Biological Molecules Notes

Cambridge IGCSE Biology: Revision notes

Key facts

  • Carbohydrates and fats contain C, H, O; proteins also contain N (sometimes S); DNA also contains N and P.
  • Large molecules are built from small subunits: starch and cellulose from glucose, proteins from amino acids, fats from glycerol and fatty acids.
  • Food tests: iodine (starch, blue-black), Benedict's (reducing sugar, brick-red), biuret (protein, purple), ethanol emulsion (fat, white), DCPIP (vitamin C).
  • DNA is a double helix with complementary base pairing: A with T, G with C.
  • Each molecule type has a role: energy, storage, structure or genetic information.

Chemical elements

Carbohydrates and fats are made of carbon, hydrogen and oxygen; proteins and DNA also contain nitrogen.

Different molecules contain different elements. Nitrogen is the element found in proteins but not in carbohydrates or fats. Proteins sometimes contain sulfur, and DNA also contains phosphorus.

012345CarbohydratesFats and oilsProteinsDNAMoleculeNumber of elements
Number of different elements always present in each molecule type (proteins sometimes also contain sulfur)

Carbohydrates

  • C, H, O

Fats and oils

  • C, H, O

Proteins

  • C, H, O, N (sometimes S)

DNA

  • C, H, O, N, P

Which element is found in proteins but not in carbohydrates?

Building blocks

Large biological molecules are chains of small subunits, and the order and arrangement of the subunits decide what the molecule does.

Starch and cellulose are both made entirely of glucose, yet starch is digestible and used for energy while cellulose is not digestible and gives structure. The way the units are joined matters as much as what they are made from. In a fat, one glycerol joins three fatty acids.

  1. 1

    Small subunits

    glucose, amino acids, glycerol and fatty acids

  2. 2

    Joined together

    in chains, or 3 fatty acids on 1 glycerol

  3. 3

    Order and arrangement

    decide how the molecule behaves

  4. 4

    Large molecule

    starch, cellulose, protein or fat

From subunits to large molecules

Starch, glycogen

Subunits:
Glucose
Role:
Energy store

Cellulose

Subunits:
Glucose
Role:
Rigid cell walls

Protein

Subunits:
Amino acids
Role:
Order decides function

Fats and oils

Subunits:
Glycerol and fatty acids
Role:
1 glycerol + 3 fatty acids

How many fatty acids join to one glycerol in a fat?

Food tests

Each food test uses a reagent that gives a particular colour change, and you must name the colour.

Learn the colours, not just the reagents: write 'blue-black', not 'dark', and 'brick-red', not 'orange'. Benedict's needs heating. Sucrose is a non-reducing sugar and does not give a positive result alone. The faster DCPIP decolourises, the more vitamin C is present.

  1. 1

    Add Benedict's solution

    to the food sample (blue)

  2. 2

    Heat in a water bath

    the test needs heating

  3. 3

    Look for a colour change

    green, yellow, orange then brick-red as more sugar is present

Benedict's test for reducing sugar

Reagent

Starch (iodine solution):
Iodine solution
Reducing sugar (Benedict's, heated):
Benedict's solution, heated
Protein (biuret):
Biuret reagent
Fat (ethanol, then water):
Ethanol, then water
Vitamin C (DCPIP):
DCPIP solution

Positive result

Starch (iodine solution):
Blue-black
Reducing sugar (Benedict's, heated):
Brick-red precipitate
Protein (biuret):
Purple or violet
Fat (ethanol, then water):
White emulsion (cloudy)
Vitamin C (DCPIP):
Blue to colourless

Worked example

A food sample turns blue-black with iodine and brick-red when heated with Benedict's solution. What does it contain?

What colour do you see when iodine solution is added to starch?

DNA

DNA is a double helix whose two strands are held together by complementary base pairs.

Complementary base pairing holds the strands together by hydrogen bonds: A pairs with T, and G pairs with C. The bases point inwards and the sugar-phosphate backbone is on the outside. The sequence of bases carries the genetic information, and pairing lets DNA be copied accurately.

  1. 1

    Backbone outside

    each strand is a sugar-phosphate backbone

  2. 2

    Bases point inwards

    A, T, C and G

  3. 3

    Complementary pairing

    A with T and G with C, joined by hydrogen bonds

  4. 4

    Base sequence

    carries the genetic information

How DNA is held together
  • Adenine (A)pairs with thymine (T)
  • Guanine (G)pairs with cytosine (C)

Worked example

One strand of DNA has the base sequence A-G-T-C. Write the complementary strand.

Which base pairs with guanine?

Roles of molecules

Each type of molecule has a job: energy, storage, structure or instructions.

Carbohydrates give quick energy, fats store energy over the long term and insulate, proteins do many jobs (structure, enzymes, antibodies) and DNA holds the instructions for making proteins.

  1. 1

    Glucose

    immediate energy, used in respiration

  2. 2

    Starch and glycogen

    compact energy stores

  3. 3

    Fats and oils

    long-term energy store and insulation

  4. 4

    Proteins

    structure, enzymes and antibodies

  5. 5

    DNA

    instructions for making proteins

What each molecule does

Role

Glucose:
Immediate energy
Starch and glycogen:
Energy store
Cellulose:
Support
Proteins:
Structure and function
Fats and oils:
Long-term energy store
DNA:
Genetic instructions

Why it suits

Glucose:
Used in respiration
Starch and glycogen:
Compact and insoluble
Cellulose:
Rigid cell walls
Proteins:
Collagen, enzymes, antibodies
Fats and oils:
Over twice the energy per gram of carbohydrate
DNA:
Makes proteins, copied in cell division

Why are starch and cellulose different although both are made of glucose?

Try an exam question

A student tests a food sample. It gives a purple colour with biuret reagent and a white emulsion with the ethanol emulsion test. State what the results show and name the elements present in the first molecule.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.