What is a membrane?

Question 1

why are membranes important?

Answer 1

• 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

Question 2

what are the functional differences between cell types due to ?

Answer 2

variations in the composition of their plasma membranes

ie different cells interact in different ways with essentially the same ECF

Question 3

what is the cell membrane compose of ? ie what are the two major constituents

Answer 3

lipid and protein

Question 4

saturated = double bond

Answer 4

unsaturated = single bond

:(

Question 5

is the phospholipid bilayer fluid or rigid?

Answer 5

it is fluid

• the phospholipids are constantly moving

Question 6

what substance aids in the stiffening of the membrane

Answer 6

cholesterol

Question 7

what kinds of molecules can cross the phospholipid bilayer ?

Answer 7

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

Question 8

what kind of appearance does the cell membrane have

Answer 8

trilaminar

Question 9

what are three important functions of the lipid bilayer?

Answer 9

• 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)

Question 10

what are the two types of membrane proteins and what are the differences between the two?

Answer 10

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

Question 11

what are some functions of integral membrane proteins (5) ?

Answer 11

-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

Question 12

other than peripheral and integral proteins, what other membrane proteins are there ?

Answer 12

docking - marker proteins

*located on the inner membrane surface, they interact with secretory vesicles leading to exocytosis of the vesicle contents

Question 13

what does it mean when a membrane is said to be ‘sugar-coated’

Answer 13

when there is a small amount of carbohydrate on the outer surface of the cell

Question 14

what is the glycocalyx

Answer 14

a layer comprised of glycoproteins and glycolipids

• short carbohydrate chains are often bound to membrane proteins and to a lesser extent - lipids ie glycolipids

Question 15

what is the significant function of membrane carbohydrates ?

Answer 15

they serve as self-identity markers ie self-antigens

Question 16

what other functions do membrane carbohydrates have?

Answer 16

• 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

Question 17

what are the three specialised cell junctions

Answer 17

1. tight junctions
2. desmosomes
3. gap junctions

*refer to notes