Protists
Three of these four investigations are CAPS's own, and the fourth is one it hides inside a key concept rather than the Investigations column. That fourth one is the best experiment in the block — it needs a bottle of vinegar and a kitchen scale, and it puts a number on the process threatening every calcium-carbonate protist in the sea.
Investigations column, block 18: "Photographs, slide show or videos" · "Study live examples (aquarium or rock-pool study)" · "Example of a protist ID chart, used to classify protists in plankton samples" · "Collect and identify plankton samples, if possible". Resources: "Live specimens · Fresh plankton sample · Microscopes · Plankton ID sheets".
📌 And the one that is easy to miss. Key concept 3 reads: "Many protists have CaCO₃ as part of their structure. (Investigate potential effects of ocean acidification.)" That parenthesis is an instruction sitting in the wrong column. It is Investigation 1, and it is the only quantitative experiment CAPS asks for anywhere in this topic.
What you need
- 🟢 kitchen White vinegar (about 500 mL) and four clean jars with lids
- 🟢 kitchen A kitchen scale reading to 0.01 g — a jeweller's or coffee scale. If yours reads to 0.1 g, use bigger pieces and see Q7.
- 🟢 kitchen Four similar pieces of blackboard chalk or seashell — mussel shell, limpet shell or oyster shell all work
- 🟢 kitchen Paper towel, a measuring jug, a marker pen
- 🟡 pharmacy pH strips, if you want to measure rather than assume
- 🌊 shore The plankton net you built in Topic 17 — a stocking, a wire hoop, a jar and string
- 🟠 hobby A student microscope (~R600–1 500). Reused from Topics 14, 17 and 22 — see Investigation 2 if you do not have one.
Investigation 2 ships with six described micrographs and a full count, the same pattern Topic 14 uses — you identify every organism and do every calculation from the descriptions. Investigation 4 ships with a described rock pool. Investigation 3 is a pencil task.
Investigation 1 needs no substitution at all. It is vinegar, chalk and a scale, and it is the investigation CAPS actually asks you to investigate.
- Vinegar is a weak acid but it still stings. Keep it out of your eyes, and rinse your hands after handling the jars.
- Do not seal the jars tightly. The reaction produces CO₂ gas — a rest the lid on top, do not screw it down.
- 🌊 Shore work: check the tide before you go, never turn your back on the sea, and replace every rock you turn over, the same way up. A rock pool is somebody's home.
Aim
To measure how the rate of calcium-carbonate loss changes with acid strength — and to work out exactly what that does and does not tell us about the sea.
Method
- Label four jars: A water (control), B 1:4 vinegar, C 1:1 vinegar, D neat vinegar.
- Make up 200 mL of liquid in each: A is plain water; B is 40 mL vinegar + 160 mL water; C is 100 mL + 100 mL; D is 200 mL neat vinegar.
- Choose four pieces of chalk or shell of similar size. Dry them, then weigh each one and record the starting mass to 0.01 g.
- Drop one piece into each jar. Rest the lids on loosely. Note what you can see happening in the first minute.
- After 24, 48 and 72 hours: lift each piece out, rinse it gently in water, blot it dry on paper towel, weigh it, and put it straight back.
- Handle the pieces carefully once they soften — the point is dissolved mass, not mass you knocked off.
Supplied results — a measured 72-hour run
Blackboard chalk, four pieces trimmed to 5.00 g each, at 22 °C. Use these if you did not run it.
| Time | A · water | B · 1:4 vinegar | C · 1:1 vinegar | D · neat vinegar |
|---|---|---|---|---|
| 0 h | 5.00 | 5.00 | 5.00 | 5.00 |
| 24 h | 4.99 | 4.71 | 4.32 | 3.68 |
| 48 h | 4.99 | 4.45 | 3.71 | 2.55 |
| 72 h | 4.98 | 4.21 | 3.18 | 1.62 |
Observations recorded at the start: jars B, C and D all produced a stream of small bubbles from the surface of the chalk, fastest in D. Jar A produced none. By 72 hours the piece in D was soft, crumbling and noticeably smaller; the piece in A looked unchanged.
Measured pH: A ≈ 7 · B ≈ 3.1 · C ≈ 2.7 · D ≈ 2.5.
Results — your own run
| Time | A · water | B · 1:4 | C · 1:1 | D · neat |
|---|---|---|---|---|
| 0 h | ||||
| 24 h | ||||
| 48 h | ||||
| 72 h |
Results — plot all four
Analysis
- Plot all four sets on one grid. Marks: axes labelled with units (2), sensible scale (2), four curves correctly plotted and labelled (4), title (1). 9
- Calculate the percentage mass lost over 72 hours for each jar. Show one full calculation. 6
- Jar A is the control. State what it is controlling for, and what its result tells you. 4
- Describe the relationship between acid strength and mass loss, quoting figures. 4
- 🔴 Look at jar D across the three intervals: it loses 1.32 g, then 1.13 g, then 0.93 g. The rate is slowing. Give two possible reasons. 5
- Bubbles came off the chalk in B, C and D but not in A. Name the gas and explain where it comes from. 4
- The method says to rinse and blot dry before each weighing. Explain why both steps matter, and what error you would introduce by skipping the drying. 4
- 🔴 The honest-limits question. Household vinegar is about pH 2.5. The ocean surface has moved from roughly pH 8.2 to 8.1 and may reach 7.8 by 2100. Explain what this experiment does establish and what it does not. 6
- Using the topic page, explain why foraminifera are threatened by acidification but radiolarians are far less so. 4
- A learner concludes: "Shells will dissolve in the sea within days, so all shellfish will die." Rewrite that as a scientifically defensible statement. 4
Aim
To collect a plankton sample from Algoa Bay and identify what is in a single drop of it — and to notice which of the things you find is not a protist at all.
Method
- Use the stocking plankton net from Topic 17. Tow it slowly through the water for about 5 metres, or hold it in a current for two minutes.
- Empty the collecting jar into a clean container and let it stand for ten minutes so the plankton settles.
- Draw one drop from the bottom with a dropper or straw, put it on a slide, and look at it at 40× and then 100×.
- Work across the drop systematically. Sketch and describe every distinct organism, and count how many of each you see in three separate fields of view.
- Classify each one using the ID chart you build in Investigation 3.
Supplied — six described organisms from one drop
A tow taken off Cape Recife in November. Counts are totals across three fields of view. One of these six is not a protist; finding it is Q15.
Roughly 300 µm. A cream-coloured shell of several chambers coiled like a tiny snail, each chamber larger than the last. Fine threads extend out from the opening and beyond the shell.
About 120 µm. A near-perfect sphere, transparent and glassy, with an intricate lattice visible in the wall and long straight spines radiating outwards in all directions.
About 40 µm. Brownish-green, with a stiff armoured surface divided into plates, two horn-like projections, and two flagella — one wrapped in a groove around the middle, one trailing behind.
About 80 µm. Sits inside a transparent vase-shaped case, open at one end, with a ring of beating hair-like structures around the opening. Moves in short darting bursts.
About 60 µm. Golden-brown, rectangular in side view, enclosed in what looks like a tiny glass box made of two halves fitting one inside the other like a petri dish. Several joined end to end in a chain.
About 1 mm — the largest thing in the drop. Transparent, with a segmented body, jointed legs, long antennae, a single dark eye spot and a forked tail. Swims in rapid jerks.
Results
| Organism | Size | Key feature used | Identification | Protist? | Count |
|---|---|---|---|---|---|
| A | |||||
| B | |||||
| C | |||||
| D | |||||
| E | |||||
| F |
Scientific drawing
Make a labelled scientific drawing of organism A or B, following the Topic 5 conventions: pencil, clear single lines, ruled label lines with no arrowheads, and a stated magnification.
Analysis
- Complete Table 2.1 for all six organisms. 12
- Make the labelled scientific drawing. Marks: outline accurate (2), at least three correct labels (3), conventions followed (2), magnification stated (1). 8
- Organisms A and B are the two CAPS names specifically. Give two differences between them, one structural and one about where they live. 4
- Calculate the percentage of the total count that organism E represents. 3
- 🔴 One of the six is not a protist. Identify it, say what it is instead, and give the feature that gives it away. 5
- That organism is nonetheless in the plankton sample and belongs in this topic. Explain how it connects to CAPS's statement that protists are at the base of the biggest food chain on Earth. 5
- Organism E is by far the most numerous. Suggest why a photosynthetic organism would dominate a sample taken in daylight near the surface. 4
- You settled the sample for ten minutes before sampling from the bottom. Explain what that step does, and one type of organism it might cause you to under-count. 4
Aim
To write a branching identification key for marine protists — and then to find out whether anyone else can actually use it.
📌 Note what CAPS asks for. Not "look at a chart" but "an example of a protist ID chart, used to classify protists in plankton samples". Using a key tests whether you can follow instructions; writing one tests whether you can observe — the same distinction Topic 15 makes.
Method
- Your key must separate these six: foraminiferan · radiolarian · dinoflagellate · diatom · ciliate (tintinnid) · amoeboid protist.
- Write it as numbered pairs of statements. Each pair must offer two mutually exclusive options, and each option leads either to another number or to a name.
- Use only features you can actually see down a microscope — shape, shell material, symmetry, presence of flagella, cilia or threads. Do not use habitat, diet or anything you would have to already know.
- Then hand your key to another person with the six descriptions from Investigation 2 and no other help. Record every point at which they hesitate or go wrong.
Analysis
- Write the key. Marks: all six organisms reachable (6), every pair genuinely exclusive (4), observable features only (2). 12
- State the route through your own key for organism B — e.g. "1b → 3a → 4b". 3
- Hand the key to another person and record where they hesitated or went wrong. 4
- 🔴 A key that only its author can use is a failed key. Using their difficulties, rewrite one pair to be less ambiguous, and explain what was wrong with the original wording. 5
- Explain why "lives in sediment" is a poor question to put in a microscope key, even though it is true of most foraminifera. 4
- Your key handles six organisms. A real Algoa Bay sample might contain forty. Explain what happens when a key meets something it was not written for, and what an honest scientist records. 4
- Suggest one feature that would let you split "shelled protists" cleanly in a single step, and state which two groups it separates. 3
- Explain why writing a key is a better test of observation than using one. 3
Aim
To observe a living marine community — and to work out how you can be certain protists are present when you cannot see a single one.
Method
- 🌊 Go to a rocky shore at low tide — Cape Recife, Schoenmakerskop or any Algoa Bay platform. Check the tide table first.
- Choose one pool and spend twenty minutes on it. Do not move on.
- Record: the pool's size and depth, water clarity and colour, what is attached to the rock, what is moving, and whether any surface is slippery or filmed.
- Take a small jar of the water. Hold it up to the light. Let it stand and look again.
- Replace every rock you turn, the same way up.
Supplied — a described pool at Cape Recife, spring low tide
The pool: about 1.5 m across, 40 cm deep at the centre, in the mid-shore zone. Water is clear but with a faint greenish tinge when held in a jar against the sky.
Attached to the rock: a dense band of small black mussels along one edge; several limpets; two sea anemones, closed; a crust of pink coralline algae over much of the submerged rock; a patch of brown-green film on the shaded side that is slippery underfoot.
Moving: three small fish darting under a ledge; a hermit crab; several tiny shrimp-like animals hopping in the weed.
The sand at the bottom of the pool: pale, fine, and under a hand lens made up partly of broken shell fragments — and partly of tiny intact coiled and chambered grains, well under a millimetre across.
The jar of water, stood for ten minutes, developed a very faint cloudiness near the bottom and a slight green tint. Nothing individual was visible to the naked eye.
Results
| Observation | What you recorded |
|---|---|
| Pool size and depth | |
| Water clarity and colour | |
| Attached (sessile) organisms | |
| Moving organisms | |
| Films, crusts or slippery surfaces | |
| The sediment on the bottom |
Analysis
- Complete Table 4.1 from your own pool or the supplied one. 6
- 🔴 You did not see a single protist. Give three separate lines of evidence from these observations that protists are nonetheless present in this pool, and explain each. 9
- The pool contains mussels — filter feeders that strain particles from the water. Explain what their presence, in numbers, implies about the water. 4
- The sand contains "tiny intact coiled and chambered grains, well under a millimetre across". Identify what these almost certainly are, and explain how they got there. 5
- Explain the faint green tint in a jar of clear-looking water. 3
- CAPS asks you to study live examples. Explain why a rock pool is a better choice than an aquarium for this topic, and give one thing the aquarium would do better. 4
- The coralline algae crust is not a protist. Using Topic 15, state which kingdom it belongs to and why that classification is easy to get wrong. 4
- Design, in three sentences, a follow-up investigation to test whether this pool has more protists at high tide or low tide. State your variable and how you would measure it. 5
Memo — worked answers and mark allocation (does not print)
Total: 173 marks — Investigation 1: 50 · Investigation 2: 45 · Investigation 3: 38 · Investigation 4: 40.
Investigation 1 — ocean acidification
- Q1 (9): As allocated. All four curves fall; A is essentially flat. Curves should be smooth and decelerating, not straight.
- Q2 (6): % loss = (5.00 − final) ÷ 5.00 × 100. A = 0.4% · B = 15.8% · C = 36.4% · D = 67.6%. (1 each, plus 2 for a fully shown calculation.)
- Q3 (4): It controls for mass lost by any cause other than acid — handling, dissolving in plain water, wetting (2). Its near-zero loss shows those causes are negligible, so the losses in B–D can be attributed to the acid (2).
- Q4 (4): The stronger the acid, the greater the mass loss (2), supported by figures — 15.8%, 36.4% and 67.6% at 72 hours (2). Accept "positive relationship"; do not accept "directly proportional", since it is not.
- Q5 (5): Any two, well explained (2 each) + 1 for recognising the deceleration is real: the acid is being used up as it reacts, so the solution weakens; the surface area of the shrinking piece falls, so there is less contact area; and dissolved products build up in the jar. Reject "the chalk got tired."
- Q6 (4): Carbon dioxide (2). The acid reacts with the calcium carbonate, breaking it down and releasing CO₂ gas (2). None in A because there is no acid to react.
- Q7 (4): Rinsing removes acid and dissolved residue that would otherwise be weighed as if it were chalk (2); drying removes surface water, which is heavy — leaving it on would make the piece look heavier and under-estimate the mass lost (2).
- Q8 (6): Does establish (3): the mechanism — acid dissolves calcium carbonate, and more acid dissolves more of it, measurably. Does not establish (3): anything about the rate in the sea. Vinegar at pH 2.5 is thousands of times more acidic than seawater at pH 8.1, which is still alkaline. The real process is far slower and works by making it harder for organisms to build shells as fast as they are lost. Full marks require both halves.
- Q9 (4): Foraminiferan tests are calcium carbonate, which acid dissolves (2). Radiolarian shells are silica — glass — which is not attacked the same way, so they are far less vulnerable (2).
- Q10 (4): Any defensible rewrite, e.g. "Increasing ocean acidity makes it progressively harder for calcium-carbonate organisms to build and maintain their shells, which threatens some species over decades — but it will not dissolve shells in the sea at the rate seen in vinegar." Marks for removing the false rate claim (2) and for retaining the real, slower threat (2).
Investigation 2 — the plankton sample
- Q11 (12): A = foraminiferan (coiled chambered test + cytoplasmic threads) · B = radiolarian (glassy sphere, lattice, radiating spines) · C = dinoflagellate (armoured plates, two flagella, one in a groove) · D = ciliate / tintinnid (vase-shaped case, ring of cilia) · E = diatom (golden-brown, two-part glass box, chain) · F = copepod — NOT a protist. (2 each.)
- Q12 (8): As allocated. Reject arrowheads on label lines.
- Q13 (4): Structural: foram test is calcium carbonate and chambered; radiolarian shell is silica with radiating spines (2). Distribution: forams are mostly benthic in seafloor sediment, radiolarians are mostly planktonic (2).
- Q14 (3): Total = 41+12+96+28+154+6 = 337 (1). 154 ÷ 337 × 100 = 45.7% (2).
- Q15 (5): F (1). It is a copepod — a crustacean, an animal (2). Give-aways, any one (2): it is multicellular with a segmented body and jointed legs, it has antennae and a compound eye, and at 1 mm it is enormous compared with a single cell.
- Q16 (5): The copepod is zooplankton that eats protists (2) — it is the next link up. It is exactly the kind of animal CAPS means: many marine animals also pass through a larval planktonic stage feeding on protists (2), so the protists at the base support everything above them (1).
- Q17 (4): Diatoms are photosynthetic (2), so they need light and are concentrated near the sunlit surface during the day, where the tow was taken (2). Accept mention of nutrient availability or an upwelling bloom.
- Q18 (4): Settling lets denser organisms sink so the sample is concentrated at the bottom (2). It would under-count anything buoyant or strongly swimming that stays in the upper water — many dinoflagellates and small flagellates (2).
Investigation 3 — the ID chart
- Q19 (12): Any working key. A sound one splits first on shelled / not shelled, then shell material, then movement structure. Mark on function, not on matching a model answer.
- Q20 (3): A correctly stated route through the learner's own key.
- Q21 (4): One mark for doing it, three for an honest record of hesitation points. A learner who reports "no problems" has probably not tested it.
- Q22 (5): The rewrite must remove a subjective or unobservable term (3) and the explanation must identify what made it ambiguous — e.g. "large" without a size, "spiky" when both spines and cilia look spiky, or a feature not visible in every orientation (2).
- Q23 (4): Because it is not observable in the sample (2) — the organism is on a slide, not in its habitat, and the user cannot check it. A key must ask only about what is in front of the person using it (2).
- Q24 (4): It will be forced down a branch it does not belong to and mis-identified (2). The honest record is that the organism does not key out, with a description and a drawing, rather than the nearest name (2).
- Q25 (3): Shell material — calcium carbonate versus silica (2) — which separates foraminifera from radiolarians in one step (1). Accept "chambered vs latticed with spines".
- Q26 (3): Using a key tests whether you can follow instructions; writing one requires you to notice which features actually differ and which are reliably visible, and to be precise enough for a stranger (3).
Investigation 4 — the rock pool
- Q27 (6): Marked on the learner's own pool or the supplied one.
- Q28 (9): Three lines, 3 each. Best answers: (i) the sand contains coiled chambered grains — these are foraminiferan tests, so forams live or lived here; (ii) the green tint and faint cloudiness in a settled jar — suspended microscopic photosynthetic organisms, largely protist phytoplankton; (iii) the slippery brown-green film on the rock — a biofilm containing diatoms and other microalgae, which is what makes it slippery. Also credit: filter feeders (mussels) in numbers require suspended microscopic food; and grazing limpets need a film to graze.
- Q29 (4): Filter feeders can only survive where there is a continuous supply of suspended microscopic food (2). A dense band of them is indirect evidence that the water carries plankton in quantity, even though none is visible (2).
- Q30 (5): They are foraminiferan tests (2) — the calcium-carbonate shells of dead forams (1). The forams lived in the pool or nearby sediment, died, and their tests remained as sand-sized grains (2). This is the same process that makes limestone, happening at a scale you can hold.
- Q31 (3): Suspended photosynthetic microorganisms — phytoplankton, largely protists — too small and too dilute to see individually but collectively tinting the water (3).
- Q32 (4): A rock pool is a real, unmanaged community with natural species composition and abundance (2). An aquarium is better for close, repeated, controlled observation of a chosen organism, and for seeing things too small or too shy to spot on a shore (2).
- Q33 (4): Chromista — accept "not Plantae" with reasoning (2). It is easy to get wrong because it is attached, pink and plant-like, and the name "algae" suggests a plant; but classification follows cell structure and pigments, not appearance (2).
- Q34 (5): Any workable design. Must state the variable (tidal state) (1), a measurable (e.g. counts per drop from a settled jar, or the same volume filtered) (2), and a fair-test control — same pool, same method, same time of day, repeated (2).
Marking note. Q8 and Q28 are the two that decide whether this sheet worked, and they pull in opposite directions on purpose. Q8 asks the learner to distrust a vivid result — the chalk really did dissolve, and it still does not prove what it appears to. Q28 asks them to trust an absence — nothing visible in the pool is a protist, and yet three independent observations show they are there. Being able to do both is most of what scientific reasoning is.