Unit 2: Cell Structure, Membrane Transport, and Compartmentalization

Vocabulary practice flashcards covering cell structure, organelles, the endomembrane system, membrane architecture, transport mechanisms, tonicity, and water potential from AP Biology Unit 2.

What polymer forms the rigid bacterial cell wall, and how does the antibiotic penicillin target it?

Peptidoglycan; it consists of a mesh-like polymer of repeating disaccharides cross-linked by short amino acid peptide chains. Penicillin works by selectively inhibiting the transpeptidase enzymes responsible for peptide cross-linking during bacterial cell wall synthesis, causing the wall to weaken and lyse.

How do Gram-positive and Gram-negative bacteria differ structurally in their cell walls and response to Gram staining?

Gram-positive bacteria possess a thick layer of peptidoglycan that traps the crystal violet stain, appearing purple. Gram-negative bacteria possess a much thinner peptidoglycan layer enclosed by an asymmetric outer membrane containing lipopolysaccharides; they shed the crystal violet during alcohol decolorization and appear pink or red from the safranin counterstain.

What are plasmids, and what evolutionary advantages do they typically confer to bacterial cells?

Plasmids are small, circular, double-stranded extra-chromosomal DNA molecules that replicate independently from the bacterial chromosome. They carry non-essential yet advantageous genes, such as those conferring antibiotic resistance, producing toxins or virulence factors, and facilitating horizontal gene exchange.

What mechanism allows bacteria to directly transfer genetic material such as plasmids between cells?

Conjugation; a form of horizontal gene transfer where a donor bacterium extends a specialized hollow pilus to establish physical contact with a recipient cell, forming a cytoplasmic bridge through which a replicated single strand of plasmid DNA is transferred.

What major evolutionary and environmental event was triggered by the emergence of photosynthetic cyanobacteria?

The Great Oxygenation Event; ancestral cyanobacteria generated molecular oxygen (O2) as a byproduct of water-splitting photosynthesis. The oxygen saturated chemical sinks in the oceans and accumulated in the atmosphere, eliminating many obligate anaerobes and providing the selective pressure for the evolution of aerobic respiration.

What hypothesis, developed by Lynn Margulis, accounts for the evolutionary origin of mitochondria and chloroplasts?

The endosymbiotic theory; it proposes that ancestral eukaryotic host cells engulfed free-living, aerobic and photosynthetic prokaryotes without digesting them. Over evolutionary time, these engulfed endosymbionts formed an obligate mutualistic relationship and evolved into mitochondria and chloroplasts, evidenced by their circular DNA, bacterial-like ribosomes, and double membranes.

What interconnected membranous network in eukaryotic cells coordinates protein synthesis, lipid metabolism, and poison detoxification?

The endomembrane system; a continuous or vesicle-linked network comprising the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the plasma membrane, which work together to synthesize, modify, package, and transport cellular macromolecules.

What structural characteristic distinguishes the rough endoplasmic reticulum, and what is its primary function?

The rough endoplasmic reticulum is studded with ribosomes on its cytosolic surface. Its membrane-bound ribosomes translate polypeptides directly into the ER lumen, where they fold into tertiary structures, undergo initial glycosylation, and are packaged into transport vesicles destined for the Golgi apparatus, lysosomes, or export from the cell.

What key metabolic processes occur within the smooth endoplasmic reticulum?

The smooth endoplasmic reticulum lacks ribosomes and functions in synthesizing diverse lipids (including phospholipids and steroid hormones), metabolizing carbohydrates, storing and releasing calcium ions, and detoxifying drugs and metabolic byproducts (such as alcohol and ammonia) via modifying enzymes.

How does the Golgi apparatus receive, process, and ship macromolecules throughout the eukaryotic cell?

The Golgi apparatus consists of flattened membranous sacs called cisternae with distinct structural polarity: its cis face receives transport vesicles from the rough ER, internal enzymes modify carbohydrate tags and phosphorylate molecules as they move through the cisternae, and its trans face sorts and packages cargo into secretory vesicles directed to specific cellular destinations or the plasma membrane.

What single-membrane organelle acts as the digestive and recycling center of animal cells, and how is its internal environment maintained?

The lysosome; it contains hydrolytic enzymes that catalyze the breakdown of biological polymers. Membrane-embedded proton pumps actively consume ATP to pump H+ into the lumen, establishing an optimal acidic internal pH of approximately 4.5 to 5.0, which protects the rest of the cell from accidental autodigestion in the neutral cytosol (pH ≈7.2 This acidic environment allows lysosomal enzymes to function effectively in degrading various macromolecules such as proteins, lipids, and nucleic acids).

What lysosomal pathway enables a cell to break down and recycle its own damaged organelles and macromolecules?

Autophagy; a catabolic process wherein intracellular debris, such as worn-out mitochondria, is sequestered into double-membrane autophagosomes (macroautophagy) or internalized directly (microautophagy and chaperone-mediated autophagy), fusing with lysosomes to yield free monomers like amino acids and fatty acids for metabolic reuse.

What structural compartmentalization allows the mitochondrion to produce high yields of ATP during aerobic respiration?

The mitochondrion has a smooth outer membrane and an extensively folded inner membrane forming cristae, which substantially increase surface area for electron transport chain complexes and ATP synthase enzymes. The internal fluid-filled matrix houses the citric acid cycle enzymes, circular mitochondrial DNA, and bacterial-like ribosomes.

What are the structural compartments of a chloroplast, and where do the distinct stages of photosynthesis occur?

Enclosed by a double membrane, the chloroplast contains a fluid stroma containing circular DNA and ribosomes where the Calvin cycle fixes CO2 into carbohydrates. Suspended within the stroma is the thylakoid network, arranged into stacks called grana, whose thylakoid membranes harbor chlorophyll pigments and photosystems for the light-dependent reactions.

What metabolic functions define peroxisomes, and how do they neutralize their own toxic reaction byproducts?

Peroxisomes are single-membrane metabolic compartments containing oxidases that strip hydrogen atoms from substrates (such as long-chain fatty acids) and transfer them to O2​, generating hydrogen peroxide (H2O2​). They simultaneously contain the enzyme catalase, which rapidly degrades toxic H2O2​ into harmless water and oxygen.

What is the structural composition of a ribosome, and what distinguishes free ribosomes from bound ribosomes?

Ribosomes are non-membranous ribonucleoprotein complexes consisting of ribosomal RNA (rRNA) and proteins organized into large and small subunits. Free ribosomes remain suspended in the cytosol to translate proteins destined to function within the cytoplasm, whereas bound ribosomes attach to the rough ER to produce membrane-embedded, organellar, or secreted proteins.

What three major protein filaments comprise the eukaryotic cytoskeleton, and what are their relative sizes and primary functions?

1. Microtubules (25 nm diameter): hollow tubulin cylinders resisting compression, acting as motor-protein tracks, and forming the mitotic spindle.

2. Microfilaments (7 nm diameter): two intertwined actin strands that bear tension, maintain cell shape, and facilitate cellular motility.

3. Intermediate filaments (8 to 12 nm diameter): rope-like fibrous proteins (such as keratin) providing permanent mechanical strength and anchoring organelles.

What is the organization and role of the centrosome in animal cell division?

The centrosome serves as the primary microtubule-organizing center (MTOC) adjacent to the nucleus. In animal cells, it houses a pair of perpendicular centrioles, each formed from nine sets of microtubule triplets in a 9+0 ring arrangement, which duplicate and coordinate the assembly of the mitotic spindle during cell division.

What takes place within the nucleolus, and why does it lack a surrounding membrane?

The nucleolus is a dense, non-membrane-bound aggregate within the nucleus organized around chromosomal regions containing rRNA genes. It serves as the assembly site where RNA polymerase transcribes rRNA and complexes it with imported ribosomal proteins to synthesize small and large ribosomal subunits before their export to the cytoplasm.

What structural arrangement and motor proteins drive the motility of eukaryotic cilia and flagella?

Eukaryotic cilia and flagella are membrane-sheathed cellular projections displaying an axoneme structure of nine doublet microtubules encircling a central pair (9+2 pattern), anchored to the cell by a basal body (9+0 pattern). Dynein motor arms use ATP hydrolysis to walk along adjacent doublets, causing sliding forces that produce rhythmic bending motions.

What constitutes the extracellular matrix (ECM) of animal cells, and how does it communicate with the cell interior?

The ECM is an intricate network composed primarily of glycoproteins such as collagen, proteoglycans, and fibronectin. Fibronectin binds to transmembrane receptor proteins called integrins, which in turn connect directly to microfilaments of the cytoskeleton, transmitting mechanical and chemical signals across the plasma membrane.

What does it mean for a membrane molecule to be amphipathic, and how does this property direct lipid bilayer formation?

An amphipathic molecule contains both a hydrophilic (polar, water-soluble) region and a hydrophobic (nonpolar, water-insoluble) region. In aqueous solutions, phospholipids spontaneously self-assemble into a bilayer where the polar phosphate heads face outwards toward water and the nonpolar fatty acid hydrocarbon tails align inward, sequestered from water.

What key principles are described by the fluid mosaic model of biological membranes?

The fluid mosaic model depicts the plasma membrane as a dynamic, fluid phospholipid bilayer in which diverse proteins, steroids (such as cholesterol), and glycolipids are embedded like a mosaic. Weak hydrophobic interactions hold the membrane together, permitting rapid lateral movement of lipids and proteins while maintaining membrane integrity and selective barrier function.

How do integral and peripheral membrane proteins differ in their physical association with the lipid bilayer?

Integral proteins penetrate into the hydrophobic core of the lipid bilayer, often spanning the entire membrane as transmembrane proteins with hydrophobic alpha-helical regions. Peripheral proteins do not penetrate the hydrophobic core; they attach loosely to the exposed hydrophilic surfaces of the membrane or to integral proteins through non-covalent interactions.

What are the major physiological roles carried out by membrane proteins?

Membrane proteins carry out six essential functions: transport of hydrophilic solutes (channels and carriers), enzymatic catalysis, signal transduction across the membrane (receptors), cell-cell recognition (glycoproteins), intercellular joining (tight junctions, desmosomes, gap junctions), and attachment to the cytoskeleton and extracellular matrix.

How does the chemical nature of the phospholipid bilayer determine the selective permeability of the plasma membrane?

The hydrophobic hydrocarbon interior readily allows small, nonpolar molecules (such as O2​, CO2, and lipid hormones) to diffuse across unassisted. In contrast, it acts as an impenetrable barrier to ions (such as Na+, K+) and large polar molecules (such as glucose), which necessitate specialized transmembrane transport proteins.

Why do cells require aquaporins when water is technically able to cross the lipid bilayer on its own?

Although small water molecules can slowly diffuse across the hydrophobic core via simple diffusion, the rate is insufficient for rapid osmotic equilibration. Aquaporins are specialized homotetrameric channel proteins that provide a narrow hydrophilic pore, enabling water molecules to pass in single file at rates up to 3×10^9 molecules/second.

What occurs at the molecular level when a diffusing substance reaches dynamic equilibrium across a membrane?

At dynamic equilibrium, solute concentrations are equal on both sides of the membrane. Molecules do not stop moving; instead, the rate of solute movement across the membrane in one direction exactly matches the rate of movement in the opposite direction, producing zero net change in concentration.

How is osmosis defined in terms of both solute concentration and water potential?

Osmosis is the net diffusion of water across a selectively permeable membrane from a region of lower solute concentration (higher water potential) to a region of higher non-penetrating solute concentration (lower water potential), diluting the more concentrated solution until equilibrium or opposing pressure is reached.

What is tonicity, and what dictates whether an extracellular solution is hypotonic, isotonic, or hypertonic to a cell?

Tonicity is the ability of an extracellular solution to cause a cell to gain or lose water, determined solely by the concentration of non-penetrating solutes relative to the cell interior. A hypotonic solution has fewer non-penetrating solutes (causing water influx), a hypertonic solution has more (causing water efflux), and an isotonic solution has equal concentrations (yielding no net water flux).

What is turgor pressure, and why does exposure to a hypotonic environment benefit plant cells while lysing animal cells?

Turgor pressure is the hydrostatic pressure exerted outward by the plant protoplast against its rigid cellulose cell wall. When placed in a hypotonic solution, water enters until the back-pressure from the wall balances the osmotic gradient, producing a firm (turgid) state essential for plant support; animal cells lack cell walls, so unregulated water influx causes them to swell and burst (lyse).

What is Water Potential ($\Psi$), and which mathematical components govern its value?

Water potential (Ψ) is a physical property measured in megapascals (MPa) or pascals (Pa) that predicts the direction in which water will flow, always moving spontaneously from high to low water potential. It is expressed by the equation Ψ=Ψs+Ψp​, where Ψs is the solute (osmotic) potential and Ψp​ is the physical pressure potential.

What mathematical formula defines solute potential (Ψs​), and how does solute ionization affect this value?

Solute potential is calculated using Ψs​=−iCRT, where i is the ionization constant, C is molar concentration, R is the pressure constant (0.0831L⋅bars/mol⋅K or 0.00831L⋅MPa/mol⋅K), and T is absolute temperature in Kelvin (K=°C+273.15). Solutes that ionize (e.g., NaCl with i=2) generate more particles than non-ionizing solutes (glucose with i=1), making Ψs​ more negative and exerting a stronger osmotic pull.

What distinguishes active transport from passive facilitated diffusion across biological membranes?

Active transport moves solutes uphill against their electrochemical or concentration gradients (from low to high concentration), requiring specific carrier transport proteins and direct or indirect cellular energy expenditure (typically via ATP hydrolysis). Passive facilitated diffusion moves substances down their gradient without energy input.

What is membrane potential, and what typical voltage exists across the resting eukaryotic plasma membrane?

Membrane potential is an electrical voltage difference across the plasma membrane generated by the unequal distribution of cations and anions on either side. In animal cells, the resting membrane potential ranges between −50mV and −200 mV, with the cytoplasmic face maintaining a net negative charge relative to the extracellular fluid.

What two combined forces establish an ion's electrochemical gradient across a cell membrane?

An electrochemical gradient is formed by two driving forces: a chemical force (the concentration gradient of the ion across the membrane) and an electrical force (the effect of the membrane potential on the ion charge). Together, they dictate the spontaneous direction of ion flow through open channel proteins.

How does a proton pump function as an electrogenic pump in plants, fungi, and bacteria?

A proton pump is a transmembrane protein that hydrolyzes ATP to actively export hydrogen ions (H+) out of the cell against their gradient. By displacing positive charge to the extracellular side, it generates a substantial membrane potential and stores potential energy in an electrochemical H+ gradient used to drive secondary active transport.

What is the stoichiometric exchange and operating mechanism of the animal sodium-potassium pump ($\text{Na}^+/\text{K}^+$ Pump)?

The Na+/K+ ATPase pump binds three intracellular Na+ ions, undergoes phosphorylation by ATP, changes conformation, and expels the 3Na+ extracellularly. Two extracellular K+ ions then bind, triggering dephosphorylation and conformation recovery to release 2K+ into the cytosol, producing a net electrogenic export of 11 positive charge per cycle.

How does cotransport (secondary active transport) utilize energy without directly hydrolyzing ATP?

Cotransport couples the energetically favorable, downhill diffusion of an ion (such as H+ or Na+) down its electrochemical gradient to the simultaneous uphill transport of a second substance (such as sucrose or glucose) against its concentration gradient. The primary energetic cost is paid indirectly by the ATP-driven ion pumps that initially establish the gradient.

How do symporters and antiporters differ in their mechanism of secondary active transport?

Both are cotransport carrier proteins: a symporter transports both the driving ion and the coupled cargo solute across the membrane simultaneously in the same physical direction (e.g., the Na+/glucose cotransporter), whereas an antiporter moves the two substances in opposite directions across the membrane (e.g., Na+/Ca2+ exchangers).

How do exocytosis and endocytosis mediate bulk transport across the plasma membrane?

Bulk transport utilizes membrane-bound vesicles to move large macromolecules and fluids across the membrane via energy-requiring processes. In exocytosis, secretory vesicles from the Golgi fuse with the plasma membrane to discharge cargo externally; in endocytosis, the plasma membrane invaginates and pinches off inward to package extracellular cargo into vesicles.

How does receptor-mediated endocytosis achieve selective bulk uptake of specific extracellular molecules?

Extracellular ligands (such as LDL particles) bind to specific transmembrane receptor proteins concentrated in clathrin-coated membrane pits. Ligand binding triggers the pit to invaginate and pinch off into a coated vesicle, allowing the cell to ingest bulk quantities of specific, scarce substances without taking in large volumes of non-specific fluid.

Why do biologists care about mols or molar concentration?

Molarity tells you how many particles are present, not how heavy they are, and particle number is what drives osmosis.