Source: Cell Biology, University of Florida | Chapter 11
Tags: plasma membrane, lipid bilayer, phospholipid, cholesterol, membrane fluidity, membrane proteins, transmembrane protein, amphipathic, flippase, scramblase, alpha helix, beta barrel, porin, BIOL 101
Difficulty: Intermediate | Prerequisites: Basic chemistry (polarity, hydrophobic/hydrophilic interactions), amino acid structure
Every cell is wrapped in a plasma membrane, and every organelle within a eukaryotic cell is enclosed by one too. This chapter focuses on what membranes are made of and how their structure gives them their properties. The lipid bilayer is the foundation: a thin sheet of amphipathic phospholipids that self-assembles in water and acts as a selective barrier. Proteins embedded in or attached to the bilayer carry out most membrane functions, from transport to signalling to anchoring. Understanding membrane structure is essential preparation for Chapters 12 (membrane transport), 15 (intracellular compartments), and any topic involving cell signalling or secretion. You should already be comfortable with hydrophobic and hydrophilic interactions before tackling this material.
Cell membranes are built from a lipid bilayer studded with proteins. Phospholipids are amphipathic (hydrophilic head, hydrophobic tails) and self-assemble into a two-layered sheet in water. The bilayer is fluid, flexible, and asymmetric. Membrane proteins carry out most functions: transport, signalling, anchoring, and enzymatic activity. They associate with the bilayer in several ways, most commonly as transmembrane proteins with alpha-helical or beta-barrel segments crossing the hydrophobic interior.
Plasma membrane
The lipid bilayer (with embedded proteins) that encloses every living cell. Serves as a selective barrier, a communication interface, and a platform for import/export of molecules.
Phospholipid
The principal lipid in cell membranes. Consists of a hydrophilic phosphate-containing head group linked to two hydrophobic hydrocarbon tails via a glycerol backbone. The most common example is phosphatidylcholine. Think of it as: a molecule with a water-loving head and two water-avoiding tails, which forces it to arrange itself into a bilayer in aqueous environments.
Amphipathic
Describes a molecule with both a hydrophilic and a hydrophobic region. Phospholipids, cholesterol, and glycolipids are all amphipathic, which is why they form bilayers.
Lipid bilayer
A thin, two-layered sheet formed by amphipathic lipids. The hydrophobic tails face inward (shielded from water), and the hydrophilic heads face outward (contacting the aqueous environment on both sides). Approximately 6 nm thick.
Cholesterol
A sterol lipid that fills spaces between phospholipids in animal cell membranes (roughly 20% of plasma membrane lipids). Its rigid planar steroid ring structure makes the membrane stiffer, less permeable, and less fluid.
Glycolipid
A membrane lipid with a sugar group attached to its head. Found exclusively on the extracellular face of the plasma membrane.
Membrane fluidity
The ability of lipid molecules and some proteins to move laterally within the plane of the bilayer. Fluidity depends on hydrocarbon tail composition (length, saturation) and cholesterol content. In simple terms, this means: the membrane behaves like a two-dimensional liquid, not a rigid wall.
Saturated vs unsaturated fatty acid tails
Saturated tails have no double bonds and pack tightly, reducing fluidity. Unsaturated tails have one or more double bonds that create kinks, preventing tight packing and increasing fluidity.
Lateral diffusion
The rapid movement of lipid molecules within the plane of one leaflet of the bilayer. This is the dominant type of lipid movement and occurs freely.
Flip-flop
The movement of a lipid molecule from one leaflet of the bilayer to the other. This occurs very rarely on its own because the hydrophilic head must pass through the hydrophobic interior. Requires enzymes (scramblases or flippases) to occur at a meaningful rate.
Scramblase
An enzyme in the ER membrane that transfers random phospholipids from one leaflet to the other, leading to equal (symmetric) distribution of lipids in both monolayers of the ER membrane.
Flippase
An ATP-dependent enzyme (found in the Golgi and plasma membrane) that transfers specific phospholipids from one leaflet to the other, maintaining the asymmetric distribution of lipids characteristic of the plasma membrane.
Membrane asymmetry
The two leaflets of the plasma membrane have different lipid compositions. For example, phosphatidylserine (PS) and phosphatidylethanolamine (PE) are concentrated in the cytosolic leaflet, while phosphatidylcholine (PC) and sphingomyelin (SM) are concentrated in the extracellular leaflet. Glycolipids are found exclusively on the extracellular face.
Transmembrane protein
A protein that spans the entire lipid bilayer, with portions exposed on both the cytosolic and extracellular sides. The membrane-spanning segments are typically alpha helices or beta barrels composed of hydrophobic amino acids.
Alpha helix (membrane-spanning)
The most common structure by which a polypeptide chain crosses the lipid bilayer. Hydrophobic amino acid side chains face outward to interact with the lipid tails. In the absence of water, backbone atoms form hydrogen bonds with one another, driving helix formation.
Beta barrel
A cylindrical structure formed by a rolled-up beta sheet that can span the lipid bilayer. Hydrophobic side chains face outward (contacting lipids), while hydrophilic side chains face inward, lining an aqueous channel. Found in porins.
Porin
A transmembrane protein that forms a water-filled channel (beta barrel) in the outer membranes of bacteria and mitochondria. Allows passage of small nutrients, metabolites, and inorganic molecules while excluding larger molecules.
Integral membrane protein
A protein that is embedded in the lipid bilayer and can only be removed by disrupting the bilayer with detergents. Includes transmembrane proteins, monolayer-associated proteins, and lipid-linked proteins.
Peripheral membrane protein
A protein that is attached to the membrane surface by non-covalent interactions with other membrane proteins. Can be released by gentle extraction methods that do not disrupt the bilayer.
Liposome
A synthetic spherical vesicle formed from pure phospholipids in water. Used experimentally to study lipid behaviour and membrane properties.
All living cells are enclosed by a plasma membrane
Two layers of lipids with proteins embedded within
Key purposes:
Barrier that prevents mixing of cellular contents with the external environment
Import of nutrients and export of waste (via channels and transporters)
Information processing (receptor proteins serve as sensors)
Capacity for growth and expansion in surface area
Self-healing: spontaneously reseals small disruptions
Phospholipids are amphipathic: hydrophilic head (phosphate group + head group such as choline, serine, ethanolamine, or inositol) linked to two hydrophobic hydrocarbon tails via glycerol
In an aqueous environment, phospholipids spontaneously arrange into a bilayer: tails face inward, heads face outward
The four major phospholipids in mammalian plasma membranes: phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylcholine (PC), and sphingomyelin (SM)
Cholesterol (a sterol) and glycolipids are also amphipathic and are important membrane components
The bilayer is a two-dimensional fluid: lipids move freely by lateral diffusion, rotation, and flexion within the plane
Flip-flop (moving between leaflets) is extremely rare without enzymatic help
Hydrocarbon tail properties that affect fluidity:
Tail length: shorter tails reduce van der Waals interactions between neighbours, increasing fluidity
Degree of unsaturation: double bonds create kinks that prevent tight packing, increasing fluidity. Saturated tails (no double bonds) pack more tightly, decreasing fluidity
Cholesterol: fills spaces between phospholipids. At physiological temperatures, cholesterol makes the membrane less fluid, stiffer, and less permeable
Allows membrane proteins to diffuse laterally and interact with one another (important for cell signalling)
Permits lipids and proteins to be redistributed to other cellular sites
Ensures molecules are evenly distributed between daughter cells during cell division
Allows membranes to fuse and mix their molecules (essential for vesicle trafficking)
New phospholipids are synthesised on the cytosolic surface of the endoplasmic reticulum (ER) by enzymes bound to that surface, using fatty acids as substrates
In the ER, scramblases transfer phospholipids randomly between the two leaflets, keeping the ER membrane roughly symmetric
In the Golgi and plasma membrane, flippases use ATP to move specific phospholipids selectively between leaflets, creating and maintaining the characteristic asymmetric distribution:
Cytosolic leaflet: enriched in PE and PS
Extracellular leaflet: enriched in PC, SM, and all glycolipids
This asymmetry is preserved during vesicle budding and fusion events: the "inside" face of an organelle membrane always corresponds to the extracellular face of the plasma membrane
Proteins account for roughly 50% of the mass of the plasma membrane in animal cells, but because proteins are much larger than lipids, there are more lipid molecules than protein molecules
Four main functional categories:
Transporters and channels: e.g. the Na+/K+ pump (actively pumps Na+ out and K+ in), K+ leak channels (allow K+ to leave cells)
Anchors: e.g. integrins (link intracellular actin filaments to extracellular matrix proteins)
Receptors: e.g. platelet-derived growth factor (PDGF) receptor (binds extracellular signal, generates intracellular signals for growth and division)
Enzymes: e.g. adenylyl cyclase (catalyses production of cAMP in response to extracellular signals)
Transmembrane: span the bilayer completely, with portions on both sides. Released only by detergents
Monolayer-associated: partially embedded in one leaflet (e.g. via an amphipathic alpha helix lying along the membrane surface)
Lipid-linked: attached to the bilayer via a covalently attached lipid anchor
Protein-attached (peripheral): bound to other membrane proteins by non-covalent interactions. Released by gentle extraction
Transmembrane and monolayer-associated proteins, plus lipid-linked proteins, are collectively called integral membrane proteins
Alpha helix: the most common way. The membrane-spanning segment is composed of hydrophobic amino acids whose side chains interact favourably with lipid tails. In the water-free interior of the bilayer, backbone atoms hydrogen-bond with one another, driving alpha-helix formation
Single-pass transmembrane proteins (one helix crossing the bilayer) often function as signalling receptors
Multipass transmembrane proteins have multiple alpha helices crossing back and forth; some form aqueous channels by arranging amphipathic helices so that hydrophilic side chains line the interior and hydrophobic side chains face the lipid tails
Beta barrel: a rolled-up beta sheet in cylindrical form. Hydrophobic residues face outward (contacting lipids), hydrophilic residues face inward (lining the channel). Found in porins in the outer membranes of bacteria and mitochondria, allowing passage of small molecules while excluding larger ones
Students often think the membrane is rigid like a wall. The lipid bilayer is a fluid, flexible, two-dimensional liquid. Lipids and many proteins move constantly within its plane.
Students often assume cholesterol is always harmful. In cell membranes, cholesterol plays an essential structural role: it moderates fluidity, reduces permeability, and increases mechanical stability.
Students often think lipids flip freely between the two leaflets. Spontaneous flip-flop is extremely rare. It requires enzymes (scramblases in the ER, flippases in the Golgi and plasma membrane) to occur at a physiologically meaningful rate.
Students often believe proteins just sit on top of the membrane. Many membrane proteins are deeply embedded in or span the entire bilayer, with hydrophobic segments anchored in the lipid interior.
⚠️ Know the structure of a phospholipid and why it spontaneously forms a bilayer in water.
⚠️ Be able to predict how changes in tail length, saturation, or cholesterol content affect membrane fluidity.
⚠️ Understand the difference between scramblases (random transfer, ER, symmetric) and flippases (selective transfer, Golgi/plasma membrane, asymmetric).
⚠️ Know the four ways proteins associate with the membrane (transmembrane, monolayer-associated, lipid-linked, protein-attached).
⚠️ Understand why transmembrane segments form alpha helices in the hydrophobic interior of the bilayer.
⚠️ Be able to describe how a multipass transmembrane channel or a beta barrel porin allows water-soluble molecules to cross the membrane.
True or False: Unsaturated fatty acid tails increase membrane fluidity because their kinks prevent tight packing.
Fill in the blank: ________ transfer random phospholipids between leaflets in the ER, while ________ selectively transfer specific phospholipids in the Golgi to maintain asymmetry.
True or False: Cholesterol increases membrane fluidity.
Fill in the blank: The membrane-spanning segment of a typical transmembrane protein folds into a(n) ________ because there is no water in the bilayer interior to hydrogen-bond with backbone atoms.
True or False: Glycolipids are found on the cytosolic face of the plasma membrane.
Answers: 1. True. 2. Scramblases; flippases. 3. False (cholesterol decreases fluidity at normal body temperature and makes the membrane stiffer). 4. Alpha helix. 5. False (glycolipids are found exclusively on the extracellular face).
Q: Why do phospholipids spontaneously form a bilayer in water?
A: Phospholipids are amphipathic: they have hydrophilic heads and hydrophobic tails. In an aqueous environment, the hydrophobic tails are driven away from water, so they cluster together, while the hydrophilic heads remain in contact with the surrounding water. The most energetically favourable arrangement is a bilayer, with tails facing inward and heads facing outward on both sides.
Q: How does the degree of unsaturation of fatty acid tails affect membrane fluidity?
A: Unsaturated tails have one or more double bonds that introduce kinks, preventing the tails from packing closely together. This increases fluidity. Saturated tails (no double bonds) are straight and pack tightly, decreasing fluidity.
Q: What is the difference between a scramblase and a flippase?
A: A scramblase (found in the ER) randomly transfers phospholipids from one leaflet to the other, keeping the membrane symmetric. A flippase (found in the Golgi and plasma membrane) uses ATP to selectively move specific phospholipids (e.g. PS and PE) to the cytosolic leaflet, creating and maintaining the asymmetric lipid distribution of the plasma membrane.
Q: How does a beta-barrel porin allow small molecules to cross a membrane?
A: A porin is a transmembrane protein folded into a cylindrical beta barrel. The amino acids on the outside of the barrel are hydrophobic and interact with the lipid bilayer. The amino acids lining the inside of the barrel are hydrophilic, forming a water-filled channel. Small nutrients, metabolites, and inorganic molecules can pass through this channel, while larger molecules are excluded.
Q: Why is membrane fluidity important for cell function?
A: Fluidity allows membrane proteins to diffuse laterally and interact with partners (essential for signalling), permits redistribution of lipids and proteins to different cellular sites, ensures even partitioning of membrane components during cell division, and enables membrane fusion events required for vesicle trafficking.
Membrane structure is the foundation for Chapter 12 (membrane transport), which explains how channels, transporters, and pumps move molecules across the bilayer.
The asymmetric lipid distribution discussed here has signalling roles: exposure of phosphatidylserine on the extracellular face is a signal for apoptosis (programmed cell death), covered later in the course.
The ER's role in phospholipid synthesis and the Golgi's role in maintaining lipid asymmetry connect to Chapter 15 (intracellular compartments and protein/lipid sorting).
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