The right shape

About this video

Chew a piece of plain bread for a minute without swallowing and it starts to taste sweet. Nobody added sugar: an enzyme in your saliva is breaking down starch, long chains of sugar units, into smaller sugars that taste sweet. It is the first step of digestion.

A cell is full of chemistry, and on their own most of its reactions would go far too slowly to keep anything alive. Enzymes speed them up. Almost all enzymes are proteins, and an enzyme is not used up by the reaction it helps: it comes out unchanged and can do the same job again and again.

Each enzyme has a pocket on its surface that only the right molecule fits, so exactly that scientists first pictured it as a key in a lock; today we know the enzyme also changes shape a little as the molecule settles in. That is why each enzyme does one job: the enzyme in saliva works on starch, but not on fat or protein.

Because an enzyme works by its shape, anything that changes the shape stops it. Strong heat does, and so does too much or too little acid, and the change usually can't be undone. You see it with the proteins in egg white: heat makes them lose their shape, and the clear, runny white turns white and firm, and stays that way when it cools. A cut apple turns brown because an enzyme uses oxygen from the air; lemon juice slows it, because acid makes the enzyme work less well. This video goes with the lesson Enzymes: The Cell's Helpers in Biology Level 1 on Knaph.

Transcript

1. 0:00Chew plain bread for a minute without swallowing. It starts to taste sweet. Why?
2. 0:10Bread is full of starch, long chains of sugar units.
3. 0:14An enzyme in your saliva cuts the chain into small sugars, and they taste sweet.
4. 0:20After each cut, the enzyme moves on, unchanged.
5. 0:25An enzyme speeds up a reaction and is not used up, so it can do the same job again and again.
6. 0:34Each enzyme has a pocket, and only the right molecule fits it. Starch slides in, and the enzyme holds it tight.
7. 0:42A fat molecule has the wrong shape. It does not fit.
8. 0:47Like a key in a lock, each enzyme does one job.
9. 0:54Heat bends an enzyme out of shape, and then it stops working, usually for good.
10. 1:03Egg white is made of proteins too. Heat it, and it turns white and firm.
11. 1:09Cool it down, and it stays white. The shape does not come back.
12. 1:16Cut an apple, and an enzyme turns its flesh brown, using oxygen from the air.
13. 1:22Put lemon juice on one half. After half an hour, it has browned much less.
14. 1:28Acid makes the enzyme work less well.
15. 1:34Enzymes make life's chemistry fast enough, each one shaped for a single job.