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IGCSE Biology 0610 · Chapter 4 notes · ~11 min read

Biological molecules & enzymes

Carbohydrates, fats, proteins and DNA, the five food tests with the exact colour changes — and enzymes: the lock-and-key model, temperature, pH, denaturation and how to investigate them.

Carbohydrates, fats, proteinsFood testsDNAEnzymesTemperature & pH
ASby Aiza Ma'am
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • Carbohydrates and fats contain C, H, O. Proteins also contain N. Large molecules are built from small building blocks: glucose, amino acids, fatty acids + glycerol.
  • Food tests: iodine (starch, blue-black), Benedict's (reducing sugar, brick red), biuret (protein, purple), ethanol emulsion (fat, cloudy white), DCPIP (vitamin C, blue → colourless).
  • Enzymes are proteins that act as biological catalysts. Each has an active site that fits only one substrate. High temperature or the wrong pH denatures them.

By the end you can

  • Name the elements in carbohydrates, fats and proteins, and their building blocks.
  • Describe the tests for starch, reducing sugars, protein, fat and vitamin C.
  • Describe the structure of DNA and complementary base pairing. EXT
  • Define catalyst and enzyme, and describe the lock-and-key idea (active site and substrate).
  • Describe and explain how temperature and pH affect enzyme activity.
  • Plan and interpret an enzyme investigation.

1Biological molecules

Which elements?

CARBOHYDRATESCHOcarbonhydrogenoxygene.g. glucose, starchFATS (LIPIDS)CHOcarbonhydrogenoxygene.g. oils, butterPROTEINSCHON+ nitrogen(sometimes S)e.g. enzymes, musclecarbon · hydrogen · oxygen in all three; only proteins also contain nitrogen
Carbohydrates and fats contain carbon, hydrogen and oxygen. Proteins contain these plus nitrogen.
  • Carbohydrates: carbon, hydrogen, oxygen. The ratio of H to O is usually 2 : 1 (for example glucose C₆H₁₂O₆).
  • Fats and oils (lipids): carbon, hydrogen, oxygen — but much less oxygen than a carbohydrate.
  • Proteins: carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur.
Memory trickC-H-O for sugars and fat, C-H-O-N for protein. If a question asks "which element is in protein but not in carbohydrate?", the answer is nitrogen.

Big molecules from small units

CARBOHYDRATESmany glucose unitsstarch, glycogen, celluloselong chains of glucose joined together(glucose is the “building block”)PROTEINSamino acids (about 20 types)proteina long chain of amino acids,folded into a specific 3D shapeFATS (LIPIDS)glycerol3 fatty acidsfat (triglyceride)1 glycerol + 3 fatty acidsBig molecules are built from many small units joined in a chain
Large biological molecules are chains of smaller units.
Large moleculeMade fromExamples and uses
starch, glycogen, cellulosemany glucose unitsstarch = plant energy store; glycogen = animal energy store (liver, muscle); cellulose = plant cell walls
proteinsmany amino acidsenzymes, haemoglobin, antibodies, muscle, hormones
fats and oils1 glycerol + 3 fatty acidsenergy store, insulation, protects organs
  • The order of amino acids in a protein decides how it folds into its 3D shape — and the shape decides what the protein does.
  • Starch, glycogen and cellulose are all made of glucose but joined differently. That is why cellulose is strong (cell walls) while starch is a store.
Common mistake"Fats and oils are made of amino acids." Fats are fatty acids and glycerol; proteins are amino acids.

2Food tests

The five tests

STARCHiodine solutionpositive result:blue-blackREDUCING SUGARBenedict’s, heat in hot waterpositive result:green → orange → brick redPROTEINbiuret solutionpositive result:purple / violetFAT (LIPID)ethanol, then add waterpositive result:cloudy white emulsionVITAMIN CDCPIP (blue dye)positive result:blue DCPIP → colourlessAlways give the colour BEFORE and AFTER: e.g. “orange-brown to blue-black”
Colour changes for each food test. Always write the colour before and after.
Test forReagent and methodPositive result
starchadd iodine solutionorange-brown → blue-black
reducing sugar (e.g. glucose)add Benedict's solution and heat in a hot water bath (about 80 °C)blue → green → yellow → orange → brick red
proteinadd biuret solution (or sodium hydroxide, then a few drops of copper sulfate)blue → purple / violet
fat (lipid)mix with ethanol, shake, then pour into waterclear → cloudy white emulsion
vitamin Cadd the food (juice) drop by drop to blue DCPIPblue → colourless
  • Benedict's colour tells you roughly how much reducing sugar there is: green = a little, brick red = a lot.
  • For solid foods, crush them with water first, then filter or use the liquid.
  • Keep safety in mind: wear eye protection and take care with the hot water bath. Ethanol is flammable — no naked flames.
Exam tipSay the colour before and after: "Benedict's: blue to brick red". "It goes red" alone loses marks.
Common mistakeSaying Benedict's test is for "sugar". It detects reducing sugars only (glucose, maltose…). Table sugar (sucrose) is not a reducing sugar.
Worked example

Orange juice A decolourises 1 cm³ of DCPIP after 10 drops. Orange juice B takes 25 drops. Which has more vitamin C?

  1. DCPIP is decolourised by vitamin C
  2. fewer drops were needed for A
  3. so A contains more vitamin C (more vitamin C in each drop)

DNAEXTENDED

DNA double helixtwo sugar–phosphatebackbonesbase pairs (rungs)joined by weak bondsComplementary base pairingAalways pairs withTCalways pairs withGstrand 1: A T G C C Astrand 2: T A C G G T
DNA is a double helix: two strands joined by pairs of bases. A always pairs with T, and C always pairs with G.

DNA — a long molecule with the shape of a double helix: two strands wound around each other. It carries the genetic code.

  • Each strand has a backbone and a row of bases: Adenine, Thymine, Cytosine, Guanine.
  • The two strands are held together by base pairs: A–T and C–G. This is complementary base pairing.
  • The order of the bases is the code for making proteins (see Inheritance, variation & selection).
Memory trickApples Tree, Car Garage — A–T and C–G.
Worked example

EXT One strand of DNA reads A T G C C A. Write the base sequence of the complementary strand.

  1. A pairs with T, T with A, G with C, C with G
  2. A→T, T→A, G→C, C→G, C→G, A→T
  3. T A C G G T
Worked example

EXT In a sample of DNA, 20% of the bases are adenine (A). Calculate the percentage of guanine (G).

  1. A pairs with T, so T is also 20%
  2. A + T = 40%, leaving 60% for C and G
  3. C and G pair, so they are equal: 60 ÷ 2 = 30% guanine

3Enzymes

What enzymes do

Catalyst — a substance that increases the rate of a chemical reaction and is not changed by the reaction.

Enzyme — a protein that acts as a biological catalyst.

  • Living cells carry out thousands of chemical reactions (metabolism). At body temperature they would be far too slow without enzymes.
  • Enzymes can break molecules down (digestion of starch) or build them up (making proteins).
  • Each enzyme works on one type of substrate: amylase on starch, protease on protein, lipase on fat. This is specificity.

The lock-and-key idea

123enzymeenzymeenzymesubstrate meetsthe active siteenzyme–substratecomplexproducts released;enzyme unchangedsubstrateactivesiteThe substrate’s shape fits the active site only: enzymes are SPECIFICThe enzyme is not used up, so it can work again and again(lock-and-key model)
The substrate fits the enzyme's active site like a key in a lock. Products leave and the enzyme is ready to be used again.

Active site — the part of the enzyme with a shape complementary to its substrate.

Substrate — the molecule an enzyme works on. Product = what it is changed into.

  • The substrate binds to the active site to make an enzyme–substrate complex EXT. The reaction happens and the products leave.
  • The enzyme itself is not used up, so a small amount of enzyme can work on a lot of substrate.
  • Only a substrate with the matching shape fits. This is why enzymes are specific.
Common mistake"Enzymes are alive and get killed by heat." Enzymes are proteins, not living things. They are denatured, not killed.

How temperature affects enzymes

temperature (°C)enzyme activity0102030405060optimum ≈ 37 °Ccooler: slow,few collisionstoo hot: active sitechanges shape —DENATUREDrate rises asparticles move fasterDenaturing is permanent: the enzyme will not work again
Activity rises with temperature up to the optimum, then drops sharply as the enzyme is denatured.

Optimum temperature — the temperature at which an enzyme works fastest. For many human enzymes it is about 37 °C.

  • Below the optimum: particles have less kinetic energy, so enzyme and substrate collide less often and the rate is slow. The enzyme is not damaged.
  • Above the optimum: the enzyme's shape starts to change. The active site is distorted, the substrate no longer fits, and the rate falls. The enzyme is denatured.

Denaturation — a permanent change in the shape of the enzyme (and its active site), so it can no longer work.

Exam tipIn a graph description: say "rate increases up to about 37 °C, then decreases rapidly to zero by about 55 °C because the enzyme is denatured."
Common mistake"The enzyme is dead / killed at high temperature." Say denatured and mention the active site changes shape.
Common mistake"At low temperature the enzyme is denatured." It is only slowed down. It can work again when warmed.

How pH affects enzymes

pHenzyme activity02468101214pepsinoptimum pH 2amylaseoptimum ≈ pH 7proteaseoptimum ≈ pH 8–9Each enzyme has its own optimum pH; extremes of pH denature it
Each enzyme has its own optimum pH. Strong acid or alkali denatures it.
  • Every enzyme has an optimum pH. Pepsin in the stomach works best at pH 2 (acid). Amylase in the mouth works best at about pH 7.
  • Moving away from the optimum changes the charges on the enzyme and bends the active site, so the substrate no longer fits. At an extreme pH the enzyme is denatured.
heat or wrong pH✗123normal enzyme:substrate fits theactive sitethe enzyme’s shapechanges: the activesite is distortedsubstrate no longerfits: NO reaction(denatured)Denaturation is permanent — cooling the enzyme does not restore its shape(low temperatures only slow enzymes down; they do not denature them)
Denaturation changes the shape of the active site. It is permanent.

Investigating enzymes

thermometerwater bath at a set temperature(try 10, 20, 30, 40, 60 °C)30 s60 s90 s120 s150 s180 sspotting tile: test every 30 sblue-black = starch still thereorange-brown = starch all digestedtime taken = measure of enzyme rate(shorter time = faster enzyme)
Mix starch and amylase at a fixed temperature. Every 30 seconds, test a drop with iodine. The time it takes for the colour to stay orange-brown measures the rate.
  • Independent variable: temperature (water baths at 10, 20, 30, 40, 60 °C) or pH (buffer solutions).
  • Dependent variable: the time taken for all the starch to be digested.
  • Controlled variables: volume and concentration of starch and amylase, the same pH (or temperature).
  • Warm each solution to the water bath temperature before mixing.
rate = 1 ÷ time
a shorter time means a faster enzyme
Worked example

Starch is digested in 120 s at 20 °C and in 60 s at 30 °C. Compare the rates.

  1. rate = 1 ÷ time
  2. at 20 °C: 1 ÷ 120 = 0.0083 per second
  3. at 30 °C: 1 ÷ 60 = 0.0167 per second
  4. the rate at 30 °C is twice as fast as at 20 °C
  • Catalase (in liver, potato) breaks down hydrogen peroxide into water and oxygen — count the bubbles or measure the height of foam to follow the rate.

4Test yourself

Test yourself — cover the answers first

Quick check

  1. Which element is in proteins but not in carbohydrates?
  2. Name the building blocks of a protein.
  3. What are fats made of?
  4. What colour change shows starch with iodine solution?
  5. What does Benedict's solution turn with glucose when heated?
  6. Which test uses a blue dye that goes colourless?
  7. What is a catalyst?
  8. Why can an enzyme only work on one substrate?
  9. EXT Which base pairs with cytosine in DNA?

Exam-style

  1. Describe how you would test a food for protein. [3]
  2. Explain why an enzyme's activity falls at high temperatures. [3]
  3. A student investigates the effect of pH on amylase. State two variables that should be kept the same. [2]
  4. EXT Explain why the enzyme pepsin works in the stomach but not in the small intestine. [3]
Show answers
  1. nitrogen
  2. amino acids
  3. glycerol and fatty acids
  4. orange-brown to blue-black
  5. blue to brick red (via green, yellow, orange)
  6. DCPIP for vitamin C
  7. a substance that speeds up a reaction and is not changed
  8. its active site has a shape complementary to only that substrate
  9. guanine
  10. crush the food and mix with water
    add biuret solution
    blue changes to purple/violet if protein is present
  11. the enzyme is denatured
    the shape of the active site changes
    the substrate no longer fits
  12. temperature
    volume and concentration of starch / amylase
  13. pepsin has an optimum pH of about 2 (acid)
    the small intestine is alkaline
    so pepsin is denatured / its active site changes shape
Practise this chapter
Biological moleculesFood testsEnzymes
Next chapterPlant nutrition →
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