plant reproduction

Define a fruit

- a structure developing from the ovary wall, containing 1 or more seed

Define a seed

- structure developed from a fertilised ovule, containing an embryonic plant and food store enclosed within a testa

What is a testa

Seed coat

- formed by integuments

How is an embryo formed?

- the zygote (2n) divides by mitosis to form an embryonic plant (2n)

Describe the embryo in a plant

- the embryo consists of the radicle and plumule and one or two cotyledons

What is a radicle

- an embryo root

What is a plumule

An embryo shoot

What is a cotyledon

- seed leaves

Describe the formation of the endosperm

- the triploid nucleus in the embryo sac divides by mitosis to from the endosperm (3n)

Outline function of endosperm

- takes over from nucellus

- will be the food supply for the embryo when the seed germinates before photosynthesis can begin

- in some seeds the endosperm itself is the food store

- in other seeds, the endosperm is absorbed by the growing cotyledons which become the food store

What seed uses the endosperm as the food store

- maize

What seed uses the cotyledons as a food store

- broad bean

State changes post-fertilisation in a plant

- ovary wall becomes a fruit

- fertilised ovule becomes a seed

- zygote forms an embryo

- triploid nucleus becomes endosperm

- integuments dry out, harden and become waterproof, with deposits of lignin and become testa

- funicle/stalk of ovule becomes funicle of the seed and attaches to the seed at the hilum

What happens to micropyle post-fertilisation

- remains as a pore and is where water will later be absorbed by osmosis triggering germination

Function of ovary wall

- develops into the fruit enclosing the seeds whose function is to protect and later to disperse the seeds

Outline seed maturation

- when a seed is mature, it becomes dehydrated

- this is a survival strategy for terrestrial life

- this seed may remain dormant for some time during which it will be dispersed so that it does not compete with the parent plant

What are types of seed structure

- monocotyledon and dicotyledon

Compare the 2 types of seed structures

- monocotyledon is a 1 seeded fruit (1 cotyledon in seed) with 1 leaf, endosperm is food store, leaf veins are parallel, sepals, petals and stamens in multiple of 3, vascular bundles scattered in stems and roots.

- Dicotyledons are 2 seeded, 2 seed leaves, endosperm is absorbed by cotyledon, leaf veins are a network, sepals, petals and stamens in multiples of 4/5, vascular bundles in a ring in stems and centre of roots

give an example of a dicotyledon

broad bean

give an example of a monocotyledon

maize

what happens after fertilisation to a broad bean

- dicotyledon

- the ovary of a broad bean flower (containing several ovules) elongates into the pod, the fruit

- the fertilised ovules mature into the seed, the beans

- the endosperm, food store for early embryo, is absorbed into the cotyledons

- the broad bean is a non-endospermic seed

what happens to maize after fertilisation

- the Testa fuses with the ovary wall so maize is a 1 seeded fruit

- the cotyledon remains small + doesn't develop further

- the endosperm remains as the food store

- maize is an endospermic seed

outline the adaptations seeds have for survival

- dormant seeds: low metabolic rate so survive very cold weather

- testa: physically protects the seed and is chemically resistant so seeds survive adverse chemical conditions

- water content: less than 10% so seeds survive very dry conditions

- endosperm/cotyledons: provide nutrient supply to last until emerging seedling can photosynthesise

- seed dispersal: away from parent plant to reduce competition and to colonise new habitats

- inhibitors: broken down in very cold weather (vernalisation) so preventing germination until suitable time of year

State adaptations of seeds for survival

- dormant seeds

- testa

- water content

- endosperm/cotyledon

- seed dispersal

- inhibitors

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)

define pollination

- transfer of pollen grains from the anther to the mature stigma of a plant of the same species

types of pollination

- self-pollination

- cross-pollination

define self pollination

- transfer of pollen from the anther of a flower to the mature stigma of the same flower or a different flower of the same plant

define cross-pollination

- transfer of pollen from the anther of a flower to a mature stigma of another flower on a different plant of the same species

- most angiosperms use this pollination type

what is the main implications of the different types of pollination

- genetic

outline the genetic implications of self-pollination

- leads to self fertilisation = inbreeding

- self-fertilised species depend only on independent assortment and crossing over during meiosis and on mutation to bring about genetic variation in the genomes of their gametes. So less genetic variation than cross fertilised species which also combine gametes from 2 different individuals

- there is a greater chance of 2 potentially harmful recessive alleles being brought together at fertilisation

- an advantage of inbreeding: it can preserve those successful genomes that are suitable to a relatively STABLE environment

outline genetic implications of cross-pollination

- leads to cross- fertilisation = outbreeding

- outbreeding combines gamete from 2 different individuals, in addition to events in meiosis and mutation so it generates more genetic variation

- outbreeding reduces chance of producing harmful allele combinations

- and has great evolutionary significance: in a particular environment, some genomes are more successful than others. This may allow a species to survive in a CHANGING environment, as there are always likely to be some members of the population with a suitable combination of alleles

state methods that flowering plants use to ensure cross-pollination takes place

- dichogamy

- genetic incompatibility

- structure: anther below the stigma + pin and thrum

- separate male and female flowers on same plant

- separate male and female plants

outline dichogamy + examples

- the stamen and stigma ripening at different times

- in protandry eg: daisy, the stamens ripen first

- in protogyny, eg: bluebell, which is rarer, stigma ripens first

Explain the pin and thrum structures of primrose

- they are pollinated by long tongued bees and the structures are such that pollen from a pin primrose is more likely to be deposited on the stigma of a thrum and vice versa when the bee visits flowers with the different structures

- in a pin-eyed primrose, the anther is below the stigma so pollen cannot fall on it

outline impact of anther being below stigma + example

- so pollen can't fall onto it

- pin-eyed primrose

explain genetic/chemical incompatibility + example

- red clover

- pollen can't germinate on the stigma of the flower that produced

- so self pollination can occur but the pollen tube cannot grow on the stigma of the same flower that produced the pollen

explain separate male and female flowers on the same plant + example

- maize

give example for a plant that has separate male and female plants

- holly

What is fertilisation

- fertilisation in the fusion of the male gamete with the female gamete to form a zygote

- the male gamete is the nucleus contained in the pollen grain

- the female gamete is in the ovule protected by the ovary

- the male gamete is delivered to the female gamete by the pollen tube

Outline germination of the pollen grain

- when a compatible pollen gran lands on the stigma it absorbs water and germinates in the sucrose solution secreted by the stigma and produces a pollen tube

- the pollen tube nucleus is at the tip of the tube with the two generative nuclei behind it

- the pollen tube grows out through the pit, a gap in the exine and down the style along a chemoattractant gradient

Outline the growth of the pollen tube

- pollen tube growth is controlled by the tube nucleus

- hydrolytic enzymes, secreted by the pollen tube, digest a path through style tissues and the products of digestion are used for growth of the pollen tube

- the direction of growth may be in response to chemicals secreted by the synergids in the ovule eg: GABA

- gamete release

Give an example of a chemoattractant

- GABA secreted from the ovule

Outline the gamete release

- the pollen tube grown through the micropyle (gap in the integuments) and passes into the embryo sac

- the tube nucleus degenerates and the tip of the tube breaks open releasing the 2 male gametes into the embryo sac

- double fertilisation

Outline double fertilisation

- One of the male gametes (n) fuses with the oosphere (n) to form the zygote (2n)

- the other male gamete (n) fuses with the diploid polar nucleus (2n) to form the triploid endosperm nucleus (3n) - this generates endosperm tissue (after mitosis)

- = double fertilisation

Function of endosperm

- takes over from nucellus in providing nutrition for developing embryo