Unit 2: Cell Structure and Function

Vocabulary flashcards covering cell structure, organelles, the endomembrane system, energy conversion, membrane architecture, and cellular transport mechanisms from Unit 2.

Prokaryotic Cell

A cell that lacks a nucleus and membrane-bound organelles; typically smaller than a eukaryotic cell.

Eukaryotic Cell

A cell containing a membrane-bound nucleus and specialized membrane-bound organelles.

Surface Area-to-Volume Ratio ($SA:V$)

The relationship of cell surface area divided by volume ($SA:V = \text{surface area} \times \text{volume}^{-1}$); a higher ratio allows more efficient exchange of materials across the plasma membrane per unit volume.

Cristae

Folds of the inner mitochondrial membrane that expand surface area for electron transport chains and ATP synthase to increase ATP synthesis.

Thylakoids

Internal flattened membrane sacs within chloroplasts that contain chlorophyll, photosystems, electron carriers, and ATP synthase for light-dependent photosynthetic reactions.

Endosymbiosis

An evolutionary process where an ancestral eukaryote engulfed bacteria that persisted inside the host, forming a mutualistic relationship and eventually evolving into mitochondria and chloroplasts.

Nuclear Envelope

A double membrane enclosing the nucleus, separated by a distinct space and perforated by nuclear pore complexes.

Nuclear Pores

Protein-lined channel complexes in the nuclear envelope that regulate molecular passage (such as RNA and proteins) between the nucleus and the cytoplasm.

Nucleolus

A specialized region inside the nucleus responsible for synthesizing ribosomal RNA (rRNA) and assembling ribosomal subunits.

Chromatin

The uncoiled complex of DNA wrapped around proteins in the nucleus that condenses into visible chromosomes during cellular division.

Ribosome

A non-membrane-bound structure composed of ribosomal RNA (rRNA) and proteins that synthesizes polypeptides by translating messenger RNA (mRNA).

Endomembrane System

A group of structurally and functionally related membranes—including the nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, vacuoles, and plasma membrane—that synthesizes, modifies, sorts, and transports proteins and lipids.

Vesicles

Small, membrane-bound sacs that store or transport substances between cellular compartments or toward the plasma membrane.

Rough Endoplasmic Reticulum (Rough ER)

A network of flattened membrane sacs studded with ribosomes on its cytosolic surface, functioning in the synthesis, initial folding, and modification of proteins destined for membranes, organelles, or secretion.

Smooth Endoplasmic Reticulum (Smooth ER)

A branching tubular membrane network devoid of ribosomes that performs lipid synthesis, carbohydrate metabolism, drug and chemical detoxification, and calcium ion storage.

Golgi Apparatus

A stack of flattened membrane sacs called cisternae with distinct receiving and shipping faces that modifies, sorts, and packages proteins and lipids into vesicles.

Cisternae

The flattened, membrane-bound disc-like sacs that comprise the functional stacks of the Golgi apparatus.

Lysosome

A membrane-enclosed acidic sac of hydrolytic enzymes that breaks down macromolecules, ingested pathogens, and worn-out cellular organelles.

Hydrolysis

A chemical reaction catalyzed by hydrolytic enzymes that cleaves covalent bonds between monomers through the addition of a water molecule.

Central Vacuole

A large membrane-bound organelle found in plant cells that sequesters water, nutrients, pigments, and metabolic wastes while generating turgor pressure against the cell wall.

Contractile Vacuole

A specialized organelle in freshwater protists that collects and periodically expels excess incoming water to prevent cellular swelling or lysis.

Mitochondrial Matrix

The fluid-filled innermost compartment of the mitochondrion containing enzymes for pyruvate oxidation and the citric acid cycle, circular DNA, and bacterial-sized ribosomes.

Stroma

The fluid surrounding thylakoids inside the chloroplast envelope containing Calvin cycle enzymes, chloroplast DNA, and ribosomes.

Grana

Dense stacks of thylakoid discs inside chloroplasts that maximize surface area for photosynthetic light absorption.

Cytoskeleton

A cytoplasmic network composed of microfilaments, intermediate filaments, and microtubules that supports cell shape, anchors organelles, and directs intracellular transport.

Microtubules

Hollow cytoskeletal cylinders composed of tubulin that form cilia, flagella, and centrioles, and serve as directional paths for motor proteins such as kinesin and dynein.

Plasmodesmata

Membrane-lined cytoplasmic channels that cross the cell walls of adjacent plant cells to permit direct intercellular exchange.

Extracellular Matrix (ECM)

An external meshwork of glycoproteins and polysaccharides secreted by animal cells that provides physical support, anchorage, and biochemical signaling.

Fluid-Mosaic Model

The structural model of biological membranes describing a fluid phospholipid bilayer wherein embedded proteins and other macromolecules freely diffuse laterally.

Integral Proteins

Membrane proteins embedded directly within the lipid bilayer, anchored firmly via interactions between their hydrophobic regions and the membrane core.

Peripheral Proteins

Proteins non-covalently attached to the exterior or interior surfaces of the plasma membrane, often associating with integral proteins or lipid headgroups.

Passive Transport

The spontaneous movement of chemical species down their concentration or electrochemical gradient across a biological membrane without the input of metabolic energy.

Diffusion

The net movement of solute particles from a region of higher concentration to one of lower concentration driven by continuous random thermal motion.

Facilitated Diffusion

Passive transport of polar or charged solutes down their concentration gradient through the bilayer via specific channel or carrier proteins.

Dynamic Equilibrium

A state achieved when opposing movements of particles balance equally so that no net concentration gradient changes occur across a membrane.

Osmosis

The net diffusion of free water molecules across a selectively permeable membrane toward a region with higher effective solute concentration (or lower water potential).

Aquaporins

Specialized channel proteins that facilitate the rapid passage of water molecules across biological membranes.

Hypertonic Solution

An external solution with a higher effective solute concentration relative to the cytoplasm, causing net osmotic efflux of water and cellular shriveling.

Hypotonic Solution

An external solution with a lower effective solute concentration relative to the cytoplasm, resulting in net osmotic influx of water that makes plant cells turgid and may cause animal cells to lyse.

Isotonic Solution

An external solution having the same effective solute concentration as the cell interior, causing no net water movement and preserving standard cell volume.

Water Potential ($\Psi$)

The physical measure of the free energy of water in a system defined by the equation $\Psi = \Psi_s + \Psi_p$, predicting that water flows toward lower water potential.

Solute Potential ($\Psi_s$)

The osmotic component of water potential determined by the equation $\Psi_s = -iCRT$, which decreases below zero with increasing concentration of dissolved solute.

Pressure Potential ($\Psi_p$)

The physical hydrostatic pressure exerted on water in a system, which equals $0$ gauge pressure in an open beaker ($\Psi_p = 0$).

Active Transport

The movement of ions or molecules across a cellular membrane against an electrochemical or concentration gradient, requiring cellular energy expenditure.

Phagocytosis

A specialized form of endocytosis ('cell eating') in which a cell engulfs large solid particles or whole microorganisms into a membrane-bound vacuole.

Pinocytosis

A non-specific type of endocytosis ('cell drinking') wherein the plasma membrane invaginates to ingest extracellular fluid and dissolved solutes in small vesicles.

Receptor-Mediated Endocytosis

A selective endocytic process whereby extracellular ligands bind to specific cell-surface receptor proteins, triggering localized vesicle formation.

Exocytosis

The transport process whereby an intracellular vesicle moves to, docks with, and fuses with the plasma membrane to expel its lumen contents into the extracellular space.