Cell Biology Terms Starting With F

F-actin is the filamentous polymer of actin formed when globular actin monomers assemble end-to-end into a double-stranded helical fiber approximately 7 nanometers in diameter that supports cell shape, movement, and division.

F-actin filaments polymerize in a polarized manner, with barbed plus ends adding subunits at roughly 10 times the rate of pointed minus ends, a difference that drives directional growth toward the cell periphery. During cell migration, Arp2/3 complexes nucleate new F-actin branches from existing filaments, creating dense networks that push the leading edge forward. Myosin motors walk along F-actin filaments using ATP hydrolysis to generate the contractile force needed for cytokinesis and muscle contraction.

Treadmilling, in which subunits add at the plus end while dissociating from the minus end, maintains filament length and drives continuous actin flow across the cell body.

Facilitated diffusion is the passive movement of molecules across a cell membrane through specific protein channels or carrier proteins, driven by a concentration gradient and requiring no energy input from the cell.

Molecules such as glucose and ions carry charges or are too large to cross the hydrophobic lipid bilayer by simple diffusion, so they rely on dedicated membrane proteins. Channel proteins form water-filled pores that open to let specific ions pass, while carrier proteins bind a molecule on one side of the membrane, change shape, and release it on the other side. Both mechanisms move substances from regions of higher concentration to regions of lower concentration, so no ATP is consumed.

GLUT1, the glucose transporter found in red blood cells and brain capillaries, can transport up to 500 glucose molecules per second when the concentration gradient is steep.

Fission is a form of asexual reproduction in which a single cell or organism divides into two or more genetically identical offspring without the fusion of gametes.

Binary fission in bacteria begins with DNA replication initiated at a single origin of replication, followed by chromosome segregation toward opposite cell poles. FtsZ protein subunits then polymerize into a Z-ring at the cell midpoint, recruiting additional proteins that synthesize a new septum and pinch the cell into two genetically identical daughters. Under optimal conditions, Escherichia coli completes this cycle in as little as 20 minutes, though nutrient-limited environments can extend division times to several hours.

Mitochondria and chloroplasts divide by a related mechanism that retains ancient bacterial fission machinery, including FtsZ homologs, reflecting their endosymbiotic origin.

Flagella are long, whip-like appendages extending from the cell surface that generate propulsive force, allowing cells such as bacteria and sperm to move through liquid environments.

Eukaryotic flagella contain a core axoneme of nine doublet microtubules surrounding two central singlets, with dynein arms that hydrolyze ATP to generate sliding forces producing wave-like bending. Bacterial flagella rotate as rigid helical propellers powered by a proton gradient across the cell membrane, reaching speeds up to 100 micrometers per second. The bacterial flagellar motor contains roughly 16 stator units that interact with a rotor embedded in the cell wall and membrane, making it one of the few rotary machines in biology.

Human sperm flagella beat at 10 to 20 hertz, generating thrust that propels the cell through viscous fluid at roughly 50 to 100 micrometers per second.

Fluid mosaic model is the widely accepted description of cell membrane structure as a dynamic bilayer of phospholipids in which proteins, cholesterol, and glycolipids move laterally and are distributed in a mosaic pattern.

Singer and Nicolson proposed the fluid mosaic model in 1972, replacing earlier static layered descriptions with a framework in which membrane components diffuse laterally. Phospholipids undergo lateral diffusion roughly 1,000 times faster than flip-flop between leaflets, allowing membranes to bend and reseal without losing integrity. Integral proteins span the entire bilayer with hydrophobic regions embedded among lipid tails, while peripheral proteins attach to the membrane surface through ionic interactions and hydrogen bonds.

Cholesterol-rich lipid rafts form microdomains that cluster specific signaling proteins and receptors, adding a layer of spatial organization to the otherwise fluid bilayer.

Focal adhesion is a large, multi-protein complex at the cell surface where integrin receptors cluster, physically linking the extracellular matrix to the intracellular actin cytoskeleton and transmitting mechanical and chemical signals into the cell.

Focal adhesions contain integrin heterodimers that connect extracellular matrix proteins such as fibronectin to bundled F-actin through adaptor proteins including talin, paxillin, and vinculin. Integrin clustering triggers recruitment of focal adhesion kinase (FAK), which phosphorylates tyrosine residues and initiates signaling cascades controlling cell survival, migration, and proliferation. Mechanical force transmitted through talin unfolds cryptic binding sites that recruit additional proteins, so adhesion size and composition change in direct response to matrix stiffness.

In migrating cells, nascent adhesions at the leading edge mature into focal adhesions under cytoskeletal tension, while adhesions at the rear disassemble rapidly as the cell body advances.

Fusion in cell biology is the merging of two lipid bilayer membranes into a single continuous membrane, a process that underlies vesicle secretion, organelle dynamics, fertilization, and viral entry into host cells.

SNARE proteins orchestrate membrane fusion by forming coiled-coil bundles that pull vesicle and target membranes together, lowering the energy barrier for lipid bilayer merger. V-SNAREs on transport vesicles recognize and bind cognate t-SNAREs on target compartments, ensuring cargo reaches the correct destination, and SNARE-mediated fusion in regulated exocytosis occurs within milliseconds once complexes fully assemble. Mitochondrial fusion involves the dynamin-related GTPases Mitofusin 1 and Mitofusin 2 on the outer membrane and OPA1 on the inner membrane, allowing mitochondria to exchange metabolites and dilute dysfunctional proteins or damaged DNA across the network.

Skeletal muscle fibers form when individual myoblasts fuse during development, producing syncytial fibers that contain hundreds of nuclei within a continuous cytoplasm and can extend the full length of a muscle, sometimes exceeding 30 centimeters in large muscles.