Sexual reproduction in plants — Combined Science 0653 - Biology Revision Notes & Topical Questions

Section 1 of 34Sexual reproduction

Sexual reproduction

Sexual reproduction fusion of the nuclei of two gametes to form a zygote, giving offspring that are genetically different from each other.

Plant gametes

• Plant gametes the male gamete is the pollen nucleus and the female gamete is in the ovule.

Haploid gametes

• Haploid gametes they have half the chromosome number of a body cell.

Cross-pollination

• Cross-pollination it increases genetic variation.

Section 2 of 34Parts of an insect-pollinated flower

Structure	Function
Sepal	Protects the unopened flower
Petal	Brightly coloured to attract insects
Stamen: anther	Produces and releases pollen
Stamen: filament	Supports the anther
Carpel: stigma	Sticky top of the female part that collects pollen
Carpel: style	Tube connecting the stigma and the ovary
Carpel: ovary	Contains the ovules
Ovule	Contains the female gamete

Section 3 of 34Insect-pollinated and wind-pollinated flowers

Feature	Insect-pollinated	Wind-pollinated
Petals	Large and brightly coloured	Small and dull, often green or brown
Scent and nectar	Produced to attract insects	Not produced
Anthers	Stiff filaments, inside the flower, brush against insects	Long filaments, hang outside the flower
Stigma	Sticky, inside the flower	Feathery, outside the flower
Pollen	Large, heavy, spiky or hooked grains	Small, light, smooth grains

Section 4 of 34Pollination and fertilisation

Pollination

Pollination transfer of pollen grains from an anther to a stigma.

Fertilisation

Fertilisation fusion of a pollen nucleus with a nucleus in an ovule.

Difference

• Difference pollination is the transfer of pollen, fertilisation is the fusion of gametes.

Section 5 of 34Pollen tube, seed and fruit

1

Pollen lands on the stigma.

2

A pollen tube grows from the pollen grain down the style to the ovary.

3

The pollen nucleus travels down the tube into the ovule.

4

The pollen nucleus fuses with the nucleus of the female gamete.

5

The ovule develops into a seed and the ovary develops into a fruit.

• Nucleus
  • it is the pollen nucleus, not the whole pollen grain, that travels down the tube.

Section 6 of 34Self-pollination and cross-pollination

Cross-pollination

Cross-pollination transfer of pollen to the stigma of another plant of the same species.

• It improves genetic variation.
• It depends on pollinators, so a fall in bee numbers is a problem for food crops.

Self-pollination

Self-pollination pollen lands on its own stigma or another flower on the same plant.

• It reduces genetic variety because all gametes come from one parent.
• Lack of variation is a disadvantage if the environment changes.

Section 7 of 34Conditions needed for germination

Germination

Germination the start of growth in a seed.

Water

• Water it makes the seed swell and lets enzymes in the embryo start working.

Oxygen

• Oxygen it is needed so that energy can be released by respiration.

Warmth

• Warmth a suitable temperature gives the best rate of enzyme-controlled reactions.

Carbon dioxide

• Carbon dioxide it is not required and does not inhibit germination.

Section 8 of 34Investigating germination

1

Set up four boiling tubes, each with 10 cress seeds on cotton wool.

2

Give tube A no water, tube B all conditions as the control, tube C no oxygen and tube D a cold temperature (about 4 °C in a fridge).

3

Keep A, B and C at about 20 °C.

4

Leave for several days and count germinated seeds in each tube.

Result

• Result only the control tube germinates, so water, oxygen and warmth are all required.

Fair test

• Fair test change one factor only and use the same number of seeds in each tube.

Section 9 of 34More from the specification

More from the specification

Floral gametogenesis and vascular system

• Floral gametogenesis and vascular system pollen mother cells undergo meiotic division inside anthers supported by xylem tissues of the floral system.

Asexual reproduction

• One parent gives genetically identical offspring

Pituitary gland

• Master gland controlling other glands

Uterus

• Organ where the embryo develops

Placenta

• Organ exchanging substances between mother and foetus

Foetus

• Developing baby once organs are in place

Hormone

• A chemical messenger carried in the blood.

Oestrogen

• The main female sex hormone.

Progesterone

• Hormone that maintains the uterus lining

Section 10 of 34Wind-pollinated and insect-pollinated flowers

1Wind-pollinated

• Wind-pollinated anthers and stigmas hang outside the flower so wind can blow pollen away and catch it on other stigmas.

2Insect-pollinated

• Insect-pollinated a sticky stigma inside the flower catches pollen when insects brush past.

Feature	Insect-pollinated flower	Wind-pollinated flower
Petals	Large, brightly coloured, scented, with nectar	Small and dull, often green or brown; no scent or nectar
Pollen grains	Large, heavy, sticky, spiky or hooked so they cling to insects	Small, smooth and light, made in large amounts
Anthers	Stiff filaments inside the flower so they brush against insects	Long filaments hanging outside the flower
Stigma	Sticky, inside the flower	Feathery, hanging outside to catch pollen

Section 11 of 34Pollen grain adaptations

Pollen grain adaptations

Large pollen grains

• Large pollen grains they are too heavy for the wind, and spikes let them catch onto the bodies of insects.

Small smooth grains

• Small smooth grains they are light enough to be carried by the wind.

Wing-like structures

• Wing-like structures they catch the wind easily.

Section 12 of 34Pollination and fertilisation

Pollination

Pollination transfer of pollen from an anther to a stigma.

Fertilisation

Fertilisation fusion of a pollen nucleus with the female gamete in the ovule.

Section 13 of 34Pollen tube growth

1

The pollen grain grows a pollen tube down the style.

2

The pollen nucleus travels down the tube to the ovule.

3

It fuses with the female gamete inside the ovule.

4

The ovule becomes a seed and the ovary a fruit.

• Nucleus
  • it is the nucleus that travels, not the pollen grain itself.

Section 14 of 34Self-pollination and cross-pollination

Cross-pollination

Cross-pollination pollen is transferred to another plant of the same species.

• It improves genetic variation.
• It relies on pollinators, so a fall in bee numbers matters because bees pollinate many food crops.
• This does not apply to wind-pollinated plants.

Self-pollination

Self-pollination pollen lands on its own stigma or another flower on the same plant.

• It reduces genetic variety because all the gametes come from one parent.

Section 15 of 34Germination investigation

1

Set up four boiling tubes, each with 10 cress seeds on cotton wool.

2

Vary one condition per tube: no water, no oxygen, a cold temperature, and a control with all conditions.

3

Leave the tubes for the same time.

4

Count the germinated seeds in each tube.

Water absent

• Water absent no seeds germinate, so water is needed.

Cold tube

• Cold tube slow or no germination, so warmth is needed.

Fair test

• Fair test change one factor, keep the rest constant and use 10 seeds to reduce the effect of chance.

Section 16 of 34More from the specification

1

Gametophyte development in flowering plants

• Gametophyte development in flowering plants specialised haploid pollen cells develop inside anther structures within the floral shoot system.

2

Oestrogen

• The main female sex hormone.

3

Progesterone

• Hormone that maintains the uterus lining

4

Asexual reproduction

• One parent gives genetically identical offspring

5

Pituitary gland

• Master gland controlling other glands

6

Reproduction

• Producing offspring of the same kind.

7

Uterus

• Organ where the embryo develops

8

Placenta

• Organ exchanging substances between mother and foetus

9

Foetus

• Developing baby once organs are in place

10

Hormone

• A chemical messenger carried in the blood.

11

Offspring

• Young produced by reproduction.

Section 17 of 34Parts of an insect-pollinated flower

Part	Function
Petals	Large and brightly coloured to attract insects
Sepals	Protect the flower in bud
Anther (on a filament)	Makes pollen grains containing the male gametes
Stigma	Sticky surface that receives pollen
Style	Tube connecting the stigma and the ovary
Ovary	Contains the ovules
Ovule	Contains the female gamete and becomes the seed
Nectary	Makes nectar to attract insects

Section 18 of 34Male and female parts

Male and female parts

Stamen

(male) — the anther and filament.

Carpel

(female) — the stigma, style and ovary.

Pollen grain

• Pollen grain it carries the male gamete nucleus.

Section 19 of 34From flower to fruit

1

A pollen tube grows down the style after pollination.

2

The pollen nucleus fuses with the female gamete in the ovule (fertilisation).

3

The fertilised ovule develops into a seed containing the embryo plant and a food store.

4

The ovary wall becomes the fruit.

5

The petals, stamens and stigma usually wither.

• Fruits
  • they protect the seeds and help dispersal, for example a bean pod, a tomato and an apple.

Section 20 of 34Seed dispersal

Purpose

• Purpose seeds are moved away from the parent so they do not compete with it for light, water and minerals.

Wind

• Wind winged or feathery seeds.

Animals

• Animals hooks catch on fur, or fleshy fruits are eaten and the seeds excreted.

Water and explosive pods

• Water and explosive pods other methods of dispersal.

Section 21 of 34Germination conditions

Water

• Water it is absorbed and activates enzymes.

Oxygen

• Oxygen it is needed for aerobic respiration to release energy.

Suitable temperature

• Suitable temperature it gives the best conditions for enzymes.

Light

• Light it is not needed by most seeds.

Section 22 of 34Germination investigation

Set-up

• Set-up cress seeds on cotton wool in boiling tubes, with one factor removed in each tube.

Result

• Result compare how many seeds germinate in each tube.

Section 23 of 34Wind-pollinated and insect-pollinated flowers compared

Feature	Insect-pollinated	Wind-pollinated
Petals	Large, colourful, scented	Small or absent
Pollen	Sticky or spiky, relatively little	Light and smooth, large amounts
Stigma	Sticky, inside the flower	Feathery, hanging out
Anthers	Inside the flower	Hang outside

Section 24 of 34More from the specification

1

Plant reproductive organ system

• Plant reproductive organ system angiosperm stamen and carpel organs form an integrated organ system housing generative gamete cells.

2

Floral anatomy and pollination mechanisms

• Flowers contain male stamens and female carpels (stigma, style, ovary); pollen transfer occurs via wind or insect pollinators.
  • (anthers producing pollen)

3

Double fertilisation in angiosperms

• One pollen sperm nucleus fuses with the female egg cell to form a diploid zygote,
• while a second sperm nucleus fuses with polar nuclei to form triploid endosperm.

4

Seed and fruit development

• Following fertilisation, the ovule develops into a protective seed containing the plant embryo,
• while the surrounding ovary wall matures into a fruit.

5

Plant sexual reproduction

• Plant sexual reproduction involves meiosis producing haploid male pollen grains and female ovules that fuse during fertilisation.

6

Mammalian uterus lining

• Mammalian uterus lining menstrual cycle hormones thicken the uterus lining to prepare for blastocyst implantation and placenta development.

7

Placental nutrient exchange

• Placental nutrient exchange maternal and foetal blood remain separated while dissolved nutrients and oxygen diffuse to the growing foetus.

8

Variation

• Differences between individuals of the same species.

9

Oestrogen

• The main female sex hormone.

10

Progesterone

• Hormone that maintains the uterus lining

11

Genetic

• Related to genes and DNA.

12

Asexual reproduction

• One parent gives genetically identical offspring

13

Pituitary gland

• Master gland controlling other glands

14

Reproduction

• Producing offspring of the same kind.

15

Uterus

• Organ where the embryo develops

16

Placenta

• Organ exchanging substances between mother and foetus

17

Foetus

• Developing baby once organs are in place

18

Hormone

• A chemical messenger carried in the blood.

Section 25 of 34Sexual and asexual reproduction compared

Feature	Sexual	Asexual
Parents	Two (or one in self-pollination)	One
Gametes	Yes, fertilisation	No
Offspring	Genetically different	Genetically identical (clones)
Variation	Yes	None

Section 26 of 34Chromosome numbers

Haploid gametes

• Haploid gametes they contain half the number of chromosomes of a diploid body cell.

Zygote

• Zygote fusion of gametes restores the full diploid number.

Section 27 of 34Asexual reproduction in plants

Definition

• Definition it involves no sex cells or fertilisation and gives genetically identical offspring from one parent.

Bulbs and tubers

• Bulbs and tubers underground food stores that grow into next year's plants.

Runners

• Runners side shoots, as in strawberries, whose plantlets root and become separate plants.

Cuttings

• Cuttings growers use them to make identical plants.

Tissue culture

• Tissue culture tiny pieces of tissue grow into many identical plantlets in sterile conditions with nutrients and hormones.

Grafting

• Grafting a shoot of one plant is joined to the root of another to combine good features.

Section 28 of 34Advantages and disadvantages

Type	Advantages	Disadvantages
Sexual	Variation allows adaptation to changing conditions; seeds can be dispersed widely	Needs pollination; slower; offspring may lose desirable features
Asexual	Fast; needs one parent; no pollinator needed; keeps desirable features	No variation, so a disease or climate change can wipe out a crop

Section 29 of 34Asexual reproduction of crops

Advantages

• Advantages desired features such as high yield, disease resistance and drought resistance are kept, products are uniform for sale, production is faster and cheaper than buying seeds.

Disadvantages

• Disadvantages crops cannot adapt to climate change, a diseased parent gives diseased offspring and the process needs human management.

Section 30 of 34Sexual reproduction of crops

Advantage

• Advantage variation may produce a variant that copes better with weather or gives a higher yield.

Disadvantage

• Disadvantage variation may give offspring less successful than the parent.

Seed banks

• Seed banks plant breeders keep wild relatives as a source of genes for resistance.

Section 31 of 34Examples of clones and variation

Potato tubers

• Potato tubers plants are clones of the parent, so all have the same susceptibility to blight.

Wheat from seed

• Wheat from seed the field has variation, so some plants may survive a new disease.

Strawberry

• Strawberry runners spread quickly while seeds maintain variation.

Tissue culture

• Tissue culture it gives disease-free stock of orchids and bananas.

Section 32 of 34More from the specification

1

Clonal propagation without reproductive system

• Clonal propagation without reproductive system vegetative propagation through stolons or rhizomes produces genetically identical offspring without requiring a complex sexual reproductive system.

2

Pollen Adaptations in Plant Reproduction

• Insect-pollinated flowers produce large, spiky pollen grains to adhere to pollinators,
• whereas wind-pollinated flowers produce light, smooth pollen grains.

3

Floral Structure and Pollination

• The male anthers produce pollen while the female carpel receives pollen on its stigma, allowing fertilisation within the flower ovary.

4

Mammalian Fertilisation and Menstrual Cycle

• In human sexual reproduction, swimming sperm cells fertilise the female egg cell,
• which embeds in the lining of the uterus during the reproductive cycle.

5

Placental Exchange for the Foetus

• The placenta develops inside the uterus to facilitate the exchange of oxygen, glucose,
• and metabolic wastes between maternal blood and the developing foetus.

6

Pollen Adaptations in Plant Reproduction

• Insect-pollinated flowers produce large, spiky pollen grains to adhere to pollinators,
• whereas wind-pollinated flowers produce light, smooth pollen grains.

7

Floral Structure and Pollination

• The male anthers produce pollen while the female carpel receives pollen on its stigma, allowing fertilisation within the flower ovary.

8

Mammalian Fertilisation and Menstrual Cycle

• In human sexual reproduction, swimming sperm cells fertilise the female egg cell,
• which embeds in the lining of the uterus during the reproductive cycle.

9

Placental Exchange for the Foetus

• The placenta develops inside the uterus to facilitate the exchange of oxygen, glucose,
• and metabolic wastes between maternal blood and the developing foetus.

10

Nucleus

• Controls the activities of the cell and contains the genetic material.

11

Germination

• Start of growth in a seed

12

Oestrogen

• The main female sex hormone.

13

Progesterone

• Hormone that maintains the uterus lining

14

Pituitary gland

• Master gland controlling other glands

15

Uterus

• Organ where the embryo develops

16

Placenta

• Organ exchanging substances between mother and foetus

17

Foetus

• Developing baby once organs are in place

Section 33 of 34Key terms

Term	Meaning
Pollination	Transfer of pollen from an anther to a stigma
Fertilisation	Fusion of a pollen nucleus with a nucleus in an ovule
Anther	Part of the stamen that produces pollen
Stigma	Sticky top of the carpel that receives pollen
Ovule	Structure in the ovary containing the female gamete; becomes the seed
Germination	Start of growth in a seed

Term	Meaning
Pollination	Transfer of pollen from an anther to a stigma
Fertilisation	Fusion of a pollen nucleus with the female gamete
Cross-pollination	Pollen transfer between different plants of the same species
Self-pollination	Pollen transfer within the same plant
Pollen tube	Tube growing from a pollen grain down the style
Germination	Start of growth in a seed

Term	Meaning
Anther	Makes pollen
Stigma	Receives pollen
Ovary	Contains ovules; becomes the fruit
Ovule	Contains the female gamete; becomes the seed
Nectary	Makes nectar to attract insects
Seed dispersal	Movement of seeds away from the parent plant

Term	Meaning
Asexual reproduction	One parent gives genetically identical offspring
Clone	Genetically identical organism
Runner	Side shoot that produces plantlets
Tissue culture	Growing many plants from small pieces of tissue
Haploid	Having half the normal chromosome number
Diploid	Having the full chromosome number

Section 34 of 34Exam technique

1

Confusing pollination with fertilisation — pollination is transfer of pollen, fertilisation is fusion of gametes.

2

Saying the pollen grain travels down the pollen tube — the nucleus does.

3

Saying wind-pollinated flowers have brightly coloured petals and nectar — they are small and dull.

4

Saying seeds need light or carbon dioxide to germinate — they need water, oxygen and warmth.

1

Confusing pollination with fertilisation.

2

Saying the pollen grain itself fuses with the ovule — the pollen nucleus does.

3

Saying wind-pollinated flowers have large bright petals and nectar.

4

Saying self-pollination increases variation — it reduces it.

1

Confusing pollination with fertilisation.

2

Saying the ovary becomes the seed — the ovule becomes the seed and the ovary becomes the fruit.

3

Mixing up stamen (male) and carpel (female).

4

Saying germinating seeds need light — most need water, oxygen and warmth only.

1

Saying asexual offspring are all different — they are identical clones.

2

Saying sexual reproduction needs one parent — it normally needs two (self-pollination is the exception).

3

Giving an advantage without linking it to variation or speed.

4

Saying asexual reproduction involves gametes — it does not.