Written for this site from the class slides. The chapter is on pages 181–204 of the course textbook. Image credits are on slide 20.
A covalent bond is a strong bond formed by sharing electrons. A carbon atom can form up to four single bonds, or a mix of single and double bonds, with carbon or other non-metals. So it can form unbranched and branched chains, and single or multiple rings.
Condensation links monomers into a polymer, releasing a water molecule for each link. Examples are polysaccharides, polypeptides and nucleic acids.
Hydrolysis digests polymers into monomers. Water is split, and it supplies the -H and -OH groups. This happens in digestion, and in the surroundings of decomposers.
In ring form, pentoses have 5 carbons, and hexoses have 6. Glucose is soluble and transportable, it is chemically stable, and it yields energy when oxidized in respiration.
Starch in plants and glycogen in animals are compact, because they coil and branch. They are relatively insoluble, because of their size, so they have little osmotic effect. Alpha-glucose monomers are easily added or removed, to build or mobilize a store.
Beta-glucose monomers alternate in orientation, which gives straight chains. Chains are grouped in bundles and cross-linked by hydrogen bonds, which gives high strength for plant cell walls.
A glycoprotein is a protein with a carbohydrate chain, which faces out from the membrane. Other cells can recognize it. ABO antigens on red blood cells differ by the sugars at the ends of their chains.
Lipids dissolve in non-polar solvents, and are only sparingly soluble in water. They include fats, oils, waxes and steroids.
By condensation, one glycerol links to three fatty acids in a triglyceride, or to two fatty acids and a phosphate group in a phospholipid.
| Type | C=C bonds | Melting point |
|---|---|---|
| Saturated | None | Highest: chains pack closely |
| Mono-unsaturated | One | Lower: a kink in the chain |
| Poly-unsaturated | Two or more | Lowest: more kinks |
Plants store energy in oils, and endotherms store it in fats, which suits their warm bodies.
Triglycerides suit long-term energy storage: they are insoluble and store about twice as much energy per gram as carbohydrate, though they mobilize slowly. They are also thermal insulators, especially as blubber in endotherms in cold water.
A phospholipid is amphipathic: it has a hydrophilic phosphate head and two hydrophobic tails. In water, heads face out and tails face in, so a bilayer forms.
Steroids, such as oestradiol and testosterone, have four fused rings. They are non-polar, so they can pass through the hydrophobic core of a membrane.
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Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
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A printable worksheet covering the chapter, from quick recall to data questions and a longer answer. It has 48 marks and takes about 50 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
Questions built on realistic experimental data, in the style of Paper 1B and Paper 2. Work them on paper and show your working. 8 marks in total. Your teacher has the mark scheme.
Thirty healthy adults were split into two groups. For three weeks, the study group ate a diet in which olive oil and nuts were replaced with butter and cream. The control group kept their usual (habitual) diet. The table shows the mean daily intake. The graph shows mean blood LDL cholesterol, ± 1 SD.
| Habitual diet | Study diet | |
|---|---|---|
| Energy / MJ day−1 | 9.4 ± 1.8 | 9.2 ± 1.1 |
| Total fat / % of energy | 34 | 34 |
| Saturated fat / % of energy | 11 | 18 |
| Monounsaturated fat / % of energy | 15 | 10 |
| Polyunsaturated fat / % of energy | 8 | 6 |
| Carbohydrate / % of energy | 48 | 47 |
You will need: Food samples, iodine solution, Benedict's solution, water bath, ethanol, test tubes
Safety: Eye protection. Ethanol is flammable, so no naked flames. Do not taste foods in the lab.
Carry this out with your teacher's supervision and follow your school's lab rules.
Advice about saturated fats has shifted as studies improved, partly because diet is hard to measure and to control.
Public advice must be given before science is settled. Consider how confidence should be communicated when evidence may change.
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
B1.1 has no HL-only content. SL and HL students learn the same material here.
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