Interactions Between Cells and the Extracellular Environment

A comprehensive set of practice flashcards covering extracellular environment, membrane transport, ion distribution, membrane potentials, signal transduction, synaptic transmission, and receptor signaling as presented in the notes.

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

The fluid outside cells with which they exchange substances in order to survive and communicate to maintain homeostasis.

What are the main functions of the plasma membrane?

To serve as a physical barrier, regulate movement into and out of the cell, establish and maintain electrochemical gradients, and participate in cell communication.

What is diffusion, and what factors affect its rate in membrane transport?

Diffusion is the passive movement of particles down their concentration gradient; its rate depends on the steepness of the concentration gradient and the temperature (kinetic energy).

Why can't large or polar molecules diffuse across the membrane, and how are they transported?

They cannot diffuse passively; they require carrier proteins, which are often specific, can show competition, and have a transport rate that saturates.

What is carrier-mediated transport and what are its key characteristics?

Transport of large or polar molecules via carrier proteins; carriers are specific, may transport more than one substance, there can be competition, and transport is saturable (Tm).

What is filtration in the context of membranes and the kidneys?

Movement of water and dissolved solutes across membranes due to pressure differences; in the kidneys, filtration occurs at the glomerulus with subsequent reabsorption and secretion along the nephron.

What are SGLT2 co-transporters in the kidneys and what is their function?

Sodium-glucose co-transporters in the proximal tubule that reabsorb glucose together with Na+; their activity saturates at high glucose concentrations.

What does transport maximum (Tm) refer to in carrier-mediated transport?

The maximum rate of transport achievable when all carrier proteins are saturated with substrate.

What is osmosis and how does tonicity relate to cells?

Osmosis is the movement of water across a semipermeable membrane toward higher solute concentration; tonicity describes a solution’s ability to change cell volume by osmosis, classifying solutions as isotonic, hypotonic, or hypertonic.

What are the typical directional distributions of Na+, K+, Cl-, and Ca2+ across the cell membrane?

Na+ is higher outside; K+ is higher inside; Cl− is generally higher outside; Ca2+ is much higher outside than inside.

What is the Ca2+ active transport pump and its role?

An active transport pump that moves Ca2+ from the cytoplasm to the extracellular fluid using ATP, keeping cytosolic Ca2+ very low.

What is the Na+/K+ pump and what does it do?

An active transporter that pumps 3 Na+ out and 2 K+ in per ATP hydrolyzed, maintaining essential ionic gradients across the membrane.

Name the three main types of cell-to-cell junctions and describe their roles.

Tight junctions (impermeable barriers); Desmosomes (anchoring junctions that resist tension); Gap junctions (channels allowing direct cytoplasmic exchange).

What are electrical signals in cell communication?

Changes in the membrane potential of a cell that carry information.

What are chemical signals, and what mediators do they include?

Mediators secreted into the extracellular fluid, including neurotransmitters, paracrine signals, autocrine signals, neurohormones, and hormones, which act via specific receptors.

Differentiate local signaling from long-distance signaling.

Local signaling includes contact-dependent, paracrine, and autocrine signaling (including gap junctions); long-distance signaling includes endocrine and neural signaling.

What is a receptor in chemical signaling?

A protein or enzyme with a specific three-dimensional structure that binds a ligand to produce a cellular response; receptors can be extracellular or intracellular.

What are ligands, and how do agonists, antagonists, inverse agonists, and partial agonists differ?

Ligands bind receptors; agonists activate receptors to produce a response, antagonists block responses, inverse agonists reduce constitutive activity, and partial agonists produce a partial response.

What is signal transduction in chemical signaling?

The process by which extracellular signals cause cellular responses, either through opening ion channels, activating second-messenger pathways, or regulating gene expression.

What is a second messenger and give an example?

A molecule inside the cell that transmits signals from a receptor to target proteins; cyclic AMP (cAMP) is a common second messenger.

How do ligand-gated ion channels function in signaling?

They open in response to ligand binding, allowing specific ions to pass through the channel directly and rapidly.

How do G-protein coupled receptors (GPCRs) signal inside the cell?

Binding of a ligand activates a G protein, which then modulates effector enzymes like adenylate cyclase or phospholipase C, leading to second messenger production and downstream effects.

What are the three main types of G proteins and their general effects on signaling?

Gs activates adenylyl cyclase to raise cAMP; Gi inhibits adenylyl cyclase to lower cAMP; Gq activates phospholipase C to produce IP3 and DAG.

What is the role of cAMP in GPCR signaling via Gs?

cAMP activates protein kinase A (PKA), which phosphorylates target proteins to alter cellular activity.

What happens in the Gi pathway when a receptor is activated?

Adenylyl cyclase is inhibited, reducing cAMP production and downstream PKA activity.

What happens in the Gq pathway when a receptor is activated?

Phospholipase C is activated, producing IP3 and DAG; IP3 releases Ca2+ from the endoplasmic reticulum and DAG activates protein kinase C.

How do ion channels and GPCR signaling differ in terms of speed and mechanism?

Ion channels produce fast, direct ionic currents; GPCR signaling is slower and uses second messengers to alter enzymes, channels, or gene expression.

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

The steady-state electrical potential across the plasma membrane of a resting cell, typically about -70 mV in neurons.

What factors influence the resting membrane potential?

Permeability differences to K+ and Na+, distribution of fixed intracellular anions, and activity of the Na+/K+ pump that maintains gradients.

Define depolarization, repolarization, and hyperpolarization.

Depolarization: membrane becomes less negative; repolarization: returns toward resting potential; hyperpolarization: becomes more negative than resting potential.

What is the difference between graded potentials and action potentials?

Graded potentials are short-lived, local changes whose magnitude varies with stimulus; action potentials are all-or-none, long-distance signals with a constant amplitude.

What are the phases of the action potential?

Resting state, depolarization, repolarization, and hyperpolarization.

What is the absolute refractory period?

A period after an action potential during which a new action potential cannot be initiated because Na+ channels are inactivated.

What is the relative refractory period?

The period after the absolute refractory period when some Na+ channels have reset but the threshold is elevated, requiring a stronger stimulus to trigger an AP.

What factors determine axonal conduction velocity?

Axon diameter (larger = faster) and myelination (myelin dramatically increases speed).

What is a synapse and what are its two main types?

A junction that mediates information transfer between neurons or to an effector; electrical and chemical synapses.

How do electrical and chemical synapses differ?

Electrical synapses use gap junctions allowing direct ion flow; chemical synapses release neurotransmitters across a synaptic cleft for unidirectional signaling.

What is the synaptic cleft and why is it important?

A fluid-filled space between presynaptic and postsynaptic neurons; ensures transmission is chemical and unidirectional.

What is a neuromuscular junction and what occurs there?

The synapse between a motor neuron and a muscle cell where acetylcholine release triggers muscle contraction; involves SNARE proteins in vesicle fusion.

How is neurotransmitter signaling terminated at the synapse?

Neurotransmitters are degraded by enzymes, reabsorbed by neurons or astrocytes, or diffuse away from the synaptic cleft.

What are EPSP and IPSP in synaptic signaling?

EPSP (excitatory postsynaptic potential) increases likelihood of reaching threshold; IPSP (inhibitory postsynaptic potential) decreases likelihood by hyperpolarizing or stabilizing the membrane.

What is receptor signaling and its two main modes of ligand interaction?

Receptors bind ligands; signaling can be direct via ligand-gated channels or indirect via second messengers (e.g., GPCRs or tyrosine kinase pathways).