Sexual reproduction in Plants (C2) Flashcards
Structures of general insect pollinated flowers? (13)
- receptacle (where stem and flower meet)
- calyx (collective name of sepals)
- sepal (leaves enclosing flower)
- corolla (all the petals on a flower)
- petal (large, brightly coloured scented)
STAMEN (anther and filament - male)
- filament (small tube like, supports anther)
- anther (site of pollen production - male)
CARPEL (stigma, style, ovary - female)
- ovary (part of female repro. organ, contains ovules)
- ovule (inside ovary)
- style (tube-like between ovary and stigma)
- stigma (sticky knob at top)
What are the differences between an insect and wind-pollinated flower? petals? anthers? stigmas? pollen?
INSECT pollinated:
• colourful, large petals, scented, nectar - attract pollinators
• anthers within flower
• stigma within flower
• small quantities of larger sticky, sculpted pollen
WIND pollinated:
• petals usually absent or inconspicuous, green, no scent or nectar
• anthers hanging outside flower
• large feathery stigmas hand outside flower
• large quantities of smaller smooth pollen
What is the role of mitosis and meiosis in the development of pollen grains in an anther? location? layer?
pollen grains are formed in the anther;
• MITOSIS to produce large numbers of diploid pollen mother cells
• followed by MEIOSIS to produce a tetrad (four) haploid cells - forming pollen grains containing male gamete
• the TAPETUM is a layer of cells around the pollen sac, providing nutrients to developing pollen grains also producing a waterproof coat
Structure of an anther and pollen sacs
ANTHER (butterfly shape):
• centre circle - upcoming filament containing vascular bundle to supply water and sucrose to anther
• four outer circles - pollen sac
• outer most thin layer - epidermis
POLLEN SAC containing 2n microspore mother cells:
• outer layer - fibrous layer
• inner most layer lining - tapetum (provide nourishment)
• pollen mother cells - from which grains develop
Describe the maturation of pollen grains and final structure
• tetrad of four haploid pollen grain cells breaks apart
• inside each pollen grain the haploid nucleus undergo mitosis - producing two haploid nuclei; a GENERATIVE nucleus and a POLLEN TUBE nucleus
(• the generative nucleus later undergoes mitosis giving rise to two male nucleus along the pollen tube nucleus)
• a mature pollen grain is formed:
- two haploid nuclei; generative and pollen tube
- inner coating INTINE
- outer coating EXINE
When are two male gametes formed in the pollen grain?
ONCE the MATURE pollen grain is developed with two haploid nuclei (generative and pollen tube)
- the generative nuclei will undergo MITOSIS to give rise to two male nuclei gametes
What are the adaptations of pollen grains? x2
- the outer pollen cell wall, the EXINE is tough and resistant to chemicals - also resisting desiccation (drying out)
- resistant to UV radiation - can be carried to high altitudes without mutation of DNA occurring (important for wind pollinated)
What is dehiscence? state the process?
a process resulting in the anther RELEASING the pollen so they can be carried by the wind or transferred by pollinators
1) the outer layers of the anther dry out - causing tension
2) dehiscence occurs, lateral grooves/walls of anther are pulled apart - edges of pollen sacs curl away
3) STOMIUM - an opening, exposes the grains to be carried way by insects or wind
What is the role of mitosis and meiosis in the development of an ovule in the ovary?
the ovule contains the female gamete and is formed in the ovary:
• MEIOSIS of a megaspore mother cell in the nucellus
produces four haploid megaspores;
• three degenerate while only one subsequently grows and develops
• the one megaspore undergoes THREE MITOTIC divisions producing 8 HAPLOID nuclei (1 female gamete, 2 polar nuclei, 2 synergids, 3 antipodal cells) within the mother cell megaspore now the embryo sac
Name the structures in a mature ovary (9)
- Ovule containing:
- embryo sac - part of ovule, containing 8 nuclei
- female gamete - fuses with a male nucleus forming a zygote
- two synergids
- two polar nuclei - fuse with a male nucleus in double fertilisation to form a triploid endosperm
- three antipodal cells
- funicle - attaches ovule to ovary wall
- integuments - protects the embryo sac
- micropyle - gap in the integuments for male nuclei to enter embryo sac
Define pollination
the TRANSFER of pollen from an anther to a stigma
Self pollination VS cross pollination: definition? genetic variation? source of variation?
SELF: transfer of pollen grains from an anther to a stigma of the SAME plant
• some genetic variation; crossing over in prophase I and independent assortment in metaphase I and II
CROSS: transfer of pollen grains from an anther to a stigma of a DIFFERENT plant of the SAME SPECIES
• increased genetic variation; crossing over in prophase I, independent assortment in metaphase I and II, mixing of potential genotypes when haploid gametes fuse
Self pollination advantages and disadvantages
+
• only one parent needed
• helps maintain genotypes which are well adapted to the current environment
-
• reduced genetic variation
• offspring less likely to survive changes in the environment, natural selection and evolution is not driven
• increased chance of homozygous recessive alleles and thus genetic diseases
What are the events involved in pollination and first stages of fertilisation? (stigma to embryo sac)
MITOSIS of the generative nucleus to produce TWO
MALE GAMETES and a POLLON TUBE NUCLEUS
- germination of a pollen grain on a compatible stigma which produces a sugary solution;
- growth of a pollen tube (under the control of the pollen tube nucleus) through the digestion of the style via the secretion of hydrolase enzymes
- entry of the pollen tube into the embryo sac through the micropyle releasing the two male nuclei
What is double fertilisation?
both male gametes are involved in separate fertilisation events in the embryo sac:
- one male gamete FUSES with the FEMALE GAMETE to produce a DIPLOID ZYGOTE
- the second male gamete FUSES with the TWO POLAR NUCLEI to form a TRIPLOD PRIMARY ENDOSPERM nucleus
