Summary of lectures given at the University of Groningen and the book Molecular Biology of the Cell. The book used is the sixth edition of Molecular Biology of the Cell.
Summary Molecular Biology of the Cell 2 (book) (WBFA007-04)
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Molecular Biology of the Cell
part II including chapters 10-17
(Summary of the lectures given
at Rijksuniversiteit Groningen)
, Chapter 10 membrane structure
Plasma membrane encloses the cell.
Nuclear envelope: consists of an inner and outer
membrane of the nucleus.
The outer membrane is being continuous with the
endoplasmic reticulum (ER) membrane. The lipid
bilayers of the inner and outer nuclear membranes are
connected at each nuclear pore.
Free ribosomes in the cytosol have a different synthesis.
ER-bound ribosomes
Golgi apparatus: vesicles split from the ER, travel from
ER to golgi apparatus and from the Golgi apparatus to
other destinations.
Peroxisome: involved in oxidizing processes.
Lysosome: peptides are degraded here.
Cytoplasm: organelles + cytosol
Prokaryotes don’t have a nucleus and other organelles.
The lipid bilayer
The membrane consists of a bilayer with an inner and
outer leaflet.
The polar head groups of the phospholipids are pointed
outside the cell and the hydrophobic tails inside.
Membrane proteins are for example receptors that
receive a signal from outside the cell and transmit it to
the inside of the cell.
Ions, glucose and other food components are
transported to the membrane by transport proteins.
Plasma membrane: 50% of mass is protein and 50% of
the mass is lipid molecules. There is no water inside of the plasma membrane.
Membrane lipids are amphiphilic molecules
Phospholipids always have:
- A phosphate group
- Glycerol unit
- Two fatty acid tails. Fatty acid tails like to be in
oil and are bended when there is a cis-double
bond between the carbon atoms.
- Nitrogen group
,Four major phospholipids in mammalian plasma membranes
(A) Head group: ethanolamine
(B) Head group composed of serine. It has an overall
negative charge. The negative charge is always present
on the inner side of the membrane (inner leaflet),
pointing towards the cytosol.
(C) The head group composed of choline is polar. The
tails are hydrophobic.
(D) Has a very different structure than A, B and C. It
doesn’t have a glycerol backbone but a sphingosine
backbone. It has one fatty acid tail and one fatty acid
chain (doesn’t contain a carboxyl group)
Lipid molecules spontaneously aggregate in water
Micelles are formed in water when the lipids own only one tail.
Lipid bilayer is formed in water by phospholipids that own two
tails.
The spontaneous closure of a phospholipid bilayer
When a flat bilayer is exposed in water, the phospholipid bilayer
folds and forms a sealed enclosed compartment by pointing the
hydrophobic tails inside.
Liposomes are commonly used as model membranes
Liposomes form a bilayer. All the hydrophobic tails are on the inside and not exposed to
water.
The lipid bilayer is a two-dimensional fluid
The lipid bilayer is very flexible, which is needed because
cells need to be able to change their shape.
Lipids can diffuse in the same leaflet of the membrane.
They almost never flip-flop from the one side of the
leaflet to the other, because it’s energetically
unfavourable for a polar head group passing a
hydrophobic membrane to expose the polar head group
to the other side of the membrane.
Flexion: extending chains
The fluidity of a lipid bilayer depends on its composition and its temperature
If the temperature goes up, fluidity increases.
By making the chains of lipids longer, there will be more hydrophobic interactions and the
membrane will be less fluid.
If there are more double bonds in the lipids of the membrane, kinks in the membranes
causes the tails can’t interact because the distance is too large, meaning more fluidity in the
membrane.
, Eukaryotic plasma membranes contain large amounts
of cholesterol
Cholesterol is not a phospholipid. It doesn’t have a
glycerol molecule. It is a sterol. It contains a rigid steroid
ring structure, to which is attached a single polar
hydroxyl group and a short nonpolar hydrocarbon
chain. The cholesterol molecules orient themselves in
the bilayer with their hydroxyl group close to the polar
head groups of nearby phospholipid molecules.
Cholesterol enhances permeability-barrier properties & inhibits phase transitions
Cholesterol is found a lot in red blood cells. Cholesterol sits in between the phospholipids of
the plasma membrane. The more cholesterol in the membrane, the less permeable the
membrane is to all kind of molecules. E. Coli bacterium doesn’t have cholesterol and does
have phosphatidylethanolamine.
Glycolipids are found on the surface of all animal cell plasma membranes
Glycolipids resemble sphingolipids, but, instead of a phosphate-linked head group, they
have sugars attached. So glycolipids consist of hydrophobic tails, connected to glycerol
where sugar is to attached. They are only on the outer leaflet of the plasma membrane.
They are pointed towards the outside of the cell and important for cell recognition. (Cell-cell
recognition to form tissues)
Glycoproteins are also present in the outer leaflet of the membrane and can be attached
with glycolipids to the membrane, but they can also have sugar molecules by themselves.
Raft domain: thicker part of membrane that is involved in cell recognition.
Asymmetrical distribution of phospholipids and glycolipids
The distribution of phospholipids is asymmetrical. Some lipids are only found on the outside
of the membrane and some only on the inside.
Lipids only found in the inner leaflet of the membrane are phosphatidylethanolamine and
phosphatidylserine.
Cholesterol is found on both sides of the leaflet
Glycolipids, phosphatidylcholine and sphingomyelin are only found in the outer leaflet of
the membrane.
Membrane proteins can be associated with the lipid bilayer in various ways
Membrane proteins are amphiphilic.
Integral membrane proteins/ trans membrane proteins
1. ∂-helix is embedded in the membrane. N-terminus is sticking to the outside and the
C-terminus to the inside. Part of the molecule is attached to a lipid molecule to the
membrane.
2. 3 ∂-helices are embedded in the membrane.
3. ß-sheet in the membrane.
Other membrane proteins
4. Embedded at the inner leaflet.
5. Attached to the inner leaflet of the membrane only through lipid molecules.
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