Exploring the Molecules of Life:  Carbohydrates

What chemical elements are found in carbohydrates?

Where do plants store energy made from photosynthesis?

What do all living beings use for energy?

The answer is carbohydrates.  They supply us with the energy we need to function.

Acetaldehyde, which is an aldehyde, is shown below.  It has a carbonyl functional group.  Can you find what molecular structure makes up a carbonyl?  This group is found in ketones too!  Remember if you place the cursor on an atom and click, the atom is identified at the lower left of your browser screen.

Other functional groups in carbohydrates are given below, where R is a carbon group, such as -CH3 .  


Carbohydrates are composed of the elements carbon, hydrogen and oxygen.  The carbonyl group consists of the bound C=O and is found in aldehydes, ketones, and as you will see later, carboxylic acids.  Below, left is an open chain molecule of glucose, a monosaccharide.  Can you find any carbonyl groups in glucose (Chime structures do not always show the double bond)?  What other functional groups can you find in glucose?


glucose                        fructose                    galactose

C __  H __ O __            C __  H __ O __            C __  H __ O __

Take a look at fructose, center, also a monosaccharide.  Count the number of carbons, hydrogens, and oxygens that make up the molecular formula of fructose.  What ratio do they occur in, in this particular order?  All carbohydrates occur in this ratio in the open chain form.  Verify your observations with galactose, right, another monosaccharide.  

What similarities does fructose have to glucose? to galactose?  Notice the molecular formulas and functional groups.  As you can see, all three are monosaccharides with the same molecular formula.  They are isomers!  Since they all have six carbons, they are called hexoses.

If an aldehyde group occurs in the chain, the monosaccharide is called an aldose, while if the ketone group occurs, it is a ketose.

Classify the monosaccharides above as aldoses or ketoses.

glucose ________        fructose ________        galactose _________

These are the ring structures of glucose (left) and fructose (center) and galactose (right).  This is how they occur most often in nature.  Does the C:H:O ratio occur in the ring structures?  What's missing from the ring that was present in the open ring form?


glucose                        fructose                    galactose

C __  H __ O __            C __  H __ O __            C __  H __ O __

When the chain closes to form the ring structure, find the new functional group containing oxygen that appears in the ring.  

What is the difference in the ring structures of glucose and galactose?

Below are sucrose (table sugar), maltose, and lactose (milk sugar). They are all disaccharides.  By examining the structures of these, what do you think disaccharide means?  Hint: Find the molecular formula (How many carbons compared to a monosaccharide?)

sucrose                     maltose                         lactose

Can you figure out what two monosaccharides joined to make sucrose? maltose? lactose?  

What is missing from sucrose's formula that was present in the individual ring formulas of glucose and fructose? 

How are the  two monosaccharides linked to form the disaccharide? 

The two monosaccharides join by dehydration synthesis, the loss of water, to form a disaccharide.  The linkage is an ether group, R-O-R, which also appears when the chain closes to form a ring structure.  The aldose and ketose character is only seen in the chain structure.  A simple example of an ether is shown to the right.  This is dimethyl ether.  When monosaccharides link together, the linkage is called a glycosidic linkage.

Monosaccharides and disaccharides are considered simple carbohydrates.  They can contain vitamins and minerals and are present in things such as fruits, milk/milk products, and vegetables.  Simple carbohydrates can also be found in processed and refined sugars such as those in candy, honey, table sugar, syrup, and regular carbonated beverages.  These refined sugars still contain calories but no longer have the fiber, minerals, and vitamins.

Look at the structure of cellulose given below.  How is it different from the others above?  You may want to rotate and then zoom in on the molecule (hold the shift key down, click, and move the mouse to zoom).

Dehydration synthesis can also join monosaccharides and disaccharides to form long polymer chains called polysaccharides.  Polysaccharides can be used as food storage or as structural components of the cell.  Amylose and amylopectin, both starches, are given below left and center.  Plants use amylose to store glucose for food.  Glycogen, not shown, is similar to amylopectin and is an animals way of storing its sugars.  Most glycogen is stored as granules in in our muscle and liver cells.  Cellulose, below right, is a structural component of plant cell walls.  Look at the structures carefully, what do they have in common?  What is the difference between them?

amylose                     amylopectin                     cellulose

How are the glucose units linked together?

The three structures above are all composed of glucose in three different arrangements.  Polysaccharides can be known as complex carbohydrates.  Although cellulose (a.k.a. fiber) is indigestible in humans due to the linkage of the glucose molecules, starch is.  Starch is present in breads, cereals, rice, starchy vegetables, and such.  Complex carbohydrates are also a good source of vitamins, minerals, and fiber.

What happens to carbohydrates, such as starch, when you eat them?

Complex carbohydrates hydrolyze (react with water) to form simpler carbohydrates in the processes of digestion and then metabolism.  These processes are aided by enzymes, which are discussed in a later activity.

What would be the products if sucrose hydrolyzed?

Carbohydrates can be present as a part of other organic molecules.  A simple sugar called deoxyribose, below left, helps make up the backbone of DNA.  Ribose, below right, another simple sugar, is present in RNA.  Can you tell the difference between the two? Hint: Pay close attention to their names.


deoxyribose                          ribose

Classify the two sugars above based on the number of carbons in their structures (triose for 3 carbons, tetrose for 4, pentose for 5, or hexose for 6).

Is table sugar, sucrose, soluble in water?  Why?  Remember your experience with the solubility of alcohols in the water activity.    How about glucose, which is blood sugar?

Sucrose and glucose are both very soluble in water as are all of the monosaccharides and disaccharides.  This is due to the polar groups, such as the -OH groups, on the molecules.

How is the solubility of the polysaccharides, such as starch and cellulose, going to behave based on their structure?

The polysaccharides are full of the polar groups, such as the -OH groups.  However, their solubilities are limited due to their high molar mass, they are very large molecules.  Most polysaccharides are insoluble in water.

Carbohydrates are energy sources for plants and animals.  In a very simple process, they burn to release energy (heat).

carbohydrate   +    oxygen       carbon dioxide   +    water   +   energy

In your body the process of metabolism is more complicated but the end result is the same.

How about the sweetness of various carbohydrates?  Here is a bar graph for some monosaccharides and disaccharides.  The polysaccharides are not sweet.


Compared to sucrose, which sugar is sweetest?  which is the least sweetest?

For the polysaccharides, what would their sweetness be compared to sucrose?

If you had a sample of starch that on standing would hydrolyze to maltose and then glucose, how would its sweetness change?

Do a web search using http://www.google.com on one of the following sugar substitutes to find out its relative sweetness to sucrose: acesulfame K, aspartame, cyclamate, saccharin, or sucralose.  How do they compare to sucrose?

Most of the structures on this page are from molecules collection available at the NSF-funded C4 Project at Cabrillo Community College.

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