Nucleic Acids, ATP, Water & Inorganic Ions 1.5

Question 1

Describe the process of DNA replication:

Answer 1

• DNA Helicase binds to DNA and breaks/hydrolyses the hydrogen bonds between two strands (using ATP)
• DNA helix unwinds & free floating nucleotides form hydrogen bonds with complementary base pairing
• Each strand is used as a template
• DNA Polymerase forms phosphodiester bonds between adjacent nucleotides (synthesis of two new DNA strands) in 5’ to 3’ direction (antiparallel)

Question 2

Describe the structure of DNA [2]

Answer 2

DNA molecule - Phosphate group, nitrogenous base and Deoxyribose sugar joined by condensation of 5’ to 3’ phosphodiester bond
(Phosphate of one strand attaches to hydroxyl group of the second strand in antiparallel 5’ to 3’ direction because of complementary base pairing)
Forms a sugar phosphate backbone.

Question 3

Humans and grasshoppers have very similar percentages of each base in their DNA but they are very different organisms.
Use your knowledge of DNA structure and function to explain how this is possible [2]

Answer 3

1. Different genes;
2. Different sequence/order of triplets;
3. So different amino acid/polypeptide/protein coded for.

Question 4

A polypeptide has 51 amino acids in its primary structure.
(i) What is the minimum number of DNA bases required to code for the amino acids in this polypeptide?
(ii) The gene for this polypeptide contains more than this number of bases - Explain why

Answer 4

(i) 153 (51x3)
(ii) Non-coding regions: introns OR start/stop code triplets
OR 2 DNA strands

Question 5

Water is vital to living organisms, making up 80% of a cell’s contents.
Give reasons why water molecules are important.

Answer 5

• A metabolite in metabolic reactions (hydrolysis & condensation)
• High specific heat capacity -> controls temp
• High latent heat of vaporisation -> controls temp
• A solvent (polar molecule, metabolic reactions take place in solution)
• Cohesive (molecules stick together) ->
  • helps cohension-tension of water transport (in plants transpiration stream)
  • helps surface tension so supports small organisms i.e. water striders

Question 6

State 4 biologically important properties of water

Answer 6

• Polar
• Metabolite
• High latent heat of vaporisation
• High specific heat capacity
• Cohesive
• Tension (surface)

Question 7

What are inorganic ions and where are they found in the body?

Answer 7

• Ions that do not contain carbon atoms
• Found in cytoplasm and extracellular fluid
• May be in high or low concentrations

Question 8

Explain the role of hydrogen ions in the body

Answer 8

High concentration of H+ = low acidic pH
H+ ions interact with H-bonds & ionic bonds in tertiary structure of proteins, which can cause them to denature.
(H+ also reduces NADP during photosynthesis)

Question 9

Explain the role of iron ions in the body

Answer 9

Fe2+ binds to haemoglobin (porphyrin ring) to form a Haem group

(Haem group has a binding site to transport 1 molecule of O2 around the bloodstream
4 Haem groups per haemoglobin molecule)

Question 10

Explain the role of sodium ions in the body

Answer 10

• Involved in Co-transport for the absorption of glucose and amino acids in lumen of gut.
• Involved in propagation of action in potentials in neurons

Question 11

Explain the role of phosphate ions

Answer 11

Component of:
• DNA
• ATP
• NADP
• cAMP

Question 12

ATP is an energy source used in many cell processes. Give the ways in which ATP is suitable as a cellular energy source (7)

Answer 12

1. Releases small, manageable amount of energy so little is lost as thermal heat energy
2. Releases energy INSTANTANEOUSLY
3. Phosphorylates other compounds to make more reactive
4. Can be rapidly re-synthesised
5. Does not leave cell/isn’t lost
6. Single-step hydrolysis
7. High energy bonds between phosphate groups

Question 13

Describe how an enzyme can be phosphorylated

Answer 13

• ATP hydrolase hydrolyses ATP -> ADP + Pi
• Attachment of inorganic phosphate ion (PO43-) to enzyme

Question 14

Describe the structure of ATP

Answer 14

Nucleotide derivative of:
adenine base
3 phosphate groups
Inorganic ribose sugar

Question 15

How does ATP work and explain the roles of ATP in cells

Answer 15

ATP Hydrolase catalyses ATP -> ADP + Pi during hydrolysis
ATP Synthase catalyses ADP + Pi -> ATP during condensation

Roles:
Phosphate group phosphorylates enzymes
Metabolic reactions
Muscle contraction (of diaphragm in inhalation)
Active Transport -> co-transport of glucose&amino acids, translocation in phloem