why are membranes important?
- they form an outer boundary of every cell
- they are selectively permeable
- they control the entry of nutrients and exit of waste (and secretory) products
- maintains differences in ion concentrations inside and outside the cell
- helps in the joining of cells to form tissues and organs ie cell junction proteins
- enables a cell to repine to changes in the cell environment
what are the functional differences between cell types due to ?
variations in the composition of their plasma membranes
ie different cells interact in different ways with essentially the same ECF
what is the cell membrane compose of ? ie what are the two major constituents
lipid and protein
saturated = double bond
unsaturated = single bond
:(
is the phospholipid bilayer fluid or rigid?
it is fluid
- the phospholipids are constantly moving
what substance aids in the stiffening of the membrane
cholesterol
what kinds of molecules can cross the phospholipid bilayer ?
small, uncharged polar molecules can cross freely ie oxygen, water and carbon dioxide
*they are impermeable to almost any water soluble substance ie ions, proteins and sugars
what kind of appearance does the cell membrane have
trilaminar
what are three important functions of the lipid bilayer?
- forms the basic structure of the membrane
- hydrophobic interior serves as a barrier
- responsible for the fluidity of the membrane (ie a RBC might need to change shape to fit through a tight lumen)
what are the two types of membrane proteins and what are the differences between the two?
peripheral and integral
peripheral: these ones are not embedded in the membrane, they are attached to the cytoplasmic or extracellular surfaces of the plasma membrane (PM)
integral: associated with the lipid bilayer either by…
1. spanning the lipid bilayer once or several times - ‘transmembrane”
2. embedded but do not cross the bilayer
3. linked to a lipid component of the membrane or a fatty acid derivative
what are some functions of integral membrane proteins (5) ?
-ligand binding receptors
-adhesion molecule;es
ie form physical contact with the surrounding ECM (important in regulating cell shape etc)
*think integrins, cadherins
-carry out transmembrane movement of water-soluble (ie as pores and channels, carriers, and as pumps)
-as enzymes
-can participate in intracellular signalling
other than peripheral and integral proteins, what other membrane proteins are there ?
docking - marker proteins
*located on the inner membrane surface, they interact with secretory vesicles leading to exocytosis of the vesicle contents
what does it mean when a membrane is said to be ‘sugar-coated’
when there is a small amount of carbohydrate on the outer surface of the cell
what is the glycocalyx
a layer comprised of glycoproteins and glycolipids
- short carbohydrate chains are often bound to membrane proteins and to a lesser extent - lipids ie glycolipids
what is the significant function of membrane carbohydrates ?
they serve as self-identity markers ie self-antigens
what other functions do membrane carbohydrates have?
- different cell types have different markers
- their role in tissue growth ie cells will not overgrow their own territory except in cancer cells (!)
- cell to cell interactions ie junctions
what are the three specialised cell junctions
- tight junctions
- desmosomes
- gap junctions
*refer to notes
