Chapter 3: The Cellular Level of Organization

Vocabulary practice flashcards covering cell structure, membrane transport mechanisms, gradients, organelles, and cytoskeletal components from Chapter 3.

Prokaryotic Cell

A type of cell (characteristic of Archaea and Bacteria) that possesses a cell wall, lacks a nucleus (DNA resides in a nucleoid region), and contains no membrane-bound organelles.

Eukaryotic Cell

A type of cell (characteristic of fungi, animals, plants, and protists) that contains DNA housed inside a membrane-bound nucleus with nuclear pores, as well as membrane-bound organelles.

Sandwich Model

A plasma membrane model proposed by Davson & Danielli in 1935 describing a phospholipid bilayer ($\approx 5\,\text{nm}$) sandwiched between two protein monolayers ($\approx 2.5\,\text{nm}$ each).

Fluid Mosaic Model

The structural model of the plasma membrane proposed by Singer and Nicolson in 1972 describing a "sea of lipids" containing a diverse mosaic of proteins that function as gatekeepers to specific molecules and ions.

Glycocalyx

A "sugary coating" surrounding the plasma membrane that acts as a molecular signal enabling cells to recognize one another.

Amphipathic

The property of possessing both polar and nonpolar regions, such as phospholipids containing a polar hydrophilic head (glycerol, phosphate, and variable group R) and two nonpolar hydrophobic tails (fatty acids).

Integral Protein

A membrane protein (also known as a transmembrane protein) that extends into or across the lipid bilayer among the fatty acid tails.

Peripheral Protein

A membrane protein that is not embedded in the lipid bilayer, typically attaching loosely to the inner or outer surface of the membrane.

Ion Channel

An integral membrane protein that allows specific ions to move through a water-filled pore across the plasma membrane.

Carrier Protein (Transporter)

An integral membrane protein that binds specific substances (such as amino acids) and changes shape to move them across the plasma membrane.

Receptor

An integral membrane protein that recognizes a specific ligand and alters cell function in response, such as antidiuretic hormone binding in the kidneys to change plasma membrane water permeability.

Linker

An integral or peripheral membrane protein that anchors filaments inside and outside the plasma membrane, providing structural stability, maintaining cell shape, participating in motility, or connecting adjacent cells.

Cell-Identity Marker

A membrane glycoprotein (such as major histocompatibility [MHC] proteins) that distinguishes an individual's cells from anyone else's.

Cholesterol (Membrane Component)

A lipid embedded within animal cell membranes that serves to stabilize membrane structure, reduce fluidity at normal temperatures, and prevent freezing as temperatures lower.

Concentration Gradient

A difference in the concentration of a chemical between one side of a plasma membrane and the other.

Electrical Gradient

A difference in electrical charge or ion concentration across the plasma membrane, creating a voltage that stores potential energy.

Electrochemical Gradient

The combined influence of the concentration gradient and the electrical gradient governing the movement of a particular ion across the membrane.

Simple Diffusion

The unassisted passive movement of small, hydrophobic, nonpolar molecules (such as $O_2$, $CO_2$, $N_2$, fatty acids, steroids, and fat-soluble vitamins) down a concentration gradient through the lipid bilayer without transport proteins.

Facilitated Diffusion

Passive transport of polar or charged solutes down their concentration gradient through the lipid bilayer mediated by transmembrane channel or carrier proteins.

Channel-Mediated Facilitated Diffusion

A passive transport process that allows specific ions to diffuse down their concentration gradient through transmembrane channel proteins.

Carrier-Mediated Facilitated Diffusion

A passive transport mechanism using carrier proteins to transport specific solutes—such as glucose, fructose, galactose, and selected vitamins—across the plasma membrane.

Osmosis

The passive net movement of solvent (water) across a selectively permeable membrane from an area of higher water concentration (lower solute concentration) to an area of lower water concentration until equilibrium is reached.

Aquaporins

Integral membrane proteins that function as water channels to allow rapid passage of water across the plasma membrane.

Tonicity

A measure of a solution's ability to change the volume of cells by altering their water content and influencing their shape.

Hemolysis

The swelling and bursting of red blood cells caused by water rushing inward when placed in a hypotonic solution.

Crenation

The shrinkage and notched appearance of red blood cells caused by water exiting the cell when placed in a hypertonic solution.

Primary Active Transport

An active transport process where energy derived directly from the hydrolysis of ATP changes the shape of a carrier protein pump to move a substance against its concentration gradient (e.g., $\text{Na}^+/\text{K}^+$ pump).

Secondary Active Transport

Coupled active transport that uses energy stored in an ion concentration gradient (typically $\text{Na}^+$ or $\text{H}^+$) maintained by primary active transport pumps to drive other substances against their own gradients.

Symporter

A secondary active transport carrier protein that moves $\text{Na}^+$ (or $\text{H}^+$) and another substance in the same direction across the membrane, such as glucose or amino acids into cells.

Antiporter

A secondary active transport carrier protein that moves $\text{Na}^+$ (or $\text{H}^+$) and another substance in opposite directions across the membrane, such as moving $\text{Ca}^{2+}$ or $\text{H}^+$ out of cells.

Endocytosis

An active vesicular transport process wherein materials move into a cell within vesicles pinched off from the plasma membrane.

Phagocytosis

A form of endocytosis ("cell eating") where pseudopods engulf large solid particles, such as bacteria, viruses, or aged cells, to form a phagosome.

Bulk-Phase Endocytosis (Pinocytosis)

A nonspecific form of endocytosis ("cell drinking") in which the plasma membrane folds inward to form vesicles containing extracellular fluid and dissolved solutes.

Receptor-Mediated Endocytosis

A highly selective endocytic process where specific extracellular ligands bind to receptors, triggering the infolding of clathrin-coated pits to internalize the ligand-receptor complexes in vesicles.

Exocytosis

An active transport process wherein membrane-enclosed secretory vesicles fuse with the plasma membrane to release contents (such as neurotransmitters, hormones, or digestive enzymes) into the extracellular fluid.

Transcytosis

A transport process combining endocytosis on one side of a cell and exocytosis on the opposite side to move substances (such as antibodies) across endothelial cells.

Cytoplasm

All the cellular contents contained within the plasma membrane, excluding the nucleus; comprised of cytosol and organelles.

Cytoskeleton

An intertwined network of protein filaments extending throughout the cytoplasm that provides structural support, determines cell shape, and facilitates movement.

Microfilaments

The thinnest elements of the cytoskeleton ($\approx 7\,\text{nm}$), composed of the protein actin, that provide structural support and generate cellular movements.

Intermediate Filaments

Cytoskeletal fibers with intermediate diameters ($\approx 8\text{--}12\,\text{nm}$), composed of proteins such as keratin, that help support cell shape and anchor organelles.

Microtubules

The thickest cytoskeletal components ($\approx 25\,\text{nm}$), composed of tubulin protein, that determine cell shape, transport organelles, form cilia and flagella, and separate chromosomes during cell division.

Centrosome

The microtubule organizing center (MTOC) located near the nucleus, consisting of a pair of centrioles and surrounding pericentriolar material that organizes mitotic spindles.

Cilia

Short, numerous hairlike cellular projections containing microtubules that perform coordinated beating movements to sweep fluid and debris along the cell surface.

Flagella

Long, solitary whiplike cellular projections composed of microtubules that move an entire cell (e.g., sperm cell motility).

Duchenne Muscular Dystrophy

A genetic disease caused by a mutation in the dystrophin gene that disrupts the anchoring of cytoskeletal elements to the plasma membrane, leading to membrane permeability, cellular rupture, and progressive muscle death.

Rough Endoplasmic Reticulum (Rough ER)

A membranous network of flattened sacs continuous with the nuclear envelope and studded with ribosomes, responsible for synthesizing glycoproteins and phospholipids.

Smooth Endoplasmic Reticulum (Smooth ER)

A network of membranous tubules lacking ribosomes that synthesizes fatty acids and steroids, detoxifies drugs, removes phosphate groups from glucose-6-phosphate, and stores and releases calcium ions in muscle cells.

Lysosome

A membrane-enclosed vesicle derived from the Golgi complex that contains digestive enzymes for autophagy (digesting worn-out organelles), autolysis (digesting entire cells), and breakdown of endocytosed substances.

Proteasome

A tiny barrel-shaped organelle containing proteases (proteolytic enzymes) that degrades unneeded, damaged, or faulty proteins by cutting them into small peptides.

Nucleolus

A prominent non-membrane-bound structure within the nucleus responsible for producing ribosomes.

Ribosome

A cellular structure composed of ribosomal RNA (rRNA) and protein, consisting of large and small subunits, that functions as the primary site of protein synthesis.