Eduqas A Level Biology: Component 2: Sexual Reproduction in Plants

What are the male parts of the plant called?

Stamen

What are the female parts of the plant called?

Carpel

What is the name for flowering plants?

Angiosperms

What are the names for the sepals and for the petals?

Sepals: calyx

Petals: corolla

What are sepals?

- first set of modified leaves

-outermost layer of the flower that protect the ovaries

State what comprises the stamen and their individual functions

1. Anther: male gamete which produces pollen

2. Filament: thin and contains vascular tissue to provide anther with nutrients

State the structures that make up the carpel and their functions.

Stigma: trap pollen

Style: hold the stigma to come into contact with insects

Ovule: female gamete

Ovary: protects ovule

What is the name of the part of the stalk where the flower is attached?

Receptacle

Describe the petals of insect and wind pollinated plants and why they are different

Insect: bright petals that produce scent and nectar for attraction

Wind; dull, scentless petals since they rely on wind

Describe the stigma of insect and wind pollinated plants and why they are different.

Insect: sticky to trap pollen and secrete chemicals to stimulate pollen tube growth

Wind; large and feathery so they have a large s.a to catch pollen grains in the wind

adaptations for insect Pollinated Flowers

Adaptations for Insect Pollination

- Large, brightly coloured flowers

- Nectar

- Scent

(These 3 to attract insects)

- Stamen and Carpel within the

petals

- Large sticky pollen (small

quantities produced)

Adaptations for Wind Pollinated Flowers

Petals are absent.

No scent is produced.

Nectaries are absent.

The anthers and stigma hang outside the flower (so that pollen can be shed easily into the air and stigma is usually feathery to increase the surface area.)

Pollen is usually small and light. (very large quantities of pollen are produced to increase the chance of pollen landing on a stigma)

pollen production

- pollen mother cells produced by mitosis of plant tissue

- meiosis 1 occurs and 2 haploid cells are formed

- meiosis 2 - produces 4 haploid cells- microspores- all genetically different

- a cell wall forms around each nucleus to form a tetrad of microspores

- these microspores then separate to form the individual pollen grains

1. Each microspore develops a thick protective wall-the intine and exine (outer sculptured layer with

adaptations for pollen dispersal)

2. Mitosis of the haploid nucleus produces 2 haploid nuclei-genetically identical -the tube nucleus and the

generative nucleus.

3. During fertilisation-the generative nucleus divides a second time to produce the two male gametes.

How is the pollen produced by insect pollinated plants different to that of wind pollinated?

Wind pollinated produce a large quantity of pollen since theres a low probability of success. Small and lightweight to be carried by the wind.

Insect pollinated makes a small quantity of pollen with barbs and hooks to attach to insect

Why are the anthers of an insect pollinated plant inside the flower unlike the wind pollinated?

Inside the flower makes it more likely to brush against insects whereas wind pollinated just needs the pollen to be blown away

State the structures found inside the anther

Lateral groove, filament, pollen sac, epidermis

Outline the structure of a pollen sac

Fibrous layer, tapetum and pollen mother cells

What provides the nutrients to a pollen sac?

Tapetum

Outline the steps to form pollen grains

Diploid pollen mother cells undergo meiosis to form a tetrad of pollen which seperates into pollen grains

What occurs inside a pollen grain before it is ready to be germinated?

The haploid undergoes mitosis to form a pollen tube nucleus and a generative nucleus. the GN becomes 2 male nuclei

How is the exine of the pollen grain adapted for protection?

Resists desiccation

Tough

chemically resistant

resistant to UV radiation

Why is it important that pollen grains are resistant to UV radiation?

at high altitudes, theyre exposed more but UV can cause DNA mutation

Dehiscence

Dehiscence- when the anthers are mature the pollen sac bursts open to

release the mature pollen

Outline the process of dehiscence

1. Outer layers dry out causing tension in lateral grooves

2. Tensions pulls the walls of the anther apart

3. Edges of pollen sacs curl away

4. An opening exposes the pollen grains for insects or the wind

What is the opening for the pollen in dehiscence called?

Stomium

Outline the steps for the formation of the ovule (megaspore)

1. The original megaspore mother cells were produced by mitosis in the ovary.

2. These cells undergo undergo meisosis I and II to produce four haploid cells.

3. 3 degenerate.

4. ONE megaspore continues to develop.

5. The haploid nucleus undergoes 3 mitotic divisions to form a total of 8 nuclei that are all genetically identical.

6. Cell membranes form around six of the nuclei to form two synergids, 1 female gamete and 3 antipodal cells while the remaining two remain as free polar nuclei.

Define pollination

The transfer of pollen grains from the anther to the stigma

self pollination advantages and disadvantages

Advantages

- Pollination is rapid with a high chance of success- therefore increasing the chance of seed production and survival of the species.

- The genes all come from the same plant and so if the plant is well-adapted to living in a particular environment

/ habitat, good genomes are preserved. If there is little change in the environment, keeping the same adaptations increases the chance of species survival.

disadvantage

Reduced genetic variation. All the plants are more or less equally adapted to the same environmental conditions

and are equally susceptible to the same disease - if there is significant environmental change, they cannot adapt quickly to the changes and few individuals will survive.

Adaptations of flowers to promote cross pollination

1. Chemical self-incompatibility

The chemical signals produced by the stigma do not stimulate pollen from the same flower / plant.

2. flower structure- The anther is below the stigma so that pollen cannot fall

onto it.

3. Development of anthers / stigmas

In many plant flowers, either the anthers mature before the stigmas or vice versa. This means that pollen and stigmas

4. Separate male and female flowers

What causes variation in self pollinated plants?

Mutation in DNA replication during Interphase, independent assortment and crossing over in meiosis

Define self pollination

Self-pollination occurs when a plant's pollen lands on the stigma of the same flower.

What are the advantages of self pollination?

Beneficial characteristics are likely to be passed on i.e farming

Preserves good genomes which is suited to a stable environment

receptacle

the top of the flower stalk from which the different parts of the flower develop

What are the disadvantages of self pollination?

Lacking genetic diversity

Interbreeding results in a greater chance of 2 recessibe alleles being combined at fertilisation

Define cross pollination

Cross-pollination occurs when a stigma

receives pollen from a different plant of the same species.

Why is it beneficial for genetic variability to be introduced in cross pollination?

The best adapted due to the variability will survive intraspecific competition

cross pollination advantages and disadvantages

Advantages:

- The offspring have increased genetic variation -variation is introduced during Prophase I and Metaphase I and II in both parents and through random fertilisation.

A wide range of genomes are produced and this increases the chance of species survival if there is significant environmental change.

disadvantage:

Good combinations of alleles can be lost due to crossing over and random fertilisation.

What are the sources of genetic variation in cross pollination?

mutations, crossing over, independent assortment, random fusion of haploid nuclei during fertilisation

What is dichogamy and what is its purpose?

the stamen and stigma ripen at different times

to stop self pollination

Name and differntiate the two types of dichogamy

1. Protandry: stamen ripens first so the stigma cant germinate the pollen

2. Protogyny; the stigma ripens first but no pollen is being produced

Apart from dichogamy, describe other ways to stop self pollination

1. Anther is below the stigma so the pollen cant fall onto the stigma with gravity

2. Genetic incompatibility so the pollen cant germinate on the stigma of the same plant

3. Seperate male and female parts of the plant

Outline the steps of fertilisation briefly

1. a compatible pollen grain lands on the stigma which releases a sugary solution

2. pollen grain germinates and pollen tube nucleus forms a pollen tube

3. The pollen tube goes down the style into the embryo sac through the micropyle

4. The pollen tube nucleus disintegrates and the two male nuclei are released into the sac

What is the micropyle?

A groove in the embryo sac which allows the entry of the pollen tube

What is the role of the integuments?

Physical protection

How does the pollen tube go from the stigma to the embryo sac?

The pollen tube nucleus produces hydrolytic enzymes which digests the style tissue on the path. it absorbs the products. The path is guided by chemioattractants secreted by the egg cell

Describe the process of double fertilisation

1. Pollen germinates and the tube nucleus directs the growth of the pollen tube towards the ovule.

2. The pollen tube contains the generative nucleus and the tube nucleus-which is at the front directing the growth

of the pollen tube.

3. The generative nucleus divides by mitosis the form 2 genetically identical male gametes.

4. The pollen tube grows into the embryo sac through the micropyle, the tube nucleus disintegrates and the 2

male gametes enter the embryo sac

Double Fertilisation

5. One male gamete (n) fuses with the female gamete (n) to form the diploid zygote.

6. One male gamete (n) fuses with the 2 polar nuclei to form the primary Triploid endosperm nucleus.

after fertilisation

ZYGOTE (2n)

This divides by mitosis to form the EMBRYO which contains:

the embryo root - the radicle

the embryo shoot - the plumule.

In dicotyledonous seeds, there are also two seed leaves formed - the COTYLEDONS that store food for the embryo.

PRIMARY ENDOSPERM NUCLEUS (3n)

This also divides by mitosis and in some seeds (monocotyledonous) forms the food storage tissue.

INTEGUMENTS

These fuse together and form the TESTA or seed coat.

OVARY WALL

This forms the fruit wall and encloses the seeds.

The fruit is often juicy or sculpted to aid in seed dispersal.

OVULES

These form the SEEDS.

Seeds-broad bean (Dicotyledon)

2 seed leaves (cotyledons)

Act as food stores

Derived from the embryo

maize (monocotyledon)

1 cotyledon

Main food store is the

endosperm-derived from the

primary endosperm nucleus

Wheat, barley, maize, grasses

List some similarities between fertilisation in plants and humans

1. internal fertilisation in humans

2. chemotrophism of male gamete towards female gametes

3. fusion of haploid gametes to form a zygote

4. secretion of digestive enzymes to let male gamete reach the female gamete

List some differences between fertilisation in plants and humans

1. double fertilisation in plants

2. triploid nucleus formed in plants

3. human male gametes can swim and dont need a tube

4. acrosome reaction in humans, pollen tube in plants

What is the endosperm tissue?

food storage tissue in plants

What structures are found in the diploid embryo?

plumule, radicle, cotyledons

What forms the fruit and what forms the seed?

Fruit; fertilised ovary

seed; ovule

What is the function of the fruit and how is it adapted for this?

Protection

To distribute the seed so its sweet smelling and tasty for animals to eat and discard the seeds

What does the integuments become, and how is it adapted for its function?

Testa; chemically resistant for its role

What is a cotyledon?

A seed leaf

Differentiate a monocotyledon and a dicotyledon

Di; nonendospermic, cotyledon absorbs endosperm and photosynthesizes itself to obtain nutrients

Mono; endospermic as it remains as a food source, one seed as the testa fuses with ovary wall

What role does the endosperm or the cotyledon play in maintaining a terrestrial life for seeds?

Supplies nutrienrs until true leaves develop and photosynthesise

How are dormant seeds adapted to terrestrial life?

low metabolic rate so they can survive the cold and low water content

Why is seed dispersal beneficial?

Spreads the plants over a large area so it reduces intra specific competition and colonises new areas

Describe 3 methods of seed dispersal

1. water; seeds are able to float e.g. coconut.

2 wind: seeds are lightweight e.g. dandelion

3. animals; spikes that attach to fur or feathers

germination requirements

Water-to mobilise enzymes, transport materials in the seed and to

increase in size through the formation of vacuoles-to split the testa

Oxygen-for aerobic respiration

Suitable temperature-for metabolic reactions to occur

When does germination occur?

environmental factors are favourable;

enough oxygen for aerobic respiration, optimum temperature for enzymes and water for the mobilisation of enzymes

Germination in a broad bean

- Water is imbibed through the

micropyle

- the testa is split allowing oxygen in

for aerobic respiration

- Food reserves in the cotyledons

(starch/protein) are mobilised

through hydrolysis

- The soluble products are used for

respiration and the ATP produced

used for cell division and growth of

the plumule and radicle.

Outline the process of germination

1. Water is imbibed through the micropyle

2. tissues swell so the testa splits and oxygen enters for aerobic respiration

3. food reserves from cotyledons are hydrolysed to soluble forms

4. the soluble products are transported to the growing points of the embryo as energy sources for growth and respiration

5. the plumule grows upwards with a hooked tip to avoud soil erosion, and leaves unfurl

6. radicle grows downwards into roots

What are the food reserves that are hydrolysed in germination and what do they become?

starch- maltose

maltose- alpha glucose

a glucose- b glucose

proteins- amino acids

In terms of phototrophism, describe the change in plumule and radicle in germination

plumule; positive

radicle; negative

in terms of geotrophism, describe the change in plumule and radicle in germination

plumule; negative

radicle; positive

What is the name of the hormone for germination control?

Gibberellins

Explain how gibberellins works.

1. after imbibition, G is released by the embryo to the aleurone layer

2. G stimulates the aleurone cells to produce hydrolytic enzymes

3. proteins in the aleurone layer are hydrolysed into amino acids

4. enzyme synthesis from these amino acids occurs to make amylase

5. amylase diffuses into endosperm and hydrolyses starch

6. these diffuse to embryo for aerobic respiration and growth

How does gibberelline stimulate the production of proteases,

By triggering transcription and translation in aleurone cells

The role of gibberellin in germination of endospermic seeds (maize)

1. Water is absorbed through the microphyle

2. Gibberellins in the cotyledon are mobilised and carried into the outer layer of the endosperm which is called aleurone

3. This causes dna to be transcribed into mRNA which is then translated into hydrolytic enzymes

4. The amino acids used to synthesise the proteins are stored in proteins in the aleurone layer and are released through the hydrolysis of these proteins

5. Amylase and maltase are produced using these amino acids. The hydrolyse the starch in the endosperm to maltose and then glucose

6. The glucose diffuses to the embryo where the radicle and plumule use the glucose for respiration to provide the energy/atp needed for growth