What you need to know
- 1.1–1.5Water, carbohydrates, lipids and food-test reasoning.
- 2.6Protein structure and function.
By the end, you should be able to…
- Retrieve key mechanisms before checking the answers.
- Apply ideas to unfamiliar numerical and experimental examples.
- Identify and correct missing steps in an explanation.
Build your understanding
Choose the relevant level
Start molecular explanations with the property asked about: polarHaving an uneven distribution of charge, so the molecule has partially charged ends. Polar molecules dissolve in water. for solubility, bond type for condensationA reaction that joins two molecules with a covalent bond and releases a molecule of water., branching for mobilisation or folding for protein function. Then explain the consequence.
Keep measurements defensible
Use units and dilution factors. A colour-standard result is an estimate or interval; describe the colour and precipitate consistently. Use Core Practical 1 for the full method.
Make your reasoning visible
Answer the command word
Describe the pattern, explain the mechanism or evaluate the evidence as requested. A correct fact may still miss the question.
Show workings and units
Write a calculation before its result, and distinguish fractions, percentages and percentage-point changes.
Use answers diagnostically
Attempt the question first, then identify the missing step rather than only checking whether your final number matches.
Avoid this misconception
The claim: A longer answer automatically earns more credit.
The correction: Relevance, accuracy and the requested reasoning matter.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Explain why glucose can be transported in plasma but a triglyceride is not freely soluble in its water.
Show suggested answer
Glucose has polar groups that interact with water, so it dissolves. A triglyceride has large non-polar hydrocarbon regions and does not interact sufficiently with water to dissolve readily; lipid transport needs other arrangements.
Practice question
Retrieval and explanation · suggested answer
Compare amylose and amylopectin and explain one advantage of branching in a storage polysaccharide.
Show suggested answer
Amylose is unbranched with α-1,4 links and coils; amylopectin also has α-1,6 branch points. Branching provides more ends for enzymes involved in mobilisation, supporting rapid glucose release.
Practice question
Retrieval and explanation · suggested answer
Describe formation of a triglyceride from its components.
Show suggested answer
One glycerol joins three fatty acids in condensation reactions, forming three ester bonds and releasing three water molecules.
Practice question
Retrieval and explanation · suggested answer
An amino-acid substitution changes a globular enzyme’s active site. Explain how primary structure can affect activity.
Show suggested answer
A changed amino-acid sequence can alter interactions responsible for folding, changing the active site’s shape or chemical properties. Substrate binding or catalysis may be impaired.
Practice question
Retrieval and explanation · suggested answer
A glucose test sample matches between the 0.4% and 0.6% Benedict’s colour standards. It was diluted fivefold before testing. Report a defensible original concentration estimate.
Show suggested answer
Approximately 2–3% in the original sample: multiply the estimated 0.4–0.6% range by five. A visual semi-quantitative comparison does not justify a precise value such as 2.437%.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
Two disaccharides have identical molecular formulae but only one is hydrolysed by an enzyme. Explain without assuming that one contains more energy.
Show a hint
Consider bonding arrangement and active-site specificity.
Explore the reasoning
Different arrangements of atoms or glycosidic links give different three-dimensional structures. Only one may form a productive enzyme–substrate complex; identical formulae do not imply identical enzyme recognition.
Recall this lesson
Lesson recall questions
Question 1 of 5: Starch is a plant ________ made of ________ and ________.
Saccharides and energy stores
Practice question
Retrieval and explanation · suggested answer
Starch is a plant ________ made of ________ and ________.
Lesson: 1A 3: Carbohydrates 2: Polysaccharides
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
A name is not a structure
1A · Thinking Bigger: TrehaloseFlyBase’s chemical entry identifies trehalose by its structure and links to chemical databases. Researchers need precise chemical identity when comparing experiments. For a food chemist, a familiar-sounding name…Specification 1.2(i) · 1.4Read & use the scienceClose article
Why this matters
FlyBase’s chemical entry identifies trehalose by its structure and links to chemical databases. Researchers need precise chemical identity when comparing experiments. For a food chemist, a familiar-sounding name is not enough to predict digestion. Two sugars can contain the same kinds of atom yet arrange their bonds differently. Whether a particular enzyme hydrolyses them depends on that arrangement and on its active site. Trehalose offers a useful unfamiliar case: apply the supplied structural evidence instead of guessing from an ending such as “-ose”. This is also why a claim about one sugar cannot automatically be transferred to every other sugar.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.2(i) know the difference between monosaccharides, disaccharides and polysaccharides, including glycogen and starch (amylose and amylopectin)
- 1.4 know how monosaccharides (glucose, fructose and galactose) join together to form disaccharides (maltose, sucrose and lactose) and polysaccharides (glycogen, amylose and amylopectin) through condensation reactions forming glycosidic bonds, and how these can be split through hydrolysis reactions
Use the science
Why is “a glucose-based sugar” too vague for a reproducible experiment?
Explore a possible response
It does not identify whether the sample is glucose, a particular disaccharide or a polymer, nor the linkages or concentration.
Explore the source
- FlyBase: trehaloseFlyBase
Can a blue test tube prove a drink has no sugar?
1A · Carbohydrates 1: Monosaccharides and DisaccharidesA food analyst and a drinks advertisement may use the word sugar in different ways. The FDA distinguishes total sugars from sugars added during manufacture: a drink can…Specification 1.2(i) · 1.3 · 1.4Read & use the scienceClose article
Why this matters
A food analyst and a drinks advertisement may use the word sugar in different ways. The FDA distinguishes total sugars from sugars added during manufacture: a drink can contain naturally occurring sugars without having sugar added. Your Benedict’s test asks another question, about reducing sugars. It is not a universal detector for every carbohydrate, nor can it tell whether a glucose molecule came from fruit or a factory. Comparing a chemical test with a label requires checking exactly what each measures, using appropriate standards and allowing for the colour of the drink itself.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.2(i) know the difference between monosaccharides, disaccharides and polysaccharides, including glycogen and starch (amylose and amylopectin)
- 1.3 CORE PRACTICAL 1 Use a semi-quantitative method with Benedict’s reagent to estimate the concentrations of reducing sugars and with iodine solution to estimate the concentrations of starch, using colour standards.
- 1.4 know how monosaccharides (glucose, fructose and galactose) join together to form disaccharides (maltose, sucrose and lactose) and polysaccharides (glycogen, amylose and amylopectin) through condensation reactions forming glycosidic bonds, and how these can be split through hydrolysis reactions
Use the science
A sucrose-containing drink gives a negative Benedict’s test before hydrolysis. Explain why the claim “laboratory-proven sugar-free” does not follow.
Explore a possible response
Sucrose is non-reducing under the test conditions. Hydrolysis produces reducing monosaccharides. A negative reducing-sugar result cannot establish absence of all sugars; appropriate controls and sensitivity also matter.