It also emulsifies fat to form small droplets which increases the surface…

The idea

Bile emulsifies fat, meaning it physically breaks large fat globules into many tiny droplets. This is a physical process, not chemical digestion, because no covalent bonds in fat molecules are broken. The droplets remain fat, but they are spread through the watery contents of the small intestine, forming an emulsion. The key benefit is a much larger total surface area of fat exposed to the enzyme lipase. Lipase molecules can then attach to more fat surface at the same time, so fat is broken down faster. A useful model is shaking oil with water: large drops become many small droplets, increasing the area available for reaction. Emulsification therefore speeds up digestion without itself producing fatty acids and glycerol.

What earns the marks

• Bile emulsifies fat by breaking large fat droplets into smaller droplets.
• Emulsification is a physical process, not chemical digestion.
• Smaller droplets have a larger total surface area for the same volume of fat.
• The increased surface area allows lipase to act faster.
• Fat molecules are not chemically changed by emulsification alone.

Common mistakes and how to put them right

• Saying bile chemically digests fat.

  Put it right by stating that bile emulsifies fat physically while lipase digests it chemically.

• Saying emulsification decreases surface area.

  Put it right by stating that many small droplets have a larger total surface area than one large globule.

• Saying bile breaks fat into fatty acids and glycerol.

  Put it right by stating that lipase produces fatty acids and glycerol.

Exam tips

• Use the phrase small droplets and link them explicitly to increased surface area.
• State that emulsification is physical, not chemical, to distinguish it from enzyme action.
• When explaining rate, connect larger surface area to more frequent collisions between lipase and fat.