Starch stores energy
Cellulose supports & strengthens

Did you know that the polymers starch and cellulose are both made by plants? In fact, plants make both starch and cellulose by connecting glucose molecules together. Every time they add a glucose to make the chain longer, a water molecule pops out! Add a glucose, out pops H2O! Add a glucose, out pops H2O! And so on and so on until the chains are really long. A starch chain can have 500 to 2 million glucose units. Cellulose can have 2,000 - 14,000 glucoses. That's a lot of sweetness!

A closer look at GLUCOSE

Glucose is a funny little molecule. Glucose likes to be in a ring, but sometimes the ring opens up. (Why? Why not? You can stand up, you can sit down. So sometimes you stand up!) When the ring closes again, the -OH can be pointed down, or it can be pointed out. Either way, it's still glucose!
Click here for an animation
of glucose changing from
α-glucose to β-glucose.
alpha glucose
Glucose:  α (alpha) form
The -OH is pointed down instead of out.
(We didn't draw in the C and H atoms that just hang out.
beta glucose
Glucose:  β (beta) form
See? The -OH is pointed outward instead of down.
glucose - open ring form
Glucose: open chain form
Look at the blue H atoms. They've moved around, but they're still there. (By the way, here in science land we call these molecules isomers, because they're made up of the same atoms that are put together differently.)
glucose - open ring form
Glucose: another open chain form
Compare this guy to the other open chain form on the left. It's almost the same, but one of the bonds turned around, making the red O point up instead of down. Yep, it's allowed to do that! It's like swinging your arm around.

Energy or Strength?

Starch to store energy

Plants really know how to use glucose. To make starch, they use α-glucose, with the -OH pointed down.

That -OH is right where the next glucose will go. Since that one -OH is pointing down, it gives the chain a built-in curve. That curve is what makes starch so good for storing glucose. The starch polymer curls around and makes a nice little package.
Many starch polymers have a lot of branches that are short chains of glucose. When plants need energy, they grab some starch and start chomping glucoses off the ends!

Cellulose for strength

Remember that plants use cellulose to make their leaves and stems (or trunks!) strong.

To make cellulose, plants use β-glucose, with the -OH pointed out. Plants flip every other glucose over, too. I know, that's weird, but it works! It works because it helps the cellulose chains to stretch out and snuggle in next to each other. (You can read more about that on the cellulose page.) That makes cellulose fibers STRONG, strong enough to make fibers - and rope, and clothes.

Click here for an animation that shows how plants make cellulose.

Also, the chains are so close to each other, that even little molecules of water can't get in. That's good - that means that your cotton t-shirt won't wash away in the rain!

Why we eat the corn but not the cob! (But cows can eat both...)

growing corn One more thing: You can eat starch, but you can't digest cellulose. Your body contains enzymes that will break starch down into glucose to fuel your body. (Click here for a way to taste this for yourself!) But we humans (and puppies and kitties) don't have enzymes that can break down cellulose.

Some bacteria can digest cellulose. Animals like cows and horses and termites have these friendly little bacteria in their stomachs (or somewhere else in their digestive system). That means that when cows eat grass and termites eat wood, they can digest it with a little help from the microbes. No such luck for us, though! When we eat cellulose (and we do - all plants have some), it isn't used for energy, but it does help us. Click here to see how.

The Starch Page
The Cellulose Page

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