4.2 Sexual Reproduction (plants) Flashcards
Define pollination
- transfer of pollen from male anther to female stigma
Differentiate between self pollination and cross pollination
Self: pollen from same plant being pollinated
Cross: pollen from a different plant to the one being pollinated
—> requires a vector ie birds, bees, wind
Structure of insect pollinated flowers
- colourful petals with nectar guides
- scent and nectar
- anthers within flower
- stigma within flower
- small quantities of sticky, sculptured pollen
- larger pollen grains
Structure of wind pollinated flowers
- petals absent or small
- no scent or nectar
- anthers hang outside flower
- long, feathery stigmas hang outside flower
- large quantities of smooth pollen
- smaller pollen grains
Describe the calyx
- comprises sepals
- protect flower in the bud
—> can be petal-like ie in lillies
Describe the Corolla
- ring of petals
- can be small or absent
- sometimes a nectary at the base which attracts pollinators
Describe the stamens
- male gamete
- filament supporting an anther which produces pollen grains
- filament contains vascular tissue which supports sucrose, mineral ions and water
- anther contains 4 pollen sacs
Describe the carpels
- female gamete
- closed structure in which one or more ovules develop
- receptive surface at top called stigma
Define dehiscence
- the opening of the anther, releasing pollen grains
Define the tapetum
- a layer of cells around the pollen sac that provide nutrients and regulatory molecules to developing grains
- plays significant role in formation of the pollen cell wall which is tough and resistant to chemicals
Describe the development of the male gamete (anther)
- diploid pollen mother cells undergo meiosis
- each forms a tetrad containing four haploid cells which form four pollen grains
- inside each pollen grain, the haploid nucleus undergoes mitosis to produce two nuclei - generative nucleus and tube nucleus
—> generative nucleus produces 2 male nuclei by mitosis
Draw and label a mature pollen grain
Describe the process of dehiscence
- when the pollen is mature the outer layers of the anther dry out, causing tension in lateral grooves
- dehiscence occurs in which tension pulls the walls of the anther apart and the edges of the pollen sacs curl away
- opening called the stomium exposes pollen grains and they are carried away by insects/wind
Describe the development of the female gamete (ovules)
- ovary contains one or more ovules
- in each ovule, a megaspore mother cells undergoes meiosis making four haploid cells
- three disintegrate
- remaining cell undergoes 3 rounds of mitosis to make 8 haploid nuclei, one of which is the female gamete (egg cell nucleus / oosphere)
- two of the haploid nuclei fuse to make a diploid nucleus called the polar nucleus
What is the nucellus?
A layer of cells which provide nutrients
—> has 2 layers around it called the integuments
—> gap in the integuments called micropyle
Explain the genetic implications of self pollination
- leads to self-fertilisation which results in inbreeding
- self fertilised display less genetic variation than cross-fertilised species
- greater chance of two potentially harmful recessive alleles
- advantage of interbreeding is that it can preserve those successful genomes that are suited to a relatively stable environment
Explain the genetic implications of cross pollination
- results in outbreeding
- creates more genetic variation
- reduces chances of producing harmful allele combinations
- evolutionary significance - allow species to survive in changing environment
Define protandry
Where the stagments of a flower ripen before the stigmas
Describe ways in which plants are adapted to ensure cross-pollination
- dichogamy - stamen and stigma ripening at different times
- anther below the stigma so pollen cannot fall onto it
- genetic incompatibility - where pollen cannot germinate on the stigma of the flower that produced it
- separate male and female flowers on the same plant
- separate male and female plants
What are the nuclei at the top and bottom of the embryo sac called?
Top = antipodals
Bottom = synergids
What does an embryo sac in a mature ovule contain?
- 3 antipodals
- 2 synergids
- 1 oosphere
- 1 polar nucleus
Describe the process of double fertilisation
- compatible pollen grain lands on the stigma, it 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 two male nuclei behind
- pollen tube grows out through a gab in the cell wall called a pit, up a gradient of chemoattractants. Pollen tube nucleus codes for the production of hydrolases and digests its way through the tissues of the style. The products are used by the growing pollen tubes
- pollen tube grows through the micropyle and passes into embryo sac
- pollen tube nucleus disintegrates
- tip of pollen tube opens, releasing two male gamete’s into the embryo sac
- one male gamete fuses with oosphere to form a zygote and the other fuses with the polar nucleus to form a triploid nucleus known as an endosperm nucleus
- endosperm nucleus divides repeatedly by mitosis and generates endosperm tissue which provides nutrition to the growing embryo
Describe the process of double fertilisation
- compatible pollen grain lands on the stigma, it 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 two male nuclei behind
- pollen tube grows out through a gab in the cell wall called a pit, up a gradient of chemoattractants. Pollen tube nucleus codes for the production of hydrolases and digests its way through the tissues of the style. The products are used by the growing pollen tubes
- pollen tube grows through the micropyle and passes into embryo sac
- pollen tube nucleus disintegrates
- tip of pollen tube opens, releasing two male gamete’s into the embryo sac
- one male gamete fuses with oosphere to form a zygote and the other fuses with the polar nucleus to form a triploid nucleus known as an endosperm nucleus
- endosperm nucleus divides repeatedly by mitosis and generates endosperm tissue which provides nutrition to the growing embryo
Describe pollen germination and pollen tube growth
- if pollen grain and stigma are chemically compatible, the pollen tube nucleus controls the germination of the pollen grain and the growth of a pollen tube
- involves digestion of a path between the cells of the style and ovary wall using hydrolase enzymes
What happens to the endosperm nucleus, integuments, ovary wall, and ovules after fertilisation?
Endosperm nucleus: divides by mitosis and becomes food storage tissue
Integuments: fuse together and form the testa
Ovary wall: forms fruit wall which encloses the seeds
Ovules: form seeds
Define a fruit
A structure developing from the ovary wall containing one or more seeds
Define a seed
Structure developed from a fertilised ovule, containing an embryo and food store enclosed within a testa
What is meant by dormant in terms of seeds
- a seed when its active growth is suspended
—> water content falls, reducing their metabolic rate
Describe the structure of monocots
- one cotyledon in seed
- leaf veins are parallel
- sepals, petals and stamens in multiples of 3
- vascular bundles scattered in roots and stems
Describe the structure of dicots
- two cotyledons in seed
- leaf veins form a network
- sepals, petals and stamens in multiples of 4 or 5
- vascular bundles in a ring in stems
- vascular bundles in centre of roots
What is meant by seed dispersal?
The movement of seeds away from the parent plant
Why is it important that seeds are dispersed from the parent?
To prevent competition in which the seedling gets outcompeted
Describe wind dispersion
- plants have sails or parachutes of stiff hairs
- pores containing seeds are shaken when wind blows and seeds are released
Describe transport dispersion
- birds, mammals, reptiles and fish eat seeds and then release them as faeces
- process called scarification where digestive enzymes weaken the testa by physical attack, enabling species to germinate
Describe rolling dispersion
- release of a conker (the seed)
- falls to ground and rolls away from parent tree
Describe bursting dispersion
- legume pods dry and split, scattering the seed
- some pods rotate as they split to send seeds in different directjins
Describe water dispersion
- seeds fall into water and are carried away
- able to float due to air cavities that make them buoyant
Describe carrying dispersion
- seeds attach to animals coats and are flown away
Describe adaptations of seeds for survival
- dormant seeds have low metabolic rate so they can survive very cold weather
- the testa is chemically resistant
- reduced water content of dormant seeds enables them to survive dry conditions
- testa can physically protect the embryo
- endosperm or cotyledons provide a supply of nutrients which lasts until the seedling can photosynthesise efficiently
- seeds dispersed from parent plant to reduce competition
- dispersal allows colonisation of new habitats
- inhibitors only allow germination to occur at certain times of year
—> broken down in vernalisation
Define germination
The biochemical and physiological processes by which a seed becomes a photosynthesising plant
What are the 3 main requirements for germination?
- suitable temp - optimum for seed is whatever is optimum for enzymes involved
- water - mobilise enzymes, transport in xylem and phloem, make cells turgid
- oxygen - aerobic respiration time release energy to fuel metabolism and growth
Describe the process of germination after the mobilisation of food reserves
- swollen tissues rupture the testa and the radicle emerges from the seed
—> positively geotrophic and negatively phototropic - plumule emerges
—> negatively geotrophic and positively phototrophic - cotyledon remains underground whilst the plumule elongates rapidly with its tip bent over to prevent soil abrasion
- once emerged it unfurls and the leaves start to photosynthesise
Germination of non-endospermic
- water enter micropyle
- water enter seed and swells cotyledons, rupturing testa
- split testa allows more O2 for aerobic respiration
- starch and proteins stored in cotyledons mobilised through hydrolysis
- starch and protein used as sources of energy for respiration and growth of radicle and plumule
Germination of endospermic seed
- water enters seed
- embryo releases gibberellin
- gibberellin diffuses to protein rich aleurone layer and amylase enzymes are made to break down stored starch in endosperm
- glucose diffuses to the embryo and used for aerobic respiration and growth
