Cell Structure and Membrane Transport: every key term you need (+ practice quiz)
25 flashcard terms for General Biology I Topic 3, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 15-question quiz — free, no account needed.
The generalization that all organisms are made of cells, that the cell is the smallest unit showing the properties of life, and that every cell arises from a preexisting cell rather than spontaneously.
Surface area to volume ratio
The geometric constraint that limits cell size: doubling the radius of a sphere multiplies surface by four and volume by eight, so exchange across the boundary falls behind metabolic demand.
Prokaryotic cell
A cell without a nucleus or membrane-bound organelles, with its chromosome in a nucleoid region. Typically one to five micrometers across, with ribosomes smaller than those of eukaryotes.
Nuclear envelope
A double membrane pierced by pore complexes that separates transcription from translation. The pores gate the passage of ribosomal subunits, messenger RNA and imported proteins carrying targeting signals.
Rough endoplasmic reticulum
Ribosome-studded membrane where proteins destined for secretion, lysosomes or the plasma membrane are threaded into the lumen, folded, checked and given their first sugar chains.
Smooth endoplasmic reticulum
Ribosome-free membrane specialized for lipid and steroid synthesis, detoxification of drugs, and calcium storage. It is abundant in liver cells and in muscle, where it releases calcium for contraction.
Golgi apparatus
A stack of flattened cisternae that receives vesicles from the endoplasmic reticulum, modifies sugar chains in sequence, and sorts finished cargo to its correct destination with address tags.
Lysosome
An acidic compartment holding hydrolases that work best near pH 5, used to digest engulfed material and worn organelles. Leaked enzymes are largely inactive at cytosolic pH, which limits self-damage.
Peroxisome
An organelle that carries out oxidative reactions generating hydrogen peroxide, then destroys it with catalase. It breaks down very long fatty acids and, in seeds, converts stored fat into sugar.
Mitochondrion
A double-membraned organelle with its own small circular genome and its own ribosomes. The inner membrane folds into cristae that hold the electron transport chain and the enzyme that makes most cellular ATP.
Chloroplast
A plastid with three membrane systems: outer, inner, and the thylakoid membrane enclosing a separate lumen. Light reactions run in the thylakoid while carbon fixation occurs in the surrounding stroma.
Endosymbiotic theory
The proposal that mitochondria and plastids descend from engulfed free-living bacteria. The evidence is circular DNA, bacterial-sized ribosomes, double membranes and division independent of the host cell.
Cytoskeleton
The internal protein scaffolding of microtubules, actin filaments and intermediate filaments that gives shape, anchors organelles, drives motor-based transport and generates cell movement and division forces.
Microtubule
A hollow tube of tubulin dimers with a plus and a minus end, undergoing dynamic instability. It forms the spindle, the tracks for kinesin and dynein, and the core of cilia and flagella.
Fluid mosaic model
The description of the membrane as a two-dimensional fluid of lipids in which proteins float and diffuse laterally. Frye and Edidin's fused mouse and human cells demonstrated the lateral mixing directly.
Membrane fluidity
The ease with which lipids move within a leaflet, raised by unsaturated tails and higher temperature. Cholesterol acts as a buffer, restraining movement when warm and preventing tight packing when cold.
Selective permeability
The property that small nonpolar molecules such as oxygen and carbon dioxide cross the bilayer freely while ions and large polar solutes require protein channels or carriers.
Simple diffusion
Net movement of a solute down its own concentration gradient by random thermal motion, requiring no protein and no energy input, and continuing until the chemical potential is equal on both sides.
Facilitated diffusion
Passive movement of a solute through a channel or carrier protein, still down the gradient and still without ATP, but showing saturation kinetics and specificity that simple diffusion lacks.
Aquaporin
A channel protein that lets water cross the membrane far faster than the lipid alone allows, while a narrow selectivity filter and fixed charges block protons and other ions.
Osmosis
The diffusion of water across a selectively permeable membrane toward the side with the lower water potential, that is, toward the more concentrated solution of non-penetrating solutes.
Tonicity
The effect a solution has on cell volume, determined only by solutes that cannot cross the membrane. Cells swell in hypotonic solutions, shrink in hypertonic ones and hold volume in isotonic ones.
Sodium potassium pump
A primary active transporter that hydrolyzes one ATP to export three sodium ions and import two potassium ions, making it electrogenic and responsible for much of the resting membrane potential.
Secondary active transport
Transport that moves a solute against its gradient by coupling it to the downhill flow of another ion, usually sodium or protons. The energy came from ATP earlier, when the ion gradient was built.
Receptor mediated endocytosis
Selective uptake in which ligands bind surface receptors that cluster into coated pits, then bud inward as vesicles. It concentrates specific cargo far more efficiently than bulk fluid uptake.