Membrane Transport

Membrane Transport for the MCAT

Part 1 of 4 — Membrane Architecture & Permeability

The Fluid Mosaic Model

The plasma membrane is a phospholipid bilayer with embedded proteins — a two-dimensional fluid in which lipids and many proteins diffuse laterally.

• Lateral diffusion (within one leaflet) is fast and common
• Transverse diffusion ("flip-flop") between leaflets is rare and slow — it requires a polar head to cross the hydrophobic core; enzymes called flippases catalyze it
• Membrane asymmetry matters: phosphatidylserine (PS) is normally on the inner leaflet; PS appearing on the outer leaflet is an "eat me" signal in apoptosis

What Crosses a Pure Lipid Bilayer?

Permeability is governed by size and polarity/charge:

Rule of thumb: charge is the strictest barrier. Even a proton — the smallest ion — cannot cross the hydrophobic core unaided.

The Partition Coefficient

A molecule's oil/water partition coefficient ($K$) predicts how readily it dissolves into the bilayer:

$P \propto \frac{K \cdot D}{\Delta x}$

where $P$ is permeability, $D$ is the diffusion coefficient within the membrane, and $\Delta x$ is membrane thickness. High $K$ (lipophilic) → high permeability. This is why steroid hormones (lipophilic) enter cells directly and bind intracellular receptors, while peptide hormones (hydrophilic) must bind cell-surface receptors.

Membrane Fluidity — What Changes It?

• Unsaturated fatty acids (cis double bonds → kinks) → more fluid
• Shorter tails → more fluid
• Higher temperature → more fluid
• Cholesterol → bidirectional buffer (moderates fluidity in both directions)

Membrane Structure & Permeability 🎯

Experimental Evidence for Membrane Fluidity — FRAP

Fluorescence Recovery After Photobleaching (FRAP) is the classic assay MCAT passages use:

1. Label membrane proteins or lipids with a fluorescent tag
2. Photobleach a small spot with an intense laser (destroys fluorescence there)
3. Watch the spot: if the membrane is fluid, unbleached molecules diffuse in and fluorescence recovers

• Fast, complete recovery → freely diffusing (mobile) molecules

• Slow or incomplete recovery → molecules anchored to the cytoskeleton or confined to domains

• Fusing a mouse cell with a human cell, each with differently labeled surface proteins, showed the proteins intermix within ~40 minutes — direct evidence of lateral mobility (mixing slows at low temperature)

• This is the Frye-Edidin experiment

Lipid Rafts and Anchored Proteins

• Lipid rafts: cholesterol- and sphingolipid-rich microdomains that are thicker and more ordered; they concentrate signaling proteins
• Some proteins are held at the membrane by a covalently attached lipid anchor rather than spanning the bilayer

• Lipid-anchor types: GPI anchors hold proteins on the outer leaflet; prenylation and palmitoylation hold them on the inner leaflet

• Cytoskeletal tethering restricts protein diffusion — a common explanation for incomplete FRAP recovery

• The textbook tether is the spectrin-ankyrin network under the red blood cell membrane

Why Membrane Asymmetry Is Maintained

• New phospholipids are synthesized on the cytosolic leaflet of the ER
• Flippases (ATP-dependent) move specific lipids (like PS) to the cytosolic leaflet
• Loss of asymmetry (PS exposed outside) → recognized by macrophages → phagocytosis of the apoptotic cell

• During apoptosis, scramblases randomize the lipids between leaflets, which is how PS reaches the outer surface

Fluidity Experiments 🎯

Key Takeaways — Part 1

• Fluid mosaic model: amphipathic phospholipid bilayer + mobile proteins; lateral diffusion fast, flip-flop rare (flippase-catalyzed)
• Permeability hierarchy: small nonpolar >> small uncharged polar >> large polar >> ions (charge is the strictest barrier)
• High partition coefficient (lipophilic) → crosses bilayer → intracellular receptors (steroids); hydrophilic ligands need surface receptors
• Fluidity increases with unsaturation, shorter tails, higher temperature; cholesterol buffers fluidity in both directions
• FRAP: fluorescence recovery = lateral mobility; incomplete recovery = immobile (anchored) fraction
• Membrane asymmetry is active and meaningful: PS on the outer leaflet is an apoptotic "eat me" signal
• Carbohydrates (glycolipids/glycoproteins) always face the extracellular side

• Low-yield extras: the mouse-human cell fusion is the Frye-Edidin experiment; lipid anchors come as GPI (outer leaflet) or prenyl/palmitoyl groups (inner leaflet); spectrin-ankyrin is the red-cell tether; scramblases randomize the leaflets during apoptosis

Membrane Transport for the MCAT

Part 2 of 4 — Passive Transport & Osmosis

Passive Transport: Down the Gradient, No ATP

All passive transport moves solutes down their electrochemical gradient ($\Delta G < 0$), so no energy input is required.

Channels vs. Carriers — The Kinetic Signature

• Channels form a continuous aqueous pore; ions flow at near-diffusion rates, orders of magnitude faster than carriers. They can be gated: voltage-gated (Na$^+$ channels in neurons), ligand-gated (nicotinic ACh receptor), or mechanically gated
• Carriers bind the solute, change conformation, and release it on the other side, so each cycle is slow. Because binding sites are limited, carrier transport shows saturation kinetics just like an enzyme:

$v = \frac{V_{max}[S]}{K_m + [S]}$

MCAT experimental signature: plot flux vs. concentration. A straight line = simple diffusion; a hyperbola that plateaus = carrier-mediated. Competitive inhibition by a structural analog also implicates a carrier.

• Typical turnover numbers: channels pass about $10^6$-$10^8$ ions/s; carriers move about $10^2$-$10^4$ molecules/s

• GLUT1 (most cells, high basal affinity) vs. GLUT4 (muscle/adipose, insulin-inserted from intracellular vesicles) vs. GLUT2 (liver/pancreatic beta cells, low affinity/high $K_m$ — acts as a glucose sensor)

Osmosis and Tonicity

Osmosis = passive movement of water across a semipermeable membrane toward the compartment with higher total solute (osmolarity). Water moves through the bilayer slowly and through aquaporins rapidly.

• Tonicity counts only impermeant solutes. Urea crosses membranes, so a urea solution that is iso-osmotic is functionally hypotonic — cells still swell
• Osmotic pressure (a colligative property): $\Pi = iMRT$, where $i$ is the van't Hoff factor (NaCl → $i \approx 2$; glucose → $i = 1$)

Equilibrium vs. Steady State

• Passive transport runs until the electrochemical gradient is dissipated (for ions, both concentration AND membrane voltage matter)
• A charged solute can be at equilibrium with unequal concentrations if the membrane potential balances the chemical gradient — the reason K$^+$ stays concentrated inside cells near the resting potential

Passive Transport 🎯

Interpreting Flux Curves — The MCAT's Favorite Graph

Aquaporins and ADH — Regulated Water Permeability

• Aquaporins are water-selective channels; they exclude ions (even H$^+$, via electrostatic barriers)
• Aquaporin-2 in kidney collecting-duct cells is inserted into the apical membrane in response to ADH (vasopressin) acting through a GPCR/cAMP pathway
• Diabetes insipidus logic: no ADH (central) or no receptor/AQP2 response (nephrogenic) → water cannot be reabsorbed → dilute urine, polyuria

Ion Gradients Store Energy

Typical mammalian cell gradients (approximate):

These gradients (built by pumps — Part 3) are potential energy: they power secondary active transport, action potentials, and neurotransmitter reuptake. The enormous (more than 10,000-fold) Ca$^{2+}$ gradient is why tiny Ca$^{2+}$ leaks make excellent signals.

Osmosis & Flux Analysis 🎯

Key Takeaways — Part 2

• Passive transport moves solutes down electrochemical gradients without ATP: simple diffusion, channels, carriers
• Carriers saturate (Michaelis-Menten-like kinetics) and show competitive inhibition; simple diffusion is linear and non-saturable
• GLUT1 = basal uptake; GLUT4 = insulin-triggered vesicle insertion (muscle/adipose); GLUT2 = high-$K_m$ glucose sensor (liver, beta cells)
• Osmosis: water toward higher impermeant-solute concentration; tonicity ≠ osmolarity (permeant solutes like urea don't count)
• $\Pi = iMRT$: count particles (NaCl ≈ 2, glucose = 1)
• Aquaporin-2 insertion is ADH-regulated — the basis of diabetes insipidus reasoning
• Ion gradients (high Na$^+$ out, high K$^+$ in, very low intracellular Ca$^{2+}$) are stored energy for transport and signaling

• Low-yield extras: turnover numbers — channels about $10^6$-$10^8$ ions/s, carriers about $10^2$-$10^4$ molecules/s

Membrane Transport for the MCAT

Part 3 of 4 — Active Transport & the Na$^+$/K$^+$ ATPase

Primary Active Transport — ATP Pays Directly

Primary active transport moves solutes against their electrochemical gradient using ATP hydrolysis directly.

The Na$^+$/K$^+$ ATPase (the pump to know cold):

$3\ \text{Na}^+_{in} + 2\ \text{K}^+_{out} + \text{ATP} \to 3\ \text{Na}^+_{out} + 2\ \text{K}^+_{in} + \text{ADP} + \text{P}_i$

• 3 Na$^+$ out, 2 K$^+$ in per ATP — net export of one positive charge → the pump is electrogenic (contributes a few mV to the negative resting potential; the K$^+$ leak channels do most of the work)
• Mechanism: phosphorylation of the pump drives conformational changes that alternate ion-binding access between the two sides

• That phosphorylated intermediate makes it a P-type ATPase, the pump family that also includes the Ca$^{2+}$ and gastric H$^+$/K$^+$ pumps

• Inhibited by ouabain and digoxin (cardiac glycosides) from the extracellular side
• Maintains the Na$^+$ gradient that powers secondary transport, osmotic balance (preventing cell swelling), and the resting membrane potential

Other primary pumps:

• The best-known multidrug-resistance pump is MDR1, also called P-glycoprotein

Secondary Active Transport — Riding the Na$^+$ Gradient

Secondary (coupled) transport uses the energy stored in one gradient (almost always Na$^+$, built by the Na$^+$/K$^+$ ATPase) to move another solute uphill. No ATP is hydrolyzed by the transporter itself — but the process is still ultimately ATP-dependent.

The Intestinal Glucose Circuit — A Complete MCAT Story

1. Basolateral Na$^+$/K$^+$ ATPase keeps intracellular Na$^+$ low
2. Apical SGLT1 symports 2 Na$^+$ (downhill) + 1 glucose (uphill) from the gut lumen — glucose can be concentrated far above luminal levels
3. Basolateral GLUT2 lets glucose exit passively into the blood
4. Water follows the absorbed solute osmotically

This is why oral rehydration therapy contains both glucose AND salt: SGLT1 needs both, and the co-absorbed solute pulls water out of the gut lumen — lifesaving in cholera.

Active Transport 🎯

Energetics — Is Transport Uphill or Downhill?

For an uncharged solute, the free energy of moving it from outside to inside:

$\Delta G = RT \ln \frac{[S]_{in}}{[S]_{out}}$

For an ion, add the electrical term:

$\Delta G = RT \ln \frac{[S]_{in}}{[S]_{out}} + zF\Delta V$

• $z$ = ion charge, $F$ = Faraday constant, $\Delta V$ = membrane potential (inside relative to outside)
• Na$^+$ entry is doubly downhill (chemical gradient inward AND negative interior attracts cations) — a rich energy source
• One ATP (~$-30$ kJ/mol under standard conditions, roughly $-50$ kJ/mol in the cell) can drive substantial concentration work; coupling stoichiometry (2 Na$^+$ per glucose in SGLT1 vs. 1 in SGLT2) sets how steep a glucose gradient can be built

CFTR and Cystic Fibrosis — Transport Pathophysiology

• CFTR is an ABC-family, ATP-gated Cl$^-$ channel on the apical surface of epithelia, activated by cAMP/PKA phosphorylation
• In cystic fibrosis, functional CFTR is missing from the apical membrane → no apical Cl$^-$ secretion
• The most common CF mutation, $\Delta$F508, is an in-frame three-nucleotide deletion that removes one phenylalanine; the protein misfolds and is degraded by ER quality control instead of reaching the membrane
• Airway logic: no Cl$^-$ (and water) secretion + excess Na$^+$ absorption → dehydrated, thick mucus → infections
• Sweat gland logic runs the other way: CFTR normally reabsorbs Cl$^-$ from sweat → CF patients have salty sweat (the diagnostic sweat chloride test)

Cholera — The Opposite Lesion

Cholera toxin locks Gs "on" → cAMP soars → CFTR is hyperactivated → massive Cl$^-$ and water secretion into the gut → secretory diarrhea. Treated with oral rehydration solution because SGLT1 (a separate, unaffected pathway) can still pull Na$^+$, glucose, and water back in.

Transport Energetics & Disease 🎯

Key Takeaways — Part 3

• Primary active transport hydrolyzes ATP directly; Na$^+$/K$^+$ ATPase: 3 Na$^+$ out / 2 K$^+$ in per ATP, electrogenic, ouabain/digoxin-inhibited
• Other pumps: SERCA/PMCA (Ca$^{2+}$), gastric H$^+$/K$^+$ ATPase (PPI target), V-type H$^+$ (lysosomes), ABC family (multidrug-resistance pumps, CFTR)
• Secondary active transport spends the Na$^+$ gradient: symport (SGLT1: 2 Na$^+$ + glucose) or antiport (Na$^+$/Ca$^{2+}$ exchanger: 3 Na$^+$ : 1 Ca$^{2+}$)
• Poisoning the Na$^+$/K$^+$ pump collapses ALL Na$^+$-coupled transport — expect this experimental logic
• Digoxin chain: pump blocked → Na$^+$ up → NCX slows → Ca$^{2+}$ up → stronger contraction
• Ion transport energetics include both chemical and electrical terms; higher coupling stoichiometry → steeper achievable gradients
• CFTR: ATP-gated Cl$^-$ channel; CF = too little secretion (thick mucus, salty sweat); cholera = too much (watery diarrhea, treated via SGLT1-based oral rehydration)

• Low-yield extras: the Na$^+$/K$^+$ pump is a P-type ATPase (phosphorylated intermediate); the classic multidrug-resistance pump is MDR1/P-glycoprotein

Membrane Transport for the MCAT

Part 4 of 4 — Vesicular Transport & MCAT Integration

Bulk Transport — Moving Cargo Too Big for Proteins

Vesicular (bulk) transport is always active (requires ATP/GTP) and moves material without it ever crossing a membrane directly.

Receptor-Mediated Endocytosis: The LDL Paradigm

The LDL receptor story is the canonical MCAT passage:

1. LDL particles bind LDL receptors clustered in clathrin-coated pits
2. The pit invaginates and pinches off (the GTPase dynamin severs the neck)
3. The clathrin coat is shed; the vesicle fuses with an early endosome
4. Endosomal acidification (V-type H$^+$ ATPase) releases LDL from its receptor
5. Receptors recycle to the surface; LDL is delivered to lysosomes, where cholesteryl esters are hydrolyzed to free cholesterol

(The pathway was worked out by Michael Brown and Joseph Goldstein, Nobel Prize 1985.)

Familial hypercholesterolemia mutations map beautifully onto this pathway:

• No receptor made → no binding, no uptake
• Receptor binds LDL but has a defective cytoplasmic tail → cannot cluster in coated pits → binding without internalization
• Either way: plasma LDL rises → early atherosclerosis

SNAREs and Regulated Exocytosis

• v-SNAREs (on the vesicle) pair with t-SNAREs (on the target membrane) to zipper membranes together
• In neurons, an action potential opens voltage-gated Ca$^{2+}$ channels; Ca$^{2+}$ binds a sensor protein on the vesicle, triggering fusion within a millisecond

• Named SNAREs: synaptobrevin (v-SNARE); syntaxin and SNAP-25 (t-SNAREs). The Ca$^{2+}$ sensor is synaptotagmin.

• Botulinum and tetanus toxins are proteases that cleave SNAREs → block neurotransmitter release (flaccid paralysis for botulinum at the neuromuscular junction; spastic paralysis for tetanus, which blocks inhibitory interneurons)

Membrane Accounting

• Endocytosis removes plasma membrane; exocytosis adds it — cells balance the two to keep surface area constant
• The lumen of a vesicle is topologically equivalent to the outside of the cell: a protein's extracellular domain faces the vesicle lumen after endocytosis, and a secreted protein never touches the cytosol

Vesicular Transport 🎯

MCAT Passage Toolkit — Transport Experiments

• Hypertonic sucrose or K$^+$ depletion disrupts the clathrin lattice, blocking receptor-mediated endocytosis specifically

Integrating the Whole Topic: One Epithelial Cell

A single intestinal enterocyte uses nearly every transport mode at once:

• Apical: SGLT1 (secondary active), amino acid symporters, facilitated fructose entry
• Basolateral: Na$^+$/K$^+$ ATPase (primary active), GLUT2 (facilitated)
• Both: aquaporins for transcellular water flow; tight junctions force transport through (not around) the cell
• Lipids: absorbed by simple diffusion/micelle delivery, re-esterified, packaged into chylomicrons, and exocytosed into lymph

• The apical fructose carrier is GLUT5

When a passage gives you a polarized epithelium, immediately sketch which transporter sits on which membrane — the answer usually falls out of the geometry.

MCAT Integration 🎯

Key Takeaways — Part 4

• Bulk transport is always active: phagocytosis (particles), pinocytosis (fluid), receptor-mediated endocytosis (specific, clathrin), exocytosis (constitutive or Ca$^{2+}$-regulated)
• LDL pathway: coated pit → dynamin scission → endosome acidification releases cargo → receptor recycles, cargo to lysosome; FH mutations can break binding OR internalization separately
• SNARE pairing (v- + t-SNARE) drives fusion; a Ca$^{2+}$ sensor triggers regulated release; botulinum/tetanus toxins cleave SNAREs
• Vesicle lumen ≡ extracellular space: topology never flips through the secretory/endocytic pathways
• Experiment signatures: 4°C blocks internalization but not binding; ATP depletion kills active + vesicular transport; a nonhydrolyzable GTP analog stalls vesicles at the pit
• Polarized epithelia: apical vs. basolateral transporter placement (SGLT1/GLUT2/Na$^+$-K$^+$ pump) explains directional absorption
• Endocytosis and exocytosis are balanced to conserve membrane surface area

• Low-yield extras: Brown and Goldstein worked out the LDL pathway; named SNAREs are synaptobrevin (v), syntaxin and SNAP-25 (t); synaptotagmin is the Ca$^{2+}$ sensor; hypertonic sucrose or K$^+$ depletion disrupts clathrin; GLUT5 is the apical fructose carrier