Everything You've Ever Wanted to Know About Carbohydrates!

Diagram of a glucose molecule.

You've probably heard quite a bit about carbohydrates.  When dieting you avoid them like the plague; they're in pastas, pizzas, and many other delicious Italian dishes.  But what, in fact, is a carbohydrate?  Carbohydrates are defined as molecules that are some form of sugar (either a polymer or monomer).  These sugars are composed of three elements:  Carbon, Hydrogen, and Oxygen in a 1:2:1 ratio. The carbohydrate most commonly found in nature is Glucose, with the molecular formula C6H12O6 (See right).  Carbohydrates perform several key functions for cells.  They store energy and they provide structure for living organisms.  The simplest sugars, such as Glucose, are referred to as monosaccharides.  Two monosaccharides can be combined to form a molecule called a disaccharide.  The process by which this occurs is called dehydration synthesis.  In dehydration synthesis, one monosaccharide donates a hydroxide ion (OH-) and another donates a hydrogen ion (H+).  These two ions form one molecule of H2O.

The result is that the two monosaccharides are held together in a glycosidic linkage.  Many monosaccharides can be combined in this way to form a polysaccharide.  Polysaccharides are the molecules that we usually think of as "carbohydrates".  They are composed of chains that have hundreds or even thousands of monosaccharides joined together via dehydration.  Interestingly enough, these chains can be broken apart using a process named hydrolysis.  Hydrolysis is, in essence, the opposite of dehydration.  In hydrolysis, the bond between two monosaccharides is broken by introducing a water molecule.  Energy is released, and the glycosidic linkage dissipates.  This is how carbohydrates store energy, by creating large polysaccharides when energy is abundant and breaking up the polysaccharides when energy is scarce.  

There are four common types of carbohydrates found in nature, described below:

1.  Starch

Potatoes, a common starch

Starch is an energy storage polysaccharide found in plants, specifically in their granules. (Want to know more about cell parts?  Click here.)  These molecules are spiral shaped, allowing for more efficient and compact storage.  The presence of starch enables a plant to stockpile its excess glucose and use it later for energy.  Starch can be broken apart by both humans and animals for energy.  Starch is, in fact, composed of two distinct substances.  The first is known as amylose.  Amylose, making up 20% of the molecule's composition, is soluble in water and has a linear shape.  Amylopectin makes up the other 80%.  Amylopectin is branched and, for the most part, not able to be dissolved by water.  Examples of starch include corn, rice, and potatoes.  

2. Glycogen

Glycogen is starch's animal counterpart.  Mostly found in the liver and muscle cells, glycogen is essential to an animal's well-being.  Without glycogen, normal body conditions cannot be maintained for long periods of time.  In fact, human beings must eat some kind of food with carbohydrates, otherwise glycogen stores will be depleted and muscle capabilities will be decreased.

3.  Cellulose

A cross-section of wood.

Cellulose provides structure to plants.  It is a straight, unbranched molecule.  Its components are held together by hydrogen bonds and mibrofibrils.  Cellulose is most commonly found in the cell walls of plants, and is the most commonly found organic compound on the planet.  Interestingly, cellulose is a polymer of glucose, with different glycosidic linkages than that of starch.  Hence, it has a flat shape while starch has a helix shape.  Cellulose is the main component in wood.  Some species of animals, such as termites, have special enzymes that enable them to digest this substance, however humans cannot digest it.

4. Chitin

Chitin is the structural carbohydrate found in many animals.  It is present in the exoskeletons of insects, and the cell walls of funguses.  Chiten has beta linkages with nitrogen attachments, creating its hard, tough surface.  

Sources:

Campbell Biology 9th AP Edition

http://www.medicalnewstoday.com/articles/161547.php

http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/carbhyd.htm
http://www.mansfield.ohio-state.edu/~sabedon/068dhsyn.gif
http://homebrewsake.com/wp-content/uploads/2010/04/glucose.gif
http://stemlynsblog.org/wp-content/uploads/2012/07/starch.jpg
http://upload.wikimedia.org/wikipedia/commons/thumb/0/0b/Taxus_wood.jpg/300px-Taxus_wood.jpg

The Properties of Water

Water is essential to the human lifestyle. Not only are we made up of water, but we drink it, wash with it, and can use it to create electricity. Water has many different properties, which contribute to the ways that we use it today.

Polarity
Polarity is a phenomenon that occurs in water because water is created by polar covalent bonds and hydrogen bonds. In a polar covalent bond, the atoms bonding do not share the electrons equally. Instead, the larger atom will pull the molecules farther towards itself. This gives the molecule a slightly positive side and a slightly negative side. The positive side of one water molecule is attracted to the negative side of other water molecules and objects and vice versa. This allows the molecules to stick together and stick to other things.
Cohesion
Cohesion, when referring to water, means that water molecules like to stick together. The cohesion happens because the molecules have polarity, and the negative side of one molecule is attracted to the positive side of another molecule, so on and so forth.

Adhesion
Adhesion is similar to cohesion. Adhesion means that the water molecules are attracted to other materials. This comes again from Polarity. If the material has a positive or negative charge, the water molecule will stick to it. An example of Adhesion is an experiment that we performed in Biology. In class, we took a balloon and charged it with static electricity by rubbing it against our heads. We then put this balloon near a stream of water. The water was drawn away from its path to the ground and towards the balloon.

Surface Tension
Surface tension is the tension created by the polar molecules in water pulling on  each other. This tension gives water a slightly taut surface. If something is not heavy enough to break the surface tension then it will float on top of the water. We experimented with surface tension in the lab with water and a paper clip. When the paper clip was horizontally on the surface of the water its weight was evenly distributed. Thus, it was not heavy enough in one place to break the surface tension of the water and remained floating on the top of the water.


Capillary Action
Capillary Action is best shown in a straw. Because of their polarity, water molecules are attracted to the sides of the straw. This allows them to slide up the straw and to the top.

Specific Heat of Water
The specific heat of any substance is the heat needed to raise the temperature of one gram of the substance one degree Celsius. Water, unlike many other substances, has a very high specific heat. The specific heat of water is so high because the attraction between molecules is strong. Thus, it takes more energy to break the bonds apart and allow the water to heat up. Similarly, water takes a long time to cool down. An everyday instance of water's high specific heat is the beach. The water at the beach is warmest in the end of the summer because the sun has been heating it up all summer.

Universal Solvent
Water is called the universal solvent because many different things are able to dissolve in water. When mixed with other polar substances, the polar water molecules attract the polar molecules of the other substance and mix together. Water is able to dissolve myriad different liquids, gases, and solids. It will not, however, mix with non-polar substances, such as oil, because the molecules are not attracted to each other.

Value of Water Density
Water has a density of one gram per milliliter when it is a liquid. Substances that are less dense will float in water, and substances that are more dense will sink in the water. However, when water is a solid the molecules in the hydrogen bonds spread farther apart. This gives ice a lower density than liquid water, which allows it to float in the water.

PH
The pH of a liquid is how acidic or basic the substance is. Water is naturally neutral, with a pH of 7 on a scale of 0 to 14. Water, however, can be ionized. Ionization is the conversion of a non-ionic compound into ions. Water is constantly becoming ionized and un-ionized. When water is broken down into ions it is made up of H-plus and OH-minus. When water has more H-plus ions it is acidic and when water has more OH-minus ions it is basic. Water being neutral is very important for living organisms. If rain becomes to acidic it is referred to as acid rain and can kill fish. Because human beings are mainly water, if we consume substances that are too acidic or basic we can die.