Membranes: The Basics Flashcards
Membrane Lipids
Phospholipids, Cholesterol and Glycolipids
Membrane Primary Functions
Keeping toxic substances out
Allowing specified transmembrane transport
Separating vital metabolic processes
Biomembrane Structure
Primarily Lipids and Proteins:
Lipids = 20-80% of membrane, giving flexibility
Proteins maintain chemical environment by mediating transport:
- Peripheral, Integral and Lipid linked (attach membrane via lipid linker)
Role of membrane proteins
Structural Support
Receptors
Transport proteins
Cell-cell communication: Glycoproteins
Nucleus’ membrane
2 membranes (inner and outer) separated by perinuclear space - protect genetic material and control import/export
ER’s membrane
Single membrane, largest organelle in cell
- membrane contact sites for inter-organelle communication
- Site of protein folding inside ER lumen
Golgi’s membrane
Fused flat enclosed cisternae stacks (40-100 in mammals)
Cis –> Medial –> Trans (Trans Golgi Network, TGN)
Vesicles
Liquid enclosed in lipid bilayer (membrane = lamellar phase)
- different environment to cytosol
Mitochondria
Outer and inner membrane
- Inner forms cristae containing matrix within for citric acid cycle
- Intermembrane space - oxidative phosphorylation
Varying membrane composition across organelles
Differing levels of lipid compositions:
- e.g. more Sphingomyelin in Plasma membrane than Golgi
- e.g. Golgi has more Phosphatidylserine than mitochondria
Lipid Bilayer Structure: Phospholipids
2x fatty acids + glycerol + phosphate group
- Long aliphatic carbon chains with terminal polar group
- Either saturated or Unsaturated (Cis double bond)
- 1 of 2 fatty acids has a kink (due to varying saturation) and a double bond giving a 30 degree bend
- Saturated = straight fatty acids, tightly packed, viscous
- Unsaturated = less tight packing, fluid membrane
Glycerophospholipids
forming different phospholipid types
Glycerol-3-phosphate - C1 and C2 esterified with fatty acids: Phosphoryl group linked to another polar group:
Phosphatidyl-
- ethanolamine (PE)
- choline (PC)
- serine (PS)
- inositol (PI)
Can be further phosphorylated into PIP, PIP2 or PIP3 each with different signal characteristics
Sphingolipids
Not synthesised from glycerol, have an amide bond between a fatty acid and sphingosine
Functions:
- structural protection from harmful environments and signalling through sphingolipid metabolism
E.g. Sphingomyelin (most common sphingolipid)
- Myelin sheath, exoplasmic leaflet of cell membrane
Broken down into ceramide by sphingomyelinase-2
Lipid Bilayer Structure: Cholesterol
A type of modified sterol (carbon rings present)
- Important in membrane structural integrity and fluidity
- Steric reason of fluidity: between 2 phospholipids, prevents them coming too close (viscous) at low temperatures)
- It’s hydroxyl group binds phosphate head; it’s non-polar hydrophobic tail attached to bilayer centre
- Structural stability at high temperatures: attracting polar -OH groups together prevents breakdown
Membrane Phase Transition Temperature
Above phase transition temperature:
- bilayer becoomes increasingly fluid until it can’t act as a barrier (fluid form)
Below phase transition temperature:
- Bilayer solidifies into a gel-like state - diffusion of lipids decreases
- Loss of functionality
Lipid Bilayer Structure: Glycolipids
Lipids, phospholipids or sphingolipids with a sugar group attached to its head group (by glycosidic bond)
- Provides membrane stability and cellular recognition
- Allows membranes to attach to form tissues
- Extracellular recognition of specific chemicals
Lipid Rafts
Dynamic, non-uniform areas of bilayer
- rich with cholesterol and sphingolipids that cluster with specific raft proteins
- important in vesicle transport
Lateral Diffusion in Membrane
The membrane bilayer is very dynamic and constantly moving
- lateral movement of lipids, phospholipids and proteins
- A typical lipid molecule exchanges place 10^7 times per second
- Observed with Fluorescence Recovery After Photobleaching (FRAP): non-fluorescent spot disappears as photobleached phospholipids quickly exchange
- Observed by fusing oppositely labelled mouse and human cells: half and half merges into one
Transverse Movement of Lipids
Moving lipids from extracellular face of bilayer to cytosolic face (and vice versa)
- Flippase proteins catalyse the otherwise slow process of diffusion of phospholipids from one face to the other
Roles of cellular lipids
Energy storage
Signalling Molecules
Storing excess lipids
Cellular lipids for energy storage
Triglycerides stored in adipose tissue
- adipocytes designed for constant synthesis and breakdown of triglycerides (controlled by activation of hormone-sensitive lipases)
- complete oxidation of fatty acids provides high caloric content
Cellular lipids as signalling molecules
Diaglycerol (DAG) and PIPs involved in calcium-mediated protein kinase C activation
- Prostaglandins used in inflammation/immunity: derived from eicosanoid
- Sphingosine-1-phosphate involved in regulating calcium mobilization, cell growth and apoptosis: derived from ceramide
Storing excess lipids in lipid droplets
Lipid ester core + surfuce phospholipid monolayer
- formed when lipid levels exceed required amount
