Science Experiments

White bread and the wonder of enzymes

Sun, 24th Sep 2006

Part of the show Catalysts for Cleaner Environments and Future Energy

What you Need

trees A slice of cheap white bread.

 

mouth A mouth to put it in.

What to do

  1. Take half a slice of bread and chew it and chew it and chew it! Even if the bread becomes disgusting, you should still keep chewing amd remember - don't swallow or you'll spoil the experiment!
  2. Pay attention to how the flavour of the bread changes.
  3. Once you think you have the answer, you can swallow the bready mush or (this is probably best!) spit it out.

What may happen

As you chew the bread, you may have noticed that it slowly tastes sweeter. Particularly the juice that is coming out.

Why does it happen?

Foods like bread, rice and potatoes are foods that are mostly made of starch. Starch is a molecule that plants make in order to store sugar. It consists of long chains of glucose (some of them branch).

 Starch

In your saliva you have an enzyme called amylase, this will cut up the large starch molecules at random until you are left with sugars made of 1,2 or 3 glucose molecules long (glucose, maltose or dextrin).

 

Starch Being Broken Down

Maltose

 

Starch doesn't taste sweet, but glucose and maltose do, so as you chew the bread it will slowly become slightly sickly sweet.

Why do plants make starch in the first place?

All living things are made up of cells these are surrounded with membranes which will let water pass through them but not larger molecules such as salt or sugar.

If there are more salt or sugar molecules on one side of the membrane than the other, water is pulled towards the salt/sugar - this is called osmosis.  This is why slugs shrivel up if you sprinkle salt on them, all the water is sucked out of them.

Plants convert water and carbon-dioxide into glucose using the energy from the sun.  If the plant is doing particularly well it will produce huge amounts of glucose in it's cells.  This will cause water to be pulled into the cell.  The problem is that this will happen until the cell explodes.

A cell

The solution is for the plant to glue all the sugar molecules together into a starch molecule, which just being one molecule rather than thousands will suck in water much less by osmosis, reducing the problem hugely.

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