Nucleic Acids, ATP, Water & Inorganic Ions

Question 1

What is a polynucleotide

Answer 1

formed when many nucleotides link to form a long chain, by condensation

Question 2

Nucleotides

Answer 2

A Nucleotide is the monomer of a nucleic acid. It is made up from 3 components.

1) Phosphate group

2) Nitrogen-containing organic base

3) Deoxyribose or Ribose sugar

Question 3

DNA vs RNA

Answer 3

DNA:
Adenine, Thymine (2 H bonds)
Cytosine, Guanine (3 H bonds)
3 bases= triplet

RNA:
Adenine, Uracil
Cytosine, Guanine
3 bases= codon

Question 4

Polynucleotide formation

Answer 4

2 strands of DNA run in opposite directions, which means they are antiparallel.
Each end of the molecule is labelled with a 5’-end and a 3’-end

Question 5

Describe the structure of DNA

Answer 5

1. Sugar-phosphate backbone so provides strength
2. Long/large molecule so can store lots of information;
3. Helical so compact;
4. Base sequence allows information to be stored
5. Double stranded so strands can act as templates;
6. Complementary base pairing so accurate replication
7. (Weak) hydrogen bonds for strand separation;
8. Many weak hydrogen bonds so stable

Question 6

Describe Semi-conservative replication

Answer 6

1. DNA helicase breaks H bonds
2. Strands separate
3. Both strands act as a template
4. (Free) nucleotides attach;
5. Complementary base pairing due to H bonds forming between bases
6. DNA polymerase joins nucleotides forming phosphodiester bonds by condensation;
7. New DNA molecules contain one old strand and one new strand;

Question 7

Describe the role of DNA polymerase in the semi-conservative replication of DNA.

Answer 7

1. Joins (adjacent DNA) nucleotides;
2. (Catalyses) condensation (reactions);
3. (Catalyses formation of) phosphodiester bonds

Question 8

Name the two scientists who proposed models of the chemical structure of DNA and of DNA replication.

Answer 8

Crick and Watson

Question 9

The Meselson Stahl Experiment

Answer 9

Grow bacteria in nitrogen heavy medium.
Transfer some bacteria to nitrogen light medium; bacterial growth continues
Take samples after 0, 20 and 40 minutes
Before bacteria reproduce for first time in light medium, all DNA is heavy
After 2 generations, DNA was intermediate

Question 10

ATP structure

Answer 10

consists of a nitrogenous organic (adenine) base, ribose sugar and three phosphate groups

Question 11

Uses of ATP

Answer 11

Provides energy for:
Active transport;
Muscle contraction;
Protein synthesis.

Phosphorylation:
Of molecules to lower Ea
make substrates more reactive

Question 12

ATP → ADP + Pi

Answer 12

• energy released (-30.6kJ)
• ATP hydrolase removes terminal phosphate
• hydrolysis reaction

Question 13

ADP + Pi → ATP

Answer 13

• energy used (+30.6kJ)
• ATP synthase adds terminal inorganic phosphate
• condensation reaction
• creates high energy bond

Question 14

Describe how an enzyme can be phosphorylated.

Answer 14

• Attachment of (inorganic) phosphate
• Released from hydrolysis of ATP

Question 15

ATP is an energy source used in many cell processes. Give two ways in which ATP is a suitable energy source for cells to use.

Answer 15

1. Little energy is lost as heat
2. Releases energy instantaneously
3. Phosphorylates other compounds, making them more reactive
4. Can be rapidly re-synthesised
5. Is not lost from cells

Question 16

Describe how ATP is resynthesized in cells

Answer 16

1. ADP + Pi
2. By ATP synthase
3. In respiration

Question 17

Adenosine triphosphate (ATP) is a nucleotide derivative.
Contrast the structures of ATP and a nucleotide found in DNA to give TWO differences

Answer 17

1. ATP has ribose and DNA nucleotide has deoxyribose
2. ATP has 3 phosphate and DNA nucleotide has 1 phosphate
3. ATP- base always adenine and in DNA nucleotide base can be different

Question 18

Explain five properties that make water important for organisms.

Answer 18

1. A metabolite in condensation
2. A solvent so (metabolic) reactions can occur
3. High heat capacity so buffers changes in temperature;
4. Large latent heat of vaporisation so provides a cooling effect
5. Cohesion (between water molecules) so supports columns of water (in plants);
6. Cohesion (between water molecules) so produces surface tension supporting (small) organisms

Question 19

Suggest why water becomes lighter as it expands

Answer 19

1. Density = mass ÷ volume;
2. Ice has same mass of water but greater volume

Question 20

Suggest one biological advantage of this property of water.

Answer 20

1. Ice is colder than water;
2. (Being lighter than water) ice floats on water;

Question 21

Scientists looking for extra-terrestrial life are looking for planets with evidence of free water. Explain why water is considered so important for life to occur. (6)

Answer 21

1. Life (thought to have) evolved in water;
2. Water provides support (for bodies of organisms);
3. Water a major component of cytoplasm;
4. Water is a universal solvent
5. Water is a metabolite in many cell reactions;
6. Water stabilises temperature;
7. Water important in cooling internal temperature (of organisms);

Question 22

Describe the roles of iron ions, sodium ions, and phosphate ions in cells.

Answer 22

Iron ions
1. Haemoglobin binds with oxygen

Sodium ions
2. Co-transport of glucose/amino acids (into cells);
3. (Because) sodium moved out by active transport
4. Creates a diffusion gradient;
5. Affects osmosis

Phosphate ions
6. Affects osmosis
7. Joins nucleotides in backbone of DNA
8. Used to produce ATP;
9. Phosphorylates other compounds making them more reactive;
10. Hydrophilic part of phospholipid bilayer