Molecules, Metabolism, Biomolecules, and Cellular Transport

Vocabulary practice flashcards covering cell structure, the endomembrane system, enzyme kinetics, bioenergetics, cellular respiration, biomolecules, and membrane transport dynamics.

Nucleoid

The non-membrane-bound region within a prokaryotic cell that stores the single, circular chromosome of DNA.

Endosymbiotic Theory

The theory that eukaryotic mitochondria and chloroplasts originated from free-living prokaryotes that were engulfed by an ancestral eukaryotic host cell.

Endomembrane System

A coordinated internal network of eukaryotic organelles—including the nuclear envelope, endoplasmic reticulum, Golgi apparatus, and transport/secretory vesicles—that functions to modify, package, and transport lipids and proteins.

Rough Endoplasmic Reticulum

A membrane-bound organelle studded with ribosomes where proteins destined for secretion or the plasma membrane are synthesized and packaged into transport vesicles.

Smooth Endoplasmic Reticulum

A region of the endoplasmic reticulum that lacks ribosomes and functions primarily in lipid synthesis and processing.

Free Cytoplasmic Ribosomes

Unattached ribosomes suspended in the cytosol that synthesize proteins destined to remain in the cytoplasm or be imported into the nucleus, mitochondria, or chloroplasts.

Microtubules

Cytoskeletal fibers that serve as structural tracks for motor proteins moving transport and secretory vesicles through the cell, and assist in chromosome separation during cell division.

Microfilaments

Cytoskeletal actin networks located near the plasma membrane that structurally stabilize or alter cell shape, particularly when portions of the membrane extend outward.

CFTR Protein Trafficking

The endomembrane pathway where the CFTR plasma membrane protein moves sequentially through the rough ER, transport vesicle, Golgi apparatus, and secretory vesicle along microtubule tracks to reach the plasma membrane.

Second Law of Thermodynamics

The physical principle stating that in every energy conversion, some energy is released as heat, causing the total entropy of any closed system to always increase.

Gibbs Free Energy ($\Delta G$)

The thermodynamic potential calculated as $\Delta G = \Delta H - T\Delta S$; a negative $\Delta G$ indicates an exergonic, spontaneous reaction, whereas a positive $\Delta G$ indicates an endergonic reaction requiring energy input.

Chemical Equilibrium

A state in which the forward and reverse reaction rates are identical ($\Delta G = 0$), resulting in no net change in reactant and product concentrations.

Steady-State (Homeostasis)

A continuous non-equilibrium condition of an organism's entire internal environment where inputs and energy dissipation maintain constant internal parameters without settling into chemical equilibrium.

$$V_{\max}$$

The maximum rate of an enzyme-catalyzed reaction achieved when all active sites are completely saturated with substrate.

Michaelis Constant ($K_m$)

The substrate concentration at which the enzymatic reaction velocity equals $\frac{1}{2}V_{\max}$; it serves as an inverse measure of an enzyme's affinity for its substrate.

Competitive Inhibition

A mechanism of enzyme regulation in which an inhibitor mimics the substrate and binds directly to the active site, physically blocking substrate binding.

Noncompetitive (Allosteric) Inhibition

Enzyme regulation wherein an inhibitor binds to a distinct regulatory (allosteric) site, altering the 3D conformation of the enzyme and its active site so the substrate can no longer bind.

Photoautotroph

An organism that captures light energy to catalyze biochemical reactions and fixes inorganic carbon ($CO_2$) from non-biological sources into organic matter.

Chemoheterotroph

An organism that derives its metabolic energy from chemical oxidation and must obtain its carbon from preformed organic biological sources.

Substrate-Level Phosphorylation

The direct metabolic synthesis of ATP through the enzymatic transfer of a phosphate group from a reactive organic intermediate molecule directly to ADP.

Oxidative Phosphorylation

The indirect synthesis of ATP driven by redox reactions along an electron transport chain, which generates a transmembrane proton gradient harnessed by ATP synthase.

Redox Reaction

A coupled chemical reaction involving the transfer of electrons, where oxidation denotes the loss of electrons (often via loss of hydrogen or gain of oxygen) and reduction denotes the gain of electrons (often via gain of hydrogen or loss of oxygen).

$NAD^+ / NADH$ Cycle

An electron shuttle mechanism where the oxidized coenzyme $NAD^+$ accepts a hydride ion ($H^-$, carrying two electrons and one proton) to become reduced $NADH$, which then transfers high-energy electrons to Complex I of the ETC.

Proton Motive Force

An electrochemical potential established by pumping protons ($H^+$) across an inner mitochondrial or cellular membrane, driving the rotary mechanism of ATP synthase to join ADP and $P_i$.

Aerobic Respiration

Catabolic respiration occurring in the presence of oxygen, which serves as the final electron acceptor with high redox potential, generating the maximum amount of ATP and producing $H_2O$.

Anaerobic Respiration

Cellular respiration occurring in the absence of oxygen that utilizes alternative terminal electron acceptors with lower redox potential, producing less ATP than aerobic respiration.

Organic Molecule

A chemical molecule containing at least one direct $C\text{--}H$ bond; by definition, inorganic molecules lack this bond.

Glycans (Polysaccharides)

Biological polymers made of monosaccharide subunits linked by glycosidic bonds, functioning in structural integrity, energy storage, and cell-cell recognition.

Lipids

A major class of biomolecules defined by their shared hydrophobicity rather than a repeated subunit or shared chemical polymer structure, used for membranes, energy storage, and signaling.

Nucleic Acids

Polymers composed of nucleotides (nitrogenous base, pentose sugar, and phosphate group) joined by phosphodiester linkages that store and process genetic information.

Primary Protein Structure

The linear sequence and order of amino acids in a polypeptide chain connected by covalent peptide bonds.

Secondary Protein Structure

Local conformations such as $\alpha$-helices and $\beta$-sheets stabilized entirely by hydrogen bonding between atoms of the polypeptide backbone, independent of amino acid R-groups.

Tertiary Protein Structure

The overall 3D shape of a single polypeptide chain formed by interactions among R-groups (and the backbone), including hydrogen bonds, hydrophobic interactions, van der Waals forces, ionic bonds, and covalent disulfide bridges.

Quaternary Protein Structure

The structural arrangement that arises when two or more distinct polypeptide subunits assemble to form a functional multi-protein complex.

Fluid Mosaic Model

A membrane model describing the lipid bilayer as a dynamic, fluid structure wherein phospholipids and proteins diffuse laterally, integrated with peripheral and embedded proteins and carbohydrates.

Archaeal Membrane Lipids

Membrane lipids characteristic of Archaea that consist of branched isoprene chains ether-linked to L-glycerol, forming either bilayers or 2-sided monolayers.

Hopanoids

Steroid-like pentacyclic lipids found in bacterial membranes that stabilize and reinforce membrane fluidity and integrity.

Cis-Unsaturated Fatty Acids

Hydrocarbon tails with cis double bonds creating rigid kinks that prevent tight lipid packing, thereby increasing membrane fluidity and permeability.

Membrane Fluidity Buffer (Cholesterol)

A eukaryotic sterol that restricts hydrocarbon tail movement to reduce permeability at normal body temperatures ($37\,^\circ\text{C}$), while preventing lipid crystallization and freezing at low temperatures.

Selective Permeability

The property of cell membranes permitting the passive diffusion of small hydrophobic molecules ($O_2$, $CO_2$) while restricting large uncharged polar molecules (glucose) and blocking charged ions ($Na^+$, $K^+$).

Facilitated Diffusion

Passive transport of solutes across a membrane down their concentration gradient via specific transmembrane channels or carriers without the expenditure of metabolic energy.

Active Transport

Carrier-mediated movement of solutes across a biological membrane against their concentration gradient, requiring the direct or indirect expenditure of cellular energy.

Hypertonic Solution

An external environment with a higher solute concentration than the cell interior, causing net water loss via osmosis and resulting in animal cell shriveling.

Hypotonic Solution

An external environment with a lower solute concentration than the cell interior, causing net osmotic influx of water that leads animal cells to swell and lyse.