Exam III Study Guide: Membrane Structure, Transport, Intracellular Trafficking, and Cell Signaling

Comprehensive vocabulary flashcards covering membrane structure, active and passive transport, intracellular trafficking, organellar protein sorting, and cell signaling cascades from Exam III lectures.

Smooth Endoplasmic Reticulum (SER)

The cellular compartment where most membrane lipids are synthesized.

Rough Endoplasmic Reticulum (RER)

The organelle studded with ribosomes where membrane proteins are synthesized.

Initial Glycosylation

The initial covalent addition of sugars/carbohydrates to proteins, occurring in the endoplasmic reticulum (ER).

Golgi Carbohydrate Processing

The further modification and processing of carbohydrates attached to proteins and lipids after leaving the ER.

Transmembrane Proteins

Integral membrane proteins that span the entire lipid bilayer, such as the $Na^+/K^+$ pump.

Monolayer-Associated Proteins

Integral membrane proteins that are embedded in only one leaflet of the lipid bilayer.

Peripheral Membrane Proteins

Proteins loosely attached to the membrane surface through non-covalent interactions with integral proteins or membrane lipids.

$\alpha$-Helix (Membrane-Spanning)

The most common membrane-spanning structure, featuring hydrophobic amino acids that interact favorably with the hydrophobic interior of the lipid bilayer.

$\beta$-Barrel

A membrane-spanning protein structure found in the outer membranes of bacteria, mitochondria, and chloroplasts that forms pores for molecular transport.

Pore

An opening through a biological membrane that permits the passage of specific molecules or ions.

Cell Cortex

A specialized network of actin filaments located beneath the plasma membrane that maintains cell shape, provides mechanical support, anchors membrane proteins, and aids cell movement.

Glycolipid

A lipid molecule covalently linked to a carbohydrate, localized on the extracellular membrane surface to function in cell recognition, protection, and signaling.

Glycoprotein

A protein with covalently attached carbohydrate chains functioning in cell-cell recognition, receptors, and immune responses.

Proteoglycan

An extracellular matrix protein bearing many long carbohydrate chains that provides structural support, hydration, and cell signaling.

Concentration Gradient

The difference in the concentration of a solute across a biological membrane.

Passive Transport

The movement of molecules down their concentration gradient without the requirement of cellular energy input (e.g., simple diffusion and facilitated diffusion).

Active Transport

The energy-requiring movement of a solute against its concentration gradient across a membrane.

Channels

Pore-forming membrane transport proteins that mediate very fast, exclusively passive movement across membranes.

Transporters

Membrane proteins that bind a specific substrate and undergo conformational changes to transfer it across the membrane via passive or active transport.

Patch Clamp

An electrophysiological technique utilized to measure the electrical activity and current through individual ion channels.

Sodium-Potassium Pump ($Na^+/K^+$ ATPase)

An active transport pump that consumes one $ATP$ to transport $3\,Na^+$ out of the cell and $2\,K^+$ into the cell, maintaining membrane potential and chemical gradients.

Symport

A coupled transporter that transfers two different solutes across the membrane in the same direction, such as the Glucose-$Na^+$ symporter.

Antiport

A coupled transporter that transfers two different solutes in opposite directions across the membrane, such as the $Na^+/Ca^{2+}$ exchanger.

Ion Channels

Selective, gated transmembrane proteins that provide hydrophilic channels for the passive transport of ions down their electrochemical gradients.

Membrane Potential

The voltage difference across a plasma membrane, ranging from roughly $-20\,\text{mV}$ to $-200\,\text{mV}$ in animal cells and typically $-70\,\text{mV}$ in resting neurons.

Depolarization

The phase of a neuronal action potential during which voltage-gated $Na^+$ channels open, allowing $Na^+$ to rush inward and making the membrane potential more positive.

Repolarization

The phase of an action potential where voltage-gated $Na^+$ channels close and $K^+$ channels open, allowing $K^+$ to exit the cell and restore a negative potential.

Endomembrane System

The interconnected group of cellular membranes consisting of the nuclear envelope, endoplasmic reticulum, Golgi apparatus, endosomes, lysosomes, transport vesicles, and plasma membrane.

Endosymbiotic Theory

The evolutionary theory stating that mitochondria and chloroplasts originated from ancestral aerobic and photosynthetic bacteria engulfed by early eukaryotic cells.

Gated Transport

Protein sorting mechanism that moves proteins between the cytosol and the nucleus through nuclear pore complexes.

Transmembrane Transport

Protein sorting mechanism where unfolded proteins cross organellar membranes via protein translocators into the ER, mitochondria, or chloroplasts.

Vesicular Transport

Protein sorting mechanism utilizing membrane-enclosed transport vesicles to move cargo between the ER, Golgi, lysosomes, and plasma membrane.

Nuclear Pore Complex

A large multi-protein channel spanning the nuclear envelope, composed of cytoplasmic fibrils, a central meshwork channel, and a nuclear basket.

Nuclear Import Receptors

Cytosolic proteins that recognize nuclear localization signals on target proteins and guide them through the nuclear pore complex into the nucleus.

Protein Translocators

Membrane-bound protein channels that thread unfolded polypeptides across organellar membranes into mitochondria, chloroplasts, and the ER.

Start-Transfer Sequence

A hydrophobic signal sequence that initiates the translocation and insertion of a nascent polypeptide into the ER membrane.

Stop-Transfer Sequence

A hydrophobic amino acid sequence that halts translocation through the ER protein translocator, creating a membrane-spanning domain.

Double-Pass Proteins

Membrane proteins that employ both a start-transfer and a stop-transfer sequence to span the bilayer twice, leaving the uncleaved start sequence embedded.

Adaptin

A coat-associated protein that binds to cargo receptors and recruits clathrin molecules during vesicle bud formation.

Clathrin

A protein that forms a basket-like outer coat on budding transport vesicles, assisting in driving vesicle curvature.

Dynamin

A GTP-binding protein that forms a ring around the neck of an invaginating vesicle and pinches it off from the donor membrane.

Rab Proteins

GTPase proteins on the surface of transport vesicles that confer targeting specificity by binding tethering proteins on the appropriate target membrane.

SNARE Proteins

Complementary membrane-fusion proteins comprising vesicle $v\text{-SNAREs}$ and target membrane $t\text{-SNAREs}$ that pull lipid bilayers together to catalyze fusion.

Exocytosis

The vesicular transport process in which intracellular transport vesicles fuse with the plasma membrane to release material into the extracellular space.

Endocytosis

The cellular uptake mechanism by which extracellular material is captured and brought into the cell via vesicle invagination.

Cis Face (Golgi)

The entry face of the Golgi apparatus that receives transport vesicles originating from the endoplasmic reticulum.

Trans Face (Golgi)

The exit face of the Golgi apparatus that sorts and ships modified proteins and lipids toward lysosomes, secretory vesicles, or the plasma membrane.

Regulated Secretory Pathway

A specialized exocytic pathway wherein secretory vesicles store soluble cargo until an extracellular signal stimulates their release (e.g., neurotransmitters).

Lysosomes

Acidic membranous organelles packed with hydrolytic enzymes dedicated to digesting macromolecules and recycling worn-out cellular parts.

Phagosomes

Large endocytic vesicles containing engulfed extracellular particles or microorganisms destined to fuse with lysosomes.

Autophagy

A catabolic degradation process where a cell encapsulates and digests its own obsolete or damaged internal components.

Ligand

A signaling molecule that binds specifically to a receptor to initiate an intracellular signaling cascade.

Receptor

A target protein that binds a specific signaling molecule (ligand) and transmits the signal across the membrane or into the nucleus.

Signal Transduction

The biochemical process by which an extracellular signal is converted into a specific, coordinated cellular response.

Endocrine Signaling

Long-distance intercellular signaling where specialized endocrine cells secrete hormones into the bloodstream to act on distant targets (e.g., insulin).

Paracrine Signaling

A form of local cell signaling in which secreted molecules diffuse through the extracellular fluid to act on nearby cells.

Autocrine Signaling

A signaling mode where a cell produces and secretes a signaling molecule that acts upon receptors located on that same cell.

Synaptic Signaling

A rapid, private mode of communication occurring across a synapse between a neuron and its target cell via released neurotransmitters.

Cell Surface Receptors

Transmembrane receptors that bind hydrophilic extracellular signaling molecules (such as peptide hormones and neurotransmitters) that cannot cross the membrane.

Intracellular Receptors

Receptors located within the cytosol or nucleus that bind hydrophobic ligands (such as steroid or thyroid hormones) and directly regulate gene transcription.

Protein Kinase

An enzyme that catalyzes the transfer of a phosphate group from $ATP$ onto a target protein to alter its activity or interactions.

Protein Phosphatase

An enzyme that removes a phosphate group from a phosphorylated target protein, reversing the effect of a kinase.

Ion-Channel Coupled Receptors

Cell surface receptors that rapidly open or close an ion channel upon direct binding of a neurotransmitter/ligand (e.g., acetylcholine receptor).

G-Protein Coupled Receptors (GPCRs)

Seven-pass transmembrane cell surface receptors that trigger intracellular second messenger cascades by activating heterotrimeric G proteins.

Enzyme-Linked Receptors

Transmembrane receptors (typically single-pass proteins such as receptor tyrosine kinases) that possess intrinsic enzymatic activity or associate directly with intracellular enzymes.

Adenylyl Cyclase

A membrane-bound enzyme activated by the $G_\alpha$ subunit that converts $ATP$ into cyclic AMP ($cAMP$).

Protein Kinase A (PKA)

A serine/threonine kinase that is activated by $cAMP$ and subsequently phosphorylates cytosolic target proteins and nuclear transcription factors such as CREB.