IB DP Biology: Carbohydrates and lipids: macromolecules, sugars, fats and phospholipids (DP Biology)

This study guide teaches IB Diploma Programme Biology topic B1.1 (Carbohydrates and lipids) from scratch. It follows the IB Biology guide, first assessment 2025, and covers statements B1.1.1 to B1.1.13. The guide notes that B1.1 has no additional higher level content, so everything here is SL and HL.

Use it with the revision notes and the practice set. Related pages: Theme B: Form and function and Water and nucleic acids. Course pages: IB DP Biology hub, printable checklist and free 10-minute diagnostics.

What this unit covers

Section	What you must be able to do	Tier
B1.1.1	Explain why carbon forms so many compounds; use SI prefixes	SL and HL
B1.1.2, B1.1.3	Condensation builds polymers; hydrolysis breaks them down	SL and HL
B1.1.4	Recognise pentoses and hexoses; link glucose properties to use	SL and HL
B1.1.5, B1.1.6	Starch, glycogen and cellulose: structure related to function	SL and HL
B1.1.7	Glycoproteins in cell-cell recognition, with ABO antigens	SL and HL
B1.1.8 to B1.1.11	Lipids, triglycerides, fatty acids, adipose tissue	SL and HL
B1.1.12, B1.1.13	Phospholipid bilayers; steroids crossing them	SL and HL

The guiding questions ask how variation in form allows diversity of function, and how carbohydrates and lipids compare as energy stores. Keep both in mind as you read.

B1.1.1 Carbon and the diversity of life’s compounds

In a covalent bond, two atoms hold a shared electron pair between them. A carbon atom has four electrons available for sharing, so it forms up to four bonds. These can be four single bonds, or a mix of single and double bonds, to other carbon atoms or to other non-metals such as hydrogen, oxygen, nitrogen, phosphorus and sulfur.

Because carbon bonds to itself so readily, it builds skeletons of many shapes:

• unbranched chains, such as the hydrocarbon tail of a fatty acid;
• branched chains, such as the side chain of the amino acid leucine;
• single rings, such as glucose and ribose;
• multiple rings, such as the four fused rings of a steroid.

Carbon-carbon and carbon-hydrogen bonds are stable at body temperature, so these skeletons persist, yet they can still be rearranged by enzymes. That combination is why life is carbon-based.

NOS: SI prefixes. Scientists agreed international conventions so measurements can be compared anywhere. You need kilo (k, 10³), centi (c, 10⁻²), milli (m, 10⁻³), micro (µ, 10⁻⁶) and nano (n, 10⁻⁹).

Worked example. A phospholipid bilayer is about 7 nm thick. (a) Convert to micrometres. (b) An electron micrograph magnified ×500 000 shows the bilayer. How thick is it on the image?

(a) 7 nm ÷ 1000 = 0.007 µm. (b) Image size = actual size × magnification = 7 nm × 500 000 = 3 500 000 nm = 3.5 mm.

B1.1.2 and B1.1.3 Building and breaking macromolecules

A monomer is a small molecule that can join to others. A polymer is a long chain of monomers, and a macromolecule is a very large molecule such as a polymer. You should know three families:

Polymer	Monomer	Link formed
Polysaccharide	monosaccharide	glycosidic bond
Polypeptide	amino acid	peptide bond
Nucleic acid	nucleotide	phosphodiester bond (sugar to phosphate)

Condensation joins two molecules and releases one molecule of water. The water is made from an -OH group on one monomer and an -H atom on the other. Hydrolysis is the reverse: a water molecule is split, and the -H and -OH are added to the two ends of the broken bond. The name means “splitting with water”.

Worked example. Two glucose molecules (C6H12O6, relative molecular mass 180.16) join to form maltose.

2 × C6H12O6 → C12H22O11 + H2O. Mass check: 2 × 180.16 − 18.02 = 342.30, which matches maltose (C12H22O11).

A chain of 1000 glucose units has 999 glycosidic bonds, so 999 water molecules are released when it forms and 999 are used when it is fully hydrolysed. In general a linear chain of n monomers needs n − 1 condensations.

Digestion is hydrolysis. Enzymes such as amylase hydrolyse starch to smaller sugars so that monomers small enough to be absorbed are released.

B1.1.4 Monosaccharides

Monosaccharides are single sugar units. In ring diagrams you can tell the type by counting carbon atoms, including the carbon outside the ring.

• Pentoses have five carbons: ribose and deoxyribose. The ring holds four carbons and one oxygen; the fifth carbon is a side group.
• Hexoses have six carbons: glucose, fructose and galactose. In glucose the ring holds five carbons and one oxygen, with carbon 6 as a side group (CH2OH).

Glucose is the model example. Link each property to its use:

Property	Why it matters
Soluble	Many -OH groups form hydrogen bonds with water, so glucose dissolves in blood plasma and cytoplasm
Transportable	Dissolved glucose moves in blood; it is small enough to cross membranes by transport proteins
Chemically stable	It does not react readily at cell pH, so it can be carried and held without being altered
Yields energy on oxidation	Oxidation in respiration releases energy used to make ATP

B1.1.5 Polysaccharides as energy stores

Starch (plants) and glycogen (animals) are polymers of alpha-glucose. Both are suited to storage because:

• Compact. The chain coils, and in amylopectin and glycogen it also branches, so a large number of glucose units fit in a small space.
• Relatively insoluble. The molecules are very large, so they do not dissolve readily and do not change the water concentration of the cell. Storing the same glucose as free molecules would draw in water by osmosis.
• Easy to add to or remove from. Glucose units are added by condensation and removed by hydrolysis. Branching creates many free ends, so many units can be mobilised at once.

Starch is a mixture of amylose (unbranched, coiled) and amylopectin (branched). Glycogen is more highly branched and is stored mainly in the liver and muscle.

B1.1.6 Cellulose

Cellulose is built from beta-glucose. In the beta form the -OH on carbon 1 points up, so each beta-glucose must be turned over (alternating orientation) relative to its neighbour to make the link. This gives a straight, unbranched chain rather than a coil.

Neighbouring chains lie side by side and are cross-linked by many hydrogen bonds. Each bond is weak but the total is large. The chains group into bundles called microfibrils, which form fibres in the cell wall. The result is a wall with high tensile strength that resists bursting when the cell takes in water.

B1.1.7 Glycoproteins and cell-cell recognition

A glycoprotein is a protein with short carbohydrate chains attached. Many sit in the plasma membrane with the carbohydrate facing outwards. Cells use them as identity markers and as binding sites for recognition.

ABO blood groups. ABO antigens are glycoproteins on the surface of red blood cells. The genotype determines the sugar added at the end of the chain, so group A, group B and group AB cells display different antigens, while group O cells lack A and B antigens. A person makes antibodies against the antigens they lack. If a recipient receives cells carrying an antigen they lack, their antibodies bind it and the cells clump.

Recipient	Antibodies in plasma	Cells they would attack
A	anti-B	B, AB
B	anti-A	A, AB
AB	none	none
O	anti-A and anti-B	A, B, AB

B1.1.8 Lipids are hydrophobic

Lipids are biological substances that readily dissolve in non-polar solvents such as hexane, yet barely dissolve in water. The group includes fats, oils, waxes and steroids. They share a large proportion of non-polar C-H bonds, which cannot form hydrogen bonds with water.

B1.1.9 Triglycerides and phospholipids

A fatty acid has a long hydrocarbon chain and a carboxyl group (-COOH) at one end. Glycerol has three -OH groups.

• Triglyceride: one glycerol + three fatty acids, joined by three ester bonds, releasing three water molecules.
• Phospholipid: one glycerol + two fatty acids + one phosphate group, joined by three condensation reactions (two with fatty acids, one with phosphate), releasing three water molecules. A further head group would add one more.

Worked example. Palmitic acid is C16H32O2 (256.43). Glycerol is C3H8O3 (92.09). The triglyceride tripalmitin forms as:

glycerol + 3 palmitic acid → tripalmitin + 3 H2O. Mass = 92.09 + 3 × 256.43 − 3 × 18.02 = 807.3, which is C51H98O6.

B1.1.10 Saturated and unsaturated fatty acids

Type	C=C bonds in the chain	Shape
Saturated	none	straight; packs tightly
Monounsaturated	one	bent at the double bond
Polyunsaturated	two or more	several bends

Bends stop the chains packing closely, so weaker attractions between molecules are easier to overcome. More C=C bonds therefore means a lower melting point.

Fatty acid (18 carbons)	C=C bonds	Approximate melting point
Stearic	0	69 °C
Oleic	1	13 °C
Linoleic	2	−5 °C

Treat these values as data given in a question. From stearic to oleic, one C=C lowers the melting point by 56 °C.

Plants usually store energy as oils rich in unsaturated fatty acids, which stay liquid. Endotherms such as mammals store fat richer in saturated fatty acids, which is solid or semi-solid at body temperature and so holds its place in adipose tissue.

B1.1.11 Adipose tissue

Adipose tissue stores triglycerides in large droplets inside fat cells.

Energy storage. Lipids store about 37 kJ per gram and carbohydrate about 17 kJ per gram (data given in questions). Lipids are insoluble, so they exert no osmotic effect, and are stable for long periods. Fat is the long-term store; glycogen is the short-term, quickly mobilised store.

Worked example. How much of each store holds 100 000 kJ? Fat: 100 000 ÷ 37 = 2703 g, or 2.70 kg. Carbohydrate: 100 000 ÷ 17 = 5882 g, or 5.88 kg. Fat is 37 ÷ 17 = 2.18 times as energy-dense, so an animal that moves carries about half the mass.

Thermal insulation. Fat conducts heat poorly. A layer under the skin reduces heat loss in endotherms, which keep a body temperature above their surroundings. Seals and whales living in cold water have thick blubber; water removes heat far faster than air. Animals in warm habitats need little. Ectotherms rely less on insulation as they do not maintain a high body temperature.

B1.1.12 Phospholipid bilayers

Phospholipids are amphipathic: part of the molecule is hydrophilic (the phosphate head, which attracts water) and part is hydrophobic (the two fatty acid tails). In water the tails are pushed away from water and the heads face it. This arranges the molecules in a double layer, the bilayer, with tails inside. It forms spontaneously and reseals if damaged. The core is a barrier to ions and large polar molecules.

B1.1.13 Steroids and the bilayer

Steroids are lipids with four fused carbon rings (three six-sided, one five-sided). You should identify a steroid from a diagram by this ring pattern. Oestradiol and testosterone are examples. They are non-polar, so they dissolve in the hydrophobic core and pass directly through the bilayer without a transport protein. Cholesterol also has this ring skeleton.

Common errors

• Writing that hydrolysis “adds water” without saying water is split into -H and -OH.
• Saying glycogen is insoluble because it is “big” with no link to osmosis or storage.
• Placing the -OH on carbon 1 wrongly for alpha and beta glucose.
• Writing “unsaturated” without naming C=C bonds.
• Stating that phospholipid tails are attracted to water.

Next, test yourself with the practice set.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Topic B1.1, Carbohydrates and lipids.