Component 2.4 - Pollination And Fertilisation

Question 1

Insect vs wind pollinated - petals

Answer 1

Insect: colourful petals with nectar guides
Wind: petals usually absent, small, green

Question 2

Insect vs wind pollinated - nectar

Answer 2

Insect: scent and nectar (mainly sucrose)
Wind: no scent or nectar

Question 3

Insect vs wind pollinated - anther

Answer 3

Insect: anthers within the flower
Wind: anthers hanging outside the flower

Question 4

Insect vs wind pollinated - stigma

Answer 4

Insect: stigma within flower
Wind: large, feathery stigma hangs outside flower

Question 5

Insect vs wind pollinated - pollen

Answer 5

Insect: small quantities of sticky, sculptured pollen
Wind: large quantities of smooth pollen

Question 6

Development of male gamete

Answer 6

1) Mitosis to form large numbers of pollen mother cells

2) Diploid mother cell forms 4 haploid cells - meiosis

Question 7

What is the role of the tapetum

Answer 7

It is a layer of cells around the pollen sac providing nutrients to the pollen grains and has a significant role in the formation of the cell wall

Question 8

What special features does the pollen cell wall have

Answer 8

It is tough and chemical resistant which means 1) it resists desiccation so the pollen can move around 2) UV light cannot penetrate seed so DNA is protected from mutation

Question 9

How is pollen released

Answer 9

Outer layers of the anther dry out causing tension in the lateral grooves which pulls the anther apart and edges of the pollen sac away

Question 10

Development of female gamete

Answer 10

1) Megaspore mother cell splits into four haploid megaspores- meiosis
2) Three of the four cells disintegrate
3) Remaining cell undergoes 3 rounds of mitosis - 8 haploid nuclei within embryo sac
4) 1 is the female gamete and 2 fuse to firm a diploid polar nucleus

Question 11

What is the role of the nucellus

Answer 11

Provide nutrients

Question 12

Disadvantages of self-fertilisation

Answer 12

1) Relies on independent assortment and crossing over for genetic variation. Less variation than cross-fertilised species
2) Greater chance if two recessive alleles being brought together in fertilisation

Question 13

Advantages of self-fertilisation

Answer 13

Can preserve successful genomes are suited to a stable environment

Question 14

What is pollination?

Answer 14

The transfer of pollen from an anther to a stigma

Question 15

Advantages of cross pollination

Answer 15

1) Combines gametes from two individuals (random fertilisation), meiosis and mutation results in increased variation
2) Outbreeding reduces chance of harmful allele combinations

Question 16

How do plants ensure cross pollination?

Answer 16

1) Anther is below stigma so pollen can’t fall on it
2) Pollen cannot germinate on the stigma of the flower that produced it
3) Separate male or female flowers/plants

Question 17

Describe the growth of the pollen tube in double fertilisation

Answer 17

1) Pollen grain germinates if on a compatible stigma
2) Mitosis of pollen grain nucleus to produce generative nucleus (which splits into two male gametes) and a pollen tube nucleus
3) Pollen tube grows under control of pollen tube nucleus through the digestion of the style by hydrolase enzymes
4) Pollen tube grows through micropyle and passes into embryo sac

Question 18

What happens after the pollen tube has reached the embryo sack

Answer 18

1) The pollen tube nucleus disintegrates
2) Pollen tube opens releasing two male gametes
3) Male gamete fuses with female gamete to make a diploid zygote
4) Male gamete + two polar nuclei fuse to form a primary endosperm nucleus

Question 19

What happens to the triploid nucleus?

Answer 19

It divides by mitosis to generate endosperm tissue which takes over from the nucellus to provide nutrients, acting as an important food storage tissue

Question 20

Describe the development of the fruit and seed

Answer 20

1) Integument dries out and becomes testa
2) Micropyle remains as a pore in the testa
3) Ovule develops into a seed
4) The ovary wall develops into a fruit wall enclosing the seeds
5) Diploid zygote divides by mitosis to form diploid embryo (plumule, radicle and one or two cotyledons)

Question 21

What’s the difference between monocotyledon and dicotyledon?

Answer 21

In a dicotyledon the endosperm absorbs into the cotyledons so it is a non-endospermic seed but in a monocotyledon the endosperm remains as food store whilst the cotyledon stays small.

Question 22

How is a one seeded fruit formed in a monocotyledon (endospermic seed)?

Answer 22

The testa of the seed fuses with the ovary wall.