Biological membranes Flashcards
What is the fluid mosaic model?
Membranes behave like fluids
A mosaic of different things
What do eukaryotic cell membranes contain?
Phospholipids
Cholesterol
Glycolipids
What does amphipathic mean?
Both hydrophobic and hydrophilic
Phospholipids are amphipathic
What is a phospholipid made of?
A hydrophilic head
A hydrophobic tail
What makes the tail of the phospholipid molecule?
Two fatty acid chains
Are fatty acids saturated or unsaturated?
Either
Unsaturated fatty acids have a kink in their chain
What is the head of the phospholipid made of?
In most cases, glycerol, phosphate and and another compound
Name the four major phospholipids
Phosphatidyl-ethanolamine
Phosphatidyl-serine
Phosphatidyl-choline
Sphingomyelin
What are the two categories of phospholipid?
Phosphoglycerides
Sphingolipid
What is a lipid micelle?
A spherical molecule of only one layer of phospholipids
Why do lipid bilayers spontaneously close to form sealed compartments?
It is energetically favourable
Why is the fluidity of a membrane important?
Allows diffusion and interaction
Fusion with other membranes
Formation of daughter cells
Cell motility
How does a membrane stay fluid at lower temperatures?
A higher number of unsaturated fatty acid chains
What is the role of cholesterol in the lipid bilayer?
Cholesterol intercalates between membrane phospholipids
This decreases the membrane permeability to small molecules
Where are lipid bilayers formed?
In the endoplasmic reticulum
How is the phospholipid molecule made?
Fatty acids are made in the cytosol and transported to the endoplasmic reticulum by binding proteins
They are embedded in the membrane
Phosphate and choline are added
Where in the endoplasmic reticulum does phospholipid synthesis occur?
The outer cystolic leaflet
What is scramblase?
Catalyses transbilayer movement so the phospholipids are distributed equally between inner and outer leaflets
Where does the inner luminal leaflet of the ER end up?
As the external leaflet of the cell membrane
What is flippase?
Flips PE and PS from extracellular leaflet to the cytosolic leaflet
What are glycolipids?
Found on the extracellular leaflet
Based on sphingosine
Contain sugar
Where does glycosylation occur?
In the lumen of the ER
Name the two categories of membrane protein
Integral
Peripheral
How are integral membrane proteins inserted into the membrane?
Synthesised by ribosomes in the cytoplasm
Contain hydrophobic signal peptide (15-20 aa) at their N-terminal end
This directs polypeptide into translocator on ER membrane
Translocated through membrane and cleaved into ER lumen
Signal peptide cleaved
How are peripheral proteins inserted into the membrane?
Contain hydrophobic signal at N-terminal
Translocated through membrane until a stop transfer signal is reached
Translocator discharges protein laterally into bilayer
Signal peptide cleaved
How are double pass peripheral proteins inserted into the membrane?
The signal peptide is internal, not found at the N-terminus
Signal peptide not cleaved
What is the function of glycoproteins?
Cell recognition
Inflammatory response
Protection
What is the glycocalyx?
Carbohydrate rich layer surrounding cells
Composed of glycoproteins and glycolipids
Protects cells against mechanical and chemical damage
What is the largest protein complex in a cell
The nuclear pore complex
Describe the nuclear pore complex
400 individual proteins
30 distinct proteins (nucleoporins)
Both integral and peripheral membrane proteins
