4.2 sexual reproduction in plants Flashcards
what is another name for flowering plants?
angiosperms
angiosperms use ____ as their reproductive structures
flowers
in angiosperms, what are the male gametes contained within?
pollen
where is pollen produced in angiosperms?
- within the anthers
what does the ovule contain?
an embryo sac with one female gamete inside
what do the sepals on an angiosperm do?
protect the flower in the bud
(the sepals are usually green)
the male parts of the flowers (stamens), consist of a filament supporting the anther which contains ___ pollen sacs
4
where are the female parts of the flowers or stamens (carpels) found in the flower
at the centre of the flower
______ contains the ovaries in angiosperms?
the carpel
- and where the ovules develop
what does the stigma do in an angiosperm?
it’s the receptive surface which collects pollen during pollination
what does an insect-pollinated flower include?
- petal (corolla)
- anther + filament = stamen
- stigma + style = carpel
- sepal (calyx)
- ovule
- ovary
- embryo sac
- receptacle
what is used to attract pollinators to insect-pollinated flowers?
- large colourful petals
- scent
- nectar
pollination definition
- the transfer of pollen from the anther of one flower to the mature stigma of another flower of the same species
what does a wind-pollinated flower include?
- stigma + style + ovary = carpel
- filament + anther = stamen
wind-pollinated flowers are usually:
- small, green and inconspicuous
- have no scent or petals
- no nectar
in insect pollinated flowers, the anther and stigmas are (outside/within) the flower
within
in wind pollinated flowers, the anther and stigmas are (outside/within) the flower
outside
why are anthers hanging outside the flower in wind pollinated flowers?
so the wind can blow pollen away
why do wind pollinated flowers have large feathery stigmas?
as they provide a large surface area to catch pollen grains
insect-pollinated flowers have a (small/large) quantity of pollen?
small
(small quantities of sticky sculptured pollen to stick to insects)
wind-pollinated flowers have a (small/large) quantity of pollen?
large
(large quantities of small, smooth, light pollen to be carried by wind)
what does a pollen sac in an angiosperm contain?
many pollen mother cells
how is a tetrad produced? (male gamete development)
- a pollen sac contains many pollen mother cells, each of which divides by meiosis to produce a tetrad
what is the tepetum? what does it do? (male gamete development)
- the surrounding nutritive layer to the tetrad
- it provides nutrients to the developing pollen grains
the pollen cell wall is tough and resistant to desiccation
dehiscence definition
the opening of the anther, releasing pollen grains
the haploid nucleus witthin the pollen grain undergoes mitosis to produce what?
- a generative nucleus
- and a pollen tube nucleus
the generative nucleus then undergoes further mitotic division to produce what? (male gamete development)
two male nuclei
as the outer layer of the anther matures and dries out, what happens? (male gamete development)
- the outer walls curl away exposing the pollen grains - a process known as dehiscence
what do the ovules contain? (female gamete development)
a megaspore mother cell
the ovules contain a megaspore mother cell which undergoes meiosis producing what?
four haploid cells
(only one of which develops further)
female gamete development:
- ovule contains a megaspore mother cell
- which undergoes meiosis producing four haploid cells
- only one develops further
- it produces eight haploid cells following three mitotic divisions
- two of these cells fuse to produce a diploid polar nucleus, leaving six haploid cells: 3 antipodals, 2 synergids and 1 oosphere, all contained within the embryo sac which is surrounded by the integuments
what does an embryo sac in a mature ovule contain?
- 3 antipodals (haploid)
- 2 synergids (haploid)
- 1 oosphere (haploid)
- 1 polar nucleus
is self-pollination a form of asexual reproduction?
no
- as pollen and ovules are produced by meiosis, some variation exists so the offspring are not genetically identical
- but variation is greatly reduced
what is it called when pollination occurs between the anther and stigma in the same flower, or different flower on the same plant?
self-pollination
what is it called when pollen is transferred to another flower on a different plant of the same species?
cross-pollination
why does self-pollination result in greatly reduced genetic variation?
- as it only occurs as a result of mutation, independent assortment and crossing over
(there is an increased risk of harmful recessive alleles coming together, but inbreeding does preserve successful genomes)
why does cross-pollination result in greater variation?
- it combines the genetic material from two different individuals
what is cross-pollination referred to as?
outbreeding
what are the advantages of cross-pollination?
- the change of harmful recessive alleles coming together is reduced
- it allows for greater genetic variation potentially leading to evolution of species over time
what are some ways in which plant species have evolved to ensure cross-pollination?
- pollen and stigma can mature at different times. when the stamens ripen first it is referred to as protandry
- the anther is located below the stigma, reducing the risk of displaced pollen falling onto it
- some plants have separate male and female flowers e.g. maize, or separate male and female plants, e.g. holly
- some plants show genetic incompatibility e.g red clover, where pollen cannot germinate on the stigma of the same plant
protandry definition
when stamens ripen before the stigmas
what is fertilisation?
the process where the male gametes fuses with the female gamete, producing a diploid zygote
when is a pollen tube produced?
- when a pollen grain lands on the mature stigma of another plant of the same species, it germinates, producing a pollen tube
what is the growth of the pollen tube controlled by?
the pollen tube nucleus
in double fertilisation, the pollen tube nucleus then disintegrates and the ______ enter the ovule?
two male gametes
in double fertilisation, in the ovule, one male nucleus fuses with the haploid female nucleus, the oosphere, to form the ____
zygote
in double fertilisation, in the ovule, the second male nucleus fuses with the diploid polar nucleus to form a _______
triploid nucleus
- which develops into the endosperm
what does the endosperm do?
provide nutrition for the developing embryo plant
two fertilisation occur:
- one with the oosphere
- and the other with the diploid nucleus
hence the term double fertilisation
- following fertilisation, the ovary wall becomes the ___
- and the ovule becomes the ____
- fruit
- seed
broad beans are an example of dicotyledonous plants. what does this mean?
- is a plant that produces seeds that contain two cotyledons/seed leaves
(which act as food stores) - these are derived from the embryo and not the primary endosperm nucleus
what does the radical of a seed form?
the root
what does the plumule of a seed form?
shoot
how many cotyledons are there in monocotyledonous plants e.g maize?
only one
do different plants have different methods of dispersing seeds?
yes
why is seed dispersal important?
- it allows seedlings to germinate away from the parent plant and so reduces competition for resources
what are some methods of seed dispersal?
- wind e.g dandelion seeds
- water e.g coconuts
- animals, attached to their fur e.g burdock
- animals, eat the fruits and egest the seeds away from the parent plant e.g cherries (the digestive system weakens the testa allowing germination to occur, and supplies its own fertiliser - faeces)
germination in the broad bean (non-endospermic seed)
- water is absorbed by the seed through the micropyle, causing the tissues to swell and activate the enzyme (now oxygen can enter through the micropyle)
- the testa (seed coat) ruptures, the radical pushes through first downwards (can absorb water and minerals from soil + anchors seedling), followed by the plumule upwards
- amylase enzyme hydrolyses starch into maltose which is transported to the growing points of the plant to be used in respiration
- during germination the cotyledons remain below ground
- the plumule is bent over in the shape of a hook to prevent damage to the tip by soil abrasion
- when the plumule emerges from the soil it unfurls and begins to produce glucose by photosynthesising as the food reserves in the cotyledons have now been depleted
what are some requirements for germination (in a broad bean)
- optimum temperature for enzyme action
- water for the mobilisation of enzymes, transport of products to growing point and to trigger secretions of gibberellin
- oxygen for aerobic respiration producing ATP for cellular processes such as protein synthesis
germination definition
the processes through which a seed becomes a photosynthesising plant
as a seed germinates the dry mass of the cotyledons (increases/decreases)
decreases
- as food reserves are used up fuelling growth of the embryo
as a seed germinates the overall mass of the seed decreases initially, until when?
until the plumule can begin photosynthesising
what does GA stand for?
gibberellic acid
what is GA? what does it do?
- a plant growth regulator
- which diffuses into the aleurone layer surrounding the endosperm
- switching on genes involved in transcription and translation
- resulting in the production of amylase and protease
what are the amino acids produced by the hydrolysis of proteins used to do?
- synthesise amylase
- which in turn hydrolyse stored starch into maltose and glucose for respiration by cells in the radicle and plumule
what is an insect-pollinated flower?
a type of flower that relies on insects to transfer pollen grains between flowers
what is the calyx?
- the first part of the flower that is formed
- consists of leafy structures that protect the flower during development
(sepal)
what is the corolla?
all the petals of a flower
what is the carpel?
- the female part of the plant
- consisting of a stigma, a style and an ovary
what is the stamen?
- the male part of the plant
- consisting of an anther and a filament
- that is involved in the production of male gametes in the form of pollen grains
describe the adaptations of insect-pollinated plants:
- internal anthers and small stigma directly touch insects
- large, bright petals and nectar from glands attract insects
- may produce chemicals to mimic scent of female insects or to intoxicate insects
- grow individually
- large, pollen grains, sometimes with projections that attach to insect
what is a wind-pollinated plant?
- a type of flower that relies on wind to transfer pollen grains between flowers
describe the adaptations of wind-pollinated plants:
- external anthers optimise pollen dispersal
- excess pollen compensates for wind wastage
- feathery stigma catches pollen from the air
- small, dull petals (no need to attract insects)
- grow densely over large areas
- light pollen grains
how does a pollen grain form in the anther?
- large numbers of pollen mother cells produced by mitosis
- meiosis of diploid mother cells in the anther forms four haploid microspores
- haploid microspores mature into pollen grains via mitosis
describe the role of the tapetum in pollen grain development
- specialised layer of cells in the anther
- provides nutrients to developing pollen grains
describe the structure of mature pollen grains:
- generative cell (haploid nucleus) produces two male gametes via mitosis
- pollen tube cell (also has its own nucleus) elongates to penetrate ovule
- outer protective coating
how does the ovule form in the ovary?
- meiosis of megaspore cell produces four haploid megaspores
- growth and development (involving three mitotic divisions) of one of the megaspores
- embryo sac forms containing eight haploid nuclei
describe the structure of the embryo sac
- two polar nuclei form endosperm
- ovum forms zygote
- two synergids help generative nucleus of pollen grains to reach ovum
- three antipodal cells
- outer protective coating
name the two types of pollination
- cross-pollination
- self-pollination
where does double fertilisation occur?
- embryo sac of ovule
how do the male nuclei reach the embryo sac?
- pollen grain from one plant lands on the stigma of another
- mitosis of pollen grain to form a pollen tube nucleus and two male gametes
- pollen tube grows from the grain down to the ovule via the digestion of the style
- pollen tube delivers two male gametes via
how does the pollen tube enter into the embryo sac?
- via the micropyle
define micropyle:
- pore in the integument of an ovule through which the pollen tube enters the embryo sac
- remains as a pore in the testa (seed coat)
what happens during double fertilisation?
in the embryo sac of ovule:
- one sperm cell fertilises an ovum to form a diploid zygote
- one sperm cell fuses with two polar nuclei to form a triploid primary endosperm
what haplens to the ovule following double fertilisation?
develops into the seed
how is the diploid embryo formed following double fertilisation?
diploid zygote undergoes mitosis to form diploid embryo
how is the endosperm formed?
- nucleus of triploid endosperm divides by mitosis to form endosperm
what structure forms from the testa?
- integuments develop into the testa (seed coat)
what happens to the ovary following double fertilisation?
it develops into a fruit wall surrounding the seed
what is the hilum?
a scar on the testa as a result of separation from its funicle
give an example of a non-endospermic seed
broad bean
describe the stages of germination in an endospermic seed:
- seed absorbs water, activating the embryo to secrete the cell-signalling plant growth factor, gibberellin
- gibberellins induce the synthesis of amylase
- amylase diffuses to the endosperm layer and causes the hydrolysis of starch to glucose
state the factors affecting seed germination
- water availability (triggers secretions of gibberellin)
- temperature
- availability of O2 for respiration
- use of fertiliser provides extra nutrients for growth
what are the advantages of self-pollination?
- pollination is rapid with a high chance of success ∴ increasing the chance of seed production and survival of the species
- the offspring have reduced genetic variation - and so if the plant is well adapted to living in a particular environment/habitat, the genes are conserved in the offspring. good genomes are preserved - if there is little change in the environment, keeping the same adaptations increases the chance of species survival
what is the main disadvantage of cross-pollination:
- good combinations of alleles can be lost due to crossing over and random fertilisation
what contains the male gametes of flowering plants?
pollen grains
what contains the female gametes of flowering plants?
ovules
what are pollen grains also called?
spores
where are pollen grains formed?
in the anthers
microspores are formed by (meiosis/mitosis)?
meiosis
- but following their production, the nuclei in each of the pollen grains undergoes further nuclear division by mitosis to produce three genetically identical nuclei in each pollen grain
what are ovules also known as?
megaspores
where are ovules produced?
within the ovary of a flower
the femal gametes are formed by meiosis within a structure called what?
the embryo sac (the megagametophyte)
in the formation of an embryo sac, the haploid nucleus undergoes three mitotic divisions to form a total of eight nuclei that are genetically (identical/different)
identical
before fertilisation can take place, what must have occurred?
- pollination
in the growth of the pollen tube, the style and ovary wall are digested by what?
hydrolase enzymes
the growth of the pollen tube is under the control of what?
the pollen tube nucleus
the pollen tube grown until it reaches what?
the micropyle
as the pollen tube grows, the TWO male gametic nuclei move down the tube behind the pollen tube nucleus
what organelles does the pollen tube contain?
- mitochondria - for ATP
- rough endoplasmic reticulum
- golgi body - process and package enzymes into vesicles
- vesicles - for exocytosis of enzymes
- vacuoles - expand as they fill with cell sap and push nuclei down the pollen tube
monocotyledonous:
- only one cotyledon
- their main food store is the endosperm
- derived from the primary endosperm nucleus
what are endospermic seeds?
seeds which use the endosperm as their food store
e.g maize
- in endospermic seeds, the endosperm is full of starch which is hydrolysed by the enzyme amylase and maltase to release glucose for respiration
- the production of these enzymes is triggered by the absorption of water
broad bean = non-endospermic seed
maize = endospermic seed
what is the epicotyl?
the stem of the plumule
- in the germination of broad beans (non-endospermic seeds), this elongates due to vacuolation of the cells and pulls the leaves backwards through the soil. once the leaves are above ground, they unfold and begin to photosynthesise
what is nectar produced by?
nectaries
what is the function of nectar?
to attract insects
explain how flowers are adapted to ensure that there is an effective pollen transfer between two flowers of the same species [1]
anther and stigma touch the same part of the bee
what is the function of the generative (male) nucleus? [1]
- fertilisation / it is the gamete / formation of the endosperm
what is the function of the tube nucleus [1]
controls the growth of the pollen tube (through the style)
a diagram shows a peanut seed and a barely seed.
- state and explain the conditions required for germination to take place.
- describe the germination of the peanut and barley seeds shown above [9QER]
CONDITIONS REQUIRED FOR GERMINATION:
- water + oxygen + suitable temperature
- water:
• cotyledons swell / testa softens
• transport - dissolve substances / fluid medium for enzyme
- oxygen:
• aerobic respiration
• energy / ATP for metabolism
- suitable temperature:
• speeds up rate of diffusion
• increases enzyme activity
GERMINATION OF PEANUT:
- non-endospermic / endosperm absorbed (into cotyledons) / no endosperm
- amylase digest starch in cotyledons to make maltose
- (proteins/fats) broken down into (amino acids/fatty acids and glycerol)
- move to (plumule / radicle / meristem / sink) - can apply to peanut or barley
- for (mitosis / growth / cell division) - can apply to peanut or barley
GERMINATION OF BARLEY:
- endospermic / endosperm present
- starch / proteins / fats in endosperm
- embryo produces (gibberellic acid / gibberellin)
- gibberellic acid (moves into / stimulates) aleurone layer
- gibberellic acid causes enzymes to break down protein into amino acids
- amino acids used to synthesise enzymes such as amylase
- amylase breaks down starch in the endosperm into maltose
a diagram shows a wheat grain.
use information in the diagram to explain why the grain is more correctly described as a fruit rather than a seed [2]
- fruit retains ovary wall / ovary wall and testa are fused
- seed would only have a testa
explain why the events taking place in the embryo sac of a wheat flower are described as a double fertilisation [2]
- one (nucleus) fuses with the (ovum / egg / female gamete) and the other fuses with (two) polar nuclei
- the first forms the zygote and the second forms the endosperm nucleus)
state 4 differences between insect pollinated and wind pollinated plants [4]
- large petals vs small insignificant petals/no petals
- brightly coloured vs dull/brown/green
- scented vs not scented
- nectar vs no nectar
- small amount of pollen vs large amount
- pollen sticky/sculpted vs pollen smooth/not as sticky
- pollen large/heavy vs pollen small/light
what part of the flower develops into the:
- fruit?
- embryo plant?
- testa?
- seed?
- fruit = FERTILISED ovary
- embryo plant = zygote
- testa = integuments
- seed = ovule
where does amylase digest starch in the plant?
- in the endosperm
(starch into maltose)
- the maltose moves into the embryo where it is hydrolysed into glucose, which is respired to generate energy
