Let's deconstruct "carbohydrate". Carbo- which means carbon. Not a big surprise there, carbon is our fundamental building block for pretty much all living things. Then the -hydrate part is for water, H2O. If you were to just write the name out for carbohydrate with a chemical formula, you'd write CH2O, right? It's carbon+water. It has to be scaled up though to really be a true carbohydrate. That formula by itself is formaldehyde - yikes. Scale it by 6× and you now have a fundamental building block of tons of carbohydrates, glucose = C6H12O6. We actually already looked at glucose in the previous chapter on polymers. Glucose is the monomer unit for cellulose and starch. We definitely don't eat cellulose for any energy or fuel concerns because we just don't digest it (turn it into energy). Starch is a big player though in our diets. So glucose plays a big role in all of this. It is a major player (or "playa" if that's more your style) in our nutritional needs.
A saccharide is a fancy, science-y way of describing a sugar. There are different types of sugars and really what that means is there are different saccharides. Let's list the types and some examples.
Monosaccharides: We have already discussed and even looked at the structure of glucose and glucose is a monosaccharide (probably the most important one). A monosaccharide means there is only one sugar unit in the formula. Another popular monosaccharide is fructose which is basically fruit sugar. Glucose is the main energy "currency" moving through our bodies. It powers your brain and numerous other processes in the body. The polysaccharides (see below) are more of our energy storage supplies for glucose. Our body stores glucose as glycogen (animals) as opposed to starch (plants) - they are similar polysaccharides.
Disaccharides: A disaccharide is a two-sugar unit molecule. Certainly the most popular one is sucrose which is table sugar. Sucrose is a combination of glucose and fructose. Another is lactose which is the sugar in milk. Lactose is a combination of galactose and glucose.
Polysaccharides: Technically, we already learned about polysaccharides in the previous chapter in the section Nature's Polymers. Starch and cellulose are both polysaccharides with 100's of units (a polymer). The subtle difference between starch and cellulose is that starch is made from α-D-glucose and cellulose is made from β-D-glucose. That difference is the positioning of the hydroxyl group on the #1 carbon. You can see this difference below in the two structures. That key hydroxyl group is purple in the figure. When that hydroxyl group is on the same side of the ring structure as the #6 C, it is β-D-glucose, when it is opposite the #6 C, it is α-D-glucose.
That 1-hydroxyl group links through the opposite 4-hydroxyl group in a condensation reaction to yield the ether links that join all the glucose units to make the chain. Below is a bit of a starch chain showing this (8 units).
click here to see a bigger view
And, now let's have a look at a cellulose chain. The β(1→4) links are in between each unit and due to the up/down nature of the hydroxyls in that link, the chain actually has every other unit flipped over 180°. You can see this if you look carefully. Once again, 8 units shown.
click here to see a bigger view
Fiber is just another way of saying cellulose - the carbohydrate that plants make and are made out of. Remember, in general, we don't directly metabolize cellulose because our bodies don't have the enzymes that are needed to breakdown the cellulose units. Cows? Yes. Humans? No. Fiber is certainly important for your health, but getting any useful metabolic energy (calories) from fiber is not an option for us.
Truth be told here... there ARE different types of fiber and they are often listed as such on a nutrition label. And yes, one of those types does actually lead to some real calories for us humans. So here are the two types:
Insoluble Fiber: This is pretty much like I already said, it is like "wood" and it is insoluble in water and has zero caloric value to use. It does have great value for digestion though and we humans really do need to make sure we have the recommended amount of insoluble fiber in our diets. Still, no calories from insoluble fiber.
Soluble Fiber: This one is a bit different. Yes, it is soluble in water and provides some thickening qualities (more viscous). When this type of fiber hits the lower intestine, the bacteria there DO start converting it into short-chain fatty acids. And, as you know, fatty acids are a major calorie source. So what is the approximate conversion? About 2 calories are available for every gram of soluble fiber. This is certainly not a major source of energy, but there is some energy content there that you can account for.