The human digestive system: Bile is made in the liver and stored in the gall bladder. It is alkaline to neutralise hydrochloric acid from the stomach. It also emulsifies fat to form small droplets which increases the surface area. The alkaline conditions and large surface area increase the rate of fat breakdown by lipase.

The idea

Bile is a digestive fluid produced by liver cells and stored in the gall bladder before release into the small intestine. It is alkaline, so it neutralises the hydrochloric acid delivered with chyme from the stomach, creating the slightly alkaline conditions that lipase needs. Bile salts emulsify large fat globules into many small droplets, greatly increasing the surface area of fat exposed to lipase. Because lipase acts on the surface of fat droplets, this larger surface area, together with the alkaline pH, raises the rate of fat breakdown into fatty acids and glycerol. Bile itself contains no lipase; it is not an enzyme.

For example: Without emulsification a single large fat droplet would be digested slowly, whereas many small droplets allow faster enzyme action.

What earns the marks

• Bile is made in the liver and stored in the gall bladder before being released into the small intestine.
• Bile is alkaline, so it neutralises hydrochloric acid from the stomach and produces alkaline conditions in the small intestine.
• Bile emulsifies fat, breaking large fat droplets into many small droplets.
• Emulsification increases the surface area of fat available for lipase to act on.
• The alkaline conditions and increased surface area together increase the rate of fat breakdown by lipase.
• Bile does not contain lipase and is not itself an enzyme; lipase is produced by the pancreas and small intestine.

Common mistakes and how to put them right

• Saying bile is an enzyme or contains lipase.

  Put it right: Bile is not an enzyme, it emulsifies fat and neutralises acid, while lipase is a separate enzyme.

• Stating that bile is made in the gall bladder.

  Put it right: Bile is made in the liver and only stored in the gall bladder.

• Claiming emulsification chemically digests fat.

  Put it right: Emulsification is a physical process that increases surface area, and lipase carries out the chemical breakdown.

Exam tips

• Link each property of bile to its effect: alkaline to neutralise acid, emulsification to increase surface area, both together to increase lipase rate.
• Use the phrase surface area to volume ratio when explaining why small droplets are digested faster than one large droplet.
• If asked why fat digestion is slow without bile, refer to both pH and surface area rather than only one factor.