In this lesson, you will discover what starch and cellulose are.
You will also learn the difference between them and how that difference affects their individual functions.
What Are Starch and Cellulose?
What is the difference between the graphite in your pencil and a diamond? They’re both composed of the same element, so why is one so strong and valuable and the other flaky and a dime-a-dozen? The answer is, their structure.Like graphite and diamonds, starch and cellulose are also composed of the same substance but with different structures. Both starch and cellulose are polysaccharides; that is, both molecules are made up of a lot of sugar molecules. In particular, starch and cellulose are made of the sugar molecule glucose.
To understand the difference between starch and cellulose structure, it’s important to know glucose structures since glucose is what starch and cellulose have in common.Glucose is a type of sugar made of carbon (C), hydrogen (H), and oxygen (O). These elements form a ring with one of the carbons sticking off the end, kind of like a ball of yarn with an unraveled string.
There are also alcohol (OH) groups attached to the carbons. There are six carbons numbered from 1 to 6, with the ‘unraveled string’ hanging off taking the 6th position.Glucose is a 3-D molecule; this means the attached elements can be pointed in different directions at any given time. There are two main forms of glucose: alpha and beta. In alpha glucose, the alcohol attached to carbon 1 is down. In the beta glucose, the alcohol attached to carbon 1 is up.When one molecule of glucose joins with another, the two alcohols attached to the carbons combine, causing the new molecule to kick away water and share oxygen.
A common way the molecules are linked is for carbon 1 on one glucose to share oxygen with carbon 4 on another.
Starch vs. Cellulose: Structure
The structure formed when the molecules share oxygen is determined by which form of glucose is linked together.
Starch contains alpha glucose, while cellulose is made of beta glucose. This difference may seem minor, but it plays a big role in the 3-D shape of the molecule. Think of a pile of wood that can be put together in different ways. If you put the wood together in one way, it makes a doghouse; put together another way, it becomes a bookcase. In the same way, starch and cellulose have different 3-D structures.
It takes anywhere from 250 to over 1,000 glucose molecules to make a single starch molecule. In alpha glucose, the OH on carbon 1 and 4 are facing the same way. This means it can link in a straight chain or branch off, like people standing next to each other all facing the same way. Because starch can be straight and faces the same way, it can roll itself up in a helical structure, kind of like rolling up a piece of paper so it takes less space. If the alpha glucose of a starch branches off, the structure is more like paper that’s folded up to make origami.
Cellulose, on the other hand, is composed of about 500 glucose molecules in the beta form. Because the OH of carbon 1 and carbon 4 are in a different place, the molecules are flip-flopped when connected, like people standing next to each other with one facing forward while the other faces the opposite direction. This pattern allows for hydrogen bonding between two molecules of cellulose.Hydrogen bonding is simply the attraction of hydrogen to other oxygens. Because of this, two molecules of cellulose usually link together, kind of like Velcro. The two linked molecules join up with other molecules, forming microfibrils.
Microfibrils are sheet-like cellulose molecules arranged parallel to each other. There can be anywhere from 40 to 70 cellulose molecules per microfibril.
Cellulose vs. Starch: Function
The functions of starch and cellulose are as different as graphite and diamonds, which is mostly due to their structure. Starch’s main function is energy storage in plants.
It can be easily broken down by plants and the digestive system of animals. Thus, both plants and animals can harness the energy stored in starch.On the other hand, animals don’t have the ability to digest cellulose. Only special microorganisms that contain the enzyme cellulase and are found in the guts of animals, like cows and termites, can break down cellulose. Because of its beta linkages and microfibrils, cellulose has a very rigid structure that lends itself well to the place where it’s found–the cell wall.
Cellulose functions to give plant cells structure and protect the cell.Have you ever tried to take a stainless steel butter knife and rip it apart? You’d have to be superhuman to do it. Because of its structure, cellulose has the strength to withstand being ripped apart, akin to a stainless steel butter knife. Cellulose also has the honor of being the most abundant polysaccharide on the planet.
It’s found in a variety of substances we use, including wood and cotton.
Starch and cellulose are both polysaccharides composed of the sugar glucose. Glucose has six carbons that most commonly link carbon 1 to carbon 4. When they link, they give off water and share an oxygen. Glucose also has two forms: alpha and beta. In alpha glucose, the alcohol group on carbon 1 faces down, while in the beta it’s up. Starch is formed from alpha glucose, while cellulose is made of beta glucose.
The difference in the linkages lends to differences in 3-D structure and function.Starch can be straight or branched and is used as energy storage for plants because it can form compact structures and is easily broken down. In cellulose, molecules are connected in opposite orientations. Cellulose is found in cell walls and gives plant cells protection and structure.