BIOL 300 Chapter 12: Transport Across Cell Membranes & Neurons

Comprehensive review flashcards covering transport across cell membranes (transporters, channels, passive and active transport, electrochemical gradients, and ion pumps) and neurobiology (membrane potentials, action potential mechanics and propagation, synaptic transmission, channel pharmacology, and brain health).

What is the permeability of a protein-free artificial lipid bilayer (such as a liposome) to water-soluble molecules?

It is impermeable to most water-soluble molecules, whereas a cell membrane contains specialized transport proteins that allow selective transfer.

What two physical properties primarily determine the rate at which a molecule can diffuse across a pure lipid bilayer?

Its size and solubility (or charge), with smaller and hydrophobic/nonpolar molecules diffusing more rapidly.

How do small nonpolar molecules, small uncharged polar molecules, larger uncharged polar molecules, and ions differ in their permeability across an artificial lipid bilayer?

Small nonpolar molecules ($O_2$, $CO_2$, $N_2$, steroid hormones) diffuse rapidly; small uncharged polar molecules ($H_2O$, ethanol, glycerol) diffuse across at a reduced rate; larger uncharged polar molecules (amino acids, glucose, nucleosides) hardly cross; and ions ($H^+$, $Na^+$, $K^+$, $Ca^{2+}$, $Cl^-$, $Mg^{2+}$, $HCO_3^-$) cannot cross at all.

Why are lipid bilayers completely impermeable to inorganic ions regardless of how small they are?

Because ions possess an electrical charge; their charge and strong electrical attraction to water molecules inhibit them from entering the hydrophobic hydrocarbon phase of the bilayer.

What are the two major classes of membrane transport proteins found in cell membranes?

Transporters and channels.

How does the functional transport mechanism of a channel differ from that of a transporter?

A channel acts as a gateway, controlling solute passage by regulating size and charge when open. A transporter acts like a turnstile, binding specific solutes and transferring them one at a time through conformational changes.

What structural feature do membrane transport proteins (channels and transporters) share regarding their orientation in the lipid bilayer?

Both proteins have polypeptide chains that traverse the lipid bilayer multiple times.

What defines passive transport across a biological membrane?

The spontaneous movement of molecules downhill from a region of high concentration to a region of low concentration without requiring an input of energy.

Which classes of transport proteins facilitate passive transport?

All channels and many transporters.

How does the glucose transporter mediate the passive transport of glucose across the cell membrane?

It undergoes reversible conformational changes that alternately expose its glucose-binding site to the extracellular space and the cytosol, passively moving glucose down its concentration gradient.

What defines active transport across a cell membrane?

The movement of a solute against its concentration gradient (uphill), which requires energy by coupling to an energy-providing process.

What are the three main energy-coupling mechanisms cells use to carry out active transport?

Coupled pumps, ATP-driven pumps, and light-driven pumps.

How does a coupled pump drive active transport across a membrane?

It couples the uphill transport of one solute across the membrane against its electrochemical gradient to the downhill transport of another solute down its gradient.

How does an ATP-driven pump power the movement of solutes against an electrochemical gradient?

It hydrolyzes ATP to ADP + Pi using the energy released from high-energy phosphate bonds to drive uphill solute transport.

In what types of organisms are light-driven pumps primarily found, and what energy source do they utilize?

They are found mainly in bacterial cells and couple uphill solute transport directly to an input of energy from light.

Which specific transport pump maintains the high acidity (low pH) within lysosomes?

A proton pump that pumps protons from the cytosol into the lysosomal lumen.

What two critical transport proteins are located in the inner mitochondrial membrane to facilitate energy production?

A pyruvate transporter (importing pyruvate) and an $ATP$/$ADP$ exchanger (exporting $ATP$ and importing $ADP$).

What two forces compose the electrochemical gradient that drives an ion across a membrane?

The driving force is the sum of the concentration gradient of the ion and the electrical potential (voltage) across the membrane.

Why does $Na^+$ tend to enter animal cells at every opportunity, whereas $K^+$ shows little net movement across resting membranes?

Na+ has a steep electrochemical gradient because its concentration gradient and the negative membrane potential pull in the same direction, whereas for K+, its concentration gradient and the membrane potential act in opposite directions.

What is the typical intracellular versus extracellular concentration of $Na^+$ in a mammalian cell?

Intracellular: $5\text{--}15\,\text{mM}$; Extracellular: $145\,\text{mM}$.

What is the typical intracellular versus extracellular concentration of $K^+$ in a mammalian cell?

Intracellular: 140 nM; Extracellular: 5 nM

What is the typical intracellular versus extracellular concentration of free $Ca^{2+}$ in a mammalian cell?

Intracellular: $10^{-4}\,\text{mM}$; Extracellular: $1\text{--}2\,\text{mM}$.

What is the typical intracellular versus extracellular concentration of $Cl^-$ in a mammalian cell?

Intracellular: $5\text{--}15\,\text{mM}$; Extracellular: $110\,\text{mM}$.

What are the typical intracellular and extracellular $H^+$ concentrations and corresponding pH values in mammalian cells?

Intracellular: 7.2 pH; Extracellular: 7.4 pH

What is the total concentration of cellular $Mg^{2+}$ and $Ca^{2+}$, and why does it differ drastically from free ion concentrations?

Total $Mg^{2+}$ is about $20\,\text{mM}$ and total $Ca^{2+}$ is $1\text{--}2\,\text{mM}$, but most is bound to proteins/substances or sequestered inside organelles.

What is the stoichiometry of ions transported per cycle by the animal cell $Na^+\text{--}K^+$ pump?

It pumps $3\,Na^+$ ions out of the cell and $2\,K^+$ ions into the cell per molecule of $ATP$ hydrolyzed.

How long does one complete cycle of the $Na^+\text{--}K^+$ pump take?

Approximately $10\,\text{milliseconds}$.

What triggers the autophosphorylation of the $Na^+\text{--}K^+$ pump during step 2 of its cycle?

The binding of cytosolic Na+ to the pump induces the pump to phosphorylate itself using ATP.

What conformational change occurs in the $Na^+\text{--}K^+$ pump following phosphorylation?

Phosphorylation triggers a conformational change that opens the pump to the extracellular space and ejects $Na^+$.

What event triggers the dephosphorylation of the $Na^+\text{--}K^+$ pump?

The binding of extracellular $K^+$ to the pump triggers its dephosphorylation.

Why is extracellular Na+ likened to a large volume of water stored behind a hydroelectric dam?

Steep electrochemical gradient of Na+ represents a large store of potential energy that the cell uses to drive active transport processes.

What conformational states does the glucose-$Na^+$ symport cycle through during the coupled transport of glucose?

Occluded-empty, outward-open, occluded-occupied, and inward-open.

Why does the glucose-$Na^+$ symport bind glucose efficiently only when facing the extracellular fluid?

Because extracellular $Na^+$ is at high concentration and binds readily, cooperatively increasing the transporter affinity for glucose.

Which transporter actively absorbs glucose from the gut lumen across the apical microvilli membrane of intestinal epithelial cells?

The Na+-driven glucose symport.

How is glucose released from the basal and lateral domains of gut epithelial cells into the extracellular fluid?

Via passive glucose uniport transporters down its concentration gradient.

What role does the basal/lateral $Na^+$ pump play in transcellular glucose transport across the gut epithelium?

It pumps $Na^+$ out into the extracellular fluid to maintain the steep $Na^+$ gradient needed for apical glucose uptake.

What is the function of tight junctions in intestinal epithelial cells regarding glucose transport?

They form a barrier that confines apical transporters to the microvillus surface and prevents them from diffusing into basal-lateral domains.

What is the primary function of the $Ca^{2+}$ pump located in the sarcoplasmic reticulum membrane of skeletal muscle cells?

It returns $Ca^{2+}$ from the cytosol back into the sarcoplasmic reticulum lumen to end muscle contraction.

Which specific amino acid residue on the phosphorylation domain of the sarcoplasmic reticulum $Ca^{2+}$ pump accepts the phosphate from $ATP$?

An aspartic acid residue.

Why is $Ca^{2+}$ described as the most tightly controlled ion in the cell?

Its resting cytosolic concentration is low (10^-4 nM), allowing brief cytosolic increases to serve as powerful intracellular signals.

What chemical lining inside the $K^+$ channel selectivity filter confers ion selectivity?

It is lined by carbonyl oxygens that bear a partial negative charge and are precisely positioned to coordinate dehydrated $K^+$ ions.

What does it mean that ion channels are gated rather than continuously open?

They switch between closed and open states in response to specific chemical, electrical, or mechanical stimuli.

What are the three main mechanisms of gating found in gated ion channels?

Voltage gating (controlled by membrane potential), ligand gating (controlled by ligand binding), and mechanical gating (controlled by mechanical stress/force).

Where can regulatory ligands bind on ligand-gated ion channels?

Either on the extracellular face (extracellular ligand) or on the cytosolic face (intracellular ligand).

What is the subunit composition and channel type of the nicotinic acetylcholine receptor?

It is a pentamer composed of 5 subunits that functions as a ligand-gated cation channel (Na+).

How many acetylcholine ($ACh$) molecules must bind to activate and open the nicotinic acetylcholine receptor?

$2$ acetylcholine molecules.

How are mechanically-gated cation channels activated in the auditory hair cells of the inner ear?

Sound vibrations tilt the stereocilia bundle, pulling on linking filaments that physically pull open the cation channels.

What is the typical resting membrane potential of animal cells, particularly neurons?

-60 mV

How do $K^+$ leak channels generate the negative resting membrane potential?

They randomly flicker between open and closed, allowing $K^+$ to flow out of the cell down its concentration gradient and leaving unbalanced negative charges behind.

What are the four principal structural components of a typical neuron?

Cell body, dendrites, axons, and terminal branches of the axon endings.

What animal model was famously utilized in early research to uncover the mechanisms of electrical signaling and action potentials?

The giant axon of the squid.

What is an action potential in a nerve cell?

A self-sustaining, all-or-nothing spike in electrical activity propagated rapidly along the membrane of an axon.

What approximate threshold potential must a depolarizing stimulus reach to trigger an action potential in a neuron resting at $-60\,\text{mV}$?

Approximately $-40\,\text{mV}$.

To what membrane potential value does the cell interior depolarize during the peak of an action potential?

It depolarizes from -60 mV up to approximately +40 mV to +50 mV.

What three distinct conformations does a voltage-gated $Na^+$ channel adopt during an action potential cycle?

Closed, open, and inactivated.

What is the refractory period of an axon membrane, and what causes it?

A period during which an axon cannot fire another action potential, caused by voltage-gated $Na^+$ channels adopting an inactivated conformation that cannot reopen until the membrane repolarizes.

Why do action potentials propagate unidirectionally down the axon away from the cell body?

Voltage-gated Na+ channels are temporarily inactivated, preventing backward propagation.

What specific ion channel opens at the presynaptic nerve terminal upon arrival of an action potential to trigger neurotransmitter release?

Voltage-gated Ca+2 channels.

How is a chemical neurotransmitter signal converted back into an electrical signal at the postsynaptic membrane?

Released neurotransmitters bind to transmitter-gated ion channels on the postsynaptic cell, opening them to permit ion flow that changes the membrane potential.

How do excitatory synapses differ from inhibitory synapses in terms of ion flow and action potential probability?

Excitatory synapses open $Na^+$ channels, causing $Na^+$ influx and depolarization to increase firing likelihood; inhibitory synapses open $Cl^-$ channels, causing $Cl^-$ influx that keeps the membrane polarized and decreases firing likelihood.

What is the typical location and physiological function of voltage-gated $K^+$ channels according to Table 12-3?

Located in the plasma membrane of nerve cell axons; their function is the return of the membrane to resting potential after initiation of an action potential.

According to Table 12-3, which two receptors function as transmitter-gated $Cl^-$ channels mediating inhibitory synaptic signaling?

The GABA receptor and the glycine receptor.

Which receptors serve as transmitter-gated cation channels mediating excitatory synaptic signaling at neuron synapses according to Table 12-3?

Glutamate receptors.

What ion channel is targeted by the cardiovascular drugs Nifedipine (Procardia) and Verapamil (Verelan), and for which disease indications?

L-type $Ca_V$ channels; indicated for angina, hypertension, and arrhythmia.

Which ion channels are targeted by Benzodiazepines (Valium), Topiramate (Topamax), Gabapentin (Neurontin), and Lidocaine?

Benzodiazepines target GABA-gated Cl- channels; Topiramate targets brain Na; Gabapentin targets Ca; Lidocaine targets Na.

Which channels are targeted by Glibenclamide (Glimepiride) and Lubiprostone, and for which indications?

Glibenclamide targets $K(\text{ATP})$ channels for diabetes; Lubiprostone targets CLC2 channels for constipation.

What physical relationship does electrophysiology measure using glass microelectrodes on living cells?

It measures electrical current or voltage changes across cell membranes, governed by Ohm's Law: V = I R

According to 'The ultimate brain workout,' which brain areas and cognitive benefits are linked to aerobic exercise versus lifting weights?

Aerobic exercise stimulates the hippocampus to enhance memory, whereas lifting weights engages the prefrontal cortex for complex thinking, reasoning, multitasking, and problem-solving.

In the neurobiology study by Joseph et al. (1999), which dietary supplement significantly improved motor performance on both the rod walking and rotarod tests in aged rats?

Blueberry supplementation (outperforming control, strawberry, and spinach diets).

According to the Seattle Longitudinal Studies (Willis et al.), at what life stage does brain function peak in logic, vocabulary, verbal memory, and spatial skills?

In late middle age (late 50s for men and 60s for women).