Cell Biology Terms Starting With P

Passive transport is the movement of substances across a cell membrane down their concentration or electrochemical gradient without the cell expending energy.

Simple diffusion moves small, nonpolar molecules such as oxygen and carbon dioxide directly across the lipid bilayer, with net flux proportional to the concentration difference across the membrane. Facilitated diffusion uses integral membrane proteins, either channel proteins that form water-filled pores or carrier proteins that undergo conformational changes, to move larger or polar molecules such as glucose and chloride ions down their gradients without ATP expenditure. Osmosis is the passive movement of water across semipermeable membranes toward regions of higher solute concentration, accelerated in most cells by aquaporin channels.

All three mechanisms continue until the electrochemical gradient across the membrane reaches equilibrium, at which point no further net movement occurs.

Peroxisome is a small, membrane-bound organelle found in nearly all eukaryotic cells that oxidizes fatty acids and neutralizes toxic compounds using hydrogen peroxide as an intermediate.

Peroxisomes contain the enzyme catalase, which decomposes hydrogen peroxide into water and oxygen, protecting the cell from oxidative damage generated during fatty acid oxidation. Beta-oxidation within peroxisomes shortens long-chain fatty acids, particularly very long-chain fatty acids with 20 or more carbons, into smaller units that mitochondria then process further. Plant leaf peroxisomes participate in photorespiration alongside chloroplasts and mitochondria, reclaiming carbon from 2-phosphoglycolate when RuBisCO oxygenates ribulose-1,5-bisphosphate.

Compartmentalizing these reactions prevents toxic hydrogen peroxide from reaching and damaging other cellular structures.

Phagocytosis is a form of endocytosis in which a cell engulfs large solid particles, such as bacteria or dead cells, by extending its plasma membrane around the target to form an internal vesicle called a phagosome.

Phagocytosis begins when receptors on immune cells such as macrophages or neutrophils recognize pathogen-associated molecular patterns or opsonins coating bacteria, triggering actin polymerization that extends pseudopodia to engulf the target into a nascent phagosome. The phagosome membrane then fuses sequentially with endosomes and lysosomes, exposing the engulfed particle to antimicrobial peptides, reactive oxygen species, and hydrolytic enzymes that disrupt bacterial cell walls and degrade proteins. Macrophages can phagocytose approximately 100 bacteria per hour under typical conditions, engulfing particles up to 10 micrometers in diameter, which contains infections before pathogens establish intracellular replication.

Phagosome maturation into a phagolysosome requires Rab GTPases, particularly Rab7, which recruit tethering factors that bring lysosomes into contact with the maturing phagosome.

Pinocytosis is a form of endocytosis in which a cell internalizes small droplets of extracellular fluid along with any dissolved molecules they contain.

Pinocytosis involves small invaginations of the plasma membrane that pinch off to form vesicles typically measuring 50 to 200 nanometers in diameter. Most animal cells carry out this process continuously, generating hundreds of vesicles per hour and internalizing dissolved nutrients, hormones, and growth factors without requiring ligand-receptor interactions. Clathrin-coated pinocytic vesicles form when adaptor proteins link clathrin trimers to cargo-bound receptors, creating a lattice that deforms the membrane inward; the clathrin coat then dissociates within seconds after vesicle formation.

Capillary endothelial cells use transcytosis, a specialized form in which pinocytic vesicles form at the luminal surface, cross the cell, and release their contents at the abluminal surface, transferring large proteins such as albumin from blood into interstitial tissue.

Plasma membrane is a selectively permeable bilayer of phospholipids and proteins that forms the outer boundary of every cell and regulates the passage of substances between the cell and its environment.

The plasma membrane consists of roughly 50 percent lipids and 50 percent proteins by mass, with cholesterol comprising 20 to 25 percent of the lipid fraction in animal cells, where it modulates membrane fluidity across temperature changes. Integral membrane proteins span the full bilayer thickness and frequently operate as ion channels, carriers, or receptors, while peripheral proteins attach to the inner or outer surface and often link to cytoskeletal filaments. The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the membrane as a dynamic structure in which lipids and proteins diffuse laterally at rates of approximately one micrometer per second at 37 degrees Celsius.

Selective permeability arises from both the hydrophobic lipid core, which blocks charged and polar molecules, and specific transport proteins that move glucose, ions, and amino acids across the otherwise impermeable barrier.

Programmed cell death is a genetically regulated process by which a cell deliberately dismantles and destroys itself in response to developmental cues, cellular damage, or infection.

Apoptosis, the best-characterized form of programmed cell death, proceeds through caspase enzyme cascades that condense the nucleus, fragment DNA into nucleosome-sized pieces, and package cellular contents into membrane-bound apoptotic bodies without rupturing the plasma membrane. During digit formation in human embryos, cells between the finger and toe buds undergo apoptosis between weeks 6 and 8, producing the separated digits visible at birth. Autophagy represents a distinct pathway in which cells digest their own organelles and cytoplasmic contents through lysosomal degradation, a process that can sustain survival during nutrient deprivation but triggers cell death when prolonged.

Both pathways prevent the inflammation that would follow uncontrolled cell rupture, and both remove cells harboring dangerous mutations or active pathogens before they can cause broader tissue damage.

Prokaryote is a single-celled organism that lacks a membrane-bound nucleus and membrane-enclosed organelles, with its genetic material residing in an unenclosed region of the cytoplasm called the nucleoid.

Prokaryotic cells typically measure 0.5 to 2 micrometers in diameter, compared to 10 to 100 micrometers for most eukaryotic cells, and their genomic DNA exists as a single circular chromosome in the nucleoid rather than inside a nuclear envelope. Cell division occurs through binary fission, in which DNA replication begins at the origin of replication and proceeds bidirectionally; under optimal conditions, Escherichia coli completes a full division cycle in roughly 20 minutes, far faster than mitosis in eukaryotes. Many prokaryotes possess flagella driven by a basal body motor that rotates the helical filament at up to 100,000 revolutions per minute, propelling the cell toward nutrients or away from toxins.

Prokaryotic cells lack membrane-bound organelles but organize metabolism efficiently through multi-protein complexes associated with the plasma membrane and through ribosomes that synthesize proteins at rates exceeding 20 amino acids per second.

Prometaphase is a stage of mitosis that follows prophase, during which the nuclear envelope disassembles and spindle microtubules attach to chromosomes at protein structures called kinetochores.

The nuclear envelope breaks down into small vesicles at the onset of prometaphase, triggered by cyclin B-CDK1 phosphorylation of lamin proteins that normally stabilize the envelope’s inner face. Kinetochores assemble at the centromeric regions of condensed chromosomes and capture spindle microtubules originating from opposite poles, generating tension that drives back-and-forth chromosome movements called congression. These oscillations help correct erroneous attachments, such as syntelic binding in which both sister kinetochores connect to the same pole rather than to opposite poles.

The spindle assembly checkpoint, enforced by the Mad2 and BubR1 proteins, delays progression to metaphase until every kinetochore achieves proper biorientation.

Proteasome is a large, barrel-shaped protein complex found in the cytosol and nucleus of eukaryotic cells that degrades damaged, misfolded, or regulatory proteins tagged with ubiquitin chains into short peptide fragments.

The 26S proteasome consists of a 20S catalytic core particle capped by one or two 19S regulatory particles; the 19S cap recognizes polyubiquitin chains of at least four ubiquitin molecules, unfolds the substrate using AAA-ATPase subunits, and threads it into the 20S barrel where three distinct protease activities cleave the chain into peptides averaging 7 to 9 amino acids in length. Substrate specificity depends on E3 ubiquitin ligases, which transfer ubiquitin to target proteins and determine which proteins are marked for degradation. Proteasomal degradation of cyclins and CDK inhibitors drives cell cycle transitions; for example, destruction of cyclin B by the anaphase-promoting complex triggers exit from mitosis.

Inhibiting the proteasome with drugs such as bortezomib causes misfolded proteins to accumulate and induces apoptosis preferentially in multiple myeloma cells, a principle that underlies its clinical use as a cancer therapy.