Re: Why higher temperatures increase the release of pigment from a vacuole

Hi there!

Sorry this took me so long. I wasn't sure exactly what you were thinking 
about at first.
I got a hint that this is a typical experiment done at high school in the 
UK, in which case you are truly talking about heating beetroots. I was a 
bit confused by your expression "secretion". By this I would mean a more 
orderly movement of something towards the outside of a cell.

Indeed, when you heat a beetroot, you disrupt the cell membranes. A 
biological membrane is made of a so-called lipid bilayer. These are formed 
because the phospholipids that construe it have one polar "water-loving" 
end and one fat end. The fat ends pack together, exposing only the polar 
ends to the water. The most effective way of doing this if there are a lot 
of these lipids around, is to create like two blankets one atop of the 
other, with the fat sides towards each other. This is the lipid bilayer. 
In a cell they form sacks. One goes all around the cell (the plasma 
membrane), others may form vacuoles (such as the tonoplast). Yet others 
may be like stacks of half empty bags (the endothelial reticulum, which is 
also continuous with the nuclear envelope). A cell is like a set of bags 
one inside the other! In these lipid seas, there are huge amounts of 
proteins in various degrees of submersion. Some span all the bilayer, thus 
being exposed on both sides. Others just drift on either of its surfaces. 
Typically, you will find that about 70% of a cell membrane is protein! The 
water around and within the compartments formed by the lipid bilayers is 
also crammed with protein. It's actually more gel-like than "watery".

So what happens when you heat this? When you heat something you give 
energy. Molecules start to spin and vibrate faster. They fly around faster 
too. First thing you might think of, is that the water will expand. This 
will have a disruptive effect on any membrane in its way. To make things 
worse, lipids become more fluid as temperature goes up (think of what 
happens when you heat butter) so the membranes become more fragile. 
Proteins are quite fantastic machinery: they're formed of coiled and 
folded strings of amino-acids. It's a bit as if one made a car motor in a 
long string that is then folded into the functional thing. Problem is, if 
you heat them too much, they will untangle and brake apart (vibrations 
again). When this happens to the proteins spanning a  lipid membrane, they 
will form holes that will definitely destroy the delicate structure. Now, 
if you have pigments in the innermost compartment they will spill out. In 
the case of beetroots, the pigments you observe are water soluble and not 
stored within the lipid bilayers, but in the watery enclosure of the 
vacuole. If you want to solve lipid-embedded pigments, try putting a leaf, 
or indeed a beetroot in an organic solvent such as acetone and see what 
you get. You brake down the structure between the phospholipids, not the 
phospholipids themselves that much. The proteins on the other hand, are 
truly destroyed.

The basic structure of the tonoplast is the same as the plasma membrane 
(as described above). In this respect, they're similar. You have a bit 
higher proportion of protein in the plasma membrane than in the tonoplast. 
Then of course, the proteins present in the plasma membrane are a complete 
different set than those found in the tonoplast. You have hundreds of 
different kinds of proteins in the plasma membrane and nearly all of them 
are unique to this membrane. Ditto for the tonoplast. In this respect 
they're very different. You will also find some differences in the 
composition of the polar heads of the phospholipids.

I hope this answers your questions.
Kind regards,
Erik