All of the information on this and the related pages (that all look like this one) should be review information. I hope nothing is startlingly new here.
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In order to understand cells, the smallest living units, one must understand the components that make them up: macromolecules. Macromolecules, or very large molecules, are not living, but they are responsible for assembling in various ways to create the parts of the cell that you know about, like the membrane or the ribosome. Macromolecules are made by putting smaller molecules (typically, monomers, also called building blocks) together. These smaller molecules are made up of many atoms stuck together by covalent bonds. So, really, one must understand atoms, bonding, small molecules, and also macromolecules in order to understand cells. In this page, and in those pages linked from this one, you will be working on understanding these simple, chemical concepts.
I am focussing here on the macromolecules that make up the cell, and am only going to give you minimal information on inorganic chemistry (like atoms, elements, and bonding). I have to assume that you know that inorganic information from your chemistry prerequisite, or that you can refresh your memory from the information in our book or from other web pages (like the chemistry portion of the Virtually Biology Course), included as links below.
All of matter is made up of chemicals. Our whole world is a big compilation of chemicals. Not all of these chemicals are used to make living things. But even inorganic chemicals (those that are not used to build living things), are important in our living world. The best example of this is water. Water contains only oxygen and hydrogen atoms; it does not contain any carbon atoms, so it is only an inorganic molecule. However, every living cell on this planet contains water within it and requires water to continue living. Water is important for life, but, like oxygen gas, cannot build the components of cells. To build our cells, we need organic molecules. But we also need to understand those inorganic molecules that are important in our lives, like water and oxygen gas.
Everything that takes up space and has some mass is matter. All matter is made up of elements. There are many different elements, all listed in the periodic table, but we only use a few of these elements in abundance in our bodies. The main elements we need to be concerned with for our organic molecules are: SPONCH. Another way to remember them is: CHNOPS (kind of like the drink, schnapps). There are other elements that do not build organic molecules but have large roles in biology, like: Na, K, Ca, and Cl.
I'm only giving you minimal information on inorganic chemistry here, but you can find other web pages that have that information. One good page to start with is a kids' page, called Chemistry 4 Kids. There are also other pages with some general information, like this page from Arizona.
All elements are made of atoms. All atoms are made up of protons, neutrons, and electrons. An oxygen atom is different from a carbon atom, though. That is because each element has a different atomic number-- which is the number of protons (and typically also neutrons) and electrons that it contains. So, an oxygen atom contains 8 protons (usually 8 neutrons) and 8 electrons, whereas a carbon atom only has 6 of each, since that is its atomic number.
It is the number of electrons in an atom that determines its properties. The protons and neutrons are found in the atomic nucleus, but the electrons are spinning madly around that nucleus, in constant motion. These electrons are distributed in various ways around the nucleus, but for the elements that we will be concerned with, we can just consider that the electrons are spinning in spherical orbits around the nucleus. The closest orbit to the nucleus is the first energy shell, the next farther out is the second energy shell, etc. Only 2 electrons fit in the first energy shell, while 8 fit in the second and third energy shells. If the outermost energy shell is full, the atom is stable, or unreactive. However, the atoms we care about for biological purposes do not have full outermost shells, so they are reactive! They will react with other atoms to make chemical bonds and become stable as molecules.
There are two main ways that atoms can bond to each other to make larger molecules: covalent bonds and ionic bonds. COVALENT BONDS ARE THE STRONGEST BONDS! Covalent bonds form molecules that remain stable in water. Atoms bond covalently by sharing electrons.
Covalent bonds can be polar or nonpolar. Polar covalent bonds occur when atoms of different elements bond together, like oxygen and hydrogen (as in water). Having polar covalent bonds gives molecules polar properties so that they are more likely to by hydrophilic (able to interact with water molecules). If molecules contain polar covalent bonds, they tend to be able to make hydrogen bonds with other molecules (see below).
Ionic bonds are weaker, and fall apart in water. Ionic bonds occur when atoms give away or take electrons from other atoms. Since electrons are negatively charged, when an atom (like sodium, "Na") gives away one electron, it becomes less negative. In the case of Na, after giving away one electron (one negative charge), it becomes stable but now is charged with one positive charge; it is called an ion after it becomes charged. With Na, it becomes the sodium ion, written as Na+. The electron cannot just be lost, it has to be given away to another atom, like chlorine, or "Cl." The Cl atom becomes stable after receiving one electron, but it also becomes negatively charged, since it took one negative charge. The chlorine atom is changed to the chloride ion after taking on the electron, and is written as Cl-. Since the Na and Cl atoms had to be close to do this transfer, afterward, the positively charged Na+ and the negatively charged Cl- are so close together that their attraction for one another causes them to stick together. That attraction of oppositely charged ions is an ionic bond. But, IONIC BONDS FALL APART IN WATER.
You have learned about hydrogen bonds before, I'm sure. These are bonds, not between atoms in order to make a molecule, but between molecules, allowing for molecules to interact with one another. When molecules are piled together, like millions of water molecules in a glass for you to drink, they do not randomly pile on top of each other. Instead, molecules orient toward one another in certain ways, and this orientation helps determine the properties that the piles of molecules will have. The way that they orient toward each other is determined by their hydrogen bonding.
NOW VISIT THESE OTHER SITES FOR MORE INFORMATION ON BONDING IF YOU NEED MORE OF A REVIEW: (keep in mind that these pages may go into more technical detail than you need)
Because water is such an important molecule to understand for biology, I have written a separate page on it. Go visit the water page!
Once we discuss molecules that contain both carbon and hydrogen, we are looking into organic chemistry-- that chemistry that is involved with understanding molecules found in or produced by living organisms. There are many molecules we could discuss that fit into this category, but we will just examine those molecules that will be involved in building cells. (You can also visit the biochemistry portion of the Virtually Biology Course).
There are four major macromolecules: LIPIDS, CARBOHYDRATES, PROTEINS, NUCLEIC ACIDS. On this page I am just giving you a little introduction to each one. But there are separate pages which go into a bit more detail for those of you who need more of a review.
In order to build a macromolecule, smaller molecules must come together and join through covalent bonds. This requires that a chemical reaction occur. Any chemical reaction that builds larger molecules can be called an anabolic chemical reaction, while those that break down larger molecules into smaller ones is called a catabolic chemical reaction. (memory tip: "cat"-abolic sounds like "cut"-abolic).
Since in the human body all chemical reactions are taking place within watery solutions, the anabolic and catabolic reactions that are taking place typically involve water. During building reactions, molecules typically join together such that their joining releases a water molecule; you may have heard of this type of chemical reaction as a condensation reaction or a dehydration synthesis reaction (whichever term you know). In order to break a large molecule, a hydrolysis reaction is required ("hydro-" for water, and "-lysis" for breaking). These reactions are summarized on pages 102- 103 of your textbook and in the figure below.
This is a good place to start understanding how macromolecules form. We have different types of lipids in our bodies, but two types, triglycerides and phospholipids form when glycerol and fatty acids join together. (The other type of lipid, steroids, will be discussed on the lipid page). A triglyceride forms when glycerol undergoes dehydration synthesis reactions with each of 3 fatty acids. After that, they are stuck together. This is shown in Figure 4.2 in your book, on page 103.
A basic principle of the cell: acquire small molecules (monomers); then assemble these into much larger molecules (polymers). The general scheme for this is shown in the figure below:
From this, you should notice that if you understand the monomers for each macromolecule, you will understand most of the macromolecule itself. The covalent bonds in the above drawing form when dehydration synthesis reactions occur between monomers.
To see more on each of the polymers above, go to their pages. You can link at the top of this page, or by clicking on any of the polymer names above.
© 2011 STCC Foundation Press