3B Fertilisation in Mammals and Plants

Question 1

the process of fertilisation in humans (3)

Answer 1

• the ovum released at ovulation has not fully completed meiosis and is surrounded by zona pellucida
• many sperm cluster around the ovum and as soon as the heads of the sperm touch the surface of the ovum, the acrosome reaction is triggered
• enzymes release from the acrosome, which digest the zona pellucida

Question 2

why must so many sperms be released during ejaculation?

Answer 2

because one sperm does not produce enough enzymes to penetrate the protective layers around the ovum

Question 3

when and why does the second meiotic divistion take place? (2)

Answer 3

• it takes place after one sperm has wiggled its way through the protective barriers and touched the surface membrane of the oocyte
• it provides a haploid ovum nucleus to fuse with the haploid male nucleus

Question 4

why is it important that no more than one sperms enters after the second meiotic division?

Answer 4

because it could result in polyspermy and would produce a nucleus containing too many sets of chromosomes

Question 5

polyspermy

Answer 5

fertilisation by more than one sperm

Question 6

acrosome reaction (3)

Answer 6

• the front of the sperm touches the zona pellucida of the oocyte and the acrosome reaction is triggered
• digestive enzymes pour out of the opened acrosome and begin to digest the zona pellucida
• projections of the sperm surface shoot forward due to actin-like proteins

Question 7

corticol reacion (3)

Answer 7

• cortical granules are released from the oocyte
• they combine with the zona pellucida to form a tough fertilisation membrane which prevents any other sperm from entering the ovum
• once complete, the male and female chromosomes combine to form a diploid zygote

Question 8

fertilisation membrane

Answer 8

the tough layer that forms around the fertilised ovum to prevent the entry of other sperm

Question 9

in plants, where is the male gamete found?

Answer 9

within the pollen grain

Question 10

in plants, where is the female gamete found?

Answer 10

embedded deep in the tissue of the ovary

Question 11

germinate

Answer 11

when pollen tube starts to grow out of the pollen grain to transfer the male nuclei to the ovule

Question 12

Describe the process of fertilisation that takes place in flowering plants (5)

Answer 12

• generative nucleus divides to form two male gametes by mitosis
• the pollen tube fuses with the embryo sac
• then double fertilisation occurs
• one male gamete fuses with the egg cell nucleus to produce a diploid zygote
• then the other male nucleus fuses with two polar nuclei to produce endosperm

Question 13

How does meiosis produce genetic variation in gametes? (3)

Answer 13

• through independant assortment and crossing over
• independant assortment gives rise to new combinations of paternal and maternal chromosomes
• crossing over involves the swapping of sections of chromosomes

Question 14

Explain the importance of meiosis in plants (2)

Answer 14

• it generates gametes

- which create genetic variation through crossing over and independant assortment

Question 15

What is the correct order of tissues through which a pollen tube grows?

Answer 15

stigma-style-micropyle

Question 16

Describe the role of the pollen tube nucleus (2)

Answer 16

• controls the growth of the pollen tube

- controls the production of enzymes and proteins

Question 17

what can be said about the nucleus of the male gamete and the nucleus of the female gamete in plants?

Answer 17

both are haploid

Question 18

Why must starch be broken down before the embryo plant can grow? (3)

Answer 18

• starch is insoluble
• glucose molecules can move into the embryo
• glucose can be used as a source of energy

Question 19

double fertilisation (2)

Answer 19

• the process that occurs in plants in which one male nucleus fuses with two polar nuclei to form a triploid endosperm nucleus
• while the other fuses with the egg cell to form the diploid zygote

Question 20

Why is internal fertilisation more sucessful than external? (2)

Answer 20

• environmental factors can destroy gametes

- it is more difficult for them to find eachother