1.3 Membrane Structure Flashcards
What are the two main components of phospholipids?
A hydrophilic polar head and two non polar hydrophobic tails
What are the hydrophilic heads composed of?
A glycerol and a phosphate molecule
What are the non polar hydrophobic tails composed of?
Fatty acid hydrocarbon chains
What are phospholipids classed as?
Amphipathic
What does amphipathic mean?
Contains both hydrophobic and hydrophilic regions
How do phospholipids arrange themselves?
Spontaneously into a bilayer
Which direction do the tails face?
Inwards
What are the tails shielded from?
Surrounding polar fluids
Which direction does the heads face?
Outwards
What fluids do the heads associate with?
Cytosolic and extracellular
What holds the phospholipid bilayer together?
Weak hydrophobic interactions between the tails
What do the the hydrophobic/hydrophilic layers restrict?
Passages of many substances
What can move within the bilayer?
Individual phospholipids
What does the fact that individual phospholipids can move within the bilayer allow the membrane to have?
Fluidity and flexibility
What does the fluidity of the phospholipid bilayer allow for?
Spontaneous breaking and reforming of membranes
What are examples of spontaneous breaking and reforming of membranes?
Endocytosis and exocytosis
What are the phospholipid bilayers embedded with?
Proteins
How long may proteins be attached to the membrane for?
Permanently or temporary
What are the two types of proteins you can get?
Integral and peripheral proteins
Are integral proteins permanently or temporarily attached to the membrane?
Permanently
Are peripheral proteins permanently or temporarily attached to the membrane?
Peripheral
How would you typically find integral proteins?
Transmembrane
How are peripheral proteins temporarily attached?
Via non covalent interactions
Where would you find a peripheral protein on the membrane?
On one surface of the membrane
What are the amino acids of a membrane protein localised in accordance to?
Polarity
How do non polar amino acids associate with the lipid bilayer?
Directly
Where are polar amino acids located?
Internally
What do polar amino acids face?
Aqueous solutions
What level of structures do transmembrane proteins typically adopt?
Tertiary
What are the two tertiary structures transmembrane proteins can adopt?
Single helices/ helical bundles
Beta barrels
What are the six functions of membrane proteins? (JET RAT)
Junctions
Enzymes
Transport
Recognition
Anchorage
Transduction
How do membrane proteins function as junctions?
They serve to connect and join two cells together?
How do membrane proteins function as enzymes?
They fix to membranes which localises metabolic pathways
How do membrane proteins function as transport?
They are responsible for facilitated diffusion and active transport
How do membrane proteins function as recognition?
May function as markers for cellular identification
How do membrane proteins function as anchorage?
They are attachment points for cytoskeleton and extracellular matrix
How do membrane proteins function as transduction?
They function as receptors for peptide hormones
What is cholesterol a component of?
Animal cell membranes
What is the function of cholesterol?
To maintain integrity and mechanical stability
Where is cholesterol absent?
In plant cells
Why is cholesterol absent in plant cells?
Their plasma membranes are surrounded and supported by a rigid cell wall made of cellulose
What type of molecule is cholesterol?
Amphipathic
What part of cholesterol is hydrophilic?
The hydroxyl group
Which way does the cholesterol hydroxyl group align?
Towards the phosphate heads of phospholipids
What parts of cholesterol is hydrophobic?
The steroid ring and hydrocarbon tail
What does the hydrophobic parts of cholesterol associate with?
The phospholipid tails
How are phospholipid bilayers fluid?
They are in constant movement relative to one another
What does the cholesterol interact with?
The fatty acid tails of the phospholipids
How does cholesterol reduce fluidity?
It functions to immobilise the outer surface of the membrane
How does cholesterol affect the permeability of the membrane?
It makes it less permeable to small water soluble molecules that otherwise would freely cross
How does cholesterol prevent crystallisation of the membrane?
It functions to separate phospholipid tails
How does cholesterol help secure peripheral proteins?
By forming high density lipid rafts capable of anchoring the protein
What are the four ways cholesterol moderates the properties of the membrane?
- reduces fluidity
- makes it less permeable
- prevents crystallisation
- Helps secure peripheral proteins
What type of model are cell membranes represented in accordance to?
A fluid mosaic model
When talking about the membrane model what does the term fluid mean?
The phospholipid bilayer is viscous and individual phospholipids can move positions
When talking about the membrane model what does the term mosaic mean?
The phospholipid bilayer is embedded with proteins causing a mosaic of components
What are the three components of the plasma membrane?
Phospholipids
Cholesterol
Proteins
Who created the first model that attempted to describe the position of proteins in the bilayer?
Davson and Danielli
What type of model did Danielli and Davson propose?
A lipo protien sandwich
What were the three things that the Danielli and Davson model assumed?
A uniform thickness
Constant lipid- protein ration
All membranes to be symmetrical
What did the Danielli and Davson model not account for?
Permeability of certain substances and temperatures
What methods helped show that the Danielli and Davson model was incorrect?
Fluorescent antibody tagging of membrane proteins and freeze fracturing
What did the fluorescent antibody tagging of membrane proteins prove?
That membrane proteins could move and did not form a static layer
What did freeze facturing prove?
That proteins were not solely localised to the outside of the membrane structure
What are the key differences of the two membrane structures?
Davson-Danielli model says that proteins form distinct layers
whereas
Singer-Nicholson model says that proteins are embedded within bilayer
