Biochemistry

Carbohydrates

Learning Objectives

• To understand the role of carbohydrates in biology.

• To understand the logic of monomer, isomer and polymer carbohydrates

What is a carbohydrate?• Carbohydrates are

‘sugars’• The name describes the

atoms they contain: Carbon, Hydrogen and Oxygen

• Glucose is the basic sugar made by plants

• The ratio of hydrogen to oxygen is always 2:1

Shortcut to Biochemistry.lnk

Shortcut to Biochemistry.lnk

Glucose• C6H12O6 is made by

photosynthesis• It is a simple sugar or

‘monomer’.• The carbons are counted in

a clockwise direction, 1, 2, 3, 4, 5, 6.

• Most other more complex sugars are chains of this

• Complex sugars have to be broken back down to glucose before they are absorbed in the small intestine.

• It is essential for respiration

Skeletal formula of glucose

Fructose

• Fructose is a monomer carbohydrate found in fruits and honey

• It has the same formula as glucose but its structure is different: it is an isomer of glucose

• Monomer carbohydrates are also known as monosaccharides

Two kinds of glucose

• There are two types of glucose:• Alpha glucose with the OH group below• Beta glucose with the OH group above.

Linking Sugar Monomers -Disaccharides

• Two conjoined monosaccharides form a disaccharide

• They conjoin by a condensation reaction (a water molecule is lost)

• The ‘join’ is called a glycosidic link

Maltose is two glucose molecules joined in a condensation reaction.For the glucose to be re-released water must be available.

Sucrose

• Sucrose is a very common disaccharide made from glucose and fructose.

• It is the sugar sold in the supermarket

• It is commonly made in plants for transport in phloem.

Sucrose is made from the joining of the isomers glucose and fructose.

Polymers (polysaccharides)• Carbohydrate Monomers linked in a long

chain are known as polysaccharides.• A common example in plants is starch, made

from long chains of alpha glucose.• Amylose Starch forms alpha 1-4 glycosidic

links.• Plants put monomers into long chains for

storage purposes• Starch doesn’t dissolve so doesn’t influence

water potential.

Amylopectin• Amylopectin is a

combination of straight chain 1,4 bonded glucose

• AND• 1, 6 bonded glucose

which causes branches to form

• It therefore forms many branches providing many ‘ends’

Glycogen (animal starch)• The surplus glucose we eat is

stored in the liver as the polysaccharide glycogen

• It’s essentially the same as Amylopectin

• Glycogen it forms branches as well as straight chains, this is because 1, 6 links form as well as 1, 4 links.

• This makes it easy to break down quickly

Hydrolysis• Hydrolysis ‘water breaking’, is just the

condensation reaction in reverse• The water molecule is added to the glycosidic link

to break the bond and re-form glucoses• It can occur in disaccharides or polysaccharides

The disaccharide lactose broken by hydrolysis

Cellulose• Plants also build starch

chains into structures such as cellulose.

• Beta glucose make straight chains

• These straight chains linked top and bottom by hydrogen bonds form cellulose.

• Cell walls are made of cellulose, these give structure.

 

Cellulose

PRESENTED BYANKIT KUMAR