Carbohydrates and lipids
Cambridge A-Level Biology (9700) · Unit 2: Biological molecules · 16 flashcards
Carbohydrates and lipids is topic 2.2 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 2 — Biological molecules , alongside Testing for biological molecules, Proteins and Water. In one line: A monomer is a small repeating unit. A polymer is a large molecule made of many monomers joined together. A macromolecule is a very large molecule, often a polymer, with a high molecular weight (.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 16 flashcards — 8 definitions and 8 key concepts — covering the precise wording mark schemes reward. Use the 8 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The terms 'monomer', 'polymer', and 'macromolecule'
A monomer is a small repeating unit. A polymer is a large molecule made of many monomers joined together. A macromolecule is a very large molecule, often a polymer, with a high molecular weight (
What the Cambridge 9700 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Draw and describe the ring structures of α-glucose and of β-glucose.
- Define each of these terms: monomer, macromolecule, polymer, monosaccharide, polysaccharide and disaccharide.
- State the part played by covalent bonds when smaller molecules are linked together into polymers.
- State that maltose, glucose and fructose are all reducing sugars, but sucrose is not (it is a non-reducing sugar).
- Describe how condensation forms a glycosidic bond, referring to disaccharides (sucrose included) and to polysaccharides.
- Describe how hydrolysis breaks glycosidic bonds in disaccharides and polysaccharides, linking this to the test for non-reducing sugars.
- Describe the molecular structure of glycogen and of starch (both amylose and amylopectin), and relate each structure to its function in living organisms.
- Describe how cellulose is built at the molecular level, and outline why the way its molecules are arranged lets plant cell walls do their job.
- State that triglycerides are hydrophobic and non-polar, and describe their molecular structure in terms of glycerol, saturated and unsaturated fatty acids, and ester bond formation.
- Relate triglyceride molecular structure to the roles triglycerides play in living organisms.
- Describe how a phospholipid molecule is built, with a phosphate head that is hydrophilic (polar) and fatty acid tails that are hydrophobic (non-polar).
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Carbohydrates and lipids
- › Amylose is unbranched with 1,4-glycosidic bonds only; amylopectin is branched and contains both 1,4 and 1,6-glycosidic bonds.
- › Recall that hydrolysis reactions use water to split molecules, whereas condensation reactions release water.
- › Specify that cellulose is a linear, unbranched polymer of beta-glucose with 1,4-glycosidic bonds and alternating 180-degree monomer rotation.
Draw the ring structure of α-glucose.
α-glucose has a six-membered ring of five carbon atoms and one oxygen atom. Numbering clockwise from C1 (to the right of the ring oxygen): C1 has -OH below the ring and H above (reversed in β-glucose); C2 -OH below; C3 -OH above; C4 -OH below; C5 carries the CH₂OH group (carbon 6) above the ring.
Define the terms 'monomer', 'polymer', and 'macromolecule'.
A monomer is a small repeating unit. A polymer is a large molecule made of many monomers joined together. A macromolecule is a very large molecule, often a polymer, with a high molecular weight (
What are monosaccharides, disaccharides and polysaccharides? Provide an example of each.
Monosaccharides are simple sugars (
What type of bond joins smaller molecules to form polymers, and how is this bond formed?
Covalent bonds join smaller molecules (monomers) to form polymers. These bonds are formed through condensation reactions, where a water molecule is removed.
Which of the following are reducing sugars: glucose, fructose, maltose, sucrose?
Glucose, fructose, and maltose are reducing sugars. Sucrose is a non-reducing sugar.
Describe the formation of a glycosidic bond.
A glycosidic bond forms between two monosaccharides by a condensation reaction. An -OH group on one sugar (e.g. on carbon 1) reacts with an -OH group on the other (e.g. on carbon 4): an -OH and an H are removed together as one water molecule, and the two sugars are linked through an oxygen atom (C-O-C).
Give an example of the formation of a glycosidic bond with reference to sucrose.
Sucrose is formed from glucose and fructose. A glycosidic bond forms between carbon 1 of glucose and carbon 2 of fructose, with the removal of a water molecule.
Give an example of the formation of a glycosidic bond with reference to polysaccharides.
Starch is formed from many α-glucose molecules. Glycosidic bonds form between carbon 1 and carbon 4 of adjacent α-glucose molecules, linking them into a long chain with the release of water molecules.
Describe the process of hydrolysis in the context of breaking down polysaccharides and disaccharides.
Hydrolysis is the breaking of a glycosidic bond by the addition of water. A water molecule (H₂O) is added, with -H attaching to one monosaccharide and -OH to the other, separating them. This process is key in digestion and is tested for with a non-reducing sugar test (
Describe the molecular structure of starch, including amylose and amylopectin, and relate their structures to their function in energy storage.
Starch is a polysaccharide composed of glucose monomers. Amylose is a long, unbranched chain of α-glucose linked by α-1,4-glycosidic bonds, forming a helical structure. Amylopectin is branched, with α-1,4-glycosidic bonds and α-1,6-glycosidic bonds at the branch points. The compact structure allows for efficient storage of glucose.
Describe the molecular structure of glycogen and relate its structure to its function in energy storage.
Glycogen is a highly branched polysaccharide of α-glucose, similar to amylopectin but with more frequent α-1,6-glycosidic branches. This branching allows for rapid hydrolysis and release of glucose when needed, important for maintaining blood sugar levels.
Describe the molecular structure of cellulose and explain how its arrangement contributes to the function of plant cell walls.
Cellulose is a linear, unbranched polysaccharide of β-glucose linked by β-1,4-glycosidic bonds. Many cellulose chains run parallel and are cross-linked by hydrogen bonds, forming strong microfibrils. These microfibrils provide tensile strength and rigidity to plant cell walls.
Explain why triglycerides are described as non-polar and hydrophobic.
Triglycerides are non-polar due to the even distribution of electrons in their hydrocarbon chains of fatty acids. This lack of polarity makes them hydrophobic, meaning they are insoluble in water because they cannot form hydrogen bonds with water molecules.
Describe the molecular structure of a triglyceride, including its components and the type of bond formed.
Triglycerides consist of a glycerol molecule bonded to three fatty acids. The bonds are ester bonds, formed by the reaction of the glycerol's hydroxyl (-OH) groups with the carboxyl (-COOH) groups of the fatty acids, releasing three water molecules.
Relate the structure of triglycerides to their function as energy stores and insulation.
The fatty acid hydrocarbon chains contain many C-H bonds, so triglycerides release about twice as much energy per gram as carbohydrate when respired. Being insoluble, they do not affect the water potential of cells and can be stored compactly. Fat under the skin is a poor conductor of heat, so it provides thermal insulation.
Describe the molecular structure of phospholipids, highlighting the hydrophilic and hydrophobic regions.
Phospholipids consist of a glycerol molecule linked to two fatty acids and a phosphate group. The phosphate group is polar and hydrophilic (water-loving), forming the 'head,' while the fatty acid tails are non-polar and hydrophobic (water-repelling).
Review the material
Read full revision notes on Carbohydrates and lipids — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 2 — Biological molecules
Carbohydrates and lipids sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Carbohydrates and lipids deck
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