Membranes are essential for life, as they define cells.
The plasma membrane separates the extracellular space from the cytosol.
Structure and Function of Cell Membranes
Selective Barrier: Membranes control the movement of substances in and out of the cell.
Thickness: Approximately 5 nm or 50 atoms thick.
Composed of a two-ply lipid sheet with embedded proteins.
Functions:
Nutrient intake.
Waste removal
Cell communication - Receptor proteins in plasma membrane enable cell to receive signals from environment
Import and export of molecules - channels and transporters in membrane enable import and export of small molecules
cell growth and motility - flexibility of membrane and its capacity for expansion allow cell to grow, change shape, and move
Membrane Dynamics
Membranes grow with the cell.
They can deform without tearing, allowing cell movement and shape changes.
Self-healing property: membranes reseal if torn.
Membranes play a role in Cell communication, Transport of small molecules, Energy generation
Tools for Studying Membranes
Laser tweezers are used to study membrane properties.
Composition and Principles
Internal membranes in eukaryotes share similar principles with the plasma membrane but vary in composition, especially in resident proteins.
Examples: nucleus and mitochondria, each enclosed by two membranes.
General Membrane Structure
Lipid bilayer: provides a permeability barrier.
Proteins: responsible for distinctive membrane functions.
The proteins extend from either side of bilayer form two closely spaced dark lines
The thin, white layer between them is lipid bilayer
The Lipid Bilayer
Lipid molecules are insoluble in water but soluble in organic solvents like benzene.
Lipid Bilayer - Flexible Two- dimensional Fluid
Fluidity of Lipid Bilayer - Depends on Its Composition
Membrane Assembly - Begins in ER
Certain Phospholipids - Confined to One Side of Membrane
Formation of Lipid Bilayers in Water
Phospholipids have a hydrophilic (water-loving) head and hydrophobic (water-fearing) tails.
Hydrophilic head: contains a phosphate group.
Hydrophobic tail: consists of a pair of hydrocarbon chains.
Phospholipids are the most abundant lipids in cell membranes.
Example: Phosphatidylcholine contains a small molecule choline attached to phosphate head
Amphipathic Nature of Membrane Lipids
Amphipathic molecules have both hydrophilic and hydrophobic regions.
Examples: phospholipids, cholesterol (in animal membranes), and glycolipids.
Triacylglycerols, unlike phospholipids, are entirely hydrophobic and have three fatty acid tails.
Driving Forces Behind Lipid Bilayer Formation
Lipid bilayer formation is driven by:
Attraction of hydrophilic heads to water.
Aggregation of hydrophobic tails with each other.
Self-Sealing Property of Lipid Bilayers
Tears in the bilayer are energetically unfavorable due to exposure to water.
Spontaneous rearrangement to eliminate free edges leads to self-sealing.
Large tears may cause the sheet to fold and break into vesicles.
Amphipathic sheets form closed spaces, creating vesicles, organelles, and cells.
Fluidity and Flexibility of Lipid Bilayers
Molecules can move and exchange places within the membrane.
Flexibility allows the lipid bilayer to deform.
Synthetic lipid bilayers (liposomes) are used to study membranes.
Liposomes range from 25 nm to 1 mm in diameter.
Molecular Movements within Lipid Bilayers
Flip-flop: the movement of a phospholipid from one monolayer to the other; rare (less than once per month).
Lateral diffusion: rapid exchange of lipid molecules with neighbors within the same monolayer.
Lipid molecules can diffuse ~2 microns per second in artificial bilayers.
Hydrocarbon tails flex and rotate rapidly (up to 500 revolutions/s).
Factors Affecting Membrane Fluidity
Fluidity is crucial for membrane function.
Depends on phospholipid composition, especially the nature of hydrocarbon tails.
Closer and more regular the packing of tails = more viscous and less fluid the bilayer.
Hydrocarbon Tail Properties
Length of hydrocarbon tails: Typically between 14 and 24 carbon atoms (18-20 most common).
Number of double bonds in hydrocarbon tails.
Double bonds create kinks, making packing difficult and increasing fluidity.
Saturated tails have single bonds, while unsaturated tails have one or more double bonds.
Bilayers with more unsaturated tails are more fluid.
Significance of Membrane Fluidity
Enables rapid diffusion and interaction of membrane proteins.
Allows lipids and proteins to diffuse from synthesis sites to other regions.
Ensures equal distribution of membrane molecules during cell division.
Allows membranes to fuse and mix molecules.
Adaptation to Temperature Changes
Bacterial and yeast cells adjust the lengths and saturation of hydrocarbon tails to maintain constant membrane fluidity.
At higher temperatures, cells produce longer H-C tails with fewer double bonds.
Practical Applications
Hydrogenation: adding hydrogen to vegetable oils to remove double bonds, creating margarine.
Plant fats are generally unsaturated and liquid at room temperature, while animal fats are saturated and solid.
Cholesterol's Role in Membrane Fluidity
Cholesterol, a short, rigid steroid ring structure, is present in large amounts in the plasma membrane (~20% of lipids by weight).
It fills spaces between phospholipids, stiffening the bilayer and making it less flexible and less permeable.
Membrane Assembly in the Endoplasmic Reticulum (ER)
In eukaryotes, phospholipids are synthesized by enzymes on the cytosolic surface of the ER using free fatty acids.
Newly made phospholipids are deposited in the cytosolic half of the bilayer only.
Scramblase – type of transporter protein that removes randomly selected phospholipids from one half of bilayer and inserts them in other à Newly made phospholipids redistributed equally between each monolayer
Scramblases
Scramblase enzymes redistribute phospholipids equally between monolayers.
Membrane Distribution
Some new membrane stays in the ER.
The remainder supplies fresh membrane to other compartments like the Golgi, plasma membrane, mitochondria, nucleus, peroxisome, lysosome, and endosome.
Transport vesicles facilitate the dynamic process of membrane budding and fusion.
Asymmetric Distribution of Phospholipids
Most cell membranes are asymmetric, with different phospholipid sets in each half of the bilayer.
Membranes emerge from the ER with an evenly assorted set of phospholipids.
Flippases
Flippases remove specific phospholipids from one side of the bilayer and flip them to the other, creating asymmetry in the Golgi apparatus.
Unlike scramblases, which move random phospholipids flippases remove specific phospholipids from side of bilayer facing exterior space and flip them into monolayer facing cytosol
Flippases selectively remove phosphatidylserine and phosphatidylethanolamine from the Golgi-lumen-facing monolayer and flip them to the cytosolic side.
This leaves phosphatidylcholine and sphingomyelin concentrated in the Golgi-lumen-facing monolayer, which may drive vesicle budding.
These flippases and similar transporters in plasma membrane start and maintain asymmetry found in animal cell membranes
Membrane Orientation
Membranes retain their orientation during transfer between cell compartments.
The cytosolic monolayer always faces the cytosol; the noncytosolic monolayer faces the cell exterior or organelle lumen.
Conservation of orientation applies to both phospholipids and membrane proteins.
Glycolipids
Glycolipids are mainly located in the plasma membrane, exclusively in the noncytosolic monolayer.
Sugar groups face the cell exterior.
They form part of a carbohydrate coat that protects animal cells.
Enzymes in the Golgi apparatus add sugar groups to lipids in the noncytosolic monolayer.
Glycolipids are trapped in the noncytosolic monolayer because there are no flippases to transfer them to the cytosolic side.
Phospholipid Distribution in Animal Cell Membranes
Phosphatidylcholine and sphingomyelin are concentrated in the noncytosolic monolayer.
Phosphatidylserine and phosphatidylethanolamine are mainly found on the cytosolic side.
Phosphatidylinositols participate in cell signaling in the cytosolic monolayer.
Cholesterol is distributed almost equally in both monolayers.
Functions of the Lipid Bilayer
Provides the basic structure of all membranes.
Serves as a permeability barrier to hydrophilic molecules.
Membrane Composition
In animals, proteins make up ~50% of the mass of most plasma membranes.
Membrane proteins carry out most membrane functions.
The remainder consists of lipids and carbohydrates (glycolipids and glycoproteins).
Lipid-to-Protein Ratio
Lipid molecules are much smaller than protein molecules.
Membranes usually have ~50 times more lipid molecules than protein molecules.