Carbohydrates and lipids — IB Diploma Biology SL
IB Biology B1.1: carbon bonding, condensation, hydrolysis, sugars, starch, glycogen, cellulose, glycoproteins, triglycerides, phospholipids and steroids.
Revision notes
8 short notes, in the order of the specification. Each one in short:
A covalent bond is a bond in which two atoms share a pair of electrons. A carbon atom can form up to four covalent bonds, single or double, with other carbon atoms or with non-metals such as hydrogen, oxygen, nitrogen and phosphorus. This lets carbon build stable chains, branches and rings, the basis of the diverse compounds of life.
A condensation reaction links two monomers by a covalent bond and releases a water molecule, so repeated condensation builds a polymer. Hydrolysis is the reverse: a water molecule is split to provide the –H and –OH groups added to the monomers, breaking the bond. Polysaccharides, polypeptides and nucleic acids are all made and digested this way.
Monosaccharides are single sugar units, such as pentoses with five carbon atoms (ribose, deoxyribose) and hexoses with six (glucose). Glucose suits its role because it is soluble, so it is easily transported in blood plasma; chemically stable, so it does not react while stored or carried; and yields energy when oxidised in cell respiration.
Starch in plants and glycogen in animals are energy storage polysaccharides of alpha-glucose. Coiling and branching make them compact, their large size makes them relatively insoluble, and glucose is easily added or removed by condensation and hydrolysis. Cellulose is made of beta-glucose in alternating orientation, giving straight chains bundled and cross-linked by hydrogen bonds into strong fibres.
Glycoproteins are proteins with carbohydrate chains attached, found on the outer surface of the plasma membrane. The carbohydrate chains act as markers that other cells recognise, which is cell–cell recognition. ABO blood group antigens are an example: the carbohydrate on red blood cells differs between groups A, B, AB and O, so mismatched transfusions trigger an immune response.
Lipids are substances in living organisms that dissolve in non-polar solvents but are only sparingly soluble in aqueous solvents. They include fats, oils, waxes and steroids. A triglyceride forms by condensation of one glycerol with three fatty acids; a phospholipid forms when glycerol links to two fatty acids and one phosphate group.
A saturated fatty acid has no C=C double bonds, a monounsaturated fatty acid has one and a polyunsaturated fatty acid has two or more. Double bonds bend the chain, so unsaturated fatty acids pack less tightly and have lower melting points. Triglycerides in adipose tissue store energy long-term and insulate endotherms, especially those in cold habitats.
Phospholipids are amphipathic: they have a hydrophilic phosphate head and two hydrophobic hydrocarbon tails. In water they form a bilayer, with heads facing the water on both sides and tails facing each other in the middle. Steroids such as oestradiol and testosterone are non-polar, so they can pass through the hydrophobic core of the bilayer.
Exam questions
8 exam-style questions (24 marks), each with its mark scheme.
Answer the questionsFlashcards
24 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of molecules (form and function) on one page, so you can see where this subtopic fits.
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Free PDFs to print or save.
Quick check questions
How many covalent bonds can a carbon atom form?
Show answer
Up to four, which can be single bonds or a combination of single and double bonds.
What is released when two monosaccharides are joined by condensation?
Show answer
A molecule of water.
Which monomer makes up cellulose?
Show answer
Beta-glucose, linked in alternating orientation.
What is meant by amphipathic?
Show answer
A molecule with both a hydrophilic region and a hydrophobic region, such as a phospholipid.
Name two steroid hormones that pass through the phospholipid bilayer.
Show answer
Oestradiol and testosterone.
Frequently asked questions
What is the difference between alpha and beta glucose?
Alpha and beta glucose differ only at carbon 1: in alpha-glucose the –OH group is below the ring and in beta-glucose it is above. This small difference matters. Alpha-glucose forms coiled, branched starch and glycogen for energy storage, while beta-glucose forms straight cellulose chains that bundle into strong fibres.
Why are starch and glycogen good energy stores?
Starch and glycogen are good energy stores because coiling and branching make them compact, and their large size makes them relatively insoluble, so they have little osmotic effect. Glucose can easily be added by condensation or removed by hydrolysis, and glycogen's many branch ends let it release glucose quickly when needed.
Why are lipids better than carbohydrates for long-term energy storage?
Triglycerides release about twice as much energy per gram as carbohydrates when oxidised. They are insoluble, so they have no osmotic effect, and they are stored without water, whereas glycogen is stored bound to water. A fat store therefore adds much less mass for the same energy, which suits long-term storage.
Why do phospholipids form a bilayer in water?
Phospholipids form a bilayer because they are amphipathic. The phosphate heads are hydrophilic and face the water on both sides, while the hydrophobic fatty acid tails point inwards, away from water, forming a hydrophobic core. This is the most stable arrangement, so the bilayer forms spontaneously.
Why can steroid hormones pass through the cell membrane?
Steroid hormones such as oestradiol and testosterone are largely non-polar, so they dissolve in the hydrophobic core of the phospholipid bilayer and diffuse straight through it. Ions and polar molecules cannot do this. Once inside the target cell, the steroid binds to a receptor protein in the cell.
Written and checked against the IB Biology SL specification · Updated October 2026