Misc

Question 1

1. The Nitrogen Cycle

Answer 1

• Nitrogen fixation (N₂ → ammonia) by nitrogen-fixing bacteria (e.g., Rhizobium) or lightning.
• Nitrification (ammonia → nitrites → nitrates) by nitrifying bacteria (Nitrosomonas & Nitrobacter).
• Assimilation (plants absorb nitrates to make amino acids & proteins).
• Ammonification (decomposition of organic N compounds → ammonia) by saprobionts.
• Denitrification (nitrates → nitrogen gas) by denitrifying bacteria under anaerobic conditions.

Question 2

Eutrophication – AQA A-Level Biology

What is it?

Answer 2

Eutrophication is the process where nutrient enrichment (usually nitrates or phosphates) in water leads to a reduction in biodiversity due to oxygen depletion.

Question 3

Eutrophication Step-by-step process:

Answer 3

1. Fertilisers leach into water
   • Rain washes nitrates/phosphates from farmland into rivers/lakes.
   
   2. Algal bloom
      • These nutrients cause rapid growth of algae on the water surface (called an algal bloom).
   3. Light blocked
      • Algae block sunlight from reaching aquatic plants below → plants die.
   4. Increased decomposition
      • Dead plants and algae are broken down by aerobic bacteria (decomposers).
   5. Oxygen depletion (hypoxia)
      • These decomposers use up oxygen during respiration → oxygen concentration in water drops.
   6. Death of aquatic life
      • Fish and invertebrates die due to lack of oxygen → biodiversity decreases.

Question 4

2. The Carbon Cycle

Answer 4

• Photosynthesis (CO₂ fixed into glucose).
• Respiration (organic compounds broken down releasing CO₂).
• Decomposition of dead organisms releasing CO₂.
• Combustion of fossil fuels releasing CO₂.
• Ocean absorption and carbon sequestration in plants and soil.

Question 5

Natural selection Step-by-step process (Mark scheme format):

Answer 5

1. Variation exists in a population due to mutations.
   
   2. Individuals with advantageous alleles (e.g. better suited to the environment) are more likely to survive.
   3. These individuals reproduce more successfully than others.
   4. They pass the beneficial alleles to offspring.
   5. Over time, the frequency of the advantageous allele increases in the population.
   6. This leads to evolution — a change in allele frequency over generations.

Question 6

Natural Selection – AQA A-Level Biology

Definition:

Answer 6

Natural selection is the process where organisms with beneficial alleles are more likely to survive, reproduce, and pass on their alleles to the next generation.

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Question 7

✅ Allopatric speciation (most commonly examined)

Step-by-step (Mark scheme style):

Answer 7

1. Geographical isolation
   • A physical barrier (e.g. river, mountain) splits a population into two groups.
   
   2. Different selection pressures
      • Each environment has different conditions, so different mutations give survival advantages in each group.
   3. Natural selection
      • Advantageous alleles are passed on → each population adapts to its environment via natural selection.
   4. Genetic divergence
      • Over time, allele frequencies change in each group → their gene pools become increasingly different.
   5. Reproductive isolation
      • Eventually, they can no longer interbreed to produce fertile offspring → they’re now separate species.

Question 8

Speciation – AQA A-Level Biology

Definition:

Answer 8

Speciation is the formation of new species from an existing population due to reproductive isolation and accumulation of genetic differences.

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Question 9

Sympatric Speciation – AQA A-Level Biology

🧠 Definition:

Answer 9

Speciation that occurs within the same geographical area, without a physical barrier. Instead, reproductive isolation happens due to behavioural, temporal or genetic differences.

Question 10

Sympatric speciation Step-by-step process (Mark scheme-style):

Answer 10

1. Mutation causes reproductive isolation
   • A mutation arises in some individuals that prevents them breeding with others in the same area.
   • This could be due to:
   • Different mating behaviours (behavioural isolation)
   • Different mating times/seasons (temporal isolation)
   • Chromosomal changes like polyploidy (especially in plants)
   
   2. Two groups stop interbreeding
      • Even though they live in the same place, they are now reproductively isolated.
   3. Different alleles passed on
      • Mutations still happen in both groups.
      • Different alleles are passed on in each due to natural selection and/or genetic drift.
   4. Genetic divergence increases
      • Over generations, the gene pools diverge further.
      • Eventually, they become genetically distinct.
   5. New species formed
      • Even if the groups come back together, they can no longer interbreed to produce fertile offspring → they’re now separate species.

Question 11

🧪 Hardy-Weinberg Principle – AQA A-Level Biology

📌 Definition:

Answer 11

The Hardy-Weinberg principle predicts that allele frequencies in a population will not change from one generation to the next if no evolutionary forces act on the population.

Question 12

✅ Conditions for equilibrium (must learn these!):

Allele frequencies remain constant if:

Answer 12

1. No mutations
   
   2. No natural selection
   3. Random mating
   4. No gene flow (immigration/emigration)
   5. Very large population

(A change in frequency = evolution’s happening)

Question 13

1. Allele frequency

Answer 13

p + q = 1
• p = frequency of dominant allele (A)
• q = frequency of recessive allele (a)