Explain how the alternation of generations in plants, involving both a sporophyte and a gametophyte phase, contributes to reproductive success.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Plants have a life cycle known as alternation of generations, in which a diploid sporophyte generation alternates with a haploid gametophyte generation. The relative dominance of each phase varies considerably across plant groups.
Model answer (4 marks)
The sporophyte is diploid (2n) and produces haploid spores by meiosis.
The gametophyte is haploid (n) and produces gametes by mitosis, which fuse at fertilisation to restore the diploid chromosome number.
Meiosis during spore production introduces genetic variation through independent assortment and/or crossing over, increasing adaptability of offspring.
Having two distinct multicellular phases allows the plant to exploit both sexual reproduction (for genetic diversity and dispersal) and asexual spore dispersal, maximising reproductive opportunities.
The gametophyte is haploid (n) and produces gametes by mitosis, which fuse at fertilisation to restore the diploid chromosome number.
Meiosis during spore production introduces genetic variation through independent assortment and/or crossing over, increasing adaptability of offspring.
Having two distinct multicellular phases allows the plant to exploit both sexual reproduction (for genetic diversity and dispersal) and asexual spore dispersal, maximising reproductive opportunities.
Examiner tips
- Use the exact terms ‘diploid’, ‘haploid’, ‘meiosis’, ‘mitosis’, ‘fertilisation’, ‘independent assortment’, ‘crossing over’.
- Show the cycle: sporophyte → meiosis → spores → gametophyte → mitosis → gametes → fertilisation → sporophyte.
- Link variation to adaptability and explain how two phases give flexibility in reproduction.
Common mistakes
- Confusing meiosis with mitosis for spore production.
- Failing to mention that gametophyte is haploid and produces gametes by mitosis.
- Omitting the link between genetic variation and adaptability.
Mark scheme (4 marks)
- The sporophyte is diploid (2n) and produces haploid spores by meiosis.
- The gametophyte is haploid (n) and produces gametes by mitosis, which fuse at fertilisation to restore the diploid chromosome number.
- Meiosis during spore production introduces genetic variation through independent assortment and/or crossing over, increasing adaptability of offspring.
- Having two distinct multicellular phases allows the plant to exploit both sexual reproduction (for genetic diversity and dispersal) and asexual spore dispersal, maximising reproductive opportunities.
Key terms in this question
alternation of generations · sporophyte · gametophyte
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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