Mini-Lesson

Form & Function: Molecules (HL)

Molecular shape dictates biological role. This HL lesson covers B1.1 Carbohydrates & lipids and B1.2 Proteins in depth.

You will link glucose isomers to starch and cellulose, phospholipids to bilayers, and the four levels of protein structure to function.

Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you are ready.

B1.1 Carbohydrates

Carbohydrates: same monomer, different form (HL)

Monosaccharides join by glycosidic bonds in condensation reactions (releasing water).

Starch and glycogen: alpha-glucose, coiled/branched — compact, easily hydrolysed energy stores.
Cellulose: beta-glucose in straight chains that hydrogen-bond into strong microfibrils — structural, and indigestible to humans.

Form and function: the same glucose atoms give very different materials simply through their bonding geometry.

B1.1 Lipids

Lipids

Triglycerides form when glycerol joins three fatty acids by condensation (ester bonds, releasing 3 water). They are compact, insoluble energy stores.

Phospholipids are amphipathic — a hydrophilic phosphate head and hydrophobic tails — so they self-assemble into bilayers in water, the basis of membranes.

Calculate

Your turn — calculate

1A single polypeptide is 124 amino acids long. Calculate the number of water molecules released when it forms by condensation.

water molecules

Hint: forming (n − 1) peptide bonds releases (n − 1) water molecules.

Calculate

Your turn — calculate

2When one glycerol molecule joins three fatty acids to form a triglyceride, calculate the number of water molecules released.

water molecules

Hint: each ester bond forms by condensation, releasing one water molecule.

B1.2 Proteins

Protein structure and function

Proteins are polymers of amino acids joined by peptide bonds.

Primary: amino acid sequence.
Secondary: helices/sheets from hydrogen bonds.
Tertiary: the folded 3D shape of one polypeptide.
Quaternary: two or more polypeptides (e.g. haemoglobin's four chains).

Fibrous proteins (collagen, keratin) are structural; globular proteins (enzymes, haemoglobin) are functional. Heat or pH extremes denature the fold.

Sort it

Sort each item into its group

Sort each molecule by type.

Fibrous protein

Globular protein

Not a protein

Quick check

Question

?Cellulose is strong and indigestible to humans because it is made of...

Quick check

Question

?The three-dimensional folded shape of a single polypeptide is its...

Quick check

Question

?Phospholipids form bilayers in water because they are...

Match it

Match each item to its partner

Match each molecular feature to its function.

Item

Partner

Quick check

Question

?Heating an enzyme above its optimum temperature denatures it by...

Recap

The big ideas to take away

B1.1 (HL): alpha-glucose (starch/glycogen) vs beta-glucose (cellulose); triglycerides

B1.1: phospholipids are amphipathic and form bilayers

B1.2: primary to quaternary structure; fibrous vs globular; denaturation

Skills: condensation of n residues releases (n − 1) water molecules

Press Finish to see your score.

🏆

Mini-lesson complete!

⭐⭐⭐

You have worked through Theme B: Form & Function — Molecules for IB Diploma Biology. 🎉

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Next: test yourself in the Evaluate stage Confidence Quiz, then lock it in with Verify.

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