Enzymes and the human digestive system

2. Organisation

An enzyme is a protein that speeds up one particular reaction inside a living organism, without being changed or used up itself. Everything about how an enzyme behaves comes back to the shape of its active site.

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187 questions5 subtopicsFoundation tier2 required practicals

What enzymes and the human digestive system covers

Every subtopic below has its own questions, mark schemes and notes.

4.2.2.1aEnzymes and how they work

4.2.2.1bTemperature, pH and rate

Enzymes only work well in a narrow range of conditions. Warming speeds them up until the point where heat starts to wreck the active site, and the wrong pH does the same damage. Required practical 5 measures this using amylase, starch and iodine.

4.2.2.1cDigestive enzymes

Food arrives as molecules far too big to get into the blood. Three groups of digestive enzyme break them into small soluble molecules, which are absorbed and then used to build the body's own molecules or released as energy in respiration.

4.2.2.1dThe digestive system and bile

The digestive system is an organ system: several organs each doing a different job so that food is broken down and absorbed. Learn which organ makes which enzyme, and keep bile separate in your mind from the enzymes — it is not one.

4.2.2.1eFood tests

Required practical 4: four reagents, four colour changes. The awkward details are the ones examiners ask about — which test needs heating, which sample is not filtered, and what a negative result looks like.

Key terms

The definitions examiners expect, in the wording that earns the mark.

Catalyst — A substance that increases the speed of a reaction without being changed or used up.

Enzyme — A protein made by a living organism that acts as a biological catalyst.

Active site — The part of an enzyme, shaped by the folding of its amino acid chain, that the substrate fits into.

Substrate — The substance an enzyme acts on.

Specific — Describes an enzyme that catalyses only one reaction, because only one substrate fits its active site.

Optimum temperature — The temperature at which an enzyme works fastest.

Optimum pH — The pH at which an enzyme works fastest.

Denatured — Describes an enzyme whose active site has permanently changed shape, so the substrate no longer fits and it no longer works.

Buffer solution — A solution that holds the mixture at a particular pH.

Continuous sampling — Taking a small sample at regular time intervals throughout a reaction to follow its progress.

Carbohydrase — An enzyme that breaks carbohydrates down into simple sugars.

Amylase — A carbohydrase that breaks starch down into maltose.

Protease — An enzyme that breaks proteins down into amino acids.

Lipase — An enzyme that breaks lipids down into glycerol and fatty acids.

Lipid — A fat or an oil.

Organ system — A group of organs working together to perform one overall function.

Bile — An alkaline liquid made in the liver that neutralises stomach acid and emulsifies fats.

Emulsify — To break a large droplet of fat into many tiny droplets, increasing the surface area.

Pepsin — The protease produced by the stomach, with an optimum pH of about 2.

Gullet — The oesophagus — the tube connecting the mouth to the stomach.

Benedict's solution — The reagent used to test for sugars; blue to green, yellow or brick-red.

Iodine solution — The reagent used to test for starch; browny-orange to blue-black.

Biuret reagent — The reagent used to test for protein; blue to purple.

Sudan III — The stain used to test for lipids; a bright red layer forms on top.

Distilled water — Water with nothing dissolved in it, used so the water itself cannot affect a test result.

Where marks get lost

The mistakes that come up again and again in this topic.

An enzyme is not alive and cannot be 'killed'. Say denatured.
The enzyme is not used up. The substrate is. Writing that enzymes are used up loses the mark for the definition of a catalyst.
The active site is not the same shape as the substrate — it is the shape the substrate fits into. Say 'complementary' or 'the substrate fits into it'.
Give both halves of the catalyst definition: speeds the reaction up, and is not changed or used up.
Denaturing changes the enzyme, not the substrate. Say the active site changes shape.
Below the optimum, nothing is being damaged — the rate rises simply because molecules are moving faster.
For rate = 1000 ÷ time, divide 1000 by the time. Doing it the other way round is the commonest slip in this practical.
Convert minutes to seconds before dividing if the answer is wanted per second.
On a graph of time taken against pH, the optimum is the lowest point, not the highest.
Amino acids come from protein; fatty acids come from fat. These two are very easily swapped under pressure.
Lipids give two products, glycerol and fatty acids. Naming only one costs a mark.
Amylase digests starch specifically, not every carbohydrate.
Chewing makes the pieces smaller but leaves the molecules unchanged. Only enzymes carry out chemical digestion.
Bile is not an enzyme. It does not catalyse anything and it does not digest fat — lipase does.
Bile is made in the liver and stored in the gall bladder. Swapping these two is the most common error in this section.
Emulsifying increases the surface area of the fat. It does not make the fat molecules smaller.
Amylase is made in three places (salivary glands, pancreas, small intestine) and protease in three (stomach, pancreas, small intestine). Only lipase has two.
Benedict's and biuret both start off blue. Benedict's needs heating; biuret does not.
The lipid sample is the one you do not filter — lipids do not dissolve and would be caught in the filter paper.
A negative result is a colour that stays the same, not no colour at all. Say 'stayed blue' or 'stayed browny-orange'.
When asked what results show, give the conclusion and the evidence for each tube, not just a list of substances.

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