Summary of the course Molecular biology of the Cell 2, including all the chapters needed to study for the course as well as the PowerPoints shown during the lectures. Studying solely from this document I was able to obtain a 9 out of 10 score.
Molecular biology of the cell chapter 10: Membrane structure
Membranes are crucial to the function of organelles
In a eukaryotic cell, a number of organelles play an important role.
o Plasma membrane à because it encloses the cytoplasm
o Cytoplasm à cytosol + all organelles in it
o Nuclear envelope à forms membrane around the
nucleus, because it controls import and export to
the nucleus à double membrane à has pores à
nuclear pores allow passage of molecules from
nucleus to cytoplasm à is continued with
endoplasmic reticulum = ER
o ER à extremely important in synthesis of lipid
molecules/synthesis of proteins à polyribosomes
bound to the ER membrane
o Mitochondrion à double membrane à inside is
matrix à extremely important in energy
metabolism in the cellà forms ATP
o Golgi apparatus à series of membranes that are
stacked on top of each other à very important in
the finetuning/processing of proteins
o Peroxisome not detailed, but important for
oxidation-reactions in the cell
o Lysosomes à degradation of food components. Enzymes degrade the food components
o Endosome à small single membrane structures. Vesicles that are released from plasma
membrane and have components form the extra cellular components à transport system of the
cell
The liver transports molecules to other parts of the cell. All the organelles in the cell are connected to
each other
A biological membrane has two layers, an outer and inner
layer, which is called the double layer. Proteins that are
embedded in the membrane are called membrane proteins.
Lipid molecules form the double membrane layer. The lipid
molecules have a large polar headgroup and hydrophobic
tails. The tails stick together to form the bilayers. The heads
are covalently attached to the hydrophobic tail. 50% of the
total mass of the plasma membrane are proteins.
An extracellular signal is passed on through membrane
proteins.
When the hydrophobic tails of membrane lipids are bend, this means there is a cis double bound
between the C atoms in the hydrophobic tails. These are unsaturated. When the tails are bended, the
lipids are more loosely packed. Straight tails mean more tightly packed lipids.
There are four major phospholipids in mammalian plasma membranes
1. Phosphatidylethanolamine
- Major phospholipid in E. Coli bacterium
2. Phosphatidylserine
- Very abundant in mammalian cells.
- Has a negative charge.
- Always on the inner leaflet of the double membrane.
3. Phosphatidylcholine
4. Sphingomyelin
- Does not have a glycerol backbone, but a sphingosine backbone.
- Has 1 fatty acid tail instead of two.
, Lipid molecules spontaneously aggregate in water
Phospholipids with one tail form micelles, with all the hydrophobic parts inside of the micelle.
Phospholipids with two tails form lipid bilayers. They form spontaneously.
The closure of a phospholipid bilayer can be spontaneous. A planar phospholipid bilayer with edges
exposed to water is energetically unfavourable. Therefore, a sealed compartment is formed by
phospholipid bilayers. This is energetically favourable, because it avoids the exposure of the
hydrophobic hydrocarbon tails to water. This is fundamental to the creation of a living cell.
Membranes tend to not pass water molecules. Fast channel proteins are needed to channel water across
the protein.
Different motions of a lipid molecule in a bilayer
1. Flexion
- Rapidly changing of position with the neighbour
phospholipid.
2. Rotation
- Individual lipid molecules rotate very rapidly.
3. Flip-flop
- This rarely occurs becaus ethe polar head-group of the
phosppholipid would have to pass through the
hydrophobic inside.
- Proteins have to facilitate the flip-flop occuring (flipase).
The fluidity of a lipid bilayer depends on its composition and its temperature
o The fluidity of the lipid bilayer goes up when the temperature goes up.
o When the number of double bonds go up, the lipid molecules cannot pack tightly and the
fluidity will go up.
o Longer tails results in more hydrophobic interactions. There is tight packing and the gluidity
goes down.
o When the chain length goes down, the fluidity goes up.
o Saturated lipid side chains mean there is no double bonds. The membrane is thicker than a
membrane that has a lot of unsaturated side chains.
Cholesterol is not a phospholipid, but another lipid molecule. It has a hydrophobic ring structure and a
hydrophobic tail, which is a hydrocarbon tail It has a very small polar headgroup.
Cholesterol enhances permeability-barrier properties and inhibits phase transitions
o Cholesterol allows tighter packing in lipid membranes, because it fills the gaps between
phospholipids in the membrane.
o When there is only phospholipids in a membrane it is a less good barrier than a barrier with
cholesterol present, because of the stiffened region caused by the cholesterol.
o When a membrane contains more cholesterol, it is harder to make the membrane fluid than
when less cholesterol is present.
Glycolipids are found on the surface of all animal cell plasma membranes
Glycolipids are found on the outside of the
membrane. All animal cells have a sugar layer on the
outside, which are the glycolipids and membrane
proteins that are glycosylated. This makes sure the
sugars are on the outside of the membrane. The
anchors are GPI that attach proteins to the lipid
membrane. Raft domains are increased
concentrations of membrane proteins or certain lipid
molecules, which make the membrane a bit thicker.
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