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

Cellular Respiration

A video I made last year for my bio class explaining cellular respiration:

Photosynthesis

It's a well known fact that plants need water and sunlight to make food, but how is that possible? Essentially, plants create their own food through a process called photosynthesis. Photosynthesis is made up of two different processes called the light dependent reaction (also called the light reaction) and the light independent reaction (also called the dark reaction or the Calvin cycle). Both of these reactions take place in the chloroplast (shown at the left). The light reactions specifically take place in the thylakoid. Inside each thylakoid there is a system very similar to the electron transport system. This reaction is shown in the image below labeled "Light dependent reaction". This reaction begins with energy from the sun. The plant can use this energy because they have several pigments, one of which is chlorophyll. These pigments allow the plant to absorb the sun's energy. One photon of energy enters into photosystem two (PSII) and bounces off of the walls of photosystem two. The photon then reaches the reaction center at the base of the photosystem. There, a water molecule (H2O) has broken apart into H+ and O2. When the water is broken apart, an electron is released. The photon excites the electron, giving is energy. This electron travels up to the top of photosystem two. It then descends across the system and entersphotosystem one (PSI). As is descends it pumps one H+ ion from the stroma into the thylakoid lumen (the area inside the thylakoid). Once the electron is inside photosystem one, it travels upward until is reaches the electron carrier. There, it reduces NADP+ to NADPH. As this process repeats, a high concentration of H+ ions is build up in the lumen. These ions then travel through the ATP synthase one by one. As they move through, they physically rotate the synthase. This creates energy, and the energy converts ADP and P to ATP. 

As a review, the inputs of the light dependent reaction are as follows:

1 H2O

Light

and the outputs are:

1 O2 (final product)

2 ATP (used in the Calvin Cycle)

1 NADPH (used in the Calvin Cycle)

Light dependent reaction

The next component of photosynthesis is the Calvin Cycle. The calvin cycle creates G3P, or PGAL, which the plant uses to make glucose. It takes three molecules of CO2 to create one molecule of PGAL, therefore this explanation will be describing the cycle in terms of three molecules of CO2. The Calvin Cycle begins with CO2, in this instance three molecules. These molecules of CO2 combine with three molecules of RuBP to form a six carbon molecule. The enzyme rubisco assists in this joining. This is a brief transition phase, and soon these molecules split into three carbon molecules, for a total of six three carbon molecules. These molecules must be rearranged and gain phosphates. Therefore, six ATP oxidize to form ADP and six NADPH oxidize to form six NADP+ and phosphate. Next, the three carbon molecules each lose a carbon. These carbons form a PGAL, and the unused substances are rearranged. The rearranging takes energy, and three ATP oxidize to become 3 ADP. The result is three Rubisco and the cycle continues. When two PGALs are created, they combine to form glucose.

As an overview, the inputs of the Calvin cycle (when one pyruvate is created) are:
3 CO2
9 ATP
6 NADPH

And the outputs are:

9 ADP
6 NADP+
6 P
1 Pyruvate

The equation for photosynthesis is: 6CO2 + 6H2O -> C6H12O6 + 6CO2


This autotrophic system of plants is amazing. Plants take in CO2 and water, two substances that are abundant on our planet, and they convert them to food and oxygen. This phenomenon is present every plant, and even some species of bacteria. It is an essential factor to maintaining the delicate balance of life on Earth. Although we have not yet studied this in class, I believe that the amount of light will assist in photosynthesis and the amount of water. Light would increase the amount of photosynthesis because light is used to excite the electron in the light reaction and water would assist because if there is a lack of water a light reaction cannot occur and therefore there will be no energy to use in the dark reaction.


Sources:

http://en.wikipedia.org/wiki/Photosynthesis

Images:

http://dft.ba/-anQ

http://micro.magnet.fsu.edu/primer/java/photosynthesis/

http://www.daviddarling.info/images/Calvin_cycle.jpg