Premed · Premed · Cell Biology
Lecture 4: Membrane Structure and Lipid Biology
Cell Biology
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the fluid mosaic model of membrane structure
- Explain the composition and asymmetry of biological membranes
- Describe the roles of cholesterol and different phospholipids in membrane properties
- Distinguish between integral and peripheral membrane proteins
- Explain how membrane fluidity is regulated and measured
Lecture Content
I. The Fluid Mosaic Model
The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the plasma membrane as a two-dimensional fluid composed of a phospholipid bilayer in which proteins are embedded. The phospholipid bilayer forms the basic structural framework, and proteins float within or associate with this lipid sea. Both lipids and proteins can move laterally within the plane of the membrane, giving it a dynamic, fluid character. Importantly, the membrane is asymmetric: the inner and outer leaflets differ in their lipid and protein composition.
Lipids and most proteins are held in the membrane by non-covalent hydrophobic interactions. The membrane is approximately 5 to 8 nanometers thick and is selectively permeable -- small nonpolar molecules pass freely through the lipid bilayer, while ions and large polar molecules require the assistance of transport proteins to cross.
II. Membrane Lipids
The lipid composition of biological membranes is remarkably complex and precisely controlled. Phospholipids are the most abundant membrane lipids. Among the glycerophospholipids, which have a glycerol backbone, phosphatidylcholine (PC) is the most abundant species in the outer leaflet, while phosphatidylethanolamine (PE) predominates in the inner leaflet. Phosphatidylserine (PS) is found exclusively in the inner leaflet under normal conditions; its exposure on the outer leaflet serves as a signal for apoptosis. Phosphatidylinositol (PI), also in the inner leaflet, plays crucial signaling roles through its phosphorylated derivatives PIP2 and PIP3. Sphingomyelin, built on a sphingosine backbone rather than glycerol, resides primarily in the outer leaflet.
Cholesterol constitutes approximately 20 to 25 percent of membrane lipids in animal cells and is virtually absent from most prokaryotic membranes. It intercalates between the fatty acid tails of phospholipids and acts as a fluidity buffer. At high temperatures, cholesterol restricts the movement of phospholipid tails and decreases fluidity. At low temperatures, it prevents tight packing and inhibits solidification. The net effect is to maintain the membrane at an intermediate fluidity across a range of temperatures.
Glycolipids, which bear sugar residues attached to a lipid, are found exclusively in the outer leaflet. Cerebrosides and gangliosides are important types. Together with glycoproteins, they form the glycocalyx on the extracellular surface and play roles in cell recognition and protection.
Lipid rafts are microdomains within the membrane that are enriched in cholesterol, sphingolipids, and specific proteins. These more ordered regions, which exist in a liquid-ordered phase relative to the surrounding membrane, are thought to serve as platforms for signaling and membrane trafficking. However, their precise size, stability, and functional significance in living cells remain subjects of ongoing debate.
<image>Detailed cross-section of the plasma membrane illustrating the fluid mosaic model. Panel A: Phospholipid bilayer with cholesterol molecules intercalated between fatty acid tails, integral membrane proteins (single-pass and multi-pass) spanning the bilayer, peripheral proteins associated with the inner leaflet, glycolipids and glycoproteins on the extracellular surface forming the glycocalyx. Panel B: Enlarged view of lipid asymmetry showing PC and sphingomyelin enriched in the outer leaflet; PE, PS, and PI enriched in the inner leaflet. Panel C: A lipid raft microdomain showing concentrated cholesterol, sphingolipids, and a GPI-anchored protein.</image>
III. Membrane Proteins
Proteins constitute approximately 50 percent of membrane mass, though this proportion varies considerably by membrane type. They are classified into three major categories based on how they associate with the bilayer.
Integral (intrinsic) membrane proteins penetrate into or span the lipid bilayer. Transmembrane proteins traverse the entire bilayer and come in several architectural forms. Single-pass transmembrane proteins, such as glycophorin, cross the membrane once with a single alpha-helix. Multi-pass proteins cross multiple times; GPCRs, for example, have seven transmembrane alpha-helices. Beta-barrel proteins, in which beta-sheets form a barrel structure, are common in the outer membranes of bacteria, mitochondria, and chloroplasts. The transmembrane domains of alpha-helical membrane proteins typically consist of about 20 to 25 nonpolar amino acids, and extracting integral proteins from the membrane requires detergents such as SDS or Triton X-100.
Peripheral (extrinsic) membrane proteins associate with the membrane surface through non-covalent interactions with integral proteins or lipid head groups. They can be removed by relatively gentle treatments such as changes in pH, ionic strength, or chelating agents like EDTA.
Lipid-anchored proteins are covalently attached to lipid molecules embedded in the bilayer. GPI-anchored proteins are tethered via a glycosylphosphatidylinositol anchor on the extracellular face. On the cytoplasmic face, proteins can be anchored through fatty acid modifications such as myristoylation (at the N-terminus) or palmitoylation (at internal cysteine residues), or through prenylation with farnesyl or geranylgeranyl groups, as seen in the Ras family of GTPases.
Membrane proteins perform a wide range of essential functions including transport (channels, carriers, and pumps), enzymatic activity (such as adenylyl cyclase), signal transduction (receptors), cell-cell recognition and adhesion, and attachment to the cytoskeleton and extracellular matrix.
IV. Membrane Fluidity
Biological membranes are fluid structures in which lipids and proteins can diffuse laterally. Lateral diffusion is rapid, occurring at rates of approximately 2 micrometers per second for lipids. Transverse diffusion, or "flip-flop," in which a lipid moves from one leaflet to the other, is extremely slow without enzymatic assistance. Three classes of enzymes catalyze transverse movement: flippases move lipids from the outer to the inner leaflet in an ATP-dependent manner, floppases move lipids in the opposite direction (also ATP-dependent), and scramblases facilitate bidirectional, non-selective movement and are activated by calcium.
Several factors influence membrane fluidity. Higher temperatures increase fluidity. Unsaturated fatty acids, with their cis double bonds that introduce kinks and prevent tight packing, increase fluidity compared to saturated fatty acids. Shorter fatty acid chains also increase fluidity. Cholesterol acts as a fluidity buffer, as described above.
Membrane fluidity can be measured experimentally using FRAP (Fluorescence Recovery After Photobleaching), which quantifies the rate of lateral diffusion, and single particle tracking (SPT), which follows the movements of individual molecules.
The landmark Frye-Edidin experiment of 1970 provided compelling visual evidence for the lateral mobility of membrane proteins. By fusing human and mouse cells and tracking the distribution of species-specific membrane proteins, the researchers demonstrated that the proteins completely intermixed within 40 minutes at 37 degrees Celsius.
<image>Experiments demonstrating membrane fluidity. Panel A: The Frye-Edidin experiment — two cells (human labeled red, mouse labeled green) are fused with Sendai virus; immediately after fusion proteins are segregated on their respective halves; after 40 minutes at 37C, red and green proteins are completely intermixed. Panel B: FRAP experiment — a fluorescently labeled membrane region is photobleached with a focused laser; over time, fluorescence recovers as unbleached molecules diffuse into the bleached area; a graph shows fluorescence intensity vs. time with recovery curve and mobile/immobile fractions labeled.</image>
V. Membrane Asymmetry
The two leaflets of the bilayer maintain distinct compositions, and this asymmetry is functionally important. Lipid asymmetry is maintained by the coordinated action of flippases, floppases, and scramblases. The outer leaflet is enriched in PC, sphingomyelin, and glycolipids, while the inner leaflet contains PE, PS, PI, and PIP2. Disruption of this asymmetry has significant biological consequences. When PS appears on the outer leaflet, it acts as an "eat me" signal that promotes phagocytic clearance, marking the cell for removal during apoptosis. PS exposure on the surface of activated platelets also plays a critical role in promoting blood coagulation.
Protein asymmetry is absolute: the orientation of a transmembrane protein is fixed once it is inserted into the ER membrane during synthesis. Extracellular domains always face the same side, and transmembrane proteins never flip across the bilayer.
The glycocalyx is a carbohydrate-rich layer on the extracellular surface, composed of oligosaccharides attached to glycoproteins, glycolipids, and proteoglycans. It serves protective, recognition, adhesion, and immune defense functions. The ABO blood group antigens, which are carbohydrate structures on glycolipids and glycoproteins, illustrate the biological importance of the glycocalyx.
VI. Experimental Approaches to Studying Membranes
Several experimental systems have been developed to study membrane properties. Liposomes are artificial vesicles made from phospholipids that are used to study membrane properties and to deliver drugs. Supported lipid bilayers are planar bilayers assembled on solid substrates for biophysical studies. Black lipid membranes are planar bilayers used for electrophysiology experiments.
Determining the structures of membrane proteins has been a major challenge because these proteins are notoriously difficult to crystallize. X-ray crystallography and cryo-EM are the primary techniques used, with approaches such as detergent solubilization or lipidic cubic phase methods employed to handle the hydrophobic nature of these proteins.
Freeze-fracture electron microscopy provides a unique view of membrane interior architecture. In this technique, the membrane is rapidly frozen and then fractured along its hydrophobic interior, exposing the P-face (protoplasmic, inner leaflet) and E-face (exoplasmic, outer leaflet). A platinum and carbon replica of the fractured surface is then viewed by TEM, revealing integral membrane proteins as bumps (intramembrane particles) on the P-face and corresponding pits on the E-face.
<image>Freeze-fracture electron microscopy of a cell membrane. Panel A: Diagram showing the freeze-fracture technique — the membrane is split along its hydrophobic interior, exposing the P-face (protoplasmic, inner leaflet) and E-face (exoplasmic, outer leaflet). Panel B: An electron micrograph showing numerous intramembrane particles (bumps) on the P-face representing integral membrane proteins, and corresponding pits on the E-face where proteins were pulled away.</image>
