Plasma Membrane – Comprehensive Study Notes

Historical Perspectives on Plasma Membrane Structure

• Ernest Overton (1890s) proposed that substances entering a cell must first dissolve in the outer boundary layer of the cell.
  • Observed with root hairs: more lipid-soluble solutes entered cells more rapidly.
  • Concluded that the dissolving power of the outer boundary layer matched that of a fatty oil.
• Gorter & Grendel (1925)
  • Extracted lipids from human red blood cells (RBCs) and measured surface area coverage when lipids were spread over water.
  • Found the lipid surface area was about twice the calculated surface area of the RBCs.
  • First to propose that the plasma membrane contains a lipid bilayer (a lipid bilayer) comprising two lipid layers for each cell.
  • Introduced the idea of a monolayer-to-bilayer equivalence using surface-area measurements.
  • Conceptual basis for the lipid bilayer theory of membranes.
• Langmuir trough and illustration in the figure (as described)
  • A sample of phospholipids spread on water forms a monolayer with hydrophilic heads toward water and hydrophobic tails into air.
  • Compression of lipid monolayers allowed estimation of the amount of lipid needed for a bilayer, leading to the conclusion that RBCs have enough lipid to form a layer two molecules thick (a bilayer).
• Davson & Danielli (1935, revised 1954)
  • 1935 model: membrane composed of a lipid bilayer sandwiched between two layers of globular proteins.
  • 1954 revision: accounted for selective permeability of membranes; proposed that the bilayer was penetrated by protein-lined pores to form conduits for polar solutes and ions.
  • Supplemental figure depicts the 1954 revised model: lipid bilayer with protein layers on both surfaces and through-protein pores.
• Singer & Nicolson (1972) – Fluid-Mosaic Model
  • Core remains the lipid bilayer, but emphasis on the physical state of the membrane.
  • Proposed that the membrane is in a fluid state; individual lipid molecules can move laterally within the bilayer plane.
  • Membrane proteins exist as a mosaic of discrete, sometimes penetrating, proteins within the lipid bilayer.
  • Membranes are dynamic and capable of large changes in composition and organization.

The Chemical Composition of Membranes

• Membrane lipids are amphipathic: they contain both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.
• Three main lipid classes in membranes:
  • Phosphoglycerides (phosphoglycerides)
  • Sphingolipids
  • Cholesterol
• Myelin sheath (illustrative example)
  • Electron micrograph shows concentric membrane layers with an extremely low protein/lipid ratio.
  • Myelin insulates nerves and increases impulse velocity along axons.

Phosphoglycerides (Phosphoglycerides)

• Most membrane lipids are phospholipids anchored to a glycerol backbone:
  • A glycerol backbone.
  • Two fatty acyl tails.
  • One phosphate group (phosphatidic acid when unmodified).
• Head groups are formed by attaching various polar moieties to the phosphate:
  • Phosphatidylserine (example) when serine is attached to the phosphate group.
• The backbone is often glycerol; thus, these are also called phosphoglycerides.
• Visual: structure of phosphoglycerides shows a glycerol backbone with two fatty acid tails and a phosphate-containing head group.

Sphingolipids

• Derived from sphingosine, an long-chain amino alcohol.
• A sphingolipid consists of a sphingosine backbone linked to a single fatty acid via an amide bond to form ceramide.
• Variants include cerebrosides and gangliosides (glycolipids) with additional sugar groups.
• Sphingolipids contribute to membrane structure and cell recognition processes.

Cholesterol

• A sterol present in certain animal membranes.
• Structure: four-ring steroid nucleus with a small hydrophilic hydroxyl group.
• Orientation: hydroxyl group faces the membrane surface; the rest of the molecule is embedded among phospholipid tails.
• Function: cholesterol stiffens the lipid hydrocarbon region, reducing movement of fatty acyl chains and modulating membrane fluidity while preserving overall fluidity.
• Visual: cholesterol molecules in the bilayer oriented with a small hydrophilic end toward the surface and hydrophobic rings intercalated among lipid tails.

Membrane Carbohydrates

• Carbohydrates in membranes are predominantly covalently linked to proteins or to lipids:
  • Glycoproteins (glycoproteins): carbohydrate portion covalently attached to protein.
  • Glycolipids: carbohydrate portion covalently attached to lipids.
• Linkages:
  • N-glycosidic linkages (asparagine, N-acetylglucosamine) are more common than O-glycosidic linkages (serine/threonine, N-acetylglucosamine).

Membrane Proteins: Classes and Roles

• Three classes of membrane proteins based on their relationship to the lipid bilayer:
  • Integral membrane proteins
  • Peripheral membrane proteins
  • Lipid-anchored membrane proteins
• Membrane Proteins Figure (descriptive):
  • (a) Integral proteins typically contain one or more transmembrane helices.
  • (b) Peripheral proteins are noncovalently bonded to the polar head groups of the lipid bilayer and/or to an integral membrane protein.
  • (c) Lipid-anchored proteins are covalently bonded to a lipid group residing within the bilayer (e.g., GPI-anchored proteins on the external face).

Integral Membrane Proteins

• Definition: transmembrane proteins that pass completely through the bilayer.
• Structure: have domains that protrude from both the extracellular and cytoplasmic sides.
• Amphipathic: transmembrane portions tend to be hydrophobic; extramembrane portions tend to be hydrophilic.
• Typical functions: receptors, channels/transporters, components of electron transport chains in photosynthesis and respiration.

Peripheral Membrane Proteins

• Located entirely outside the lipid bilayer on cytoplasmic or extracellular sides.
• Attached noncovalently to lipid head groups or to integral proteins.
• Dynamic association with membranes; recruitment/removal depends on cellular conditions.

Lipid-Anchored Membrane Proteins

• Located outside of the bilayer, covalently linked to a lipid anchor embedded in the membrane.
• GPI-anchored proteins: attached to a glycosylphosphatidylinositol lipid anchor on the external face of the membrane.
• Other lipid anchors can include prenyl groups or fatty acids.

Transition Temperature and Membrane Fluidity

• The lipid bilayer behaves like a phase system whose state depends on temperature:
  • Above the transition temperature, lipids are fluid; molecules rotate and diffuse laterally within the bilayer plane.
  • Below the transition temperature, movement is restricted and the bilayer becomes a crystalline gel.
• The transition temperature is influenced by:
  • Fatty acid saturation: greater unsaturation (more double bonds) lowers the transition temperature, increasing fluidity at a given temperature.
  • Fatty acid chain length: shorter chains lower the transition temperature, increasing fluidity.
• Maintaining membrane fluidity via temperature changes:
  • Cells regulate fluidity using membrane remodeling enzymes and changes in phospholipid composition.
  • Enzymes involved:
  • Desaturases: introduce double bonds into fatty acyl chains.
  • Phospholipases: reshuffle lipid chains by cleaving fatty acids from glycerol backbones.
  • Acyltransferases: transfer fatty acids between phospholipids.
  • Cells can synthesize more unsaturated phospholipids to maintain appropriate fluidity.

Movement of Substances Across Cell Membranes: Overview

• Cells move solutes passively by diffusion or actively via energy-coupled transport.
• Four basic mechanisms (illustrated in the text):
  • Simple diffusion through the lipid bilayer or through a protein-lined aqueous channel (down the concentration gradient).
  • Facilitated diffusion via a membrane transporter that binds solute and undergoes conformational changes to expose the binding site on the opposite side (still down the gradient).
  • Active transport requiring energy (ATP or another energy source) to move solutes against the gradient via a pump.
• The net flux of solutes is determined by the driving forces and the availability of transport pathways.

Simple Diffusion vs Facilitated Diffusion vs Active Transport

• Simple diffusion: through bilayer or aqueous channel; energy-independent; moves down the concentration gradient.
• Facilitated diffusion: requires a carrier/transporter that binds the solute and flips orientation; still down the gradient.
• Active transport: uses energy to move solutes against their gradient; includes primary and secondary active transport mechanisms.

Osmosis and Water Permeability

• Water moves rapidly through membranes compared to dissolved ions or small polar molecules; membranes are semipermeable.
• Osmosis: movement of water from regions of lower solute concentration to higher solute concentration.
• Hypotonic solutions: lower external solute concentration; cells swell as water enters.
• Hypertonic solutions: higher external solute concentration; cells shrink as water leaves.
• Isotonic solutions: equal solute concentrations; constant cell volume.
• Plant cells and plasmolysis:
  • Plant cells in hypotonic environments experience turgor pressure due to water uptake.
  • In hypertonic environments, plant cells lose water and the plasma membrane may pull away from the cell wall (plasmolysis).

The Diffusion of Ions Through Membranes

• Ions cross membranes primarily through ion channels: integral proteins forming an aqueous pore.
• Channels can be open or closed and are often gated (regulated).
• Types of gating:
  • Voltage-gated channels: gating depends on ionic charge differences across the membrane.
  • Ligand-gated channels: gating depends on binding of a specific molecule (ligand).
  • Mechano-gated channels: gating depends on mechanical forces.

Gated Potassium Channels: KcsA as a Model

• The bacterial KcsA channel is a widely used model for Kv channels.
• Structure: four identical subunits; each subunit contains two transmembrane helices (M1, M2) and a pore region (P).
• The P region forms a narrow selectivity filter with a conserved pentapeptide motif (GYGVT) that lines the channel and coordinates K+ ions via carbonyl oxygens (C=O) to mimic hydration shells.
• Gating mechanism (hinge-bending model):
  • In the closed state, the inner M2 helices form a tight bundle that seals the cytoplasmic face of the pore.
  • Opening occurs when the M2 helices bend at a hinge point near a glycine residue, widening the intracellular gate.
  • More than 10^7 potassium ions can pass per second when the channel is open.
• Voltage-gated K+ channels in eukaryotes have a similar architecture but with six transmembrane helices per subunit (S1–S6).
  • The P region corresponds to S5–S6, and the voltage-sensing domain comprises S1–S4.
  • The S4 helix contains positively charged residues at every ~third residue, acting as the primary voltage sensor.
  • The first cloned Kv channel (Shaker) was identified in 1987.

Inactivation of Kv Channels

• Inactivation is mediated by a small peptide that dangles from the cytoplasmic side and can move into the pore, blocking ion flow (inactivation peptide).
• This process gates the channel after it has opened, providing a rapid off-switch for ion conductance.

Facilitated Diffusion

• Facilitated diffusion relies on a membrane-spanning transporter that binds the solute specifically and undergoes conformational changes to expose the binding site to the opposite side.
• A classic schematic example is the glucose transporter, which alternates exposure of the binding site to either side of the membrane to move glucose down its concentration gradient.
• The transporter-mediated diffusion is still driven by the existing concentration gradient, not by ATP hydrolysis.

Active Transport: Primary and Secondary

Primary Active Transport: Coupling Transport Directly to ATP Hydrolysis

• Active transport requires energy to move substances against their concentration gradient via a transporter (pump).
• Primary active transport couples ATP hydrolysis directly to substrate transport.
• The Na+/K+-ATPase (the sodium–potassium pump) is the classic example:
  • Discovered by Jens Skou in 1957.
  • Pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell per ATP hydrolyzed.
  • The cycle is energetically electrogenic, contributing to the membrane potential due to the net outward movement of positive charge.
• Quantitative expression of the ion exchange ratio:
  • The pumped ratio is $ext{Na}^+: ext{K}^+ = 3:2$ per ATP hydrolyzed.
  • Equivalently, the charge transfer per cycle is uneven, contributing to the cell's electrochemical gradient.
• Figure reference: simplified schematic of the Na+/K+-ATPase transport cycle.

Secondary Active Transport: Coupling to Existing Ion Gradients

• Secondary active transport uses energy stored in pre-existing ion gradients (usually Na+ or H+) generated by primary pumps.
• The gradient’s free energy can drive the transport of another solute against its gradient (cotransport).
• Example in intestinal epithelium:
  • Na+/glucose cotransporter on the apical membrane uses the Na+ gradient to bring glucose into the cell against its gradient.
  • Once inside, glucose exits at the basal surface via a glucose facilitator (GLUT) transporter down its gradient into the bloodstream.
• The Na+/K+-ATPase maintaining the Na+ gradient is essential for providing the energy stored in the gradient for cotransport.
• Conceptual figure captures secondary transport exploiting ion gradients to perform work.

Connections and Real-World Relevance

• The evolution of membrane models—from Overton’s boundary-layer dissolution concept to Gorter–Grendel’s lipid bilayer, Davson–Danielli’s protein-sandwiched bilayer, and Singer–Nicolson’s fluid-mosaic—maps the historical progression of how we understand cellular membranes.
• The lipid bilayer provides the structural basis for selective permeability, membrane protein function, and the dynamic regulation of cellular processes.
• Understanding membrane fluidity and remodeling is crucial for interpreting how cells adapt to stress (temperature changes) and how membrane composition influences signaling, transport, and metabolism.
• Ion channels, pumps, and transporters are fundamental to nerve impulses, muscle contraction, nutrient uptake, and fluid balance, with implications for health and disease (e.g., channelopathies, electrolyte disorders).

Key Equations and Notable Numerical Details

• Na+/K+-ATPase ion exchange ratio per ATP hydrolyzed:
  • $ext{Na}^+: ext{K}^+ = 3:2 \,.$
• Ion throughput of Kv channels (KcsA model):
  • More than $10^7$ potassium ions can pass per second when the channel is open. $10^7$ is often represented as $10^7\, ext{ions s}^{-1}$.
• Bilayer thickness concept (from lipid spreading experiments): RBC membrane lipids cover a surface area about twice that calculated from RBC surface area, implying a bilayer structure (two molecular layers).
• Transition temperature concepts (qualitative): above transition temperature, membranes are fluid; below, they are in a crystalline gel state. Fluidity increases with increased unsaturation and shorter fatty acyl chains.
• The role of cholesterol in membranes: orientation with hydroxyl group toward the surface; hydrophobic rings intercalate with phospholipid tails to modulate fluidity and stability.

Quick Reference – Terminology to Remember

• Amphipathic: molecules with both hydrophilic and hydrophobic regions (typical of membrane lipids).
• Lipid bilayer: two-layer sheet of lipids forming the core structure of the membrane.
• Glycoprotein vs Glycolipid: carbohydrate moieties attached to proteins vs lipids.
• Glycosylphosphatidylinositol (GPI): lipid anchor used to attach proteins to the external leaflet of the plasma membrane.
• Integral vs Peripheral vs Lipid-anchored proteins: distinct modes of association with the lipid bilayer.
• Transition temperature: temperature at which the bilayer shifts from fluid to gel-like state.
• Osmosis: movement of water across semipermeable membranes toward higher solute concentrations.
• Plasmolysis: plant cell membrane detaching from the cell wall in hypertonic conditions.
• Guarded gating of ion channels: voltage-gated, ligand-gated, and mechano-gated channels control ion flow.
• Selectivity filter: narrow region in ion channels that determines which ions pass, often via specific motifs (e.g., GYGVT).