Biological Molecules

Question 1

How do hydrogen bonds form between water molecules?

Answer 1

Water is polar: O is more electronegative than H so attracts electron density in covalent bond more strongly.
This creates a delta negative charge on the O and a delta positive charge on the H.
There are intermolecular forces between a lone pair on the delta negative O of one molecule and the delta positive H on an adjacent molecule.

Question 2

State 7 biologically important properties of water.

Answer 2

• high latent heat of vaporisation (energy to change from liquid to gas) therefore evaporation of sweat is an effective cooling mechanism
• High specific heat capacity which makes it a thermally stable environment for aquatic organisms
• Hydrogen bonds make the ice form a lattice when it solidifies so it expands which provides a habitat for some animals and insulates the water underneath due to ice being less dense than water
• Effective solvent so allows plants to take up nitrates through their roots
• Cohesion (water molecules sticking together) helps transpiration stream
• Adhesion (water molecules sticking to other things) allows lignin to be attracted to xylem which helps the transpiration stream
• Surface tension as hydrogen bonds are strong enough to allow invertebrates to walk om water providing a good habitat
  Water is transparent allowing plants underwater to photosynthesise

Question 3

Why is the incompressible nature of water important for organisms?

Answer 3

• Provides turgidity to plant cells

Question 4

Define monomer and polymer.

Answer 4

monomer = smaller units that join together to make larger molecules
polymer = molecules formed when many monomers join together

Question 5

Give some examples of monomers.

Answer 5

monosaccharides (glucose, fructose, ribose, galactose)
amino acids
nucleotides

Question 6

Give some examples of polymers.

Answer 6

Polysaccharides (starch, cellulose, glycogen)
proteins
DNA

Question 7

What happens in condensation reactions?

Answer 7

Chemical bond forms between two molecules and water molecule is produced

Question 8

What happens in a hydrolysis reaction?

Answer 8

A water molecule is used to break a chemical bond between two molecules

Question 9

Give some examples of hydrolysis reactions.

Answer 9

Breakingof a chemical bond between two molecules:
- Peptide bonds in proteins
- Ester bonds between fatty acids
- Glycerol in lipids

Question 10

Name the elements found in carbohydrates, lipids, proteins and nucleic acids.

Answer 10

• Carbohydrates and lipids: C, H, O
• Proteins: C, H, O, N, S
  Nucleic acids: C, H, O, N, P

Question 11

How do you draw alpha glucose?

Answer 11

hexose monosaccharides (6 carbons) OH goes below the H

Question 12

How do you draw beta glucose?

Answer 12

hexose monosaccharides (6 carbons) OH goes above the H

Question 13

Describe the properties of alpha glucose.

Answer 13

• Small and water soluble so easily transported in bloodstream

Question 14

How do you draw the structure of ribose?

Answer 14

Pentose monosaccharide (5 carbons) OH is above the H

Question 15

What types of bond forms when monosaccharides react?

Answer 15

1,4 or 1.6 glycosidic bond
If there are 2 monomers then only 1 bond will form and it will create a disaccharide
If there are many monomers then many bonds will form and it will create a polysaccharide.

Question 16

Name 3 disaccharides and describe how they form.

Answer 16

Condensation reaction forms glycosidic bonds between two monomers:
maltose = glucose + glucose
sucrose = glucose + fructose
lactose = glucose + galactose

Question 17

Describe the structure and function of starch.

Answer 17

Starch is a storage polymer of alpha glucose in plant cells as it is :
- insoluble so has no osmotic effect on cells
- large so doesnt diffuse out of cells
Starch is made from amylose and amylopectin.

Question 18

What is the difference between amylose and amylopectin?

Answer 18

Amylose has:
- 1,4 glycosidic bonds
- Alpha glucose
- is coiled structure with intermolecular hydrogen bonds - is compact
Amylopectin has:
- 1,4 and 1,6 glycosidic bonds
- coiled structure
- alpha glucose
- is branched so has many terminal ends for hydrolysis into glucose

Question 19

Describe the structure and function of glycogen.

Answer 19

Main storage polymer of alpha glucose in animal cells
Has 1,4 and 1,6 glycosidic bonds
Branched so has more terminal ends for hydrolysis into glucose
Insoluble so has no osmotic effect and does not diffuse out of cells
Compact

Question 20

Describe the structure and function of cellulose.

Answer 20

Polymer of beta glucose which aids rigidity of plant cell walls
1,4 glycosidic bonds
Straight chain unbranched molecule
Alternate glucose molecules are rotated 180 degrees so the OH can bond with the H and make water (condensation reaction)
Hydrogen bond cross-links between parallel strands form microfibrils

Question 21

Contrast saturated and unsaturated fatty acids.

Answer 21

Saturated:
- Contain only single bonds
- Straight chain molecules have many contact points
- highermelting points - solid at room temp
- found in animal fats
Unsaturated:
- Contains 1 or more carbon carbon double bonds
- “kinked molcules” have fewer contact points
- lower melting point = liquid at room temperature
- found in plant oils

Question 22

How do triglycerides form?

Answer 22

Condensation reactions occur between one molecule of glycerol and 3 fatty acids which forms ester bonds

Question 23

Relate the structure of triglycerides to their function.

Answer 23

• High energy: mass ratio = high calorific value from oxidation
• Insoluble hydrocarbon chain = no effect on water potential of cells and used for waterproofing
• Slow conductor of heat = thermal insulation
• Less dense than water = buoyancy of aquatic animals

Question 24

Describe the structure and function of phospholipids.

Answer 24

Amphipathic: glycerol backbone attached to 2 hydrophobic fatty acid tails and 1 hydrophilic polar phosphate head
- Forms phospholipid bilayer in water = component of membranes
- Tails can splay outwards = waterproofing e.g for skin

Question 25

Are phospholipids and triglycerides polymers?

Answer 25

No theyre made from a small repeating unit. They are macromolecules.

Question 26

Describe the structure and function of cholesterol.

Answer 26

Steroid structure of 4 hydrocarbon rings
Hydrocarbon tail on one side, hydroxyl group (-OH) on the other side
Adds stability to cell surface phospholipid bilayer by connecting molecules and reducing fluidity.

Question 27

Describe how to test for lipids in a sample.

Answer 27

• Dissolve solid samples in ethanol
• Add an equal volume of water and shake
• Positive result: milky white emulsion forms

Question 28

Describe how to test for reducing sugars.

Answer 28

• Add an equal volume of benedicts reagent to a sample
• Heat the mixture in an electric water bath at 100 degrees for 5 minutes
• Positive result: colour changes from blue to orange and brick red precipitate forms

Question 29

Describe the benedicts test for non reducing sugars.

Answer 29

• If the sugar is not reducing the Benedicts reagent remains blue
• So you take a new sample of the test solution and adding dilute HCl and heating in a water bath
  -Then neutralise with sodium hydrogencarbonate.
  -Then carry out a normal benedicts test for sugar (add benedicts reagent and heat)
  -If the test is positive for a non reducing sugar then a coloured precipitate will form
• If there is no sugar present the solution will say blue

Question 30

What are microfibrils?

Answer 30

Around 50-100 beta glucose molecules cross linking via their hydrogen bonds to form bundles

Question 31

What are macrofibrils?

Answer 31

Bundles of microfibrils cross linking via their hydrogen bonds

Question 32

What are cellulose fibres?

Answer 32

Bundles of macrofibrils

Question 33

State the structure and function of cell walls?

Answer 33

Provide strength to cells and whole plant
Prevents bursting and keeps cells turgid
Arrangement of macrofibrils permits passage of water
Arrangement allows cells to grow or change shape
Cell walls can be reinforced with other substances so that they can be waterproof e.g lignin

Question 34

Which ion is required for the hydrolysis of starch by an enzyme?

Answer 34

Cl-