Explain the Structures and Roles of Carbohydrates, Proteins and Lipids

Have you ever heard the phrase, ‘You are what you eat’?

Well, it’s actually kind of true! Our body is a collection of the nutrients we eat to sustain us.

 

Food is categorised into three main types of nutrients:

 

                

CARBOHYDRATES                      PROTEINS                           LIPIDS

 

In the body, nutrients are broken down for two reasons:

 Firstly, these molecules are often too big to be absorbed through the gut wall into the blood.

 Also, humans need their own versions of the nutrients, so they need to be collapsed and rebuilt differently

 

Let’s work through each nutrient:

 

 

Carbohydrates:

 

 

Carbohydrate sources include bread, rice and pasta, and they give us energy.

Carbohydrates are actually long polymer chains of single sugar molecules like glucose.

 

From plants, we digest starch which is plant sugar storage and cellulose from plant cell walls.

However, humans and animals store sugar as glycogen (not starch), so starch needs to be broken down into glucose first, absorbed by the gut and can then be used for respiration or reassembled as glycogen storage.

However, cellulose is too difficult to break down further and so it’s egested from the gut as the body can’t store sugar in this form.

 

 

Proteins:

 

 

They come from sources like meat and eggs and are important for growth and repair.

Proteins are polymers of 100s to 1000s of amino acids that are linked in a specific sequence.

They are broken down after eating and reassembled into human proteins in protein synthesis.

Unlike sugar, amino acids can’t be stored, so any that are leftover are broken down by the liver and urinated out.

 

 

Lipids:

 

 

They provide energy but also are the key part of cell membranes.

Lipids are fats (like butter) and oils and are made of two different units: fatty acids and glycerol.

 

The arrangement is three fatty acids attached to one glycerol molecule, altogether making one lipid molecule which a chemist would call an ester.

Again, they must be digested first because they are too big to pass through the gut wall, after which they are reassembled.

 

 

Chemical Qualitative Tests:

 

We can use different qualitative tests to detect whether we have a carbohydrate, protein or lipid.

 

 

Carbohydrate Tests	Protein Tests	Lipid Tests
Benedict's Test for reducing sugars:    This blue solution creates a red/brown precipitate with reducing sugars like glucose and heat.   If there is only some reducing sugar present, the solution turns green/yellow.	Biuret Test:   Proteins turn a clear blue Biuret solution into a purple/mauve colour.	Emulsion test:   1. Add ethanol to the food. 2. Pour the liquid from this mix into another test tube with water. 3. A cloudy liquid or emulsion shows lipids are present.
Iodine test for starch:   Starch turns blue/black when it comes into contact with dark brown iodine solution		Sudan III test: 1. Equal quantities of water and test sample are put in a test tube 2. Add drops of Sudan III into the mix and shake. 3. If a layer of red appears on top of the mix, lipids are present.

 

Now it's time for some questions.