Discuss alternative models of membrane structure including evidence for or against each model.

Outline the process used to load organic compounds into phloem sieve tubes.

a. early evidence showed membranes are partially permeable AND organic solvents penetrate faster than water 

b. suggests they have non-polar regions 

c. chemical analysis showed membranes consist mainly of proteins and lipids 

d. layer of phospholipids spread over water, orientate themselves into monolayer with nonpolar/hydrophobic tails out of water and polar/hydrophilic heads in water surface 

e. when shaken with water form micelles/particles with tails inwards away from water 

f. Davson–Danielli model proposed phospholipid bilayer coated with protein molecules on both surfaces 

g. evidence from electron microscopy «supported Davson–Danielli model» 

h. three-layered structure/ sandwich/railway tracks/two dark bands with a light band between 

i. model could not account for hydrophobic proteins / artifacts due to low resolution 

j. fluorescent labelling / freeze fracturing later used to investigate membrane structure 

k. led to Singer-Nicholson / fluid mosaic model of protein molecules floating in fluid lipid bilayer 

l. shows particles/proteins project partially and sometimes right through lipid bilayer 

m. indicates peripheral and integral proteins present

Accept any of the points clearly explained in an annotated diagram.

a. active transport/loading of sucrose/amino acids/organic metabolites 

b. sucrose moves by apoplastic / symplastic routes 

c. «loading» at source into companion cells «of sieve tubes» 

d. movement «of sucrose» through plasmodesmata 

e. high concentration of solutes in phloem leads to water movement by osmosis