1.3 Membrane Structure Flashcards
What is the structure of a phospholipid? (head)
Consist of a polar head (hydrophilic) composed of a glycerol and a phosphate molecule
What is the structure of a phospholipid? (body)
Consist of two non-polar tails (hydrophobic) composed of fatty acid (hydrocarbon) chains
What are phospholipids named in terms of water?
phospholipids contain both hydrophilic (water-loving) and lipophilic (fat-loving) regions, they are classed as amphipathic
What can phospholipids spontaneously form?
Phospholipids spontaneously arrange into a bilayer
How are the phospholipids arranged in the plasma membrane?
The hydrophobic tail regions face inwards and are shielded from the surrounding polar fluids, while the two hydrophilic head regions associate with the cytosolic and extracellular fluids respectively
What holds the bilayer together?
The bilayer is held together by weak hydrophobic interactions between the tails
What effect do the Hydrophilic / hydrophobic layers have on the passage of substances?
Hydrophilic / hydrophobic layers restrict the passage of many substances
What does it mean that the membrane is fluid, in terms of phospholipids?
Individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility
Why does the membrane need to be fluid?
This fluidity allows for the spontaneous breaking and reforming of membranes (endocytosis / exocytosis)
What is embedded in the phospholipid bilayer?
Phospholipid bilayers are embedded with proteins, which may be either permanently or temporarily attached to the membrane
What does it mean if a protein is integral?
Integral proteins are permanently attached to the membrane and are typically transmembrane (they span across the bilayer)
What does it mean if a protein is peripheral?
Peripheral proteins are temporarily attached by non-covalent interactions and associate with one surface of the membrane
According to which factor are membrane proteins arranged?
The amino acids of a membrane protein are localised according to polarity
How are non-polar amino acids localised?
Non-polar (hydrophobic) amino acids associate directly with the lipid bilayer
How are polar amino acids localised?
Polar (hydrophilic) amino acids are located internally and face aqueous solutions
What 2 structures can transmembrane proteins have?
Single helices / helical bundles
Beta barrels (common in channel proteins)
What 6 functions can membrane proteins have?
Junctions Enzymes Transport Recognition Anchorage Transduction
What do membrane proteins associated with junctions do?
Serve to connect and join two cells together
What do membrane proteins associated with enzymes do?
Fixing to membranes localises metabolic pathways
What do membrane proteins associated with transport do?
Responsible for facilitated diffusion and active transport
What do membrane proteins associated with recognition do?
May function as markers for cellular identification
What do membrane proteins associated with anchorage do?
Attachment points for cytoskeleton and extracellular matrix
What do membrane proteins associated with transduction do?
Function as receptors for peptide hormones
What is the role of cholesterol in the cell membrane?
it functions to maintain integrity and mechanical stability
Is cholesterol present in plant cells? Why?
NO
It is absent in plant cells, as these plasma membranes are surrounded and supported by a rigid cell wall made of cellulose
What type of molecule (relating to polarity) is cholesterol?
Cholesterol is an amphipathic molecule (like phospholipids), meaning it has both hydrophilic and hydrophobic regions
Which part of cholesterol is hydrophilic?
Cholesterol’s hydroxyl (-OH) group is hydrophilic
How does cholesterol’s hydroxyl group align in the cell membrane?
aligns towards the phosphate heads of phospholipids
How is the remainder of cholesterol (everything apart from the hydroxyl group) arranged in the cell membrane?
The remainder of the molecule (steroid ring and hydrocarbon tail) is hydrophobic and associates with the phospholipid tails
What does cholesterol interact with to moderate properties of membrane?
Cholesterol interacts with the fatty acid tails of phospholipids to moderate the properties of the membrane
How does cholesterol affect the fluidity of the cell membrane?
Cholesterol functions to immobilise the outer surface of the membrane, reducing fluidity
How does cholesterol affect the permeability of the cell membrane?
It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross
How does cholesterol affect the crystallisation of the cell membrane?
It functions to separate phospholipid tails and so prevent crystallisation of the membrane
How does cholesterol affect the proteins of the cell membrane?
It helps secure peripheral proteins by forming high-density lipid rafts capable of anchoring the protein
What is the model of the cell membrane used today called?
fluid-mosaic model
How is the membrane a mosaic?
the phospholipid bilayer is embedded with proteins, resulting in a mosaic of components
What are the 3 components of the plasma membrane?
- phospholipids
- cholesterol
- proteins
What was the first model of the cell membrane (relating to proteins) called and what was its purpose?
The first model that attempted to describe the position of proteins within the bilayer was proposed by Hugh Davson and James Danielli in 1935
How do membranes look like under a microscope?
When viewed under a transmission electron microscope, membranes exhibit a characteristic ‘trilaminar’ appearance
Trilaminar = 3 layers (two dark outer layers and a lighter inner region)
How did the davson-danielli model look like?
Danielli and Davson proposed a model whereby two layers of protein flanked a central phospholipid bilayer
What evidence was there for the davson-danielli model?
The dark segments seen under electron microscope were identified (wrongly) as representing the two protein layers
What were 4 problems with the davson-danielli model?
It assumed all membranes were of a uniform thickness and would have a constant lipid-protein ratio
It assumed all membranes would have symmetrical internal and external surfaces (i.e. not bifacial)
It did not account for the permeability of certain substances (did not recognise the need for hydrophilic pores)
The temperatures at which membranes solidified did not correlate with those expected under the proposed model
What were 2 basic pieces of falsification for the davson-danielli model?
Membrane proteins were discovered to be insoluble in water (indicating hydrophobic surfaces) and varied in size
How would the various nature of proteins be a piece of falsification evidence against the davson-danielli model?
Such proteins would not be able to form a uniform and continuous layer around the outer surface of a membrane
What two methods were used to disprove the davson-danielli model?
- fluorescent antibody-tagging
2. freeze fracturing
How did fluorescent antibody tagging disprove the davson danielli model?
Fluorescent antibody tagging of membrane proteins showed they were mobile and not fixed in place
What 2 steps were involved in fluorescent antibody tagging? What conclusion was formed?
Membrane proteins from two different cells were tagged with red and green fluorescent markers respectively
When the two cells were fused, the markers became mixed throughout the membrane of the fused cell
This demonstrated that the membrane proteins could move and did not form a static layer (as per Davson-Danielli)
How did freeze-fracturing disprove the davson-danielli model?
Freeze fracturing was used to split open the membrane and revealed irregular rough surfaces within the membrane
What did the rough surfaces seen as a result of freeze fracturing represent and how did this disprove the davson-danielli model?
These rough surfaces were interpreted as being transmembrane proteins, demonstrating that proteins were not solely localised to the outside of the membrane structure
What was the name of the new model that replaced the D-D model and who was it proposed by?
In light of these limitations, a new model was proposed by Seymour Singer and Garth Nicolson in 1972
called the fluid-mosaic model
What was the key difference between the davson-danielli and fluid-mosaic model?
According to this model, proteins were embedded within the lipid bilayer rather than existing as separate layers
