Reproduction in plants-2

this has 85 flashcards,prettyyy long~!!

Structure of flowers

Calyx

The outermost layer which comprises the sepals.

Usually green

Protect the flower in the bud

Corolla

Ring of petals

Attract insects

Male part of the flower - stamen

A filament supporting an anther which produces pollen grains.

Filament

Contains vascular tissue, which transports sucrose, mineral ions and water to the developing pollen grains

Anther

Contains four pollen sacs arranged in two pairs.

When mature, the pollen sacs dehisce = open and release the pollen

Carpel

Female parts of the flower.

Ovary

Style

Stigma

Carpel

Pollination definition

The transfer of pollen grains from the anther to the mature stigma of a plant of the same species

Protandry definition

The stamens of a flower ripen before the stigmas

Self pollination

The pollen from the anthers of a flower is transferred to the mature stigma of the same flower/ a flower of the same plant

Genetic implications of self pollination

Depend only on independent assortment and crossing over during meiosis and mutation to bring about genetic variation in genomes of the gametes = display less genetic variation

Greater chance of 2 potentially harmful recessive alleles being brought together at fertilisation

Advantage of self pollination

It can preserve those successful genomes that are suited to a relatively stable environment

Cross-pollination

Pollen is transferred from the anthers of one flower to the mature stigma of a flower on another plant

Genetic implications of cross-pollination

Combines gametes from 2 individuals = more genetic variation

Reduces the chance of producing harmful allele combinations

In the struggle for survival, some genomes are more successful than others. It may allow a species to survive in a changing environment, as there are always likely to be some members of a population with a suitable combination of alleles

How is cross-pollination ensured?

1. Separate male + female flowers on the same plant

2. Separate male and female plants

3. Dichogamy - stamen + stigma ripen at different times

4. Anther below the stigma = pollen can't fall onto it

5. Genetic incompatibility

Protogyny

Stigma ripens first

Insect pollinated flowers

Colourful petals

Scent and nectar

Anthers within the flower

Stigma within the flower

Small quantities of a sticky pollen

Larger pollen grains

Wind pollinated flowers

Petals usually absent

No scent or nectar

Anthers hanging outside the flower

Large, feathery stigmas hang outside flower

Large quantities of smooth pollen

Small pollen grains

Wind pollinated flowers

Where are male gametes developed?

In pollen sacs of the anther

Process of male gamete formation

Diploid microspore mother cells undergo meiosis

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4 immature pollen grains in a tetrad = four haploid cells ( 0.5 number of chromosomes)

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In the pollen grain, the haploid nucleus undergoes mitosis to produce 2 nuclei = a generative nucleus and tube nucleus

The generative nucleus undergoes mitosis to produce 2 male nuclei = the male gametes

How does the pollen get out?

When the pollen is mature, the outer layers of the anther wall will dry out causing tension in the lateral grooves.

Dehiscence occurs

What is dehiscence?

The walls of the anther are pulled 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 wind or insects

Pollen grain diagram

Development of the female gamete

- The ovary contains one or more ovules

- In each ovule, a megaspore cell, surrounded by cells of the nucellus, undergoes meiosis making 4 haploid cells

- 3 disintegrate

- The remaining cell undergoes 3 rounds of mitosis, producing 8 haploid nuclei, one of which is the female gamete

- 2 of the haploid nuclei fuse to make a diploid nucleus called the polar nucleus

What does each embryo sac contain?

3 antipodals = haploid

2 synergids = haploid

1 oosphere = haploid

1 polar nucleus = diploid

Diagram of a mature ovule

Cross section of an anther

Parts of the anther

- Pollen sac

- Surrounding the pollen sac is the tapetum

-Lateral groove

- Vascular strand

Fertilisation definition

The fusion of a female and male gamete, producing a zygote.

Double fertilisation

1. When a compatible pollen grain lands on the stigma, it germinates in the sucrose solution secreted by the stigma and produces a pollen tube

2. Pollen tube nucleus = infront of 2 male nuclei

3. Pollen tube grows out of the pollen grain through a gap in the cell wall, called a pit, + down the style, up a gradient of chemoattractants

4. The pollen tube nucleus codes for the production of hydrolases, including cellulases and proteases, and it digests its way through the tissues of the style

5. The pollen tube grows through the gap in the integuments, the micropyle and passes into the embryo sac

6. The pollen tube nucleus disintergates having completed its function of controlling the growth of the pollen tube

7. The tip of the pollen tube opens, releasing the 2 male gametes into the embryo sac.

8. Male + female gametes are haploid. One of the male gametes fuses with the oosphere to form a zygote - diploid

9. Other male gamete fuses with the diploid polar nucleus to form a triploid nucleus = endosperm

Development of endosperm tissue

Endosperm divides repeatedly by mitosis, it generates the endosperm tissue, which takes over from the nucellus in providing nutrition for the developing embryo.

Fate of zygote

- Divides by mitosis to form the diploid embryo

What does the embryo consist of?

Plumule

Radicle

Cotyledon/s

Fate of triploid endosperm nucleus

- Divides by mitosis to form endosperm tissue

What happens to the outer integument?

Dries out

Hardens

Waterproof w/ deposits of lignin

Becomes the testa ( seed coat )

Micropyle remains as a pore in the seed

What happens to the ovule ?

Becomes the seed

What happens to the funicle?

Becomes the funicle of the seed. It attaches to the seed of the hilum

What happens to the ovary?

Becomes the fruity

- In a cherry: the wall becomes sweet, juicy and pigmented

- Almond - wall is dry and hard

Example of a dicotyledon

e.g. broad bean

Its seeds have 2 cotyledons. The embryo lies between them

Plumule = shoot

Radicle = root

Endosperm = absorbed into the cotyledons = a non-endospermic seed

Example of Monocotyledon

e.g. maize

Has only one cotyledon

Endosperm remains as the food store = endospermic

Cotyledon remains small and does not develop further

Testa fuses with the ovary wall = one-seeded fruit

Monocots

- One cotyledon in seed

-Leaf veins = parallel

- Sepals, petals and stamens in multiples of three

- Vascular bundles scattered in stems

- Vascular bundles in roots

Dicots

- 2 cotyledons in one seed

- Leaf veins form a network

- Sepals, petals, and stamens in multiples of 4 or 5

- Vascular bundles in a ring

- Vascular bundles in center of root

What is seed dispersal?

The movement of seeds away from the parent plant

Why is seed dispersal important?

- If a seed grows near its parent, the parent plant would be more successful at obtaining water and minerals from the soil

- This plant would be taller meaning it would cast shade over the seedling + prevent it from photosynthesizing properly

- Those dispersal methods have been subject to natural selection

Different methods of seed dispersal

Wind

Transport e.g. birds egest feces

Rolling

Bursting

Water

Carrying

Seed survival methods - 1

A low metabolic rate meaning they can survive very cold weather

Seed survival methods - 2

Testa is chemically resistant meaning seeds survive adverse chemical conditions

Seed survival method - 3

The water content of a dormant seed is reduced below 10% and so seeds can survive very dry conditions

Seed survival method - 4

The testa can physically protect the embryo

Seed survival method - 5

The endosperm for cotyledons provide a supply of nutrients which lasts until the emerging seedling can photosynthesize adequately

Seed survival method - 6

They can be dispersed great distances from the parent plant and so do not compete with it

Seed survival method - 7

Dispersal allows the colonisation of new habitats

Seed survival method - 8

Inhibitors may only allow germination at a suitable time of year. They are broken down in very cold weather, in a process called vernalisation so that the seed can germinate in spring

Cabbage = in seed

Tomato = in fruit

Why is a suitable temperature needed for germination?

The optimum temperature for germination is the optimum for the enzymes involved in the process.

Usually between 5C and 30C

Why is water needed for germination?

To mobilise enzymes for transport in the xylem and phloem, and to vacuolate cells, making them turgid

Why is oxygen needed for germination?

Aerobic respiration releases energy, which fuels metabolism and growth

How does light affect different seed species?

Some need light to germinate, others need darkness

How is water taken in to the seed?

Through the micropyle

What is the effect of water being taken up?

Causes the tissues to swell and provides suitable conditions for enzyme activity

Why must the food reserves be broken down?

Food reserves in seeds are insoluble in water and cannot be transported to the embryo

What does amylase do?

Hydrolyses starch into maltose

What do proteases do?

hydrolyse proteins to amino acids

What happens to soluble products?

Transported to the embryo and carried in the phloem to the apical meristems of the plumule and radicle

What happens to sugars?

Converted to cellulose for cell wall synthesis

What does aerobic respiration do?

Releases energy from sugars

What are amino acids used for?

Synthesise proteins

Emergence of the radicle

Swollen tissues rupture the testa

The radicle is positively geotrophic, meaning it grows downwards

Emergence of the plumule

Positively phototrophic meaning it grows upwards

What happens during germination? - step 1

1. The cotyledons of the broad bean remain below ground. The part of the plumule above the ground btwn the embryo and cotyledons elongates rapidly, pushing the plumule upwards

What happens during germination? - step 2

2. Plume = shape of a hook which protects it from soil abrasion

What happens during germination? - step 3

3. If a seed is planted at the correct depth in the soil, when the plumule emerges, the hook straightens and the leaves begin to photosynthesise

Food reserves in cotyledons are depleted

What industry are barley seeds used in?

The brewing industry to make beer

The effect of gibberellic acid step 1

The barley embryo secretes a plant growth regulator (GA), which diffuses through the endosperm to the aleurone layer

What is the aleurone layer?

The cells towards the outside of the seed which has a high protein content

Effect of gibberellin step 2

The GA switches on genes in the cells of the aleurone layer, resulting in transcription and translation, producing enzymes e.g. proteases, amylase

Step 3, what doe proteases do?

Hydrolyse protein in aleurone layer to amino acids, which are used to make amylase

Step 4, what does amylase do?

Diffuses out of the aleurone layer and hydrolyses the starch stored in the endosperm cells

Step 5, what do maltose and glucose do?

Diffuse back through the endosperm to the plumule and radicle of the embryo

Step 6, what are maltose and glucose required for?

Energy, which fuels biosynthesis and cell division and which brings the seed out of dormancy

How are gibberellins used?

The brewing industry to speed up germination so that barley seeds need to kept warm for less time and more malted barley can be produced

Germination in non endosperm seeds

1. Water imbibed through the micropyle

2. Cotyledons swell

3. testa splits - allows entry of more oxyegn for aerobic respiration

4. Food reserves from the cotyledons are mobilised through hydrolysis

Provides sources of energy for respiration and growth of the plumule and radicle

Germination and mass graph

Hilum plant