Carbohydrates

Plants use sunlight, carbon dioxide and water to make an important class of foods called carbohydrates in a process called photosynthesis.

  

 
The importance of Carbohydrates

• Carbohydrates are an important food for animals.

• Respiration is important for all living things because it provides energy when glucose is burned or broken down in the body.
  Glucose + oxygen ----------> carbon dioxide + water + energy

  C₆H₁₂O₆ + 6O₂ ----------> 6CO₂ + 6H₂O

• Carbohydrates release energy when burned, and make carbon dioxide and water.

• The release of energy can be seen in the custard-powder tin experiment.

• Animals use this energy for many things such as movement, and warmth to keep our body temperature at 37°C.

 
What are carbohydrates?

Carbohydrates are compounds which contain the elements carbon, hydrogen and oxygen only.
In these compounds the hydrogen and oxygen are in the ratio of two to one - in other words there is always twice as many hydrogen atoms as oxygen atoms.

There are actually two different types of carbohydrates: sugars and starches.

 
Sugars

Sugars are small carbohydrate molecules that can dissolve in water.

These small sugar molecules can be dissolved in our bloodstream so they can be carried to parts of the body that require energy.

Some examples of sugars are: glucose, fructose, maltose and sucrose (which is also known as table sugar).

Most sugars can be detected by the Benedict's test; sucrose is an exception.
In this test, Benedict's solution turns from blue to brick-red in colour after heating with a sugar in a water bath. This is a positive test for sugars (except sucrose).

 
Starches

When molecules join together and water molecules are eliminated when they join, the reaction is called condensation.

If many glucose molecules join together and a polymer is made, the reaction is called condensation polymerisation.

Starch is a natural condensation polymer made of many glucose molecules linked together.

Starch can be detected using the iodine test. In this test starch turns iodine solution form reddish-brown to blue-black.

Plants convert glucose into starch for storing energy.

To show that starch is made in leaves by photosynthesis

1. Put plant in dark for 2 days to remove all food from the leaf.
2. Put plant in the light for one day to let it make food.
3. Remove one leaf and boil in water to kill it.
4. Put in hot alcohol to remove chlorophyll - the leaf will be white.
5. Wash in water to remove alcohol and add iodine solution to it.
6. The leaf will be blue-black proving that starch had been made.

During digestion, the large starch molecules are hydrolysed (broken down by the action of water) into smaller glucose molecules that can then be transported around the body in our bloodstream.
This reaction is the reverse reaction of the formation of starch from glucose as shown above.

Acids and enzymes can help the digestion of starch.

 
Enzymes

Enzymes are biological catalyst and were covered in Unit 1: Reaction rates.

Enzymes such as amylase help to break down starch into glucose molecules.

These enzymes function best at body temperature (37°C) and are destroyed at higher temperatures.

 
You can quickly test your knowledge of the above information.

 
Proteins

• Proteins form an important class of food made by plants.

• When animals eat plants, we take in these proteins and use/modify them for our own purposes.

• Animals (including humans) use proteins as the main material to build tissue, muscle, hair, fingernail, etc.

• Proteins are also used in the maintenance and regulation of life processes and include enzymes and many hormones, e.g. insulin and haemoglobin.

 
How are proteins made?

Proteins are made when amino acids join together.

Amino acids are molecules that contain an amine group (-NH₂) at one end of the molecule and a carboxylic acid group at the other (-COOH).

In between these groups is a carbon chain (shown by the blue box) that can vary in its composition. This makes it possible to have a huge variety of different amino acids.

When these molecules join together, they eliminate water molecules and form a large molecule called a protein.

Proteins are formed by condensation polymerisation of amino acids as shown below.

The peptide link

The structure of a section of protein is based on the constituent amino acids.

Condensation of amino acids produces the peptide (amide) link which is shown on the left. The peptide link is formed by the reaction of an amine group with a carboxyl group.

During digestion, enzymes break apart the large insoluble protein molecules (by hydrolysis) to produce the component amino acid molecules. This is basically the reverse of the reaction above.

The amino acid molecules are small enough to be soluble in the bloodstream and can be transported around the body to the location that they are needed.

The body then recombines these amino acids into the desired order so as to produce a specific protein that the body requires.

It is possible to identify the individual amino acids in each protein as each amino acid in the protein is separated from the next by the peptide link.

The amino acids produced when a protein is hydrolysed can be identified by chromatography. Hydrolysis is the process where the amine link is broken using water molecules. Hydrolysis is the reverse of condensation.

 
You can quickly test your knowledge of the above information.

 
Fats and oils

Fats and oils in the diet supply the body with energy and are a more concentrated source of energy than carbohydrates.

Natural fats and oils can be classified according to their origin:

Fats and oils can also be termed as saturated and unsaturated. These terms were covered in the 'Reactions of carbon compounds' topic.

• Saturated compounds do not contain carbon to carbon double or triple bonds - only single carbon to carbon bonds are present.

• Unsaturated compounds contain carbon to carbon double and/or triple bonds.

As with alkenes, the test for an unsaturated fat or oil molecule is that it decolourises bromine water rapidly.

 
Differences between fats and oils

Fats are solid and oils are liquids at room temperature.

Fats tend to contain more saturated molecules (less double bonds) than oils and this causes them to have higher melting points.

The lower melting points of oils compared to those of fats is related to the higher unsaturation (more double bonds) of oil molecules.

Oils can be converted into hardened fats by the partial removal of unsaturation by addition of hydrogen. (Addition reactions involving hydrogen were covered in the 'Reactions of carbon compounds' topic). This reaction is called hydrogenation.

 
The structure of fats and oils

Fats and oils are esters. This means that they are formed when molecules that contain carboxylic acid groups join with molecules containing an alcohol group. (this was covered in the 'Reactions of carbon compounds' topic.)

The acids that make fats and oils are called fatty acids and are straight chain saturated or unsaturated carboxylic acids.

Capric acid is a fatty acid found in goats milk. It is a completely saturated fatty acid

An example of an two unsaturated fatty acids are shown below.

In fats and oils, the alcohol groups are provided by the compound propane-1,2,3-triol, which is commonly known as glycerol.

Since glycerol contains three alcohol (-OH) groups, it can combine with three fatty acids to produce a fat or an oil.

The reaction between three fatty acids and glycerol is shown below.

The reverse reaction can also take place, when the fat/oil is hydrolysed to form glycerol and three fatty acids.

 
You can quickly test your knowledge of the above information.

New words and their meanings

CARBOHYDRATE - a compound containing carbon, hydrogen and oxygen in which the ratio of hydrogen:oxygen is the same as in water (2 hydrogens for each oxygen).

PHOTOSYNTHESIS - a process in plants in which carbon dioxide and water are changed into carbohydrates and oxygen with the help of sunlight and chlorophyll.

RESPIRATION - a process in living things where oxygen is used to break up food and produce water, carbon dioxide and energy.

SUGAR - a molecule made by plants that give us energy. This is a simple carbohydrate.

STARCH - a polymer made from glucose molecules. This is the form that plants store their carbohydrates.

BENEDICT'S TEST - a test that detects sugars (except sucrose). In the test, Benedict's solution turns from blue to brick red if a sugar is present.

IODINE TEST - a test that detects starch. Iodine solution turns from reddish-brown to blue-black in the presence of starch.

PROTEIN - a molecule that is made from many amino acids. Proteins are used for body-building and body repair.

AMINO ACIDS - molecules that contain an amine group and a carboxylic acid group at either end of the molecule. These join together to form proteins.

PEPTIDE LINK - the link that is between amino acids when they form a protein. The link is -CONH-

FAT - solid esters formed when fatty acids join up with glycerol. They are an important class of foodstuffs.

OIL - liquid esters formed when fatty acids join up with glycerol. They are an important class of foodstuffs..

SATURATED - compounds that only contain single bonds between carbon atoms.

UNSATURATED - compounds that contain double (or triple) carbon to carbon bonds.

HYDROGENATION - The process where hydrogen is added across a double carbon to carbon bond in an unsaturated fat/oil to reduce the level of unsaturation (or increase the level of saturation).

FATTY ACIDS - a group of molecules that contain a straight carbon chain with a carboxylic acid group at one end. Fatty acids are used to make fats and oils.

GLYCEROL - a molecule that contains three alcohol groups (-OH) that is used in the formation of fats/oils when it combines with three fatty acids.