Gametogenesis - plants

Describe structure of the immature ovule

- made up of a megaspore mother cell, surrounded by a nucellus protected by 2 layers called integuments

- ovule is attached to wall of the ovary by the funicle/funiculus

Outline female gametogenesis

- Meiosis in the ovule: the megaspore mother cell (2n) divides by meiosis to form 4 haploid megaspores (n)

- nuclei degeneration: 3/4 haploid cells degenerate leaving 1 remaining megaspore

- mitosis in the ovule: remaining haploid megaspore undergoes 3 rounds of mitosis to form 8 haploid nuclei in the embryo sac.

What does the embryo sac in a mature ovule contain

- the oosphere (n) (egg cell)

- 2 synergids (n)

- 3 antipodals (n)

- 2 polar nuclei (n) that fuse to form 1 secondary nucleus (2n)

Define ovule

- female reproductive cell - megaspore mother cell, nucellus, integuments

Nucellus

- layer of cells surrounding the embryo sac

- provides nutrients

Integuments

- 2 layers of protective cells around the nucellus

- later become the seed coat/testa

Micropyle

- gap in the integument so pollen tube can grow + enter embryo sac

- remains as a pore in seed coat for water to enter

Funicle

- connects ovule to the ovary wall

Megaspore mother cell

- 2n cell, meiosis, n cell, 3x mitosis, embryo sac with 8x n nuclei/cells

Embryo sac/ovum

- contains the nuclei/cells after megaspore mother cell undergoes meiosis + mitosis (x3)

Polar nuclei

- within the embryo sac, develop into endosperm after fertilisation

Egg cell/ovum

- female gamete -> zygote -> embryo plant after fertilisation

Synergids

- on either side of the egg cell, release a chemical signal to guide the pollen tube to the egg cell, will disintegrate after fertilisation

Antipodal cells

- opposite end of embryo sac - no known function, disintegrate after fertilisation

What is the structure of the pollen sac

- as the pollen sac develops, many rounds of mitosis produce a large number of cells called pollen mother cell (aka microspore mother cells)

- tapetum around the the pollen sac provides nutrients and regulatory molecules to the developing pollen grains and in the formation of the pollen cell wall

Describe the formation of the male gamete: pollen

- meiosis of the microspore mother cells in the pollen sac: the pollen mother cell undergoes meiosis to produce a tetrad of 4 haploid cells. These separate and become pollen grains

- mitosis in pollen grain: the haploid nucleus undergoes mitosis to form 2 haploid nuclei, generative and tube nuclei

- the pollen grain develops a specialised wall with a thin inner layer (intine) + thick outer layer (exine) with thinner areas for pollen tube to grow

- generative nucleus then undergoes mitosis to form 2 haploid nuclei = male gametes

Describe the pollen cell wall

- tough, resistant to chemicals, desiccation and UV as protection against drying out + mutation

Pollen sac

- 4 pollen sacs in the anther - it is where pollen grains are formed

Tapetum

- layer of cells surrounding the pollen sac, supplying nutrients and regulatory molecules

Intine

- thinner inner wall of pollen grain

Exine

- thicker outer wall of pollen grain, often textured (aids sticking to stigma

Microspore mother cell

- 2, undergoes meiosis -> tetrad of pollen grains

Tetrad

- 4 haploid pollen grains

- before secretion of pollen grain

Pollen grain

- contains male gamete + tube nucleus provides

- provides protection

- vehicle for transport of male nucleus to female gamete

Generative nucleus

- male nucleus/gamete (x2 haploid after mitosis)

Tube nucleus

Controls the growth of the pollen tube

Lateral groves

- groves between lobes of the anther

Dehiscence

- process of the opening and release of the mature pollen grains

Stomium

An opening in the anther/pollen sac, exposes the pollen grains for transport (wind/animal)