2.7 - 2.14B - Biological molecules

Question 1

What are the 3 major molecules and what elements are present in them?

Answer 1

Question 2

What are the structures of carbohydrates?

Answer 2

Carbohydrates contain the elements carbon, hydrogen and oxygen
A monosaccharide is a simple sugar e.g. glucose (C6H12O6) or fructose
Glucose molecules contain lots of energy which can be released in respiration by breaking the bonds between the carbon atoms
A disaccharide is made when two monosaccharides join together
Maltose is formed from two glucose molecules
Sucrose is formed from one glucose and one fructose molecule
A polysaccharide is formed when lots of monosaccharides join together
Starch, glycogen or cellulose are all formed when lots of glucose molecules join together
Polysaccharides are insoluble and therefore useful as storage molecules

Question 3

What is the structure of fats?

Answer 3

Most fats (lipids) in the body are made up of triglycerides
Their basic unit is one glycerol molecule chemically bonded to three fatty acid chains
The fatty acids vary in size and structure
Lipids are divided into fats (solids at room temperature) and oils (liquids at room temperature)

Question 4

What are the structures of proteins

Answer 4

Proteins are formed from long chains of amino acids
There are 20 different amino acids
When amino acids are joined together a protein is formed
Amino acids can be arranged in any order, resulting in hundreds of thousands of different proteins
Examples of proteins include enzymes, haemoglobin, ligaments and keratin

Question 5

What is the shape of proteins?

Answer 5

Even a small difference in the amino acid sequence will result in a completely different protein being formed
The different sequences of amino acids cause the polypeptide chains to fold in different ways and this gives rise to the different shapes of proteins
In this way, every protein has a unique 3-D shape that enables it to carry out its function
The shape of a protein determines its function
For example:
Enzymes have a specifically shaped active site - this is where a specific substrate molecule fits in order for a reaction to take place
If the shape of the active site does not match the shape of the molecule that fits into it, the reaction will not take place
Antibodies are proteins produced by certain types of white blood cells that attach to antigens on the surface of pathogens
The shape of the antibody must match the shape of the antigen so that it can attach to it and signal it for destruction

Question 6

Before food tests how do you prepare a sample?

Answer 6

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

Question 7

How to test for glucose?

Answer 7

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

Question 8

How to test for starch?

Answer 8

We can use iodine 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

Question 9

How to test for proteins?

Answer 9

Add drops of Biuret solution to the food sample
A positive test will show a colour change from blue to violet / purple

Question 10

How to test for lipids?

Answer 10

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

Question 11

What are the important hazards for carrying out preparing a food sample?

Answer 11

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

Question 12

What are enzymes?

Answer 12

Enzymes are proteins that act as biological catalysts to speed up the rate of a chemical reaction without being changed or used up in the reaction
They are biological because they are made in living cells
Enzymes are necessary to all living organisms as they maintain reaction speeds of all metabolic reactions at a rate that can sustain life

Question 13

What is the mechanism of enzyme action?

Answer 13

Enzymes are specific to one particular substrate(s) as the active site of the enzyme, where the substrate attaches, is a complementary shape to the substrate
When the substrate moves into the enzyme’s active site they become known as the enzyme-substrate complex
After the reaction has occurred, the products leave the enzyme’s active site as they no longer fit it and it is free to take up another substrate

Question 14

What the stages of forming an enzyme substrate complex?

Answer 14

Step One: Enzymes and substrates randomly move about in solution
Step Two: When an enzyme and its complementary substrate randomly collide an enzyme-substrate complex forms, and the reaction occurs
Step Three: A product (or products) forms from the substrate(s) which are then released from the active site. The enzyme is unchanged and will go on to catalyse further reactions

Question 15

What effect does tempreture have on enzyme activity?

Answer 15

Enzymes are proteins and have a specific shape, determined by the amino acids that make the enzyme and held in place by bonds
This is extremely important around the active site as the specific shape is what ensures the substrate will fit into the active site and enable the reaction to proceed
Enzymes work fastest at their ‘optimum temperature’
In the human body, the optimum temperature is 37⁰C

Question 16

What happenes if the tempreture is above that of the optimum for the enzyme?

Answer 16

Heating to high temperatures (beyond the optimum) will break the bonds that hold the enzyme together and it will lose its shape
This is known as denaturation
Substrates cannot fit into denatured enzymes as the shape of their active site has been lost
Denaturation is irreversible - once enzymes are denatured they cannot regain their proper shape and activity will stop

Question 17

Why does increasing the tempreture increase enzyme activity?

Answer 17

Increasing the temperature towards the optimum increases the activity of enzymes as the more kinetic energy the molecules have the faster they move and the number of collisions with the substrate molecules increases, leading to a faster rate of reaction
This means that low temperatures do not denature enzymes, they just make them work more slowly due to a lack of kinetic energy

Question 18

What is the optimum pH for most enzymes?

Answer 18

The optimum pH for most enzymes is 7
Some enzymes that are produced in acidic conditions, such as the stomach, have a lower optimum pH (pH 2)
Some that are produced in alkaline conditions, such as the duodenum, have a higher optimum pH (pH 8 or 9)
If the pH is too high or too low, the bonds that hold the amino acid chain together to make up the protein can be disrupted/destroyed
This will change the shape of the active site, so the substrate can no longer fit into it, reducing the rate of activity
Moving too far away from the optimum pH will cause the enzyme to denature and activity will stop