3.1.2 The digestive system provides an interface with the environment. Digestion involves enzymic hydrolysis producing smaller molecules that can be absorbed and assimilated.

Carbohydrate digestion

• Within this unit, carbohydrates should be studied in the context of the following
• starch, the role of salivary and pancreatic amylases and of maltase located in the intestinal epithelium
• disaccharides, sucrase and lactase.
• Biological molecules such as carbohydrates and proteins are often polymers and are based on a small number of chemical elements.
• Monosaccharides are the basic molecular units (monomers) of which carbohydrates are composed.
• The structure of a-glucose and the linking of a-glucose by glycosidic bonds formed by condensation to form maltose and starch.
• Sucrose is a disaccharide formed by condensation of glucose and fructose. 
• Lactose is a disaccharide formed by condensation of glucose and galactose. 
• Lactose intolerance.
• Biochemical tests using Benedict’s reagent for reducing sugars and non-reducing sugars. Iodine/potassium iodide solution for starch.

Carbohydrates, proteins and fats are considered macromolecules because they are polymers (long chains of molecules). Each is actually composed of a series of monomers, which are small individual units. 

In carbohydrates, monosaccharides are the monomers, forming the basic molecular unit of carbohydrates. 

• Monosaccharides are: 
  • Sweet 🍬
  • Water-soluble 🌊
  • Have the general formula (CH₂O)_n, where n is between 3 and 7 inclusive
• Examples: glucose, galactose, and fructose
  • For GCE AQA Biology, you will need to know how to draw the structure of alpha-glucose, one type of the glucose molecule
  • Remember the up/down orientation of the hydroxyl groups (-OH) in clockwise order is down, down, up, down. This is important to distinguish alpha-glucose from the other variant of glucose, beta-glucose. 

2 monosaccharides can be linked up to form a single disaccharide. 

• For example: 
  • Alpha-glucose + alpha-glucose = maltose 
  • Glucose + fructose = sucrose 
  • Glucose + galactose = lactose 
• The monosaccharides are linked up by a condensation reaction. The reaction is termed ‘condensation’ because it produces a molecule of water. 🌊
  • For example, the following equation depicts the condensation of 2 alpha-glucose molecules to form a single molecule of maltose
  • The alpha-glucose molecules are linked by a glycosidic bond, highlighted in pink 
  • The eliminated hydroxyl group (-OH) and hydrogen (H) form a molecule of water as a side product 

Many monosaccharides can also be joined up to form long chains of carbohydrate polymers known as polysaccharides. 

• For example, starch is a long polysaccharide composed of many alpha-glucoses linked up via glycosidic bonds. 
• Cellulose is another polysaccharide, composed of many beta-glucoses (another type of glucose) joined together.  

Carbohydrates can thus be divided into three levels of organisation: 

We have just discussed the building of carbohydrates from the smallest building blocks of monosaccharides. In GCE AQA Biology, you should also be familiar with the breakdown of carbohydrates from its largest polysaccharides or smaller disaccharides into its constituent monomers. This process of breaking down often requires the presence of specific enzymes. 

For example, let’s follow the breakdown of starch (found in many foods like bread 🍞or pasta 🍝) in the human digestive system. 

• Starch is broken down by an enzyme called amylase, into the disaccharide maltose. 
  • Amylase is found in saliva secreted into the mouth by salivary glands, and also in pancreatic juice, which is secreted into the small intestine by the pancreas. 
• The maltose is then broken down by the enzyme, maltase, into 2 alpha-glucose molecules. 
  • Maltase is produced by the epithelial lining of the small intestine. The enzyme is bound to the cells that line the inner lining of the small intestine. 
• The breakdown of starch is known as a hydrolytic process because the breakdown requires water to break the glycosidic bonds connecting the monosaccharides. 
  • Remember that this is the reverse of the condensation reaction discussed earlier. 

Other disaccharides are also broken down by specific enzymes: 

• Sucrase → glucose + fructose, catalyzed by sucrase 
• Lactose → glucose + galactose, catalyzed by lactase 

Carbohydrate breakdown - what happens when it goes wrong: Lactose intolerance 

• Milk contains the disaccharide lactose. 🥛
• Lactose needs to be broken down by the lactase enzyme.
• As we get older, milk becomes a smaller and smaller part of our diet. Our bodies start to produce less lactase enzyme. 
• In some adults, the diminishment is so great that they produce very little or no lactase. 
  • These adults are unable to digest the lactose in milk and other dairy products in the small intestine. 
• When the undigested lactose enters the large intestine, microorganisms like bacteria break lactose down.  
  • This leads to the production of gas, leading to burping or discomfort as the gas accumulates. 
  • It also leads to diarrhoea, as the products of the breakdown by bacteria include soluble molecules that reduce water potential, causing water to move into the large intestine. 
• This problem is known as lactose intolerance. 

Biochemical tests 

GCE AQA Biology includes biochemical tests for the presence of specific carbohydrates: 

1. Test for reducing sugars: Benedict’s test 
2. Test for non-reducing sugars: Benedict’s test, followed by hydrolysis, followed by Benedict's test again 
3. Test for starch: Iodine test 

1. Test for reducing sugars: Benedict’s test 

• Reducing sugar: a sugar that can reduce (donate electrons to) another chemical. 
  • All monosaccharides are reducing sugars 
  • Some disaccharides, like maltose, are reducing sugars 
• Principle of Benedict’s test: 
  • Benedict’s reagent is copper(II) sulfate solution. This is a blue solution.
  • When a reducing sugar reacts with Benedict’s reagent, the copper(II) sulfate is reduced and produces an insoluble red precipitate - copper(I) oxide. 
  • The overall change when a reducing sugar is present is from a blue solution to a brick-red precipitate. 
  • An increasing concentration of reducing sugar produces a colour closer and closer to red. 

Procedure: 

1. Add the food sample (in liquid form) to a test tube. If the sample is solid, grind it up in water to make it liquid 
2. Add an equal volume of Benedict’s reagent to the test tube. 
3. Heat the mixture in a warm water bath for 5 minutes. 

• Positive result: brick red precipitate
• Negative result: the solution remains blue 

2. Test for non-reducing sugars 

• Non-reducing sugars include some disaccharides and all polysaccharides. These are all composed of individual monosaccharides joined together via condensation reactions. 
• Principle of test for non-reducing sugars: 
  • Benedict’s test is first performed (same procedure as above). The test should give a negative result if it is a non-reducing sugar. 
  • We must then hydrolyse the sample. If it is a non-reducing sugar, hydrolysis of the sample should produce monosaccharides, which are reducing sugars. 
  • Hydrolysis is carried out by reaction with dilute acid. 
  • The new solution, containing monosaccharides, can then be tested again for reducing sugars using Benedict’s reagent. 
• Procedure

1. Carry out the procedure (steps 1 to 3) for Benedict’s test detailed above. If the test for reducing sugars initially tests negative, we can proceed. 
2. Add a sample of the food sample (liquid form) to an equal volume of dilute hydrochloric acid in a test tube. 
3. Place the test tube in a warm water bath for 5 minutes. 
4. Add some sodium hydrogencarbonate solution to neutralise the hydrochloric acid, as Benedict’s reagent needs to be in alkaline conditions. Make sure that the solution is alkaline by testing it with pH paper. 
5. Repeat the procedure for Benedict’s test with the new solution. 

• Positive: The re-test with Benedict’s test should give a positive result, producing a brick-red precipitate. 
• Negative: The re-test with Benedict’s test continues to give a negative result, remaining blue. 

3. Test for starch: iodine test 

• Principle of iodine test: Iodine changes colour from yellow-brown to blue-black in the presence of starch 
• Procedure 

1. Add a few drops of the liquid sample of the food to a spotting tile. 
2. Add two drops of dilute iodine solution to the food. 

• Positive: iodine solution turns blue-black 
• Negative: remains yellow-brown  

Wanna Boost Up Your GCE AQA Biology? Check out our study tips here!

Written by Justine | Biology Specialist @ Tuttee

 References
Toole, G. & Toole, S. 2015. AQA Biology (2nd edition). Oxford University Press

Images from:

1. https://commons.wikimedia.org/wiki/File:Alpha-D-glucose_Haworth.svg 
2. https://www.quora.com/When-two-glucose-C6H12O6-are-combined-to-form-a-molecule-of-maltose-C12H22O11-the-formula-of-maltose-is-not-C12H24O12-Why 
3. https://biochemphilic.wordpress.com/2013/03/24/its-all-about-structure/ 
4. http://biologyaqaalevelnotes.blogspot.com/2017/04/biological-molecule-non-reducing-sugar.html
5. https://sites.google.com/a/wrps.net/cns-ontl/cns-2nd-semester-weblinks/unit-7-resources---lab/chemical-tests-to-identify-biomolecules
6. https://www.pexels.com/photo/biscuits-black-background-breads-breakfast-2041879/