Cell Membranes

Question 1

state what the 3 functions of cell membranes are

Answer 1

1. a selective barrier to the passage of molecules
2. detecting chemical signals from other cells
3. anchoring cells to adjacent cells and to the extracellular matrix of connective tissue membranes

Question 2

state 3/5 facts about cell membranes

Answer 2

1. contains two types of proteins (Integral and peripheral)
2. extracellular surface contains carbohydrates
3. cholesterol + it’s polar region forms H-bonds with the polar regions of the phospholipids
4. no chemical bonds between phospholipids so can move freely (fluid mosaic model)
5. 6-10 nm thick

Question 3

state what it is meant by the key term ‘integral membrane proteins’

Answer 3

proteins closely associated with membrane lipids and can’t be extracted from the membrane without disturbing the phospholipid bilayer

Question 4

state 4 facts about the chemical structure of integral membrane proteins

Answer 4

1. amphipathic so arranged in same orientation as phospholipids
2. some move laterally with membrane and some are fixed to peripheral proteins so are immobilised
3. most integral proteins span the entire membrane and are known as ‘transmembrane proteins’
4. polar regions may expand past the perimeter of the cell surface membrane

Question 5

state what the 3 roles of integral cell membrane proteins can be

Answer 5

1. some form channels for the transport of ions/minerals in and out of cells
2. others are associated with the transmission of chemical signals across the membrane
3. the anchoring of extracellular and intracellular protein filaments to the plasma membrane

Question 6

state what it is meant by ‘peripheral membrane proteins’

Answer 6

non-amphipathic proteins that don’t associate with the non-polar regions of the lipids in the interior of the membrane

Question 7

where are peripheral membrane proteins located

Answer 7

located on the cytosolic (interior) surface of the membrane where they’re bound to the polar regions of the integral membrane proteins and also sometimes with the polar regions of the phospholipids

Question 8

state 3 functions of peripheral member proteins

Answer 8

1. enzymes that mediate the metabolism of membrane components
2. local transport of small molecules
3. associate with cytoskeletal elements involving cell shape and mobility

Question 9

how are carbohydrates bound to the cell membrane

Answer 9

carbohydrates are covalently bound to proteins or lipids on the cell surface membrane

Question 10

state the function of the cell surface membrane carbohydrates (2 points)

Answer 10

1. monosaccharides which extend from the cell surface into the extracellular fluid where they form a layer called the Glycolax
2. enable cells to identify and interact with each other

Question 11

what is the exception in cell surface membranes which contradicts the fluid mosaic model

Answer 11

isolated regions contain certain membrane proteins that are anchored to cytoplasmic proteins or covalently linked to membrane lipids to form ‘Lipid Rafts’

Question 12

state what it is meant by the key term ‘lipid rafts’

Answer 12

cholesterol rich regions of reduced cell membrane fluidity which serve as organising centres for the generation of intracellular signals (eg - signals may arise when the cell binds to a hormone of a paracrine molecule)

Question 13

state what it is meant by the key term ‘Integrins’

Answer 13

proteins on cell surface membranes which bind to specific proteins in the extracellular matrix and link cells to proteins on the membrane of adjacent cells

Question 14

state what the three membrane junctions are

Answer 14

1. Desmosomes
2. Tight junctions
3. Gap junctions

Question 15

define what it is meant by the key term ‘Desmosomes’

Answer 15

1. desmosomes consist of a region between two adjacent cells where the apposed plasma membranes are separated by approximately 2nm
2. desmosomes hold adjacent cells together in areas subject to stretching (such as skin)

Question 16

what are desmosomes characterised by

Answer 16

desmosomes are characterised by accumulations of proteins called ‘dense plaques’ along the cytoplasmic surface of the cell membrane and these proteins serve as an anchoring point of ‘Cadherins’

Question 17

define what it is meant by the key term ‘cadherins’

Answer 17

1. cadherins are proteins that extend from the cell into the extracellular space where they link up and bind cadherins from adjacent cells
2. the presence of more desmosomes between cells increase the structural stability of tissue

Question 18

what is the function of ‘keratin filiments’

Answer 18

keratin filaments anchor the cytoplasmic surface in desmosomes to interior structures of the cell

Question 19

describe what it is meant by the key term ‘tight junctions’

Answer 19

tight junctions form when extracellular surfaces of adjacent plasma membranes join together so there is no extracellular space left between them

Question 20

state the 2 functions of tight junctions

Answer 20

1. forms a bond around the entire circumference of the cell surface membrane
2. tight junctions impediment of molecular movement between cells is not absolute, water and ions can still get through

Question 21

describe what it is meant by ‘gap junctions’

Answer 21

gap junctions consist of protein channels linking the cytosols of adjacent cells

Question 22

describe the structure of gap junctions

Answer 22

within the gap junction region, opposing plasma membranes come within 2-4nm within each other which allows connexins (specific proteins) from the two membranes to join which forms small protein lined channels connecting the two cells together

Question 23

what is the function of gap junctions

Answer 23

the small diameter (1.5nm) of gap junctions limits what can pass between the cytosols of the two connected cells to small ions and molecules and excludes the exchange of large proteins

Question 24

microscopic observation of cells:

Answer 24

1. 1000 um (micro) in 1 mm

2. 1 nm (nano) = 0.001 um