Membranes Overview Flashcards
What are the primary functions of cell membranes?
Three primary functions:
1. Keep toxic substances out of cells
- Have receptors and channels that allow specific molecules, such as ions, nutrients and metabolic products to pass between organelles and between the cell and the outside environment
- Separate vital metabolic processes within organelles
Concentrate and isolate enzymes and reactants in a smaller volume, thereby increasing the rate andefficiencyof chemical reactions
What is the basic structure of cell membranes?
Primarily composed of proteins and lipids
Depending on the membrane location, lipids can make up 20 to 80 % of the membrane.
Lipids give membranes flexibility - very dynamic
Proteins maintain the chemical environment of the cell by assisting the transfer of molecules across membranes
Fluid-mosaic model
Integral proteins ‘float’ in a 2D lipid ‘sea’ in a random or mosaic distribution
Not rigid - bends/flexes in 3D but maintains integrity
Many non-covalent interactions between proteins and lipids hold membranes together
Describe lipids in the lipid bilayer?
Lipids - any hydrophobic molecule insoluble in water but soluble in organic solvents (e.g. chloroform)
Main organelle for lipid biosynthesis is the ER: produces the bulk of the structural phospholipids and cholesterol
Lipid composition of different organelle membranes varies
3 main types:
- Phospholipids- major component of cell membranes.
- Cholesterol- dispersed between phospholipids. Stops membranes from becoming stiff and provide stability.
- Glycolipids- located on cell membrane surface. Have carbohydrate sugars attached. Important during cell recognition
Describe the membrane proteins - within the lipid bilayer?
- Peripheral - connected to membranes by interactions with other proteins
Do not directly interact with the hydrophobic core of the bilayer; temporarily attached by non-covalent interactions and they associate with one surface of the membrane - Integral - They span the entire membrane and are permanently attached
Portions of these transmembrane proteins are exposed on both sides of the membrane - Lipid linked – attached to membranes via a lipid linker that increases hydrophobicity, allows for control of the protein
What are some roles of membrane proteins?
Structural - give the cell support/shape (to things like cytoskeletal components)
Receptors - allow communication with external/internal environment through hormones, neurotransmitters etc
Transport proteins - transport molecules across cell membranes throughfacilitated diffusion
Glycoproteins - have a carbohydrate chain attached, aids cellto cell communication and transport across the membrane
What are other lipid membranes?
Organelle membranes
Most components of a typical animal cell have a membrane - to protect their contents and separate chemical reactions
E.g.
Ones that do - nucleus, ER, golgi, vesicles and mitochondria
Ones that don’t - nucleolus, ribosomes and cytoskeleton
Describe phospholipids?
Composed of two fatty acids, a glycerol unit, a phosphate group, and a polar molecule
The phosphate group is the negatively-charged polar head, hydrophilic
The fatty acid chains are the uncharged tails, hydrophobic
DEscribe the fatty acid tails of phospholipids?
Long aliphatic carbon chains with a terminal carboxylic acid
Unbranched chains
Usually even number of CH2 groups
Usually asymmetric (non-identical)
They may differ in length and/or the presence or absence of double bonds
Either
Saturated - no double bonds = viscous
Unsaturated - contains double bond(s) = more fluid
Polyunsaturated fatty acids - More than one C=C bond = lower melting temperature
Double bond: bend of ~30 degrees in the string of carbons
Kinks” in their tails elbow adjacent phospholipid molecules away, maintaining some space between the phospholipid molecules
What are the types of phospholipid?
Glycerophospholipid
Sphingolipid
What are Glycerophospholipids?
Glycerol-3-phosphate
C1 and C2 are esterified with fatty acids
C3 has a phosphoryl group usually linked to another polar group (X)
X helps determine the name
= Amphiphilic
Non-polar aliphatic tails
Polar phosphoryl-X heads
What are some glycerophospholipid names based on different X groups?
Ethanolamine - Phosphatidylethanolamine
Choline - Phosphatidylcholine
Serine - Phosphatidylserine
Myo-inositol - Phosphatidylinositol
What are some facts of the different glycerophospholipids?
Phosphatidylcholine - most abundant in cell membranes
Phosphatidylethanolamine - 2nd most abundant, helps proteins to be positioned in the membrane
Phosphatidylserine - normally on cytoplasmic side, so presence on the outer membrane ofdying cells signalsmacrophages to digest them
Phosphatidylinositol - can be phosphorylated, important signalling molecules and abundant in thebrain
What are sphingolipids?
Derivatives of amino alcohol sphingosine (= bond in trans)
General structure: sphingosine, fatty acid, PO4, amino alcohol
The N-acyl fatty acid derivatives of sphingosine are known as ceramides
Ceramides are the parent compounds of the more abundant sphingolipids
Broken down into ceramide by sphingomyelinase-2
Largely found in the exoplasmic leaflet
Give some functions of sphingolipids in cell membranes?
Structural: protect cell from harmful environmental factors
Signalling: sphingolipid metabolism creates products that play significant roles in cells
e.g. sphingomyelin is prominent in the myelin sheaths of neurons
Multiple sclerosis - myelin sheath is attacked (autoimmune)
What are some types of sphingolipids?
Sphingomyelins
Cerebrosides
Gangleosides
What are steroids?
Eukaryotic origin - contains 4 fused rings (A-D)
A-C has 6C and D has 5C
Can be further classified as a sterol due to C3-OH group e.g. in cholesterol
Describe cholesterol?
Forms 30-40% of plasma membrane lipid
Modified steroid
Fused ring provides great rigidity
Essential to maintain both membrane structural integrity andfluidity
Fluidity allows cells to change shape rapidly (unlike bacteria and plant cells, which are restricted by their cell walls)
Sandwiched between 2 phospholipids - due tosteric reasons
Prevents 2 phospholipid molecules from coming too close so when the temperature is low it stops membrane from becoming too rigid
Interaction of the polar -OH groups with the polar head groups of the phospholipids also holds the membrane together
At high temperatures, they attract each other and prevent the membrane from breaking down - maintains thestabilityof the membrane
Describe glycolipids?
They have a mono/oligosaccharide attached (sugar)
Lipids with acarbohydrate attached by aglyosidic bond (least common membrane lipid)
Role in membrane stability and cellular recognition - carbohydrates are found on the surface of alleukaryotic cell membranes
Extend into the extracellular, environment, allow phospholipid bilayer to act as a recognition site for specific chemicals
Allows cell attachment: tissue formation - they dictate blood type
What property do lipid bilayers have?
Fluid-like properties - the membranes are 2D fluids
Lateral diffusion - movement of lipids within its own plane of the bilayer (rapid)
and rotation
Transverse diffusion (flip-flop) - transfer of a lipid molecule across a bilayer (slow and rare)
Why doesn’t transverse diffusion happen often?
The hydrated polar head has to pass through the anhydrous hydrocarbon core of the bilayer to the other side
Requires flippases to catalyse the movement from one layer of the bilayer to the other
What are the types of flippases?
P-type - inward movement, ATP dependent
ABC - outward movement, ATP dependent
Scramblase - bi-directional, not energy dependent but Ca2+ dependent
What are some other roles of cellular lipids?
Energy storage
Triglycerides: stored in adipose tissue
Adipocyte (fat cell) designed for continuous synthesis and breakdown of triglycerides in animals: controlled by the activation of hormone-sensitive lipases
Complete oxidation of fatty acids provides high caloric content - long term energy storage
Cell signalling
Diacylglycerol(DAG) & phosphatidylinositolphosphates (PIPs) are involved in calcium-mediated activation ofprotein kinase C
Prostaglandins: type of fatty-acid derived eicosanoid involved in inflammation and immunity
Sphingosine-1-phosphate: sphingolipid derived from ceramide involved in regulating calcium mobilization, cell growth and apoptosis
What do cells do with excess lipids?
Store them in droplets
Independent organelles that are composed of alipidester core and a surface phospholipid monolayer
Formed when lipid levels exceed those required for membranes
Storage of lipids and energy storage
What are lipid rafts?
Membrane subdomains (lateral organisation of lipids and proteins)
Example: Epithilial cells
Apical domain - faces the interior lumen
Basolateral domain - covers the remainder of the cell
The domains don’t mix and have different lipid/protein compositions
What do lipid rafts tend to consist of?
Glycosphingolipids and cholesterol filling the gaps between its tails
Forms an ordered crystalline arrangment
GPI-linked proteins preferentially associate with the rafts
Therefore lipid rafts are involved in intercellular signalling
What is the fluidity of a bilayer dependent on?
Temperature
Above the transition state: Highly mobile (liquid crystal)
Below the transition state: gel like solid (loses fluidity)
The transition state temperature of the bilayer increases with the chain length and the degree of saturation
Cholesterol decreases fluidity due to rigidity
