1.4Unit 1 · Living Organisms & Cells
Biological Molecules — Edexcel IGCSE Biology Modular - UNIT 1 Revision Notes & Topical Questions
10 min read 7 parts Lesson 5 of 6 Unit quiz
Lesson summary
Identify the chemical elements and building blocks of carbohydrates, proteins and lipids.Enzymes are proteins; their shape is specific to one substrate (lock and key model).Most human enzymes have an optimum near 37 °C.Above the optimum, bonds holding the active site break and th…
- Structure of biological molecules
- Practical — food tests
- Role of enzymes
- Temperature and enzyme function
- Practical — temperature and enzymes
- pH and enzyme function
Section 1 of 7Biological Molecules
Essential knowledge
Structure of biological molecules
Structure of biological molecules carbohydrates contain C, H and O; simple sugars such as glucose join to make starch (plants) and glycogen (animals). Proteins are chains of amino acids and contain C, H, O and N. Lipids are made from glycerol and fatty acids and contain C, H and O.
Practical — food tests
Practical — food tests iodine turns blue-black with starch.
- Benedict's solution heated in a water bath changes from blue to green, yellow, orange or brick-red with glucose depending on concentration.
- Biuret reagent turns purple with protein.
- For fat, dissolve the food in ethanol then add water: a cloudy white emulsion shows lipid.
Role of enzymes
Role of enzymes enzymes are proteins that catalyse reactions in cells, including digestion, respiration and photosynthesis. The substrate binds to the complementary active site forming an enzyme-substrate complex, the reaction happens and products are released; the enzyme can be reused.
Temperature and enzyme function
Temperature and enzyme function as temperature rises, molecules move faster and collide more often, so rate increases up to the optimum. Above the optimum, the active site changes shape (denaturation) and the rate falls quickly.
Practical — temperature and enzymes
Practical — temperature and enzymes amylase and starch are incubated at different temperatures, sampling into iodine at regular intervals. Control pH with a buffer and use the same volumes and concentrations. The shortest time for starch to disappear shows the optimum.
pH and enzyme function
pH and enzyme function each enzyme has an optimum pH. pH values away from the optimum alter the bonds holding the active site in shape so the substrate fits less well; extreme pH denatures the enzyme.
Practical — pH and enzymes
Practical — pH and enzymes use buffer solutions of different pH with amylase and starch at a constant temperature (e.g. a 35 °C water bath), timing how long until iodine no longer turns blue-black. Alternatively measure oxygen produced by catalase from hydrogen peroxide at each pH.
Sugars to large carbohydrates
Glucose is a simple sugar. Two glucose units join to make maltose, the product of amylase digesting starch. Many glucose units join to make starch, glycogen or cellulose, which are insoluble and therefore good for storage or structure.
Protein shape and function
Amino acids can be joined in any order, so a huge variety of proteins exist. The sequence of amino acids decides how the chain folds, giving each protein a particular shape, and that shape decides its job (e.g. an enzyme's active site).
Fats and oils
Lipids that are solid at room temperature are called fats; those that are liquid are called oils. Both are made from glycerol and fatty acids and contain only carbon, hydrogen and oxygen.
Preparing food for testing
Grind a solid food with a pestle and mortar, mix it with distilled water, then filter to obtain a solution for the tests. Do not taste samples, and wear eye protection throughout.
Food test hazards
Biuret reagent contains copper(II) sulfate and corrosive sodium hydroxide, so wear goggles and wash splashes off skin. Ethanol for the emulsion test is highly flammable, so keep it away from Bunsen flames. Heat Benedict's tests in a water bath rather than over a flame.
Why organisms need enzymes
Without enzymes, metabolic reactions at body temperature would be far too slow to keep an organism alive; for example, digesting a meal would take weeks instead of hours. The product of one enzyme-controlled reaction is often the substrate for the next.
Range of optimum pH
Most enzymes work best near pH 7, but enzymes vary with where they act: stomach protease (pepsin) works best near pH 2, while enzymes in the duodenum work best in slightly alkaline conditions of about pH 8–9.
Improving the amylase practical
Judging when iodine stops turning blue-black is subjective; a colorimeter gives an objective reading of colour intensity instead. Heating with a Bunsen makes temperature hard to hold steady, so use thermostatic water baths and check with a thermometer.
CORMS for planning
Change: the independent variable (e.g. temperature). Organism: same species or source of material. Repeat: several times for reliability. Measure: what is measured and when. Same: control variables, e.g. volumes and concentrations of starch, amylase and iodine.
Section 2 of 7Process & sequence
1
Set water baths at a range of temperatures (e.g. 20, 30, 40, 50, 60 °C).
2
Equilibrate starch solution and amylase separately in each bath.
3
Mix them and start the stopwatch.
4
Every 30 seconds, place a drop of the mixture into iodine on a spotting tile.
5
Record the time when iodine no longer turns blue-black (starch fully digested).
6
Calculate rate = 1 ÷ time and plot rate against temperature.
How an enzyme workswrite these steps in this order; each step is a marking point.
1
Substrate collides with enzyme
2
Fits complementary active site
3
Enzyme-substrate complex forms
4
Reaction takes place
5
Products released, enzyme reused
Section 3 of 7Compare
| Substance | Reagent / method | Positive result |
|---|---|---|
| Starch | Iodine solution | Orange-brown to blue-black |
| Glucose (reducing sugar) | Benedict's solution, heat in a water bath | Blue to green, yellow or brick-red |
| Protein | Biuret reagent | Blue to purple / lilac |
| Lipid (fat) | Shake with ethanol, then add water | Cloudy white emulsion |
| Large molecule | Made from | Elements |
|---|---|---|
| Starch / glycogen | Glucose units | C, H, O |
| Protein | Amino acids | C, H, O, N (sometimes S) |
| Lipid | Glycerol + fatty acids | C, H, O |
Section 4 of 7Worked examples
1
At 40 °C amylase digested all the starch in 50 s. Calculate the rate of reaction.
- Rate = 1 ÷ time = 1 ÷ 50 = 0.020 s-1.
2Explain why the rate of an enzyme reaction falls rapidly at 60 °C.
- The high temperature breaks bonds holding the enzyme in shape, so the active site changes shape.
- The substrate no longer fits.
- (no enzyme-substrate complexes form)
- The enzyme is denatured, so the rate falls.
- The substrate no longer fits.
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Carbohydrate · Protein · Lipid · Enzyme · Active site · Substrate · Denatured · Optimum · Catalyst
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
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