Sexual Reproduction in Plants Flashcards
(15 cards)
Flowers & Pollination
The flower is the organ responsible for sexual reproduction in a plant.
The gametes are made, released and combine in the flower.
The female gamete is the ovule and the male gamete is the pollen.
Pollination is the transfer of pollen to the stigma
Self Pollination
Pollen from the plant is transferred to the stigma of the same plant
The only variation that occurs in the offspring comes from mutation, independent assortment and crossing over in meiosis of gamete formation
Good for successful plants in a stable environment, but species will be unable to adapt to change
Cross Pollination
Pollen from one plant is transferred to the stigma of a different plant of the same species
Variation is from meiosis, mutation and having genetic material from 2 different parent plants
Species more likely to survive a changing environment due to genetic variation in offspring
Plants prevent self pollination by….
-Chemical self-incompatibility, gametes from the same plant cannot combine
-Irregular flower structure for example different positioning of anthers and stigmas or different ripening times of anthers or stigmas; some plants like holly have male and female plants
Dicotyledonous - Insect Pollinated Flower Structure
-stigma
-style
-ovary filled with ovules
-filament
-anther
-receptacle and the calyx, the outer ring of sepals that cover the flower in bud
-A corolla of petals. Colourful and scented to attract insects. In a wind pollinated flower, petals are often lacing or if present small and green
-The carpel (female) made of a sticky stigma to collect pollen from the insects’ bodies. In a wind pollinated flower, this is large and feathery and hangs outside the flower to collect pollen blown past
-The stamen (male) made of the anther and filament is tucked inside the flower, so the insect rubs past picking up pollen
-In a wind pollinated flower the anther is large and hangs outside the flower, so the small light pollen gets carried away
Anther
The pollen grains develop in anthers
Tapetum - a layer of cells that provide nutrients to developing pollen grains
Tension in lateral grooves increases as the anther dries out
Dehiscence occurs when walls of the pollen sac curl away exposing pollen grains to wind or insects
Gametes & Fertilisation
-Pollen mother cells develop by mitosis
-Meiosis occurs to produce a tetrad of 4 haploid cells
-In each haploid, pollen grain mitotic division of the nucleus forms a generative nucleus (that mitotically divides to produce 2 male nuclei) and a tube nucleus
Formation of the Ovule
The ovules develop in the ovary
-A megaspore mother cell carries out meiosis and 4 haploid nuclei are formed
-3 nuclei degenerate and 1 divides by mitosis three times to produce 3 antipodal cells, 2 polar nuclei and a female gamete with 2 synergids either side
Fertilisation
Pollen grain on a compatible stigma produces hydrolase enzymes, forming a pollen tube leading to the micropyle of the embryo sac.
Pollen tube formation is under the control of the tube nucleus.
Pollen tube growth is negatively aerotrophic (away from air), and positively chemotrophic (towards the embryo sac)
One male gamete enters the embryo sac and fuses with the female gamete to produce a diploid zygote
The second male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus
Seeds & Germination
Seeds are an adaptation to terrestrial life.
They can survive very dry conditions, have enough of a food store to provide food until the new plant can photosynthesise and plants have developed different mechanisms for dispersal of the seeds to reduce competition with parent plants
Following fertilisation, the structures develop…..
Ovule - develops into a seed
Diploid Zygote - divides by mitosis to form the diploid embryo, this is made of the plumule, radical and 1 or 2 cotyledons
Triploid Endosperm Nucleus - divides by mitosis to form endosperm tissue, an important food storage tissue
Integuments - develop into the testa
Micropyle - a pore in the testa
Ovary - develops into a fruit wall, enclosing seeds
Germination
Germination is the series of biochemical and physiological processes through which a seed becomes a photosynthesising plant, independent of the food stores in the cotyledons
Conditions for Germination
Temperature - A suitable temperature would be between 5 degrees and 30 degrees. This is the optimum temperature for the enzymes needed in germination
Water - Water is required to make cells turgid, to transport substances and to mobilise enzymes
Oxygen - Required for aerobic respiration
Non-endospermic Seed
(broad bean)
-Germination begins with water entering the micropyle
-Water enters the seed and swells the cotyledons, splitting the testa
-The split testa allows more oxygen in for aerobic respiration
-Starch and proteins stored in the cotyledons are mobilised through hydrolysis
-The starch and proteins are used as sources of energy for use in respiration and the growth of radicle and plumule
Endosperm Seed
(maize)
-Water enters the seed
-The embryo releases the plant hormone gibberellin
-Gibberellin diffuses to the protein rich aleurone layer and amylase enzymes are made to break down stored starch in the endosperm
-Glucose diffuses to the embryo and used for aerobic respiration and growth
