structure of cell membrane - matteus de silva Flashcards
Nuclear envelope
membrane enclosing the nucleus . the protein lined pores allow material to move in and out
chromatin
DNA plus associated protein
nucleolus
condensed region where ribosomes are formed
peroxisome
metabolises waste
rough endoplasmic reticulum
associated with ribosomes ; makes secretory and membrane proteins
smooth endoplasmic reticulum
makes lipids
microtubules
form the mitotic spindle and maintain the cell shape
centrosome
microtubule organising centre
intermediate filaments
fibrous protein that hold organelles in place
microfilaments
fibrous proteins that form the cellular Cortex
lysosome
digests food and waste material
Golgi apparatus
modifies proteins
mitochondria
produces energy
functions of the cell membrane
protective barrier
contains cytoplasm (fluid in cell)
regulated transport in and out of cell (selectively permeable)
interlocking surfaces bind cells together
allow cell recognition
provide binding site for enzymes
provide anchoring sites for filaments of cytoskeleton
receives signals from outside the cell for internal cellular activities
imports and exports molecules
movement of the cell
general structure of the cell membrane
a lipid bilayer that contains 2 sheets of lipids inter dispensed with proteins
Lipid structure
hydrophilic head- H2O loving due to polar group in the head
hydrophobic tail - H20 hating due to the long hydrocarbon tail
Amphiphatic molecules
contain both hydrophobic and hydrophilic parts
Lipids
most abundant lipid is the phospholipid
phospholipids have a PO4 group in the third OH- group of the glycerol instead of hydrocarbon
this can attach a hydrophilic group
- choline - phosphylicitycholine
-polar amino acids like serine - phosphatidylserine
properties of the lipid bilayer
due to the amphiphatic property of the membrane can reseal after an ‘injury’
bilayer is fluid - the orientation of the lipids and the outer aqueous surroundings keep the lipids in the bilayer
- the lipid can move around the layer
not the same as flexible - entire membrane being bended
importance of hydrocarbon on properties of layer
hydrocarbon tail will determine the fluidity of the membrane just as it does in fats and oils
two components are important
^the length of the hydrocarbon chain
- 14 to 24c but usually 18-20 present
^level of unsaturation (number of double c bonds)
- 1 tail has 1 or more double c bonds
- other tail is saturated
membrane fluidity
enables the membrane proteins to diffuse rapidly . simple means of distributing lipids and proteins allows membranes to fuse with one another . evenly distributed during daughter cell formation
cholesterol in the membrane
cholesterol is added to areas that have lots of unsaturated lipids to help fill in the gaps between the tails
helps to stiffen and stabilise the bilayer
> less fluid and less permeable
vesicles
the size of the membrane is increased and decreased with the interaction of vesicles . vesicles bringing material to the membrane to be secreted . increase the surface area of the plasma membrane and through the process of endocytosis , the surface area of the cell membrane decreases
endocytosis
process in which a substance gains entry into a cell without passing through the cell membrane .
results in the formation of an intracellular vesicle by virtue of the invagination of the plasma membrane
example : antibodies
phagocytosis
process by which some living cells ingest or engulf other cells or particles
the phagocyte may be a free living one celled organism such as an amoeba , or one of the body cells such as a white blood cell
example : white blood cell engulfing bacteria
pinocytosis
small particles suspended in extracellular fluid are brought into the cell through an invagination of the cell membrane
receptor- mediated endocytosis
binding of ligands allow uptake of specific molecules
example cholesterol intake:
low density lipid proteins bind cholesterol and carry it in the blood
LDL’s act as ligands binding receptors on cell surface and cholesterol is taking into the cell
exocytosis
exocytosis is the process by which cells move materials from within the cell into the extracellular fluid
exocytosis occurs when a vesicle fuses with the plasma membrane , allowing its contents to be released outside the cell
requires atp for the active transport
example : cell expels chemical transmitters such as hormones
example : insulin being released by pancreas cells using exocytosis
functions of cell membrane proteins
transport
enzymatic activity
intercellular joining
attachment to the cytoskeleton and extracellular matrix
signal transduction
cell-cell recognition
Proteins
integral/trans membrane
-proteins that cross both layers of the phospholipid bilayer
peripheral
-proteins that are only on the top half or the bottom half of the phospholipid bilayer
trans-membrane proteins carry out the function
transporters- na+ pump to move na+ across
linkers - integrins to link intercellular components to extracellular ones
receptors - to bind a compound that sends a signal to the rest of the cell
enzymes - perform chemical reactions in the membrane
association with the membranes
transmembrane - span the entire membrane
linked by lipids - on either surface of the membrane interaction with transmembrane proteins
integral membrane proteins
are amphiphatic , in that they have hydrophilic and hydrophobic regions that are oriented in the same regions in the lipid bilayer
examples :
na+-k+ ATPase
glucose permeate
hormone receptors
ion channels and gates
histocompatibility antigens
certain enzymes of the electron transport chains
gap junction proteins
single transmembrane segment proteins
usually alpha-helical = 20-25 residues , mostly non polar
example : glycophorin of the erythrocyte , antibodies
multiple transmembrane segment proteins
7 alpha helix motif is very common
example : highly selective potassium ion channel
membrane spanning regions can also be beta barrel structure
multiple trans membrane segments form B sheets that line a cylinder
example : porins (non specific bacterial channels )
peripheral membrane proteins
associated with membranes but do not penetrate the hydrophobic core of the membrane . found in association with integral membrane proteins
examples : the NADH-CoQ reductase complex of enzymes and cytochrome c reductase complex of enzymes are integral membrane proteins of the mitochondrial electron transport chain
