Cells, Grade 10: The Endoplasmic Reticulum Gets Three Different Jobs in One Topic

The endoplasmic reticulum is given three different jobs in this one topic, and one of them belongs to the Golgi body.

On the organelle pages the smooth ER "packages proteins for transport to other parts of the cell". One page later the Golgi body "collects molecules such as proteins and lipids from the endoplasmic reticulum, processes the proteins and lipids, and packs them into vesicles". In the revision summary both kinds of ER have been reduced to one line that says they transport substances.

Packaging proteins into vesicles is the Golgi body's work. Smooth ER has no ribosomes and does not handle proteins at all. It makes lipids, stores calcium and breaks down toxic substances. A learner asked what the smooth ER does now has three answers to choose from and two of them are wrong.

This page gives both units, six places where the teaching material is wrong or contradicts itself, what a school microscope will really show you, and a free 80-mark worksheet with its memo.

Where this fits in the curriculum

Subject Life Sciences
Grade 10
Term 1
Topic Topic 2, Cells: the basic units of life
Units Unit 1 the discovery of cells: history of microscopy, using the light microscope, measuring the size of cells. Unit 2 cell structure and function: basic structure, organelles, plant and animal cells compared, drawing from micrographs, movement across membranes
Formal assessment None. There is an end-of-topic test
Marks Our worksheet is 80

Finding 1: one organelle, three different jobs

Where it appears What the smooth ER is said to do
Organelle pages "Packages proteins for transport to other parts of the cell", then, in the same paragraph, makes lipids and breaks down toxic substances
The next page Nothing. The Golgi body is the one that packs proteins and lipids into vesicles
Revision summary "Rough and smooth endoplasmic reticulum transport substances from one part of the cell to another." The lipids and the detoxification have disappeared

What each one actually does

Organelle Function
Rough ER Studded with ribosomes. Transports the proteins those ribosomes make, for membranes or for export
Smooth ER No ribosomes. Makes lipids, including the lipids for new membranes. Stores calcium. Breaks down toxic substances
Golgi body Receives, processes and packs proteins and lipids into vesicles for distribution or secretion. Also forms lysosomes

If a learner writes that the smooth ER packages proteins, that is a wrong answer, and it came from a sentence in their own teaching material. Worth saying out loud before the test.

Finding 2: the model drawing breaks the rules printed beside it

The rules given for drawing a cell from a micrograph are good ones:

  1. Keep the same size and shape as the micrograph
  2. Draw organelles the size and shape they appear in the micrograph
  3. Draw them in the same positions
  4. Outlines only, not fine internal detail
  5. Avoid shading, and avoid dots that do not represent a structure
  6. Ruled label lines
  7. Give the diagram a heading

Beside that list sits a micrograph of flat, angular cheek cells, and beside that the diagram offered as the model answer. Readers are told to notice how well the drawing represents the specimen.

Rule The model drawing
Same shape Round. The cells in the micrograph are flat and angular
Only what is visible About ten mitochondria. There are none in the micrograph, and a light microscope could not show them
Only what is visible Small vacuoles, also absent from the micrograph
No stippling Stippled from edge to edge

Four of the seven rules, broken by the example meant to demonstrate them. Drawing questions are marked against the rules, so use the rules and ignore the picture.

Finding 3: the scale formula adds where it should divide

The formula is printed like this:

True size = measured size x true length of scale line + measured length of scale line

That plus sign should be a divide. The worked example directly underneath divides, correctly: a 44 mm measurement against a 12 mm scale line representing 1 micrometre gives 3,67 micrometres.

Apply the formula as printed and you get 44 x 1 + 12 = 56, which is not a length of anything.

The activity on the facing page asks learners to use that formula twice, for 10 marks.

How to do it, and how to check it

Step Example
Measure the scale line on the page, in mm 15 mm
Read what it represents 2 micrometres
Measure the structure, in mm 60 mm
Multiply by the true length, divide by the measured length 60 x 2 / 15
Answer 8 micrometres

Check it is believable. Most plant and animal cells are 10 to 100 micrometres. A mitochondrion is about 1. An answer in the hundreds means the sum went the wrong way up.

Finding 4: osmosis, renamed

"The process of water moving into and out of a cell vacuole is called osmoregulation."

It is called osmosis, and two sentences earlier the same page says so: water moves into the cell sap by osmosis.

Osmoregulation is a different thing at a different level. It is how a whole organism keeps its water and salt balance steady: your kidneys, the contractile vacuole of a single-celled pond organism. It is a Grade 11 and 12 word, and meeting it here attached to the wrong process is worse than not meeting it at all.

Finding 5: the potato practical tells you to read the wrong container

The osmosis investigation is the classic one. Hollow out two potatoes. Fill one with concentrated sugar solution and add red dye to it. Fill the other with tap water as the control. Stand both in beakers of water.

Then: "Mark the water level in the beakers", and later, "Observe the beakers and record the level of water in each beaker."

Where the change happens Will you see it?
Inside the potato cavity, where the dyed sugar solution rises Yes, clearly, in one to two hours
In the beaker, where the level falls No. The same small volume spread across a wide beaker moves the level by a fraction of a millimetre

The dye gives the method away. There is only one reason to colour the sugar solution, and that is so you can see it climb inside the potato. The instruction then sends you to look at the beaker.

The fix costs nothing

  • Mark the starting level of the dyed solution on the potato with the same marking pen the materials list already includes
  • Make the cavity narrow rather than wide. The narrower the cavity, the further the same volume of water pushes the level up
  • Leave it overnight if you can. Two hours works, overnight is unmistakable
  • The control should show no rise at all, and that is the whole point of having it

Finding 6: the end-of-topic test adds up three different ways

Where Total given
Heading of the test 73 marks
Foot of the test 50
Adding the printed subtotals 65

8 + 5 + 6 + 14 + 19 + 13 = 65. Every individual subtotal is internally correct. It is only the two stated totals that are not.

One matching question in the same test lists six items and then instructs learners to answer five, while allocating six marks.

If you set this test as written, say 65 before they start. Learners budget their time against the total on the page, and both printed totals are wrong.

Three lists of plant and animal differences, and they disagree

List What it says is animal only
Organelle pages Vesicles, some vacuoles, lysosomes
Comparison table Irregular shape, small vacuoles, no cell wall, lysosomes, no plastids
Revision summary Lysosomes

Vesicles are not animal only. The plant cell diagram in the same topic labels a Golgi vesicle. Plant cells have Golgi bodies, Golgi bodies pinch off vesicles, so plant cells have vesicles.

The centrosome is the structure that really is animal only, and it appears in none of the three lists, although it is drawn and labelled on the animal cell diagram.

What is actually different

Plant cell Animal cell
Cell wall of cellulose No cell wall
One large central vacuole with a tonoplast, full of cell sap Small vacuoles and vesicles, or none
Plastids: chloroplasts, chromoplasts, leucoplasts No plastids
No centrosome Centrosome
Lysosomes rare; the vacuole does that work Lysosomes
Regular shape, held by the wall Shape varies

Nucleus, nucleolus, mitochondria, ribosomes, rough and smooth ER, Golgi bodies, cytoplasm and cell membrane are in both. The two diagrams label different subsets of that list, which makes it look as though they differ when they do not.

What a school light microscope actually shows you

Two cell diagrams in this topic are captioned as a typical cell as seen with a light microscope, and both label mitochondria. The next spread then explains that electron microscopes are needed to see the structures inside cells, and draws them again.

Both cannot be true. A mitochondrion is about one micrometre across and, unstained in a wet mount, it is simply not visible.

In an onion wet mount at 400x you will see You will not see
Cell wall, as a clear boundary Mitochondria
Cytoplasm Ribosomes
Nucleus, a dark dot, darker with iodine Endoplasmic reticulum
The edge of the vacuole, if the stain is light Golgi bodies
The cell membrane, pressed against the wall and hard to separate from it Lysosomes

The magnification you are told to expect, and what a microscope does

The teaching pages give the light microscope a range of 100 to 2 000x.

Eyepiece Objective Total
10x 4x 40x
10x 10x 100x
10x 40x 400x
16x 40x 640x
10x 100x oil immersion 1 000x

2 000x is beyond what light can resolve. Past roughly 1 500x the image gets bigger without getting clearer, which is called empty magnification. It is also why the honest number to shop on is the objective set, not the headline figure on the box.

Both micrographs of real cells in this topic are labelled 400x, which is exactly what a 10x eyepiece and a 40x objective give you. That is the whole topic covered.

Three safety gaps, and all three are easy to close

One: the cheek cell smear has no disposal step

Learners wipe the inside of their own cheek with a cotton bud, smear it on a glass slide, add a stain and put it under a microscope. There is no safety note beside that method at all.

This is human tissue and saliva. It needs three lines that are not there:

  • One fresh cotton bud per learner. Never a second use, never a shared packet, never back in the box
  • Nobody looks at anybody else's slide. Not swapped, not passed down the bench
  • Slides into disinfectant before washing. Household bleach at one part in ten, ten minutes, then wash. Or use the slide once and bin it

Two: boiling water, with nothing said about it

The diffusion demonstration asks a group to put boiling water into a container and drop in a tea bag. There is no warning of any kind anywhere near it.

Hot water from the tap shows the same thing. Diffusion is faster when it is hotter, which is the point, and hot tap water is plenty hot enough to watch the colour spread. If you do use a kettle, one adult pours, the container stays on the bench, and nobody carries it.

Three: iodine and methylene blue, with no gloves mentioned

Both stains are on the materials lists. Neither list mentions gloves or eye protection, and the only safety note on those pages is about broken glass.

Iodine stains skin brown for days and clothes permanently. Methylene blue is worse on fabric. Neither is dangerous in a school dilution, but a class of thirty will produce several ruined shirts, and the step that does it is the one that says to mop up the excess liquid around the cover slip with paper.

Mop with the paper held in forceps, or wear gloves. And note that the method puts the onion straight into neat iodine, which over-stains it. Mount in water first, look at it, then draw the iodine under the cover slip from one side with paper pulling from the other. The class then sees the same cell before and after staining, which is a better lesson anyway.

What the material gets right, and it is worth saying

  • The plant and animal comparison table is correct. Of the three lists in this topic it is the one to trust
  • The drawing rules are the right rules. It is only the example that lets them down
  • The history is properly told, with the cork, the first drawings, the 170 quiet years and the two scientists whose work in 1838 and 1839 led to cell theory
  • Cell theory itself is given in four clear statements on the teaching pages, even though the revision summary drops them and says only that cell theory was developed

One name is missing from the history: Rudolf Virchow. The statement that all cells arise from pre-existing cells is his, from 1855, and it is the one statement the two earlier scientists could not have made, because they thought cells formed from non-living material. It is the single most useful date in the topic and it is not there.

What this topic costs

This is the topic where equipment actually matters, so here are real prices and real stock rather than a wish list.

Consumables for a class of 30, working in groups of three

Item Price Quantity Cost
Microscope slides, single frosted end, pack of 50 R45,00 1 pack R45,00
Cover glass 24 x 24 mm, pack of 100 R16,00 1 pack R16,00
Plastic tweezers, 110 mm R5,00 10 R50,00
Disposable droppers, 5 ml, pack of 100 R104,00 1 pack R104,00
Dropper bottles, 50 ml, to decant stain into R5,00 4 R20,00
Total R235,00

Under R8 a learner, and the slides and cover slips will run more than one class. Onion, potato and a tea bag come from a kitchen.

Buy the frosted slides, not the plain ones

The plain slides are the obvious choice and we are out of stock of them. The single frosted slides at R45 for 50 are the better buy anyway, by R12 a pack, because the frosted strip takes a pencil mark. Thirty learners each making a wet mount produces thirty identical pieces of glass, and the frosted end is the difference between a lesson and a lost-property pile.

The microscope

Microscope Magnification Price Right for this topic?
Monocular student microscope 40x, 100x, 400x R1 990,00 Yes. Everything in this topic is seen at 100x and 400x
Student kit with 16x eyepiece up to 640x R3 350,00 Yes, with spare slides and accessories in the box
Compound binocular school microscope up to 1 000x R6 500,00 More than this topic needs. Two eyes is easier on a long practical

Do not buy magnification you cannot use. The micrographs in this topic are at 400x. Getting to 1 000x means a 100x oil immersion objective and immersion oil, which nobody uses on onion cells. The full range is here, and the compound microscopes are here.

What we cannot supply

  • Iodine solution. We do not stock it. A pharmacy or a chemical supplier is where to get it, and a weak solution is all this needs
  • Methylene blue. We list it at R16 for 50 ml and we are out of stock. Ask before you plan a lesson around it
  • Dissecting needles on their own. They come only inside a 20-piece dissecting set at R499, which is a lot of money for one needle. A wooden skewer lowers a cover slip just as well
  • Onions, potatoes and tea bags. Obviously

What you should be able to do by the end

  • Name the parts of a light microscope and say what each does
  • Calculate total magnification from the eyepiece and objective
  • Focus a microscope without breaking the slide, coarse at low power and fine thereafter
  • Make a wet mount of onion epidermis and of cheek cells, and dispose of both safely
  • Calculate a true size from a scale line, dividing rather than adding
  • Name every organelle and give one function each, with the ER and the Golgi body kept apart
  • Give the real differences between plant and animal cells, and the structures that are in both
  • Draw a cell from a micrograph showing only what is in the micrograph
  • Define diffusion, osmosis and active transport and say which needs energy and which goes against the gradient

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

  • Learner worksheet, 80 marks, across both units: the microscope, organelles, plant and animal cells, movement across membranes, and drawing to scale
  • Marking memorandum, with the magnification and scale calculations worked through

There is also a labelled microscope diagram and a blank labelling worksheet on our free teaching resources page, both A4 and both free.

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