The alkaline conditions and large surface area increase the rate of fat…

The idea

Lipase is the enzyme that chemically digests fat into fatty acids and glycerol. Its activity in the small intestine depends on suitable conditions. Bile provides two of these: it neutralises stomach acid so the pH is close to the optimum for lipase, and it emulsifies fat so the total surface area available to lipase is greatly increased. A larger surface area means more lipase molecules can contact fat at the same time, so the rate of breakdown rises. The alkaline pH also prevents denaturation of lipase by acid. Together, these effects mean fat is digested faster than it would be by lipase alone in acidic, unemulsified conditions.

What earns the marks

• Lipase breaks fat down into fatty acids and glycerol.
• Bile neutralises stomach acid, producing alkaline conditions near the optimum pH for lipase.
• Emulsification increases the surface area of fat available to lipase.
• A larger surface area increases the rate of fat breakdown by lipase.
• Alkaline conditions prevent acid from denaturing lipase.

Common mistakes and how to put them right

• Saying bile itself breaks fat into fatty acids and glycerol.

  Put it right by stating that lipase carries out this chemical digestion.

• Saying alkaline conditions slow lipase.

  Put it right by stating that alkaline conditions are closer to the optimum pH for lipase in the small intestine.

• Saying surface area affects only the amount of fat digested, not the rate.

  Put it right by stating that increased surface area increases the rate of reaction.

Exam tips

• Name lipase as the enzyme and fatty acids and glycerol as the products.
• Link both alkaline conditions and increased surface area to a faster rate of fat breakdown.
• Use the idea of enzyme-substrate contact when explaining why surface area matters.