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Topics in Depth

The Theme of Carbohydrates in Biomolecules and the Chemistry of Life

Carbs are awesome, and not because they’re responsible for some of the tastiest comfort foods known to humankind. To quote Homer Simpson, "Mmmmmm. Donuts." Carbohydrates are an important source of energy for cells and provide a means of transporting and storing that energy. They are also good for other things, like providing structural support.

If you remember anything about carbohydrates, it should be these three things:

  1. Carbohydrates = energy for cells.
     
  2. Carbohydrates are made of carbon (C), hydrogen (H), and oxygen (O), or CHO, in an approximate ratio of 1:2:1.  
     
  3. All sugars are carbohydrates. Another word for sugar is saccharide. Now the following paragraph will make sense. Please proceed.

Monosaccharides (mono = one, saccharide = sugar, aka "one sugar") are monomers of carbohydrates; they are small sugars, and when they dissolve in water, they form little rings. Monosaccharides vary in how many carbons they contain, but most commonly, they have three, five, or six, and as many as seven carbons.

All monosaccharides have two things:

  1. A carbonyl group, or a carbon that forms a double bond with an oxygen, written as C=O, and two other atom friends that we call A and B (4 bonds total for carbon), and
     
  2. Some hydroxyl groups (–OH).

If you'd like to get a little more into the chemistry, we are happy to oblige. 

  • If the carbonyl group is at the end of the carbon backbone, and it makes a single covalent bond with H as well as one other atom friend, we call that sugar an aldehyde. The aldehyde group is written as RCHO. R = unknown atom friend.
     
  • If the carbonyl group is in the middle of the chain and bound to two other atom friends, then it’s a ketone. The ketone group is written as RCOR'. R and R' = two unknown friends that may or may not be different from each other.
     
  • The placement of other atoms and atom groups matter as well. In fact, some sugars differ only in the arrangement of hydrogen and hydroxyl groups along the carbon backbone.

All these differences might seem pretty minor and technical, but they actually have substantial effects on the behavior of the overall molecule. To put this in perspective, some of us humans take for granted that our heads are attached to the top-end of our spinal columns. If, however, our heads joined with one of the vertebrae in the middle of our backs instead, we’d really need to change the way we do things. Now you know where these sugars are coming from when they place importance on the arrangement of their atoms.

Common monosaccharides include

  • Glucose, C6H12O6
     
  • Galactose, C6H12O6
     
  • Fructose, C6H12O6

You may have noticed that they all have the same molecular formula. How can this be, you ask? Arrangement of atoms!

You have probably heard of glucose, and it is the most common sugar because it is what plants produce during photosynthesis (see our photosynthesis section), but regardless of type, all monosaccharides serve as an immediate source of fuel for cells.

When two monosaccharide rings join (by dehydration synthesis, no less) to form a bigger sugar, they are called a disaccharide ("two sugars"). Sucrose, or table sugar, is a disaccharide that is formed from glucose and fructose. Lactose is a disaccharide composed of glucose and galactose. Many adult humans can’t digest lactose, or "milk sugar." (Visit "In the Real World: Health" to learn more!)

Here is our sweet friend, sucrose. Notice the distinct structure of each monosaccharide unit (pink and blue parts), despite the similar molecular formula.

For long-term storage of excess sugars, the best thing for a cell to do is turn them into polysaccharides ("many sugars"). Some polysaccharides are also used for structural support. If carbohydrates are just carbon, hydrogen, and oxygen, or CHO, packed into little units and strung together, how do we have substances as diverse as the starch in your bowl of pasta and the stringy things that make celery so annoying to chew? Check out our "Big Theme: Unity and Diversity" to find out!

Brain Snack

The most abundant carb on the planet is cellulose—the hard stuff plants are made of—but few organisms can actually break it down to eat it.

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