A Rant About Cells.

No one warned me that A Level Biology was going to be this hard. I’m shocked and amazed that I managed to drag myself through my homework this morning. So here I am to revise. I’d like to think I’m on my blog, but my internets down so this is just in Word until it’s fixed and I can put it up. So there.

I’m gonna go over Cell Structure today because it’s what I’m struggling with the most; I’ll probably do Neucleic Acids another time.

Okay, so cells are the basic unit of life, and there are two types. Type one is Eukaryotic, which are cells that have a nucleus, such as plant and animal cells. Type two is Prokaryotic, which are cells with no nucleus, such as bacterium. There are two types of microscopes we can use to look at cells – light microscopes and electron microscopes. We can see in a much finer detail with an electron mircoscope because the wavelength of electrons is shorter than the wavelength of light. Electron microscopes revel the fine structure or the ultrastructure of a cell, which is what I’m going to be going into more detail about in a moment.

Okay, so a Eukaryotic cell is surrounded by a plasma membrane, and is filled with cytoplasm. I’m trying to think of a good anology for this, but all I can come up with at the moment is a blister or a water balloon. Oh, and there are structures inside the cell which are called organelles.

These organelles can be divided into two groups –

Structures composed of or surrounded by a membrane:

•          Nucleus
•          Chloroplast
•          Mitochondrion
•          Endoplasmic reticulum
•          Golgi apparatus
•         Lysosomes

And structures not composed or surrounded by a membrane:

•        Ribosomes
•          Centrioles

And now I need to go into detail about all the cell boundaries before I can go into detail about organelles.

So then. The most obvious cell boundary is probably the cell membrane, but for A Level it’s been renamed plasma membrane. Now, the term ‘unit membrane’ is a term used to describe all the following:

•          Plasma membrane
•          Membranes of the nucleus, mitochondrion, chloroplast, endoplasmic reticulum…etc
•          And to describe the structure (see section 1.3)

What’s important to remember is that membranes within the cell cordon off areas where this is no cytoplasm so other things can be stored such as harmful chemicals or enzymes. They’re kept within membranes so they can’t damage the rest of the cell, sort of like when they put someone in isolation at school, its so they can’t infect the rest of the student body.

Also, membranes within the cell provide a huuuuuuge surface area for various things such as the attachment of enzymes (eg. in the synthesis of ATP in mitochondrion membranes), and guess what, that’s not all they do. They also provide a transport system within the cell (eg. endoplasmic reticulum).

One final note on the plasma membrane – microvilli are infoldings of the membrane and are used to increase surface area (eg. in the digestive system).

Now then, the cell wall. It’s fully permeable, so pretty much anything can get through it. It’s like putting a wimp in charge of entrance to a nightclub, anyone’s gonna get in. But, really, that wimp is made of tough fibres embedded in the matrix of polysaccharides (and other sugars). But, despite letting anyone and anything in, the cell wall does keep the cell nice a strong and rigid. On the downside, there are a couple of gaps in the wall, AKA pores, which allow cytoplasmic channels called plasmodesmata to flow between cells.

Now those are all done with, I can get on with all the different organelles. These are the little bits and pieces you get floating around in the cytoplasm. Like the little bits you get in non-smooth orange juice.

So, most importantly is the nucleus. It’s so important because it regulates all the functions within the cell. Essentially it’s the brain of the cell, but I’m not allowed to say that in the exam. Like the huamn brain is housed inside the skull, the nucleus is housed in a a nuclear membrane or envelope. This membrane, like many others I’m going to mention, is a double membrane.

The outer membrane is joined up with the endoplasmic reticulum and possess ribosomes, wheras the inner membrane has nothing to do with the outside world. Both the membranes fuse every so often to make pores so things can get in and out.

Inside the nucleus is an interesting place – it’s filled with a material called nucleoplasm (gotta give it to those Biologists, they’re original with they’re naming of things). Anyway, there are some structures within this nucleoplasm. Joy, there are just structures within structures within structures. I can’t cope. But must plough on.

Right, so what does the nucleolus do? For one thing it makes RNA and assembles ribosomes. The nucleolus is also the home of all the little chromosomes, which contain hereditry DNA and are pretty much the instruction manuals for the cell.

And that’s pretty much it for the nucleus. Ready to move on? No? Too confusing? I’m gonna say tough, just like my teacher did….

So then, the mitochondrion provides the energy for the cell to do all its bits and pieces. They’re rod shaped and vary in size (0.5µm wide and 7µm long). You can find the mitochondrion doing the most work in active cells such as in the liver, muscles and kidney, because these cells require the most ATP (energy).

Again, this is a double membrane organelle. The outer membrane is smooooooooth, whereas the inner is folded to form cristae. This is needed for a large surface area for enzyme attachment. Cristae are also the home of Oxidative phosphorylation (a stage in aerobic respiration). Oh, and the cristae also have stalked particles where the enzymes for ATM synthesis are kept.

The inner membrane has just as much action as the outer – it contains ribosomes, a loop of DNA and enzymes.

And that’s a neat little way of finishing off the mitochondria.

Next up are chloroplasts, which everyone knows are involved in photosynthesis in plants. They convert light energy into chemical energy which is stored in food molecules for the plant to gobble up. Chloroplasts are disc shaped and are like tinnnnnny, I’m talking 1-4µm in diametre and 1µm thick. Oh, and they’re green because of the presence of chlorophyl, so there’s no need to stain the cell if you want to see them with a light microscope.

This is another organelle with a double membrane, but unlike the mitochondrion, both membranes are smooooooth and have no folds. Inside the inner membrane is the stroma, where DNA, ribosomes and starch grains are stored. Like the nucleus, this is an organelle with bits and bobs all inside of each other. Inside the choroplast there are internal membranes called thylakoids (when you stack them together you have to call them a granum). These thylakoid thingies increase surface area for the attachment of chlorophyll pigments.

Oh, and chloroplasts also have enzymes for the light-independent reaction of photosynthesis.

As you might have noticed, chloroplasts and mitochondria were quite similar, but what’s next are rather different.

Ribosomes are these tiny little dots that seem o just float about a bit. They’re actually very small particles of RNA and are made in the nucleolus. They’re made of two sub units, the light unit and the heavy unit (if I look back in my notes I can see the measurements because I can’t be bothered to figure out how to type that little symbol for microns again. Anyway, ribosomes are found floating free in the cytoplasm and boung to membranes such as the rough endoplasmic reticulum. These guys are kinda like that person you can always see wandering your local shopping centre. They’re always there, just bopping around with not much to do. BUT! Ribosomes do have something to do. They make protein. Dun dun dunnnnnnnn.

And that’s all I have to say on ribosomes.

Endoplasmic reticulum however, is going to take a long time. Basically, the ER is a network or membranes running through the cytoplasm of every cell. The places between the membranes are called cisternae. There are two types of ER, rough ER and smooth ER, so to save myself the bother of witty prose, I’m gonna bullet point on each type.

Rough ER:

·         Ribosomes (which make it rough)

·         Involved in making and transporting proteins

·         Very prominent in enzyme secreting cells, such as the pancreas

Smooth ER:

·         No ribosomes

·         Involved in making and transporting lipids (fats) and steroids

·         Detoxifies poison

·         Extensive in steroid hormone-secreting cells, such as the testes

Magically, that’s the endoplasmic reticulum all done and dusted.

Now, lets tackle the golgi body/apparatus/complex. This is an organelle made of membranes, they’re all curved and stacked one on top of the other. The spaces between these membranes are called the cisternae. And basically, what the golgi body does is act like a post office, in that it receives, sorts and delivers loads of different molecules.

The two main functions of the golgi body are; sectretion, when it produces secretory vesicles, and ; intracellular digestion, when it produces lysosomes.  How does it do these things? At this point I should be flicking forward a few pages in my file to find Figure 5, 6 and 7 because they talk me through the whole thing in laymans terms. Well, ish.

Now, in the whole golgi body process thingy, something called vesicles are mentioned. These are small membrane bound structures and they come in two types – secretory vesicles and lysosomes (funny, where have I heard those terms before? *Note: in a sarcastic tone*)

Lets talk about these two different types of vesicles. The secretory vesicles contain (for example) mucin and are produced by cells lining the gut, reproductive and respiritory systems, or (for example) digestive enzymes which are produced by pancreatic cells. The contents of the secretory vesicles is determained by the type of cell.

Lysosomes on the other hand, are not usually released from the cell and contain digestive enzymes.

Woo, only two more organelles which I haven’t covered yet, but I’ll get back to those as soon as we’ve done then in class J

I’m back, finally, because we’ve finished Eukaryotic cells in class, so I’m just gonna go over vacuoles and centrioles.

Vacuoles are different in plants and animals so I’ll just list them.

Plants;

·         Biggggg permenant vacuole

·         Single membrane called a tonoplast

·         Contains a fluid called cell sap which contains chemicals like glucose

·         It provides support of young tissues

Animals;

·         Large numbers of small tempory vacuoles/vesicles

·         Functions: taking up food and removing waste

NB – Vacuoles are larger than vesicles, but otherwise there’s no structural difference.

Now, finally centrioles. They’re only found in animal cells and are basically hollow cylinders and come in pairs. The separate from each other during the early stages of mitosis and are associated with the separation of chromosomes during cell division.

And that’s all I need to know on Eukaryotic cells. I’m dreading Prokaryotic, if I’m honest. Now this is being posted to the blog.

Wowwww, this is long.