Topic 3A - Membrane Composition Flashcards
Describe lateral diffusion as it relates to membranes
Movement of phospholipids in the membrane within their monolayer
Describe flexion as it relates to memrbanes
Movement of the hydrophobic tails of a single phospholipid
What movement of membrane phospholipids rarely occurs?
Flip-flop
How dynamic is the plasma membrane?
Extremely dynamic
What are the 3 kinds of membrane lipids in mammals?
- Phospholipids
- Glycolipids
- Sterols
What are the 2 kinds of phospholipids?
- Sphingolipids
2. Phosphoglycerides
The most common kind of phospholipid (phosphoglyceride) in mammalian membranes is…
Phosphatidylcholine
The primary phospholipid (phosphoglyceride) in bacteria is…
Phophatidylethanolamine
What is distinct about phosphatidyl -serine? (2)
- Head group is negative
2. Only found on the cytosolic side monolayer of the plasma memrbane
Phosphatidylserine in healthy cells is present only on the (1) side of the plasma mebrane
Cytosolic
Describe what would happen if phosphatidylserine was present on the extracellular side
Cell would undergo apoptosis, macrophages would engulf the cell
What is the difference between phosphoglycerides and sphingolipids?
Phosphoglycerides = fatty acid tail Sphingolipids = Fatty chain (no acid group)
What is added to a sphingosine molecule to make sphingomyelin? (2)
- Fatty acid tail (different from fatty chain)
2. Head group
Cholesterol embedded in the membrane can easily perform this movement
Flip-flop
What is the role of membrane cholesterol at low temperatures?
Prevents the membrane from getting too stiff
What is the role of membrane cholesterol at high temperatures?
Prevents the membrane from getting too fluid
Do bacteria have cholesterol?
No, they have other primary sterols
Do plants and fungi have cholesterol in their membranes?
Yes, in low amounts
On which side of the membrane can glycolipids be found?
Extracellular side only
What are the components of a glycolipid? (2)
Sphingosine (phospholipid) and sugar head group
Why are glycolipids exclusively found on the extracellular side of the membrane?
Because when they are trafficked from the ER to vesicles to the membrane the way the vesicle fuses with the plasma membrane means the sugar will always be found on the extracellular side
How many lipids have sugars? (%)
5%
Sugars are added to lipids in the…
ER
What is the difference between bacterial lipids and archaeal lipids? (3)
- Branched side chains in archaea
- Ether-linked in archaea, ester-linked in bacteria
- Some archaeal lipids form a monolayer (2 phospholipids which have fused together)
The branched side chains of archaeal lipids are composed of repeated (1) units
Isoprene
Plants use this kind of enzyme to create double bonds in their lipid chains
Desaturase
Unsaturated bonds in phospholipid tails keep membranes fluid at (1) temperatures
Low temperatures - prevents stiffness in the membrane due to fewer interactions
Why don’t humans have desaturases?
We regulate our internal temperature and therefore have no need for desaturases
How does the ratio of unsaturated:saturated fatty acids change in plants as the temperature drops?
Unsaturated population grows
Why is the asymmetry of the lipid bilayer important?
Certain lipids have functions on one particular side of the membrane, maintain functions of the cell and do not trigger early apoptosis
How many different kinds of lipids can be in the membrane of a human cell?
500-2000
What are 3 key functions of lipids in the cell?
- Act as docking sites for molecules
- Participate in intracellular signalling pathways
- Act as membrane markers
The abundance of inositol lipids are (%)
5-10% of lipids
What method can we use to assess how many transmembrane regions a TM protein has when we only know the sequence?
Hydropathy plots
Beta barrel transmembrane proteins are common in… (3)
- Bacteria
- Mitochondria
- Chloroplasts
Describe the stability of a protein linked to a membrane lipid via an amide linkage
Stable bond: this protein stays there
Describe the dynamics of a protein linked to a membrane lipid via a thioester linkage
This bond is partially stable, will likely form and reform
What are the three types of lipid anchors which anchor proteins to the membrane? What kind of linkages do they use?
- Myristoyl (amide)
- Palmitoyl (thioester)
- Farnesyl (thioether) [in archaea membranes]
What is the function of the extracellular carbohydrate layer of a cell membrane? (2)
- Act as a scaffold to protect from mechanical and chemical damage under pressure
- Cell-cell recognition
What is a glycoprotein?
Membrane proteins with sugar chains (carbohydrates) on their extracellular sides
Is the cytosol a reducing or oxidizing environment? What does this mean for cysteine/disulfide bonds?
Reducing, means no cysteine bridges form
Is the extracellular matrix a reducing or oxidizing environment? What does this mean for cysteine/disulfide bonds?
Oxidizing, means there are disulfide bonds btw cysteines
In an experiment where protein mobility is being studied, a heterocaryon is expressing mouse and human proteins. Why can’t secondary antibodies be used in this experiment?
Constant region of both is “rabbit”, therefore will be read as the same protein, need differently labelled primary antibodies
Are proteins mobile in the “lipid sea” without any restrictions?
No, they typically stay within a zone. They only switch zones rarely, proteins are localized within specific regions of a cell, and can be bound to other structures which restricts their movement
What 4 things can proteins be bound to to restrict their movement?
- Each other
- The cytoskeleton
- Elements in the matrix
- Other cells
What method is used to determine where one specific protein moves over time?
High-speed single particle tracking
What 3 features can form bends in a membrane?
- Large proteins associated with only one side of the lipid bilayer
- Coats of vesicles
- Large lipid head groups embedded in the membrane
