Fertilisation, zygote, embryo, germination | How do organisms reproduce | Biology | Khan Academy

Summary

This 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

• 🌼 Pollination involves the transfer of pollen grains onto the stigma of a flower.
• 🌱 After pollination, the pollen grain germinates and grows a tube that extends to the ovule within the ovary.
• 💧 The pollen tube is guided towards the ovule by chemical signals secreted by the ovule, a process known as chemitropism.
• 🚀 Sperm cells are transported through the pollen tube to reach the egg cells inside the ovule.
• 🤝 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.
• 🌱 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.
• 🌿 The embryo develops within the ovule, which eventually becomes a seed, complete with cotyledons that provide nourishment.
• 🌳 The structure of the embryo includes the plumule, which will grow into the new shoot, and the radical, which will become the new root.
• 🍅 The ovary of the flower develops into a fruit, containing seeds that are the result of fertilized ovules.
• 🌱 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.
• 🌿 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

00:00

🌼 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.

05:03

🌱 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.

10:04

🌱 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.

Keywords

💡Pollination

Pollination is the process of transferring pollen grains onto the stigma of a flower, which is a crucial step in plant reproduction. In the video's context, pollination is the starting point for the discussion on how sperm cells reach the egg cells and how a new plant is conceived. The script describes how many pollen grains land on the stigma and begin the process of fertilization.

💡Stigma

The stigma is the part of the female reproductive organ of a flower where pollen grains adhere and germinate. It is depicted in the script as the landing site for pollen grains, which is the first step in the process of fertilization after pollination.

💡Pollen Grain

Pollen grains are the male reproductive cells of plants that contain sperm cells. The script uses the term to describe the cells that germinate after landing on the stigma, initiating the process that leads to fertilization.

💡Sperm Cells

Sperm cells are the male gametes in plants, which are produced inside pollen grains. The script explains that these cells are transported through a pollen tube to reach the egg cells within the ovule, a vital part of the fertilization process.

💡Ovule

An ovule is the female reproductive structure in the flower that contains the egg cell. The script describes how sperm cells travel through the pollen tube to reach the ovule, where fertilization occurs.

💡Pollen Tube

A pollen tube is a tube-like structure that grows from a germinating pollen grain. As explained in the script, it serves as a conduit for sperm cells to travel from the pollen grain to the ovule, facilitating fertilization.

💡Chemitropism

Chemitropism is the movement of a cell or organism in response to a chemical stimulus. The script mentions chemitropism as the mechanism by which the pollen tube senses chemicals secreted by the ovule and grows towards it, ensuring the sperm cells reach their destination.

💡Double Fertilization

Double fertilization is a unique process in flowering plants where two sperm cells from the pollen grain unite with different cells within the ovule. The script describes this process, where one sperm cell fuses with the egg cell, and the other fuses with two nuclei in the ovule, leading to the formation of a zygote and a separate structure.

💡Zygote

A zygote is the initial cell formed when two gamete cells (sperm and egg) fuse during fertilization. The script explains that the fusion of a sperm cell with the egg cell results in the formation of a zygote, which is the first step in the development of a new plant.

💡Embryo

An embryo in the context of plants is the early developmental stage that forms from a zygote and eventually grows into a new plant. The script details how the zygote divides and forms an embryo, which contains structures like cotyledons, plumule, and radical that will develop into different parts of the new plant.

💡Cotyledons

Cotyledons are seed leaves that provide nourishment to the developing embryo. The script explains that they play a crucial role in supplying food to the growing plant before it can produce its own food through photosynthesis.

💡Germination

Germination is the process by which a plant grows from a seed. The script describes germination as the development of the embryo from the seed, where the radical develops into roots, and the plumule into the shoot, eventually leading to the growth of a new plant.

💡Radical

The radical is the embryonic root of a seedling. The script mentions that the radical is where the root of the new plant develops from during germination, establishing the foundation for the plant's growth.

💡Plumule

The plumule is the embryonic shoot of a seed, which will develop into the stem and leaves of the new plant. The script explains that the plumule grows from the embryo during germination, leading to the formation of the plant's aerial parts.

💡Fruit

A fruit is the mature ovary of a flower, typically containing seeds. The script describes how the ovary swells and becomes a fruit after fertilization, which can contain seeds for the propagation of the plant.

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