1.4Unit 1 · Living Organisms & Cells
Biological Molecules — Unit 1 — Edexcel IGCSE Biology Modular - UNIT 1 Revision Notes & Topical Questions
10 min read 4 parts Lesson 5 of 6 Unit quiz
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 4Biological Molecules — Unit 1
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:
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:
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.
- The substrate binds to the complementary active site forming an enzyme-substrate complex, the reaction happens and products are released;
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:
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:
- 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.
- 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;
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.
- Lipids that are solid at room temperature are called fats;
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 4Process & sequence
- Set water baths at a range of temperatures (e.g. 20, 30, 40, 50, 60 °C).
- Equilibrate starch solution and amylase separately in each bath.
- Mix them and start the stopwatch.
- Every 30 seconds, place a drop of the mixture into iodine on a spotting tile.
- Record the time when iodine no longer turns blue-black (starch fully digested).
- Calculate rate = 1 ÷ time and plot rate against temperature.
How an enzyme workswrite these steps in this order; each step is a marking point.
- Substrate collides with enzyme
- Fits complementary active site
- Enzyme-substrate complex forms
- Reaction takes place
- Products released, enzyme reused
Section 3 of 4Compare
| 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 4Worked examples
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.
Explain 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.
- The high temperature breaks bonds holding the enzyme in shape, so the active site changes shape.
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