2.3 Biological Molecules

REVISION NOTES

IGCSE Edexcel Biology

2.3.1 Identify the chemical elements present in carbohydrates, proteins and lipids (fats and oils)

Chemical elements in biological molecules:

- Carbohydrates and Lipids contain:

Carbon

Oxygen

Hydrogen

- Proteins contain:
  - Carbon
  - Oxygen
  - Hydrogen

Nitrogen

- Small amounts of other elements

2.3.2 Describe the structure of carbohydrates, proteins and lipids as large molecules made up from smaller basic units: starch and glycogen from simple sugars, protein from amino acids, and lipid from fatty acids and glycerol

Structure of Carbohydrates:

Monosaccharides are simple sugars:
- Glucose (C6H12O6)
- Fructose

Disaccharide are two monosaccharides joined together
- Maltose is made up of two glucose molecules
- Sucrose is made up of one glucose and one fructose molecule
- Lactose

Polysaccharides are more than two monosaccharides joined together
- Starch
- Glycogen
- Cellulose
- All 3 polysaccharides are made up of multiple glucose molecules joined together
- Polysaccharides are insoluble and used for storage

Structure of Fats:

Lipids are made from three fatty acid chains bound to a glycerol molecule

Vary in size and structure based on:
- Length of the fatty acid chain
- Saturated or unsaturated
Fats are solids at room temperature

Oils are liquids at room temperature

Structure of Proteins:

Proteins are made up of multiple amino acids bound to each other
20 amino acids that can be used for protein production
Order of amino acids determine the protein being made
- Amino acid sequence contributes to 3-D shape and function
Examples of proteins:
- Enzymes
- Haemoglobin
- Ligaments
- Keratin

2.3.3 Practical: investigate food samples for the presence of glucose, starch, protein and fat

All food samples should be prepared before conducting any food tests

Chop the food into smaller pieces
Transfer food to a test tube and add distilled water
Mix the food and water by stirring
Run the mixture through a filter paper and collect the solution
Use the solution for food tests

Glucose test:

Add benedict’s solution to food sample in a test tube
Heat up the test tube in a water bath for 5-10 mins till the temperature is above 50oC
Remove the test tube and observe the colour
Repeat step 1-2 two more times

Results:

Solution turns red or orange in the presence of glucose
Solution remains blue in the absence of glucose

Starch test:

Add a few drops of iodine solution to the food sample
Record colour of solution after 2 mins
Repeat step 1-2 two more times

Results:

Solution turns blue or black in the presence of starch
Solution remains orange or brown in the absence of starch

Protein test:

Add a few drops of Biuret solution to the food sample
Record colour of solution after 2 mins
Repeat step 1-2 two more times

Results:

Solution turns violet or purple in the presence of protein
Solution remains blue in the absence of protein

Lipid test:

Add 3cm3 of ethanol to the food sample and properly mix it
Let the sample dissolve into the ethanol
Strain the ethanol solution into a test tube and add 3cm3 of distilled water
Repeat steps 1-3 two more times

Results:

Solution becomes cloudy in the presence of lipids
Solution remains clear in the absence of lipids

2.3.4 Understand the role of enzymes as biological catalysts in metabolic reactions

Enzymes:

Enzymes are biological catalysts for cellular reactions
- Are proteins
- Speed up the rate of reaction
- Are not changed or used up
Involved in metabolic reactions
Required to maintain speed of metabolic reactions
- E.g. ensure digestion occurs in 4-6 hours

Products of enzymatic reactions can be reactants in other reactions

Enzymatic reactions:

Enzymes are specific to substrate
- Based on the shape of the active site
- Active site is complementary to the shape of the substrate
Shape of the enzyme and substrate is based on amino acid sequence

Order of enzymatic reactions:

Enzymes and substrates randomly move around in a solution
They randomly collide to form enzyme-substrate complex
Enzyme-substrate complex formation leads to reaction
Product(s) formed are released from the active site
Enzyme remains unchanged and can catalyse other reactions

2.3.5 Understand how temperature changes can affect enzyme function, including changes to the shape of active site

Effect of temperature on enzymatic reactions:

Rate of reaction is fastest at optimum temperature
Optimum temperature is 37oC
Enzymes are amino acid polymers
Shape of the active site determines the reaction it can catalyse
Temperature increases towards optimum, rate of reaction increases
Temperature increases beyond optimum, rate of reaction decreases

Enzymes at temperatures above optimum:

Bonds between the amino acids break
Enzyme is denatured
- Shape of active site changes
Substrate can no longer fit
Metabolic reaction no longer proceeds
Enzyme activity stops

Enzymes at temperatures below optimum:

Enzymes do not denature
They have less kinetic energy at temperatures below optimum
Less kinetic energy leads to lower chances of colliding with substrate
Slower rate of reaction
Increase in kinetic energy as temperature increases towards optimum
Enzymes move faster

Increase in collision with substrates which increases rate of reaction

Irreversible effect

2.3.6 Practical: investigate how enzyme activity can be affected by changes in temperature

1. Starch is digested into maltose by an enzyme called amylase

Method:

Add 5ml of starch solution to a test tube that is then heated to a particular temperature in a water bath
Using a syringe, add 2ml of amylase to the starch solution from step 1
Add 1 drop of iodine to each well of the spotting tile
Every minute, transfer one drop of starch-amylase solution to a new well of the spotting tile
Keep transferring a drop of the starch-amylase solution until the iodine stops turning blue/black (when the amylase has broken down all the starch)
Record the time taken for all of the starch to be broken down by the amylase
Repeat this experiment for a range of different temperatures

Results:

Amylase breaks down starch, thus the faster the enzyme is working, the quicker the iodine will stop turning blue/black
At the optimum temperature, the iodine stops turning blue/black the fastest
1. Enzyme work at their fastest rate at optimum temperatures so starch is digested faster
At temperatures below optimum, the iodine takes a longer time turning blue/black
1. Enzymes has less kinetic energy and thus have a decreased rate of reaction
At temperatures above optimum, the iodine stays blue/black throughout the experiment
1. Enzyme denatures and can no longer breakdown starch

Control Variables:

Temperature
1. Water baths at each temperature
2. Re-confirm using a thermometer
Change in iodine solution colour when compared to a control
1. Colorimeter to accurately measure colour change

2.3.7 Understand how enzyme function can be affected by changes in pH altering the active
site

Optimum pH for most enzymes is 7
Exceptions are:
- Stomach enzymes have an optimum pH of 2
- Enzymes in the duodenum have an optimum pH of 8 or 9
pH is above or below optimum
- Break the bonds within the amino acid chain
- Changes the shape of the active site
pH is significantly away from the optimum pH
- Enzyme denaturation
- Stops all enzymatic activity

2.3.8B Practical: investigate how enzyme activity can be affected by changes in pH

Method:

Add 1 drop of iodine to each well of a spotting tile or tray
Add 4cm3 of amylase solution into a test tube
Add 4cm3 of starch to the test tube
Add 2cm3 of buffer solution of the testing pH to the test tube
Mix the solution using a glass rod
Start a stopwatch and transfer a droplet of the solution to a well of spotting tile after every 20s
Continue transferring a drop of the solution into a spotting tile for 5 mins

Record the time taken for all of the starch to be broken down by the amylase
1. The colour of the solution remains orange/brown
Repeat this experiment for a range of different pH levels

Results: