Formation of the embryo sac | Sexual reproduction in flowering plants | Biology | Khan Academy

0:00 in our previous videos we have discussed about the structure of an ovule in an ovary of a flower and we also saw how a megaspore is formed inside an ovule so here this is the structure of an ovule and this green thing inside that you see is the megaspore now this mega spore is not sufficient it is it is not capable to fuse with a male gamete and form new plant babies so what else is required because it has already divided meiotically remember in our previous video we discussed that the megaspore is formed by meiotic division and therefore it has half the number of chromosomes so gametes also have half the number of chromosomes so what else is required

0:46 well here it is seen that a very special type of cell division take place so that something called the female gametophyte or the embryo sac is formed and this gametophyte or the embryo sac will have the egg cell and that egg cell will be capable to fuse with the sperm cell and give rise to new plant babies so let's understand how this embryo sac is formed so this megaspore will undergo multiple cellular division so what kind of division would it be well it will undergo mitosis it will undergo mitosis and will form an embryo sac now you must be thinking that hey we we

1:31 know what mitosis is uh it is something that looks like this one where one cell gradually divides into two where the nucleus divides first and then the cytoplasm divides but let me tell you that this mitotic division won't look anything like this here we will see that the nucleus would divide but the cytoplasm won't divide for a very long time let us see how that happens so for better understanding let me take out the megaspore and this is the nucleus now like any mitotic division first the nucleus would divide so here the nucleus divides into two and then gradually both this nuclei would

2:17 start moving towards the opposite poles so this nucleus a would move here and the other nuclei would gradually start moving towards the opposite pole now in a normal mitosis the cytoplasm would have divided right but here the cytoplasm won't divide instead the nucleus would divide again yes this nucleus here it would divide into two and the nucleus at the top it would also divide into two now what the cell has four nuclei is this the time for cytoplasmic division well the answer is no because this four nuclei will undergo further mitosis and will give rise to eight nuclei and let's see how

3:02 so this nucleus will divide into two and this one here will divide into two okay then again let's move to the top this nucleus here would divide into two and then this one here would also divide into two so we have eight nuclei in just one cell and then finally the cell decides that hey it's time for cytoplasmic division but before the cytoplasm divides something unusual happens again one nucleus from the two poles slowly starts moving towards the center so both of them starts moving simultaneously and they meet somewhere here at the center

3:48 and once they meet somewhere at the center gradually the cytoplasmic wall starts to form so for better understanding i have enlarged that again so this is how the cytoplasmic wall starts forming around each nucleus here as you can see this three nuclei on top it formed cell walls around it so we have got three individual cells at the top and then this three nuclei at the bottom it has also got cell walls around it so we got three individual cells down here at the bottom but this two nuclei that was at the center they didn't form individual cell walls around each of them instead they stuck together in this large central cell and this typical arrangement that

4:35 you see this is called the embryo sac and this also answers a very classical question which is name the eight nucleate and seven cell stage of a flowering plant so now we know the answer it is the embryo sac right because it has seven cells and eight nucleus let's count let's count and see so this is one two three four five six and this big large one it's the seventh cell and it has got one extra nucleus that means it has eight nucleus so the answer to this classical question is the embryo sag and now we know why it is called eight nucleate and seven cell right all right now let's move on to talk

5:23 about what is the need of all these different cells well we will see that each cell here will differentiate and will perform a specific function each cell will have a specific role to play and we will begin with the most important cell that is the egg cell now can you guess where will that egg still be placed will it be somewhere here in the micro pile area or down here at the chalazar land let me label it where do you think will the egg be placed well it will be near the micropile area so that when the pollen tube enters it can easily fuse with the egg cell that's here so that means if we consider this to be the micropile area

6:09 then it is seen that our egg cell is placed somewhere here all right now that we know where the egg is this brings us to another question how will the pollen tube know where the egg is because pollen tube don't use google maps like we do right there is no google map to tell him that go straight and then turn left so how will this pollen tube know where exactly is the egg in this entire embryo sac well to help the pollen tube these two cells that you see on top develop special structure called the filiform apparatus so this one is the filiform apparatus and philly form comes from the word filament so this looks like filament and

6:57 this actually guides the pollen tube into the egg cell interesting right so let me clear the board so that we can label things properly so this is the filiform structure which is developed from these two cells on top and these two cells are called the synergies and this word synergies actually comes from a greek word which means working together okay so here this two cells works together in order to help the pollen tube reach the egg cell okay let us label the egg cell as well so this one is the egg cell and this egg cell along with these two supporting cells are together called as the egg apparatus let's call the egg

7:44 apparatus all right so if this three comprises of the egg apparatus and does the main job of fusing with the male gamete what is the role of the other cells well the scene that the three cells here at the bottom or we can say at the cellazil end are called the antipodal cells antipodal simply means opposite and since it's opposite to the egg cell these are called antipodal cells and later it is seen that these three cells played no major role in the development of the growing embryo okay and therefore the cells slowly they degenerate and disappear okay but for now let's keep these three cells so that we know that these three cells are called antipodal cells

8:30 all right now let's move on to this huge big cell with two nuclei at the center this big cell is called the central cell and the two nuclei is called the polar nuclei in our later videos we will see that the central cell along with the polar nuclei will develop into endosperm in some case into juicy ripe fruits and we will see how that happens in our future videos but for now we will call this large cell as the central cell and this two nuclei as the polar nuclei now you may think why is this two nuclei called the polar nuclei it's not on the opposite poles or something it's in the center it should be called the central nuclei but it is called the polar nuclei because it has come from the opposite

9:16 poles remember we discussed that the two nuclei from each pole slowly migrates towards the center and that is how it got its name the polar nuclei all right so this is all about embryo sac or gametophyte in our future video we will talk about how fertilization takes place and what are the events that takes place post-fertilization but for now let's quickly summarize what we discussed in this video so in this video we saw that first the megaspore divided mitotically where the nuclear division took place without cytoplasmic division finally giving rise to eight nuclei and seven cell stage where the cells slowly differentiates

10:02 and evolves into specialized cells like the synergies egg cells antipodal cells and this entire structure is called the embryo sac and the most important thing to remember here is that the embryo sag has got eight nucleus and seven cells