Cell Membranes and Membrane Transport

Comprehensive practice flashcards covering cell membrane composition, lipid bilayer properties, phase transition, membrane proteins, active and passive transport, ATP-driven pumps, electrochemical gradients, and channel gating mechanisms.

What are the primary biological functions of cellular membranes?

They enclose reaction compartments, maintain chemical concentration gradients, determine selective permeability for what enters and exits, and provide an interactive surface with the surrounding environment.

What thermodynamic force drives the spontaneous assembly of lipid bilayers in an aqueous environment?

Hydrophobic interactions (the hydrophobic effect), which minimize the thermodynamically unfavorable exposure of nonpolar hydrocarbon tails to the water dipole.

What is the relationship between the terms 'amphiphilic' and 'amphipathic'?

They are synonymous terms describing molecules possessing both hydrophilic (polar) and hydrophobic (nonpolar) parts.

What four basic chemical components make up a phospholipid molecule?

A glycerol backbone, two fatty acyl groups, a phosphate group, and a polar head group.

Which four headgroups are commonly found attached to the phosphate group in phosphoglycerides?

Ethanolamine ($\text{PE}$), choline ($\text{PC}$), serine ($\text{PS}$), and inositol ($\text{PI}$).

What net charge does phosphatidylserine ($\text{PS}$) carry, and where is it predominantly localized in the plasma membrane?

It carries a negative charge and is predominantly localized to the inner (cytosolic) leaflet of the lipid bilayer, producing a charge difference across the bilayer.

Which two major lipids are primarily concentrated in the outer leaflet of the plasma membrane?

Phosphatidylcholine and sphingomyelin.

How does cholesterol affect lipid bilayer fluidity at high and low temperatures?

Cholesterol intercalates between acyl chains to decrease fluidity at high temperatures and increase fluidity at low temperatures, maintaining relatively constant fluidity across temperature ranges.

In which cellular membranes is cholesterol present, and where is it absent?

It is present in eukaryotic plasma membranes, but absent in prokaryotic membranes and intracellular organellar membranes.

Which two factors determine the phase transition of a lipid bilayer between an ordered gel phase and a disordered liquid crystalline phase?

The fatty acyl chain composition (chain length and degree of saturation) and temperature.

What is the saturated fatty acid content of butter compared to olive oil?

Butter contains $51\%$ saturated fatty acids (making it solid at room temperature), whereas olive oil contains $14\%$ saturated fatty acids (keeping it liquid).

What is the rate of lateral diffusion of lipids within a single membrane leaflet?

Lateral diffusion is rapid, occurring on a timescale of $10^{-8}\,\text{s}$, allowing a lipid to traverse the length of an entire bacterial cell in approximately $1\,\text{s}$ at $37^\circ\text{C}$.

Why is transverse diffusion (flip-flop) of lipids extremely slow compared to lateral diffusion, and how is it accelerated in cells?

It takes $6\text{ to }20\,\text{hours}$ because the polar headgroup must shed its extensive hydration shell to cross the hydrophobic core; it is catalyzed by flippase enzymes.

What is the typical thickness of a biological lipid bilayer?

$4\text{ to }6\,\text{nm}$.

What is the typical size range of liposomes (vesicles) formed from lipid bilayers?

They range in diameter from $50\,\text{nm}$ to $>10\,\mu\text{m}$.

What are the typical mass percentages of lipids and proteins in biological membranes?

Lipids account for $25-50\%$ by mass, and proteins account for $50-75\%$ by mass.

What is the glycocalyx and what are its constituents?

A carbohydrate-rich outer surface layer composed of oligosaccharide chains attached to transmembrane glycoproteins, glycolipids, and transmembrane proteoglycans.

Identify the three structural classes of integral membrane proteins shown in the diagram.

Single $\alpha$-helix (recognition/receptors), helical bundle (enzymes/transporters/receptors), and $\beta$-barrel (transporters/channel proteins).

What four structural mechanisms allow peripheral membrane proteins to associate with the lipid bilayer?

1. Amphiphilic $\alpha$-helices; 2. Hydrophobic loops; 3. Lipid modifications/tails (lipid anchors); 4. Electrostatic or ionic interactions.

Identify the three lipid aggregates illustrated in the diagram.

Liposome (lipid bilayer vesicle enclosing an aqueous center), micelle (spherical monolayer of amphiphiles), and planar bilayer sheet.

How does membrane compartmentalization differ between prokaryotes and eukaryotes?

Prokaryotes possess a plasma membrane (and sometimes an outer protective membrane separated by a periplasm) without cholesterol, whereas eukaryotes feature a plasma membrane and internal membrane-bound compartments (such as mitochondria, nucleus, and endoplasmic reticulum).

How does permeability across a pure lipid bilayer differ between nonpolar gases and inorganic ions?

Nonpolar gases ($\text{CO}_2$, $\text{O}_2$) freely permeate the hydrophobic core by simple diffusion, whereas inorganic ions ($\text{Na}^+$, $\text{K}^+$, $\text{Cl}^-$) are virtually impermeable due to their charge and hydration shell.

What are the operational differences between channel proteins and transporter/carrier proteins?

Channels form aqueous pores open to both sides simultaneously, are gated, and conduct rapidly ($>10^6\,\text{ions/s}$); carriers bind specific solutes and undergo alternating conformational changes.

What percentage of membrane proteins are dedicated to transport, and what is their general membrane-spanning topology?

$15-30\%$ of membrane proteins encode transport proteins, and all membrane transport proteins are multipass transmembrane proteins.

What percentage of total cellular ATP is utilized to establish and maintain $\text{Na}^+$ and $\text{K}^+$ gradients?

$30-70\%$ of total cellular ATP.

What condition marks the endpoint of passive transport across a selectively permeable membrane?

Net transport continues until the chemical potential is dissipated, meaning the concentration outside equals the concentration inside ($c_o = c_i$).

What distinguishes active transport from passive transport?

Active transport moves solutes against their concentration or electrochemical gradient and requires an input of metabolic energy, whereas passive transport proceeds down the gradient without energy expenditure.

What are the three major classes of primary ATP-driven transport pumps?

$\text{P}$-type pumps, $\text{F}$-type (and $\text{V}$-type) proton pumps, and $\text{ABC}$ transporters.

What is the function of the ABC transporter P-glycoprotein in cells?

It utilizes ATP binding and hydrolysis to export toxic molecules and xenobiotics out of the cell against their concentration gradient.

What is the standard free energy change ($\Delta G^{\circ \prime}$) for the hydrolysis of ATP into ADP and $\text{P}_i$?

$-32\,\text{kJ}\,\text{mol}^{-1}$ (or $-7.3\,\text{kcal}\,\text{mol}^{-1}$).

Identify the three modes of carrier-mediated transport shown in the diagram.

Uniport (transports a single molecule), symport (transports two different molecules in the same direction), and antiport (transports two different molecules in opposite directions).

How does the $\text{Na}^+$-glucose symporter power the active uptake of glucose?

It couples the energetically favorable movement of $\text{Na}^+$ down its electrochemical gradient to drive glucose uphill against its concentration gradient.

What does cooperative binding mean in the context of coupled co-transporters?

The binding of one solute enhances the binding affinity of the other solute; if either solute is absent, the other fails to bind and transport cannot proceed.

How do apical and basolateral transporters work together during glucose absorption across the intestinal epithelium?

An apical $\text{Na}^+$-driven symporter actively imports glucose from the lumen; a basolateral uniport carrier passively exports glucose into extracellular fluid; and a basolateral $\text{Na}^+/\text{K}^+$ pump maintains the $\text{Na}^+$ gradient.

What formula defines the Nernst equation for calculating the equilibrium membrane potential for an ion?

$V = \frac{RT}{zF} \ln\left(\frac{c_o}{c_i}\right)$, where $R$ is the gas constant, $T$ is temperature in Kelvin, $z$ is ion valence, $F$ is Faraday's constant, and $c_o$ and $c_i$ are extracellular and intracellular ion concentrations.

What are the typical cytoplasmic versus extracellular concentrations of $\text{Na}^+$ and $\text{K}^+$ in mammalian cells?

$\text{Na}^+$: $5-15\,\text{mM}$ cytoplasmic vs. $145\,\text{mM}$ extracellular; $\text{K}^+$: $140\,\text{mM}$ cytoplasmic vs. $5\,\text{mM}$ extracellular.

What are the typical free cytoplasmic versus extracellular concentrations of $\text{Ca}^{2+}$ and $\text{Cl}^-$ in mammalian cells?

Free $\text{Ca}^{2+}$: $10^{-4}\,\text{mM}$ cytoplasmic vs. $1-2\,\text{mM}$ extracellular; $\text{Cl}^-$: $5-15\,\text{mM}$ cytoplasmic vs. $110\,\text{mM}$ extracellular.

What is the pumping stoichiometry and ATP usage of the $\text{Na}^+/\text{K}^+$ ATPase pump per operational cycle?

It pumps $3\,\text{Na}^+$ ions out of the cell and $2\,\text{K}^+$ ions into the cell per $1\,\text{ATP}$ molecule hydrolyzed.

Identify the four gating mechanisms regulating ion channel permeability shown in the diagram.

Voltage-gated, ligand-gated (extracellular ligand), ligand-gated (intracellular ligand), and mechanically gated.

Which specific ion fluxes underlie the depolarisation and repolarisation phases of a neuronal action potential?

Depolarisation is caused by $\text{Na}^+$ influx through voltage-gated $\text{Na}^+$ channels (shifting potential from $-70\,\text{mV}$ toward $+30\text{ to }+40\,\text{mV}$); repolarisation is driven by $\text{K}^+$ efflux through voltage-gated $\text{K}^+$ channels.