Reproduction In Plants

Question 1

State 4 differences in the features of insect pollinated and wind pollinated flowers

Answer 1

• Insect pollinated have colourful large petals, scent and nectar whereas in wind pollinated, petals are often absent or small, no scent or nectar
• In insect pollinated, anthers are within the flower whereas in wind pollinated anthers hang outside the flowers
• Insect pollinated have the stigma within the flower whereas wind pollinated have large feathery stigmas that hang outside the flower
• Insect pollinated have small quantities of larger, sticky sculpted pollen, wind pollinated have large quantities of smaller, smooth pollen

Question 2

Describe the formation of a pollen grain (male gamete)

Answer 2

• Pollen grains are formed in the anther by mitosis and meiosis
• Mitosis occurs to produce large numbers of diploid pollen mother cells
• The mother cells undergo meiosis to produce a tetrad (four) haploid cells. These cells form pollen grains that contain the male gamete
• Inside each pollen grain the haploid nucleus undergoes mitosis to produce 2 nuclei: a generative nucleus and a pollen tube nucleus. The generative nucleus will later undergo mitosis to give rise to 2 male nuclei

Question 3

What is the tapetum and its function?

Answer 3

A layer of cells around the pollen sac. It provides nutrients to the developing pollen grains and produces a waterproof coat for the pollen grain.

Question 4

What is the function of the pollen wall (exine)?

Answer 4

It is tough and resistant to chemicals. It resists desiccation.

Question 5

Pollen is resistant to UV radiation. What is the advantage of this?

Answer 5

Pollen can be carried at high altitudes without DNA mutation occurring

Question 6

Describe the release of pollen (dehiscence)

Answer 6

• The outer layers of the anther dry out, causing tension
• Dehiscence occurs, tension pulls the walls of the anther apart and the edges of the pollen sacs curl away
• An opening called the stomium exposes the pollen grains and they are carried away by insects or the wind

Question 7

Describe development of the ovule

Answer 7

The ovule contains the female gamete and is formed in the ovary by mitosis and meiosis
- The megaspore mother cell in the nucellus undergoes meiosis to produce 4 haploid megaspores
- Three of the megaspore cells degenerate and only one grows and develops
- This one megaspore undergoes 3 mitotic divisions to produce 8 haploid nuclei within the embryo sac of the ovule (1 female gamete, 2 polar nuclei, 2 synergids, 3 antipodal cells)

Question 8

What is the micropyle?

Answer 8

A gap in the integuments for the male nuclei to enter the embryo sac

Question 9

What is the function of the integuments?

Answer 9

To protect the embryo sac

Question 10

What is the function of the funicle?

Answer 10

It attaches the ovule to the ovary wall

Question 11

Define pollination

Answer 11

The transfer of pollen grains from the anther to the stigma
Necessary so pollen grains, containing male gametes, are brought into contact with the female part of the flower so fertilisation can occur

Question 12

Define self pollination

Answer 12

Transfer of pollen grains from an anther to a stigma of the same plant
Some genetic variation through crossing over and independent assortment

Question 13

State 2 advantages of self pollination

Answer 13

• Only 1 parent needed
• Can help maintain genotypes which are well adapted to the current environment

Question 14

State 2 disadvantages of self pollination

Answer 14

• Reduces genetic variation - offspring may be less likely to survive changes in the environment
• Increases the chance of offspring being homozygous for recessive alleles which could lead to genetic diseases

Question 15

Define cross pollination

Answer 15

Transfer of pollen grains from an anther to a stigma of a different plant of the same species
Increased genetic variation through crossing over, independent assortment and mixing of parental genotypes when haploid gametes fuse

Question 16

Where do insect pollinated flowers have their nectaries located? Why is this an advantage?

Answer 16

Below the anther/stigma. This promotes insects transferring pollen grains as they will brush past the anther/stigma when going down to the nectaries.

Question 17

State 4 processes that ensure cross pollination occurs

Answer 17

1) The stamen and stigma ripen at different times
- Even if pollen produced by the anther lands on the stigma it will not germinate and form a pollen tube
- No pollen will fall on the ripe stigma as its not being produced by the anther
2) The anther is located below the stigma to pollen can’t fall from the anther onto the stigma
3) Genetic/chemical incompatibility Pollen is unable to germinate of the stigma of the flower that produced it
4) Separate male and female plants (unisexual). The plant does not have both male and female sex organs so can’t self pollinate

Question 18

Describe fertilisation

Answer 18

1. When a compatible pollen grain lands on a stigma, the stigma secretes a sugary solution
2. The pollen grain then germinates, and the pollen tube nucleus controls the growth of the pollen tube
3. The pollen tube nucleus codes for hydrolytic enzymes that digest through the tissues of the style. The product of digestion are used by the pollen tube
4. The pollen tube nucleus is at the tip of the pollen tube with the two male nuclei behind
5. The pollen tube enters the embryo sac through the micropyle
6. The pollen tube nucleus now disintegrates and the tip of the pollen tube opens, releasing the two male nuclei into the embryo sac

Question 19

Describe double fertilisation

Answer 19

• One male gamete fuses with the female gamete to produce a diploid zygote
• The second male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus

Question 20

What are 3 similarities between fertilisation in humans and in flowering plants?

Answer 20

• Both involve the fusion of haploid gametes to form a diploid zygote
• Both involve internal fertilisation in the female
• Both involve the secretion of digestive enzymes to digest a path for the male gamete to reach the female gamete

Question 21

What are 3 differences between fertilisation in humans and in flowering plants?

Answer 21

• Double fertilisation occurs in plants
• Pollen tube formation in plants/acrosome reaction in humans
• Male gamete is able to swim in humans but in plants the male gamete cannot

Question 22

What is the role of the diploid zygote in the formation of the seed and fruit?

Answer 22

Undergoes mitosis to form a diploid embryo, consisting of plumule (developing shoot), radicle (developing root) and one or two cotyledons

Question 23

What is the role of the triploid endosperm nucleus in the formation of the seed and fruit?

Answer 23

Undergoes mitosis to form endosperm tissue, an important food storage tissue in cereal grains

Question 24

What is the role of the ovule in the formation of the seed and fruit?

Answer 24

Becomes the seed

Question 25

What is the role of the ovary wall in the formation of the seed and fruit?

Answer 25

Develops into a fruit wall, enclosing the seeds

Question 26

What is the role of the integuments in the formation of the seed and fruit?

Answer 26

Develop into the testa

Question 27

What is the role of the micropyle in the formation of the seed and fruit?

Answer 27

Remains as a pore in the testa

Question 28

What is a cotyledon?

Answer 28

A seed leaf

Question 29

Maize is a _____________ so only has one ____________. The ______________ acts as a food store, therefore maize is ____________.

Answer 29

Monocotyledon, cotyledon, endosperm, endospermic

Question 30

Broad beans are ____________, they have two __________ with the embryo lying between them. Broad beans use the ____________ as food storage tissue. This means they do not have an _________ so are _________________.

Answer 30

Dicotyledons, cotyledons, cotyledons, endosperm, non-endospermic

Question 31

Name 4 seed adaptations for life on land

Answer 31

1. The testa is chemically resistant so seeds survive adverse chemical conditions, it also provides physical protection
2. Endosperm/cotyledons provide nutrients until photosynthesis can occur
3. Dormant seeds have a low metabolic rate so they can survive over winter in time for higher temperatures in spring/summer
4. Very low water content to survive dry conditions

Question 32

Why is seed dispersal important for plants?

Answer 32

Reduces intraspecific competition with the parent plant and other offspring plants
Colonise new areas

Question 33

Why are fruit important in seed dispersal?

Answer 33

Fruits are eaten by animals, the seeds pass through the animals digestive system and are then egested. This disperses the seeds.
Fruit may be adapted for a specific mode of dispersal eg hooked to attach to animals

Question 34

State and explain the 3 main requirements for germination

Answer 34

1. Oxygen - aerobic respiration releases ATP. Energy is required for metabolism and growth
2. Suitable temperature - optimum for the enzymes. Increases rate of diffusion
3. Water - transport medium for enzymes and softens the testa

Question 35

What do the first steps of germination involve?

Answer 35

• Water is imbibed (taken in) through the micropyle
• The tissues then swell, splitting or softening the testa, allowing entry of more oxygen
• Oxygen can then be used in aerobic respiration to produce ATP energy for metabolism

Question 36

Describe germination in non-endospermic seeds eg broad bean

Answer 36

• Mobilised enzymes hydrolyse large insoluble food stores
  Amylase: starch -> maltose
  Lipase: lipids -> fatty acids and glycerol
  Protease: proteins -> amino acids
• These soluble products are transported to the plumule and the radicle for use in respiration. Aerobic respiration can then produce ATP energy for mitosis and growth
• The plumule grows upwards and the radicle grows downwards
• The plumule emerges from the soil, the leaves unfurl and begin to photosynthesise. Plant is now independent of food stores in the seed

Question 37

Describe germination in endospermic seeds eg maize

Answer 37

• Following imbibition of water, gibberellin is released by the embryo and diffuses to the aleurone layer
• Gibberellins cause transcription and translation to occur, producing hydrolytic enzymes eg protease
• Protease digests proteins in the aleurone layer to form amino acids, these can be used in further enzyme synthesis eg amylase
• Amylase diffuses into the endosperm and hydrolyses the starch stored there into maltose. Proteins and fats in the endosperm are also hydrolysed into amino acids and fatty acids & glycerol
• Maltose, glucose and other nutrients diffuse to the embryo where they are used in aerobic respiration to provide ATP for the growth of the plumule and the radicle

Question 38

As maize seeds germinate, they produce the enzyme amylase which hydrolyses starch in the endosperm into maltose. Describe the method that can be used to demonstrate this in a laboratory

Answer 38

• Soak maize seeds in water
• Cut seeds in half and place onto starch agar
• Remove the seeds after 24 hours and add iodine solution to the starch agar
• Clear zones around the seeds indicate amylase activity
• Boiled then cooled seeds are set up as a control