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
