Plant Reproduction

• Female parts of a plant

  the stigma, style and ovary form the carpel
• Male parts of a plant

  the anther, filament, and pollen form the stamen
• How are Pollen grains formed?

  1. Diploid microspore mother cells undergo meiosis to form 4 haploid microscpores which develop
  2. The nuclei of the Haploid microspores undergo mitosis to mature into pollen grains, containing a generative and pollen tube nucleus.
• How are embryo sacs formed?

  1. Diploid megaspore mother cells undergo meiosis to produce 4 haploid megaspores
  2. three of these degenerate, whilst one grows Until it fills most of the ovule
  3. The nucleus of this megastore divides by mitosis 3 times to form 8 haploid nuclei.
  4. The cytoplasm of the immature embryo sac then divides to form a mature embryo sac.
  5. The mature embryo sac contains 3 antipodal cells, 2 polar nuclei, 2 synergic cells and 1 female gamete.
• what is pollination

  the transfer of pollen grains from the anther to the stigma of a plant of the same species.
• what are the 2 types of pollination

  self pollination

  cross pollination
• WHat are the two ways plants can cross pollinate?

  insect pollination

  wind pollination
• features of wind pollinated plants:

  • no scent
  • dull colours
  • external stamens
  • exposed stigma
  • feathery stigma
  • no nectaries
  • smaller, smoother pollen
• features of insect pollinated plants

  • scented
  • bright colours
  • internal stamen
  • enclosed stigma
  • sticky stigma
  • nectaries
  • larger, stickier pollen
• describe the structure of pollen grains

  • generative nucleus which produces 2 male gametes via mitosis
  • Pollen tube nucleus which elongates to penetrate ovule
  • outer protective coating
• describe the structure of the embryo sac

  • 2 polar nuclei which form endosperm
  • the ovum forms a zygote
  • 2 synergids which help generative nucleus of pollen grain reach ovum
  • 3 antipodal cells
  • outer protective coating
• how do male nuclei reach the embryo sac?

  • pollen grain germinates on the stigma
  • pollen tube emerges from the split and grows down the style towards the ovary
  • the pollen tube secretes enzymes to digest the tissue around it , and it uses these enzymes as a nutrient source.
  • When the pollen tube reaches the ovary, it enters the female embryo sac through the micropyle.
  • The pollen tube nucleus degenerates whilst the 2 male gametes enter the embryo sac.
• where does double fertilisation occur? What happens?

  in the embryo sac of the ovule.

  • one male gamete fertilises the female gamete to form a diploid zygote.
  • the other male gamete fuses with 2 polar nuclei to form a triploid primary endosperm cell
• what is the purpose of a triploid endosperm?

  it acts as a nutrient source for the embryo
• what happens after fertilisation when the flower develops into a seed?

  1. zygote develops into the embryo. once dispersed it will germinate and form shoots and roots, growing into a new plant.
  2. the primary endosperm grows and develops into a food store which nourishes the embryo.
  3. the embryo sac develops into the seed surrounding the embryo, protecting it during dispersal and germination.
  4. the ovary wall develops into a fruit to aid seed dispersal
• Suggest why pollen in peat bogs is preserved for many years

  • due to low oxygen, microorganisms can’t respire, so bacteria cannot produce enzymes to break down sporopollenin
  • the low ph reduces enzyme activity
  • very slow decomposition due to the lack of microorganisms.
• give 3 other roles of water in a plant other than for pollen tube growth

  1. photosynthesis
  2. transport medium in translocation and transpiration
  3. turgor changes
• suggest a stimulus, other than water, that causes the pollen tube to grow towards the micropyle

  gravity
• suggest the role of digestive enzymes in the growth of the pollen tube

  • enzymes digest the surrounding tissue in the style, clearing a pathway for the pollen tube to grow down.
  • the enzymes break down pectin
  • enzymes act as a nutrient source for growth of pollen tube.
• suggest the advantages to flowering plants of increased pollen tube growth

  • pollen tube is more likely to reach the ovule
  • so fertilisation happens faster
• explain how meiosis produces genetic variation in gametes

  • both independent assortment and crossing over occur.
  • independent assortment gives rise to new combinations of chromosomes
  • crossing over allows sections of chromosomes to break off at chiasmata and join other chromosomes
• in a pollen tube growth experiment, why is temperature kept constant?

  • enzymes in the pollen tube are affected by temperature
  • therefore it makes the investigation valid
• describe how the structure of a pollen grain differs from that of a sperm cell

  • pollen grains do not have a flagellum
  • pollen grains do not have an acrosome
  • pollen grains contain 2 nuclei
• What is the purpose of the investigation described in the study material?

  To determine the effect of pH on the rate of growth of pollen tubes
• What pH values are used in the investigation?

  Five different pH values ranging from 0.0 to 1 mol/dm^-1
• How should the pH solutions be prepared for the investigation?

  By measuring out equal volumes of the 5 different pH values and adding them to 5 slides
• What additional substance is added to each slide along with the pH solutions?

  Equal volumes of sucrose concentration
• What is the method used to introduce pollen grains onto the slides?

  Using a mounted needle to rub the anther of a flower
• How long should the slides with pollen grains be left in the pétri dish?

  24 hours
• What tool is used to measure the length of the pollen tubes formed?

  A microscope with a stage micrometer and graticule
• How is the rate of growth of the pollen tubes calculated?

  By dividing the length of the pollen tube by the time
• What should be ensured when repeating the process for each concentration?

  That the same species of flower are used and collected from the same plant
• How can humidity be controlled during the investigation?

  By ensuring a lid is on the pétri dish
• What should be calculated at the end of the investigation?

  A mean of the results