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Samenvatting molecular biology of the cell 2
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Samenvatting molecular biology of the cell 2 (pharmacie)
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MOLECULAR BIOLOGY OF THE CELL 2CHAPTER 10
Membranes are crucial to the function of organelles. In a eukaryotic cell, a number of
organelles play an important role:
- Plasma membrane (the membrane that encloses the cytoplasm)
- Cytosol (the liquid in the cell)
- Cytoplasm (cytosol + all the organelles; protein synthesis and degradation)
- Nucleus (the core of the cell, DNA and RNA are made here)
- Nuclear envelop (the membrane of the nucleus, it controls import and export to
the nucleus. It has a double membrane and pores; these pores allow passage
of molecules from nucleus to cytoplasm. It extends in the ER)
- Endoplasmic reticulum (important in the synthesis of lipid molecules and
proteins. Polyribosomes are bound to the membrane of the ER. Store Ca2+)
- Mitochondria (the organelles that provide the energy of the cell by producing
ATP; energy packages)
- Golgi-apparatus (the organelle that finishes proteins by adding, for example,
phosphate groups or sugars. It is made of membranes
stacked on top of each other. Sends the proteins to
final destination)
- Lysosomes (important for degradation of food
compounds; enzymes)
- Endosomes (small vesicles that are released from the
plasma membrane to transport food compounds to the
lysosome by fusing with them; transport system of the
cell)
A biological membrane has two layers, an outer and inner layer, which is called the
double membrane layer. Proteins that are embedded in the membrane are called
membrane proteins. Lipid molecules form the double membrane layer (lipid bilayer).
Lipid molecules have a hydrophilic head and a hydrophobic tail.
The heads are covalently attached to the hydrophobic tail. The
plasma membrane consists of 50% proteins and 50% fats.
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.
All the lipid molecules in cell membranes are amphiphilic, they have a hydrophilic
(polar) head and a hydrophobic (nonpolar) tail. The most common membrane lipids
are phospholipids, these have a polar head group containing a phosphate group and
two hydrophobic hydrocarbon tails. These tails are usually fatty acids, of which one
,usually is saturated and the other contains one or more cis-
double bonds, unsaturated. Phosphoglycerides are the main
phospholipids in animal cell membranes. They have a three-
carbon glycerol backbone and two long-chain fatty acids are
linked through ester bonds to adjacent carbon atoms of the
glycerol, and the third carbon atom of the glycerol is attached to
a phosphate group, which in turn is linked to one of several types
of head group. By combining several different fatty acids and
head groups, cells make many different phosphoglycerides.
The other 4 major phospholipids in mammalian
plasmamembranes are:
- Phosphatidylethanolamine (phosphoglyceride)
- Phosphatidylserine (phosphoglyceride) (-)
- Phosphatidylcholine (phosphoglyceride)
- Sphingolipids (they are built from sphingosine
rather than glycerol)
Sphingosine is a long acyl chain with an amino
group (NH2) and two hydroxyl groups (OH) at one
end. In sphingomyelin, the most common
sphingolipid, a fatty acid tail is attached to the
amino group, and a phosphocholine group is
attached to the terminal hydroxyl group.
Next to phospholipids, the lipid bilayer contains glycolipids and cholesterol.
Phospholipid molecules form bilayers spontaneously in
aquatic environments. They aggregate to bury their
hydrophobic tails in the interior, shielded from the water,
and they expose their hydrophilic heads to the water.
This way they can either form micelles (1 hydrophobic
tail) or bilayers. This also gives them the ability to self-
sealing properties, because contact with water inside a
bilayer is energetically unfavorable. This behavior was
fundamental for the creation of living
cells. This structure is called a liposome
and is energetically very favorable.
Liposome →
Lipids are in state to dynamically shift. One of those shifts is a flip-flop in which the
membrane switches places, this rarely occurs, unless it’s cholesterol (can flip-flop
rapidly). Phospholipid molecules in bilayers migrate from the monolayer on one side
to that on the other, but phospholipid molecules are manufactured in only one
monolayer of a membrane, mainly in the cytosolic monolayer. If none of these newly
made molecules could migrate reasonably promptly to the noncytosolic monolayer,
,new lipid bilayer could not be made. The problem is solved by a
special class of membrane proteins called phospholipid translocators,
or flippases, which catalyze the rapid flip-flop of phospholipids from
one monolayer to the other.
Flip-flop rarely occurs, because the hydrophilic head wants to be in
contact with the water and it don’t want to be in contact with the
hydrophobic region on the tail, so it is energetically unfavorable to flip
flop.
Individual lipid molecules also rotate very rapidly about their long axis and have
flexible hydrocarbon chains. And they also exchange places with their neighbors
within a monolayer, this is called lateral diffusion.
The fluidity of a lipid bilayer depends on its composition and temperature. A shorter
chain length reduces the tendency of the hydrocarbon tails to interact with one
another, and cis-double bonds produce kinks in the chains that make them more
difficult to pack together, so that the membrane remains fluid at
lower temperatures, so all increasing the fluidity. Also, an increase in
temperature causes molecules to stir more, creating a higher fluidity.
Although cholesterol tightens the packing of the lipids in a bilayer, it
does not make membranes any less fluid. Cholesterol enhances the
permeability-barrier properties but inhibits phase transitions, so it
prevents lipid bilayers going from rigid (stijf) easily. It also fills up
spaces between phospholipids.
So to increase fluidity:
- Short chain length
- Much double bonds
- High temperature
Important: E-coli has no cholesterol and red blood cells have
membranes rich in cholesterol
There has been a long debate about whether the lipid molecules in the plasma
membrane similarly segregate into specialized domains, called lipid rafts. Specific
membrane proteins and lipids are seen to
concentrate in a temporary, dynamic fashion
facilitated by protein-protein interactions that
allow the transient formation of specialized
membrane regions. Note that because of their
composition, raft domains have an increased
membrane thickness.
The lipid composition of the 2 monolayers of the lipid bilayer is very different. Lipid
asymmetry is functionally important, especially in converting extracellular signals into
intracellular ones. Asymmetric means that you find different lipids in the two different
layers (but you find cholesterol in both layers). They also help distinguish living cells
from dead ones, in dead cells, phosphatidylserine goes to the outer monolayer
, signaling other cells to digest the dead cell. So, keeping the layers in the way they
are, is very important. Important to know is that the phosphatidylcholine and
sphingomyelin are on the outside opposed to phosphatidylethanolamine and
phosphatidylserine on the inside. Next to that, glycolipids are also on the outside.
All membranes have different proteins bound to their membrane, because of the
function of the cell. So a liver cell has different proteins than a red blood cell (different
numbers in graphic).
The translocation of the phosphatidylserine in apoptotic cells is thought to occur by
two mechanisms:
1. The phospholipid translocator that normally transports this lipid from the outer
monolayer to the inner monolayer is inactivated.
2. A “scramblase” that transfers phospholipids nonspecifically in both directions
between the two monolayers is activated.
Blue = glycolipids
Red = phosphatidylcholine
Brown = sphingomyelin
Yellow = phosphatidylethanolamine
Green = phosphatidylserine
Sugar-containing lipid molecules are called glycolipids. These are only found in the
monolayer facing away from the cytosol. They are made of sphingosine. These
molecules tend to self-associate through hydrogen bonds between their sugars and
through Van der Waals forces between their hydrocarbon chains, creating lipid raft
phases. Sugar layer on the outside → cell-cell communication & tissue formation.
When on the cell surface, they contribute to many
interactions of the cell with its surroundings and are very
important for cell recognition and play a role in cell protection.
Another type of glycolipids are the gangliosides, these
contain oligosaccharides with one or more sialic acid moieties
(samenstellingen), which gives them a negative charge. This
negative charge may be important for altering the electrical
field across the membrane surface.
Glycolipids are made of: a sphingosine, a fatty chain, a fatty
acid tail and a sugar (negative charged in gangliosides).
The membrane proteins perform most of the membrane’s specific tasks and therefore
give each type of cell membrane its characteristic functions. Membrane proteins vary
in structure and in the way they associate with the lipid bilayer, which reflects their
diverse functions.
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