1.3 Membrane structure Flashcards
What are plasma membranes?
They are the boundary of a cell. They control movement of molecules across the cell.
What does a phospholipid molecule look like?
It has a hydrophilic phosphate head and two hydrophobic fatty acid tails
What is the term for something that is hydrophilic and hydrophobic?
Amphipathic
How does being amphipathic affect arrangement in membrane?
- Phospholipids spontaneously arrange in a bilayer
- Tail regions face inwards and are shielded from polar fluid.
- Head regions face outside and are in contact with extracellular environment
How are phospholipids held together?
By weak hydrophobic interactions
What properties of bilayer help fluidity and why is it useful?
- Phospholipids with more short or unsaturated fatty acids are more fluid
- Phospholipids can move horizontally or laterally
- Fluidity allows for breaking or remaking of membranes in exo and endocytosis
What structure did Davson-Danielli propose for membrane?
They proposed the ‘Sandwich model’. It showed a layer of proteins surrounding a phospholipid bilayer on both sides.
What did the ‘Sandwich model’ help explain?
It would explain how despite the thinness of membranes, they are an effective barrier to some substances.
How did the electron microscopy support the ‘Sandwich model’?
It showed two parallel dark lines and a lighter portion in between
Which process did Singer-Nicholson use to disprove the ‘Sandwich model’?
They used freeze fracture
What happens in freeze fracture?
The cells are rapidly frozen and fractured. Fracture occurs across line of weakness (in this case, center of inner and outer membrane)
What did freeze fracture show and how did it disprove the Sandwich model?
It showed proteins in both membranes and hence were interpreted as transmembrane. It disproved the Sandwich model as it showed proteins were not only on the outside
How does protein structure undermine Sandwich model?
Proteins are varied in size and globular and so cannot form layers. They are also hydrophobic on parts of the surface so they would be attracted to hydrocarbon tails in the center
How was fluorescent antibody tagging used in locating proteins?
Cells containing red and green antibody-tagged proteins were fused together. Within 40min, markers were mixed throughout membrane showing that they are free to move rather than fixed in a layer.
What was the Singer-Nicholson model?
Fluid phospholipid molecules form a bilayer. Peripheral proteins are bound either to inner or outer surface while integral proteins permeate membrane
Anagram for membrane protein function
TRACIE: Transport, receptors, anchorage, cell recognition, intercellular joining, enzymatic activity
Function of membrane protein: Transport
Protein channels and pumps facilitate movement of substances that cannot diffuse
Function of membrane protein: Receptors
For peptide based hormones like insulin and glucagon
Function of membrane protein: Anchorage
Cytoskeleton attachments and extracellular matrix
Function of membrane protein: Cell recognition
Has glycoproteins for MHC proteins and antigens to recognize
Function of membrane protein: Intercellular joining
Tight junctions to connect cells
Function of membrane protein: Enzymatic activity
Fixing to membranes localizes metabolic pathways (eg. photosynthesis)
Function of glycoproteins
Important for cell recognition by immune system and they act as hormone receptors
Cholesterol belongs to which group?
Steroids
Where is cholesterol located in membrane?
Located between phospholipid bilayer
Relationship between properties of cholesterol and its position
Has a hydrophilic part (hydroxyl group) which attracts to phospholipid head. Rest is hydrophobic so it connects to tail of phospholipid
How does cholesterol affect membrane and hence movement of molecules?
It restricts movement as membrane becomes less fluid
How does cholesterol increase membrane flexibility + importance
It disrupts regular packing of hydrocarbon tails. This prevents tails from crystallizing and becoming a solid hence more flexibility
What chemical properties of membrane are affected by cholesterol?
It reduces permeability to hydrophilic molecules and ions like Na and H
How does cholesterol aid exo and endocytosis?
It encourages membrane to take on a concave shape
Why are saturated fatty acid tails not kinked and why does it help?
They have no double bonds making it easier to pack
What is a micelle?
They are single layered and arranged in a circular form so tails are kept dry
What are liposomes?
Double layered and arranged where heads are on the outside to keep the tails dry
