Biological Molecules - A

Question 1

All about carbohydrates

Answer 1

They’re polymers - which are composed of monomers e.g. monosaccharides, nucleotides or amino acids

They contain C, H and O

They’re made from monosaccharides e.g. glucose, fructose or galactose

Glucose is a hexose sugar

Two isomers - alpha or beta

Question 2

How are monosaccharides joined

Answer 2

Condensation reactions

A glycosidic bond forms

A disaccharide is formed when 2 monosaccharides join together

Two a-glucose = maltose
Glucose + fructose = sucrose
Glucose + galactose = lactose

Question 3

Test for reducing sugars

Answer 3

Include all monosaccharides (glucose) and some disaccharides (maltose and lactose)

Add blue Benedict’s reagent to sample and heat in water bath

Use BR in excess to ensure all sugar reacts

If the test’s positive it will form a coloured precipitate
(Blue - green - yellow - orange - brick red)

Alternatively, filter solution and weigh precipitate

Question 4

Test for non-reducing sugars

Answer 4

If the result of the reducing sugars is negative a non-reducing sugar could be present

First have to break them down into monosaccharides

Add dilute hydrochloride acid and heat in water bath

Neutralise with sodium hydrogen carbonate

Add BR and heat further

If positive, coloured precipitate will form

If negative solution will stay blue

Question 5

What is a polysaccharide

Answer 5

Formed when more than two monosaccharides join together by a condensation reaction

Question 6

Test for starch

Answer 6

Add iodine dissolved in potassium iodide solution

If starch is present the sample turns blue-black

Question 7

All about starch

Answer 7

Plants get energy from glucose
Plants store excess glucose as starch

Starch is a mixture of two polysaccharides of a-glucose - amylose and amylopectin

Good for storage as it’s insoluble in water and doesn’t effect water potential so water can’t enter cells via osmosis

Question 8

What is amylose

Answer 8

A long and unbranched chain of a-glucose

Angles of the glycosidic bonds give it a coiled structure, making it compact so good for storage

Question 9

What is amylopectin

Answer 9

A long and branched chain of a-glucose
Its side branches give enzymes more surface area to break down glycosidic bonds allowing glucose to be released rapidly

Question 10

All about glycogen

Answer 10

Animals store excess glucose as glycogen, instead of starch like plants

Glycogen is a polysaccharide of a-glucose

Similar structure to amylopectin but a lot more side branches

Compact so good for storage

Question 11

All about cellulose

Answer 11

The major component of cell walls in plants
Made up of long, unbranched chains of b-glucose
When b-glucose molecules bond they form straight cellulose chains linked by hydrogen bonds
This forms strong fibres called microfibrils
Cellulose provides structural support in plant cells
Formed by a condensation reaction which forms a glycosidic bond
Bonds are hard to break
Resists action of enzymes/digestion

Question 12

Structure of trigycerides

Answer 12

A type of lipid

One glycerol

Three fatty acids

Fatty acids have tails made from hydrocarbons
The tails are hydrophobic making lipids insoluble in water. All fatty acids have the same basic structure but the hydrocarbon tail varies

Question 13

How are triglycerides formed?

Answer 13

By condensation reactions between the glycerol and fatty acid
An ester bond is formed and a molecule of water released

Question 14

Double bonds in fatty acid tails

Answer 14

Can be saturated or unsaturated
Saturated - no double bonds
Unsaturated - at least one double bond

Unsaturated fatty acids cause the chain to kink

Question 15

All about phospholipids

Answer 15

The lipids found in cell membranes
Similar to triglycerides but one fatty acid is replaced by a phosphate group

Phospholipids make up the bilayer of cell membranes
The phosphate group is hydrophilic and the fatty acid tails hydrophobic so they form a double layer with their heads facing out towards the water on either side
The centre of the bilayer is hydrophobic so water-soluble substances can’t pass through

Question 16

Structure of triglycerides relating to their function

Answer 16

Mainly used as energy storage molecules which they’re good for because -
The long hydrocarbon tails contain lots of chemical energy which is released when they’re broken down
They’re insoluble so don’t affect water potential of the cell and cause water to enter cells via osmosis

Question 17

Emulsion test for lipids

Answer 17

Shake sample in test tube with ethanol so it dissolves
Add same amount of water
Any lipid will show up as a milky emulsion

Question 18

Proteins and amino acids

Answer 18

Amino acids are the monomers of proteins
A dipeptide is formed when two amino acids join together
A polypeptide is formed when more than two amino acids join together
Proteins are made up of one or more polypeptides

Question 19

How are polypeptides formed?

Answer 19

By condensation reactions
The bonds formed between amino acids are called peptide bonds

Question 20

What is the primary structure of a protein?

Answer 20

The amino acid sequence in the polypeptide chain

Question 21

What is the secondary structure of a protein?

Answer 21

Hydrogen bonds form between amino acids in the chain
This makes it coil into either an a-helix structure or beta pleated sheet

Question 22

What is the tertiary structure of a protein?

Answer 22

Coiled or folded chain of amino acids is folded further
New bonds form - hydrogen and ionic
Disulfide bridges also form when two cystine amino acids come close together. The sulfur atom in one binds to the sulfur atom in the othe.

For proteins made from a single polypeptide chain the tertiary structure forms their final 3D structure

Question 23

What is the quaternary structure of a protein?

Answer 23

The way multiple polypeptide chains are arranged
For proteins made from more than one polypeptide (e.g. insulin, collagen, haemoglobin) the quaternary structure is the proteins final 3D structure

Question 24

Test for proteins

Answer 24

Test solution needs to be alkaline so first add a few drops of sodium hydroxide solution
Add a few drops of copper(II) sulfate solution

Positive result solution turns purple
Negative result solution stays blue

Question 25

What is an enzyme?

Answer 25

Biological catalysts
They catalyse biological reactions at a cellular level and for the organism as a whole
Enzymes can affect structures in an organism
They can be intracellular or extracellular
They’re proteins
They’re highly specific due to their tertiary structure

Question 26

How do enzymes lower the activation energy of a reaction?

Answer 26

Allow reactions to happen at a lower temperature
When the enzyme-substrate complex is formed it lowers the activation energy because
- repulsion is reduced between molecules so they can bond more easily
- If the enzyme is catalysing a breakdown reaction fitting the substrate into the active site puts strain on the bonds so they break more easily

Question 27

How do enzyme properties relate to their tertiary structure

Answer 27

Enzymes are very specific
One complementary substrate will fit into the active site
The active site’s shape is determined by the enzymes tertiary structure
Each different enzyme has a different tertiary structure

Question 28

Effect of temperature on enzyme activity

Answer 28

Increase in temperature results in more kinetic energy
So molecules move faster and collide more frequently
More enzyme-substrate complexes formed
Increase in temperature causes bonds to break in the enzymes active site
The active site changes shape and the substrate no longer fits
The enzyme is denatured

Question 29

Effect of pH on enzyme activity

Answer 29

All enzymes have an optimum pH value
Above or below optimum ionic and hydrogen bonding within the tertiary structure break, the active site changes shape and the enzyme becomes denatured

Question 30

Effect of enzyme concentration on the rate of reaction

Answer 30

The higher the enzyme concentration the more likely a substrate molecule is to collide and form an enzyme-substrate complex
So reaction rate increases
If the amount of substrate is limited, increasing enzyme concentration further has no effect

Question 31

Effect of substrate concentration on the rate of reaction

Answer 31

The higher the substrate concentration the faster the rate of reaction as substrate and enzyme are more likely to collide
This is only true up to a saturation point - until all active sites are full
Substrate concentration decreases with time during a reaction - unless more is added
So if no other variables are changed the rate will decrease over time too
This makes the initial rate of reaction the highest

Question 32

Competitive inhibition

Answer 32

Competitive inhibitor molecules have a similar shape to the substrate
They compete with the substrate to bind to the active site though no reaction takes place so they block the active site
Increasing substrate concentration will increase the rate of reaction up to a certain point

Question 33

Non-competitive inhibition

Answer 33

Molecules bind to the enzyme away from the active site
This causes the active site to change shape Substrate molecules can no longer bind because the active site and substrate are no longer complementary
They don’t compete with the substrate as they are a different shape
Increasing the concentration of the substrate won’t make a difference to the reaction rate as the enzyme activity will be inhibited regardless

Question 34

Investigation into the effect of a named variable on the rate of an enzyme controlled reaction PAG

Answer 34

Independent variables - enzyme concentration, pH, temperature, substrate concentration

In this method the named variable is temperature
Make two control samples:
Two boiling tubes
Add 5cm3 of milk suspension to each tube.
Add 5cm3 of distilled water to one tube- this control will indicate the
absence of enzyme activity.
Add 5cm3 of hydrochloric acid to the other- this control indicates the
colour of a completely hydrolysed sample.
2. Take three test tubes and measure 5cm3 milk into each. Place in water bath at
10°C for 5 minutes to equilibrate.
3. Add 5cm3
trypsin to each test tube simultaneously and start the timer
immediately.
4. Record how long it takes for the milk samples to completely hydrolyse and
become colourless.
5. Repeat steps 2-3 at temperatures of 20°C, 30°C, 40°C and 50°C.
6. Find the mean time for the milk to be hydrolysed at each temperature and use
this to work out the rate of reaction.