(c) Biological molecules
- Syllabus
- 2024
- Topic
- —
- Level
- —
Learning objectives
2.7Elements in biological moleculesIdentify the chemical elements present in carbohydrates, proteins and lipids (fats and oils).2.8Biological molecule structureDescribe the structure of carbohydrates, proteins and lipids as large molecules made up from smaller basic units: starch and glycogen from simple sugars, protein from amino acids, and lipid from fatty acids and glycerol.2.9Food testsPractical: investigate food samples for the presence of glucose, starch, protein and fat.2.10Enzymes as biological catalystsUnderstand the role of enzymes as biological catalysts in metabolic reactions.2.11Temperature and enzyme functionUnderstand how temperature changes can affect enzyme function, including changes to the shape of active site.2.12Enzyme activity practicalPractical: investigate how enzyme activity can be affected by changes in temperature.2.13pH and enzyme functionUnderstand how enzyme function can be affected by changes in pH altering the active site.214B Effect of pH on enzyme activity practicalPractical: investigate how enzyme activity can be affected by changes in pH.
Identify the elements in biological molecules
Carbohydrates, proteins and lipids are carbon-based biological molecules, but their usual elemental compositions are not identical.
| Molecule group | Elements present | Symbols |
|---|---|---|
| carbohydrate | carbon, hydrogen and oxygen | C, H, O |
| lipid (fat or oil) | carbon, hydrogen and oxygen | C, H, O |
| protein | carbon, hydrogen, oxygen and nitrogen | C, H, O, N |
Carbon, hydrogen and oxygen occur in all three groups. Nitrogen is the distinguishing element in the syllabus comparison because proteins are built from amino acids.
Do not infer the molecule group from carbon alone. Carbohydrates and lipids contain the same three named elements, so their structure—not a different element list—distinguishes them.
Build large biological molecules from smaller units
Large biological molecules are assembled from smaller basic units; the type and arrangement of those units determine the molecule group.
| Large molecule | Smaller basic unit or units |
|---|---|
| starch | many simple sugar units |
| glycogen | many simple sugar units |
| protein | amino acids |
| lipid | fatty acids and glycerol |
Starch and glycogen are both large carbohydrate molecules made from simple sugars, but they are different storage molecules. A protein uses amino acids rather than sugars; a lipid combines glycerol with fatty acids.
A lipid is not described here as a chain of only one repeating unit: it requires both glycerol and fatty acids. Do not confuse a molecule's building units with the chemical elements inside those units.
Test food samples for four biological molecules
A food test is valid only when the reagent, treatment and positive colour change are matched to the molecule being detected.
| Molecule | Method | Positive result |
|---|---|---|
| glucose | add Benedict's solution and heat in a hot-water bath | blue changes through green, yellow or orange to brick-red |
| starch | add iodine solution | orange-brown changes to blue-black |
| protein | add Biuret reagent | blue changes to lilac or purple |
| lipid (fat) | shake with ethanol, then add water | a cloudy white emulsion forms |
Crush a solid sample and mix it with water when an extract is needed. Use comparable sample and reagent volumes when comparing foods, and include known positive and negative samples if the test must be checked.
Wear eye protection. Heat Benedict's tubes in a water bath rather than directly in a flame; ethanol is flammable, so keep it away from ignition sources. A negative result means the test did not detect the molecule, not that the food contains nothing else.
Explain how enzymes catalyse metabolism
An enzyme is a biological catalyst: it increases the rate of a metabolic reaction without being used up by that reaction.
A substrate with a complementary shape binds to the enzyme's active site, forming an enzyme–substrate complex. The reaction occurs, products leave, and the unchanged enzyme can catalyse another reaction.
ext{enzyme}+ ext{substrate}
ightarrow ext{enzyme–substrate complex}
ightarrow ext{enzyme}+ ext{products}
Metabolism means the chemical reactions occurring in cells or organisms. Enzymes make these reactions fast enough under cellular conditions, and active-site shape gives each enzyme specificity for particular substrate shapes.
An enzyme changes reaction rate, not the final products, and it is not consumed as a reactant. A substrate must fit the active site; merely colliding with any enzyme is insufficient.
Explain the effect of temperature on enzymes
Enzyme activity usually rises with temperature to an optimum, then falls sharply when higher temperature changes the active site.
| Temperature region | Molecular explanation | Effect on rate |
|---|---|---|
| below optimum | enzyme and substrate have less kinetic energy, so successful collisions and complexes form less often | rate is lower but rises as temperature increases |
| at optimum | active sites retain their shape and successful complex formation is most frequent | maximum rate |
| above optimum | bonds maintaining enzyme shape are disrupted; the active site changes shape | substrate fits less well or not at all, so rate falls |
When the active site changes permanently enough that the substrate is no longer complementary, the enzyme is denatured. The reaction may stop even though substrate remains.
Low temperature does not usually denature an enzyme; it slows molecular movement and is often reversible on warming. Do not say the enzyme itself is killed—enzymes are molecules, not living organisms.
Investigate temperature and enzyme activity
To test temperature fairly, change only temperature and measure enzyme activity with a repeatable endpoint or rate.
| Step | Action |
|---|---|
| 1 | prepare water baths across a suitable temperature range |
| 2 | keep enzyme and substrate volumes and concentrations, pH and total volume constant |
| 3 | equilibrate enzyme and substrate separately at each temperature, then mix and start timing |
| 4 | for amylase and starch, sample at fixed intervals onto iodine until iodine remains orange-brown |
| 5 | repeat each temperature, calculate a mean time and compare rate using 1/exttime |
Use smaller temperature intervals around the fastest result to estimate the optimum more accurately. A shorter endpoint time means a faster reaction, so time itself is inversely related to rate.
Use water baths and eye protection. Equilibrating before mixing prevents the reaction starting at an unintended temperature; a single reading cannot reveal random variation, so repeats are required.
Explain the effect of pH on enzymes
Each enzyme has an optimum pH at which its active site has the most effective shape for binding its substrate.
Changing pH changes the chemical conditions around the enzyme. Away from the optimum, bonds maintaining its three-dimensional shape can be disrupted, altering the active site so fewer enzyme–substrate complexes form.
| Condition | Active site and rate |
|---|---|
| optimum pH | substrate is complementary; complex formation and rate are greatest |
| moderately away from optimum | fewer successful complexes form; rate decreases |
| sufficiently extreme pH | active site may change so much that the enzyme is denatured |
Optimum pH is enzyme-specific; it is not always neutral. pH changes enzyme activity by altering the active site, not by changing the amount of substrate present.
Investigate pH and enzyme activity
A pH investigation uses buffer solutions to set different pH values while every other factor affecting enzyme rate is controlled.
| Step | Action |
|---|---|
| 1 | prepare labelled tubes containing buffers across a suitable pH range |
| 2 | add equal volumes and concentrations of amylase and starch; keep temperature constant in a water bath |
| 3 | mix one pH treatment, start timing and sample at fixed intervals onto iodine |
| 4 | record the time when iodine stays orange-brown, showing no starch is detected |
| 5 | repeat each pH, calculate mean time and compare rate using 1/exttime |
Independent variable: buffer pH. Dependent variable: time to the starch endpoint or calculated rate. Controls include temperature, enzyme and substrate concentration and volume, total volume, sampling interval and iodine volume.
Use a buffer rather than an unmeasured amount of acid or alkali so pH is known and stable. Test smaller pH intervals around the fastest result before claiming an optimum, and wear eye protection when handling reagents.