Sample Qs -Double fertilisation

Diagram 8 shows a cross section through the ovary and pollen tube of a flowering plant just before fertilisation.(a) With the aid of Diagram 8, explain the development of an embryonic sac [5 marks]

(b)(i)Explain the mechanism of double fertilisation. [10 marks]

(ii)Explain the importance of double fertilization for the survival of flowering plants

[5 marks]

Suggested Answer:

(a)

-The complete development of each ovule takes place beginning with an embryosac mother cell
-The embryonic mother cell (diploid cells) divide by meiosis to form haploid cells
-3 of them degenerate
-The nucleus of the 1 haploid cell remaining divides by mitosis
-This produces 2 haploid nuclei, which again divide by mitosis
-This produces 4 haploid nuclei, which again divide by mitosis
-Finally this produces 8 haploid nuclei, and the structure is known as a mature embryo sac

Any 3 points [ 3 marks]

Drawing 2 marks: Correct drawing and Labels.

* May use diagram from any reference book

(b)(i)
-Pollen reaches a stigma and sticks to the surface
-A pollen tube begins to grow from the pollen grain
-The proteins for this tube are produced with the help of the tube nucleus which remains near the tip of the tube
-The generative nucleus divides into 2 male nuclei which follow down the tube
-The pollen tube grows into an ovule
-One male nucleus fuses with the egg cell forming a diploid zygote;
-this (divides many time to ) form the embryo of the seed
-The other male nucleus fuses with two polar nuclei forming a triploid cell;
-this (divides many times to )form the endosperm of the mature seed.

-This is double fertilization because of the fusion to form the diploid zygote and the primary endosperm cell. [10 maximum]

(ii)

F1 This results in genetic and morphological diversity:
P1 During fertilization there is a random fusion of gametes
with the egg cells
P2 As a result every plant is different from the mother plant as
well as each other

F2 This diversity helps plant to adapt to climatic changes and
wider range of habitats:
P1 Therefore plants are able to colonize large areas of land

F3 The development of seeds with its tough coat protect them
from unfavourable condition
P1 Germination will only take place under favourable
condition
P2 The development of colourful and sweet tasting fruits
helps in the dispersal of seeds.
Or any suitable anwer [ Max 5 marks]