Chapter 8: Biological Membranes

The fluid mosaic model (Singer and Nicolson, 1972) is the standard picture of a cell membrane. Two leaflets of phospholipids with embedded proteins. “Fluid” because lipids and proteins can move laterally; “mosaic” because proteins are scattered throughout the lipid sea.

Components

Asymmetry

The two leaflets are not identical. Phosphatidylcholine and sphingomyelin are in the outer leaflet. Phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol are in the inner leaflet. Glycans are always on the outer leaflet. Flippases and floppases (ATP-driven) maintain this asymmetry. Apoptosis flips phosphatidylserine to the outer leaflet as an “eat me” signal.

Membrane Fluidity

Fluidity depends on:

• Temperature: higher T → more fluid.
• Saturation: more cis double bonds (kinks) → more fluid.
• Chain length: shorter tails → more fluid.
• Cholesterol: bidirectional buffer - reduces fluidity at high T, prevents gel formation at low T.

Lipid Rafts

Special microdomains enriched in cholesterol, sphingolipids, and certain proteins. Rafts are more ordered than the surrounding membrane. Many signaling receptors cluster in rafts, concentrating their downstream signaling partners together.

Half of every plasma membrane by mass is protein. Proteins do most of the work: transport, signaling, catalysis, structural anchoring. They sit in the membrane in specific ways depending on their structure.

Integral vs. Peripheral

• Integral membrane proteins: embedded in the bilayer, held there by hydrophobic amino acid interactions with the lipid tails. They can only be removed by disrupting the membrane (detergents). Most span the entire membrane (transmembrane proteins).
• Peripheral membrane proteins: stuck to the surface via interactions with integral proteins or with lipid head groups. They can be removed by mild treatments (salt washes, pH changes) without disrupting the membrane.

Transmembrane Structure

A transmembrane protein crosses the bilayer through one or more membrane-spanning segments. Two common structural motifs:

• Alpha helices: the most common transmembrane motif. A 20-residue hydrophobic alpha helix is long enough to span the 30 Å hydrophobic core of the bilayer. Examples: most receptors, channels, transporters.
• Beta barrels: stacks of antiparallel beta strands form a cylindrical pore. Rare in the plasma membrane but common in the outer membranes of bacteria, mitochondria, and chloroplasts. Example: porins.

Lipid-Anchored Proteins

Some proteins that are not transmembrane nonetheless attach to the membrane via a covalent lipid modification. Examples include:

• GPI anchor (glycosylphosphatidylinositol): links the protein’s C-terminus to the outer leaflet via a GPI moiety. Prion protein (PrP) is GPI-anchored.
• Myristoylation: 14-carbon myristate attached to the N-terminal glycine. Signaling kinases like Src.
• Palmitoylation: 16-carbon palmitate attached to cysteine. Reversible and regulatory.
• Prenylation: attachment of a farnesyl (15C) or geranylgeranyl (20C) isoprenoid. Small GTPases like Ras.

Four Functional Classes

1. Transporters and channels: move molecules across the membrane. Covered in the next sections.
2. Receptors: bind extracellular signals and transmit them inside (GPCRs, RTKs, ion channel receptors).
3. Enzymes: catalyze reactions at the membrane (adenylyl cyclase, phospholipase C, acetylcholinesterase).
4. Anchors and structural proteins: connect the membrane to the cytoskeleton or extracellular matrix (integrins, cadherins, spectrin-ankyrin network).

Passive transport moves substances down their concentration or electrochemical gradient without consuming energy. Two varieties: simple diffusion (through the lipid bilayer) and facilitated diffusion (through a protein channel or carrier).

Simple Diffusion

Small, nonpolar molecules cross the bilayer by dissolving through it. O2, CO2, N2, steroid hormones, small uncharged molecules like ethanol - all cross freely. Rate depends on concentration gradient, temperature, and the molecule’s lipid solubility.

Charged molecules and large polar molecules (ions, glucose, amino acids) cannot cross the hydrophobic core effectively. They need a protein to help.

Facilitated Diffusion

Still passive (down the gradient, no ATP), but requires a protein. Two types of proteins:

• Channels: hollow tunnels. Ions or small polar molecules flow through. Rate is fast ($10^{7}$-$10^{8}$ per second). Channels can be gated (open/close in response to voltage, ligand, or mechanical stimulus). Examples: voltage-gated Na+ channels, aquaporins for water.
• Carriers: bind the substrate, change conformation, release it on the other side. Slower than channels ($10^{2}$-$10^{4}$ per second) but more selective. Examples: GLUT transporters for glucose, GABA transporters.

Factors Affecting Rate

• Gradient: steeper gradient, faster transport.
• Temperature: higher T, faster (Arrhenius).
• Surface area: more membrane, more transporters, faster.
• Molecule properties: for simple diffusion, smaller and more lipophilic = faster. For facilitated diffusion, specificity matters.

Aquaporins

Water can slowly diffuse through the bilayer but much faster through aquaporins - protein channels dedicated to water. Kidneys and some brain regions have high aquaporin expression and can move water rapidly across membranes. Antidiuretic hormone (ADH) inserts aquaporin-2 into collecting duct cells, concentrating urine.

Active transport moves molecules against their gradient. This requires energy, either directly from ATP (primary) or from another ion gradient (secondary).

Primary Active Transport

The pump uses ATP directly. Classic example: Na+/K+ ATPase.

Each cycle:

1. Pump binds 3 Na+ from inside + 1 ATP.
2. ATP hydrolysis phosphorylates the pump; it changes shape and expels 3 Na+ to outside.
3. Pump binds 2 K+ from outside.
4. Phosphate is released; the pump changes shape again and releases 2 K+ inside.

Net result per ATP: 3 Na+ out, 2 K+ in. Because 3 positive charges leave while only 2 enter, the pump is electrogenic and contributes directly to the resting membrane potential (more negative inside).

Other major ATP-driven pumps:

• Ca2+ ATPase (SERCA): pumps Ca2+ from cytoplasm into the ER. Keeps cytoplasmic Ca2+ extremely low (~100 nM) so small releases produce dramatic signaling.
• H+/K+ ATPase: pumps H+ into stomach lumen, producing gastric acid. Target of proton pump inhibitors.
• V-type ATPase: pumps H+ into lysosomes and other acidic compartments.

Secondary Active Transport

The transporter does NOT use ATP directly. Instead, it uses the energy stored in an ion gradient (usually Na+, built by the Na+/K+ ATPase). The Na+ moves down its gradient through the transporter, and the energy drags another molecule UP its gradient as a passenger.

Two classes:

• Symporter (cotransporter): Na+ and the co-transported molecule move in the SAME direction. Example: SGLT1 in intestinal cells absorbs glucose against its gradient by coupling to Na+ flowing down its gradient.
• Antiporter (exchanger): Na+ and the co-transported molecule move in OPPOSITE directions. Example: Na+/Ca2+ exchanger in heart cells pumps Ca2+ out using the Na+ gradient.

Osmosis is the movement of water across a semipermeable membrane from low solute concentration to high solute concentration. It is passive - driven by the concentration gradient of water itself. Tonicity describes what happens to a cell placed in a solution relative to the cell’s own interior.

Osmosis

A semipermeable membrane allows water through but blocks solute. Water moves from the side with less solute (high water concentration) to the side with more solute (low water concentration) until the solute concentrations equalize or until a physical pressure stops the flow.

Water can cross the bilayer directly at a slow rate, but most physiologic water movement goes through aquaporins - specialized channel proteins that let water pour through at much higher rates while excluding ions and other solutes. Kidney collecting-duct cells express aquaporin-2 under vasopressin (ADH) control, which is how urine is concentrated on demand.

Osmotic Pressure

The pressure needed to prevent osmosis is called osmotic pressure. For dilute solutions:

$\Pi = iMRT$

• Π = osmotic pressure.
• i = van’t Hoff factor (number of particles a solute dissociates into; NaCl → Na+ and Cl-, i = 2).
• M = molar concentration of solute.
• R = gas constant.
• T = absolute temperature.

A solution with more dissolved particles exerts more osmotic pressure. This is why IV solutions are designed to match blood osmolarity (~300 mOsm/L).

The Three Tonicities

Always stated from the perspective of the SOLUTION compared to the CELL’s cytoplasm.

| Tonicity | Solution vs. cell | Water flows | RBC outcome |
|----------|-------------------|-------------|-------------|
| Hypertonic | Higher solute | Water OUT of cell | Cell shrinks (crenation) |
| Hypotonic | Lower solute | Water INTO cell | Cell swells (lysis, hemolysis) |
| Isotonic | Equal solute | Balanced | No net change |

Plant Cells and Turgor Pressure

Plant cells have a rigid cell wall outside the membrane. When placed in a hypotonic solution, they absorb water and expand against the wall until the wall resists further expansion. That pressure inside the cell is turgor pressure - it is what keeps non-woody plants upright. In a hypertonic solution, plant cells lose water and the membrane pulls away from the wall (plasmolysis) - wilting.

Some cargoes are too big to cross the membrane. The cell wraps the membrane around them (endocytosis) or wraps the membrane around something inside to release it (exocytosis). Both use vesicles.

Endocytosis Types

Phagocytosis (“cell eating”)

Engulfing solid particles (bacteria, dead cells, debris). Used mostly by specialized immune cells (macrophages, neutrophils). The cell extends pseudopodia around the target, forming a large vesicle (phagosome). The phagosome fuses with a lysosome for degradation.

Pinocytosis (“cell drinking”)

Non-specific uptake of fluid and small dissolved molecules. The cell surface pinches in continuously, engulfing whatever fluid is nearby. No specific receptors involved. Happens constantly in most cells.

Receptor-Mediated Endocytosis

Highly specific. A receptor on the cell surface binds a specific ligand; the receptor-ligand complex clusters in a clathrin-coated pit; the pit pinches off into a vesicle coated with clathrin. The vesicle uncoats and enters the endosomal pathway. This is how cells efficiently import specific cargo (LDL, transferrin, many hormones) even when their blood concentrations are low.

Exocytosis

The reverse of endocytosis. An intracellular vesicle (usually carrying secretory protein cargo, like insulin, a neurotransmitter, or a hormone) fuses with the plasma membrane and releases its contents to the outside. SNARE proteins mediate the fusion. Exocytosis is how cells secrete proteins, discard waste, or deliver new membrane components to the cell surface.

Two main types:

• Constitutive: continuous, unregulated secretion. Delivers membrane lipids, ECM proteins, and constitutive secretory proteins.
• Regulated: vesicles wait near the membrane until a stimulus triggers fusion. Examples: neurotransmitter release at synapses (triggered by Ca2+), insulin release from beta cells (triggered by glucose), hormone secretion.

Most cells have a negative voltage inside (about -70 mV in neurons at rest). This membrane potential powers signaling in neurons and muscle, drives secondary active transport, and keeps membranes ready to respond to stimuli.

Why the Inside Is Negative

Two main reasons:

1. The Na+/K+ ATPase pumps 3 positive charges out per 2 positive charges in, directly making the inside more negative (electrogenic).
2. K+ has high cytoplasmic concentration and is the most permeable ion at rest (because K+ leak channels are open). K+ tends to flow down its gradient out of the cell, leaving behind a net negative charge inside. This sets the resting potential close to the K+ equilibrium potential.

The Nernst Equation

For a single ion at equilibrium, the voltage difference across the membrane that would stop net ion flow is the equilibrium potential (Eion). The Nernst equation calculates it:

$E_{\text{ion}} = \frac{RT}{zF} \ln\frac{[\text{ion}]_{\text{out}}}{[\text{ion}]_{\text{in}}}$

• R = gas constant, T = temperature in K, z = ion charge, F = Faraday constant.
• At 37°C, this simplifies to E (mV) = (61.5 / z) × log([out]/[in]) for a single ion.

At typical cellular concentrations:

• $E_{K}$ ≈ -90 mV
• $E_{\text{Na}}$ ≈ +60 mV
• $E_{\text{Cl}}$ ≈ -65 mV
• $E_{\text{Ca}}$ ≈ +120 mV

The Goldman Equation

The Nernst equation handles one ion at a time. The actual resting potential depends on all permeable ions. The Goldman-Hodgkin-Katz equation weighs each ion by its permeability:

$V_m = \frac{RT}{F} \ln\frac{P_K[\text{K}]_o + P_{\text{Na}}[\text{Na}]_o + P_{\text{Cl}}[\text{Cl}]_i}{P_K[\text{K}]_i + P_{\text{Na}}[\text{Na}]_i + P_{\text{Cl}}[\text{Cl}]_o}$

At rest, K+ has the highest permeability, so Vm is close to $E_{K}$ but not exactly equal because some Na+ and Cl- leak through too. When Na+ channels open (action potential), permeability shifts toward Na+, and Vm swings toward $E_{\text{Na}}$.

Most extracellular signals do not enter the cell directly. They bind receptors on the outside of the plasma membrane, which transmit the signal to the inside. Three main classes of cell surface receptors dominate the MCAT:

GPCRs (Reviewed from Chapter 3)

Seven transmembrane alpha helices. Coupled to heterotrimeric G proteins inside. Ligand binding triggers GDP-GTP exchange on the Gα subunit, which dissociates from Gβγ and activates downstream effectors (adenylyl cyclase, phospholipase C, etc.). Major second messengers: cAMP (Gs), decreased cAMP (Gi), IP3/DAG/Ca2+ (Gq).

Examples: beta-adrenergic receptor (Gs), muscarinic acetylcholine receptor (Gi), alpha-1 adrenergic receptor (Gq).

Receptor Tyrosine Kinases (RTKs)

Single transmembrane protein. The cytoplasmic domain is a tyrosine kinase. Ligand binding dimerizes two receptor monomers, triggering cross-phosphorylation on specific tyrosine residues. Phosphotyrosines become docking sites for SH2-domain proteins that activate downstream pathways (Ras/MAPK, PI3K/AKT, PLCγ).

Examples: insulin receptor, EGF receptor (EGFR), PDGF receptor, FGF receptor, VEGF receptor.

Ligand-Gated Ion Channels

Receptors that double as ion channels. Ligand binding opens or closes the channel directly. Fast electrical response (milliseconds).

Examples:

• Nicotinic acetylcholine receptor: nonselective cation channel at the neuromuscular junction. Opening depolarizes the muscle cell.
• GABA-A receptor: Cl- channel in the CNS. Opening hyperpolarizes neurons (inhibitory). Target of benzodiazepines and many anesthetics.
• NMDA receptor: Ca2+ and Na+ channel. Requires both glutamate and membrane depolarization to open. Critical for learning and memory (long-term potentiation).

A second messenger is a small intracellular molecule produced in response to an extracellular signal. It carries the signal from the receptor into the cell and amplifies it. Five main ones dominate the MCAT.

The Five

| Second messenger | Made by | Activates | Receptor type |
|------------------|---------|-----------|---------------|
| cAMP | Adenylyl cyclase (from ATP) | Protein kinase A | Gs-coupled GPCR |
| cGMP | Guanylyl cyclase (from GTP) | Protein kinase G | Some hormones, NO |
| IP3 | Phospholipase C (from PIP2) | Opens ER Ca2+ channels | Gq-coupled GPCR |
| DAG | Phospholipase C (from PIP2) | Protein kinase C | Gq-coupled GPCR |
| Ca2+ | Released from ER by IP3 or entering via channels | Calmodulin, PKC, many enzymes | Various |

cAMP

Adenylyl cyclase converts ATP to cAMP (+ pyrophosphate). cAMP activates PKA, which phosphorylates many targets. cAMP is degraded by phosphodiesterase (PDE). Caffeine inhibits PDE, so caffeine raises cAMP indirectly.

Examples of pathways using cAMP:

• Epinephrine → beta-adrenergic receptor → Gs → adenylyl cyclase → cAMP → PKA → phosphorylates glycogen phosphorylase kinase → glycogen breakdown.
• Cholera toxin locks Gs in the active form, producing persistent cAMP in gut cells and massive watery diarrhea.

IP3 and DAG

Phospholipase C (PLC) cleaves PIP2 into IP3 (diffusible, cytoplasmic) and DAG (stays in the membrane). IP3 binds receptors on the ER and releases stored Ca2+. DAG, together with Ca2+, activates PKC. PKC phosphorylates a different set of substrates than PKA.

Calcium as Second Messenger

Cytoplasmic free Ca2+ is kept very low (~100 nM) by Ca2+ ATPases and Na+/Ca2+ exchangers. Small Ca2+ releases produce dramatic changes. Ca2+ binds calmodulin, activating many calmodulin-dependent kinases (CaMK). Ca2+ also activates PKC (with DAG), triggers muscle contraction (binds troponin C), and triggers neurotransmitter release at synapses.

Amplification Numbers

For a typical GPCR-cAMP-PKA cascade:

1. 1 hormone → 1 active receptor
2. 1 active receptor activates ~100 G proteins
3. Each G protein’s Gα produces ~1000 cAMPs (over its active lifetime)
4. Each PKA molecule (4 cAMPs activate 1 PKA) phosphorylates ~100 targets

Net: one hormone molecule → ~10,000-1,000,000 phosphorylation events, depending on kinetics.