Cell Membrane Overview

Cell Membrane Overview

• Cell membranes act as barriers enclosing reaction compartments
• They maintain concentration gradients
• They determine what enters & exits the compartments
• They provide an interaction surface with the environment

Learning Objectives

• Properties and locations of membranes
• Structure of membrane lipids
• How membrane lipids order to form bilayers
• How lipid bilayers form membranes
• How membranes are selective
• How biological membranes are formed

Membranes as Barriers

• Enclose reaction compartment
• Maintain concentration gradients
• Control what enters/exits the compartment
• Provide an interaction surface with the environment

A Barrier in Aqueous Environments

• Membranes form due to phase separation from water's insolubility in non-polar solvents (like hexane) and vice versa.
• Water is a strong dipole while compounds like hexane are apolar.

Aqueous Solubility

• Charged/polar molecules dissolve readily in water due to interactions with the water dipole.
• Non-polar molecules are insoluble in water because there is no interaction with the water dipole.

Compartment of Amphipathic Molecules

• Phase separation between water and organic solvents (like hexane) is due to the insolubility of each
• Surfactants/detergents are amphiphilic (both polar and non-polar)
• Amphipathic molecules organize at the interphase (boundary layer) between water and non-polar solvents.

Behaviour of Amphipathic Molecules in Aqueous Solution

• Micelle formation reduces exposure of lipophilic (non-polar) residues to the aqueous environment
• Hydrophillic heads face the water, hydrophobic tails cluster inside.

Behaviour of Amphipathic Molecules (continued)

• Micelles form in polar solvents
• Inverse micelles form in non-polar solvents

How Detergents Work

• Soaps/detergents dissolve in water
• Surfactants orientate themselves in grease and water
• Agitation separates grease from the surface
• Process continues until cleaning is complete.

Locations of Membranes: Prokaryotes

• Some prokaryotes have a single membrane
• Outer membrane provides protection
• Outer membrane is relatively permeable to small molecules
• Inner membrane acts as a permeability barrier.
• Region between the membranes is called the periplasm.

Locations of Membranes: Eukaryotes

• Eukaryotic cells have a single lipid bilayer cell membrane
• Internal compartments are surrounded by specialized membranes (e.g., mitochondria, nucleus, endoplasmic reticulum)

Terminology

• Hydrophilic: Water-loving, interactions with water are thermodynamically favorable.
• Hydrophobic: Water-fearing, non-polar molecules can’t interact with water.
• Lipophilic: Lipid-loving, tending to combine or dissolve in lipids/fats
• Amphiphilic: Molecules with both hydrophilic and hydrophobic parts
• Amphipathic: Molecules with both hydrophilic and hydrophobic parts.

Phospholipids

• Naturally occurring amphipathic molecules
• Have a glycerol backbone
• Two fatty acyl groups
• A phosphate group
• A head group

Example Phospholipids

• PE (phosphatidylethanolamine)
• PC (phosphatidylcholine)
• PS (phosphatidylserine)
• PI (phosphatidylinositol)

Cholesterol

• Intercalates with acyl chains, reducing mobility
• Decreases fluidity at high temperatures
• Increases fluidity at low temperatures
• Maintains fairly constant fluidity across temperature ranges
• Found in eukaryotic but not prokaryotic intracellular membranes.

Phase Transition

• Membrane fluidity depends on acyl chain composition and temperature.
• Examples: compositions of butter and olive oil.

Spontaneous Formation of Membranes

• Lipid bilayers form spontaneously
• Growth of bimolecular sheets is rapid and spontaneous in water.
• Driven by hydrophobic interactions
• Leads to creation of cell membranes.

Lipid Structures in Aqueous Environments

• Planar bilayers are energetically unfavorable
• Micelles (polar head group outside, non-polar tails inside)
• Vesicles/liposomes are formed from planar bilayers and are energetically favorable (sealed compartments formed by phospholipid bilayers)

Movement of Lipids in Membranes: Lateral Diffusion

• Lipids move laterally within the leaflet.
• 2D lateral diffusion is fast (e.g., bacterial cell length in 1 second at 37°C)

Movement of Lipids in Membranes: Transverse Diffusion (Flip-Flop)

• Movement between leaflets
• Polar heads have large solvation shells that must be shed
• Massive energy requirements, much slower than lateral diffusion(~20 hours).
• Catalyzed by flippases.

Lipid Structures (continued)

• Bilayer membranes are two lipid sheets (monolayers or leaflets)
• Polar heads are on outside, hydrophobic tails are inside
• Typically 4-6 nm thick
• Liposomes/vesicles: aqueous environments enclosed by lipid bilayers
• Useful experimental tools, vary in size (50 nm to >10 µm).

Membranes are Asymmetric

• Compare head groups to understand how they affect membrane properties.
• Phosphatidylcholine & Phosphatidylserine

Membranes are Asymmetric (continued)

• Phosphatidylcholine and Sphingomyelin are in the outer monolayer
• Phosphatidylserine is in the inner leaflet (is - charged, a difference in charge).
• Membrane asymmetry is functionally important

Membrane Composition Inside the Cell

• Diagram illustrating pathways of endocytosis (internalization of materials by the cell).
• Membrane-bound components are labeled and described

Composition of Biological Membranes

• Lipids (25-50% by mass) – phospholipids, glycosphingolipids, cholesterol
• Carbohydrates (glycolipids and glycoproteins)
• Proteins (50-75% by mass)

Glycocalyx

• Carbohydrate layer on the cell surface.
• Consists of glycoproteins, glycolipids etc

Integral vs Peripheral Membrane Proteins

• Integral proteins are embedded within the membrane
• Peripheral proteins are associated with the membrane surface.
• Various classifications of integral proteins are mentioned, including α-helices, helical bundles, and β-barrels with descriptions.