Practical 13
Marine Biology
CAPS block 4 · Term 1 · 5 hours
Life Processes & the Chemistry of Life
These are the only practical investigations CAPS sets in the entire Chemistry of Life section, and they are the classic three: find out whether a food contains carbohydrate, lipid or protein by watching a colour change. Two of the reagents cannot honestly be put in a home kitchen, so those two arrive as measured results instead — and you answer exactly the same analysis questions either way.
What CAPS asks for
Investigations column, block 4, four-hour half, in full: "Practical investigations · Test for carbohydrates · Test for lipids · Test for proteins" and "Homework research — The name and function of 2 or 3 each of water soluble and fat soluble Vitamins". Resources column: "Chemicals for food tests" — the only resource listed for the whole four hours.
The one-hour half adds one more: "Classify organisms into living and non-living", with the resource "Acronym: MRS GREN". That is Investigation 5, and it is deliberately last, because the best version of that question needs the chemistry you will have just done.
What you need
- 🟡 pharmacy Tincture of iodine — the small brown bottle in the first-aid section, about R40
- 🟢 kitchen Small clear glasses or jars, a teaspoon, and a dropper (or a clean straw)
- 🟢 kitchen Plain white paper — for the grease-spot test
- 🟡 hardware Methylated spirits — for the emulsion test
- 🟢 kitchen A kettle, and a bowl for a hot-water bath
- 🟢 kitchen Test foods: bread, potato, apple or apple juice, milk, table sugar, cooking oil, butter, an egg, and a piece of fish if you have one
- 🟡 optional Glucose test strips (diabetic urine strips) — a legitimate stand-in for Benedict's on sugars
- 🔴 lab only Benedict's solution and biuret reagent — supplied as results, see below
Two reagents you will not have. That is expected, and it is planned for.
Benedict's solution and biuret reagent are school-laboratory chemicals. Biuret in particular is made with sodium hydroxide, and mixing that in a kitchen is not something this course is going to ask you to do. You will not have them, and it does not matter.
Investigations 1 and 3 both ship with a full measured result set. Every table, every graph, every analysis question and every mark works identically from the supplied data. Nothing on this sheet is gated on owning a laboratory — and no mark anywhere in this block is awarded for having personally held a test tube.
What you can do at home is real and worth doing: the iodine test for starch is a proper experiment with a pharmacy bottle, and both lipid tests need nothing but paper and methylated spirits.
Safety — short, and all of it real
- Iodine stains everything — skin, clothes, worktops — and is an irritant. Work over a plate, and do not get it in your eyes. It is not for drinking, and the bottle says so.
- Methylated spirits is flammable. Keep it away from the kettle and any flame, use it in a ventilated room, and put the lid back on between steps.
- Use boiled-then-poured water for the water bath, not a pot on the stove. You need about 80 °C, not boiling, and a bowl of just-boiled water is safer and works.
- Do not eat anything you have tested. Once a reagent has touched it, it is a sample, not food.
1
Test for Carbohydrates
CAPS Investigation, verbatim
Aim
To test a range of foods for starch (a polysaccharide) using iodine, and for reducing sugars using Benedict's solution — and to find out why those are two separate tests rather than one "sugar" test.
Method — part A, the starch test (you can do this one)
- Put a small amount of each test food into a separate glass. Solids can go in as they are; add a little water to help.
- Add 2–3 drops of iodine solution to each. Add it to a glass of plain water as well — that is your control, and it tells you what a negative looks like.
- Record the colour immediately. A positive is fast and unmistakable.
- Iodine is orange-brown to begin with. Blue-black means starch. Anything still orange-brown is negative.
Method — part B, the reducing-sugar test (read the supplied results)
- Benedict's solution is blue. A few drops go into the sample, and the tube is stood in a hot water bath for about five minutes.
- The colour it ends on shows how much reducing sugar is present: blue (none) → green (trace) → yellow → orange → brick-red (a lot). It is one of the few school tests that is semi-quantitative — the result carries a quantity, not just a yes or no.
- If you managed to get glucose test strips, run them on the same samples and compare. They detect glucose specifically, so they will agree with Benedict's on some samples and not others — which is itself worth a sentence in Q7.
Supplied results — both tests, nine samples
A measured run. Note sample 6: it is sample 5 after being boiled with a little acid, which splits the sucrose into its two monosaccharides. Compare 5 and 6 carefully — that pair is the whole point of the investigation.
| # | Sample | Iodine result | Benedict's result |
|---|---|---|---|
| 1 | Potato, mashed in water | Blue-black | Stayed blue |
| 2 | White bread | Blue-black | Pale green (trace) |
| 3 | Apple juice | Orange-brown | Brick-red |
| 4 | Milk | Orange-brown | Orange |
| 5 | Table sugar (sucrose) solution | Orange-brown | Stayed blue |
| 6 | The same sucrose solution, boiled with acid first | Orange-brown | Brick-red |
| 7 | Glucose solution | Orange-brown | Brick-red |
| 8 | Egg white in water | Orange-brown | Stayed blue |
| 9 | Cooking oil | Orange-brown | Stayed blue |
Results — your own starch test
| Sample | Colour before | Colour after iodine | Starch present? |
|---|---|---|---|
| Water (control) | |||
Analysis
- Complete Table 1.1 for your own samples, including the water control. 6
- State the reagent, the starting colour and the positive colour for both carbohydrate tests. 6
- Why was a glass of plain water included? Name what it is and explain what it is for. 3
- 🔴 Sample 5 is obviously a sugar, yet it gave a negative Benedict's result. Explain why. 4
- Sample 6 is sample 5 after boiling with acid, and it turned brick-red. Explain what the acid did, and what that proves about sample 5. 5
- A learner writes: "The Benedict's stayed blue, therefore the sample contains no sugar." Rewrite that conclusion so it is scientifically correct. 3
- Benedict's gives a range of colours rather than just positive or negative. State what the range tells you, and explain why that makes it more useful than the iodine test. 4
- Sample 1 is starch-positive but sugar-negative; sample 7 is the reverse. Both are carbohydrates. Explain the difference in terms of monosaccharides and polysaccharides. 4
2
Test for Lipids
CAPS Investigation, verbatim · fully doable at home
Aim
To detect lipids by two independent methods, and to work out which is the better test and why.
Method — the grease-spot test
- Rub a small amount of each sample onto a square of plain white paper. Label each square in pencil.
- Leave them somewhere warm for 20 minutes, so any water evaporates.
- Hold each square up to a window or a lamp. A lipid leaves a translucent spot that light passes through and that does not dry out. Water leaves a mark that disappears completely.
Method — the emulsion test
- Put a small amount of sample into a glass and add about 2 cm of methylated spirits. Shake, and let it settle for a minute — the lipid dissolves into the alcohol.
- Carefully pour the liquid off into a second glass containing plain water.
- A cloudy white emulsion means lipid. If it stays clear, there was none. The lipid comes out of solution as microscopic droplets the moment it meets water — which is exactly why oil and water do not mix.
Results
| Sample | Grease spot after 20 min | Emulsion test | Lipid present? |
|---|---|---|---|
| Water (control) | |||
| Cooking oil | |||
| Butter | |||
| Milk | |||
| Egg white | |||
Supplied results — if you could not run it
| Sample | Grease spot | Emulsion |
|---|---|---|
| Water (control) | Dried completely, no mark | Stayed clear |
| Cooking oil | Large translucent spot | Dense white emulsion |
| Butter | Translucent spot | White emulsion |
| Milk | Faint translucent spot | Cloudy white |
| Egg white | Dried, slightly stiff, not translucent | Stayed clear |
| Avocado | Translucent spot | White emulsion |
Analysis
- Complete Table 2.1. 6
- Explain why the grease-spot squares must be left for 20 minutes before being read. 3
- Explain, in terms of solubility, why the emulsion test needs two liquids — alcohol first, then water. 5
- Milk gave a weaker result than cooking oil in both tests. What does that tell you, and what does it not tell you? 4
- Egg white is negative here but will be strongly positive in Investigation 3. Explain what that shows about food and about food tests. 4
- Whale blubber is roughly 60–70% lipid. Using the functions of lipids from the topic page, give two reasons a Southern Right whale in the Southern Ocean needs that much. 4
- Which of the two lipid tests would you trust more, and why? Give one advantage of each before deciding. 4
3
Test for Proteins
CAPS Investigation, verbatim · supplied result set
This one is read, not run — and here is exactly why
Biuret reagent is dilute copper sulfate in sodium hydroxide. Sodium hydroxide is caustic soda: it burns skin and blinds eyes, and there is no version of "make it in the kitchen" that this course is willing to print. So Investigation 3 is supplied as a measured result set.
You lose nothing that carries marks. Read the method carefully anyway — describing a method you have not personally performed is a standard exam task, and CAPS assesses the interpretation, not the pouring.
Aim
To identify which foods contain protein, and to interpret a test that reports presence rather than amount.
Method (for reading and describing)
- A small amount of the food is placed in a test tube with a little water.
- Biuret reagent is added and the tube is shaken. No heating — unlike Benedict's, this test works cold.
- The reagent is pale blue. A violet or purple colour means protein is present. The colour comes from copper ions reacting with the peptide bonds that link amino acids together — so it is really a test for peptide bonds, which is why it detects protein and not free amino acids.
Supplied results — seven samples
| # | Sample | Result | Protein? |
|---|---|---|---|
| 1 | Egg white in water | Deep violet | Yes, strongly |
| 2 | Milk | Violet | Yes |
| 3 | Fish, mashed in water | Violet | Yes |
| 4 | Gelatine solution | Violet | Yes |
| 5 | Glucose solution | Stayed blue | No |
| 6 | Cooking oil | Stayed blue | No |
| 7 | Starch suspension | Stayed blue | No |
One extra observation from the same run: the egg-white sample was split in two before testing, and one half was boiled for five minutes first. The boiled half still gave a deep violet result.
Analysis
- Describe the biuret method in full, as if instructing someone who has never done it. Include the reagent, the procedure and the positive result. 5
- Which samples contain protein? Give the evidence, not just the names. 4
- Unlike Benedict's, biuret needs no heating. Give one practical advantage of that. 2
- 🔴 The boiled egg white still tested positive. But boiling an egg denatures its protein. Explain why a denatured protein still gives a positive biuret result — and what that tells you about what the test actually detects. 6
- Using samples 1, 5, 6 and 7, explain why a full food analysis needs three separate tests rather than one. 4
- Fish is a protein food. Using the ammonia–urea chain from the topic page, explain what happens if an animal eats far more protein than it needs. 5
4
Vitamin Research
CAPS homework research task, verbatim
Aim
To find and record the name and function of 2–3 water-soluble and 2–3 fat-soluble vitamins — CAPS's own wording — and to explain why the solubility split changes how often you must eat them.
Method
- Fill in Table 4.1 from your own research first, before opening the supplied panel. Use the topic page, your booklet, or any reliable source.
- For each vitamin record: its letter, its chemical name if you can find it, what it does in the body, and what happens if you do not get enough.
- Then open the supplied table and mark yourself. Add anything you missed in a different colour.
Results
| Vitamin | Fat- or water-soluble? | Function in the body | Deficiency causes… |
|---|---|---|---|
Supplied — a reference table to mark yourself against
| Vitamin | Solubility | Function | Deficiency |
|---|---|---|---|
| C (ascorbic acid) | Water | Antioxidant; needed to make collagen, the protein holding skin, gums and blood vessels together | Scurvy |
| B1 (thiamine) | Water | Assists the enzymes of carbohydrate metabolism; needed for nerve function | Beriberi |
| B12 (cobalamin) | Water | Red blood cell formation and nerve maintenance | Anaemia |
| A (retinol) | Fat | Vision in dim light; maintains skin and lining tissues; immune function | Night blindness |
| D (calciferol) | Fat | Regulates calcium uptake for bones and teeth. Made in skin using sunlight | Rickets |
| K | Fat | Blood clotting | Excessive bleeding |
Two facts worth having for the analysis:
Scurvy and the sea. Vitamin C is water-soluble, so it cannot be stored and must be eaten regularly — and it is destroyed by prolonged cooking. On long voyages, before refrigeration, sailors ate salted and dried food for months and got none. Scurvy killed more sailors than storms and combat combined, and the fix — citrus — was a dietary one, not a medical one.
Hypervitaminosis, and a genuinely extreme case. Vitamin A is fat-soluble and is stored in the liver. Predators at the top of long marine food chains accumulate enormous amounts: polar bear and bearded seal liver contains so much vitamin A that eating it can poison a human, and Arctic expeditions have been made seriously ill this way. Inuit hunters have long known not to eat it.
Analysis
- Complete Table 4.1 with at least 3 water-soluble and 3 fat-soluble vitamins, with functions and deficiencies. 12
- State the four general jobs vitamins do in an organism. 4
- Explain why water-soluble vitamins must be eaten regularly but fat-soluble ones need not. 4
- Explain why hypervitaminosis is almost always a risk with fat-soluble vitamins rather than water-soluble ones. 4
- Scurvy is a vitamin C deficiency. Using two properties of vitamin C, explain why it was specifically a sailors' disease. 4
- Polar bear liver can poison a human. Explain the mechanism, naming the vitamin, its solubility, and why a top predator accumulates it. 5
5
Living or Non-Living?
CAPS Investigation, one-hour half · resource: "Acronym MRS GREN"
Aim
To classify a set of things as living or non-living using MRS GREN — and to discover the two cases where the seven processes are not enough on their own.
Method
- For each item in Table 5.1, work through the seven processes and tick the ones it genuinely performs. Be strict: could it, or does it?
- Then give a verdict: living, non-living, or once part of a living thing — that third category matters and most lists leave it out.
- Two items are deliberately difficult. Do not skip them; they are Q33 and Q34.
- 🌊 shore Extension: do this on an actual rocky shore — Cape Recife, Schoenmakerskop or any Algoa Bay platform at low tide. Add five things you find. A shore is unusually good for this because so much of it is skeleton: shells, tests, coral rubble and old holdfasts are everywhere, and sorting "alive" from "was alive" is genuinely hard in a way a classroom list is not.
Results
| Item | M | R | S | G | R | E | N | Verdict |
|---|---|---|---|---|---|---|---|---|
| A limpet on a rock | ||||||||
| An empty limpet shell | ||||||||
| A kelp holdfast, attached | ||||||||
| Driftwood | ||||||||
| Sea foam | ||||||||
| Coralline algae crust on a rock | ||||||||
| A salt crystal growing in a pool | ||||||||
| A dormant seed | ||||||||
| 🔴 A fire | ||||||||
| 🔴 A virus |
Analysis
- Complete Table 5.1 with a verdict for every item. 10
- Name the seven life processes in order and state what each one means. 7
- Explain why an empty limpet shell and driftwood need a category of their own rather than simply "non-living". 4
- A dormant seed performs almost none of the seven processes right now. Argue that it is nonetheless alive. 4
- Explain why movement alone is worthless as a test for life, using two items from the table as evidence. 4
- 🔴 A fire moves, grows, needs oxygen, gives off waste gases, responds to wind, and spreads to make more fires. On MRS GREN alone it scores alarmingly well. Explain precisely why it is not alive — and name the criterion that settles it. 6
- 🔴 A virus carries nucleic acids and reproduces, but only inside a host cell, and does none of the seven processes on its own. Decide whether it is alive, and justify your decision by reference to the eighth criterion. 6
- Both hard cases are settled by the same clause, which CAPS hides inside criterion 6. State that clause, and explain why the seven processes are not sufficient without it. 5
Memo — worked answers and mark allocation (does not print)
Total: 170 marks — Investigation 1: 35 · Investigation 2: 30 · Investigation 3: 26 · Investigation 4: 33 · Investigation 5: 46.
Investigation 1 — carbohydrates
- Q1 (6): Marked on the learner's own samples. The water control must be recorded as staying orange-brown. Bread and potato blue-black; apple, oil and sugar solution negative.
- Q2 (6): Starch: iodine solution (1), orange-brown → blue-black (2). Reducing sugar: Benedict's solution with heating (1), blue → green/yellow/orange/brick-red (2).
- Q3 (3): It is a control (1). It shows what a negative result looks like (1), so any colour change in the real samples can be attributed to the food rather than to the reagent itself (1).
- Q4 (4): Benedict's detects only reducing sugars (2). Sucrose is not a reducing sugar (2), so despite being a sugar it leaves the solution blue.
- Q5 (5): The acid (with heat) hydrolysed / split the sucrose into its two monosaccharide units (3) — glucose and fructose, both of which are reducing sugars (1). It proves sample 5 genuinely contained sugar all along, and that the first negative was a limitation of the test, not an absence of sugar (1).
- Q6 (3): Any correct rewrite, e.g. "The Benedict's stayed blue, therefore the sample contains no reducing sugar." (2) Full marks require the word reducing (1).
- Q7 (4): The colour indicates how much reducing sugar is present — it is semi-quantitative (2). Iodine is only positive/negative, so Benedict's gives an estimate of concentration as well as presence (2).
- Q8 (4): Sample 1 (potato) contains starch, a polysaccharide — many monosaccharide units joined, large and insoluble, so it gives no reducing-sugar result (2). Sample 7 is glucose, a monosaccharide — a single soluble unit, so it is reducing but is not starch (2).
Investigation 2 — lipids
- Q9 (6): Oil, butter and avocado positive on both; water negative on both; egg white negative; milk weakly positive on both.
- Q10 (3): So that any water in the sample evaporates (2). Water also makes paper translucent while it is wet, so reading it early gives a false positive (1).
- Q11 (5): Lipids are insoluble in water but soluble in alcohol (2), so the alcohol dissolves the lipid out of the food (1). When that solution is poured into water the lipid can no longer stay dissolved and comes out as microscopic droplets suspended through the water (2) — the cloudiness is those droplets.
- Q12 (4): It tells you milk contains less lipid per unit volume than oil (2). It does not tell you a quantity — neither test is properly quantitative, so "less" is as far as the evidence goes (2).
- Q13 (4): A single food commonly contains more than one class of nutrient (2), and each test detects only what it is designed for — a negative in one test says nothing about the others (2).
- Q14 (4): Any two, 2 each: long-term energy storage at high energy density; thermal insulation in very cold water, since fat conducts heat poorly; buoyancy; and a food reserve for the migration and calving season when the whale is not feeding.
- Q15 (4): Marked on reasoning. Grease-spot: needs no chemicals, no hazard, very simple — but slow and easily confused by water. Emulsion: faster, clearer, harder to fake — but needs a flammable solvent and more care. Either may be preferred if the trade-off is argued (advantages 2, justified decision 2).
Investigation 3 — proteins
- Q16 (5): Sample plus a little water in a tube (1); add biuret reagent (1); shake, no heating (1); pale blue → violet/purple is positive (2).
- Q17 (4): Samples 1, 2, 3 and 4 — egg white, milk, fish and gelatine (2), evidenced by the reagent turning violet where the others stayed blue (2).
- Q18 (2): Any one: no heat source needed, so it is quicker, safer and usable in the field; and no risk of altering the sample by heating it.
- Q19 (6): Biuret detects peptide bonds — the links between amino acids in the chain (2). Denaturing changes the protein's 3D shape but does not break the peptide bonds; the chain is still a chain (2). So the test still reads positive — which tells you biuret detects the presence of protein, not whether that protein still works (2). 📌 This is the strongest question on the sheet: it separates "is it there" from "is it functional", which is exactly the distinction denaturing turns on.
- Q20 (4): Sample 1 is protein-positive only, 5 is sugar-positive only, 6 is lipid-positive only, 7 is starch-positive only (2). Each reagent responds to one class of compound, so a single test can only ever answer one question — three tests answer three (2).
- Q21 (5): Excess protein is broken down (catabolic) for energy or storage (1). Protein contains nitrogen, which carbohydrates and fats do not (1), and that nitrogen is released as ammonia, which is toxic (2). The body converts it to the less toxic urea and excretes it (1).
Investigation 4 — vitamins
- Q22 (12): 2 marks per vitamin for a correct name, solubility, function and deficiency — minimum 3 water-soluble and 3 fat-soluble. Accept any correct vitamins, not only the six supplied.
- Q23 (4): Regulate mineral uptake · regulate growth and cell division · assist specific enzyme-catalysed processes · act as antioxidants. (1 each.)
- Q24 (4): Fat-soluble vitamins dissolve in and are stored in the body's fatty tissue, so a reserve builds up (2). Water-soluble ones are not stored and are excreted easily in urine, so the supply runs out and must be replaced (2).
- Q25 (4): Because the fat-soluble ones accumulate rather than being excreted (2), so repeated excess intake builds to a harmful level; water-soluble excess is simply passed out (2).
- Q26 (4): Vitamin C is water-soluble and therefore not stored, so it must be eaten continuously (2); and it is destroyed by prolonged cooking and not present in salted or dried provisions, which is all long voyages carried before refrigeration (2).
- Q27 (5): Vitamin A (1), which is fat-soluble and therefore stored rather than excreted (2), concentrating in the liver. A top predator eats prey that has itself accumulated it, so the concentration rises at each step of the food chain (2). Eating that liver delivers a dose far above what a human can handle — acute hypervitaminosis A.
Investigation 5 — living or non-living
- Q28 (10): Limpet living · empty shell once part of a living thing · kelp holdfast living · driftwood once living · sea foam non-living (it is whipped-up organic matter, so "contains once-living material" also earns the mark if argued) · coralline algae living · salt crystal non-living · dormant seed living · fire non-living · virus see Q34.
- Q29 (7): Movement · Respiration · Sensitivity · Growth · Reproduction · Excretion · Nutrition, each with a correct one-line meaning. (1 each.)
- Q30 (4): They are made of once-living material but perform none of the seven processes now (2). Calling them simply "non-living" loses the information that they were biologically produced — which on a shore is most of what you are looking at (2).
- Q31 (4): It retains intact cells and nucleic acids and is capable of every process once conditions allow (2); dormancy is a suspended state, not death, and germination proves it (2).
- Q32 (4): The salt crystal grows and sea foam moves, yet neither is alive (2). Individual processes occur in non-living systems too, so life is indicated by the full set together, not by any one (2).
- Q33 (6): A fire genuinely does appear to move, grow, consume oxygen, produce waste and spread (1). But it is not made of cells and has no nucleic acids governing it (3) — the eighth criterion — and its "growth" is only the spread of a chemical reaction, its "reproduction" passes on no hereditary information, and it has no organised metabolism (2). Full marks require naming the cellular/nucleic-acid criterion, not just asserting it is a chemical reaction.
- Q34 (6): Either verdict earns full marks if justified against the criterion. The stronger answer: a virus is not a cell and cannot perform the seven processes independently (3), so on the eighth criterion it fails — it carries nucleic acids but is not a membrane-bound cellular unit doing its own metabolism (2). It is best described as being on the boundary, active only inside a host (1).
- Q35 (5): The clause is that the organism is made up of cellular units governed by nucleic acids (3). Without it, MRS GREN describes behaviours, and non-living systems such as fire can imitate several of them — the clause adds a requirement about what a living thing is made of, not just what it does (2).
Marking note. Q4–Q6 and Q19 are the four questions that decide whether this practical worked. They all test the same habit: knowing what a negative result actually rules out. A learner who can say "no reducing sugar" rather than "no sugar", and who understands that biuret detects a bond rather than a working protein, has learned more from this sheet than one who simply recorded nine colours correctly.