4.2 Sexual Reproduction (plants)

Question 1

Define pollination

Answer 1

• transfer of pollen from male anther to female stigma

Question 2

Differentiate between self pollination and cross pollination

Answer 2

Self: pollen from same plant being pollinated
Cross: pollen from a different plant to the one being pollinated
—> requires a vector ie birds, bees, wind

Question 3

Structure of insect pollinated flowers

Answer 3

• colourful petals with nectar guides
• scent and nectar
• anthers within flower
• stigma within flower
• small quantities of sticky, sculptured pollen
• larger pollen grains

Question 4

Structure of wind pollinated flowers

Answer 4

• petals absent or small
• no scent or nectar
• anthers hang outside flower
• long, feathery stigmas hang outside flower
• large quantities of smooth pollen
• smaller pollen grains

Question 5

Describe the calyx

Answer 5

• comprises sepals
• protect flower in the bud
  —> can be petal-like ie in lillies

Question 6

Describe the Corolla

Answer 6

• ring of petals
• can be small or absent
• sometimes a nectary at the base which attracts pollinators

Question 7

Describe the stamens

Answer 7

• male gamete
• filament supporting an anther which produces pollen grains
• filament contains vascular tissue which supports sucrose, mineral ions and water
• anther contains 4 pollen sacs

Question 8

Describe the carpels

Answer 8

• female gamete
• closed structure in which one or more ovules develop
• receptive surface at top called stigma

Question 9

Define dehiscence

Answer 9

• the opening of the anther, releasing pollen grains

Question 10

Define the tapetum

Answer 10

• a layer of cells around the pollen sac that provide nutrients and regulatory molecules to developing grains
• plays significant role in formation of the pollen cell wall which is tough and resistant to chemicals

Question 11

Describe the development of the male gamete (anther)

Answer 11

• diploid pollen mother cells undergo meiosis
• each forms a tetrad containing four haploid cells which form four pollen grains
• inside each pollen grain, the haploid nucleus undergoes mitosis to produce two nuclei - generative nucleus and tube nucleus
  —> generative nucleus produces 2 male nuclei by mitosis

Question 12

Draw and label a mature pollen grain

Question 13

Describe the process of dehiscence

Answer 13

• when the pollen is mature the outer layers of the anther dry out, causing tension in lateral grooves
• dehiscence occurs in which tension pulls the walls of the anther apart and the edges of the pollen sacs curl away
• opening called the stomium exposes pollen grains and they are carried away by insects/wind

Question 14

Describe the development of the female gamete (ovules)

Answer 14

• ovary contains one or more ovules
• in each ovule, a megaspore mother cells undergoes meiosis making four haploid cells
• three disintegrate
• remaining cell undergoes 3 rounds of mitosis to make 8 haploid nuclei, one of which is the female gamete (egg cell nucleus / oosphere)
• two of the haploid nuclei fuse to make a diploid nucleus called the polar nucleus

Question 15

What is the nucellus?

Answer 15

A layer of cells which provide nutrients
—> has 2 layers around it called the integuments
—> gap in the integuments called micropyle

Question 16

Explain the genetic implications of self pollination

Answer 16

• leads to self-fertilisation which results in inbreeding
• self fertilised display less genetic variation than cross-fertilised species
• greater chance of two potentially harmful recessive alleles
• advantage of interbreeding is that it can preserve those successful genomes that are suited to a relatively stable environment

Question 17

Explain the genetic implications of cross pollination

Answer 17

• results in outbreeding
• creates more genetic variation
• reduces chances of producing harmful allele combinations
• evolutionary significance - allow species to survive in changing environment

Question 18

Define protandry

Answer 18

Where the stagments of a flower ripen before the stigmas

Question 19

Describe ways in which plants are adapted to ensure cross-pollination

Answer 19

• dichogamy - stamen and stigma ripening at different times
• anther below the stigma so pollen cannot fall onto it
• genetic incompatibility - where pollen cannot germinate on the stigma of the flower that produced it
• separate male and female flowers on the same plant
• separate male and female plants

Question 20

What are the nuclei at the top and bottom of the embryo sac called?

Answer 20

Top = antipodals
Bottom = synergids

Question 21

What does an embryo sac in a mature ovule contain?

Answer 21

• 3 antipodals
• 2 synergids
• 1 oosphere
• 1 polar nucleus

Question 22

Describe the process of double fertilisation

Answer 22

• compatible pollen grain lands on the stigma, it germinates in the sucrose solution secreted by the stigma and produces a pollen tube
• the pollen tube nucleus is at the tip of the tube with two male nuclei behind
• pollen tube grows out through a gab in the cell wall called a pit, up a gradient of chemoattractants. Pollen tube nucleus codes for the production of hydrolases and digests its way through the tissues of the style. The products are used by the growing pollen tubes
• pollen tube grows through the micropyle and passes into embryo sac
• pollen tube nucleus disintegrates
• tip of pollen tube opens, releasing two male gamete’s into the embryo sac
• one male gamete fuses with oosphere to form a zygote and the other fuses with the polar nucleus to form a triploid nucleus known as an endosperm nucleus
• endosperm nucleus divides repeatedly by mitosis and generates endosperm tissue which provides nutrition to the growing embryo

Question 23

Describe the process of double fertilisation

Answer 23

• compatible pollen grain lands on the stigma, it germinates in the sucrose solution secreted by the stigma and produces a pollen tube
• the pollen tube nucleus is at the tip of the tube with two male nuclei behind
• pollen tube grows out through a gab in the cell wall called a pit, up a gradient of chemoattractants. Pollen tube nucleus codes for the production of hydrolases and digests its way through the tissues of the style. The products are used by the growing pollen tubes
• pollen tube grows through the micropyle and passes into embryo sac
• pollen tube nucleus disintegrates
• tip of pollen tube opens, releasing two male gamete’s into the embryo sac
• one male gamete fuses with oosphere to form a zygote and the other fuses with the polar nucleus to form a triploid nucleus known as an endosperm nucleus
• endosperm nucleus divides repeatedly by mitosis and generates endosperm tissue which provides nutrition to the growing embryo

Question 24

Describe pollen germination and pollen tube growth

Answer 24

• if pollen grain and stigma are chemically compatible, the pollen tube nucleus controls the germination of the pollen grain and the growth of a pollen tube
• involves digestion of a path between the cells of the style and ovary wall using hydrolase enzymes

Question 25

What happens to the endosperm nucleus, integuments, ovary wall, and ovules after fertilisation?

Answer 25

Endosperm nucleus: divides by mitosis and becomes food storage tissue Integuments: fuse together and form the testa Ovary wall: forms fruit wall which encloses the seeds Ovules: form seeds

Question 26

Define a fruit

Answer 26

A structure developing from the ovary wall containing one or more seeds

Question 27

Define a seed

Answer 27

Structure developed from a fertilised ovule, containing an embryo and food store enclosed within a testa

Question 28

What is meant by dormant in terms of seeds

Answer 28

- a seed when its active growth is suspended —> water content falls, reducing their metabolic rate

Question 29

Describe the structure of monocots

Answer 29

- one cotyledon in seed - leaf veins are parallel - sepals, petals and stamens in multiples of 3 - vascular bundles scattered in roots and stems

Question 30

Describe the structure of dicots

Answer 30

- two cotyledons in seed - leaf veins form a network - sepals, petals and stamens in multiples of 4 or 5 - vascular bundles in a ring in stems - vascular bundles in centre of roots

Question 31

What is meant by seed dispersal?

Answer 31

The movement of seeds away from the parent plant

Question 32

Why is it important that seeds are dispersed from the parent?

Answer 32

To prevent competition in which the seedling gets outcompeted

Question 33

Describe wind dispersion

Answer 33

- plants have sails or parachutes of stiff hairs - pores containing seeds are shaken when wind blows and seeds are released

Question 34

Describe transport dispersion

Answer 34

- birds, mammals, reptiles and fish eat seeds and then release them as faeces - process called scarification where digestive enzymes weaken the testa by physical attack, enabling species to germinate

Question 35

Describe rolling dispersion

Answer 35

- release of a conker (the seed) - falls to ground and rolls away from parent tree

Question 36

Describe bursting dispersion

Answer 36

- legume pods dry and split, scattering the seed - some pods rotate as they split to send seeds in different directjins

Question 37

Describe water dispersion

Answer 37

- seeds fall into water and are carried away - able to float due to air cavities that make them buoyant

Question 38

Describe carrying dispersion

Answer 38

- seeds attach to animals coats and are flown away

Question 39

Describe adaptations of seeds for survival

Answer 39

- dormant seeds have low metabolic rate so they can survive very cold weather - the testa is chemically resistant - reduced water content of dormant seeds enables them to survive dry conditions - testa can physically protect the embryo - endosperm or cotyledons provide a supply of nutrients which lasts until the seedling can photosynthesise efficiently - seeds dispersed from parent plant to reduce competition - dispersal allows colonisation of new habitats - inhibitors only allow germination to occur at certain times of year —> broken down in vernalisation

Question 40

Define germination

Answer 40

The biochemical and physiological processes by which a seed becomes a photosynthesising plant

Question 41

What are the 3 main requirements for germination?

Answer 41

- suitable temp - optimum for seed is whatever is optimum for enzymes involved - water - mobilise enzymes, transport in xylem and phloem, make cells turgid - oxygen - aerobic respiration time release energy to fuel metabolism and growth

Question 42

Describe the process of germination after the mobilisation of food reserves

Answer 42

- swollen tissues rupture the testa and the radicle emerges from the seed —> positively geotrophic and negatively phototropic - plumule emerges —> negatively geotrophic and positively phototrophic - cotyledon remains underground whilst the plumule elongates rapidly with its tip bent over to prevent soil abrasion - once emerged it unfurls and the leaves start to photosynthesise

Question 43

Germination of non-endospermic

Answer 43

- water enter micropyle - water enter seed and swells cotyledons, rupturing testa - split testa allows more O2 for aerobic respiration - starch and proteins stored in cotyledons mobilised through hydrolysis - starch and protein used as sources of energy for respiration and growth of radicle and plumule

Question 44

Germination of endospermic seed

Answer 44

- water enters seed - embryo releases gibberellin - gibberellin diffuses to protein rich aleurone layer and amylase enzymes are made to break down stored starch in endosperm - glucose diffuses to the embryo and used for aerobic respiration and growth