Sexual reproduction in plants 4.2

Question 1

What are the characteristics of insect pollinated plants?

Answer 1

1: Petals - coloured to attract insects
2: a smell of some kind may be produced to attract insects
3: Nectar may be produced for the same reason
4: The anthers and stigma are generally enclosed within petals
5: Pollen is often large and sticky to stick to insects
6: Only a small amount of pollen is produced.

Question 2

What is the male reproductive organ of the flower?

Answer 2

Stamen made of the anther and filament.

Question 3

What is the female reproductive organ of the flower?

Answer 3

Carpel made of the ovary, ovule, style and stigma.

Question 4

What are the characteristics of wind pollinated flowers?

Answer 4

1: Petals are absent
2: No smell is produced
3: Nectaries are absent
4: Anthers and stigma hang outside the flower so pollen can be shed easily into the air.
5: Pollen is usually small and light containing air sacs
6: Large quantities of pollen
example of wind pollinated flowers is grass.

Question 5

How are microspores formed?

Answer 5

Meiosis but following their production the nuclei in each of pollen grains undergoes mitosis to produce 3 genetically identical nuclei in each pollen grain.

Question 6

What is the process that pollen grains are formed by?

Answer 6

1: Many microsporocytes are formed by mitosis within each anther.
2: These diploid cells undergo meiosis to produce 2 then 4 haploid nuclei
3: These nuclei are genetically identical to each other
4: A cell wall forms around each nucleus to form a tetrad of microspores. These microspores then separate to form the individual pollen grains.

Question 7

What is the thick protective wall that microspores develop?

Answer 7

• Exine is sculpted into various shapes that are characteristic of a particular species and are adaptations to help the pollen transport from an anther to a stigma - pits in the surface enable gaseous exchange.
• Intine is thinner and more porous.

Question 8

What are the three nuclei that pollen grains contain?

Answer 8

1: 2x male gamete nucleus that are genetically identical.
2: Pollen tube nucleus

Question 9

How are the female gametes formed?

Answer 9

By meiosis within a structure called the embryo sac (megagametophyte).

Question 10

Where are ovules produced?

Answer 10

In the ovary of a flower.

Question 11

What is the process of ovule production?

Answer 11

1:Megasporocytes undergo meiosis I and II as normal.
2: 3 of the haploid cells die and 1 develops.
3: Haploid nucleus undergoes 3 mitotic divisions to form 8 genetically identical nuclei.
4: Cell membranes form around 6 of the nuclei while 2 remain as free nuclei.

Question 12

What is pollination?

Answer 12

The process by which pollen is transferred from anther to stigma.

Question 13

What is self pollination?

Answer 13

Plant’s own pollen lands on stigma of same flower.

Question 14

What are the advantages of self pollination?

Answer 14

• Pollination is rapid with a high chance of success so increases chance of seed production and survival of species.
• Offspring have reduced genetic variation - genes all come from same plant, if plant is well-adapted to living in a particular environment the genomes are preserved in the offspring. Good genes presereved.

Question 15

What are the disadvantages of self pollination?

Answer 15

• Reduced genetic variation so all plants susceptible to the same disease. If there is significant environmental change they can’t adapt quickly to the changes and few individuals survive.

Question 16

What is cross-pollination?

Answer 16

Stigma receives pollen from anther of a different plant.

Question 17

What are the advantages of cross-pollination?

Answer 17

Offspring have increased genetic variation. Wide range of genomes are produced which increases chance of species survival if there are significant environmental change.

Question 18

How is variation introduced in meiosis?

Answer 18

1: Independent assortment at anaphase 1
2: Random fertilisation at metaphase 1
3: Crossing over at prophase 1

Question 19

What is the main disadvantage of cross-pollination?

Answer 19

Due to crossing over and random fertilisation, good combinations of genes can be lost.

Question 20

How does chemical self incompatibility assure fertilisation achieved?

Answer 20

In many plants, pollen won’t germinate on the stigma of a flower from the same plant - Chemical signals produced by stigma don’t stimulate pollen from same flower. This ensures that pollen must come from a different individual and so increase genetic variation.

Question 21

How does flower structure assure fertilisation is achieved?

Answer 21

1: Pin eyed - stigma is above the anthers - pollen can’t fall up so the flower is pollinated only when a visiting insect carries pollen from another flower.
2: Thrum ‘eyed’ - Where stigma is below the anthers - pollen falls on the stigma so the flower can be self pollinated.

Question 22

What is double fertilisation?

Answer 22

Two separate fusions of male and female haploid nuclei.

Question 23

What is the pollen tube nucleus?

Answer 23

It controls the growth of the pollen tube that grows down through the style. It also causes the secretion of digestive enzymes to enable the pollen tube to move forward.

Question 24

Where does the pollen tube grow into?

Answer 24

The embryo sac through the micropyle and the two male nuclei pass into the embryo sac.

Question 25

What happens in the embryo sac in double fertilisation?

Answer 25

One male nucleus fuses with the female nucleus to form the diploid zygote.
The other male nucleus fuses with Two polar nuclei to form a triploid endosperm nucleus.

Question 26

What forms as a result of double fertilisation?

Answer 26

1: Zygote divides by mitosis to form the embryo which contains the embryo root - the radicle and embryo shoot - the plumule. In dicotyledonous seeds there are also seed leaves formed - the cotyledons that store food for the embryo.
2: PEN also divides by mitosis and in monocotyledonous seeds it forms the food storage tissue.
3: Integuments fuse together and form the testa or seed coat.
4: Ovary wall forms the fruit wall and encloses the seeds.
5: Ovules form the seeds.

Question 27

What is an example of a dicotyledonous seed?

Answer 27

The broad bean seed.

Question 28

How is the outer coat formed in monocotyledonous seeds?

Answer 28

Only one cotyledon and the ovary wall and testa fuse to form the outer coat, which is really a fruit not a seed.

Question 29

What is an example of a dicotyledonous seed?

Answer 29

Maize.

Question 30

What needs to happen before germination can take place?

Answer 30

1: Seeds need to absorb water through the micropyle
- This mobilises the enzymes
- Increases cell size through formation of vacuoles
- Transport of materials in the seed
2: Absorb oxygen through the micropyle.
3: The temperature also needs to be suitable and some seeds require light to trigger germination.

Question 31

What is the first part of the seedling to grow?

Answer 31

The radicle.

Question 32

What part of the stem elongates?

Answer 32

Epicotyl is the stem of the plumule - elongates due to vacuolation of the cells and pulls the leaves backward through the soil. Once leaves are above the ground they unfold and begin to photosynthesise.

Question 33

Describe the effect of gibberellin in the process of germination in endospermic seeds such as maize.

Answer 33

Following water inhibition, gibberellin is mobilised. It diffuses to the aleurone layer and stimulates enzyme production, protease to digest protein to amino acids. The amino acids form amylase which diffuses into the endosperm where it digests starch to maltose. Maltase digests maltose to glucose, glucose diffuses into embryo growing tips.

Question 34

Describe the process of germination in a non-endospermic seed.

Answer 34

Water imbibed through the micropyle which causes the tissues to swell and causes the testa to split allowing more room for oxygen to come in and aerobic respiration to take place. Food reserves are broken down into soluble molecules - amylase hydrolyses starch into maltose and proteases hydrolyse proteins to amino acids. This provides sources of energy for respiration and growth of the plumule and radical.

Question 35

Why are some seeds described as non-endospermic?

Answer 35

Because the cotyledons have taken over from the endosperm as the food store.