during development.

Anaphase is the stage of mitosis or meiosis during which chromosomes separate and move toward opposite poles of the dividing cell.

Anaphase begins when the protease separase cleaves cohesin proteins holding sister chromatids together, freeing them to move poleward along spindle microtubules at approximately 1 micrometer per second. Two distinct forces drive this movement: kinetochore-associated motor proteins pull chromatids toward shortening microtubule ends, while polar ejection forces push chromosome arms away from the spindle midzone. In mitosis, each separated chromatid is now counted as an individual chromosome, so a human cell briefly contains 92 chromosomes moving toward two poles.

During meiosis I, whole homologous chromosome pairs separate rather than sister chromatids, a distinction that halves the chromosome number.

Aquaporin is a membrane-spanning channel protein that selectively conducts water molecules across cell membranes at rates far exceeding passive diffusion through the lipid bilayer.

Aquaporins form tetramers in the membrane, with each subunit containing a narrow pore that passes up to one billion water molecules per second while excluding ions and most other solutes through a combination of electrostatic repulsion and steric filtering. In plant roots, aquaporin abundance increases during water stress, accelerating osmotic water uptake from soil. Human kidney collecting duct cells deploy aquaporin-2 inserts into the apical membrane in response to vasopressin, increasing water reabsorption and concentrating urine.

Peter Agre received the 2003 Nobel Prize in Chemistry for discovering aquaporins, work that began with the purification of aquaporin-1 from red blood cells in 1988.

ATPase is an enzyme that catalyzes the hydrolysis of ATP, releasing chemical energy that cells use to perform mechanical, transport, and biosynthetic work.

ATPases catalyze the hydrolysis of ATP to ADP and inorganic phosphate, releasing approximately 7.3 kilocalories per mole under standard cellular conditions. This released energy drives active transport pumps like the sodium-potassium ATPase, which exchanges three sodium ions outward for two potassium ions inward per ATP molecule consumed. Motor proteins like myosin and kinesin couple ATPase activity to directed movement along cytoskeletal filaments.

Chaperone ATPases such as Hsp70 use ATP hydrolysis to unfold and refold misfolded proteins, preventing toxic protein aggregation.

Autocrine signaling is a mode of cell communication in which a cell secretes signaling molecules that bind to receptors on its own surface, triggering responses that alter that same cell's behavior.

In autocrine signaling, a cell releases ligands that diffuse locally and bind to receptors displayed on the same cell’s plasma membrane, initiating intracellular signaling cascades without requiring hormonal circulation. Activated fibroblasts release transforming growth factor beta, which binds to their own TGF-beta receptors and promotes collagen synthesis, sustaining wound-healing responses. T lymphocytes secrete interleukin-2 and simultaneously express IL-2 receptors, creating a positive feedback loop that amplifies clonal expansion during an immune response.

Many cancer cells exploit autocrine loops involving epidermal growth factor receptor ligands to drive continuous proliferation independent of external growth signals.

Autophagy is a cellular degradation process in which cytoplasmic contents, including damaged organelles and misfolded proteins, are enclosed in a double-membrane vesicle and delivered to lysosomes for breakdown and recycling.

Autophagy begins when a cup-shaped membrane called the phagophore expands around cytoplasmic cargo, sealing into a double-membrane autophagosome roughly 500 to 1500 nanometers in diameter. The autophagosome then fuses with lysosomes, whose acid hydrolases degrade the contents into amino acids, fatty acids, and nucleotides that re-enter biosynthetic pathways. During nutrient starvation, autophagy can increase protein breakdown up to ten-fold, sustaining protein synthesis when dietary amino acids are unavailable.

Selective autophagy variants target specific cargo: mitophagy removes damaged mitochondria, and xenophagy degrades intracellular bacteria, contributing to innate immune defense.

Axon is a single elongated projection of a neuron that conducts action potentials away from the cell body toward synaptic terminals on target cells.

An axon originates at a specialized region called the axon hillock, where action potentials are initiated, and extends from micrometers in short interneurons to over one meter in motor neurons that innervate muscles of the human leg. Most axons in the central nervous system are wrapped in myelin sheaths produced by oligodendrocytes, while Schwann cells myelinate peripheral axons; myelination increases conduction velocity from roughly 1 meter per second in unmyelinated fibers to up to 120 meters per second in large myelinated ones. Axons maintain their structure through axonal transport systems that carry proteins, mitochondria, and vesicles from the cell body at rates of 1 to 400 millimeters per day, depending on cargo type.