Biological molecules

What are the chemical elements present in carbohydrates, proteins and lipids?

MOLECULE	CHEMICAL ELEMENTS
CARBOHYDRATE	Carbon, Oxygen and Hydrogen
PROTEIN	Carbon, Oxygen, Hydrogen, Sulfur and Nitrogen
LIPIDS	Carbon, Oxygen and Hydrogen

What are the basic units that make up carbohydrates, proteins and lipids?

MOLECULE	STRUCTURE
STARCH AND GLYCOGEN	Composed of simple sugars
PROTEIN	Composed of amino acids
LIPID	Composed of fatty acids and glycerol

How do you investigate food samples for the presence of glucose, starch, protein and fat?

Practical: Food Tests

Preparing a sample

• Before you can carry out any of the food tests described below, you may need to prepare a food sample first (especially for solid foods to be tested)

• To do this:

  • Break up the food using a pestle and mortar

  • Transfer to a test tube and add distilled water

  • Mix the food with the water by stirring with a glass rod

  • Filter the mixture using a funnel and filter paper, collecting the solution

  • Proceed with the food tests

Test for glucose (a reducing sugar)

• Add Benedict's solution to the sample solution in a test tube

• Heat in a boiling water bath for 5 minutes

• Take the test tube out of the water bath and observe the colour

• A positive test will show a colour change from blue to orange / brick red

The Benedict's test for glucose

Test for starch using iodine solution

• We can use iodine solution to test for the presence or absence of starch in a food sample

• Add drops of iodine solution to the food sample

• A positive test will show a colour change from orange-brown to blue-black

In the presence of starch, iodine solution will turn from brown to blue-black

Protein food test

• Add drops of Biuret solution to the food sample

• A positive test will show a colour change from blue to violet / purple

The Biuret test for protein

Test for lipids (fats)

• Mix the food sample with 4cm³ of ethanol and shake

• Allow time for the sample to dissolve in the ethanol

• Strain the ethanol solution into another test tube

• Add the ethanol solution to an equal volume of cold distilled water (4cm³)

• A positive test will show a cloudy emulsion forming

The ethanol test for lipids

Food Test Results Table

Important hazards

• Whilst carrying out this practical you should try to identify the main hazards and be thinking of ways to reduce harm

  • Biuret solution contains copper (II) sulfate which is dangerous particularly if it gets in the eyes, so always wear goggles

  • Iodine solution is also an irritant to the eyes

  • Sodium hydroxide in biuret solution is corrosive, if any chemicals get onto your skin wash your hands immediately

  • Ethanol is highly flammable; keep it away from any Bunsen burner

  • The Bunsen burner itself is a hazard due to the open flame

What is the role of enzymes in metabolic reactions?

ENZYMES: Protein molecule that acts as a biological catalyst by speeding up the rate of specific reaction without being used up or affected

• Enzymes are biological catalyst in metabolic reactions (digestion) as they allow substrates to bind onto their active site, breaking them down into products

                                   

COMMON ENZYMES:

ENZYME	REACTION
AMYLASE / MALTASE	Starch → Glucose
PROTEASE	Protein → Amino Acids
LIPASE	Lipids → Fatty Acids & Glycerol

How can temperature changes can affect enzyme function, including changes to the shape of the active site?

ENZYMES: Protein molecule that acts as a biological catalyst by speeding up the rate of specific reaction without being used up or affected

• Enzymes are biological catalyst in metabolic reactions (digestion) as they allow substrates to bind onto their active site, breaking them down into products

EFFECT OF TEMPERATURE ON ENZYME ACTIVITY

Diagram showing the Effect of Temperature on Enzyme Activity

EXPLANATION: As the temperature increases towards optimum, the rate of enzyme activity increases This is because as temperature increases, enzymes and substrates will gain kinetic energy, vibrating and moving faster This allows more frequent successful enzyme-substrate collisions to occur, increasing the rate in which substrates are broken down, hence increasing enzyme activity However, as temperatures exceed optimum, enzymes will denature This is because high temperatures causes bonds to vibrate more, increasing chance of bonds within enzyme breaking When bonds in enzymes break, shape and structure of active site will change, diminishing the ability of substrate to bind to active site of enzyme Therefore, enzymes denature and metabolic reactions cannot take place

Show how enzyme activity can be affected by changes in temperature.

INVESTIGATING EFFECT OF TEMPERATURE ON ENZYME ACTIVITY

Diagram Showing the Spots of Starch and Amylase in Wells

INDEPENDENT VARIABLE: Temperature of Starch Solution (0, 10, 20, 30, 40, 50, 60°C)

METHOD: Test tube of Starch solution is placed in Water bath of set temperature for several minutes (0, 10, 20, 30, 40, 50, 60°C) Solution of Amylase enzyme is added into test tube of Starch solution Immediately after Amylase is added into test tube, pipette is used to add droplets of mixture into wells Immediately after mixture is added into wells, a few drops of Iodine solution is added Process is repeated in increments of fixed time (seconds) until Iodine turns orange, indicating that Starch has completely broken down into Glucose Time taken for Starch to be completely broken down is compared to that of each respective temperature (time taken for Iodine solution to turn orange with each respective temperature)

RESULT: As temperature increases towards optimum (37°C), the rate of amylase activity increases as shown with the decrease in time taken for Iodine solution to turn orange (as temperature increases towards optimum) This is because as temperature increases, Amylase and Starch will gain kinetic energy, vibrating and moving faster This allows more frequent successful enzyme-substrate collisions to occur, increasing the rate in which Starch is broken down into Glucose (which causes Iodine solution to turn orange quicker as Starch is not present), hence increasing enzyme activity) However, as temperature exceeds optimum, Amylase will denature as shown with the increase in time taken for Iodine solution to turn orange (as temperature increases away from optimum) This is because high temperatures causes bonds to vibrate more, increasing chance fo bonds within Amylase enzyme to break When bonds in enzyme break, shape and structure of active site will change, diminishing the ability of Starch (substrate) to bind to active site of Amylase Therefore, Amylase denatures and Starch cannot be broken down into Glucose (which causes Iodine solution to turn orange slower as Starch is present), hence decreasing enzyme activity

How can enzyme function can be affected by changes in pH altering the active site?

ENZYMES: Protein molecule that acts as a biological catalyst by speeding up the rate of specific reaction without being used up or affected

• Enzymes are biological catalyst in metabolic reactions (digestion) as they allow substrates to bind onto their active site, breaking them down into products

EFFECT OF pH ON ENZYME ACTIVITY

Diagram showing the Effect of pH on Enzyme Activity

EXPLANATION: As the pH increases towards optimum, the rate of enzyme activity increases This is because as pH increases, the shape and structure of the active site, and the charge of substrate molecule will both be optimal to allow the substrate to bind to the active site of enzyme This allows more frequent successful enzyme-substrate collisions to occur, increasing the rate in which substrates are broken down, hence increasing enzyme activity However, as pH exceeds optimum, enzymes will denature This is because the increase/decrease from optimum pH causes the shape and structure of active site to change, additionally changing the charge fo substrates, therefore diminishing the ability of substrate to bind to active site of enzymes Therefore, enzymes denature and metabolic reactions cannot take place

Show how enzyme activity can be affected by changes in pH.

INVESTIGATING EFFECT OF pH ON ENZYME ACTIVITY

Diagram Showing the Spots of Starch and Amylase in Wells

INDEPENDENT VARIABLE: pH of Buffer Solution (pH 2, 4, 6 , 8, 10, 12) - buffer solution maintains reaction mixture at a constant pH

METHOD: Test tube of Starch solution is placed in Water bath of set temperature (30 - 40°C as optimum temperature of Amylase is approximately 37°C) Solution of Amylase enzyme is added into test tube of Starch solution Immediately after, buffer solution of set pH is added which will maintain reaction mixture at set pH Pipette is used to add droplets of mixture into wells on tile A few drops of Iodine solution is added Process is repeated in increments of fixed time (seconds) until Iodine turns orange, indicating that Starch has completely broken down into Glucose Time taken for Starch to be completely broken down is compared to that of each respective pH (time taken for Iodine solution to turn orange with each respective pH of buffer solution)

RESULT: As pH increases towards optimum (pH 6.7 - 7.0), the rate of amylase activity increases as shown with the decrease in time taken for Iodine solution to turn orange (as pH increases towards optimum) This is because as pH increases, the shape and structure of the active site, and the charge of substrate molecule will both be optimal to allow Starch to bind to the active site of Amylase This allows more frequent successful enzyme-substrate collisions to occur, increasing the rate in which Starch is broken down into Glucose (which causes Iodine solution to turn orange quicker as Starch is not present), hence increasing enzyme activity However, as pH exceeds optimum, Amylase will denature as shown with the increase in time taken for Iodine solution to turn orange (as pH increases away from optimum) This is because the increase/decrease from optimum pH causes the shape and structure of active site to change, additionally changing the charge fo substrates, therefore diminishing the ability of Starch to bind to active site of Amylase Therefore, Amylase denatures and Starch cannot be broken down into Glucose (which causes Iodine solution to turn orange slower as Starch is present), hence decreasing enzyme activity