Practical 14 — Cell Biology
All six CAPS investigations. Two of them need nothing but a kitchen. The other four want a microscope — so each of those carries a described micrograph and real measurements, letting you do the identification, the labelled drawing, the scale calculation and the analysis whether or not you own one.
Four of these six want one, and that is the first real equipment barrier in this course. A basic student microscope (roughly R600–R1 500, 40×–400×) does every single one of them and will serve you again in Topics 17, 18 and 22. If you are going to buy one piece of equipment for this subject, it is this.
If you are not: every microscope investigation below has a Supplied observations panel with a described micrograph, real dimensions and a scale bar. You can complete the labelled drawing, the comparison table, the magnification calculation and every analysis question from it. Those are the parts CAPS assesses — no question in this block awards a mark for having personally focused an eyepiece.
Aim
To prepare and examine a plant cell and an animal cell, and to identify the structural differences between them.
Apparatus
- 🟠 microscope Light microscope, slides and cover slips
- 🟢 kitchen An onion, tweezers, a cotton bud
- 🟡 pharmacy Iodine solution (tincture of iodine from any pharmacy, diluted about 1:10 with water)
- 🟡 pharmacy Methylene blue — aquarium shops sell it cheaply as a fish treatment
Method — plant cell
- Cut a segment of onion and snap it backwards so the thin, transparent epidermis peels away from the inner curve.
- Lay a small piece flat on a slide in a drop of water. Do not let it fold — a folded layer is the commonest reason a slide looks like a mess.
- Add one drop of iodine. It highlights the cell wall and nucleus.
- Lower a cover slip at an angle, edge first, to avoid trapping air bubbles.
- Examine on low power first, then high. Do the focusing on low power every time.
Method — animal cell
- Gently scrape the inside of your cheek with a clean cotton bud.
- Smear it thinly on a clean slide, add one drop of methylene blue (which highlights organelles), and cover slip.
- Examine. Cheek cells are far fewer, more scattered and much less regular than onion cells — you will need to hunt.
🔬 Supplied observations — use these if you have no microscope
Onion epidermis · iodine stain · × 400
A tightly packed sheet of rectangular, brick-like cells in neat rows with no gaps between them. Each cell is bounded by a thick, straight, sharply defined cell wall stained yellow-brown. Inside, a single small round nucleus stains darker brown and sits pressed against one side of the cell, not in the centre. The bulk of the cell interior appears empty and pale — that space is the central vacuole. No chloroplasts are visible (this tissue grows underground).
Field of view 0.45 mm across. A typical cell measures 250 µm long × 70 µm wide.
Human cheek cells · methylene blue · × 400
Scattered, well-separated cells — nothing like the onion's continuous sheet. Each is an irregular, flattened, roughly circular flake with a soft, wavy outline that changes shape from cell to cell. The boundary is a thin cell membrane only; there is no straight rigid edge. A single round nucleus stains deep blue and sits near the centre. The surrounding cytoplasm is pale blue and granular. No large vacuole, no wall.
Field of view 0.45 mm across. A typical cell measures 60 µm across.
Results — your scientific drawings
Two drawings, side by side, applying all nine rules of the Drafter's Code (Section 7 of the topic page). Label at minimum: cell wall, cell membrane, nucleus, cytoplasm, vacuole. Give each an underlined heading and state the magnification beneath it.
Analysis
- 6Draw a table comparing the onion cell and the cheek cell under these headings: shape · outer boundary · position of nucleus · vacuole · arrangement of cells.
- 3Give three structures present in the onion cell that are absent from the cheek cell.
- 3Explain how the cell wall accounts for two of the differences you listed in Q1.
- 2Onion epidermis has no chloroplasts even though it is plant tissue. Explain why.
- 3Explain why a stain is necessary, and state which structure each of your two stains was chosen to highlight.
- 2The onion cell is 250 µm long. Your drawing of it is 100 mm long. Calculate the magnification of your drawing.
Extension — the marine version
Scrape the green film off a rock in a tide pool at Cape Recife, or squeeze a drop from a piece of fresh seaweed, and mount it in seawater. You will see plant-type cells that do have chloroplasts. Compare them with your onion.
Aim
To calculate magnification and actual size from microscope observations, and to apply the prescribed scientific drawing conventions.
Part A — the calculations
No apparatus needed. Show every step, and state the unit in every answer. Convert to a single unit before you divide.
| Question | Working | Answer |
|---|---|---|
| a) A cell is 60 µm wide, viewed at × 400. How wide is the image? | ||
| b) A drawing is 90 mm wide at × 300. What is the cell's actual size, in µm? | ||
| c) A diatom is 80 µm across; the micrograph shows it 40 mm across. Find the magnification. | ||
| d) A scale bar marked 10 µm measures 25 mm. A cell beside it measures 45 mm. How wide is the cell? | ||
| e) How many 20 µm cells fit end to end across 1 mm? | ||
| f) An electron microscope works at × 10 000 000. A ribosome is 25 nm across. How large would it appear? |
Part B — the drawing test
Draw the plant cell from your topic page (Section 3), applying all nine rules. Then swap with someone and mark each other's against the list.
Analysis
- 6Complete all six calculations in Part A with full working.
- 4State the magnification range of a light microscope and of an electron microscope, and give one thing each is used for that the other cannot do.
- 2Why does a magnification value have no unit?
- 3Explain why a scale bar is more reliable on a published micrograph than a stated magnification.
- 9List all nine drawing conventions from memory. (One mark each — this is a free nine marks if you have learnt them.)
Aim
To demonstrate osmosis using a potato osmometer, and to show that the direction of water movement depends on the concentration gradient.
Hypothesis
Write your own. Predict what happens in the salt-filled well and in the water-filled control, and give the reason.
Apparatus
- 🟢 kitchen Two large raw potatoes, a knife and a teaspoon
- 🟢 kitchen Salt, water, two saucers
- 🟢 kitchen A ruler or a marker pen to record the level
Method
- Peel both potatoes and cut a flat base on each so they stand upright.
- Hollow a deep well in the top of each, roughly 3 cm across.
- Stand each potato in a saucer holding about 1 cm of plain water.
- Potato A: half-fill the well with a strong salt solution (2 teaspoons in 50 mℓ). Mark the level.
- Potato B (the control): half-fill with plain water. Mark the level.
- Leave both for 24 hours. Measure the level in each well again.
| Potato | In the well | Level at start | Level after 24 h | Change |
|---|---|---|---|---|
| A | Salt solution | |||
| B (control) | Plain water |
📊 Supplied results
Potato A (salt): level rose from 18 mm to 31 mm — a rise of 13 mm. The potato tissue around the well felt firm.
Potato B (control, water): level went from 18 mm to 17 mm — effectively no change (1 mm is within measurement error and evaporation).
Analysis
- 4Explain, using the words water potential, semi-permeable membrane and osmosis, why the level in potato A rose.
- 3What is the purpose of potato B? Explain what you would have concluded if both levels had risen.
- 2Identify the semi-permeable membrane in this experiment.
- 3Name three variables you controlled, and explain why one of them mattered.
- 3Predict the result if you had used boiled potato, and explain your prediction.
- 3Relate this to a marine fish: is seawater hypertonic or hypotonic to its cells, and which way does its water move?
Aim
To measure the mass gained by dried fruit placed in water, and to explain it using osmosis.
Your booklet's sample question 3 is exactly this experiment — Thandi's overnight oats and raisins. Doing it makes that question free.
Apparatus
- 🟢 kitchen About 30 raisins
- 🟡 pharmacy A kitchen scale reading to 0.1 g if possible (1 g will do if you weigh ten at a time)
- 🟢 kitchen Three cups: fresh water · strong salt solution · empty (dry control)
Method
- Count out three groups of ten raisins. Weigh each group and record.
- Group 1 into fresh water. Group 2 into strong salt solution. Group 3 stays dry.
- Leave all three overnight (about 12 hours).
- Lift each group out, blot gently dry — surface water is not absorbed water — and reweigh.
- Calculate the change in mass, and the percentage change, for each group.
| Group | Mass at start (g) | Mass after 12 h (g) | Change (g) | % change |
|---|---|---|---|---|
| 1 · fresh water | ||||
| 2 · salt solution | ||||
| 3 · dry (control) |
📊 Supplied results
| Group | Start (g) | After 12 h (g) |
|---|---|---|
| 1 · fresh water | 8.4 | 15.1 |
| 2 · salt solution | 8.6 | 9.2 |
| 3 · dry control | 8.5 | 8.5 |
The fresh-water raisins were plump and smooth, like small grapes. The salt-solution raisins were barely changed and still wrinkled.
Analysis
- 3Calculate the percentage change in mass for each group. Show your working.
- 4Explain the result for group 1 using osmosis and water potential. Treat the raisin skin as a semi-permeable membrane.
- 3Group 2 gained far less. Explain why.
- 2What was the purpose of group 3?
- 2Why must the raisins be blotted before reweighing?
- 3Draw a bar graph of percentage change for the three groups.
- 3Predict what would happen to a fresh grape left in strong salt solution, and name the process.
Aim
To observe plasmolysis in a living plant cell placed in a hypertonic solution, and to reverse it.
Apparatus
- 🟠 microscope Light microscope, slide, cover slip
- 🟢 kitchen Onion epidermis (as in Investigation 1), strong salt solution, fresh water
- 🟢 kitchen Paper towel torn into a small wick
Method
- Mount onion epidermis in water and find a clear group of cells. Do not move the slide again.
- Draw what you see — the cytoplasm pressed out against the wall all the way round.
- Put a drop of strong salt solution at one edge of the cover slip and hold the paper wick at the opposite edge. The wick draws the salt solution right across the specimen while you watch.
- Watch the same cells for two minutes. Draw again.
- Now flush fresh water through the same way, and watch for another few minutes. Draw a third time.
🔬 Supplied observations
Stage 1 · in water — turgid
The cytoplasm and its lining sit pressed firmly against the cell wall around the entire perimeter. There is no visible gap. The cell looks full and rigid.
Stage 2 · in strong salt solution — plasmolysed
Within about 90 seconds the cell contents shrink away from the cell wall, pulling inward into a rounded blob near the centre. A clear gap opens between the shrunken membrane and the wall, which itself does not change shape at all — it stays a rigid rectangle. The nucleus is dragged inward with the cytoplasm.
Stage 3 · flushed with fresh water — deplasmolysed
Over about four minutes the contents swell back out and re-press against the wall. The cell returns to near its original appearance. The cells were alive throughout — a dead cell does not recover.
Results — three drawings
Analysis
- 2Define plasmolysis.
- 4Explain what happened at stage 2 in terms of water potential and the direction of water movement.
- 3The cell contents shrank but the cell wall did not. Explain what that tells you about the wall.
- 3The cells recovered at stage 3. Explain why this proves they were alive, and what would have happened had they been dead.
- 3An animal cell in the same salt solution behaves differently from an animal cell in pure water. Describe both, and explain why a plant cell survives pure water while an animal cell may not.
- 3Relate this to seaweed left in a rock pool that evaporates in the sun on a hot Gqeberha afternoon.
Aim
To culture a community of micro-organisms and to identify and draw the different cell types that appear.
Apparatus
- 🟢 kitchen A handful of dried grass or hay, a jar, water (rainwater or pond water is best; let tap water stand 24 h first)
- 🟠 microscope Microscope, slides, cover slips, a dropper
Method
- Put the hay in the jar, cover with water, and leave it uncovered in a warm place out of direct sun for 5–7 days. It will smell. That is the point.
- From day 3 onward, take a drop from just below the surface scum each day and examine it.
- Sketch every distinct organism you find, and note how it moves — how something moves is the fastest way to identify it.
- Keep going for several days. The community changes: bacteria peak first, then the things that eat bacteria.
🔬 Supplied observations — a real 7-day infusion
Day 3 · × 400 — bacteria dominate
Vast numbers of tiny rods and dots, 1–3 µm long, in constant jittery motion. No internal structure visible at this magnification. Prokaryotes.
Day 5 · × 400 — ciliates arrive
Slipper-shaped cells about 200 µm long, covered in fine beating hairs, swimming fast in a spiralling path. A visible oral groove sweeps particles inward. Ciliate protists — the Paramecium body plan.
Also present: smaller oval cells, ~40 µm, with a single long whip-like flagellum — flagellates.
Day 7 · × 400 — amoeboid forms and green cells
Shapeless creeping blobs, 100–400 µm, with no fixed outline, flowing along by pushing out blunt pseudopodia. Also small bright-green spherical cells about 10 µm across with a visible chloroplast — photosynthetic algae that have grown in the light.
Results
| Day | Sketch ref | Approx. size | How it moves | Plant-like or animal-like? |
|---|---|---|---|---|
Analysis
- 6Make labelled scientific drawings of three different cell types you found (or from the supplied observations), each with a heading and a stated magnification.
- 3For each, state whether it is a prokaryote or a eukaryote, and give your reason.
- 3The community changed over the week. Explain why bacteria appeared first and the larger protists later.
- 2One organism you found is bright green. What organelle does it have that the others lack, and what does it do?
- 3The 200 µm ciliate lives in fresh water. Explain the osmotic problem it faces and name the organelle that solves it.
- 2Explain how this activity links forward to Topic 18 (Protists).
✅ Memo & marking guide — try the analysis first
Total: 118 marks — Investigation 1: 19 · Investigation 2: 24 · Investigation 3: 18 · Investigation 4: 20 · Investigation 5: 18 · Investigation 6: 19.
This memo does not print with the worksheet.
Investigation 1 — onion and cheek
- Q1 (6): Onion — regular rectangular / rigid cell wall + membrane / nucleus at the side / one large vacuole / packed in continuous rows. Cheek — irregular rounded / membrane only / nucleus central / no large vacuole / scattered and separate.
- Q2 (3): Cell wall · large central vacuole · plastids (present in plant cells generally, though absent in this underground tissue).
- Q3 (3): The wall is rigid, so it fixes the cell into a regular geometric shape and lets cells pack together without gaps; it also resists the vacuole's turgor, which is what pushes the nucleus to the edge.
- Q4 (2): Onion bulbs grow underground, receive no light, so chloroplasts would serve no purpose. Structure follows function.
- Q5 (3): Cell contents are transparent and nearly colourless, so there is too little contrast to see structures. Iodine → cell wall and nucleus. Methylene blue → organelles and nucleus.
- Q6 (2): 100 mm = 100 000 µm; M = 100 000 ÷ 250 = × 400.
Investigation 2 — scale and drawing
- Q7 (6): (a) I = M × A = 400 × 60 = 24 000 µm = 24 mm · (b) A = 90 ÷ 300 = 0.3 mm = 300 µm · (c) 40 mm = 40 000 µm; M = 40 000 ÷ 80 = × 500 · (d) 1 mm = 10 ÷ 25 = 0.4 µm; 45 × 0.4 = 18 µm · (e) 1 mm = 1 000 µm; 1 000 ÷ 20 = 50 cells · (f) 25 nm = 0.025 µm; × 10 000 000 = 250 000 µm = 250 mm.
- Q8 (4): Light 40–400× (1000× for bacteria); electron up to 10 000 000×. Light can view living material; electron reveals internal organelle detail such as cristae and grana.
- Q9 (2): It is a ratio of two lengths, so the units cancel.
- Q10 (3): A stated magnification is only true at the original print size — resize, crop or photocopy the image and it becomes wrong. A scale bar is part of the image, so it scales with it and stays correct.
- Q11 (9): One mark per convention — dark sharp pencil · neat single lines, no sketching · no overlapping at the starting point of an enclosed structure · colour/shading only if asked · ruler for label lines · no arrows · labels aligned in line under each other · neat legible print · descriptive underlined heading.
Investigation 3 — potato osmometer
- Q12 (4): The salt solution has a lower (more negative) water potential than the water in the saucer and in the potato cells. Water therefore moves by osmosis through the living potato tissue, which acts as a semi-permeable membrane, from high to low water potential — into the well, raising the level.
- Q13 (3): It is the control: it shows the rise is caused by the salt gradient and not by the apparatus, capillarity or the potato itself. If both had risen, the salt would not be the cause and the experiment would prove nothing.
- Q14 (2): The living cell membranes of the potato tissue between the saucer and the well.
- Q15 (3): Any three — same potato variety, same well size, same volume, same temperature, same time. E.g. a bigger well surface would allow more water through and confound the comparison.
- Q16 (3): No rise. Boiling kills the cells and denatures the membrane proteins, destroying selective permeability, so osmosis cannot occur.
- Q17 (3): Seawater (~35 g/L) is hypertonic to fish body fluids (~9 g/L), so water moves out of the fish by osmosis; it compensates by drinking seawater and excreting salt through its gills.
Investigation 4 — raisins
- Q18 (3): Supplied data — group 1: (15.1−8.4)/8.4 × 100 = +79.8% · group 2: (9.2−8.6)/8.6 × 100 = +7.0% · group 3: 0%.
- Q19 (4): The dried raisin's contents are highly concentrated, so their water potential is much lower than the surrounding fresh water. Water moves by osmosis through the skin (semi-permeable) from high to low water potential, into the raisin, which swells.
- Q20 (3): The salt solution's water potential is much closer to the raisin's own, so the gradient is far smaller and much less water moves in.
- Q21 (2): The control — shows mass change is caused by water uptake, not by handling, weighing error or air humidity.
- Q22 (2): Surface water adds mass without having entered the raisin, which would overstate the uptake.
- Q23 (3): Bar graph — groups on x, % change on y ✓ · axes labelled with units ✓ · heading ✓. Bar, not line: the x-axis is categories.
- Q24 (3): It would shrink and wrinkle — water leaves the cells into the hypertonic solution. In plant cells this is plasmolysis.
Investigation 5 — plasmolysis
- Q25 (2): The shrinking of the cytoplasm and membrane away from the cell wall when a plant cell loses water in a hypertonic solution.
- Q26 (4): The salt solution has a lower water potential than the cell sap, so water leaves the vacuole and cytoplasm by osmosis; the protoplast loses volume and pulls inward, away from the wall.
- Q27 (3): The wall is rigid and fully permeable — it is not affected by osmosis and does not change shape. It gives the cell its fixed form regardless of internal water content.
- Q28 (3): Recovery (deplasmolysis) requires an intact, selectively permeable living membrane. A dead cell's membrane has lost that selectivity, so it could not re-accumulate water and would stay collapsed.
- Q29 (3): An animal cell in salt solution shrinks/crenates; in pure water it swells and bursts (lysis). A plant cell survives pure water because the rigid wall resists expansion and generates turgor pressure, stopping further water entry.
- Q30 (3): As the pool evaporates its salinity rises, becoming increasingly hypertonic to the seaweed's cells; water is drawn out, the cells plasmolyse, and prolonged exposure kills the tissue.
Investigation 6 — hay infusion
- Q31 (6): Two marks each — recognisable drawing ✓, correct labels ✓, underlined heading with magnification ✓.
- Q32 (3): Bacteria = prokaryote (tiny, no visible internal structure, no nucleus). Ciliates, flagellates, amoebae and green algae = eukaryotes (large, visible internal organelles, a nucleus).
- Q33 (3): Bacteria reproduce fastest and feed directly on the decaying hay, so they bloom first. The larger protists feed on the bacteria, so their population can only rise once there is enough food — a predator–prey lag, and a microbial food chain.
- Q34 (2): A chloroplast — it carries out photosynthesis, making the organism's own food from light.
- Q35 (3): Fresh water is hypotonic to its cytoplasm, so water floods in constantly by osmosis and would burst it. A contractile vacuole collects the excess and expels it through the membrane.
- Q36 (2): Nearly every organism found is a protist — the "everything else" eukaryote kingdom Topic 18 covers, including the amoeboid and ciliate body plans whose marine relatives are the foraminifera and tintinnids.