Interactions Between Cells and the Extracellular Environment

A comprehensive set of flashcards covering extracellular environment, membrane transport, membrane potentials, synapses, neurotransmission, and receptor signaling as described in the lecture notes.

What is the extracellular environment (or extracellular fluid) and why must cells exchange substances with it?

Cells must exchange substances with the extracellular environment to survive and to communicate in order to maintain homeostasis.

Name the organs listed as organs of elimination.

Liver, kidneys, lungs, and intestines.

What components make up the extracellular matrix (ECM)?

Glycoproteins and proteoglycans, elastin/fibrin fibers, and collagen protein fibers.

List four key functions of the plasma membrane.

Acts as a barrier, regulates movement into and out of the cell, establishes and maintains an electrochemical gradient, and participates in cell communication.

Define passive transport.

Transport that does not require cellular energy (no ATP) and moves substances down their concentration gradient.

What is the role of ATPase in active transport?

ATPases hydrolyze ATP to power active transport, directly using energy to move substances against their gradient (e.g., Na+/K+ ATPase).

Differentiate primary from secondary active transport.

Primary active transport uses ATP directly; secondary active transport uses the energy from an existing ion gradient (built by primary transport) to move another substance.

What factors affect the rate of diffusion?

Steepness of the concentration gradient and temperature; membrane thickness can also influence transport rate.

What is facilitated diffusion?

Diffusion that uses carrier proteins to move substances down their gradient; it is specific and can be saturable.

What is osmosis?

Movement of water across a semipermeable membrane, often through aquaporins.

Define isotonic, hypotonic, and hypertonic solutions.

Isotonic: same solute concentration as inside the cell; hypotonic: lower outside; hypertonic: higher outside.

What is tonicity?

The ability of a solution to change the volume or pressure of a cell by osmosis.

What do SGLT2 co-transporters do in kidneys?

Sodium-glucose co-transporters reabsorb glucose with sodium; they can saturate, leading to glucose remaining in urine (glycosuria) in diabetes.

Where does glucose reabsorption predominantly occur in the nephron?

Proximal tubule.

What is resting membrane potential (RMP) and its typical value?

The electrical potential across the membrane of a resting cell, typically around -70 millivolts (mV).

Which ions contribute to the resting membrane potential and where are they concentrated?

K+ is higher inside the cell; Na+ and Cl− are higher outside; fixed intracellular anions contribute to the negative potential.

What maintains the resting membrane potential?

Differences in ion permeabilities (especially to K+) and the Na+/K+ pump help maintain the RMP.

Name the main types of ion channels.

Leak (always open) channels, ligand-gated channels, and voltage-gated channels.

What triggers an action potential (AP)?

Depolarization to threshold (~ -55 to -50 mV) due to increased Na+ permeability.

List the phases of the action potential.

Resting state, depolarization, repolarization, and hyperpolarization.

What occurs during depolarization?

Na+ permeability increases; Na+ channels open, driving the membrane potential toward a more positive value.

What occurs during repolarization?

Na+ gates close and K+ gates open; K+ exits the cell, restoring negative membrane potential.

What is hyperpolarization?

The membrane becomes more negative than the resting potential due to continued K+ efflux.

What contributes to the resting membrane potential besides ion permeabilities?

The uneven distribution of ions across the membrane and the Na+/K+ pump maintaining gradients.

What is the absolute refractory period?

A period during which a new action potential cannot be generated, ensuring one-way transmission.

What is the relative refractory period?

The interval after the absolute refractory period when a stronger stimulus can trigger another AP because the threshold is elevated.

What factors affect conduction velocity in axons?

Axon diameter (larger is faster) and the presence of myelin (myelination speeds conduction).

What are electrical and chemical synapses?

Electrical synapses use gap junctions for direct ionic flow; chemical synapses use neurotransmitter release into the synaptic cleft.

What are gap junctions?

Direct cytoplasmic connections between adjacent cells formed by connexons, allowing ions to pass.

What is a synaptic cleft?

The fluid-filled space between presynaptic and postsynaptic neurons where chemical transmission occurs.

What triggers neurotransmitter release at chemical synapses?

Arrival of an action potential opens voltage-gated Ca2+ channels; Ca2+ triggers vesicle fusion and transmitter release.

What is the neuromuscular junction (NMJ)?

The synapse between a motor neuron and a muscle fiber; acetylcholine is released to stimulate muscle contraction.

What are SNARE proteins?

Proteins (including synaptobrevin, syntaxin, SNAP-25) that mediate vesicle fusion with the plasma membrane.

What happens to acetylcholine after release at the NMJ?

It binds to receptors causing depolarization; it is degraded by acetylcholinesterase or diffuses away.

What is an EPSP?

Excitatory post-synaptic potential; a depolarizing post-synaptic potential.

What is an IPSP?

Inhibitory post-synaptic potential; a hyperpolarizing post-synaptic effect.

What are receptors?

Three-dimensional protein structures that bind specific ligands and elicit a cellular response.

What are the four types of ligands discussed?

Agonists, antagonists, partial agonists, and inverse agonists.

What is second-messenger signaling?

Indirect signaling where receptor activation triggers intracellular second messengers (e.g., cAMP, Ca2+, DAG) that modulate target proteins.

What is the role of cAMP in signaling?

A common second messenger produced by adenylyl cyclase that activates protein kinase A and other downstream effects.

Which receptors use direct signaling versus second-messenger signaling?

Ligand-gated ion channels provide direct signaling; G-protein coupled receptors and receptor tyrosine kinases use second messengers or kinase cascades.

What are G-protein coupled receptors (GPCRs) and the major G proteins?

GPCRs are receptors that activate G proteins (Gs, Gi, Gq) to propagate signals through second messengers.

How does the Gs pathway operate?

A ligand binds the receptor, Gs activates adenylyl cyclase, increasing cAMP, which activates protein kinase A.

How does the Gi pathway operate?

A ligand activates Gi, which inhibits adenylyl cyclase, decreasing cAMP and downstream signaling.

How does the Gq pathway operate?

A ligand activates Gq, which activates phospholipase C, producing IP3 and DAG; IP3 releases Ca2+ from the ER and DAG activates PKC.

What are IP3 and DAG in GPCR signaling?

IP3 triggers Ca2+ release from the endoplasmic reticulum; DAG activates protein kinase C.