Cell Structure, Membranes, and Transport

Comprehensive vocabulary flashcards generated from lecture notes covering cell theory, prokaryotic and eukaryotic features, membrane dynamics, transport mechanisms, and the endomembrane system.

Cell theory

The foundational biological principle formulated by Matthias Schleiden, Theodor Schwann, Rudolf Virchow, and others stating that all organisms are composed of cells, the cell is the fundamental unit of life, and all cells arise from preexisting cells.

Prokaryotes

Single-celled organisms, including bacteria and archaea, that lack a nucleus and extensive internal membrane compartmentalization, with cells typically measuring $1\text{--}2 photograph\text{ }\frac{1}{1,000,000}\text{ m}$ ($1\text{--}2\text{ }\bar{\text{m}}\text{m}$) in diameter.

Eukaryotes

Organisms, including animals, plants, fungi, and protists, whose cells contain a nucleus housing linear chromosomes and an extensive array of internal membrane-bound organelles.

Nucleoid

The discrete region within a prokaryotic cell where its genetic material, organized as a single circular chromosome, is concentrated.

Organelles

Membrane-bound compartments within eukaryotic cells that divide the cell contents into smaller spaces specialized for distinct functions.

Cytoplasm

The entire contents of a cell bounded by the cell membrane, excluding the nucleus if present.

Cytosol

The jelly-like internal fluid environment of the cell that surrounds the organelles inside the cell membrane.

Amphipathic

Refers to a molecule, such as a phospholipid or cholesterol, containing both hydrophilic (polar) and hydrophobic (nonpolar) domains.

Micelles

Spherical structures formed in water by wedge-shaped lipids possessing bulky head groups and a single hydrophobic fatty acid tail.

Lipid bilayer

A two-layer sheet of lipid molecules in which hydrophilic phosphate heads face the external aqueous environments and hydrophobic fatty acid tails are sandwiched inside away from water.

Liposomes

Spherical, self-healing bilayer structures enclosing a central space that form spontaneously when phospholipids are added to water at neutral $pH = 7$.

Fluid mosaic model

Model proposed by S. Jonathan Singer and Garth Nicolson in 1972 stating that cell membranes are fluid structures wherein proteins and carbohydrates are embedded and move laterally within a lipid bilayer.

Saturated fatty acids

Fatty acid tails lacking carbon-carbon double bonds, making them straight and capable of tight packing, which reduces membrane fluidity.

Unsaturated fatty acids

Fatty acid tails containing carbon-carbon double bonds that create kinks, preventing tight packing and increasing membrane fluidity.

Cholesterol

An amphipathic lipid comprising four interconnected carbon rings and a hydroxyl group that makes up approximately $30\text{ }\text{percent}$ by mass of animal cell membrane lipids and stabilizes membrane fluidity against temperature changes.

Lipid rafts

Defined regions or patches in the cell membrane where specific lipids (such as sphingolipids), cholesterol, and membrane proteins accumulate.

Lipid flip-flop

The rare spontaneous movement of a lipid molecule from one layer of the lipid bilayer to the opposite layer.

Integral membrane proteins

Proteins permanently associated with cell membranes that cannot be separated from the membrane without destroying the lipid bilayer itself.

Peripheral membrane proteins

Proteins temporarily associated with the lipid bilayer or integral proteins through weak noncovalent interactions, easily separated by simple procedures.

Transmembrane proteins

Integral membrane proteins that span the entire lipid bilayer, featuring two hydrophilic regions on either face and a hydrophobic region spanning the interior.

Fluorescence recovery after photobleaching (FRAP)

An experimental technique demonstrating membrane protein mobility by photobleaching fluorescently labeled surface proteins with a laser and monitoring the return of fluorescence.

Homeostasis

The active maintenance of stable intracellular conditions (such as specific pH ranges or salt concentrations) required for life.

Selectively permeable

The characteristic of the cell membrane that regulates internal conditions by allowing specific molecules to pass freely, restricting others, and blocking the rest completely.

Diffusion

The net movement of ions or molecules from an area of higher concentration to an area of lower concentration resulting from random thermal motion.

Simple diffusion

The unassisted passive movement of hydrophobic or small uncharged molecules directly through the hydrophobic interior of the lipid bilayer down a concentration gradient.

Facilitated diffusion

Passive transport of molecules down a concentration gradient across a cell membrane mediated by specific protein transporters.

Channel protein

A membrane transport protein providing an open pathway between the inside and outside of the cell through which specific ions or molecules pass according to shape and charge.

Carrier protein

A membrane transport protein that binds a specific molecule and undergoes a conformational change to transfer the molecule across the lipid bilayer.

Aquaporins

Specialized membrane channel proteins that facilitate the rapid movement of water molecules across cell membranes by facilitated diffusion.

Osmosis

The net movement of a solvent, such as water, across a selectively permeable membrane toward a region of higher solute concentration.

Osmotic pressure

The force required to prevent the tendency of a solution to draw water in by osmosis, which increases as solute concentration rises.

Active transport

The energy-requiring, uphill movement of substances across a membrane against their concentration gradient.

Primary active transport

Active transport that directly utilizes the chemical energy of ATP to move substances across a membrane against a concentration gradient.

Antiporters

Transporter proteins that move two different molecules or ions across a membrane in opposite directions, such as the sodium-potassium pump.

Symporters

Transporter proteins that move two different molecules or ions across a membrane simultaneously in the same direction.

Secondary active transport

Active transport driven by the potential energy stored in an electrochemical gradient built by a primary active transport pump, rather than direct ATP breakdown.

Electrochemical gradient

A transmembrane gradient consisting of both a chemical concentration difference and an electrical charge difference across the membrane.

Contractile vacuoles

Organelles in freshwater single-celled eukaryotes like Paramecium that collect excess water that enters by osmosis and contract to pump it out of the cell.

Turgor pressure

The hydrostatic force exerted by water pushing against the cell wall of plants, fungi, algae, or bacteria, providing mechanical support and shape.

Cytoskeleton

An ancient system of protein filaments present in cells that provides internal structural support, maintains cell shape, and enables movement.

Endomembrane system

A dynamic eukaryotic system of interconnected membranes including the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, cell membrane, and transport vesicles.

Vesicles

Small, membrane-enclosed sacs that bud off from organelles and fuse with other membranes to transport lipids and proteins within or outside the cell.

Exocytosis

The cellular process where a membrane-bound vesicle fuses with the cell membrane, discharging its soluble contents into the extracellular space.

Endocytosis

The process where the cell membrane invaginates and buds inward to form a vesicle, bringing extracellular contents into the cell interior.

Nuclear envelope

A double-membrane barrier consisting of inner and outer lipid bilayers perforated by nuclear pores that encloses the nucleus and regulates nucleocytoplasmic traffic.

Nuclear pores

Large protein complexes spanning both membranes of the nuclear envelope that regulate molecular transport into and out of the nucleus.

Rough endoplasmic reticulum

The region of the endoplasmic reticulum studded with ribosomes on its outer surface, specialized for synthesizing transmembrane, organelle, and secreted proteins.

Smooth endoplasmic reticulum

The ribosome-free portion of the endoplasmic reticulum involved in fatty acid and phospholipid synthesis, steroid hormone production, and drug detoxification.

Golgi apparatus

An organelle consisting of flattened membrane stacks called cisternae that chemically modifies, sorts, and packages proteins and lipids synthesized in the ER.

Glycosylation

The enzymatic covalent attachment of sugars to proteins or lipids in the Golgi apparatus to produce glycoproteins and glycolipids.

Lysosomes

Specialized vesicles formed by the Golgi apparatus that contain hydrolytic enzymes optimal at acidic $pH \thicksim 5$ to degrade damaged organelles and macromolecules.