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ALevel

Biology-9700

Carbohydrates And Lipids

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Carbohydrates And Lipids

Carbohydrates And Lipids


Objective

The objective is to be familiar with the structure, function and different types of carbohydrates and lipids present in living organisms.

Biolmolecules

4 major types of macromolecules are found in the living organisms. Most of these macromolecules are polymers and made up of small building units called monomers.

Monomers

Smallest unit which cannot be further broken down.

Polymers

Polymers are made up of many monomers joined together.

Carbohydrates

Carbohydrates are hydrated carbons as they contain the general formula (CH2O)n.  Most carbohydrates have a sweet taste and are soluble in water. They can be classified into different types based on the number of monomers present.

Monosaccharides

These are the simplest carbohydrates. Thus, they cannot be further broken down.

They have a sweet taste. There are different types of monosaccharides based on the number of carbons present.

Example: glucose, galactose, ribose, etc

Ring Structure Of Monosaccharides

Monosaccharides exist in 2 structural forms. They are either in a linear form or folds to make a ring structure.  These ring structures are formed when the aldehyde group on C 1 reacts with a hydroxyl group on C4.

There are two types of ring structures of glucose.

It can be alpha glucose or beta glucose. Both of them are isomers. That is they are the mirror image of each other.

If the hydroxyl group is present below the ring the molecule is alpha glucose.

If the hydroxyl group is present above the ring the molecule is beta glucose.

Role Of Monosaccharide In Living Organisms

They serve as the major source of energy due to the abundant carbon and hydrogen bonds.

Besides, they also take part in the formation of other macromolecules such as sucrose, starch, etc.

Disaccharides

Disaccharides are formed when two monosaccharides join together.

The bond between them is called glycosidic.


Glucose + Glucose
             Maltose
Glucose + Fructose
             Sucrose
Glucose + Galactose
             Lactose

Formation Of Dissacharide By Condensation Reaction

When two monosaccharides are joined together and a molecule of water is removed.  This reaction is called a condensation reaction.

Example

When fructose and glucose are joined together a molecule of sucrose will be formed with the removal of a water molecule.

Splitting Of Dissacharides

Disaccharides can be split into monosaccharides by adding water molecules. This type of reaction is called hydrolysis.

Example

Sucrose can be broken down into fructose and glucose with hydrolysis.

Benedict Test:

Sucrose is a non-reducing sugar that is why it gives a negative Benedict test. But when it is hydrolyzed it forms glucose and fructose. These are reducing sugar thus after hydrolysis sucrose gives a positive Benedict test.

In the same way, other disaccharides can be broken down into monosaccharide by hydrolysis.

Polysaccharides

Polysaccharides contain numerous monosaccharide linked together by condensation.

They do not dissolve in water thus form a good compound for storing energy or are used for making strong structural compounds.

They can be broken down into disaccharides by hydrolysis which can be further broken down into monosaccharide.

There are different types of polysaccharides.

Glycogen

Glycogen is polymers of alpha glucose. Most of the bonds between them are alpha 1-4. That means carbon 1 of the first molecule is linked to carbon 4 of the other molecule.

There are few alpha 1-6 bonds as well. These bonds form the branches in the compound. These branches increase the rate of hydrolysis as they provide numerous sites for the enzyme to act on.

Glycogen is found in animals and fungi.

Their function is to store energy.

Starch

Starch is found in plants only. They consist of amylose and amylopectin. Both of them are polymers of alpha glucose.

Natural starch has 10-20% of amylose and the rest of it contains amylopectin.

Amylose

Amylose consist of alpha 1-4 bonds only. Thus it forms a long chain of glucose. The chain spirals on itself and is held together by hydrogen bonds.

Amylopectin

It consists of alpha 1-6 bonds as well. Thus, they have a highly branched structure. Short branches of around 30 glucose molecules are held by 1-6 linkage. These branches are present after every 20-30 glucose molecules.

Cellulose

Cellulose consists of long chains of Beta glucose which are held together by beta 1-4 bonds.

The difference in cellulose and other polysaccharide is that it is a polymer of Beta glucose.

It has a straight structure and there is no spiraling. It forms a very strong structure which is difficult to digest as very few animals have enzymes that break Beta 1-4 links.

Thus, cellulose is usually found in the plant cell wall to give them support.

Role Of Polysaccharides In Living Organisms

Starch and glycogen are used by cells for storing energy. This is because they are complex and do not dissolve in water.

Cellulose due to its strong structure is a major component of the plant cell wall.

Lipids

Lipids consist of a class of different molecules that are insoluble in water.

The most common of them is triglycerides.

Triglycerides

It consists of three fatty acid chains joined with glycerol molecule hence called triacylglycerol or triglycerides. They are linked together by ester bonds.

They are nonpolar molecules thus insoluble in water but dissolves in organic solvents.

Types Of Triglycerides


Saturated Fatty Acids/ Fats

Saturated fatty acids are also called as fats.

They contain fatty acids having only single covalent bonds. That is why they are usually solid at room temperature.

Saturated fatty acids do not have a kink in their structure. Thus they organize in a regular fashion which makes them solid.

Unsaturated Fatty Acids/ Oils

Unsaturated fatty acids are also called oils.

They contain fatty acids having one or more double or triple covalent bonds. That is why they are liquid at room temperature.

Why Unsaturated Fatty Acids Are Liquid

Unsaturated bonds create a kink in the structure of fatty acid chains. This kink prevents the molecules from organizing regularly. Thus, making them liquid at room temperature.

Role Of Tryglycerides

They are a long term reservoir of energy. Lipids store more amount of energy than carbohydrates as it contains more carbon and hydrogen bonds.

Fats are stored in various parts of the human body. Below the skin, it functions as an insulator to keep the body warm.

Phospholipids

In phospholipid one of the three fatty acids is replaced by a phosphate group.

Thus, one end of the molecule is hydrophilic( water-loving) and the other end is hydrophobic( does not dissolve in water).

Due to this dual nature, it is called amphipathic.

Role Of Phospholipid

Phospholipid plays a very important role in the cell membrane of living organisms.

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