Cell
Biology
Every fish in the ocean is slowly losing water to the sea — even though it's surrounded by it. A river fish is doing the exact opposite, fighting off too much water. Both are true. The reason is hidden inside every cell — in the chemistry of the membrane, the pull of osmosis, and the quiet machinery of the organelles that keep an organism alive.
If you drop a fish from the river into the sea, it dies. If you drop a fish from the sea into a river, it dies just as fast. Both fish are surrounded by water — the same molecule. So what kills them?
Osmosis. Every cell in every fish is wrapped in a thin cell membrane with tiny pores in it. Water flows through those pores from whichever side has less dissolved salt to whichever side has more. A river fish's body is much saltier than river water, so water rushes in through its skin and gills — its cells would burst if it didn't constantly pump the excess out. A marine fish's body is much less salty than seawater, so water rushes out — it has to drink seawater all day just to stay hydrated, and its gills work overtime pumping out the extra salt.
Swap the two fish, and you've reversed the direction of the pressure. The river fish in the sea dries out; the sea fish in the river bursts. Both die not because of the water itself but because their cells are no longer in osmotic balance with their surroundings. This is why cell biology isn't just biology — for a marine scientist, it's the biology. Every life process in this course, from kelp photosynthesis to a whale's metabolism, happens inside a cell.
The Cell — A Self-Contained Factory
A cell is the smallest unit of life — a self-contained factory bound by a membrane, filled with specialised structures (organelles) that each do a specific job.
Every living organism is made of one or more cells. Some are single-celled (like a phytoplankton diatom drifting near the surface) — that one cell does everything the organism needs to do. Others are multicellular — like a tuna, a kelp blade, or you — built from trillions of cells that have differentiated into specialised types working together.
Inside every cell is a chemical soup (the cytoplasm) containing all the macromolecules from Topic 13. Floating in that soup are organelles — tiny machines bound by their own membranes. The cell's outer boundary, the plasma membrane, is a "fluid mosaic" of phospholipids and proteins that decides what gets in and out. Water moves passively through pores by osmosis; bigger or charged molecules need active transport, paid for with ATP. Get this layer right and the rest of biology becomes a lot less mysterious.
What a Cell Is Actually Made Of
Before the organelles, the chemistry. CAPS opens this block with the cell's molecular constituents — and then asks something more interesting: what kind of mixture is the inside of a cell? The answer is that it is three different kinds at once, and that turns out to explain a lot.
Cells are mostly water — and then proteins, carbohydrates, lipids and nucleic acids.
You met all four macromolecule families in Topic 13. Here they are again in their working home. Proteins build structures and act as enzymes. Carbohydrates supply and store energy. Lipids build every membrane in the cell. Nucleic acids (DNA and RNA) carry the instructions. And water — 70–80% of most cells — is the medium all of it happens in.
The "soup mug" — three mixtures in one cell
Your booklet calls this the soup mug model. A mug of soup has salt dissolved invisibly in the broth, protein and starch making it thick and cloudy, and lumps of vegetable floating in it — three different kinds of mixture in one container. The cytoplasm is exactly that. Tap each one.
The exam question hiding in here
Your booklet's sample question 2 is "How does a colloidal solution differ (2.1) from a true solution? (2.2) from a suspension?" — so learn the comparison in both directions, not just the three definitions.
Against a true solution: the colloidal particles are much larger and do not dissolve — they stay suspended intact, rather than dissociating into ions.
Against a suspension: the colloidal particles are smaller and stay up on their own — a suspension's particles would settle out the moment the cytoplasm stopped circulating.
Inside the Cell
Below: a typical animal cell drawn as it would appear under an electron microscope. Tap each organelle to see what it does, what it's made of, and why it matters. Plant cells (kelp, phytoplankton) have most of these too — plus a few extras we'll look at in a moment.
~ and the same cell as a plant / algal cell — three extra parts
Local connection — osmoregulation along the SA coast
The cells in your body sit at roughly the same salt concentration as your blood — about 9 grams of salt per litre. Seawater is about 35 grams per litre, almost four times saltier. That means every marine fish (tuna, snoek, galjoen) and every diving sea bird is constantly fighting osmotic dehydration. They've evolved clever solutions: marine fish drink seawater and pump excess salt out through specialised cells in their gills; the African Penguin at Boulders Beach has a pair of salt glands above its eyes that excrete concentrated brine, leaving it free to drink seawater all day. Kelp (Ecklonia maxima) along the West Coast has cell walls strong enough to withstand the same osmotic pressure that would shrivel a land plant. The Two Oceans Aquarium in Cape Town runs its tanks at the same salinity — every animal in there is performing osmoregulation in real time.
The Cell Membrane — Security Checkpoint
The plasma membrane is what separates "inside the cell" from "outside the cell". Scientists describe its structure with the Fluid Mosaic Model: a double layer of phospholipids with pores and proteins embedded in it like floating tiles.
~ the fluid mosaic model · cross-section
🌊 Passive transport
~ no energy needed
Diffusion & osmosis. Small molecules (water, oxygen, CO₂) and ions slip through pores from where there's more of them to where there's less — down the concentration gradient. The cell pays nothing for this; it just happens.
⚡ Active transport
~ uses ATP
Protein pumps. Specific membrane proteins burn ATP to force substances against the concentration gradient — from low-concentration to high-concentration. This is how a marine fish's gill pumps out the salt it has accumulated; how kelp pulls scarce minerals in from seawater. (Note that kelp has no roots — its holdfast only grips the rock. Every nutrient it needs is absorbed straight through the surface of the blade, against the gradient, using ATP.)
Osmosis & Water Potential
Cells live or die by osmosis — the movement of water across the semi-permeable membrane from higher water potential (less salty, more dilute) to lower water potential (saltier, more concentrated). The direction of water flow depends on what surrounds the cell. Three classic scenarios:
Isotonic
~ same concentration both sides
Equal water potential inside and outside. Water moves in both directions equally — the cell stays the same size. This is what your blood looks like, and it's the "rest state" cells prefer.
~ a fresh-killed shark cell in shark blood
Hypotonic (outside)
~ outside is less salty
Outside water potential is higher; water floods into the cell. An animal cell would burst (lyse) unless it actively pumps water back out. A plant cell with a rigid cell wall just becomes turgid — its internal pressure rises until it pushes back.
~ a marine fish dropped into a river
Hypertonic (outside)
~ outside is saltier
Outside water potential is lower; water drains out of the cell, which shrinks. The cell would dehydrate and die unless it actively pumps water in (or excretes the salt). Most marine animals are in this state every day.
~ a river fish dropped into the sea
The water-potential rule
Water always moves from high water potential to low water potential. Pure water has the maximum value of zero. Every solution has a negative water potential — and the saltier the solution, the more negative. So when you see "hypertonic" think "more negative water potential"; water flows towards it. Marine fish live in the most negative water potential on Earth — that's why they drink seawater all day.
Plastids — and the Chloroplast in Detail
CAPS is unusually specific here. Of the eleven organelles in this block, it says to "look at detailed structure only of nucleus, mitochondrion and chloroplast" — because those three get referred back to when you reach cell division, respiration and photosynthesis. You have had the nucleus and the mitochondrion. Here is the third, and it is the one that matters most in a marine course.
The plastid family — tap each type
Local connection — why Algoa Bay's seaweed isn't green
Walk the rocks at Cape Recife and you will find seaweeds that are brown and red, not green. They all still run on chlorophyll, which is green — so where has the green gone?
It is masked. Marine phytoplankton and macroalgae pack their chloroplasts with accessory pigments — fucoxanthin in the browns, phycoerythrin in the reds — sitting alongside the chlorophyll. Those pigments absorb the blue-green light that penetrates deepest into seawater, wavelengths chlorophyll itself is poor at catching, and hand the energy over to it. The colour you see is the pigment that is reflecting, not the one doing the work.
This is why red algae can grow deeper than green ones, and it is the direct link from this organelle to Topic 20 (light in water) and Topics 22–24 (the algae themselves).
Similar: both have a double membrane · both contain their own DNA and ribosomes (so both can partly self-replicate) · both have a hugely folded or stacked internal surface to maximise the area available for reactions (cristae / grana).
Different: the mitochondrion releases energy from glucose (respiration) while the chloroplast captures energy into glucose (photosynthesis) · mitochondria occur in nearly all eukaryotic cells, chloroplasts only in plants, algae and phytoplankton.
Seeing Cells — Microscopes, Scale and the Drafter's Code
CAPS puts two things in this block that are easy to skip and expensive to lose marks on: "Scale and measurements at microscopic level should be explained and understood" and "Scientific drawing conventions to be introduced and used." Both are content, not optional extras.
🔬 Light microscope
40× – 400×
- Up to 1000× when looking at bacteria.
- Uses beams of light passing through the specimen.
- Shows overall cell structure — the shape, the wall, the nucleus.
- Specimens are usually stained: iodine highlights the cell wall and nucleus.
- This is the one you would actually use. Living material can be viewed.
⚡ Electron microscope
up to 10 000 000×
- Uses beams of electrons, not light.
- Essential for the intricate internal organelles — cristae, grana, ribosomes.
- Methylene blue and heavy-metal stains highlight organelles.
- Every "electron micrograph" in your textbook came from one of these.
- The specimen must be dead — you cannot watch a living cell with it.
Scale — the units, and the calculation
Cells are measured in micrometres (µm). A typical animal cell is about 20 µm across — you could line up fifty of them inside a millimetre.
So 1 mm = 1 000 µm, and 1 µm = 0.001 mm. Almost every scale question is really a units question.
The magnification formula
The rule that saves you: both measurements must be in the same unit before you divide. Convert first, then calculate. Magnification itself has no unit — it is written "× 400".
Worked example 1 — how big is the real cell?
You draw a cell 60 mm wide from a view magnified × 400. What is its actual size?
A = I ÷ M = 60 mm ÷ 400 = 0.15 mm
Convert to the sensible unit: 0.15 × 1 000 = 150 µm
A sanity check is worth a mark: 150 µm is large for a single cell but perfectly reasonable for something like a big algal cell. If you had got 150 mm, you would know instantly you had slipped a factor of a thousand.
Worked example 2 — what magnification was used?
A diatom is genuinely 80 µm across. In a micrograph it measures 40 mm. Find the magnification.
Same units first: 40 mm = 40 × 1 000 = 40 000 µm
M = I ÷ A = 40 000 ÷ 80 = × 500
Worked example 3 — reading a scale bar
A micrograph carries a scale bar labelled 10 µm that measures 25 mm on the page. A cell beside it measures 45 mm. How wide is the cell?
The bar tells you 25 mm on the page = 10 µm in life, so 1 mm = 10 ÷ 25 = 0.4 µm.
Cell = 45 × 0.4 = 18 µm
A scale bar is more reliable than a stated magnification, because it survives the image being resized or photocopied. That is exactly why journals print one.
The Drafter's Code — nine rules, and they carry marks
Your booklet lists these as "the conventions for drawings in the Marine Sciences curriculum", and sample question 1.3 asks you to spot three breaches in someone else's drawing. That is a question you can only answer if you know the list.
- Use a dark, sharp pencil — never pen, never blunt.
- Draw neat single lines. No sketching, no feathery repeated strokes.
- Do not overlap your starting point when you close an enclosed structure.
- Colour or shading only if asked, and only to highlight a specific structure.
- Use a ruler for every label line.
- No arrows on label lines — plain straight lines only.
- Labels sit in line, under each other, on one or both sides of the diagram.
- Labels in neat legible print — not cursive.
- Give it a descriptive, underlined heading, above or below the drawing.
Cell Types & the Hierarchy of Life
Not every cell looks the same. Two huge distinctions matter for marine biology.
Plant cells vs animal cells
🐠 Animal cells
- Bound only by a flexible cell membrane — no rigid wall.
- Have centrosomes (with centrioles) for cell division.
- Small, temporary vacuoles only.
- Heterotrophic — must eat other organisms. No plastids.
- Examples: every fish, whale, octopus, shark, you.
🌿 Plant & algal cells
- Have a rigid cell wall of cellulose (stiffened with lignin in woody land plants) outside the membrane — provides structural support.
- Contain plastids, especially chloroplasts with chlorophyll for photosynthesis.
- Have one big central vacuole that keeps the cell turgid.
- Autotrophic — make their own food from light.
- Centrosomes: land plants have lost them and organise cell division without one. Most algae — including kelp — still have them, which is one of the clues that algae are not plants.
- Examples: kelp, phytoplankton, seagrass, land plants.
Prokaryotes vs eukaryotes
The deeper divide is between prokaryotic cells (no membrane-bound nucleus — DNA floats freely in the cytoplasm) and eukaryotic cells (DNA enclosed in a proper nucleus). Almost all primitive, single-celled organisms — like the cyanobacteria that pumped Earth's first oxygen into the atmosphere (Topic 9) — are prokaryotes. Every animal, plant, fungus and most single-celled algae are eukaryotes. Prokaryotes also lack most of the membrane-bound organelles above (no mitochondria, no ER, no Golgi). The first eukaryote evolved about 2 billion years ago when one prokaryote engulfed another — and the engulfed cell became the first mitochondrion. That's why mitochondria still have their own DNA and ribosomes.
The hierarchy of life
CAPS writes this ladder as "Undifferentiated → tissues → organs → systems", and that first rung is the one everyone forgets. It is also the most interesting, because it is the step where multicellular life actually began.
Cell
One living unit — the smallest level of life.
Colony
Undifferentiated. Many identical cells living together, but each still doing every job itself.
Tissue
Cells that have differentiated — specialised to one job (e.g. fish muscle).
Organ
Several tissues doing one big job (e.g. gill).
System
Several organs working together (e.g. respiratory system).
The colony step — where multicellular life came from
A colony is the missing link between "one cell" and "one body". The cells stick together but stay undifferentiated — every cell still feeds itself, still moves itself, still reproduces itself. Nothing is specialised, so nothing depends on anything else.
The moment some cells in a colony give up doing everything and specialise — these ones only swim, those ones only digest — you have differentiation, and you have crossed from a colony into a true multicellular organism. Every animal, plant and seaweed on Earth is descended from that step.
The sea is the best place to watch it, because it still contains organisms at every rung. Volvox is a hollow ball of near-identical flagellated cells — a colony. A sponge (Topic 29) has a handful of specialised cell types but no true tissues. A jellyfish (Topic 30) has real tissues. A fish has the whole ladder.
In the Lab — the CAPS Investigations
This is the most practical block in the whole year — CAPS sets six investigations and gives it 12 hours, more than any other topic. Some of it needs a microscope. Most of it does not.
🔬 The six investigations
- Onion epithelium and cheek cells. The classic plant-vs-animal comparison — peel the thin skin from an onion scale, stain with iodine, and compare it with your own cheek cells stained with methylene blue. 🟠 microscope
- Use microscopes and stains — and draw what you see. Not a diagram copied from a book: your drawing, of your slide, following all nine rules of the Drafter's Code. 🟠 microscope
- Osmosis in a hollowed-out potato. The classic potato osmometer — no microscope needed, and it produces real numbers you can graph. 🟢 kitchen
- Osmosis with raisins. Your booklet's sample question 3 is literally this experiment (Thandi's overnight oats). Soak, weigh, explain. 🟢 kitchen
- Plasmolysis of onion epithelium. Watch a living cell shrink away from its own wall in salt water — then bring it back with fresh water. The single best demonstration of hypertonic vs hypotonic there is. 🟠 microscope
- Hay diffusion — find, identify and draw different cell types. Leave grass in water for a week and a whole ecosystem of protists appears. Also the bridge to Topic 18. 🟠 microscope
Four of the six want a microscope — the first real equipment barrier in this course. So the practical sheet does two things about it: it gives you the full method if you have one (a basic student scope handles all of this), and for every microscope investigation it supplies described micrographs and real measurements so you can still do the identification, the labelled drawing, the scale calculation and the analysis. Those are the parts CAPS assesses.
📄 Practical sheet: Open the Topic 14 practical →
Key Vocabulary
Tap each card to flip it. Twenty cards, because this is a 12-hour block — the largest allocation in your year — and every one of these appears somewhere in CAPS or in your booklet's examinable-definitions list. Use water potential, hypertonic, hypotonic and isotonic precisely: CAPS names those four outright.
The scientific model that describes the cell membrane structure: a double phospholipid layer, with pores just big enough for water molecules to pass and proteins embedded at intervals — some acting as pumps (active transport), others as identity markers.
The movement of water across a semi-permeable membrane from a solution of higher water potential to one of lower water potential — i.e. from the more dilute side to the more concentrated side. Happens without energy (passive transport).
A numerical measure of a solution's tendency to move by osmosis. Pure water = 0 (the highest possible). Every solution has a negative value — the saltier the solution, the more negative. Water always flows from higher water potential to lower water potential.
A solution that is more concentrated than the one on the other side of the membrane (more salt, lower water potential). Water flows towards it through the membrane. Seawater is hypertonic to a marine fish's body fluids — water flows out of the fish.
A solution that is less concentrated than the one on the other side of the membrane (less salt, higher water potential). Water flows away from it. River water is hypotonic to a freshwater fish's body fluids — water flows into the fish.
Two solutions of equal concentration on either side of the membrane — equal water potential, equal flow in both directions, no net movement. The cell stays the same size. Cells inside your body sit in roughly isotonic interstitial fluid.
The movement of substances across the cell membrane against the concentration gradient — from low to high concentration. Done by membrane protein pumps that burn ATP. Opposite of passive transport (diffusion, osmosis).
An organism whose cells have a nucleus enclosed in a membrane, plus other membrane-bound organelles (mitochondria, ER, Golgi). Includes every animal, plant, fungus, and most algae. Contrasts with prokaryote — primitive single-celled organisms (bacteria, archaea) whose DNA floats free.
A fluid of ionic compounds dissolved in water — the water is the solvent, and the compounds have dissociated into ions, which are the solute. Sodium chloride and calcium carbonate sit in cell contents this way. Particles are far too small to see or settle.
A fluid containing larger organic compounds (like proteins) that do not dissolve but are still small enough to stay suspended. Its defining property: it can shift between a liquid sol state and a jelly-like gel state.
A fluid holding particles too large to dissolve — inside a cell, the organelles themselves. They would settle out if the cytoplasm ever stopped circulating. This is the largest of the three particle sizes.
A family of organelles in plant, algal and phytoplankton cells for producing and storing food and pigments. Three types: chloroplasts (green, photosynthesis), amyloplasts/leucoplasts (colourless, starch storage), chromoplasts (red/orange/yellow pigment).
The internals of a chloroplast. Grana are stacks of disc-shaped thylakoid membranes where the chlorophyll sits and light is captured. Stroma is the fluid around them where sugar is built. Lamellae are the membrane bridges joining one granum to the next.
A pigment sitting alongside chlorophyll in a chloroplast that absorbs wavelengths chlorophyll cannot — and passes the energy to it. In the sea they absorb the blue-green light that reaches deepest, and they mask the green, which is why seaweeds look brown or red.
How many times larger an image is than the real object. M = image size ÷ actual size. Both measurements must be in the same unit first. Magnification has no unit — write "× 400". Light microscope: 40–400× (1000× for bacteria). Electron microscope: up to 10 million ×.
One thousandth of a millimetre — the working unit of cell biology. 1 mm = 1 000 µm. A typical animal cell is about 20 µm across, so roughly fifty of them fit inside one millimetre.
The fluid inside the nucleus, held in by the nuclear envelope — the nucleus's own equivalent of cytoplasm. The chromatin network rests coiled in it between cell divisions.
The folding inward of the cell membrane to engulf something. Step one of phagocytosis: the membrane wraps around a food particle, pinches off, and becomes a phagosome inside the cell, which a lysosome then digests.
The state where concentrations have evened out and there is no net movement in either direction. Molecules are still moving — they just cross in equal numbers both ways. An isotonic cell is at equilibrium with its surroundings.
The process by which cells become specialised for one job. It is the step that separates an undifferentiated colony (every cell doing everything) from a true multicellular organism (cells depending on each other). Differentiated cells of the same kind form a tissue.
Quick Check
Question 1
Well done!
Next: Basic Classification — how scientists organise the millions of cell-based organisms into a single tree of life.