Double fertilization| Sexual reproduction in flowering plants | Biology | Khan Academy

0:00 in our previous videos we have discussed so much about reproduction in flowering plants we have discussed in detail about how the male gametes are formed and how the female gametes are formed and in this video we are going to talk about how both these gametes fuse how the fertilization actually takes place so for that we have taken a beautiful bisexual flower here that means it has both the female and the male parts in it and also most importantly this flower is self-pollinating which means the male gamete of this flower has the capacity to fuse or fertilize the female gamut of the same flower alright having said that let's begin the story of fertilization so it begins with a release of pollen from the enthalpes the small particles

0:47 that you see here are pollen grains next this pollen grains fall onto the female part of the flower it can happen through vein that can happen through other pollinating agents like bees or other insects so here let me be that pollinating agent i will carry the pollen all the way to the female reproductive part here this yellow pollen grain actually acts like a bag that holds the male gametes now can you recall how many male gametes a pollen grain contains well each pollen grain will have two male gametes and both the male gametes will be haploid all right now let's talk about the female gametes where are they

1:33 well the female gametes lie somewhere in the small small lobe-like structures that you see in the ovary they remain hidden somewhere in these ovules each of you will contain one female gamete or we can say a single embryo sac so this male gametes that has come all the way from the anther to the stigma has to do some more traveling it has to travel all the way down the style to finally reach the ovule and they do this with the help of something called the pollen tube the pollen tube helps the male gametes to reach the destination and this is where the fusion or the fertilization takes place now how about we zoom into this part the ovule and the tip of the pollen tube to

2:19 have a clear picture have you noticed something new well this green color lobe-like structure here seems new right well this is our female gametophyte or the embryo sag that contains the female gamete and this was not visible in this tiny picture here how about we enlarge it even further let's enlarge the female gametophyte now take a moment to look at the structure of this embryo sac you will see that it has one large cell and few small cells so let's begin with these three cells here these three cells here are called antipodal cells now the role of this antipodal cells in

3:05 fertilization is not known some research shows that this persists all throughout fertilization and some also believes that it disintegrates so like in most common textbooks let's consider that it disintegrates so now we have three cells on top and two nuclei at the center of this large cell and we will start with this two cells on top these two cells are called the synergies without them the pollen tube won't even reach the embryo sac the pollen tube gets the direction with the help of these pink structures that you see the finger like projections these are called filiform structures and they actually show the pollen tube the part through which they have to enter so let's bring

3:52 in the pollen tube now this is the pollen tube here and these are the two male gametes that it brings with it so as soon as the pollen tube reaches this synergies one of the synergied actually makes path for the pollen tube to enter it this integrates and slowly these male gametes it starts entering the synergy itself so one by one both these male gametes will enter the synergied cell and slowly we will see that both the synergy after they have done their work it will disintegrate it will disappear because their job is done now we are left with the egg cell that is the female gamete and then we have

4:40 two nucleus in the center these two nucleus are called polar nuclei now let's talk about the ploidy of this embryo sac what do you think it will be well it is seen that every nuclei that we just discussed or every cell that we saw inside this embryo sag were all haploid so this egg cell nucleus is haploid and this two polar nuclei here are also haploid now if you have no clue about how they became haploid then i would recommend you to go back and watch the video of megasporogenesis where we discussed how and at which point during the

5:27 development meiosis took place and all these cells became haploid all right moving ahead let's talk about this pollen tube now after the release of the male gametes this pollen tube has no role to play and therefore it also disintegrates and disappears now all we are left with are these polar nuclei this egg cell here and this two meal gametes now this is the most important part of the entire story one of the male nuclei moves towards the egg cell and fuses with the egg cell this process is called fertilization or syngemi where the male gamut and the female gamut fuses and they fuse to give

6:15 rise to a diploid psychotic we saw that male gamete and female gamete both haploid fused therefore the zygote will be a diploid one and this zygote later forms an embryo and that embryo gives rise to a baby plant wonderful then what is the role of this three nuclei that's just lying there well the other male gamete this one will slowly come and with the polar nuclei and they together form something called the primary endosperm nucleus this is nothing but a nutritive tissue that provides nourishment as the zygote

7:02 develops so what do you think should develop first the zygote or the primary endosperm nucleus well the primary endosperm nucleus develops first because without food without nourishment how will the zygote even develop right now can you tell me what will be the ploidy of this primary endosperm nucleus it had two haploid female nuclei and one male gamete nuclei right so the ply d will be 3n or triploid and also since three different nuclei fused together to give rise to this triploid structure we call

7:47 it triple fusion the fusion of three nuclei and also since the male gamut fused twice once to form the zygote and next to form the primary endosperm nucleus this entire process is called double fertilization which is typical for an angiospermic plant all right now that we have seen how fusions of male and female gametes takes place we know what is double fertilization and triple fusion how about we try and solve a question related to it so the question is if each haploid nucleus in an embryo sac contains 10 chromosomes so this is an

8:35 embryo sac and and every nucleus has 10 chromosomes then at the end of double fertilization how many chromosomes will the primary endosperm nucleus have so this is the primary endosperm nucleus what do you think how many nucleus sorry how many chromosomes will primary endosperm nucleus have at the end of double fertilization pause the video for a while and try find out the answer all right so the question is asking about the number of chromosomes in the primary endosperm nucleus and we know that it is formed by the fusion of three nuclei of which two are female and one is a male nuclei

9:22 right and the haploid female nuclei of the embryo sac has 10 chromosomes which is mentioned in the question so every female nuclei will have 10 chromosomes each and what will be the number of chromosomes of the male nucleus well the male gamete or the male nucleus here also comes from the same species right therefore it's haploid number of chromosomes would be same as that of the female chromosome therefore it will also have 10 chromosomes in its haploid nucleus so here is the answer the primary endosperm nucleus which has three haploid nucleus will have a total of 30 chromosomes