The first thing you need to know is that there are two basic kinds of cell: Prokaryotic and Eukaryotic...
Prokaryotic Cells:
Prokaryotic cells are found as single-celled organisms such as bacteria. Things that live on their own. They are different in the fact that:
- They have less organelles,
- They have different types of organelle (like plasmids - more about them later...),
- They are smaller in size,
- They are less complex,
- They don't have a nucleus,
- They have smaller ribosomes (more about them later...),
- They don't have mitochondria,
Eukaryotic cells work together to build up multi-celled organisms like you, or plants, or your pet cat or jellyfish. They are different in the fact that:
- They have more organelles,
- They are larger,
- They are more complex,
- They make up bigger organisms,
Let's just classify the four things that differs plant cells from animal (epithelial) cells:
- Plant cells have a vacuole, animal cells do not,
- Plant cells have chloroplasts, animal cells do not,
- Plant cells have a cell wall, animal cells do not,
- Plant cells have plasmodesmata (see below), animal cells do not,
Other than this, animal and plant cells have exactly the same organelles.
Okay, so what actually is an organelle (other than a good name for a cat)? Here's a nifty definition from Google:
"any of a number of organized or specialized structures within a living cell."
Yep! So, within you, you have organs, within them are cells, and within the cells are organelles! You are going to have to learn all of them (sorry - but it's actually ok!) and there's about 21. Well, no, I will talk about 21 things you find in a cell (starting from most complicated to least) and chances are they'll be useful in the exam. Okay... Let's go!
Chloroplasts!
Yes! Chloroplasts! You'll hopefully remember them from GCSE and know that they are found only in plant cells and use chlorophyll to carry out photosynthesis. If not, well then now you know! However, our chloroplast is a lot more complex than it seems...
- Chloroplasts have a double membrane (the books seem keen you mention this),
- Grana and lamellae (plurals of a granum and a lamella) are made of the effective 'third membrane' of the chloroplast called thylakoid membrane,
- The stroma is a thick fluid inside the chloroplast,
- Grana are linked by lamellae,
- Lamellae are thin, flat, fluid-filled sacks,
- Photosynthesis is carried out in the grana and the stroma,
Sounds cool - is cool. Here's what one looks like:
- The Golgi Apparatus is a group of fluid-filled, flattened sacs,
- It packages new proteins and lipids,
- It processes new proteins (protein synthesis) and lipids,
- It makes lysosomes (see below),
- It makes vesicles (see below),
You've probably heard of these, and know they carry out respiration. Time to find out about their innards! Here's what one looks like:
There are another 5 things to know about mitochondria:
- Carries out aerobic respiration,
- Has a double membrane,
- The inner membrane is all folded to make 'structures' called cristae (singular = crista),
- Within the cristae is the matrix (no, not from the film!) and that is where respiration takes place,
- Mitochondria provide energy so lots are found in cells that need a lot of energy - like muscle cells for example.
It's not just a blob... Here's a closer look:
Again, 5 things to remember:
- The nuclear envelope is a double membrane,
- The nucleus is filled with a fluid called chromatin and it's made of DNA and proteins,
- Some nuclei have a nucleolus which makes ribosomes but not all,
- The nuclear pores (gaps in the nuclear envelope) act like gates and allow substances like RNA to move between the cytoplasm and the nucleus,
- The DNA in the nucleus is in charge of controlling the cells activities,
The Cell Membrane!This encompasses the entire cell and all the organelles inside it. Don't get confused about organelles having their own membranes - the cell membrane is like the king of all membranes!
4 things to remember this time (it's getting less complicated now...)!
- Found in all cells - all kinds of cells! Even the bacteria ones!
- Made of lipids and proteins,
- Regulates the movement of substances in and out of the cell,
- Contains receptor molecules on the surface that respond to chemical signals - like hormones and stuff,
These organelles keep getting sillier and cilia! Sorry - bad joke - I'll move on...
4 things to remember about these...
- They're found on a variety of specialised cells, like the ciliated cells in your nose when you sneeze, or in your lungs when you cough - they help brush the mucus out,
- They contain a ring of 9 pairs of microtubules (hollow protein thread things) with two in the middle,
- They are covered by the cell's membrane (i.e. don't have one of their own),
- The microtubules allow the cilia to move,
Flagella!
Often you associate a flagellum to be on a bacterium (though not all bacteria have them) but don't forget they can appear on some animal cells too - namely sperm.
4 things for these...
- Just like cilia, they have the same formation of microtubules - allowing them to move,
- Just like cilia, they are covered by the cell's membrane,
- Bacterial flagella have a different internal structure,
- The flagella acts like an 'outboard motor' and rotates in such a way that it allows the cell to move forward,
Plasmids!
Now, I don't really know if these are counted as an organelle, but they certainly are worth knowing. They're only ever found in bacterial cells, but even then, not all bacteria have them.
4 things to remember about plasmids...
- They are tiny loops of DNA found in the cytoplasm of bacterial cells,
- They're not actually a part of the DNA of the bacterium itself,
- They can be passed between bacteria (like a text message!),
- They contain genes for things like antibiotic resistance,
The Rough Endoplasmic Reticulum!
Wow! Big, confusing name - surprisingly simple job. Here's what one looks like:
4 things to remember about the rough ER...- It's 'roughness' comes from the fact it's covered in ribosomes (the little dot things),
- It's made of a load of fluid-filled membranes - much like the Golgi apparatus,
- It processes proteins (namely the ones made by the ribosomes attached to it),
- It makes vesicles (see below),
Hopefully you should remember this one from GCSE. This one's easy!
Just 3 things to remember here! It's getting easier...
- The cell wall provides structure and shape,
- It's found in plant and bacterial cells,
- In plants, it's made of the carbohydrate, cellulose, but in bacteria it's made of something else,
These are quite cool and very, very easy to draw!
- Lysosomes have no clear internal structure,
- They consist of a single membrane enclosing some digestive enzymes,
- These enzymes can be used to break down worn-out parts of a cell (like the cell's very own dustbin!) or kill and digest invading cells (like pathogens),
Surprisingly simple given they have such an important job!
- They carry out protein synthesis (making proteins. Proteins do pretty much everything in your body so these are unbelievably important!),
- They're found either on the rough ER or free-floating (just chillin') in the cytoplasm,
- They're made of a large and small subunit,
Another long-worded one! Here's what it looks like:
3 things for the smooth ER:
- Synthesises and processes lipids (fats),
- It's made of membranes filled with fluid (much like the rough ER),
- Makes vesicles (see below),
The cell kingdom's favourite post-person!
3 points for this tiny thing:
- A tiny fluid-filled sack,
- Transports substances between organelles and in and out of the cell membrane (like a post-person),
- Made by the Golgi apparatus, both endoplasmic reticulums and at the cell surface,
Simply bacterial goo! The capsule in the picture is the wobbly, 'gooey stuff' around the bacterium. Again, I'm not entirely sure it's counted as an organelle - nevertheless, it's fairly interesting.
- It's made of secreted slime,
- It protects certain bacteria (not all types have a capsule) from attack from white blood cells (so the bacterium can continue making you ill! Grr...),
I haven't fully worked out how these work yet, but I'm sure all will come clear when I cover mitosis. Here's what they look like:
2 things here:
- Centrioles comprise of a ring of hollow cylinders containing a ring of microtubules (microtubules are the things inside flagella and cilia),
- They are involved in the separation of chromosomes during cell division (though I have no idea how yet),
You only need to know this for AQA but it's still fairly interesting!
Just 2 points again:
- Microvilli are folds in the cell membrane,
- Like most folds in biology, they increase the surface area through which substances can be exchanged,
Ooh - spell check doesn't like this one! It's a brilliant word, though! Only found in plants, this one!
2 points here:
- Plasmodesmata are gaps in the cell wall of plant cells,
- They allow the cells to exchange substances with adjacent cells,
Not entirely sure this counts as an organelle - but you need to know it! This one is also incredibly easy to draw...
Just one point now!
- Bacterial DNA is simply one, large, squished-up circle of DNA acting as a substitute nucleus (bacteria don't have nuclei),
Again, this is not an organelle but is very important and found in all cells!
- The cytoplasm is the substance in which all organelles are suspended,
Easy peasy!
1 point:
- A vacuole is a big bag of sap found in all plant cells,
That's all 21 of the things I wanted to talk about! Now, I got most of my facts from the brilliant CGP AS-Level biology book which I thoroughly recommend getting and I'm hopefully correct on most things. If I'm not, please let me know because I hate being wrong! Also, if you want to know what each organelle looks like and where it is in a cell, you can find images on Google for complicated cell pictures.
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