UNIT 3: Sexual Reproduction In The Flowering Plant Flashcards
sexual reproduction
production of a new individual from two parents
structure of the flower: receptacle
tissue from which all other parts originate
structure of the flower: sepal
thick, green, leaf-like structures that protect the developing flower when it is in bud form
structure of the flower: petals
large and brightly coloured in animal-pollinated plants; small and usually green in wind-pollinated plants
structure of the flower: stamen
male organ consisting of:
- anther - pollen formation
- filament - supports the anther in a position where pollen will be easily transferred.
structure of the flower: carpel
female organ consisting of:
- stigma - pollen lands on stigma.
- style - supports the stigma in a position where pollen will have a good chance of landing.
- ovary - where ovules develop
gamete
haploid sex cell
gamete formation
the male gamete is the pollen grain that is a tough-walled single cell with two nucei:
1. tube nucleus
2. generative nucleus
the female gamete is the egg cell that is a large cell present in the embryo sac of the ovule.
embryo sac development
- the ovary is located at the bottom of the flower with the style and stigma above it.
- within the ovary are a number of ovules.
- each ovule is composed of two outer walls called integuments.
- integuments have a small opening at the base of the ovule, called the micropyle, that allows the pollen tube to enter.
- the inner layer of each ovule has a layer called the nucellus - which nourishes the developing embryo sac.
- within each ovule are a number of diploid cells - one of which develops further to become the megaspore mother cell.
- the megaspore mother cell divides by meiosis to produce 4 haploid cells.
- 3 of these haploid cells degenerate and one survives to become the embryo sac.
- the embryo sac (megaspore) enlarges and the haploid nucleus divides by first round of mitosis to form 2 haploid nuclei.
- the 2 haploid nuclei then undergo a second round of mitosis to form 4 haploid nuclei within the 1 embryo sac.
- a third and final round of mitosis occurs to produce 8 haploid nuclei.
- the 8 haploid nuclei move to various areas of the embryo sac.
- cell membranes and a thin cell wall form around 6 of the haploid nuclei.
- the 2 remaining haploid nuclei remain free and are called polar nuclei.
- the egg cell is present at the bottom of the embryo sac.
pollen grain development
- anther has 4 chambers called pollen sacs.
- pollen sacs are where the millions of pollen grains develop and mature.
- each pollen sac has an outer fibrous layer (dermal tissue) that protects.
- inside the protective layer is the tapetum - which nourishes the developing pollen grains.
- on the innermost layer of the pollen sac is a layer of diploid cells (containing 2 sets of chromosomes) called microspore mother cells.
- microspore mother cells divide by meiosis to produce 4 immature, haploid cells.
- the immature, haploid pollen grains (microspores) then mature over time and develop a tough outer wall called the exine and a soft inner wall called the intine.
- mitosis of the haploid nucleus in each microspore also occurs during maturation - this produces a pollen grain with 2 haploid nuclei.
- tube nucleus: burrows into stigma and style
- generative nucleus: fertilizes egg
pollination
the transfer of pollen from anther to stigma of a flower of the same species.
self-pollination
where a flower allows pollen to fertilise the egg cell within the ovary of the same plant - disadvantageous to species as resulting seeds less likely to form healthy plant.
cross-pollination
where a flower transfers pollen from anther to stigma of different plant of same species - more advantageous as greater variation is shown.
pollination methods: wind
pollen is produced in very large amounts by the flower and is usually small, light and smooth to allow easy transfer by wind e.g. grasses
pollination methods: animal
pollen is produced in relatively small amounts. grains are larger and stickier and they are usually transferred by insects e.g. dandelions, daisies, tulips, roses
fertilisation
union of the male and female gametes to form a diploid zygote.
- once the pollen grain is trapped by the stigma, the pollen tube forms by action of the tube nucleus.
- the generative nucleus enters the pollen tube and divides by mitosis to form 2 haploid nuclei called sperm nuclei.
- the sperm nuclei enter the embryo sac and ‘double fertilisation’ occurs.
- an adapatation of of angiosperms to life on dry land is pollen tube formation as no external water is required for fertilisation to occur.
double fertilisation
- one fertilises the egg - diploid (2n) zygote results.
- other fuses with the 2 polar nuclei to form triploid (3n) endosperm cell which goes on to absorb nutrients and functions as a food store.
seed formation
- the ovule develops in the seed.
- integuments become the testa (seed coat).
- the diploid zygote becomes the plant embryo.
- the embryo develops further into the radicle, plumule, and cotyledon(s).
- triploid endosperm nucleus divides repeatedly by mitosis to produce many cells that swell with food.
endospermic seeds
the plant embryo increases in size and only absorbs some of the endosperm e.g. corn
non-endospermic seeds
the plant embryo increases in size absorbing all of the endosperm in the process e.g. broad bean
monocot seeds
- tend to be endospermic e.g. corn
- one cotyledon
- when germinating, the food is obtained mainly from the endosperm.
- tend to send up single shoot with no leaves (grasses)
dicot seeds
- tend to be non-endospermic e.g. broad beans
- two cotyledons
- when germinating the food is obtained mainly from the cotyledons.
- send up shoots with leaves
fruit formation
- fruits are formed from the ovary under the influence of auxins.
- fruits can also form from the receptacle of the flower (false fruits) e.g. apple.
- fruits protect seeds and attract animals to eat them so that seeds can be dispersed.
seed dispersal
dispersal is the transfer of the seeds away from the parent plant.
advantages:
- avoid competition
- increases chances of surviving winter
- colonise new habitats
- increase the number of the species.
wind dispersal
seeds are generally very light and usually have some anatomical adaptation (hairs, wings) that enables them to be transported a long distance from parent plant e.g. dandelions, sycamore
water dispersal
seeds are usually enclosed within an air-filled fruit that is capable of floating e.g. water lillies, coconuts
animal dispersal
seeds may be enclosed within a sticky fruit e.g. burdock, goosegrass
seeds may be enclosed by a fleshy fruit e.g. strawberry, blackberries
self-dispersal
seeds are enclosed within a pod that explodes open when it becomes dry e.g. pea pods
dormancy
a resting period in which the seed undergoes no growth and has a very low metabolism.
advantages:
- allows plant to avoid harsh conditions of winter
- gives embryo time to fully develop
- provides extra time for dispersal
biotechnological issues: seedless fruits and larger fruits
parthenocarpy is the process of growing fruit that does not have seeds.
carried out in two ways:
1. breeding of plants in such a way as to produce seedless fruit (pollination occurs but no fertilisation)
2. use of auxins - sprayed onto plant to stimulate fruit formation
biotechnological issues: ethene as a ripening agent
ethene is a hydrocarbon (C2H4) gas that causes fruit to ripen (turn from green to characteristic colour)
germination
the regrowth of the embryo, following a period of dormancy, when the environmental conditions are suitable factors necessary: - water - oxygen - suitable temperature
digestion and respiration in germination
- digestion of food substances is required during germination as food stores in the form of oils and starch need to be mobilised and converted to usable forms - like fatty acids and glycerol and glucose
- respiration is required to produce ATP as the embryo is growing and so anabolic reactions are occurring all the time
stages of seedling growth
- cotyledons remain below the soil e.g. broad bean
2. cotyledons move above the soil e.g. sunflower
Radicle
The part of the plant embryo that develops into a root
Plumule
The part of the plant embryo that develops into the shoot
Non endospermic seed
Has no endosperm when fully formed
Endospermic seed
Contains endosperm when fully formed
Fruit
A developed ovary
