Digestive enzymes (amylase, proteases, lipases)

Biology · Anatomy

made here isn't always works here

The salivary glands, the stomach, the pancreas and the small intestine all make enzymes, and the liver makes bile. Enzyme-making is shared out — no single organ does the job.

Switch between the two views, then tap an organ to find what it makes and where that enzyme goes to work.

One enzyme, one encounter

Step through it — the second moment is the one that explains why each enzyme has only one job.

1. 1Molecules collide with the enzymeFood molecules of every kind are knocking against the enzyme. Each one meets the same dent in its surface, called the active site.+
2. 2Most are turned awayA molecule whose shape does not match the active site cannot settle into it, so it drifts off unchanged. Most collisions end here.+
3. 3The matching substrate fitsThen a molecule with the matching shape arrives and slots into the active site, the way one key fits one lock and no other.+
4. 4The bond is brokenHeld in the active site, the substrate is split, and the large molecule becomes smaller ones.+
5. 5The products leave and the enzyme goes againThe smaller products no longer fit the active site, so they fall away and leave it empty, ready for the next substrate molecule.+

Tap any cell to reveal it. Tap a column header to read one property down every item.

Biology · Practical investigation

change the pH → watch the row get shorter

At pH 4 the iodine was still darkening sample after sample. The starch finally disappeared after 300 s — the slowest run of the five.

Pick a pH on the left, then read the row of iodine tests. Tap a part of the apparatus to find out what it is for.

How bile speeds up the breakdown of fat

Tap each box. Two different routes arrive at the same result.

What you need to know

• Carbohydrases break carbohydrates down into simple sugars. Amylase is the carbohydrase that breaks down starch.
• Proteases break proteins down into amino acids.
• Lipases break lipids down into glycerol and fatty acids.
• Amylase is made in the salivary glands, the pancreas and the small intestine; protease in the stomach, the pancreas and the small intestine; lipase in the pancreas and the small intestine.
• Bile is made in the liver and stored in the gall bladder. It is alkaline, so it neutralises the hydrochloric acid from the stomach, and it emulsifies fat into small droplets, which increases the surface area.
• Those alkaline conditions and that larger surface area together increase the rate at which lipase breaks fat down.
• The products of digestion are used to build new carbohydrates, lipids and proteins, and some of the glucose is used in respiration.

The big picture

Digestive enzymes break the large, insoluble molecules in food into small, soluble ones that can be absorbed into the bloodstream. Amylase breaks down starch, proteases break down proteins and lipases break down lipids, and each one is specific because of the shape of its active site. They are made in several different organs and act in several different places, which is why the digestive system is described as an organ system. Bile, made in the liver and stored in the gall bladder, is not an enzyme: it neutralises stomach acid and emulsifies fat, and both of those changes speed up the breakdown of fat by lipase.

⚠ Watch out

Writing that bile digests fat. Bile is not an enzyme and breaks no bonds — lipase does the digesting. Bile only improves the conditions: it neutralises acid from the stomach and emulsifies fat into droplets, so lipase has more surface to work on.

Memory hook

Three enzymes, three foods, one purpose — and bile is the pit crew, not the driver. It never breaks a single bond; it just makes the track faster for lipase.

Check yourself

Cover the page. Name the three enzyme classes, what each breaks down and what each produces. Then say where each is made, where it acts, and the two things bile does.