Re: how does the temperature affect rate of osmosis and pigment leakage

Hi Sara!
This is a standard investigation for many students of 17-18yrs here in the 
UK, where we have exams at both ages leading to university entrance.  As 
part of their biology exmas, students have to do two extended individual 
investigations ('coursework') and the question you have asked is one that 
is often use by students here.

The pigment in beetroot is held in the vaculoe in the centre of the cell 
and so has two membranes (tonoplast and plasmalemma) to cross before it 
can get to the cellulose cell wall.  This wall is freely permeable to both 
the pigment and water, so the question boils down to (sorry!) the effect 
of temperature on the membranes and on the movement of the pigment 
molecules (by diffusion).

Anthocyanins are water-soluble, so they cannot cross the phopholipid 
bilayer of the two membranes, providing these membranes are undamaged.  
When the beetroot is cut open, the cells next to the cut will be damaged, 
so some pigment will leak out of the cells and so, after cutting the 
beetroot into standard-sized pieces, they must be quickly washed to remove 
this pigment.  the easiest way to make even-sized pieces is to cut a core 
out of the beet witha  cork-borer (8-10mm dia is great) and then to use a 
knife or scalpel to cut these into disks about 1-2mm thick - rather like 
the TV chefs do when cutting up vegetables.

Anthocyanins are also heat labile, so high temperatures (over about 70C) 
cause the purple pigment to break down into a yellow-orange pigment; for 
that reason, you should confine your experiments to lower temperatures.

Below 0C, the beet cells freeze, causing the water inside them to expand 
and the cells' membranes to rupture.  Once the beet thaws out, the pigment 
will rapidly leak from the cells into the surrounding solution.
Over 0C, as the temperature rises, so does the speed of movement (ie the 
kineteic energy) of the molecules.  Eventually, the molecules making up 
the membranes are moving so quickly that they allow the anthocyanins to 
leak through them; this normally starts to happen at about 50C.  However, 
from about 40C onwards, the proteins in the membranes start to denature 
and taht can cause 'holes' to appear which allows the anthocyanins to 
begin to leak out well below 60C.

Finally, the rate of diffusion of (any) molecule is prioportional to the 
ABSOLUTE temperature, so the molecules are moving faster and diffusion at 
30C will be faster than at 20C just because of that.  Many biologists 
think this effect is greater than it really is, so be careful in your 
answer (!) - the main effect of rising temperatures is on the structure of 
the mebranes, rather than on the rate of diffusion per se.

Because thsi experiment is a 'standard' one over here, some detailed 
background information has been published at:
 http://www.biologymad.com/resources/BEETROOT%20PIGMENT2.doc http://www.biologymad.com/resources/BEETROOT%20COURSEWORK%20PRACTICALS.doc http://www.biologymad.com/resources/beetroot.doc

The entry of water into the cells by osmosis is not of any great 
importance as the cells will already be turgid and so little (if any) 
EXTRA water will go in to them (though the cells next to the cut surface 
will no longer be under pressure from surrounding cells, so a LITTLE extra 
water will enter them)

I hope that helps!
HD2