Plant reproduction

• What are flowering plants known as?

  angiosperms
• What is the calyx?


  Collective name for sepals
• What is the corolla?


  All the petals on a flower
• Insect pollinated flower features:


  -Colourful petals, scent, nectar

  -Anthers within the flower

  -Stigma within the flower

  -Small quantities of larger, sticky sculpted pollen

  
• Wind pollinated flower features:


  - Petals are usually absent or small

  -Anthers are hanging outside the flower

  -Large feathery stigmas hang outside the flower

  -Large quantities of smaller, smooth pollen.

  
• Formation of a pollen grain


  - Formed in the anther by mitosis and meiosis

  - Mitosis occurs to produce large number of diploid pollen mother cells.

  - The mother cells undergo meiosis to produce a tetrad haploid cells

  -These cells form pollen grains that contain the male gamete.

  -The tapetum is a layer of cells around the pollen sac.

  
• What does the tapetum do?


  It is a layer of cells around the pollen sac and provides nutrients to the developing pollen grains and produces a waterproof coat for the pollen grain.
• hapolid gametes


  In each pollen grain, the haploid nucleus undergoes mitosis to produce two nuclei: a generative nucleus and a pollen tube nucleus. The generative nucleus will later undergo mitosis to give rise to 2 male nuclei.
• Exine


  -Pollen cell wall

  -Tough and resistant to chemicals

  resists desiccation
• Pollen is resistant to UV radiation; what does that mean.


  Meaning it can be carried at high altitudes without DNA mutation occurring.
• Dehiscence (release of pollen)


  -Outer layers of the anther dry out, causing tension.

  Tension pulls the walls of the anther apart and the edge of the pollen sacs curl away.

  -The stomium exposes the pollen grains and they are carried away by insect or wind

  
• Development of the ovule


  - The ovule contains the female gamete and is formed in the ovary by mitosis and meiosis.

  -The megaspore mother cell in the nucellus undergoes mitosis to produce 4 haploid megaspores.

  -Three of the megaspores degenerate and only one grows and develops .

  -This one megaspore undergoes 3 mitotic divisions to produce 8 haploid nuclei within the embryo sac of the ovule.

  
• Megaspore (embryo sac)


  Haploid cell that undergoes mitosis three times to form:

  - 1 female gamete

  - 2 polar nuclei

  - 2 synergies

  - 3 antipodal cells.
• Micropyle


  A gap in the integuments for the male nuclei to enter the embryo sac.
• Integuments


  Protect the embryo sac
• Funicle


  Attaches the ovule to the ovary wall
• polar nuclei


  fuse with a male nucleus in double fertilisation to form a triploid endosperm
• Female gamete


  fuses with a male nucleus to form a zygote
• Pollination


  The transfer of pollen from male reproductive structures to female reproductive structures in plants
• self-pollination


  the transfer of pollen grains from an anther to the stigma of the same flower or to the stigma of another flower on the same plant
• cross-pollination


  transfer of pollen grains from the anther of a flower to the stigma of a flower on a different plant of the same species
• Advantages of self pollination


  • only one parent plant

  • offspring inherits ALL GENES from parent plant (more likely to pass down beneficial qualities)

  • does not rely on external factors (eg insects or wind for pollination)

  • higher chance for pollination (as compared to cross pollination)

  • less pollen/energy wasted

  • relatively faster
• Disadvantages of self pollination


  • less genetic variation in offspring

  →less well adapted to changes in environment

  • higher probability of harmful recessive alleles being expressed in offspring

  • continued selfP lead to offspring becoming weaker, smaller, less resistant to diseases
• Why is cross pollination of greater evolutionary significance than self pollination?


  Increases genetic variation so a species can adapt to environmental change-natural selection.
• Fertalisation (plants)


  - Pollen grows a tube to reach the ovary.

  - Tube growth occurs only if pollen lands on the right stigma.

  - The ovary contains ovules, each with an ovum for fertilization.

  - Once pollen and ovum nuclei fuse, fertilization occurs.

  - A zygote is formed as a result of nuclei fusion.

  - The zygote starts to divide and eventually becomes a seed within the ovule.

  - Different plant species have varying ovule counts, explaining the diversity in the number of seeds in fruits.

  
• Double fertilisation


  -One male fuses with the female gamete to produce a diploid zygote.

  -The second male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus.
• Zygote undergoes mitosis


  forms diploid embryo, consisting of plumule, radicle and one or two cotyledons.
• Triploid endosperm nucleus undergoes mitosis


  Forms endosperm tissue, an important food storage tissue in cereal grains.
• Cotyledons


  A seed leaf
• Monocotyledon


  Maize is a monocotyledon so only has one cotyledon.

  The endosperm acts as a food store therefore maize is endospermic.

  
• Dicotyledon


  Broad beans are dicotyledons, they have two cotyledons and an embryo lying between them.

  Broad bean seeds use the cotyledons as food storage tissue this means they do not have an endosperm so are non-endospermic.
• Gibberellin


  Plant hormones involved in the germination of monocotyledons (endospermic)
• Germination


  the process whereby seeds or spores sprout and begin to grow
• Three main requirements for successful germination


  -Oxygen - aerobic respiration to produce ATP for metabolism.

  -Temperature - optimum temperate speeds up the rate of diffusion and increases enzyme activity.

  -Water - transport medium for enzymes and softens the testa.
• first steps of germination


  1) Water is taken in through the micropyle

  2) Tissues then swell, splitting or softening the testa, allowing entry of more oxygen.

  3)Oxygen can then be used in aerobic respiration to produce ATP energy for metabolism.
• Germination in non-endospermic seeds (broad beans)


  1) Mobilised enzymes hydrolyse large, insoluble food stores. Amylase hydrolyses starch into maltose.

  Lipase hydrolyses lipids into fatty acids & glycerol.

  Protease hydrolyses proteins into amino acids.

  2) These soluble products are transported to the plumule and the radicle for use in respiration. Aerobic respiration can then produce ATP energy for mitosis and growth.

  3)The plumule grows upwards. The radicle grows downwards.

  4)The plumule emerges from the soil, the leaves unfurl and begin to photosynthesise. The plant is not independent of food stores in the seed.

  
• Germination in endospermic seeds (maize)


  1)Following imbibition of water, gibberellin is released by the embryo and diffuses to the aleurone layer.

  2)Gibberellins cause gene expression (transcription & translation) to then occurs, producing hydrolytic enzymes.

  3)Protease digests the proteins in the aleurone layer to form amino acids, these can be used in further enzyme synthesis.

  4)Amylase diffuses into the endosperm and hydrolyses the starch stored there into maltose. Proteins and fats in the endosperm are also hydrolysed into amino acids and fatty acids& glycerol.

  5) Maltose, glycerol and other nutrients diffuse to the embryo where they are used in aerobic respiration to provide ATP for the growth of the plumule and radicle.

  
• Aleurone layer


  A protein layer found in monocotyledon seeds which is digested to provide amino acids for protein synthesis.