A level Biology B · Edexcel 9BI0 · Topic 2F
Sexual reproduction in mammals and plants
Key points
- Mammalian sperm have a haploid nucleus, an acrosome of digestive enzymes, many mitochondria and a tail; the egg is surrounded by the zona pellucida.
- Acrosome reaction: enzymes digest the zona pellucida so the sperm can fuse with the egg membrane.
- Cortical reaction: cortical granules release enzymes that harden the zona pellucida, preventing polyspermy.
- In flowering plants, pollen lands on the stigma and a pollen tube grows down the style, guided by chemicals, carrying two male nuclei.
- Double fertilisation: one male nucleus fuses with the egg to form the zygote; the other fuses with the two polar nuclei to form the triploid endosperm (food store).
- Plants also have life cycles that alternate between haploid and diploid generations.
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Sexual reproduction joins two haploid gametes to make a diploid zygote. Mammals and flowering plants solve the same problems in different ways: making gametes, getting the male nucleus to the egg, and stopping more than one male gamete…
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