Bile

Bile is a greenish-yellow liquid that helps with the digestion of food. You need to know exactly where it comes from:

• Bile is made (produced) in the liver.
• Bile is then stored in the gall bladder.
• When food enters the small intestine, bile is released down the bile duct into the first part of the small intestine (the duodenum).

So the journey is: liver → gall bladder → bile duct → small intestine (duodenum).

Exam tip. Two very common errors: writing that bile is made in the gall bladder (it is only stored there) and confusing bile with stomach acid. Keep them apart: the liver makes bile, the gall bladder stores it.

Food leaving the stomach is strongly acidic — the stomach makes hydrochloric acid, giving a low pH (about pH 2). But the enzymes in the small intestine work best in alkaline conditions.

• Bile is alkaline.
• When bile mixes with the acidic food entering the duodenum, it neutralises the acid and raises the pH.
• This provides the suitable (alkaline) pH that the small intestine's enzymes need to work efficiently.

If the food stayed acidic, the small intestine's enzymes would be the wrong shape for that pH and would work much more slowly — so this step matters for digestion to continue.

Exam tip. Link it to enzymes: don't just say "bile neutralises acid" — add "to give the right (alkaline) pH for the enzymes in the small intestine to work." That extra clause is usually the marking point.

Lipids (fats and oils) do not mix with the watery contents of the gut — they form a few large droplets. This is a problem because the enzyme lipase can only act on the surface of a droplet.

Bile fixes this by emulsifying the lipids:

• Bile breaks the large fat droplets into many tiny droplets.
• Many small droplets have a much larger total surface area than a few large ones.
• A larger surface area means lipase can act faster, so the fat is digested more quickly.

Important: emulsification is a physical change — bile only breaks the droplets up; it does not break any chemical bonds. The actual chemical digestion of fat (into fatty acids and glycerol) is done by lipase, not by bile.

This is a favourite Edexcel trap, so make it stick:

• An enzyme is a biological catalyst that speeds up a reaction by breaking (or joining) chemical bonds — e.g. lipase breaks lipids into fatty acids and glycerol.
• Bile is NOT an enzyme. It does not break any chemical bonds.
• Bile only emulsifies fat (a physical change) and neutralises acid.

A neat way to remember the teamwork:

Substance	What it does	Type of change
Bile	emulsifies fat into small droplets; neutralises acid	physical (no bonds broken)
Lipase (an enzyme)	digests fat into fatty acids + glycerol	chemical (bonds broken)

So bile helps lipase work faster, but lipase does the actual chemical digestion.

Watch out. Never write "bile digests fat" or "bile breaks down fat into fatty acids". Bile emulsifies fat; lipase digests it.

When acidic food arrives in the duodenum, bile does two jobs at the same time:

1. It neutralises the acid, raising the pH to the alkaline level the small intestine's enzymes need.
2. It emulsifies the lipids, increasing the surface area for lipase.

Both jobs make digestion in the small intestine faster and more efficient — but neither job is "digestion" by bile itself. Bile sets up the conditions; the enzymes (including lipase) do the digesting.

Exam tip. A 4-mark "describe the role of bile" question wants both jobs and the consequence of each: neutralises acid → right pH for enzymes, and emulsifies fat → larger surface area → faster action of lipase. Naming lipase and the phrase "surface area" are reliable marks.