K224 / K225 / 2027

Biological molecules

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Which pairing correctly gives a main role?

  1. Carbohydrates are used mainly for cell repair.
  2. Proteins form the insulating layer under the skin.
  3. Fats provide long-term energy storage and insulation.
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Fats provide long-term energy storage and insulation.

The two functions are storing chemical energy and reducing heat loss.

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Which list correctly gives the smaller units for protein and fat?

  1. Protein: amino acids; fat: glycerol and fatty acids.
  2. Protein: fatty acids and glycerol; fat: glucose and amino acids.
  3. Protein: glucose; fat: amino acids.
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Protein: amino acids; fat: glycerol and fatty acids.

Keep both components when describing the basic units of a fat.

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Benedict's solution stays blue after being added to glucose solution, but the mixture was not heated. What is the best next step?

  1. Repeat correctly with water-bath heating and appropriate controls.
  2. Conclude that glucose is not a reducing sugar.
  3. Add iodine and use its colour to measure the glucose concentration.
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Repeat correctly with water-bath heating and appropriate controls.

Missing heating makes the method incomplete. A known positive control checks that the method and reagent work.

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Invented results for four foods: P: iodine blue-black, Benedict's blue. Q: iodine brown, Benedict's brick-red. R: iodine blue-black, Benedict's green. S: iodine brown, Benedict's blue. Which food contains starch and a small amount of reducing sugar?

  1. S
  2. P
  3. R
  4. Q
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R

Blue-black iodine shows starch, and a green Benedict's result shows a small amount of reducing sugar.

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Which sequence is appropriate for the ethanol emulsion test?

  1. Shake the sample with water and record any cloudiness as a positive result.
  2. Add biuret reagent, then heat strongly over a flame.
  3. Mix the sample with ethanol, then add the ethanol extract to water.
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Mix the sample with ethanol, then add the ethanol extract to water.

A cloudy white emulsion indicates fat. Keep ethanol away from flames and heat sources.

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A student adds biuret solution to egg white, milk, glucose solution and cooking oil. Which pair would turn violet?

  1. Egg white and milk
  2. Glucose solution and milk
  3. Glucose solution and cooking oil
  4. Cooking oil and egg white
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Egg white and milk

Both contain protein, which gives a violet colour with biuret solution.

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Why does maltase act on maltose rather than starch?

  1. Maltose has a shape complementary to the maltase active site.
  2. Maltase is a starch molecule that is used up.
  3. Every enzyme acts on any molecule if enough is added.
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Maltose has a shape complementary to the maltase active site.

This allows a productive enzyme-substrate complex to form. Specificity depends on a suitable fit.

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A diagram shows an enzyme with a notch-shaped active site. Molecule P fits the notch exactly; molecule Q is a different shape. Which statement is correct?

  1. Q can bind if more of it is added, because enzymes act on any substrate.
  2. P and Q both bind, but P is broken down faster because it is smaller.
  3. P can form an enzyme-substrate complex because its shape is complementary.
  4. P binds and the enzyme is used up as P is changed into products.
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P can form an enzyme-substrate complex because its shape is complementary.

The 3D shape of the active site is complementary to P, so P binds and is changed into products. Q does not fit.

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An enzyme is slow at 5 degrees C but becomes active when warmed to 30 degrees C. Which explanation fits?

  1. The enzyme died in the cold and came back to life.
  2. The cold denatured it, and warming to 30 degrees C let the active site refold into shape.
  3. Cold slowed collisions without permanently changing its active site.
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Cold slowed collisions without permanently changing its active site.

The recovered activity supports a reversible effect of cooling rather than permanent denaturation.

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Invented data: amylase takes 120 s to digest starch at 20 degrees C, 40 s at 40 degrees C and 300 s at 60 degrees C. Which statement fits the data?

  1. The enzyme has an optimum of exactly 40 degrees C for all conditions.
  2. The rate is highest at 40 degrees C and falls at 60 degrees C.
  3. The rate at 20 degrees C is three times the rate at 40 degrees C.
  4. The rate is highest at 60 degrees C, because it takes the longest.
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The rate is highest at 40 degrees C and falls at 60 degrees C.

The shortest time means the fastest rate, so 40 degrees C is fastest; 300 s at 60 degrees C is much slower.

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Which method best isolates the effect of pH on amylase activity?

  1. Change pH, temperature and the amount of enzyme together.
  2. Use different pH buffers but keep temperature and enzyme and substrate amounts constant.
  3. Test each pH at a different temperature, so that the full range of conditions is covered.
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Use different pH buffers but keep temperature and enzyme and substrate amounts constant.

A consistent endpoint and repeated measurements then support a fair comparison of pH conditions.

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Why does an amylase experiment include a tube of boiled and cooled amylase with starch?

  1. To check that the iodine solution is working and still turns blue-black with starch.
  2. To give a faster reaction for comparison.
  3. To show that starch disappears only when active enzyme is present.
  4. To keep the temperature of every tube the same.
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To show that starch disappears only when active enzyme is present.

Boiling denatures the enzyme. If starch remains in that tube, the change in the test tubes was caused by active amylase.

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Write a short answer, then compare it with a sample and check for missing ideas.

Question 1 of 11

Connect the role to the need

4 marks

A growing animal needs energy for movement, a long-term energy store, insulation and new cell material. Match carbohydrates, fats and proteins to these needs and distinguish insulation from energy release. [4]

Give a function for each group; one group can meet more than one need.

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