Carbohydrates
Carbohydrates range from simple sugars to the complex chains they form. The monomers for carbohydrates are called monosaccharides. The polymers are called polysaccharides. Carbohydrates found in food are our main source of energy.
Monosaccharides
These are 1-ring sugars like glucose, fructose, and galactose. They are the most basic forms of sugar. They are made of either 3, 5, or 6 carbons. 3-carbon sugars are called triose sugars. 5-carbon sugars are called pentose sugars, and 6-carbon sugars are called hexose sugars.
Each monosaccharide has 1 carbonyl group (=O). The location of this functional group determines whether the sugar is aldose or keytose. If the group is on the end of the carbon chain, then the sugar is aldose. If the group is in the middle of the chain, then it is considered ketose. Glucose, galactose, and ribose are considered aldose sugars, while fructose and ribulose are ketone sugars.
Each monosaccharide has 1 carbonyl group (=O). The location of this functional group determines whether the sugar is aldose or keytose. If the group is on the end of the carbon chain, then the sugar is aldose. If the group is in the middle of the chain, then it is considered ketose. Glucose, galactose, and ribose are considered aldose sugars, while fructose and ribulose are ketone sugars.
Disaccharides
Disaccharides are basically two monosaccharides joined together by a dehydration reaction. A glycosidic linkage forms between the involved molecules. Common examples include sucrose, maltose, and lactose. Below is a set of monosaccharide -> disaccharide equations.
glucose + glucose -> maltose + water
glucose + fructose -> sucrose + water
glucose + galactose -> lactose +water
Note that maltose is commonly used in beer, sucrose is common table sugar, and lactose is found in milk. The above disaccharides can be broken down back into its component monomers by hydrolysis.
glucose + glucose -> maltose + water
glucose + fructose -> sucrose + water
glucose + galactose -> lactose +water
Note that maltose is commonly used in beer, sucrose is common table sugar, and lactose is found in milk. The above disaccharides can be broken down back into its component monomers by hydrolysis.
Starch
Starch is the primary energy storage molecule in plants. Essentially, this polysaccharide is made from a long chain of glucose molecules joined by dehydration reactions. It can be branched (amylopectin) or unbranched (amylose). Plants usually store their starch in organelles called pastids. Humans can digest starch, which is why we can get energy from eating vegetables and grains. Note that starch is made from alpha glucose. This kind of glucose has its hydroxyl groups from the 1st and 4th carbons on the same side of the ring (see picture on top right). Starch has a helix shape, but do not confuse it with DNA or secondary protein structure.
Glycogen
Glycogen is the primary energy storage in animals. This polysaccharide is also made from a long chain of glucose molecules joined by dehydration. However, it has more branches than starch. Animals tend to store glycogen in liver and muscle cells. When more glucose is needed, it is broken down by hydrolysis.
Cellulose
Cellulose is a structural polysaccharide that helps plants maintain their shape. Unlike starch, it is made from beta glucose molecules. Therefore each glucose is upside down relative to adjacent glucose molecules. The shape of cellulose is straight, so they can be bundled together into strong microfibrils, which make up the plant cell walls. Cellulose cannot be digested by humans since we cannot break the bonds between beta glucose molecules, so they pass through our digestive system as feces. However, some other animals harbour bacteria in their gut which can digest cellulose. This is why herbivores can eat grass.
Chitin
Chitin is another structural polysaccharide, it is used by arthropods and fungi. Similar to cellulose, it consists of beta glucose molecules, but this time a nitrogen appendage is attached onto it. Arthropods use it to build their cell walls. Chitin itself is soft, but it can become hardened by calcium bicarbonate. This is why the insect shells are hard. Fungi use it to build their cell walls. Again, because we cannot break down the bonds between beta glucose molecules, we cannot digest chitin.