1.3 membrane structure

Question 1

what was the the Davson-Danielli model

Answer 1

• invented in 1930
• proposed that the membrane structure was like a sandwich
  two layers of protein sandwiching a bilayer of phospholipids

Question 2

what was the evidence for the Davson- Danielli model

Answer 2

• chemical analysis showed that the membrane was composed of phospholipids and protein
• electron micrograph images showed the membranes as two dark lines separated by a lighter area
• because membranes are semipermeable the proteins could be acting as a barrier to some substances

Question 3

evidence against the Davson-Danielli model

Answer 3

• electron microscopes showed globular proteins were in the centre of the phospholipid layer
• analysis of the protein showed that some were hydrophobic
• some proteins extended all the way through and/or within the phospholipid layer due to the hydrophobic properties
• an experiment was done were fluorescent markers were attached to antibodies which binded to membrane proteins of mouse cell (red) and human cells (green), which were then fused together. after 40 minutes both green and red markers were mixed through the membrane of the fused cell
• this proved that membrane proteins are free to move within the membrane rather being fixed in a peripheral layer

Question 4

What is the Fluid Mosaic model

Answer 4

-created by Singer Nicolson
proposes that:
-all cells have a cell membrane
-cell membrane is made of a double layer of phospholipids with proteins and cholesterol embedded in it.
-the cell membrane is semipermeable

Question 5

what does semipermeable mean

Answer 5

• allows some molecules through but not all molecules.

this is useful as the cell membrane controls what goes in and out of the cells

Question 6

phospholipid

Answer 6

phospholipid molecules are composed of a polar (charged) phospate head which is hydrophilic and two non-polar fatty tails which are hydrophobic.

because the head is hydrophilic (attracted to water) and the tails hydrophobic (repelled by water) the head is positioned on the outside of the membrane and the tails on the inside.

the head behaves as a solid and the tails as a liquid so the over overall effect is that the cell membrane is ‘fluid’

the attraction of the tails to each other cause the membrane to be stable and flexible.

Question 7

the phospholipids are amphipathic

Answer 7

part of the molecule (the head) is attracted to water and part (the tails) are repelled by water.

Question 8

cholesterol

Answer 8

• a component of animal cells (about 20%)
• embedded in the phospholipids
• have hydrophilic and hydrophobic ends
• reduce fluidity and permeability of the membrane to ions e.g. sodium and Hydrogen ions

Question 9

where is cholesterol in the cell

Answer 9

-embedded in the phospholipids

Question 10

intergal proteins

Answer 10

• transmembrane
• spans the entire way through the cell membrane
• embedded in the bilayer

Question 11

what do membrane proteins do

Answer 11

-they have individual effects that contribute to the semi-permeable nature of the membrane

Question 12

peripheral proteins

Answer 12

• attached to the outer surface of the cell and are involved in cell to cell communications

Question 13

channel proteins

Answer 13

-act as pumps- transport

Question 14

glycoproteins

Answer 14

• sugar units attached to the outer surface of the membrane which are used for cell recognition by the immune system and working as hormone receptors

Question 15

membrane proteins are diverse in terms of function

what are the different function types

Answer 15

T.R.A.C.I.E

• Transport
• Receptors
• Anchorage
• Cell recognition
• Intercellular joinings
• Enzymatic activity

Question 16

T.r.a.c.i.e.

Transport

Answer 16

• channel proteins which use facilitated diffusion

- protein pumps which use active transport

Question 17

T.r.a.c.i.e.

receptors

Answer 17

• used for hormones

e. g. insulin, glucagon

Question 18

t.r.a.c.i.e.

anchorage

Answer 18

cytoskeleton and cell wall attachments

Question 19

t.r.a.c.i.e

cell recognition

Answer 19

immune system proteins

Question 20

t.r.a.c.i.e

intercellular joinings

Answer 20

cells attached to eachother

e.g. plasmodesmata

Question 21

t.r.a.c.i.e.

enzymatic activity

Answer 21

the movement of ions back and forth across the membrane

Question 22

what are membrane proteins diverse in

Answer 22

structure
position in the membrane
function

Question 23

phospholipids have amphipathic, what does this mean

Answer 23

part of them is hydrophobic and part is. hydrophilic

• hydrophobic fatty acid tails
• hydrophilic head