Biological Molecules

Question 1

Benedict’s test for reducing sugars

Answer 1

Include all monosaccharides + some disaccharides
1. Add Benedict’s reagent to a sample + heat in a water bath that’s been brought to the boil
2. Positive = coloured precipitate (blue to green/yellow/orange/brick red)

Question 2

Benedict’s test for non-reducing sugars

Answer 2

To test for non-reducing sugars, like sucrose, first you have to break them down into monosaccharides.

1. Add dilute hydrochloric acid to a new sample of the test solution, carefully heat in a water bath that’s been brought to the boil
2. Neutralise it with sodium hydrogencarbonate
3. Carry out the Benedict’s test as you would for a reducing sugar.
4. Positive = coloured precipitate
   Negative = stay blue, which means it doesn’t contain any sugar (either reducing or non-reducing)

Question 3

Starch is a mixture of which 2 polysaccharides?

Answer 3

Amylose + amylopectin

Question 4

Structure of amylose

Answer 4

Long, unbranched chain of alpha-glucose
Angles of the glycosidic bonds give it a coiled structure - makes it compact, so good for storage because you can fit more in to a small space

Question 5

Structure of amylopectin

Answer 5

Long, branched chain of a-glucose
Side branches allow the enzymes that break down the molecule to get at the glycosidic bonds easily -> glucose can be released quickly

Question 6

Iodine test for starch

Answer 6

1. Add iodine dissolved in potassium iodide solution to test sample
2. Positive = colour change from brown/orange to blue/black

Question 7

Glycogen structure

Answer 7

Structure is similar to amylopectin but has lots more side branches -> stored glucose can be released quickly to release energy in animals
Very compact molecule so good for storage

Question 8

Cellulose structure

Answer 8

Long, unbranched chains of beta-glucose
When beta-glucose molecules bond, they form straight cellulose chains.
Cellulose chains are linked together by hydrogen bonds to form strong fibres called microfibrils -> provides structural support for plant cell wall

Question 9

Structure of triglycerides

Answer 9

1 molecule of glycerol + 3 fatty acids
Condensation reaction -> ester bond

Question 10

Fatty acids structure

Answer 10

Long hydrophobic tails made of hydrocarbons -> insoluble in water
Saturated or unsaturated (in R group)

Question 11

How does the structure of triglycerides relate to their function?

Answer 11

1. Long hydrocarbon tails of fatty acids contain lots of chemical energy so lots of energy released when broken down
2. Insoluble so don’t affect water potential of cell + cause water to enter cell by osmosis.
   Triglyceride clump together as insoluble droplets in cells with hydrophobic tails facing inwards and hydrophilic glycerol heads facing outwards

Question 12

Phospholipid structure

Answer 12

1 glycerol molecule + 2 fatty acids + 1 phosphate group

Question 13

How does the structure of phospholipids relate to their function?

Answer 13

1. Heads are hydrophilic + tails are hydrophobic so they form a double layer with heads facing out towards water
2. Centre of bilayer is hydrophobic so water-soluble substances cannot easily pass through -> membrane acts as a barrier

Question 14

Emulsion test for lipids

Answer 14

1. Shake test substance with ethanol for a minute so that it dissolves, then pour the solution into water
2. Any lipid will show up as a milky emulsion (more lipid = more noticiable milky colour)

Question 15

Structure of amino acids

Answer 15

Carboxyl group (-COOH), amine group (-NH2) and R group

Question 16

What bond if formed between amino acids?

Answer 16

Peptide bonds

Question 17

What is the secondary structure of a protein?

Answer 17

Hydrogen bonds form between amino acids in the polypeptide chain.
Coils into an alpha helix or folds into a beta pleated sheet

Question 18

What is the quaternary structure of a protein?

Answer 18

The way several different polypeptide chains are held together by bonds

Question 19

Biuret test for proteins

Answer 19

1. Test solution needs to be alkaline so add a few drops of sodium hydroxide solution
2. Add copper(II) sulfate solution
   Positive = purple
   Negative = stay blue

Question 20

How do enzymes speed up the rate of reaction?

Answer 20

Enzymes lower the activation energy by forming an enzyme-substrate complex
Joining reaction: substrates attached to the enzyme holds them close together, reducing any repulsion between the molecules so they can bond more easily
Breakdown reaction: fitting into the active site puts a strain on bonds in the substrate, so the substrate molecules break up more easily

Question 21

What is the induced fit model?

Answer 21

1. Prior to binding, the substrate and active site and not completely complementary in shape
2. When the substrate binds, the active site changes shape and moulds around the substrate

Explains why the enzymes are so specific and only bind to one particular substrate

Question 22

Why do enzymes denature?

Answer 22

1. Higher temperature makes the enzyme’s molecules vibrate more
2. If the temperature goes above a certain level, this vibration breaks some of the bonds that hold the enzyme in shape
3. Active site changes shape so enzyme + substrate no longer fit together

Question 23

What is the saturation point in enzyme activity?

Answer 23

All active sites are full so increasing the substrate concentration will have no effect

Question 24

What is competitive inhibition?

Answer 24

1. Competitive inhibitor molecules have a similar shape to substrate molecules so compete with substrate molecules to bind to active site
2. Block the active site so no substrate molecules can fit in it
   Higher concentration of inhibitor = nearly all active sites full
   Higher concentration of substrate = substrate’s chances of getting to an active site before inhibitor increase

Question 25

What is non-competitive inhibition?

Answer 25

1. Non-competitive inhibitor molecules bind to enzyme away from its active site 2. Causes active site to change shape so substrate molecules can no longer bind to it Increasing the concentration of substrate will not increase reaction rate

Question 26

Explain how 2 features of DNA are important in semi-conservative replication?

Answer 26

2 stands so both can act as a template Easily broken hydrogen bonds between bases allows to the 2 stands to separate Complementary base pairing allows accurate replication

Question 27

Explain why DNA replicates semi-conservatively

Answer 27

Ensures genetic continuity between generations of cells

Question 28

How does DNA polymerase move along the antiparallel strands?

Answer 28

active site of DNA polymerase is only complementary to the 3' end of the newly forming DNA strand so the new strand is made in a 5' to 3' direction + DNA polymerase moves down the template strand in a 3' to 5' direction DNA polymerase working on 1 template strand moves in the opposite direction to the DNA polymerase working on the other template strand

Question 29

Why does water have hydrogen bonding?

Answer 29

The slightly negatively-charged oxygen atoms attract the slightly-positively charged hydrogen atoms of other water molecules

Question 30

Use of water as an important metabolite

Answer 30

many important reaction involve a condensation or hydrolysis reaction -> hydrolysis reactions require a molecule of water to break a bond condensation reactions release a molecule of water as a new bond is formed

Question 31

Use of high latent heat of vaporisation of water

Answer 31

Takes a lot of energy to break hydrogen bonds between water molecules so a lot of energy is used up when water evaporates -> organisms use evaporation to cool down without losing too much water

Question 32

Use of high specific heat capacity of water

Answer 32

Hydrogen bonds absorb a lot of energy so it takes a lot of energy to heat it up -> water doesn't experience rapid temperature changes -> keeps aquatic habitats and internal body temperature stable

Question 33

Use of water as a good solvent

Answer 33

water is polar so positive end will be attracted to the negative ion + negative end will be attracted to the positive ion -> ions will get totally surrounded by water molecules (dissolve) -> can be easily transported

Question 34

Use of strong cohesion between water molecules

Answer 34

Very cohesive because they are polar -> helps water to flow so good for transporting substances e.g. xylem Also means it has a high surface tension when it comes into contact with air -> sweat droplets + pond skaters

Question 35

ATP structure

Answer 35

adenine (nucleotide base) + ribose sugar + 3 phosphate groups (high energy bonds between) -> nucleotide derivative