BIOLOGY - MCAT
Nucleus
Contains all the genetic mateoral necessary for replication of the cell. It is
surrounded by the nuclear membrane or envelope, which is a double membrane that
maintains a nuclear environment seperate and distinct from the cytoplasm.
Nucler pores in the nuclear membrne allow selective two-way exchange of material
between the cytoplasm and the nucleus.
DNA contains coding regions - genes. Linear DNA is wound around organizing
proteins known as histones, and is then further wound into linear strands called
chromosomes
Nucleolus - where ribosomal RNA - rRNA is produced- makes up about 25% of the
nucleus
Plasma membrane (eukaryotes)
Amphipathic phospholipids that have a polar head and a non-polar tail- bilayer
membrane- polar head face the intracellular and extracellular (aqueous)
environments and the non-polar tails remain inside the membrane.
mostly phospholipid with proteins and cholesterol
The presence of cholesterol and lipid rafts within the plasma membrane help
contribute to the fluidity of the membrane at lower temperatures and to its stability at
higher temperatures.
can add fluidity at low temperatures - allows the cell membrane to be stable at high
temps
The bilayer is permeable to very small uncharged molecules and to lipid-soluble
molecules- which can freely diffuse through the cell membrane. The membrane is
not permeable to larger, hydrophilic molecules such as glucose.
Phospholipids are involved in cell signaling, second messengers
Second messengers are small molecules and ions that relay signals received by
cell-surface receptors to effector proteins.
membrane fludity
Is determined by the cocnentration of cholesterol and the tail length of fatty acids
Glycerophospholipids (phosphoglycerides)
phospholipids that contain a glycerol backbone bonded by ester linkages to two fatty
acids and by a phosphodiester linkage to a highly polar head group
,most common type of phospholipid
each of the bonds to the glycerol backbone can undergo hydrolysis, which breaks
the bond as water is added - when all bonds are broken, glycerol is released, along
with two fatty acids and the phosphate headgroup
the initial concentration of glycerophospholipids was 20mM because one glycerol
molecule is released per glycerophospholipid, the final concentration of glycerol
approaches 20mM as the reaction proceeds
because two fatty acids are initially attached to each glycerol molecule, the final
concentration of fatty acids (40 mM) is twice that of glycerol
glycerophospholipids vs triglycerides
glycerophospholipids have two fatty acid chainds and a phosphate group -
triglyceride has 3 fatty acids and a glycerol group
Differentiating phospholipids
Each phospholipid has a unique mass,charge, and solubility due to distinct features
in the backbone, poalr head groups, and fatty acid chains - they may be separated
based on each of the following properties
Composition of the head groups and the length of the tails create lipids with distinct
charge,mass, and solubility
charge on phospholipids
Is determined by the structure of the polar heads - the phosphate component of the
head group may be linked to various chemical groups - positively charged groups
such as choline or ethanolamine neutralize the negative charge of the phosphate
whereas neutral groups such as serine allow the ehad to maintain a net negative
charge
Mass of phospholipids
Is established by the molecular weight of the backbone and the polar head groups -
backbones can contain either a glycerol with two fatty acid tails or sphingosine with
one fatty acid tail
Glycerophospholipids tend to be larger than sphingophospholipids because thay
have two fatty acid tails instead of one - polar head groups may be as small as a
phosphate (phosphatidic acid) or as large as phosphatidylglycerol. They can be
separated by size-exclusion chromatography
,solubility of phospholipids
Depends on both the hydrophobic and hydrophilic regions of the molecule - charged
head groups are more soluble than neutral groups because they more readily form
hydrogen bonds wheras lipids with long hydrophobic chains have decreased
solubility
can be separated by thin-layer chromatography
Sphingolipids
long fatty acid chain
polar head group
backbone=amino alcohol (not glycerol)
short unsaturated fatty acid tails increase membrane fluidity by preventing
phospholipids fro clustering together, but longer saturated tails such as those
typically found in sphingolipids induce lipid clustering and decrease fluidity
Lipid rafts
regions of relatively high order (low entropy)- help localize distincy processes or
pathways to specfic regions of the membrane by recruiting the proteins and other
components involved and holding them in proximity to each other (rather than being
allowed to drift freely)
To maintain a more ordered state, lipid rafts tend to be rich in sterols (cholesterol in
animals and similair molecules called phytosterols in plants) and are relatively poor
in phospohlipids compared to other regions of the membrane
High temperatures - provides rgidity and stabilizes the membrane
Lowe temepratures - increase membrane dluifity by preventing phospohlipids from
clustering together
membrane lipid
Cholesterol stabolizes the membrane fluidity by both decreasing fluidity at higher
temperatures and increasing fluidity at lower temepratures
Able to accomplish this because its hydrophobic portion is made of four fused rings -
this structure restricts the rotation of the bonds involved, imparting rigidity and
decreasing fluidity at high temperatures
, It also impairs packing of the fatty acyl tails of the phospholipid component of the
membrane, increasing fluidity at low temperatures
esterification of cholesterol would replace the hydrophilic hydroxyl group with a much
less polar ester bond connected to a large acyl group - this modification reduces the
ability of cholesteryl esters to interact with water and destabilizes the localization of
cholesteryl esters on the exterior-facing surface of membranes
Hydrophobic interactions push the added acyl tail to the interior side of the
phospohlipid monolayer, and the overall nonpolar chelesterl ester diffuses from the
surface to the core of the lipoprotein particle
fluid mosaic model
model that describes the arrangement and movement of the molecules that make up
a cell membrane
Composed largely of phopholipids
Said to be fluid as its various non-phospohlipid components can migrate laterally
(side to side) through the entire phospholipid-rich surface of the cell in any direction
Other important comonents include cholesterol, glycoproteinsl, and glycolopids
transmembrane proteins are other components that can move laterally
fluidity of cell membranes
affected by several factors, such as temperature and membrane composition -
increasing the temperature increases the average kinetic energy of the membrane
components - increasing fluidity
vice versa with a decrease in temperature
under prolonged exposure to an external stimulus that affect fluidity, cells may initiate
a homeostatic response to counter this change - such as by changing their
membrane composition
So, for example: in a colder environment, since fluidity is decreased, to maintain
homeostasis, the phospholipid would increase in unsaturated fatty acids - since they
have a double bond and more pi bonds - they increase fluidity
LDL cholesterol
bad cholesterol, an increased amount of LDL cholesterol change the composition of
the cell membrane, making it more rigid, which can impair the proper formation of
trafficking vesicles
integral proteins
penetrate the hydrophobic interior of the lipid bilayer
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