Biology homework question -- martianbishop or other biologists?

[frogs]

Dd1 is quite a way into her biology homework make a 3-D model of a cell(!) It's quite a fine object so far eggbox lid for the cell wall, lined with a nice plastic membrane, yellow bouncy ball for the nucleus and a selection of items she's created from a polymer science kit someone gave her last christmas for the variole etc.

Following on from this, and watching the Harvard Cell animation that was posted recently, she has now asked the very reasonable question: Why, if all animal and plant cells have specific structures as shown in her textbook, can they produce differentiated body parts and have different functions as shown in the video?

Please save me from embarrassing myself by admitting I don't have a clue? [slightly pleading emoticon]

[Blandmum]

OK, first of all, excellent question!

We all originaly derive from one cell, the zygote that is made when the egg and sperm join together. Every cell in the body is made from that original cell. Every cell in the body (except red blood cells and eggs and sperm) contains *exactly the same genetic information as that original cell.

In every cell with a nucleus(except the things I have listed) there is aroubnd 3 M of DNA. THis takes the corm of 46 strands of DNA. Between those 46 strands are arounf 30,000 genes.....lenghts of DNA that hold the information needed to make a protein.

The reason that all our different tissues are made up of very different types of cells is that not all 30,000 genes are switched on or expressed at any one time.

For example cells in the pancreas will express the gene for the protein insulin, but cells in the brain will make the protein acetylcholine esterase.

This of the nucleus of the cell being a huge recipie book that contains all the details to make every dish in the world. The book is in ever resturant, but different types of resturants choose to use different informationm.

The differentiation of cells also happens in the uterus and is under the control of growth hormones and I will put my hands up and say I don't know much about that!

HTH

[Blandmum]

oh and sex cells have half the number of strands of DNA exg. Red bloodcells have no nucleus in humans and so have no ordinary dna

[frogs]

Ah, thank you. So the standard-issue biology-textbook cell which she's laboriously making a Blue Peter-style model of is actually a kind of proto-cell? Do eg. hair cells, skin cells or retinal cells look different from the standard model when viewed under the microscope, or are the differences you can see on the video to do with the way particular proteins bind to the cell?

[Blandmum]

oh god, they look wildy different!

(how old is she btw cos that was a corker of a question!?)

look at these

rods and cones by this a styalised diagram not an actual picture

an actual scanning electron microscope picture of a nerve cell


muscle cells

cells from the lining of the trachea

[Blandmum]

oh and just to amuse you, the human egg cell is tha largest cell in the human body, and the sperm is the smallest

[suzywong]

I LOVE your mighty biology brain MB, I feel very sorry that I didn't pay attentionat school and you have helped me plug that gap

[suzywong]

Oi!!! listen to Mrs Bishop and stop thinking about Dairylea Lunchables and fingering or what ever is the current vogue and PAY ATTENTION you lot

[Blandmum]

[frogs]

Thank you, mb. She is 11 I'm guessing that higher-up the school they ditch the Blue-Peter style model making in favour of real work... But her current aim is to read veterinary medicine at Cambridge or failing that at Edinburgh, so is paying special attention in science. This is the same dd who had run-ins with her primary teachers over the little matter of evolution and whether or not women had an extra rib than men, so having proper science lessons seems to be a twice-weekly source of joy to her! And she's after a science sticker as well the school has a system where you get subject-stickers for particularly good pieces of work or coming first in tests etc. Four stickers in a term and you get a 'praise postcard' sent home -- bless.

Those cell photos are beautiful -- I'm with suzywong in wishing I'd paid more attention in third year biology. Then I might even have been able to answer my own children's questions.

Am I right in assuming this is one of those school moments where they teach you something that you assume is always and everywhere true, only to discover as you move onwards and upwards that most of what you learnt earlier on isn't actually true at all?

[Blandmum]

I must say (and I risk being pelted by eggs from some of MN) that a question like that from a child of that age shows the potential to be a real high fligher. The really high end kids are not distinguised by the questions that answer, but by the questions they ask!

Re the models.....I used them with my very able year 11s....what you do with them at that point is ask them to find the flaws in the model.....so the cell membane...a plastic bag, doesn't let things in and out as the real membrane does etc.

TBH, all the things in the models is present in the differentiated cells, they just look odd. So a neurone will still have nucleus, cell bemebrae etc etc, it just looks bloody wierd!

Every thing at GCSE is sort of right, but we make vast simplifications. At A level the stuff is more right, and so on and so on.

We don't do this to be mean, but simply because you cant do all of everything at the ultimate depth in year 7, or 11!

I used to work in the Vet school in Edinburgh! And lieased with the vet school in Cambridge! Two very fine institutions!

[frogs]

Okay, I think I'm getting this. So we could say that the illustration in the textbook is a kind of standard-issue cell, pretty much like the house we would draw if asked to explain one to a martian (square box, four windows, door in the middle, pitched roof, chimney la di dah), but then qualify it by pointing out that lots of houses look a bit different from that, some look so different as to be unrecogniseable, but that they all have common features (roof, walls, windows, door)?

What kinds of cells would look most like the standard illustration one?

mb, I suspect dd1 has the potential to be a bit of a high-flier if she chooses to go the academic route. But the lovely thing is she's finally getting positive rewards for it at her lovely new secondary school, after 7 years of being a lone member of the awkward squad at primary. It's surprising how many teachers really, really don't like kids who ask tricky questions. Only a very few of her primary teachers showed any sign of actually liking her, and some showed an active dislike that was positively unprofessional. And now she's so happy, she gets good grades and feels that all the teachers like her.

[tamum]

That's a good analogy frogs. The other thing to bear in mind is that tissues are not just made up of cells, there's extracellular matrix and stuff around them that can also change the structure of the tissue. Lung cells look fairly standard individually (well, ish) but they are put together quite differently with loads of air space in between them compared with a liver cell, for example.

Tell her if she applies to Edinburgh not to stress membership of the Pony Club on her personal statement

[frogs]

Oh god, she would so love to join the pony club! Luckily she is sufficiently in touch with reality to have clocked that a 40' london back garden isn't really where it's at in the pony stakes...

[tamum]

[Blandmum]

I would say in answer to the 'what cells look most 'standard'

Squamous eplithlial cells for the animal ' blob in a bag'. These are the cells that line the mouth.

For the plant cells, pallisade mesophyll cells ...the second layer of cells in a cross section of a leaf (first layer are atypical since in most plants they do not have choroplasts)

I'm glad she is enjoying this so much, making a model is great, btw, since it re-inforces that the cells are 3d not a 'flat' picture.

Tamum, I've been teaching mitosis to my sixth form this week. they had just got the concept of homologous pairs, then the concept that chromosomes only exsist post replication, and each has two chromatids totaly blasted what little understanding they had!

Oh God I have them again tomorrow!

[shewhoneverdusts]

Not wishing to hijack this thread too much, but can I please just say to MB, why couldn't you be my tutor?. I have just left my AS level human biology course because the tutor wasn't prepared to explain anything. I was doing it at night school for 2 1/2 hours a week, whereas daytime students are doing 5 hours a week, and the tutor just kept saying "we don't have time to go over anything during the lesson", so I felt a complete failure and left.

[Blandmum]

Oh, that is so sad, and so not on! How awful

Whenever a child askes a question, particularly if it is' I am confused, please explain it again' I always say thank you! Since you can bet your bottom doller that there are another 5 in the class just as confused who are not brave enough to ask!

And if we run out of time, it goes into break and lunch, and if that isn't enough I have a website for my sixth formers to ask for help from me, and more importanly each other!