A Guide to Cell Structure
Cells are the basic units of function, and of structure, in living things. There are numerous cells in the body, up to estimates over about 1014, or one hundred trillion. Human beings are very complex, however it is puzzling to ponder that we humans began as one measly cell. Some organisms comprise of one single cell, one's thoughts turn to bacteria and other microscopic organisms. You've heard of the atom, right? The cell is like the 'atom' of biology. Every field of Biology, to increase studies, must deal with the cell. If one wants to learn of Biology, the natural place to start is the function and structure of cells. However, as not everyone is knowing of Biology, here is a guide of sorts to the chemical basis of life that can direct into the study of the cell:
We have combinations called compounds in Biology. Compounds are divided into two groups for classification: inorganic compounds and organic compounds. Inorganic compounds are compounds that do not contain carbon; carbon is a unique element in that it can form covalent bonds. Organic compounds, as you would guess, are compounds that contain carbon. Organic compounds are widely-studied; the study of organic chemistry is an intriguing topic. There are four sub-groups of organic compounds: lipids, proteins, nucleic acid and carbohydrates.
Lipids are organic compounds that are very oily, or waxy. Lipids generally have three uses in the human body. 1) Lipids can be used to store energy. 2) Lipids can be used as chemical messengers. And 3) Lipids can be used to form membranes. Many lipids are formed by combining glycerol with fatty acids. Fatty acids are long chains of carbon and hydrogen atoms. This formation occurs when two (or perhaps three) fatty acids are attached to the glycerol, and thus a lipid is formed. A good example of lipids in use is hibernation in animals. Specifically, animals use lipids as a means of storing energy and when animals hibernate they cease to eat, move and eliminate waste. Lipids provide the energy that the animals need, and these are stored within the animal. There are many variations of lipids; however two important kinds are the sterols and phospholipids. The most common sterol is cholesterol. Sterol lipids play an important part in the body: they have roles in building cells and carry messages across the body. Phospholipids are molecules that contain both parts that can dissolve in water and parts that can not dissolve in water. However, you may ask, how does this molecule function if it is composed of opposite parts that undertake opposite functions? Well, when water is mixed with phospholipids they transform into balloon-like structures, we call these structures liposomes. Each liposome is formed from a double layer of lipids, and the production of liposomes occurs spontaneously (Ha! Take that, Spallanzani, Redi and Pasteur!). This phospholipid bilayer of which I speak is important to the creation and formation of cell membranes.
Proteins are organic compounds that contain carbon, obviously, but also nitrogen. A protein, a complete protein at least, is comprised of a polypeptide chain. A polypeptide chain is a long chain of amino acids. Proteins are responsible for a number of things: They are responsible for making cells move! Quite an important responsibility. They also regulate pumping small molecules in and out of the cell. In addition, they assist in creating chemical reactions.
Nucleic acids are complex organic molecules. Nucleic acids are very large compounds that are comprised of a large number of smaller compounds known as nucleotides. Nucleic acids, as well as being comprised of nucleotides, are comprised of nitrogen, oxygen, carbon and other chemical substances. There are two main types of nucleic acids, and I'm quite sure you know of them well: deoxyribonucleic acid, commonly referred to as 'DNA', and ribonucleic acid, commonly known as 'RNA'. So, in case you do not know of the functions of DNA and RNA, they store and transmit genetic information...which makes them technically responsible for your life.
Carbohydrates are the molecules that we call sugars and starches. They are comprised of hydrogen, carbon and oxygen. There are many variations of carbohydrates as well, for example the monosaccharides, which includes glucose, fructose and galactose. Monosaccharides are single sugars, so disaccharides are double sugars. Disaccharides are produced by the process of dehydration synthesis, where two monosaccharides are combined to form large molecules. An example of a disaccharide is table salt. In addition, when numerous monosaccharides, as opposed to only two, are combined a unit known as a polysaccharide is formed. Starches, for example, are polysaccharides. Plants such as potatoes store an excess of sugar, but do so in the form of starch. Since there is a process to create large molecules comprised of individual monosaccharids, there is also a process where polysaccharides are broken down back into monosaccharides. This process is called hydrolysis.
Now that was some of the terms that will re-appear while discussing the structure and functions of the cell. Just to repeat from what was said earlier, cells are the basic units of structure in living things. Over the years, scientific observations and contributions were compiled into a basic theory that describes cells. Fortunately, it is not very complex and can be memorized with ease; we call this theory the cell theory. The cell theory states that 1) all cells come from pre-existing, or already existing, cells. 2) Cells are the basic units of structure in living things. And 3) all living things are composed of cells. Remember these three points of the cell theory and you've already grasped the general meaning of cells.
Goodie! On to the difficult stuff! Upon looking at animals, plants and other organisms we can note many many many differences; however in one way we relate is in the structure of our cells. All plants and animals have three basic structures within their cells: The cell membrane, or the outer borders of the cell, the nucleus, or the center where the cell functions and is controlled, and the cytoplasm, the material that is in between the membrane and nucleus. The cell membrane separates the cell from its surrounding. In a way, it is like the walls that surround a house. The walls basically keep the whole structure of the house stable, and they protect the insides of the house from the outsides. This is the same for the cell membrane. Moreover, messages, fuel and waste are carried in and out of the cell membrane, just as the walls of a house have telephone, television and electric lines. Also, just like a house, we need a door to carry out and take in waste and food. All of these are present in the cell membrane; and remember the example of the walls of a house to facilitate your understanding of the cell membrane concept. Delving deeper into the cell membrane, the cell membrane is composed of lipids with a double layer of lipids forming the basic unit from which the cell membrane is constructed. In addition, carbohydrates and protein also relate to the cell membrane in that some proteins manage to stick to the surface of the double lipid layer; these proteins can channel molecules to pass through the cell membrane. The said carbohydrates act as chemical identification cards, meaning they permit cells to recognize and interact with others. These carbohydrates usually get attached to either the proteins or the lipid layer. In organisms such as plants and some types of bacteria, a cell wall is present. The cell wall holds similar responsibilities to the membrane in that its job is to help protect and support the cell. The cell wall is composed of cellulose and is porous since materials such as water can transverse through the cell wall with relative ease.
Next, the nucleus. Organisms can be divided into two basic groups: prokaryotes and eukaryotes. The classification of these two groups depends on the absence or presence of a nucleus in the organisms' cell. Many unicellular organisms and bacteria have not a nucleus; so these organisms are referred to as prokaryotes. A little trick I use to draw a difference between the two is remember that 'pro' for prokaryote rhymes with 'no' meaning that the organism lacks a nucleus. The responsibilities of the nucleus are dire. The nucleus has been described as the 'information center' of the cell; it retains this reputation because the instructions to make thousands of molecules are found, and executed, in the nucleus. In basic terms, the nucleus directs all the activities that may occur in the cell. Molecules moving into the nucleus most pass through nuclear pores that are known as the structure of the nuclear envelope. The nuclear envelope is in the form of two membranes of sorts that surround the nucleus and regulate and facilitate the movement of molecules in and out of the nucleus. In addition to the nuclear envelope, a nucleus usually contains a small region known as the nucleolus. The nucleolus is made up of ribonucleic acid (RNA) and proteins; it is in this structure that ribosomes are produced. Ribosomes aid the production of protein in the cell. However since the ribonucleic acid is located in the nucleolus, where would the deoxyribonucleic acid (DNA) be? The answer is that DNA is locared on structures we call chromosomes.
As stated earlier, the cytoplasm is the material in between the cell membrane and nucleus. Inside the cytoplasm are numerous structures that are of monumental importance to the functioning of the cell. These structures are collectively called organelles and they perform a designated function for the furthering of the cell. Two of the most important of these organelles are the mitochondrion and the chloroplast. The chloroplast inherits and traps sunlight and other forms of energy and transforms it into chemical energy. The mitochondrion is important because it alters the chemical energy from the chloroplast in the cell into compounds which are better suited to the cell. The structure of the mitochondrion and the chloroplast is quite similar; they both contain two unique membranes. The first membrane makes up the outside of the organelle and the second membrane is a plethora of folds located inside the organelle to increase the overall surface area. Innovative scientific discussion has been spurred over the topic of whether the mitochondrion and the chloroplast may have been descended from independent organisms since these two organelles have a degree of independence within the cell. Another organelle, and one mentioned earlier, is the ribosome. Ribosomes are composed ribonucleic acid and proteins and produce protein themselves. Ribosomes are located both in the membrane and in the cytoplasm and are one of the smallest organelles. All of these is key to the functioning of the cell, but how do materials move in and around the cell? The endoplasmic reticulum is a complex group of small little sacs that transport materials to and fro in the cell. The endoplasmic reticulum is divided into two groups: the smooth endoplasmic reticulum and the rough endoplasmic reticulum. The difference is that the walls of the smooth endoplasmic reticulum, well, appear smooth. In addition, little granules, a type of particles, are not connected to the smooth endoplasmic reticulum while on the other hand the rough endoplasmic reticulum does have little granules studded onto itself. The roles these two organelles have is to 1) store chemicals (this is the smooth endoplasmic reticulum's job) and 2) synthesize proteins (accordingly, this is the rough endoplasmic reticulum's job). Next we have structures call lysosomes. Lysosomes contain chemicals that enable it to digest or break down smaller particles; this is precisely the burden of the lysosome...to dispose of or break down foreign particles. However, lysosomes also break down fellow organelles but only when the said organelle is losing its usefulness. The lysosomes are vital to the cell because they in a way dispose of the junk that could accumulate and be a future burden for the functioning of the cell. Although present in animal cells, the lysosomes are not present in plant cells. Exclusively in cells, we have an organelle named the vacuole. The vacuole makes it possible for plants to support large structures such as flowers and leaves. This is because the vacuole is the structure that stores materials the cell sourly needs such as water, proteins and carbohydrates. Finally, we have the cytoskeleton. For quite a bit of years, scientists imagined of a material that was called protoplasm that roamed the cell; the protoplasm supposedly held unique charateristics filled the cytoplasm. However as scientific studies carried on we have discovered there is actually a cytoskeleton at work. The cytoskeleton is composed of microtubules. These microtubules are a life-line of the cell in that they provide and maintain the very shape of the cell. In addition, the cytoplasm has a property of microfilaments that aid with the movement and function of the cell.
As you can see, the inside workings of the cell are comprehensive to understand, and even more so to memorize. I hope this helped a bit, if not, I apologize for possibly giving your head a head-ache.
Works Consulted:
The excellent work of Kenneth Miller, Ph. D, Professor of Biology, Brown University
And the excellent work of Joseph Levine, Ph. D, Assistant Professor of Biology, Boston College
And last, but not least, Wikipedia. Despite some rumours on the credibility of Wikipedia, specifically the scientific section contains thorough information..and correct information.
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