Difficulty: Intermediate | Prerequisites: Chapter 9 material (genomics wrap-up), basic chemistry of lipids and aqueous solutions.
This is the beginning of the cell structure unit. The course has finished covering molecular components and genomes and now shifts to how eukaryotic cells are physically organised, starting with membranes. Membranes are not passive wrappers; they define what a cell is, create internal compartments, control what enters and exits, and carry signals. The guiding principle for this unit is "form is associated with function, shape is associated with activity," so every structural detail connects to a biological consequence. If you are behind, make sure you understand basic lipid chemistry and the concept of hydrophobic versus hydrophilic interactions before reading these notes.
Cell membranes are phospholipid bilayers whose structure arises from the amphipathic nature of their lipid components. Lipids move freely within a leaflet but rarely flip between leaflets without enzymes. Cholesterol buffers fluidity. The ER, Golgi, and plasma membrane maintain a conserved topology during vesicular transport, and the Golgi generates deliberate leaflet asymmetry that has functional consequences, including the "eat me" signal of apoptosis.
Amphipathic
A molecule possessing both hydrophobic (nonpolar) and hydrophilic (polar) regions.
In simple terms, one end of the molecule likes water and the other end avoids it. This dual nature is what drives membrane formation.
Phospholipid
A lipid built on a glycerol backbone with two fatty-acyl chains (hydrophobic tails), a phosphate group, and a variable head group (e.g. choline, serine).
In simple terms, think of it as a lollipop with two sticks: the candy (head) sits in water and the sticks (tails) hide inside the membrane.
Bilayer
Two layers of phospholipids arranged so that hydrophobic tails face inward (shielded from water) and hydrophilic heads face outward toward the aqueous environment on both sides.
In simple terms, it is a sandwich where the fatty tails are the filling and the polar heads are the bread.
Phosphate group
The negatively charged component of a phospholipid that contributes to the molecule's amphipathic nature. It always carries at least one negative charge.
Cholesterol
An amphipathic lipid (one hydroxyl group provides polarity; the rest is hydrophobic) that inserts into the membrane and buffers fluidity, preventing extremes at both high and low temperatures.
In simple terms, cholesterol acts like a thermostat for the membrane: it stops it from getting too runny when warm or too stiff when cold.
Flippase
An enzyme that moves phospholipids from the non-cytoplasmic leaflet to the cytoplasmic leaflet.
In simple terms, it pulls lipids inward, toward the cell's interior face.
Floppase
An enzyme that moves phospholipids from the cytoplasmic leaflet to the non-cytoplasmic (extracellular) leaflet.
In simple terms, it pushes lipids outward, away from the cytoplasm.
Scramblase
An enzyme that moves phospholipids bidirectionally and randomly between leaflets, equalising distribution.
In simple terms, it shuffles lipids between the two layers without preference for direction. The ER uses scramblases to keep both leaflets growing evenly.
Glycolipid / Glycoprotein
A lipid or protein with one or more sugar groups attached. Sugars are typically added in the ER and further modified in the Golgi. These sugar-bearing molecules face the extracellular space on the plasma membrane.
Selectively permeable membrane
A membrane that allows certain substances to cross while blocking others. Selectivity is primarily conferred by membrane proteins, not by the lipid bilayer alone.
Topologically equivalent
Two compartments that maintain the same spatial orientation through membrane trafficking. The lumen of the ER, the lumen of the Golgi, the interior of a vesicle, and the extracellular space are all topologically equivalent.
In simple terms, whatever faces the inside of the ER will end up facing the outside of the cell once it reaches the plasma membrane. The orientation is preserved throughout the journey.
A cell is fundamentally defined by its plasma membrane: a boundary separating an internal aqueous environment from the external one.
Membranes serve four major functions:
Waterproofing barrier
Selectively permeable boundary (controls molecular traffic)
Compartmentalisation (organelles as specialised "little test tubes" for concentrated biochemistry)
Signalling platform (receptors on the membrane surface alter gene expression and cell behaviour)
Eukaryotic compartmentalisation is why eukaryotic cells can be orders of magnitude larger than prokaryotic cells while maintaining efficient biochemistry.
Origin-of-life hypothesis: early life likely involved a self-replicating RNA molecule trapped inside a lipid droplet (primitive membrane), concentrating ribozymes and separating them from the environment. These proto-membranes evolved into modern phospholipid bilayers.
Phospholipid anatomy:
Glycerol backbone + two fatty-acyl chains (tails) + phosphate group + variable head group
Head-group examples: choline (phosphatidylcholine), serine (phosphatidylserine)
Tails are nonpolar and hydrophobic; the head group is polar and hydrophilic
The phosphate always carries a negative charge. Some head groups (e.g. phosphatidylserine) add further negative charge.
Why bilayers and not micelles:
Phospholipids have two fatty-acyl tails, which favours bilayer formation.
Single-chain amphiphiles (e.g. detergents) form micelles (spherical monolayers) instead.
Bilayer behaviour in water:
Phospholipids spontaneously assemble into bilayers in aqueous environments, tails inward, heads outward.
Free-floating bilayers curl into closed vesicles (spheres) to avoid exposing hydrophobic edges to water. A flat sheet is unstable unless physically constrained.
Under electron microscopy, the bilayer appears as two distinct electron-dense layers corresponding to the head groups.
Chain length and saturation effects:
Longer fatty-acyl chains produce thicker, more tightly packed membranes.
Unsaturated chains (containing double bonds) introduce kinks that increase fluidity by preventing tight packing.
Cholesterol is amphipathic (one hydroxyl group is polar; the rest of the molecule is hydrophobic). It intercalates between phospholipids and buffers membrane fluidity:
At high temperature, it reduces fluidity (stops the membrane becoming too runny).
At low temperature, it prevents tight packing (stops solidification).
Glycolipids are formed when a sugar group replaces the phosphate head (lecture example: galactose). Glycoproteins are proteins with attached sugars.
Glycosylation begins in the ER and is further modified in the Golgi.
Sugar-modified molecules that face the ER/Golgi lumen will face the extracellular space once they reach the plasma membrane (topological equivalence).
All membrane lipids are amphipathic: tails orient inward, polar regions face the aqueous environment.
Within a single leaflet, phospholipids move freely:
Lateral diffusion (sliding side to side) and rotation happen constantly via Brownian motion.
This mobility also allows membrane proteins to diffuse across the plane of the membrane.
Between leaflets (transbilayer movement or "flip-flop"), spontaneous movement is extremely rare because dragging a polar head group through the hydrophobic core is energetically unfavourable.
Appreciable flip-flop rates require enzymes: flippases, floppases, and scramblases.
Mnemonic from lecture: "flip aces" (flippases), "flop aces" (floppases), "scramble aces" (scramblases).
Fluidity and temperature:
Unsaturated fatty acids and tail flexibility give membranes a fluid character with a melting behaviour (temperature-dependent transition between solid-like and fluid-like states).
Single-celled organisms actively adjust lipid composition (chain length, degree of unsaturation) in response to ambient temperature to maintain appropriate fluidity.
Higher eukaryotes do this too: winter wheat and summer wheat have different mitochondrial membrane compositions to function at their respective temperatures.
Cholesterol buffers these fluidity changes and prevents extremes of phase behaviour.
Phospholipid synthesis in the ER:
New phospholipids are made on the cytoplasmic face of the ER membrane (the biosynthetic enzymes sit on that side).
This means new lipids are initially inserted into the cytoplasmic leaflet only, which would cause one-sided growth if left unchecked.
Scramblases equalise the ER membrane:
In the ER, scramblases randomise phospholipids between the two leaflets, producing a symmetric distribution so both sides grow evenly.
Vesicular transport conserves topology:
Membrane and luminal contents travel between organelles via vesicles (membrane-bound bubbles that bud from one compartment and fuse with the next).
The cytoplasmic face stays cytoplasmic through ER, Golgi, and plasma membrane. The lumenal face of the ER/Golgi corresponds to the extracellular face of the plasma membrane.
Sugars added to proteins or lipids in the ER lumen will therefore face the extracellular space when those molecules reach the cell surface.
The Golgi generates asymmetry:
Flippases and floppases in the Golgi selectively sort phospholipids between leaflets.
Negatively charged phospholipids (e.g. phosphatidylserine) are preferentially kept on the cytoplasmic leaflet.
This creates a charge differential: the cytoplasmic face is more negative, the extracellular face more positive. This charge difference is the beginning of membrane voltage, which becomes critical in later physiology topics.
Phosphatidylserine is normally confined to the cytoplasmic leaflet of the plasma membrane.
During apoptosis (programmed cell death), phosphatidylserine is actively flipped to the extracellular face.
This exposure serves as an "eat me" signal: macrophages and other phagocytic cells recognise it and engulf the dying cell.
Glycosylation on the extracellular face enables cell-surface recognition and signalling more broadly.
No mathematical formulas in this lecture. Key diagrams to study:
Phospholipid structure diagram (glycerol backbone, two tails, phosphate, head group)
Bilayer cross-section showing hydrophobic interior and hydrophilic surfaces
Vesicular transport pathway: ER to Golgi to plasma membrane, with topology arrows showing which face is which
Comparison: bilayer formation (two-tailed lipids) versus micelle formation (single-tailed amphiphiles)
Liposomes (artificial phospholipid vesicles) are used in drug delivery because they exploit the same bilayer self-assembly described here, encapsulating medicines and fusing with cell membranes to release their contents. The mRNA COVID-19 vaccines, for instance, wrapped fragile mRNA in lipid nanoparticles to get it safely into cells.
Students often think phospholipids are static once in a membrane. They are not; lateral diffusion and rotation within a leaflet are constant and rapid. Only transbilayer flip-flop is rare without enzymes.
Students frequently confuse flippases, floppases, and scramblases. Remember: flippases pull lipids toward the cytoplasm, floppases push them away from the cytoplasm, scramblases randomise in both directions.
It is a common mistake to think cholesterol only increases membrane rigidity. Cholesterol is a buffer: it reduces fluidity when it is too high and prevents solidification when temperature drops.
Students sometimes assume that the ER membrane is asymmetric like the plasma membrane. It is not; scramblases in the ER keep it symmetric. Asymmetry is generated later, in the Golgi.
⚠️ Know the physical basis for bilayer formation: two fatty-acyl tails favour bilayers; single-chain amphiphiles form micelles.
⚠️ Be able to explain why spontaneous flip-flop is energetically unfavourable and name the three enzyme classes that mediate translocation (flippases, floppases, scramblases) with their directionality.
⚠️ Understand vesicular topology conservation: lumenal face = extracellular face; cytoplasmic face stays cytoplasmic throughout the secretory pathway.
⚠️ Know that phosphatidylserine exposure on the extracellular face is an apoptotic "eat me" signal.
⚠️ Cholesterol as a fluidity buffer (both directions) is a commonly tested concept.
⚠️ Membrane proteins determine selectivity and signalling (to be covered in the next lecture, but flagged here as important).
True or false: Phospholipids spontaneously flip-flop between membrane leaflets at high rates. (False. Spontaneous flip-flop is extremely rare; enzymes are required.)
Fill in the blank: In the ER, ________ enzymes randomise phospholipids between leaflets to maintain symmetric growth. (Scramblases.)
True or false: Cholesterol makes membranes more rigid at all temperatures. (False. It buffers fluidity in both directions: reduces fluidity at high temperatures, prevents solidification at low temperatures.)
Fill in the blank: The lumen of the ER is topologically equivalent to the ________ space. (Extracellular.)
True or false: Glycosylated proteins in the ER lumen will face the cytoplasm when they reach the plasma membrane. (False. They will face the extracellular space.)
Q: Why do phospholipids form bilayers rather than micelles in aqueous solution?
A: Phospholipids have two fatty-acyl tails, which are too bulky to pack into the curved monolayer of a micelle. The geometry of two tails favours the flat (or gently curved) arrangement of a bilayer. Single-chain amphiphiles, by contrast, fit into micelles.
Q: Describe the roles of scramblases, flippases, and floppases and state where each primarily operates.
A: Scramblases randomise phospholipids bidirectionally between leaflets and are prominent in the ER, where they keep the membrane symmetric. Flippases move phospholipids to the cytoplasmic leaflet and floppases move them to the non-cytoplasmic leaflet; both operate primarily in the Golgi to generate the characteristic asymmetry of the plasma membrane.
Q: Explain how vesicular transport conserves membrane topology from the ER to the plasma membrane.
A: When a vesicle buds from the ER, a patch of membrane pinches off, preserving the orientation of each leaflet. The cytoplasmic face of the ER membrane remains cytoplasmic on the vesicle, and when the vesicle fuses with the Golgi (and later the plasma membrane), that orientation is maintained. Molecules facing the ER lumen end up facing the extracellular space.
Q: What is the biological significance of phosphatidylserine appearing on the extracellular face of the plasma membrane?
A: Phosphatidylserine is normally confined to the cytoplasmic leaflet. Its appearance on the extracellular surface is an "eat me" signal associated with apoptosis. Macrophages and phagocytic cells recognise this signal and engulf the dying cell for removal.
Q: How do organisms maintain appropriate membrane fluidity as environmental temperature changes?
A: Single-celled organisms adjust the lipid composition of their membranes, altering fatty-acyl chain length and degree of unsaturation to maintain fluidity at a given temperature. Higher eukaryotes use similar strategies (e.g. different mitochondrial membrane compositions in winter vs summer wheat). Cholesterol also buffers fluidity across a range of temperatures.
This material leads directly into membrane proteins (next lecture, rest of Ch. 11), which determine the selective permeability and signalling functions that the lipid bilayer alone cannot provide. It also connects forward to Chapter 12 on transport across membranes (electrical and concentration gradients, membrane potential). The apoptosis signalling touched on here will reappear in cell-death and cell-cycle regulation topics later in the course.
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