Review Seminar 1: Cell Biology, Membranes, Viruses, Respiration, and Apoptosis

Vocabulary flashcards covering the structure and function of prokaryotic and eukaryotic cells, viruses, biological membranes, membrane transport mechanisms, signal transduction, the extracellular matrix, cellular respiration, and apoptosis.

Prokaryotic Cell

A single-celled organism typically $1\text{--}10\,\mu\text{m}$ in size that lacks a nucleus and membrane-bound organelles, has small ribosomes, reproduces asexually, and contains circular DNA (plasmid) located in a nucleoid region.

Eukaryotic Cell

A cell type (typically $10\text{--}100\,\mu\text{m}$ in size) found in multicellular organisms and unicellular protozoa, characterized by a membrane-bound nucleus containing linear chromosomes, membrane-bound organelles, and large ribosomes.

Virion

An inanimate viral particle existing outside host cells, consisting of a small nucleic acid core (DNA or RNA) encoding a few to hundreds of genes enclosed by a protein capsid.

Capsid

The protective protein capsule or shell that encloses the viral nucleic acid genome in a virion.

Bacteriophage

A virus that infects bacteria, typically characterized by a capsid head containing nucleic acid (DNA), collar, sheath, baseplate, spikes, and tail fibers.

Host Range

The specific variety of host species or cell types a virus can infect, determined by viral surface proteins binding to matching host surface receptors.

Lytic Infection

A type of viral infection where the virus hijacks the host cell machinery to produce new virus particles, ultimately rupturing and killing the host cell (e.g., influenza, T4 bacteriophage).

Non-Lytic Infection (Lysogenic / Integrative)

A viral infection in which viral DNA integrates into the host chromosome as a provirus, allowing the host cell to survive and replicate, often with impaired function (e.g., HIV, Chicken Pox).

Provirus

Viral DNA that has integrated into the genome of a host cell during a non-lytic (lysogenic or integrative) viral cycle.

Plasma Membrane

A trilaminar biological membrane approximately $6\,\text{nm}$ thick composed of a phospholipid bilayer that defines the cell boundary, encloses compartments, regulates transport, and provides a scaffold for biochemical activities.

Amphipathic

The structural property of having both hydrophilic (polar) and hydrophobic (non-polar) regions on the same molecule, such as in phospholipids and cholesterol.

Fluid Mosaic Model

A model of biological membrane structure where individual lipid molecules move laterally (fluidity) within a bilayer embedded and penetrated by a diverse array of proteins, carbohydrates, and cholesterol (mosaic).

Membrane Asymmetry

The structural difference in lipid and protein composition between the inner and outer leaflets of the plasma membrane, with the outer leaflet typically enriched in glycolipids and glycoproteins.

Transition Temperature

The characteristic temperature at which a biological membrane undergoes a phase shift between a rigid crystalline gel state (at lower temperatures) and a fluid liquid crystal state (at higher temperatures).

Saturated Lipids

Lipids with hydrocarbon tails containing only single carbon-carbon bonds, enabling tight intermolecular packing that decreases membrane fluidity.

Unsaturated Lipids

Lipids with hydrocarbon tails containing one or more carbon-carbon double bonds, producing kinks that disrupt tight packing and increase membrane fluidity.

Cholesterol (Membrane Role)

An amphipathic lipid that modulates membrane fluidity by altering lipid packing and flexibility, decreasing fluidity in a liquid crystal membrane and increasing fluidity in a crystalline gel membrane.

Integral Membrane Proteins

Transmembrane proteins that completely span the lipid bilayer, functioning in nutrient and ion transport, cell-cell communication (such as gap junctions), and attachment.

Peripheral Membrane Proteins

Proteins that associate non-covalently with the polar surfaces of the lipid bilayer rather than embedding in the hydrophobic interior.

Lipid-Anchored Proteins

Membrane-associated proteins located on the bilayer surface that are covalently tethered directly to a lipid molecule residing within the bilayer.

Simple Diffusion

A passive, non-mediated transport mechanism whereby small, uncharged molecules like $\text{O}_2$, $\text{CO}_2$, and $\text{NO}$ move freely across the phospholipid bilayer downhill along their concentration gradient.

Ion Channel

An integral membrane protein consisting of multiple subunits lining an aqueous pore that allows selective, passive, downhill flow of small charged ions such as $\text{Na}^+$, $\text{K}^+$, $\text{Ca}^{2+}$, and $\text{Cl}^-$.

Voltage-Gated Channel

An ion channel that opens or closes its pore in response to alterations in electrical charge or voltage potential across the biological membrane (e.g., $\text{Na}^+$ and $\text{K}^+$ channels).

Ligand-Gated Channel

An ion channel whose pore opening or closing is triggered by a conformational change induced upon the binding of a specific chemical molecule (ligand) to its receptor site.

Facilitated Diffusion

Carrier-mediated passive transport across a membrane where a compound specifically binds to an integral facilitative transporter, inducing a conformational change that moves the compound downhill without energy expenditure.

Uniporter

A carrier protein that selectively facilitates the movement of a single type of solute down its concentration gradient across a biological membrane.

Symporter

A membrane transport protein that couples the simultaneous movement of two different molecules or ions across the membrane in the same direction.

Antiporter

A secondary active transport carrier protein that harnesses the favorable downhill concentration gradient of one molecule to drive the movement of a second molecule in the opposite direction.

Active Transport

The movement of substances across a biological membrane against their concentration gradient via an active transporter, directly requiring input of metabolic energy through $\text{ATP}$ hydrolysis.

Sodium-Potassium Pump

An active transport integral membrane protein that hydrolyzes $\text{ATP}$ to pump $3\,\text{Na}^+$ ions out of the cell and $2\,\text{K}^+$ ions into the cell against their electrochemical gradients.

Ligand

A small signaling molecule that binds specifically to a cell-surface receptor protein without entering the cell, inducing a conformational change that initiates downstream signal transduction.

Signal Transduction

The cellular pathway through which an extracellular ligand binding to a receptor stimulates intracellular second messengers (e.g., $\text{cAMP}$, calcium, G-proteins) to activate a molecular cascade that drives a specific cellular response.

Extracellular Matrix (ECM)

A dynamic meshwork of secreted macromolecules including collagens, proteoglycans, glycosaminoglycans, and glycoproteins that provides bulk, shape, structural strength, and signaling cues to animal tissues.

Integrins

Transmembrane anchor proteins that function at the cell surface as receptors for specific extracellular matrix (ECM) components, activating intracellular signaling cascades upon ligand binding.

Plant Cell Wall

A rigid extracellular structure in plants composed of cellulose, hemicellulose, pectin, and lignin that provides mechanical support and defense against pathogens.

Substrate-Level Phosphorylation

A mechanism of $\text{ATP}$ synthesis where a coupled hydrolysis reaction releases sufficient chemical energy to directly transfer a phosphate to $\text{ADP}$, producing approximately $12\%$ of total $\text{ATP}$ in cellular respiration.

Oxidative Phosphorylation

The major ATP-generating process in aerobic cells (producing approximately $88\%$ of total $\text{ATP}$), driven by an electrochemical proton ($\text{H}^+$) gradient across the inner mitochondrial membrane generated by the electron transport chain.

Electron Transport Chain (ETC)

A series of multiprotein assemblies (Complexes I–IV) in the inner mitochondrial membrane that transfer electrons from $\text{NADH}$ and $\text{FADH}_2$ to oxygen ($\text{O}_2$) while translocating protons ($\text{H}^+$) to establish an electrochemical gradient.

Apoptosis

A regulated process of programmed cell death characterized morphologically by cell shrinkage, plasma membrane blebbing, nuclear and DNA fragmentation, loss of attachment, and eventual phagocytic engulfment.

Intrinsic Pathway of Apoptosis

A mitochondria-driven cell suicide pathway activated by intracellular damage, where pro-apoptotic proteins BAX and BAK permeabilize the outer mitochondrial membrane to leak Cytochrome c into the cytosol.

Cytochrome c

A mitochondrial intermembrane protein that, upon release into the cytoplasm during the intrinsic pathway of apoptosis, binds to cytoplasmic factors like Apaf-1 and procaspase-9 to form the apoptosome and initiate cell dismantling.