Download File - LC Biology 2012-2013

Sexual reproduction in
the flowering plant
(Extended study)
Learning Objectives


Outline pollen grain development from
microspore mother cells
Outline embryonic sac development
Pollen Grain
development
Pollen grain
development

An anther consists of
four chambers called
pollen sacs
Pollen
Sac


Each pollen sac is
protected by a fibrous
layer
Inside the fibrous
layer is the tapetum –
a food store that
supplies energy for
cell divisions in the
pollen sac
Fibrous Layer
Tapetum
Pollen
Fibrous Layer
Tapetum
Pollen
Pollen Grain development
Microspore mother cells (2n) are located in
the pollen sacs
 They are diploid
 They divide by meiosis to produce a group
of four haploid cells called a tetrad
 Each tetrad breaks up to form four
separate haploid pollen grains

Pollen Grain development
Microspore mother cell
(2n)
Tetrad of pollen
Meiosis
Separation
Pollen Grains
(n)
Pollen Grain development

Pollen grains divide by mitosis producing two
haploid nuclei:
nucleus – forms the pollen tube
 Generative nucleus – form the male gametes
 Tube
Mitosis
Tube nucleus
Generative
nucleus
Pollen Grain development


When pollen grains
have matured the
walls of the anther dry
and split
Pollen grains are then
exposed and are
ready for pollination
Pollen Grain development


After pollination the
generative nucleus
(n) divides by mitosis
to form two sperm
nuclei (n)
These will each play a
role in double
fertilisation
Pollen Grain development
Learning Check




How many pollen sacs does an anther have?
What form of cell division takes place in a pollen
mother cell?
What is the group of four pollen cells called
before they separate?
What form of cell division takes place to produce
the generative nucleus and tube nucleus?
Embryo sac
development




Each ovary contains one or
more ovules
An ovary has two walls
called integuments
Between the integuments
is a small opening
(micropyle) through which
a pollen tube can enter.
The nucellus provides
nutrients for growth in the
ovule
Integuments
micropyle
nucellus
Megaspore
mother cell
(2n)



One cell in the ovule,
the megaspore
mother cell (2n)
divides by meiosis to
form four haploid cells
Three of these cells
disintegrate
The remaining cell is
called the embryosac.
Meiosis
Embryo sac
(n)




The nucleus of the
embryo sac (n)
divides by mitosis
three times forming
eight haploid nuclei
These are still
contained within the
embryo sac
Five of these nuclei
will later disintegrate
The remaining three
are the gametes
Gametes
Polar nuclei
(n)
Gamete – Egg cell
(2n)
Two of these form the polar nuclei in the
embryo sac
 The remaining female gamete forms an
egg cell

Megaspore
mother cell (2n)
Meiosis
Embryo sac
(n)
Mitosis (3 times)
Polar nuclei
Egg cell
Recall
Pollen grains divide by mitosis producing
two haploid nuclei:
 Tube nucleus – forms the pollen tube
 Generative nucleus – form the male
gametes

Events leading to fertilisation


Once pollination has
occurred the pollen tube
nucleus forms the pollen
tube which moves down
the style to the ovule in
the ovary
The tube nucleus moves
towards chemicals
released from the ovule
(chemotropism)
Events leading to fertilisation
The generative nucleus (n) divides by
mitosis as it moves down the pollen tube
 Two haploid sperm nuclei result
 These are the male gametes
 On reaching the embryo sac a double
fertilisation will take place

Fertilisation


One sperm nucleus (n) fuses with the egg
nucleus (n) to form a diploid zygote. This
will develop into an embryo
The second sperm nucleus (n) joins with
the two polar nuclei (2n) forming a triploid
(3n) endosperm nucleus
Syllabus
Depth of treatment

Pollen grain development from microspore
mother cells: meiotic division, mitotic
division, generative and tube nuclei
production, formation of pollen grain
Depth of treatment

Embryo sac development: megaspore
mother cell, meiotic division, cell
disintegration, mitotic division in the
production of eight cells of the embryo
sac, one of which is the egg cell.