If you remember anything about carbohydrates, it should be these three things:
- Carbohydrates = energy for cells.
- Carbohydrates are made of carbon (C), hydrogen (H), and oxygen (O), or CHO, in an approximate ratio of 1:2:1.
- All sugars are carbohydrates. Another word for sugar is saccharide. Now the following paragraph will make sense. Please proceed.
All monosaccharides have two things:
- 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
- Some hydroxyl groups (–OH).
- 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.
Common monosaccharides include
- Glucose, C6H12O6
- Galactose, C6H12O6
- Fructose, C6H12O6
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!
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.