Almost everything in a living cell is built from a handful of elements. Carbohydrates and fats use just three; proteins add nitrogen and sometimes sulfur. Join hundreds of small molecules end to end and you get the big ones: starch, glycogen and cellulose from glucose, proteins from amino acids, fats and oils from fatty acids and glycerol.
Carbohydrates and fats contain carbon, hydrogen and oxygen only. Proteins contain carbon, hydrogen, oxygen and nitrogen (some also contain sulfur). Large biological molecules are made by joining many small molecules together: starch, glycogen and cellulose are made from glucose; proteins from amino acids; fats and oils from fatty acids and glycerol.
Fig. 1: three families of large molecule and the small molecules each is built from; the hexagons are glucose units, the coloured beads are amino acids
Section 01
The elements inside.
Turn the glucose molecule in the sim and press each element chip. Carbon atoms are dark grey, hydrogen white, oxygen red, nitrogen blue and sulfur yellow. Then switch to the fat, to alanine and to cysteine, and press nitrogen and sulfur again.
Group of molecules
Example in the sim
Elements
carbohydrates
glucose, C6H12O6
carbon, hydrogen, oxygen
fats and oils
a fat (glycerol + three fatty acids)
carbon, hydrogen, oxygen (much less oxygen than a carbohydrate)
proteins
alanine, cysteine (amino acids)
carbon, hydrogen, oxygen, nitrogen; some amino acids also contain sulfur
No nitrogen in sugars or fats: press the nitrogen chip with glucose or the fat showing and every atom fades: there are none.
Nitrogen in amino acids: alanine has one blue nitrogen atom in its amino group. That nitrogen is why plants need nitrate ions to make proteins.
Sulfur: only some amino acids, such as cysteine, contain sulfur, so a protein may contain C, H, O, N and S.
Simplified models: the fatty acid tails in the sim are cut short; real ones have about 16 or 18 carbon atoms.
Examiner trap
Nitrogen is the element that separates proteins from the other two groups. "Carbon, hydrogen and oxygen" is a correct answer for carbohydrates and for fats, but it loses the mark for proteins unless you add nitrogen.
Section 02
Big molecules from small ones.
Each large molecule is a chain of many copies of a small molecule. In the sim the units start scattered, as they are after digestion. Press Join them up to build the large molecule, then Break them down to split it again. Count the units and read what each one is made from.
Large molecule
Made from
Arrangement in the sim
Role
starch
glucose
long chain coiled into a spiral
energy store in plants
glycogen
glucose
branched chains
energy store in animals (liver and muscles)
cellulose
glucose
straight chains lying side by side
plant cell walls; fibre in our diet
protein
amino acids (about 20 different kinds)
chain folded into a particular shape
enzymes, muscle, antibodies and more
fat or oil
fatty acids and glycerol
one glycerol joined to three fatty acids
energy store, insulation
Same unit, different molecule: starch, glycogen and cellulose are all glucose, but the way the chains are coiled, branched or lined up gives them very different properties.
Order matters in proteins: the sequence of the coloured amino acids decides how the chain folds, and so the shape and job of the protein.
Not a long chain: a fat molecule is only four units: one glycerol and three fatty acids. The sim counts 6 units because it draws glycerol as three cubes, one for each of its three carbon atoms, and hangs one fatty acid tail on each.
Digestion in reverse: amylase splits starch back into glucose, proteases split proteins into amino acids and lipase splits fats into fatty acids and glycerol (details in Topic 7).
Examiner trap
"Fats are made from fatty acids" is only half an answer. Always name both parts: fatty acids and glycerol. Likewise, cellulose is made from glucose, not from starch.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the Elements tab, glucose is showing and you press the nitrogen chip. What does the sim report?
Quick check+10 XP
Which molecule in the Elements tab contains a yellow sulfur atom?
Quick check+10 XP
In the Big from small tab, starch, glycogen and cellulose are all built from the same small molecule. Which?
Quick check+10 XP
When you join up the fat or oil model in the sim, what does each fat molecule consist of?
Section 03
Why the building blocks matter.
These facts are the foundation for several later topics, so it is worth seeing where they lead.
Digestion (Topic 7): large molecules cannot cross the wall of the gut. They are broken down into their small molecules, which are absorbed and then rebuilt into the large molecules the body needs.
Plant nutrition (Topic 6): glucose from photosynthesis is turned into starch for storage and cellulose for new cell walls. To make amino acids, and so proteins, the plant also needs nitrogen from nitrate ions in the soil.
Enzymes (Topic 5): enzymes are proteins. Their shape, set by the order of their amino acids, is what lets them work.
Food tests (next lesson): each test detects one of these groups: starch, reducing sugar, protein, fat or vitamin C.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which elements are found in all proteins?
Exam style+20 XP
A student analyses a molecule and finds it contains only carbon, hydrogen and oxygen, with very little oxygen compared with carbon. The molecule is most likely:
Exam style+20 XP
Which row correctly matches each large molecule with the small molecules it is made from?
Exam style+20 XP
Which large molecule is NOT made from glucose?
Skill unlocked
Large molecules from small ones, mastered.
This skill is now recorded on your profile. Keep the chain going.