BiologyISC Class 11 · Biomolecules

Carbohydrates, Amino Acids, Proteins and Lipids: Structure and Function

Updated 17 October 2026 · Fact-checked

Carbohydrates are monosaccharides, disaccharides and polysaccharides. Proteins are polymers of α-amino acids joined by peptide bonds and have four levels of structure (primary to quaternary). Lipids include fats and oils (esters of glycerol and fatty acids), phospholipids and cholesterol.

Follow the arrows: a chain of amino acids (primary) coils (secondary), folds (tertiary), then joins other chains (quaternary), as in haemoglobin.

Illustration drawn for QuestionBuddy.

Understand Carbohydrates, Amino Acids, Proteins and Lipids

Carbohydrates. *Monosaccharides:* glucose, fructose, ribose; *disaccharides:* sucrose (glucose + fructose), maltose (glucose + glucose), lactose (glucose + galactose), joined by glycosidic bonds; *polysaccharides:* starch (plant reserve; amylose and amylopectin), glycogen (animal reserve), cellulose (a structural polymer of β-glucose in plant walls) and chitin. All monosaccharides, maltose and lactose are reducing sugars (they reduce Benedict's solution); sucrose is a non-reducing sugar. Functions: energy, storage and structure.

Amino acids. An α-amino acid has an amino group, a carboxyl group, a hydrogen and an R group on the α-carbon. In water at neutral pH they exist as zwitterions (dipolar ions), carrying both NH₃⁺ and COO⁻. They can be acidic, basic or neutral; essential amino acids must be obtained from the diet.

Peptide bond and proteins. A peptide bond (–CO–NH–) forms between the –COOH of one amino acid and the –NH₂ of another with loss of water. Proteins are polypeptides. *Primary structure:* the sequence of amino acids. *Secondary:* α-helix and β-pleated sheets. *Tertiary:* the overall three-dimensional folding, held by hydrogen, ionic, disulphide and hydrophobic interactions. *Quaternary:* arrangement of more than one polypeptide chain (haemoglobin: 2 α + 2 β chains). Collagen is the most abundant protein in the animal world and RuBisCO the most abundant in the biosphere.

Lipids. Water-insoluble; fats (solid) and oils (liquid) are triglycerides (glycerol + 3 fatty acids). Saturated fatty acids lack double bonds (animal fats); unsaturated ones have one or more double bonds (plant oils). Phospholipids have a phosphate-containing polar head and two fatty acid tails (lecithin) and form membranes. Cholesterol is a sterol in animal cell membranes.

Carbohydrates, Amino Acids, Proteins and Lipids: key facts

Disaccharides: $S u cr ose = g l u cose + f r u c t ose ∣ M a l t ose = g l u cose + g l u cose ∣ L a c t ose = g l u cose + g a l a c t ose$; Joined by glycosidic bonds.
Peptide bond: $R - C O - O H + H - N H - R' \to R - C O - N H - R' + H_{2} O$; Formed by dehydration.
Protein structure: $P r ima r y : se q u e n ce ∣ S eco n d a r y : h e l i x, s h ee t ∣ T er t ia r y : f o l d in g ∣ Q u a t er na r y : s u b u ni t s$; Haemoglobin is quaternary.
Counting: $P e pt i d e b o n d s = n - 1 f or namin o a c i d s ∣ P oss ib l e t r i p e pt i d es = 2 0^{3}$; For a linear peptide.

Carbohydrates, Amino Acids, Proteins and Lipids in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Counting bonds in a hormone

Suppose a hormone is a linear chain of 51 amino acids (as in insulin, taken here as a model of two chains, 21 + 30).

Find the number of peptide bonds in the whole hormone and the number of water molecules lost in forming it.

Try it first, then show the solutionSolution

Chain A has 21 amino acids, so 20 peptide bonds; chain B has 30, so 29 peptide bonds.
Total peptide bonds = 20 + 29 = 49.
Each peptide bond releases one water molecule: 49 molecules of water are lost.

Answer: 49 peptide bonds; 49 water molecules lost (the two chains are held by disulphide bonds).

Memory hook: Peptide bonds in a chain are one fewer than residues.

Tricks to remember Carbohydrates, Amino Acids, Proteins and Lipids

Short memory aids. Say each one aloud once, then use it on a question the same day.

n − 1
A chain of n amino acids has n − 1 peptide bonds.
Use it when: Numericals on peptides.
Sucrose is the odd one out
Among common sugars, sucrose is non-reducing.
Use it when: Benedict's test questions.

Carbohydrates, Amino Acids, Proteins and Lipids: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Reducing sugar · 1 mark · MCQBoard level

Which of the following is a non-reducing sugar?
(a) glucose
(b) maltose
(c) lactose
(d) sucrose

Try it first, then show the solutionSolution

In sucrose the reducing groups of glucose and fructose are linked together, so it does not reduce Benedict's solution.

Answer: (d) sucrose

Example 2: Peptide bond · 3 marks · Short answerBoard level

Define a peptide bond and calculate the molar mass of the dipeptide glycine–alanine (glycine 75, alanine 89 g mol⁻¹).

Try it first, then show the solutionSolution

A peptide bond is the –CO–NH– linkage between the carboxyl group of one amino acid and the amino group of the next, formed by loss of water. (1 mark)
Molar mass of the dipeptide = 75 + 89 − 18. (1 mark)
= 146 g mol⁻¹. (1 mark)

Answer: Peptide bond: –CO–NH–; dipeptide 146 g mol⁻¹.

Example 3: Polysaccharides · 4 marks · Short answerHarder board level

Compare starch, glycogen and cellulose on the organisms in which they occur, the monomer, and the function.

Try it first, then show the solutionSolution

Starch: plants; glucose units (amylose and amylopectin); reserve food. (1.3 marks)
Glycogen: animals and fungi; highly branched glucose polymer; reserve food in liver and muscle. (1.3 marks)
Cellulose: plant cell walls; β-glucose units, unbranched; structural. (1.4 marks)

Answer: Starch (plant store), glycogen (animal store), cellulose (structural).

Example 4: Levels of protein structure · 4 marks · Short answerHarder board level

Explain the four levels of protein structure using haemoglobin as an example. Calculate how many different tripeptides can be formed from 20 amino acids.

Try it first, then show the solutionSolution

Primary: the amino acid sequence; secondary: α-helix and β-sheet through hydrogen bonds. (1 mark)
Tertiary: the three-dimensional folding of the chain; quaternary: arrangement of the subunits. (1 mark)
Haemoglobin has four chains (two α and two β), so it shows quaternary structure. (1 mark)
Number of tripeptides = 20 × 20 × 20 = 8000. (1 mark)

Answer: Primary to quaternary; haemoglobin has 4 chains; 8000 tripeptides.

Example 5: Amino acids and the zwitterion · 5 marks · Long answerHOTS / competency

Draw or describe the zwitterion of glycine, explain why amino acids are amphoteric and show that a tripeptide of Gly (75), Ala (89) and Ser (105 g mol⁻¹) has a molar mass of 233 g mol⁻¹.

Try it first, then show the solutionSolution

Glycine zwitterion: ⁺H₃N–CH₂–COO⁻, with the amino group protonated and the carboxyl group ionised. (1.5 marks)
They can act as acid (donating H⁺ from NH₃⁺) and as base (accepting H⁺ at COO⁻), so they are amphoteric. (1.5 marks)
A tripeptide has two peptide bonds: 2 H₂O are lost. (1 mark)
Molar mass = 75 + 89 + 105 − 2 × 18 = 269 − 36 = 233 g mol⁻¹. (1 mark)

Answer: Zwitterion ⁺H₃N–CH₂–COO⁻; amphoteric; tripeptide 233 g mol⁻¹.

Where students lose marks in Carbohydrates, Amino Acids, Proteins and Lipids

The slips examiners see most often, and what to write instead.

Mistake: Forgetting water loss in peptide formation
Adding the masses of amino acids directly.
Do this: Subtract 18 for each peptide bond formed.
Usually costs: 2 marks.
Mistake: Calling sucrose a reducing sugar
Listing sucrose with glucose.
Do this: Sucrose is non-reducing.
Usually costs: 1 mark.
Mistake: Mixing the levels of structure
Calling haemoglobin tertiary.
Do this: Haemoglobin has quaternary structure (four subunits).
Usually costs: 1 mark.

Exam tips

Write structures of zwitterions clearly.
Subtract 18 for each peptide bond when calculating molar masses.
Give examples for every class of biomolecule.
Link structure to function: cellulose to strength, glycogen to storage.

Practice questions on Carbohydrates, Amino Acids, Proteins and Lipids

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The bond joining amino acids in proteins is:
(a) glycosidic
(b) peptide
(c) ester
(d) hydrogen
Hint
It forms between –COOH and –NH₂.
Answer
(b) peptide
Q2 · 2 marks · Short answerBoard level
Name two essential functions of lipids in the body.
Hint
Energy and membranes.
Answer
Energy storage (fats and oils give more energy per gram than carbohydrates) and structural components of membranes (phospholipids, cholesterol); they also insulate and protect organs.
Q3 · 3 marks · Short answerHarder board level
How many peptide bonds are there in a protein made of one chain of 150 amino acids, and how many water molecules are released?
Hint
n − 1.
Answer
149 peptide bonds; 149 water molecules are released.
Q4 · 3 marks · Short answerHarder board level
Distinguish between saturated and unsaturated fatty acids with examples of their physical states.
Hint
Think about double bonds.
Answer
Saturated fatty acids have no double bond in the chain and fats rich in them (animal fats) are solid at room temperature. Unsaturated ones contain double bonds and oils rich in them (plant oils) are liquid at room temperature.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): Cellulose cannot be digested by humans. Reason (R): Humans lack the enzyme that breaks the β-glycosidic bonds of cellulose. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Think about the enzyme.
Answer
(a) Both are true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a nutrition student tests a food sample with Benedict's solution and gets an orange-red precipitate.
(i) What does the test show? (1 mark)
(ii) Name two such sugars. (1 mark)
(iii) Name one sugar which does not give this test. (1 mark)
(iv) Explain why. (1 mark)
Hint
Reducing sugars give a coloured precipitate.
Answer
(i) A reducing sugar is present. (ii) Glucose and maltose (or lactose). (iii) Sucrose. (iv) Its reducing groups are bonded together in the glycosidic link.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Sugar in milk?Show answer
Lactose.
2.Most abundant protein in animals?Show answer
Collagen.
3.Peptide bonds in a chain of n residues?Show answer
n − 1.
4.Reserve carbohydrate of animals?Show answer
Glycogen.

Carbohydrates, Amino Acids, Proteins and Lipids: frequently asked questions

Why are some amino acids "essential"?

The body cannot make them, so they must come from the diet.

What is the difference between a fat and an oil?

Fats are solid at room temperature and have saturated fatty acids; oils are liquid and have unsaturated fatty acids.

Is every protein an enzyme?

No. Proteins serve many functions: structural, transport, hormonal, defensive and others; enzymes are one group.