Fertilisation, zygote, embryo, germination | How do organisms reproduce | Biology | Khan Academy
Summary
TLDRThis educational video script delves into the intricate process of plant fertilization and germination. Beginning with pollination, it describes the journey of pollen grains to the stigma and their germination to form a pollen tube guiding sperm cells to the egg. The script explains double fertilization, where one sperm fuses with the egg to form a zygote, and another unites with polar nuclei. The zygote then develops into an embryo, which eventually grows into a new plant. The transformation of the ovary into fruit and the plant's life cycle, from seed germination to maturation and reproduction, is also covered, offering a comprehensive look at plant reproduction.
Takeaways
- πΌ Pollination involves the transfer of pollen grains onto the stigma of a flower.
- π± After pollination, the pollen grain germinates and grows a tube that extends to the ovule within the ovary.
- π§ The pollen tube is guided towards the ovule by chemical signals secreted by the ovule, a process known as chemitropism.
- π Sperm cells are transported through the pollen tube to reach the egg cells inside the ovule.
- π€ In flowering plants, a unique process called double fertilization occurs, where one sperm cell fuses with the egg cell and another fuses with the polar nuclei.
- π± The fusion of a sperm and an egg cell results in a zygote, which then divides to form an embryo, the precursor to a new plant.
- πΏ The embryo develops within the ovule, which eventually becomes a seed, complete with cotyledons that provide nourishment.
- π³ The structure of the embryo includes the plumule, which will grow into the new shoot, and the radical, which will become the new root.
- π The ovary of the flower develops into a fruit, containing seeds that are the result of fertilized ovules.
- π± Germination is the process by which a new plant grows from a seed, starting with the growth of roots from the radical and the shoot from the plumule.
- πΏ As the new plant grows, the cotyledons provide nourishment until the plant can produce its own food through photosynthesis, after which the cotyledons are no longer needed.
Q & A
What is the primary function of pollination in plants?
-Pollination is the process of transferring pollen grains onto the stigma of a plant, which is essential for fertilization and the production of seeds.
What happens after pollen grains land on the stigma?
-After landing on the stigma, pollen grains germinate, absorbing secretions from the stigma, and a tube grows from the grain that extends into the ovary and ovule to facilitate the movement of sperm cells to the egg cells.
How does the pollen tube know where to grow towards the ovules?
-The ovules secrete chemicals that the pollen tube senses, guiding its growth towards them. This type of movement in response to chemicals is known as chemitropism.
What is the significance of double fertilization in flowering plants?
-Double fertilization is a unique process where one sperm cell fuses with the egg cell, and another sperm cell fuses with the two polar nuclei in the ovule, leading to the formation of a zygote and the endosperm, respectively.
What is a zygote and how is it formed?
-A zygote is the initial cell formed when a sperm cell fuses with an egg cell. It is the first stage in the development of a new plant and will eventually divide and grow into an embryo.
What are cotyledons and what is their role in seed development?
-Cotyledons are leaf-like structures within the embryo that provide nourishment to the developing plant before it can produce its own food through photosynthesis.
How does the structure of a seed develop from a fertilized ovule?
-After fertilization, the zygote divides to form an embryo, which includes the cotyledons, plumule, and radicle. The outer wall of the ovule hardens, and the entire structure becomes a seed, ready for germination.
What changes occur in a flower after pollination and fertilization?
-Following pollination and fertilization, the stamens and petals of the flower wither and fall off. The ovary swells and develops into a fruit, containing the seeds formed from the fertilized ovules.
What is germination and how does it relate to the growth of a new plant from a seed?
-Germination is the process by which a seed develops into a new plant. It involves the growth of roots from the radicle and the development of a shoot from the plumule, with the cotyledons providing initial nourishment.
How do monocots and dicots differ in terms of their seed structure?
-Monocots have seeds with a single cotyledon, while dicots have seeds with two cotyledons. This difference in the number of cotyledons is a key characteristic that distinguishes these two major groups of flowering plants.
Outlines
πΌ The Process of Pollination and Fertilization
This paragraph explains the fundamental process of pollination and the subsequent steps leading to fertilization in plants. Pollination is the transfer of pollen grains to the stigma, which initiates the germination of the pollen grain. The pollen grain absorbs secretions from the stigma, leading to the growth of a pollen tube that extends into the ovary and ovule. The ovules secrete chemicals that guide the tube's growth through chemitropism. Sperm cells are produced within the pollen grains, and egg cells are formed inside the ovules. The sperm cells travel through the pollen tube to reach the egg cells. The paragraph also discusses the unique phenomenon of double fertilization in flowering plants, where one sperm cell fuses with the egg cell to form a zygote, and the other fuses with two nuclei in the ovule.
π± Formation of the Embryo and the Development of Seeds
Following the fertilization process, the zygote undergoes multiple divisions to form an embryo, which represents the future plant. The embryo's structure includes cotyledons, which provide nutrition to the developing plant, and parts that will eventually grow into the shoot (plumule) and root (radical). The outer wall of the ovule hardens, transforming it into a seed. The changes in the flower are also described, where the fertilized ovules become seeds, stamens dry off, petals fall, and the ovary swells to become a fruit containing seeds. The paragraph concludes with a brief mention of sowing seeds and the potential for germination.
π± Germination and the Life Cycle of a Plant
This paragraph delves into the process of germination, where seeds develop into new plants. Initially, the root emerges from the radical, and the shoot develops from the plumule. The cotyledons swell to provide nourishment for the growing plant. As the plant grows, true leaves emerge and begin photosynthesis, making the cotyledons obsolete, leading to their eventual shrinkage and disappearance. The plant continues to grow, and once mature, it can produce flowers, which can be pollinated to continue the cycle of plant reproduction. The paragraph summarizes the key points of fertilization, embryo formation, and the transition from seed to mature plant.
Mindmap
Keywords
π‘Pollination
π‘Stigma
π‘Pollen Grain
π‘Sperm Cells
π‘Ovule
π‘Pollen Tube
π‘Chemitropism
π‘Double Fertilization
π‘Zygote
π‘Embryo
π‘Cotyledons
π‘Germination
π‘Radical
π‘Plumule
π‘Fruit
Highlights
Pollination is the transfer of pollen grains onto the stigma.
Pollen grains germinate after landing on the stigma, absorbing its secretions.
A pollen tube grows from the grain, extending into the ovary and ovule.
Ovules secrete chemicals guiding the pollen tube through chemitropism.
Sperm cells move through the pollen tube to reach the egg cells.
Double fertilization occurs in flowering plants with two sperm cells.
One sperm cell fuses with the egg cell to form a zygote.
The zygote undergoes cell division to form an embryo, the future plant.
Cotyledons in the embryo provide nutrition for the growing plant.
Plants can be classified as dicots with two cotyledons or monocots with one.
The plumule develops into the new shoot, and the radical into the new root.
The fertilized ovule transforms into a seed with a hard outer wall.
The ovary swells to become a fruit containing seeds.
Sowing seeds initiates germination, with the radical sprouting the root.
The plumule develops into the shoot, and cotyledons provide nourishment.
As the plant matures, cotyledons shrink, and true leaves take over photosynthesis.
The cycle of plant life continues as the mature plant produces flowers for pollination.
Transcripts
pollination is the act of transferring
the pollen grains on to the stigma
great we have transferred pollen grains
onto the stigma but what happens next
that's basically what we're going to
talk about in this video in this video
we're going to see exactly how the sperm
cells are gonna reach the eggs and
eventually how a new baby plant is born
so let's zoom in on this female part and
see what happens after pollination
so here is the pollen grain which has
landed on the stigma now a couple of
things is first of all there will be a
lot of pollen grains and pollen grains
are usually much smaller than what i've
shown over here but i've shown them big
and i've shown one so that we can
clearly focus on and see what's going on
so the sperm cells are going to be
produced inside these pollen grains and
the egg cells will be formed inside the
ovules over here so we need to get the
sperms from here to there but how does
it happen
it is for this reason the pollen grain
starts germinating so what's going to
happen is the pollen grain starts
absorbing some of the secretions that
are done by the stigma the sticky stigma
and then the pollen grain starts
germinating and a tube starts growing
from this grain so let me show you so a
tube starts growing
from this pollen grain and the tube will
eventually
the tube will eventually go all the way
into the ovary this is the ovary it will
go all the way into the ovary and all
the way in to the ovule so it makes a
path for the sperms and leads them all
the way to the ovule where the egg cells
are lying now at first you may be
wondering well how does how does the
pollen tube know exactly where the
ovules are well it turns out that the
ovules over here start secreting some
chemicals and the pollen tube basically
senses the chemicals and starts growing
towards it now we've talked about such
motion before we call this the
chemitropism tropism means movement and
chemi means movement due to chemicals
anyways don't worry too much about that
a pollen grown a pollen tube has now
grown and now we will see some sperm
cells
which i'm drawing over here let me draw
that over here a couple of sperm cells
they will start moving through the
pollen tube and they will enter
into the overview
great what's going to happen next
well let's zoom in on this ovu and find
out so here is the zoomed in version of
that overview this is that same poland
tube
and this is that ovule the single ovule
have zoomed in over here now if you look
inside the ovule you'll find
the egg cell that's where the sperms
need to reach
but here's the thing the ovule doesn't
only contain egg cells it also contains
some other cells as well
in fact if you count it turns out there
are one two three
four five six
and this whole big total seven cells
this big cell has two nuclei inside of
it all right so it turns out there are
total eight nuclei inside this ovu but
don't worry about it we don't have to
worry too much about it right now so
let's not worry about these other cells
and so now from this pollen tube the
sperms are coming in so let me just draw
those sperm cells
so these are the nuclei
of this pumps
they will move they'll move and
eventually
the pollen tube guides them into the
ovule now what happens
well one of the sperm will unite with
the egg cell
so one of the sperms unites with dx
cells and the other sperm
not this one
the other sperm cell
will go and unite with the two nuclei
remember i showed that there are two
nuclei over here sharing one entire
cytoplasm so
that other other sperm is going to unite
with that
now the reason why i'm telling you this
even though that's not really important
the reason i'm telling you this is
because because there are two sperms
uniting one uniting with the egg and one
uniting with the other cells we say
there are two fertilizations happening
in this plant and that's why usually in
flowering plants we say double
fertilization takes place fertilization
is basically the uniting of the sperm
cells with one of these cells
okay but anyways we don't have to worry
about that other sperm and that other
thing so let's
get rid of that we will only consider
the fusion of
the sperm cell with the egg cell so when
that
fusion happens we now get a new cell
and this new cell which is basically the
fusion of the two we give a name to that
we call this
the zygote
zy
goat so what exactly is zygote zygote is
the
fusion of sperm
sperm
and the egg
great we now have a zygote what happens
next
well next a lot of changes start
happening first of all the pollen tube
has done its job so it will start
disintegrating then we will see that the
zygote will start dividing from one to
two two to four four to many so let me
show you that
so here it is
we can now see the zygote is multiplying
it's going into dividing into many many
many more cells
and eventually you will find
that the zygote would eventually have
divided into a lot many cells to give us
this kind of structure
this structure
is now ready to grow a future baby plant
so this now
is called let me write that down
this
structure is now called
the
embryo
so you can think of embryo as the future
future
plant
it's going to grow
into the future plant
in fact we can actually identify the
parts of the embryo if you look at these
two leaf-like structures they are called
the cotyledons let me just write that
down
so these two leaflet structures are
called
the cotyledons
and their main job
is to provide the embryo with all the
nutrition all the food that they need
once it starts growing it will require a
lot of food and remember we don't have
leaves yet so until the new leaves get
formed the cotyledons are the one that's
going to provide them with nourishment
when it starts growing eventually we'll
look at that in a second
and then you can see this part over here
this
and important to see them
and of course over here i have drawn two
cotyledons but not all plants will have
two cotyledons some will have two which
is called dicots and some plants will
have just one cartilage and we call them
as monocots you may have learned about
that previously anyways this part as you
can see in between the two cotyledons
this is the part that eventually in the
future will grow into a new shoot and
that part is called the plumule
plumu
this will grow into a new shoot
and finally this part over here
let me write that in a different color
this part
this is something that eventually come
out from here and grow into a new root
so this is called the radical
so the radical grows into a new root the
plumel grows into a new shoot and the
cotyledons kind of act like leaves
they're not really the leaves but they
act like leaves in the sense they give
them all the nourishment needed once it
starts growing
and a lot of other changes happen as
well for example the outer wall starts
to become a little hard
and now
we no longer have an overview we are
going to call this
as a seed
so the fertilized ovule has now finally
got converted into a seed
great but what happens to the flower
well let's see so here is the flower
here is our flower we are zooming back
out now
all right here is the flower here is
that particular overview we were talking
about what are the changes that are
happening first of all we will see now a
lot of ovules would have gotten
fertilized remember i just showed one
pollen grain but there are multiple
pollen grains which are gonna fertilize
multiple ovules so all these ovals have
now gotten converted into seeds they're
now seeds and the unfertilized ones are
eventually going to disintegrate
what else is going to happen well we no
longer need the stamens so that stamens
will start drying off we no longer need
the petals so that you will see the
petals fall off and we will now find
eventually the ovary
this ovary starts swelling and starts
becoming bigger and bigger
here it is
the over starts becoming big and big and
eventually it's the ori that's going to
grow
into a fruit
and there will be seeds
inside that particular fruit so over
here i've shown a tomato as an example
but it can be any fruit with seeds in
them
then you can eat that fruit and then
soak the seeds
so maybe you can't do that for tomato
but for some other fruit you may be able
to sow the seeds so
next question is what happens once you
sow the seeds well once you start sowing
the seeds you will now see that
first
root starts developing from the radical
radical is the part where the root
develops right
then we will find
the shoot starts developing from the
plumeu remember we just talked about
that let me show you that
so from the radical the roots are coming
from the plymouth the shoot is coming
and the cotyledons these cotyledons
these two things are the quarterly the
cotyledons become massive they become
this swell up and they almost take up
the entire seed these are now the
cotyledons it's the cotyledons are
providing them with all the nourishment
needed to grow they're giving them all
the food to grow
and then as time passes by we will now
find that eventually that shoot starts
growing into new leaves
and now the leaves the first leaves can
now true leaves we call them can now
start photosynthesizing and start and
can start making the new food and now no
longer cotyledons are needed so the
cotyledons eventually get drained they
will now start shrinking and eventually
the cotyledons go away they will shrink
and finally
a new baby plant is born the roots can
keep on growing the plant can keep on
growing and once the plant becomes
mature enough
the plants can grow new flowers
and the flowers can get pollinated and
now the cycle can repeat
this process of new plants growing from
a seed by the way is what we call
germination
and so this in short explains
how fertilization happens
in plants
so to quickly summarize the important
points after pollination a pollen tube
starts growing
and it directs the sperm cells all the
way into the ovule
once the sperm cells reach the ovule one
of the sperm
fuses with the excel this process of
fusion is what we call fertilization
and the new fused cell is what we call a
zygote the zygote can now multiply
divide into many cells and eventually
forms what we call the embryo the embryo
is the future plant
the cotyledons of the embryo are going
to nourish it once the embryo starts
gener once the seed starts germinating
the radical is where the root grows and
the plumeu is where the shoot starts
growing at the same time
the flower loses all its other parts the
ovary swells and becomes the fruit
and eventually as the plant grows the
cotyledons shrink and now the
sleeves take care of all the nourishment
and the food requirement of the plant
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