AS Biology - Carbohydrates!

3 Distinguish between monosaccharides, disaccharides and polysaccharides (glycogen and starch — amylose and amylopectin) and relate their structures to their roles in providing and storing energy (β-glucose and cellulose are not required in this topic).

This website is your saviour for AS Biology biological molecules (carbohydrates, proteins, lipids and nucleic acids): http://www.mrothery.co.uk/biochm/biochmnotes.htm#CARBOHYDRATES

-Monosaccharides are the building blocks (monomers) are disaccharides and polysaccharides. These 'saccharides' are all carbohydrates.

-Two monosaccharides react = a disaccharide. The bond formed is called a glycosidic bond. It looks like this: [monosaccharide]---O---[monosaccharide]. Type of reaction: condensation reaction (water molecule given off!). One hydrogen from -OH group and one whole -OH group combine to give water. Therefore, the reverse of this reaction involves adding water to split the glycosidic bond (hydrolysis).

-Monosaccharides: glucose, fructose, galactose.

-Disaccharides: maltose (formed from 2 alpha glucose molecules), sucrose (formed from alpha glucose and fructose), lactose (formed from alpha glucose and galactose)

-KNOW THE STRUCTURE OF GLUCOSE (RING STRUCTURE FROM THAT WEBSITE). BE SURE TO BE ABLE TO DRAW HOW DISACCHARIDE FORMS FROM TWO MONOSACCHARIDES AND HOW THE WATER IS FORMED.

-KNOW WHAT ALPHA GLUCOSE IS (MIGHT AS WELL KNOW BETA AS WELL). The -OH on carbon one is BELOW the ring plane in alpha glucose. In beta glucose, the -OH on carbon one is ABOVE the ring plane. Please know this distinction. It is very important. Google it! Look at pictures to see the glucose rings and see how the -OH is arranged in the beta and alpha forms.

-Polysaccharides: poly=many; made up of many glucose (or other monosaccharides) bonded by glycosidic bonds. The polysaccharides you have to know are made up of only glucose: glycogen and starch.

-Glycogen: form of energy storage in ANIMAL CELLS. Made up of alpha glucose only! Glycosidic bonds are: 1-4 glycosidic bonds and 1-6 glycosidic bonds. 1-4 bonds are straight, 1-6 bonds are bent. So, the 1-6 bonds give glycogen a branched structure. Structure to function: the main function of glycogen is to act as an energy store in animal cells. Glucose is polymerised into glycogen (glycogenesis) and this glycogen is compacted into glycogen granules (this is great because it enables animals cells to store more glucose in a smaller space). Because glycogen is highly branched due to 1-6 links, it is easily hydrolysed into glucose (glycogenolysis). However, unlike glucose, glycogen is insoluble (this is good because it will not diffuse out of cells and it will have no osmotic effect on the cells).

-Starch: form of energy storage in PLANT CELLS. Starch is made up of alpha glucose only! Starch, like glycogen, contains both 1-4 and 1-6 links (know how these links are formed and how they look! Do your research, it's quick and easy!!!). HOWEVER, starch is made up of two forms of polysaccharides: amylopectin and amlyose. Amylose is a straight chain polymer and can form spirals; it is made up of ONLY 1-4 links hence it is straight chained. Amylopectin, on the other hand, is branched; it is made up of BOTH 1-4 AND 1-6 links (like glycogen). Because starch contains the highly branched amlyopectin, it can be easily hydrolysed into glucose. Additionally, the glucose can be formed into starch and compacted into starch grains/granules resulting in more glucose/energy being stored in smaller cell space. Starch, like glycogen, is insoluble meaning it will not have an osmotic effect in plant cells nor will it diffuse out of cells!

-Structure and function of monosaccharides and disaccharides: they're all a direct source of energy. Disaccharides can be quickly broken down into their respective monosaccharides. They're soluble and can exist in solution in living things. Glucose exists in blood and because it is soluble it can easily be transported around the body (it is small and can be trafficked into and out of cells). Sucrose in solution is used to transport glucose around plants.

-Remember: glucose = energy

4 Describe how monosaccharides join to form disaccharides (sucrose, lactose and maltose) and polysaccharides (glycogen and amylose) through condensation reactions forming glycosidic bonds, and how these can be split through hydrolysis reactions.

Check the website I sent above. Be able to draw how disaccharides are formed, how water is given off, know the formation of 1-4 and 1-6 links and how straight and branched chains come about. Know what a glycosidic bond is and know condesation/hydrolysis.

Next post will deal with lipids!