ANSWERS

Question 1

what is tissue culture?

Answer 1

when a group of cells is grown into a new organism

Question 2

Organic substances decay into simpler substances.

The leaves fall off many trees in autumn.

The dead leaves contain carbon compounds and nitrogen compounds.

Describe how carbon and nitrogen in compounds in the leaves are recycled and used by living trees.
You should include a description of:

• how the leaves are broken down (so decomposers)
• how substances are taken in and used by the trees (roots and active transport, photosynthesis).

Answer 2

• Microorganisms cause the decay of the dead leaves by using enzymes to break them down.
• As the microorganisms carry out decomposition they respire which releases carbon dioxide into the air.
• Carbon dioxide in the air is taken up by the stomata in the trees and is used by the leaves for photosynthesis to make glucose.
• At the same time, the microorganisms release nitrate ions into the soil as they decompose the leaves.
• These nitrate ions are absorbed by the tree roots to combine with glucose to make amino acids

Question 3

Students investigated the effect of temperature on the decay of milk. This is the method used.
1. Place 25 cm3 of fresh milk into each of three beakers.
2. Keep one beaker of milk at 5 °C.
3. Keep one beaker of milk at 15 °C.
4. Keep one beaker of milk at 25 °C.
5. Record the pH of the milk in each beaker every day for 4 days.
Table 1 shows the results.
Time in days
5 C
Table 1
pH of milk 15 C 6.8
6.1
5.5
5.1
4.8
25 C
6.8 5.3 4.9 4.8 4.8
0 6.8
1 6.5
2 6.2
3 5.9
4 5.6
04.2 Suggest one improvement the students could have made to the method.

Answer 3

anyone from:
- Use the same type of milk
- use more temperatures
- measure the pH at smaller time intervals

Question 4

Thyroxine is a hormone produced by the thyroid gland.

A decrease in body temperature causes an increase in thyroxine production.

Explain how the production of thyroxine causes an increase in body temperature.

Answer 4

• Thyroxine increases the body’s basal metabolic rate.
• So rate of respiration is increased
• Which releases energy to increase the body temperature.

Question 5

The concentration of ADH in a man’s blood was 3.75 nanograms/dm3.

The concentration of ADH in his blood decreased to 1.25 nanograms/dm3.

Explain how the decrease in the concentration of ADH would cause the changes to urine shown in table 2.

Answer 5

• Decrease in ADH means kidney tubules less permeable to water
• This means less water is reabsorbed into the bloodstream.
• So there now is more water in urine causing the rate of urine being produced to increase.
• More water in urine causing lower concentration of dissolved substances.

Question 6

A wide variety of species exists on Earth.

Most scientists accept Darwin’s theory of evolution by natural selection as the explanation for this variety of species.

Explain how our understanding of evolution has developed due to:

• fossil evidence
• increased understanding of the mechanisms of genetics (Mendel)

Answer 6

Indicative content Fossil evidence:
* Fossils show evidence of life in the past and how change in organisms has occurred over time.
*So fossils show the development in a species over time which suggests that

• fossil record shows development of species over time
• fossils show evidence of extinction
• fossil record shows how organisms from the past are related to
  species alive today
• gaps in fossil record
• gaps in fossil record are being filled in with new evidence
• ref to evolutionary trees
• description of how fossils are formed
  Genetics:
• ref to Mendel’s breeding experiments with plants
• Mendel’s description / idea of units / factors of inheritance
• dominant and recessive units / alleles / genes
• observation of chromosome behaviour during cell division
• chromosome behaviour and Mendel’s units work in similar ways * structure of DNA worked out
• gene mechanism in determining protein synthesis worked out
• (genetic) variation in a species
• (variation) due to mutation or change in gene (structure)
• individuals with advantageous characteristics more likely to
  survive
• individuals with advantageous characteristics more likely to
  reproduce
• (survivors) pass on (advantageous) alleles / genes
• eg of evolution (such as antibiotic resistance in bacteria)
• new species arise when sufficient changes occur to prevent
  (successful) reproduction
  For Level 3, the response must include details about fossils and about the mechanisms of genetics
  For Level 2, the response should include descriptions about fossils and / or genetics

Question 7

how does meiosis produce offspring that are genetically different?

Answer 7

random chromosome from each pair (of chromosomes)
moves to one end of the cell