8 - Sexual Reproduction in Plants

Flower Structure
1. Angiosperms = flowering plants
2. Pollination = the transfer of pollen from the anther of one flower to the mature stigma of another flower of the same species
3. Insect-pollinated flower
  1. Large, colourful petals, scent and nectar to attract pollinators
  2. Anthers within flower, which transfer pollen to insects when they feed on nectar
  3. Stigma within flower to collect pollen from insect when it feeds on nectar
  4. Small quantities of sticky sculptured pollen to stick to insect
4. Wind-pollinated flower
  1. Small, green and inconspicuous, no scent, petals usually absent
  2. Anthers hanging on outside so wind can blow pollen away
  3. Large feathery stigma = large SA to catch pollen grains
  4. Large quantities of small, smooth, light pollen to be carried by wind
5. Anther produces Pollen = male gamete, stamen is male
6. Ovule contains embryo sac with one female gamete inside
  1. Carpel is female
7. Sepal protects flower in the bud
Gamete development
1. Development of male gamete
  1. Dehiscence = the opening of the anther, releasing the pollen grains
  2. Pollen sac contains many pollen cells
  3. Tapetum provides nutrients to developing pollen grains
  4. Pollen cell wall tough + resistant to desiccation
  5. Generative nucleus undergoes mitosis producing two male nuclei
2. Development of female gamete
  1. Embryo sac contains: 3 antipodals (haploid), 2 synergids (haploid), 1 oosphere (haploid), 1 polar nucleus (diploid)
3. Ovary structure
Self-pollination
1. Pollination = the transfer of pollen from the anther of one flower to the stigma of another of the same species
2. = when pollination occurs between the anther and stigma of the same flower/plant
3. results in:
  1. little genetic variation
  2. increased risk of harmful recessive alleles coming together
  3. preservation of successful genomes
4. Inbreeding
Cross-pollination
1. = pollination of different flowers/plants of the same species
2. results in:
  1. greater genetic variation
  2. reduced chance of harmful recessive alleles coming together
3. Outbreeding
4. Ensuring cross-pollination:
  1. Stamen and stigma ripen at different times
    1. Protandry = stamen ripens before the stigmas
  2. Anther located below stigma, reducing risk of pollen falling on it
  3. Some plants have seperate male and female flowers/plants
  4. Some plants have genetic incompatibility
    1. pollen cannot germinate of stigma of same plant
Double fertilisation
1. Fertilisation = fusion of the male gamete with the female gamete, producing a diploid zygote
2. 1. when pollen grain lands on stigma of another plant it germinates, producing a pollen tube
  1. 2. growth of tube controlled by pollen tube nucleus, also producing hydrolases e.g. celluloses + proteases
    1. 3. hydrolyses digest a path through style towards micropyle and into embryo sac, guided by chemical attractants e.g. GABA
      1. 4. tube nucleus disintegrates, two male gametes enter ovule
        5. one male nucleus fuses with haploid female nucleus (oosphere), forming zygote
        6. second male nucleus fuses with diploid polar nucleus, forming triploid nucleus, which develops into endosperm, which will provide nutrition for developing embryo
Development of fruit and seed
1. fruit = a structure developing from the ovary wall, containing one or more seeds
2. seed = structure developed from a fertilised ovule, containing an embryo and food store enclosed within a testa
3. 1. diploid zygote divides by mitosis, forming embryo
  1. 2. triploid endosperm nucleus develops into food store
    1. 3. outer integument dries out, hardens and waterproofs, containing lignin, becomes seed coat/testa
      1. 4. funicle of ovule becomes funicle of seed, attaches at hilum
        5. ovary becomes fruit
4. Structure
  1. Dicotyledonous plants (broad beans) = two cotyledons, which absorb endosperm/ food store
  2. Radicle becomes root
  3. plumule forms shoot
Seed dispersal
1. allows seeds to germinate away from parent, reduces competition
2. wind
3. water
4. attached to animal fur
5. animals eat seeds, come out in faeces
Germination of broad bean
1. Germination = the biochemical and physiological processes through which a seed becomes a photosynthesising plant
2. 1. water absorbed by seed, tissues swell + enzymes mobilise
  1. 2. testa ruptures, radicle pushes through downwards, plumule upwards
    1. 3. amylase hydrolyses starch into maltose, which is transported to growing parts
      1. 4. cotyledons remain below ground
        5. plumule bent over in shape of hook, preventing damage to tip by soil abrasion
        6. plumule emerges + unfurls + begins to photosynthesise
3. Requirements:
  1. Optimum temperature for enzymes
  2. water for mobilisation of enzymes + transport of products to growing points
  3. Oxygen for aerobic respiration
4. As seed germinates, mass decreases until plumule photosynthesises
5. Effect of gibberellin
  1. Gibberellin acid (GA) = plant growth regulator
    1. diffuses into aleurone layer surrounding endosperm, switching on genes involved in protein synthesis, resulting in production of amylases + proteases
  2. amino acids produced by hydrolysis of proteins used to synthesise amylases, which turn stored starch into maltose + glucose for respiration

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8 - Sexual Reproduction in Plants

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	Created by Rosie Horwood over 5 years ago