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B1.1

Carbohydrates and lipids

Theme B · Form and function · Molecules · SL and HL · no additional higher level

There is no additional higher level content in this topic. It is the chemistry that the rest of Theme B builds on: how carbon makes large molecules, how those molecules are built and broken down, and why small differences in structure — an alpha or a beta glucose, a single double bond in a fatty acid — make the difference between food and wood, or between butter and olive oil.

🎯What you need to be able to do

  • Explain the bonding properties of carbon that allow such a diversity of compounds.
  • Explain condensation and hydrolysis reactions, with examples of polysaccharides, polypeptides and nucleic acids.
  • Recognize pentoses and hexoses in ring diagrams, and relate the properties of glucose to its uses.
  • Explain how the structures of starch and glycogen suit energy storage, and how the structure of cellulose suits its structural role.
  • Outline the role of glycoproteins in cell recognition, using ABO antigens.
  • Explain the hydrophobic nature of lipids, and how triglycerides and phospholipids form by condensation.
  • Distinguish saturated, monounsaturated and polyunsaturated fatty acids, and relate them to melting point.
  • Explain why triglycerides suit long-term energy storage and thermal insulation.
  • Explain how phospholipids form bilayers (amphipathic), and why steroids can pass through membranes.

📚The biology

Why carbon?

Living things are built on carbon compounds because of how carbon bonds. A covalent bond is a shared pair of electrons between two atoms, and it is strong and stable. A carbon atom can form four covalent bonds, which may be:

  • four single bonds, or a combination of single and double bonds;
  • with other carbon atoms, so that carbon can form long chains, branched chains and rings (single or multiple);
  • with other non-metallic elements, especially hydrogen, oxygen, nitrogen, phosphorus and sulfur.

These possibilities give an almost unlimited variety of molecules of different shapes and sizes — the diversity on which life depends. Glucose is a single ring; cellulose is an unbranched chain of rings; glycogen is a heavily branched chain; steroids have four fused rings; fatty acids are long unbranched chains.

Condensation: building polymers

Large biological molecules (macromolecules) are made by linking small molecules (monomers) into chains (polymers). Each link is made by a condensation reaction: an –OH from one monomer and an –H from the other are removed and combine to form water, and a covalent bond forms between the monomers.

Polysaccharides
monomers: monosaccharides (e.g. glucose)
link: glycosidic bond
examples: starch, glycogen, cellulose
Polypeptides
monomers: amino acids
link: peptide bond
examples: insulin, collagen, enzymes (B1.2)
Nucleic acids
monomers: nucleotides
link: phosphodiester bond
examples: DNA, RNA (A1.2)

Hydrolysis: breaking polymers

Hydrolysis is the reverse: a polymer is broken into monomers by adding water. A water molecule is split, and its –H and –OH are added to the two monomers on either side of the bond being broken — hence the name, “splitting with water”. Digestion is hydrolysis catalysed by enzymes: starch to maltose and glucose, proteins to amino acids, triglycerides to fatty acids and glycerol.

\[ \text{condensation: } \text{monomer} + \text{monomer} \rightarrow \text{dimer} + \mathrm{H_2O} \qquad \text{hydrolysis: } \text{dimer} + \mathrm{H_2O} \rightarrow \text{monomer} + \text{monomer} \]

Monosaccharides

Monosaccharides are single sugar units. They are classified by the number of carbon atoms:

  • Pentoses have five carbons. Ribose and deoxyribose are pentoses; in ring form they make a five-membered ring (four carbons and an oxygen) with one carbon outside the ring.
  • Hexoses have six carbons. Glucose, fructose and galactose are hexoses; glucose forms a six-membered ring (five carbons and an oxygen) with the sixth carbon outside the ring.

To identify one in a diagram, count the carbons, including any outside the ring.

Glucose (C6H12O6) shows how properties match uses:

Soluble
Its many –OH groups form hydrogen bonds with water, so it dissolves readily and can be used in metabolism in the cytoplasm.
Transportable
Because it is soluble and small, it is carried dissolved in blood plasma to every cell.
Chemically stable
It does not react readily on its own, so it can be transported and stored without breaking down.
Energy yield from oxidation
Oxidizing it in respiration releases a large amount of energy, used to make ATP (C1.2).

Glucose exists in two ring forms, alpha and beta, which differ only in the position of the –OH on carbon 1: below the ring in alpha-glucose, above it in beta-glucose. That single difference decides whether a polymer is a food store or a structural material.

Starch and glycogen: energy stores

Starch (in plants) and glycogen (in animals and fungi) are polymers of alpha-glucose. Starch is a mixture of amylose (unbranched chains that coil into a helix) and amylopectin (branched); glycogen is similar to amylopectin but much more highly branched.

  • Compact. Coiling and branching during polymerization pack a great many glucose units into a small space.
  • Relatively insoluble, because the molecules are so large. They therefore do not dissolve in the cytoplasm and do not draw water into the cell by osmosis, which a store of free glucose would.
  • Easy to build up and break down. Glucose can be added to the ends of the chains by condensation when glucose is plentiful, and removed by hydrolysis when it is needed. Branching gives many ends, so glycogen in particular can be mobilized very rapidly — useful for an active animal.

Cellulose: a structural polysaccharide

Cellulose, the main component of plant cell walls, is a polymer of beta-glucose. Because of the position of the –OH groups, each beta-glucose in the chain must be inverted relative to its neighbours for the bonds to form. This alternating orientation produces straight, unbranched chains.

Straight chains can lie side by side in bundles, with –OH groups on neighbouring chains forming many hydrogen bonds that cross-link them. Bundles form microfibrils, which are extremely strong in tension — strong enough to let plant cells withstand the pressure of water pushing outwards. Most animals cannot digest cellulose because they lack enzymes that hydrolyse the bonds between beta-glucose units.

Glycoproteins and cell recognition

Glycoproteins are proteins with carbohydrate chains attached. In membranes, the carbohydrate part projects outwards from the cell surface, where it can be recognized by other cells (B2.1).

The ABO blood groups are an example. All red blood cells carry a basic carbohydrate chain on surface glycoproteins (and glycolipids). In people of group A an extra sugar is added that makes the A antigen; in group B, a different sugar makes the B antigen; group AB has both; group O has neither, only the basic chain. The immune system recognizes its own antigens as “self”. If blood with a different antigen is transfused, antibodies bind to the foreign antigens and cause the red blood cells to clump, which is why blood groups must be matched (D3.2).

Lipids are hydrophobic

Lipids are substances in living organisms that dissolve in non-polar solvents such as ethanol but are only sparingly soluble in water. They are made mostly of carbon and hydrogen in non-polar hydrocarbon chains or rings, which cannot form hydrogen bonds with water. The group includes fats, oils, waxes and steroids.

Triglycerides and phospholipids

Both are made by condensation reactions using glycerol, a three-carbon molecule with three –OH groups:

Triglyceride
one glycerol + three fatty acids
three condensation reactions, releasing 3 H2O
bonds formed: ester bonds
Phospholipid
one glycerol + two fatty acids + one phosphate group
the phosphate group is often linked to another small polar group

Saturated and unsaturated fatty acids

A fatty acid is a hydrocarbon chain with a carboxyl group (–COOH) at one end. Fatty acids differ in the number of carbon–carbon double bonds (C=C) in the chain:

Saturated
No C=C double bonds; every carbon carries the maximum number of hydrogens. Chains are straight and pack closely.
Monounsaturated
One C=C double bond. A cis double bond puts a kink in the chain.
Polyunsaturated
Two or more C=C double bonds, and more kinks.

Kinks stop chains packing tightly together, so the attractions between them are weaker and less energy is needed to separate them. The more double bonds, the lower the melting point.

This explains where each type is found. Plants store energy mainly as oils rich in unsaturated fatty acids, which stay liquid at the temperatures plants experience. Endotherms (birds and mammals) store fats richer in saturated fatty acids, which are solid at room temperature but soft at body temperature (around 37 °C).

Triglycerides for energy storage and insulation

Triglycerides are stored in adipose tissue. Their properties suit long-term energy storage:

  • High energy yield per gram — about 37 kJ g−1, more than double the 17 kJ g−1 of carbohydrates, because they contain little oxygen and a lot of oxidizable carbon and hydrogen.
  • Insoluble, so they do not affect osmosis and are stored without water. Glycogen is stored with several grams of water per gram, so as a store it is much heavier.
  • Chemically stable, so they can be kept for long periods.

Because energy is stored in less mass, fat is the better store for animals that move, and especially for those that fly or migrate. Glycogen is used for short-term storage, because it can be mobilized faster and used in anaerobic respiration.

Adipose tissue is also a thermal insulator: fat conducts heat poorly, so a layer under the skin reduces heat loss. The thickness relates to body temperature and habitat: animals that keep a high body temperature in cold environments, such as seals, whales and polar bears, have thick layers of blubber, especially in cold water, which conducts heat away far faster than air (A1.1).

Phospholipid bilayers

A phospholipid has two very different regions. The phosphate head is charged or polar and so hydrophilic; the two fatty acid tails are non-polar and so hydrophobic. A molecule with both a hydrophilic and a hydrophobic region is amphipathic.

In water, phospholipids arrange themselves so that the heads face the water and the tails are hidden from it. The result is a bilayer: two layers of phospholipids with the tails pointing inwards towards each other and the heads facing the water on both sides. Bilayers form spontaneously and are the basis of every cell membrane (B2.1).

Steroids

Steroids are lipids with a characteristic structure of four fused carbon rings — three six-membered rings and one five-membered ring. You can identify a molecule as a steroid from this ring system. Examples include cholesterol and the sex hormones oestradiol and testosterone.

Steroids are largely non-polar, so they can dissolve in the hydrophobic core of the membrane and pass directly through the phospholipid bilayer. This is why steroid hormones do not need membrane receptors: they enter their target cells and bind to receptors inside (C2.1).

✏️Worked example

An adult human stores about 10 kg of triglyceride in adipose tissue. Triglyceride yields about 37 kJ g−1 when respired; glycogen yields about 17 kJ g−1, and each gram of stored glycogen is associated with about 3 g of water.
(a) Calculate the energy stored in 10 kg of triglyceride, in megajoules.
(b) Calculate the mass of glycogen, and the total mass of glycogen plus its water, that would store the same energy.
(c) Use your answers to explain why animals store most of their energy as lipid.

(a) 10 kg = 10 000 g.

\[ 10\,000\ \mathrm{g} \times 37\ \mathrm{kJ\,g^{-1}} = 370\,000\ \mathrm{kJ} = 370\ \mathrm{MJ} \]

(b) Mass of glycogen needed:

\[ \frac{370\,000\ \mathrm{kJ}}{17\ \mathrm{kJ\,g^{-1}}} = 21\,800\ \mathrm{g} \approx 22\ \mathrm{kg} \]

Each gram of glycogen comes with 3 g of water, so the store would weigh 4 g for every gram of glycogen:

\[ 21.8\ \mathrm{kg} \times 4 = 87\ \mathrm{kg} \]

(c) Storing the same energy as glycogen would add roughly 87 kg to body mass instead of 10 kg — nearly nine times as much. Lipid releases more than twice as much energy per gram, and because triglycerides are insoluble they are stored without water. Carrying less mass costs less energy when moving, which matters greatly for animals that run, fly or migrate. Glycogen is kept only for smaller, short-term stores that can be mobilized quickly.

Check it. Glycogen yields less than half the energy per gram, so it must take more than twice the mass — 22 kg against 10 kg is about 2.2 times, consistent with 37 ÷ 17. Then the water quadruples it. If your glycogen mass came out less than 10 kg, you multiplied by 17 instead of dividing.
Saying lipids “contain more energy” without saying per gram. The comparison only makes sense per unit mass. And give the reason, not just the fact: lipids contain less oxygen and more hydrogen and carbon that can be oxidized, and they are stored without water.

📝Practise

Work through these on paper, then reveal the answer.

1. Distinguish between condensation and hydrolysis reactions, giving one example of each.
Condensation joins two monomers by a covalent bond, releasing a molecule of water (formed from an –OH of one monomer and an –H of the other). Example: two glucose molecules forming maltose; amino acids forming a polypeptide. Hydrolysis breaks the bond between monomers using a molecule of water, whose –H and –OH are added to the products. Example: digestion of starch to maltose, or of a triglyceride to glycerol and fatty acids.
2. Explain how the structure of glycogen makes it suitable as an energy store in animals.
Glycogen is a polymer of alpha-glucose that is highly branched and coiled, so it is compact and stores many glucose units in a small space. Its very large molecules make it insoluble, so it does not affect the water potential of cells or diffuse out of them. Branching gives many ends, where glucose can be rapidly removed by hydrolysis when energy is needed, or added by condensation when glucose is plentiful.
3. Explain how the structure of cellulose is related to its function in plant cell walls.
Cellulose is made of beta-glucose, and each monomer is inverted relative to the next, which produces straight, unbranched chains. Straight chains can lie parallel to one another in bundles, and –OH groups on adjacent chains form many hydrogen bonds that cross-link them into microfibrils. This gives very high tensile strength, allowing the cell wall to resist the outward pressure of water entering the cell by osmosis without bursting.
4. Explain why olive oil is liquid at room temperature while butter is solid.
Olive oil is rich in unsaturated fatty acids (mainly monounsaturated), which contain C=C double bonds that put kinks in the hydrocarbon chains. The kinked chains cannot pack closely, so the attractions between molecules are weaker and the melting point is lower — below room temperature. Butter is rich in saturated fatty acids with no double bonds; their straight chains pack tightly, the attractions are stronger, and the melting point is above room temperature.
5. Explain why phospholipids form bilayers in water.
Phospholipids are amphipathic: the phosphate head is polar/charged and hydrophilic, attracted to water, while the two fatty acid tails are non-polar and hydrophobic. In water they arrange themselves so that the heads are in contact with water and the tails are shielded from it. A bilayer achieves this on both sides: two layers with tails facing inwards, towards each other, forming a hydrophobic core, and heads facing the water on the outside of each layer.
6. Testosterone is a steroid hormone. Explain how it is able to enter its target cells, and state how a steroid can be identified from its molecular structure.
Testosterone is a lipid and largely non-polar, so it can dissolve in the hydrophobic core of the phospholipid bilayer and diffuse directly through the membrane into the cell, without needing a channel or transporter. A steroid can be identified by its structure of four fused carbon rings (three with six carbons and one with five).

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Khan Academy — Carbohydrates and Lipids videos, with clear drawings of alpha and beta glucose and of saturated and unsaturated fatty acids.
  • MolView or the PubChem 3D viewer — free tools for looking at glucose, cellulose chains and steroid rings in three dimensions, which makes the alpha/beta difference obvious.
  • Royal Society of Chemistry, Learn Chemistry — background on covalent bonding and the chemistry of carbon compounds.