DNA, ATP, water & inorganic ions

Question 1

Describe the structure of DNA

Answer 1

1. Sugar-phosphate (backbone)/double stranded/helix so provides strength/stability /protects bases/protects hydrogen bonds;
2. Long/large molecule so can store lots of information;
3. Helix/coiled so compact;
4. Base sequence allows information to be stored/ base sequence codes for amino acids/protein;
5. Double stranded so replication can occur semi-conservatively/ strands can act as templates;
6. Complementary base pairing / A-T and G-C so accurate replication/identical copies can be made;
7. (Weak) hydrogen bonds for replication/ unzipping/strand separation;
8. Many weak hydrogen bonds so stable/strong molecule;

Question 2

Describe Semi-conservative replication.

Answer 2

. Strands separate / H-bonds break;
2. DNA helicase (involved);
3. Both strands/each strand act(s) as (a) template(s);
4. (Free) nucleotides attach;
5. Complementary/specific base pairing due to H bonds forming between bases/ Adenine to Thymine and GC;
6. DNA polymerase joins nucleotides (on new strand) forming phoshodiester bonds by condensation;
7. Semi-conservative replication / new DNA molecules contain one old strand and one new strand;

Question 3

Give two features of DNA and explain how each one is important in the semi-conservative replication of DNA.

Answer 3

. Weak / easily broken hydrogen bonds between bases allow two strands to separate / unzip;
2. Two strands, so both can act as templates;
3. Complementary base pairing allows accurate replication;

Question 4

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 4

1. Releases relatively small amount of energy/ 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/ does not leave cells;

Question 5

Describe how an enzyme can be phosphorylated.

Answer 5

• Attachment/association of (inorganic) phosphate (to the enzyme);
• 2. (Released from) hydrolysis of ATP OR (Released from) ATP to ADP + Pi;

Question 6

Explain five properties that make water important for organisms.

Answer 6

. A metabolite in condensation/hydrolysis/ photosynthesis/respiration; 2. A solvent so (metabolic) reactions can occur OR A solvent so allowing transport of substances;
3. High heat capacity so buffers changes in temperature;
4. Large latent heat of vaporisation so provides a cooling effect (through evaporation);
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 7

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

Answer 7

Iron ions
1. Haemoglobin binds/associates with oxygen OR Haemoglobin transports/loads oxygen;
Sodium ions
2. Co-transport of glucose/amino acids (into cells);
3. (Because) sodium moved out by active transport/Na – K pump;
4. Creates a sodium concentration/diffusion gradient;
5. Affects osmosis/water potential;
Phosphate ions
6. Affects osmosis/water potential;
7. Joins nucleotides/in phosphodiester bond/in backbone of DNA/RNA/in nucleotides;
8. Used in/to produce ATP;
9. Phosphorylates other compounds (usually) making them more reactive;
10. Hydrophilic/water soluble part of phospholipid bilayer/membrane