Sexual Reproduction in Flowering Plants - Chp 39

Characteristics of Asexual reproduction

• 1 parent involved

• mitosis

• no gametes

• offspring genetically identical to parents

Characteristics of Sexual reproduction

• 2 parents involved

• meiosis

• gametes-zygotes produced

• offspring genetically different from parents

Asexual reproduction

production of organisms from one parent only

Sexual reproduction

production of a new individual from the fusion of gametes from both parents

Gametes

haploid cells capable of fusion

Diagram of the structure of a flower

Sepal structure + function

• green, leaf like structure - turns brown when the flower opens

  • protect the flower when its a bud

Petals structure + function

• large, brightly coloured, scented

  • attract insects for pollination

Stamen structure + function

• male part of the plant made up of the filament + anther

  • anther - produces pollen grains by meiosis

  • filament - supports the anther, supplies it with food + water

Carpel structure + function

• female part of the plant made up of stigma, style + ovary

  • stigma - where pollen lands

  • style - where the pollen tube grows towards the ovary

  • ovary - contains one or more ovules

Receptacle structure + function

• where flowering parts arise

  • supports the plant

Describe the structure of the anther

• made up of 4 chambers called pollen sacs

• each pollen sac is surrounded by the protective epidermis

• underneath is the fibrous layer

• underneath is the nutrient rich tapetum

• inside is a diploid microspore mothercell aka pollen mothercell

Diagram of a transverse section of the anther

Describe the formation of the male gamete

• the diploid microspore mother cell/pollen grain mother cell divides by MEOISIS

• this produces a cluster of 4 haploid cells called a tetrad

• the single tetrad breaks up to form 4 SEPARATE haploid pollen grains aka microspores

• each pollen grain divides by MITOSIS to form 2 haploid nuclei: tube nucleus + generative nucleus

Describe the structure of a pollen grain:

• Pollen grains are NOT gametes, it produces gametes

  • exine - thick, very strong outer wall

  • intine - inner lining

What happens once the pollen grians have matured?

• the walls of the anther become dry and shrivel

• they split (dehiscing), burst and expose the pollen grains

Describe the structure of the ovule:

• Inside the ovary are a number of ovules, which are attached to the ovary wall by a stalk

  • nucellus - the bulk of the ovule + supplies nutrients

  • integuments - 2 layered walls for protection

  • micropyle - small pores for fertilisation

  • megaspore mother cell/emrbyo sac mother cell

Describe the formation of the female gametes:

• the diploid megaspore mothercell divides by MEIOSIS 

• this forms 4 haploid cells

• 3 of these degenerate, the remaining is the embryo sac/megaspore

• the haploid embryo sac/megaspore divides by MITOSIS 3 times

• this forms 8 haploid nuclei

• 5 of these degenerate, the remaining 3 form the female gametes

• 2 of these form the polar nuclei

• the remaining one forms a thin cell wall and becomes the egg cell

Diagram of a longitudinal section of a carpal + mature embryo sac

Pollination

transfer of pollen from anther to stigma of a flower from the same species

Self-pollination

transfer of pollen from anther to stigma on the same flowerC

Cross-pollination

transfer of pollen from anther to stigma on a different flower of the same species

Why is cross pollination more desirable for horticulturists than self pollination?

self pollination results in a less robust plant than cross pollination

What are 2 methods of pollination?

• wind pollination

• animal pollination

Example of a wind pollinated flower

grass

Example of an animal pollinated flower

wallflower

What are the differences between wind and animal pollinated plants?

Wind pollinated

• small green petals

• no scent or nectar

• large amounts of pollen

• large anther outside of petals

• large and feathery stigma

Animal pollinated

• large coloured petals

• scent and nectar

• smaller amounts of pollen

• sticky pollen

• small anther inside petals

• small stigma inside petals

Name a condition in humans caused by a sensitivity to pollen:

Hay fever

• caused by an allergic reaction to pollen

• symptoms include sneezing, stuffy nose, watery itchy eyes

Fertilisation

union of male and female gametes to form a diploid zygote

Describe the growth of the pollen tube for fertilisation:

• pollen grain lands on stigma

• pollen grain is stimulated to grow by sugars produced by stigma to produce pollen tube

• pollen grain divides by MITOSIS to produce the tube nucleus and generative nucleus

• growth of the pollen tube is controlled by the tube nucleus

• pollen tube grows from the stigma, down through the style, towards the embryo sac of the ovary

• chemotropism - grows towards chemicals released from ovule

• pollen tube stops growing when it meets the micropyle

• 2 haploid male gametes made by the generative nucleus move down through the pollen tube, and enter the embryo sac at the micropyle without the need for external water

How many fertilisations occur in flowering plants?

2 - double fertilisation

What are the two fertilisations which take place + their products?

1st male gamete + Egg → Zygote

(n) + (n) = (2n)

2nd male gamete + 2 Polar nuclei → Endosperm (nucleus)

(n) + (n) + (n) = (3n)

Describe the formation of a seed:

• the fertilised ovule becomes the seed

• the integuments dry up and become the wall of the seed called the testa

• the zygote divides by MITOSIS to become the young embryo plant

• the young embryo plant is made up of the future root the radicle, the future shoot the plumule and the future seed leaves the cotyledons

• at the same time the endosperm nucleus divides by MITOSIS to produce many triploid endosperm cells

• these expand and absorb the nucellus and act as a food store of fats, proteins and starch

ovule → seed

integuments → testa

zygote → young embryo plant

radicle → root

plumule → shoot

cotyledons → seed leaves

Immature seed diagram

What are the differences between seeds that are monocots and dicots

Monocots

• endospermic seeds

• cotyledons only absorbs some of the endosperm

• food stored in, and absorbed from the endosperm

Dicots

• non-endospermic seeds

• cotyledons grow a lot so all the endosperm in absorbed

• food stored in the cotyledons

Example of a monocot seed

Maize

Example of a dicot seed

broad bean

Describe the formation of a fruit

• a fruit is a developed ovary

• growth regulators/auxins are produced by the seed, these influence the ovary to become a fruit

• the ovary walll become the fruit wall called the pericarp

• fruits are designed to protect seeds and help disperse them

What are the changes that occur in a flower after fertilisation?

How are seedless fruits produced by horticulturists?

• genetically

• naturally

• special breeding programmes

  • sprayed by growth regulators: if a large amount of auxin is sprayed a fruit can form without fertilisation

Dispersal 

transfer of fruit or seed as far away as possible from the parent plant (ensuring survival and minimising competition)

What are the methods of dispersal?

Wind dispersal

• seeds are small and light

• seeds have parachutes eg. dandelion

Water dispersal

• light air filled fruits float eg. water lily

Animal dispersal

• fruits stick to the animals body with hooks called burrs eg. buttercup

• animals eat the fruit and egest seeds

Self dispersal

• fruit dries and and bursts to release seeds

Dormancy

a resting phase of reduced metabolic activity where seeds undergo no growth

What are the causes of dormancy?

• chemical growth inhibitor (eg. abscisic acid) in testa

• testa impermeable to water and oxygen

• testa to hard for embryo to emerge

• embryo is slow to develop to to lack of growth regulatr (can be due to reduced light or low temp like in winter)

How can dormancy be broken?

• soaking seeds in water

• physical damage to break the testa: abrasion, alcohol

• exposing them to the light (or dark?)

• exposing them to cold temperature, then a suitable temperature

What are the advantages of dormancy?

• allows seed to withstand cold winter + recover from harsh conditions

• allows germination in spring

• allows time for dispersal

• allows time for immature embryo to develop

Germination

is the regrowth of the embryo/seed into a new plant after a period of dormancy if the environmental conditions are suitable

What conditions are need for germination?

MOST

Moisture - enzyme activity + formation of cell sap

Oxygen - aerobic respiration

Suitable Temperature - enzymer activity

Describe what happens during germination:

• begins when the seed absorbs water, through the micropyle, allowing acitvation of enzymes

• food reserves are digested:

  • oils -lipase→fatty acids

  • starch -amylase→ glucose

  • protein -protease→ amino acids

• glucose + amino acids → cell walls + enzymes

• fats and glucose used to produce energy

• products of digestion move to the growing embryo

• the mass/dry weight of the seed falls as food stores are used up,

• the embryo weight increases as it uses food to grow

• radicle burst through the testa, the plumule and its leaves are visible above ground

• the first leaves start to photosynthesise, and the dry weight increases again, but the overall dry mass decreases due to energy lost in respiration

Why is dry weight used?

different amounts of water in different seeds would affect the weight readings

Explain this graph

• mass of the seed falls

• respiration occurs and food is used by embryo

• then photosynthesis occurs and the seed grows

Explain this graph

• loss of mass in the endosperm/seed is matched by a rise in mass of the embryo

• food is passing from the seed to the embryo

What are the 2 types of seedling growth + examples?

Hypogeal - cotyledon remains below the soil eg. broad bean

Epigeal - cotyledon moves above the soil eg. sunflower

Describe hypogeal seedling growth:

• germination occurs, the radicle bursts through testa and grows downwards due to geotropism

• plumule emerges and grows upwards, hooked over to protect leaves

• cotyledons/endosperm shrival as food is transferred from them

• once above ground the plumule straightens and its leaves begin to photosynthesise

Describe epigeal seedling growth:

• cotyledon is forced out of the soils

• once above soil the pericarp falls to the ground

• the cotyledons open and photosynthesise

• the plumule emerges from the cotyledons and forms the first true leaves