I was lucky to have an extremely good Biology teacher at high school. Our teacher, Ms Lawson, taught us with such passion and commitment that were all transfixed on her and her stories during our Biology lessons. Sometimes understanding comes from hearing something explained in the right way. I hope to be able to do this for you, so that you can understand one of the great miracles of life, the cell; without which we would be just a pool of liquid on the ground!
Many people don't understand how big a human cell is relative to something like a human hair, or a molecule. So I would like to start by giving you a sense of scale. On the smallest end of the (Biological) scale are atoms and molecules. They are so tiny that you could fit around 100,000 molecules such as a water molecule (1 nanometer wide apx.) into the width of a human hair. An average cell is about 25,000 times bigger than a molecule. You can fit around 10 cells into the thickness of a human hair. A cell is just a bit too small to see with the naked eye. We can see objects around 0.4m wide, or 400,000 nanometers. A cell is around 25,000 nanometers (25 micrometers) and so we need a microscope to view them.
The Scale Of Biology
The literal meaning of a cell is from the Latin word 'cella', meaning "small room". The development of the cell was one of the very first steps in evolution of life on Earth. In its simplest form, a cell is essentially a membrane bound capsule that envelops all of the things that are needed to sustain life. It is this membrane that is absolutely key to life as it creates the boundary between life and the harsh and fluctuating external environment. We need the cell to stabilize the internal environment. Before a cell existed, Earth was a pool of materials and biological building blocks that together had the potential of life. Whilst the materials of life float around randomly, there is no way for life to form. It is like trying to make a milkshake without having a cup. You need the cup to provide form and boundaries so that the ingredients can be contained and separated from the rest.
So at its most basic, a cell is a vessel to hold and protect the ingredients of life. The cell is the simplest functional unit of biology. Despite being incredibly small, there is more going on in an cell than you can possibly imagine. To give you a sense of the miracles happening here, we can say that roughly speaking there are something like 1 Billion chemical reactions happening in one cell each second . Yes, you heard me right! So don't be surprised that scientists have only really figured out a very small percentage of what goes on in one tiny cell!
Everything that happens in a biological process happens in a cell. There really isn't much that happens outside of them. The very first cells appeared on Earth around 4 billion years ago. These were the simplest kind of cells, called prokaryotic cells, and were basically a kind of bacteria. Bacterial cells have much less going on than animal and plant cells. They have a circular DNA inside them, and have very few other internal organelles. I'll be talking more about organelles soon. Bacteria or prokatyotic cells are are unicellular (One Independent Cell) and each one is more or less the same as, or a clone, of the other. Any differences are a result of random mutations in the DNA. This mutation process happens all the time, and is one of the ways that bacteria evolve and change. One interesting appendage that bacteria have is a tail. This tail, or flagellum, is able to provide motility to bacteria so they can swim about in any kind of liquid to find nutrients and space. The flagella is another example of a miracle of life and probably the most efficient motor ever created. The tail of a bacteria can spin at up to around 18,000 rpm! This is enough to propel it quite quickly and they can dart about very rapidly. In fact, a bacteria can swim 15-60 times it's own length in 1 second! This would be like a person of height 1.8 m (6 ft) swimming at 100 km/h (60 mph)!!! Check out the the diagram of a flagellum below. Don't worry about the details, just observe how it s made of parts and elements that resemble the way in which a normal motor is made today. The only difference is that this motor is a molecularly perfect fit! It requires no lubrication, won't overheat, and runs with incredible efficiency! One day I think that we will make our motors in this way!
The Flagellum
As life evolved, so did the complexity of cells. Plant cells and animal cells are called Eukaryotic cells, and have many features not found in bacteria. Most notably is that they have many different kinds of organelles, as well as great differentiation or specialization where a particular cell is adapted to suit a slightly different purpose. Eukaryotic cells are part of multicellular organisms like plants and animals and therefore need to have different kinds of cells, such as blood cells, muscle cells, neurone cells, skin cells etc. There are around 200 different types of cell in the Human body that together can all perform the many functions needed by a complex multicellular organism.
The Cell Membrane
I would like to focus on what goes on, in general, in a human cell, starting with the membrane. The cell membrane is responsible for letting in and out whatever a cell decides it needs. You can liken the cell membrane to a very strict border crossing. Nothing gets in or out except that which the cell wishes. The cell membrane is made of a type of fat called a lipid, and some proteins. There are also very important Sodium and Potassium pumps that are built in to the cell membrane. Sodium and potassium ions are pumped in opposite directions across the membrane building up a chemical and electrical gradient for each. These gradients can be used to drive other transport processes. In nerve cells the sodium pump is used to generate electrical gradients of both sodium and potassium ions that allow it to alter the salt balance. These gradients are used to propagate electrical signals that travel along nerves. It is estimated that around three quarters of all the energy we need is used for Sodium Pumps!
The cell membrane is said to be selectively permeable because it can actively decide what to push in and out. Movement of things across the membrane also happens naturally by diffusion and osmosis. The main concept here is that the cell can alter these gradients by the use of their pumps. This allows them to maintain a comfortable internal concentration of salts, sugars and other elements that will cause toxicity if they get out of balance. This is why we shouldn't eat too much salt for example, as we can dehydrate ourselves since the salt concentration is too out of balance for our sodium (Salt) pumps to be able to compensate.
The Cell Membrane
What else can you find in a cell membrane? Some very important things! Cells identify themselves to other cells by the use of antigens that are embedded in the outside surface of each cell. They serve to help identify and classify cells. This is important in many ways, but especially for our immune system so that it can recognize its own cells and and won't attack them! Without the antigens there would be no way for our immune cells to know what is us and what is them, and we would end up destroying ourselves! Our white blood cells do this by producing antibodies that attach to a part of the antigen called the epitope. These epitopes are like fingerprints, and are created in a mindboggling process of DNA magic that is extremely hard to understand. This deactivates and marks unwanted cells ready for their destruction by white blood cells! There are many types of white blood cells (e.g. Phagocytes, macrophages, lymphocytes) that help in different ways. They are all involved in immunity and can destroy foreign cells and deal with toxicity. All white blood cells are are made in the bone marrow and then supplied to the whole body via the blood stream. The cells of the immune system, such as lymphocytes, are under highly complex control, understandably so, because they can set off the alarm that sets a full-blown immune response into motion. After lymphocytes recognize another cell their response to this recognition is influenced by dozens of further “handshakes”, a whole committee of checks and double-checks. These “checkpoints” have evolved to ensure that immune responses are always appropriate, and never occur without a good reason. Cells are VERY smart and are constantly doing more to keep us alive then we could possibly imagine! Here is an image showing how antibodies bind and mark tumor cells for destruction!
Inside a Cell - Organelles
I would like to introduce you to some of the things you can find inside a cell! You can look at a cell as the ultimate biological factory. Cells are so busy doing so many things (one billion things a second!), that they have to specialize certain areas and deal with individual process in their own micro structures and environments. These cellular structures are called organelles, which literally means 'Small Parts'. They offer the site or location of production and management of cellular processes. Organelles live in the sea of the cell, also called the cytoplasm. There are many organelles, and each one could be a post on its own! I would like to mention a few important ones. The mitochondria are very small bacteria like organelles that live in our cells. Their main role is to produce energy for us in the form of a chemical called ATP. Mitochondrion make ATP for us to use in our cellular processes, such as sodium pumps. This process is called respiration, and uses oxygen to power the process. The main reason we need to breath and take in oxygen is to allow our mitochondrion to produce ATP for us. It is thought that our mitochondrion were originally bacteria that forged a symbiotic relationship with our cells. They even have their own DNA, which is very unusual for an organelle!
Plants have a very special organelle called a chloroplast. Plants are unique in having this, and it allows for photosynthesis to happen. This is the opposite reaction to Respiration, whereby plants take up sunlight and carbon dioxide, and produce sugars and food for themselves. It is the chlorophyll inside these chloroplasts that give plants their green colour. Plants can both respire and photosynthesize depending on their needs and the time of day.
Another organelle is called a ribosome. These small slightly round structures are often attached to another organelle called the Endoplasmic Reticulum. It is at these ribosomes that protein synthesis happens. This process is another mind boggling example of biology at its best! Once proteins are made, they often head to another organelle called the Golgi Body. It is here that a proteins is folded into its usable and active shape or form so that the body can make use of it. The Golgi body might also add fats or sugars to the proteins to improve their activity. Rather than even try to attempt to describe protein synthesis in less than 100,000 words let me instead show you a very short movie that will give you a sense of the mechanical nature of this process!
Protein Synthesis Movie
The final organelle I would like to mention is the cytoskelaton. These are small but long micro filaments and micro-tubules that are made of protein. The cytoskelaton not only keeps our cells round and in good shape, but also provide a kind of railway track for other proteins to be transported around our cells.
If you want to learn more about organelles, you can watch this short 2 part movie below to learn more!
PART 1
PART 2
I hope you have found this as interesting as I have! Most people have no idea what our cells are, even though we are made up of them! In fact we are made up of around 37.2 Trillion Cells! That is more than the number of galaxies in the entire universe!!! I hope you agree that Biology is amazing, which means we are amazing!! ;-)
Please do let me know in the comments what you thought of this post, and also what detail you like me to cover in my future Biology Posts! I have tried to introduce you to the basic themes without going into too much detail. I think its important to understand the basics first so that we have a good foundation on which to build.
SOURCES
http://www.10forio.info/how-does-the-immune-system-work#question-5-how-is-the-immune-system-controlled
https://en.wikipedia.org/wiki/Bone_marrow
https://en.wikipedia.org/wiki/Epitope
https://www.genscript.com/cell_marker_antibody.html
http://sciencenetlinks.com/student-teacher-sheets/cells-your-body/
https://www.quora.com/On-average-how-many-chemical-reactions-happen-in-the-body-in-one-second
https://en.wikipedia.org/wiki/Cell_(biology)
http://book.bionumbers.org/what-is-the-frequency-of-rotary-molecular-motors/
https://en.wikipedia.org/wiki/Flagellum
https://creation.com/the-amazing-motorized-germ
https://biology.stackexchange.com/questions/13433/homework-sodium-potassium-pump
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