CHAPTER 16: REPRODUCTION IN PLANTS
Nuclear division
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Chromosome: a thread-like structure of DNA, made up of a string of genes
Gene: a length of DNA that is the unit of heredity and codes for a specific protein. A gene may be
copied and passed on to the next generation
Haploid nucleus: a nucleus containing a single set of unpaired chromosomes (e.g. sperm and egg)
Diploid nucleus: a nucleus containing two sets of chromosomes (e.g. in body cells)
In a diploid cell there is a pair of each type of chromosome and in a human diploid cell there are 23
pairs
Mitosis: a nuclear division giving rise to genetically identical cells in which the chromosome number
is maintained (details of stages are not required)
Importance of mitosis:
- Growth: in animals each tissue provides its own newcells when they are needed.
- Repair of damaged tissues: for example, when you cut your skin, mitosis provides new cells
to cover up cut.
- Replacement of worn out cell.
- Asexual reproduction: in plants
Meiosis: a reduction division in which the chromosome number is halved from diploid to haploid
resulting in genetically different cells (details of stages are not required)
Importance of meiosis:
- Meiosis is involved in the production of gametes
- Meiosis maintain the continuity of life.
- Meiosis results in genetic variation so the cells produced are not all genetically identical.
Stem cells: unspecialised cells that divide by mitosis to produce daughter cells that can become
specialised for specific functions
Cancers form as a result of uncontrolled cell division
Number of cells at beginning
Type of cell at beginning
Number of cells at end (daughter
cells)
Type of cell at end
Number of divisions
Identical or non-identical cells
Used for
Where it occurs
Mitosis
One
Diploid body cell (23 pairs of
chromosomes in humans)
Four
Meiosis
One
Diploid body cell (23 pairs of
chromosomes in humans)
Four
Diploid body cell (23 pairs of
chromosomes in humans)
One
Identical
Growth and repair
Everywhere except the sex
organs
heploid gamete (23
chromosomes in humans)
Two
Non-identical
Producing gametes
Sex organs (varies and testes in
mammals)
Reproduction:
Formation of young one of one’s own kind is an essential character of all living things. However, this is
needed for the survival of a specie.
Species:
A group of similar organisms which can reproduce freely in nature and produce fertile off-springs.
Types of reproduction:
1. Asexual reproduction
2. Sexual reproduction
Asexual reproduction
It is a process resulting in the production of genetically identical offspring from one parent
Sexual reproduction
It is the process involving the fusion of haploid nuclei (fertilisation) to form a diploid zygote and the
production of genetically different offspring
Asexual reproduction:
Natural vegetative propagation
● Involves the separation of a part from the parent plant
● The piece of the parent plant develops into new plant
Example
● Tubere.g potato
● Rhizome e.g Ginger
● Bulb e.g Onion
● Corme.g Water
Chestnut
Artificial vegetative propagation
● Most artificial methods to propagate plants are
based on the ability of plants to form
adventitious root
Example
● Cutting
● Layering
● Marcotting
● Budding
● Grafting
Asexual reproductioninPlants
● Single parent
● No sexcell
● No zygote
● Mitosis occurs
● Offspring genetically same as of its parents
● Fast
● Fertilization does not occur
Sexual reproduction
● Two parents
● Gametes formed
● Diploid zygote
● Meiosis occurs
● Offspring geneticallydifferent from his parents
● Slow
● Fertilization is must.
Advantages and disadvantages of asexual reproduction
Advantages
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No mate is needed.
No gametes are needed.
All the good characteristics of the parent are passed on to the offspring. This results in uniform
crops and stable varieties.
Where there is no dispersal (e.g. with potato tubers), offspring will grow in the same favourable
environment as the parent.
Plants that reproduce asexually usually store large amounts of food that allow rapid growth when
conditions are suitable.
Disadvantages
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There is little variation created, so adaptation to a changing environment (evolution) is unlikely.
If the parent has no resistance to a particular disease, none of the offspring will have resistance. This
could affect all of a crop.
Lack of dispersal (e.g. with potato tubers) can lead to competition for nutrients, water and light.
Advantages
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There is variation in the offspring, so adaptation to a changing or new environment is likely, enabling
survival of the species.
New varieties can be created, which may have resistance to disease.
In plants, seeds are produced, which allow dispersal away from the parent plant, reducing
competition.
Disadvantages
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Two parents are usually needed (though not always -some plants can self-pollinate).
Growth of a new plant to maturity from a seed is slow.
Advantages and disadvantages of sexual reproduction
Sexual reproduction
Partof flower
Pedicel
Structure
● Stalk of a flower
Function
● To support flower
Receptacle
(Thalamus)
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Swollen end of stalk
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Attachment for flower parts
Some become fleshy after fertilization e.g
apple
Calyx(Sepals)
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Outer most whorl
Green in colour
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Protect other part sof flower
Corolla (petals)
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Second whorl
Coloured
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Attract insects for pollination
Androecium
(stamen)
(i) Filament
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Third whorl
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Male part of a flower
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Stalk that holds & another
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Supports pollen sacs
(ii) Anther
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Contains two pollen sacs
which contains
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Release pollen grains
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Pollen grains (male gamete)
Forth whorl
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Female pollen of a flower
Gynoecium (pistil
/carpel)
(i) Stigma
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Swollen structure at the end
of style
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Receives part grains
Stimulate germination of Pollen grains
(ii) Style
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Stalk that connect the
stigmatotheovary
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Holdstigma inasuitableposition
(iii)
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Contains one or more ovules
which contains egg cell
(female gamete)
● Develops into fruit and ovule in to seeds
Ovary
Pollination
The transfer of pollen grains from the anther to the stigma
1. Self-pollination
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Transfer of pollen grains to the stigma of same flower or different flower in the same plant.
Factors that promote self-pollination:
(a)
Bisexual flowers with anthers and stigma maturing at the same time
(b) Stigma being located directly below the anthers, allowing pollen grains to fall onto it
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Advantages of self-pollination:
(a) Not dependent on external agents of pollination such as insects or wind
(b) Less wastage of pollen and energy. During wind and insect pollination, a great number of pollen
grains are lost as only a few pollen grains come into contact with a stigma of a flower of the
same species.
(c) Only one parent plant is required.
Disadvantage of self-pollination
(d) (a) is less genetic variation,
(e) (b) hence the offspring is less adapted to environmental changes.
2. Cross-pollination
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Transfer of pollen grains of one flower to a flower in another plant of the same kind
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Factors that promote cross-pollination are:
(a) Plants bearing only male or female flowers. These plants are called dioecious plants.
(b) In plants with bisexual flowers, the anthers and the stigmas mature at different times.
(c) Self-incompatibility – When a pollen grain of a flower happens to land on the stigma of the
same flower or another flower on the same plant, a biochemical block prevents the pollen
grain from germinating.
Advantages of cross-pollination:
(a) Greater genetic variation, hence the offspring has a higher chance of surviving environmental
changes.
(b) Offspring may have inherited beneficial qualities from both parents.
Disadvantages of cross-pollination:
(a)
(b)
(c)
(d)
Energy-consuming – lots of energy is required to make large amounts of pollen grains.
A great number of pollen grains are wasted due to the randomness of the dispersal methods.
External agents of pollination i.e. wind, insects are required.
Two parent plants are required.
Agents of pollination
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Wind
Insect
Water
Feature
Flower
Wind pollinated flower
● Usually small
● Dull coloured
● Scentless
Insect pollination flower
● Usually large
● Brightly colored
● Scented
Petals
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Small / absent
Dull coloured
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Large
Brightly colored
Nectar
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No nectar
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Produce nectar
Nectar
guides
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Absent
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Nectar guides present to attract and
guide insects
Stigma
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Large
Feathery
protrude outside
Long style
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Small
Compact / not feathery
Do not protrude out
Small style
Stamens
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Long pendulous filament
Anthers Protrude outside
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Not pendulous.
Do not protrude out.
Pollen
grains
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Abundant
Small
Dry
Light
Smooth but may have
Extension like "wing"
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Less in number
Large
Sticky
Heavy
Rough surface / spiky
Insect pollination flower
Example: Foxglove
Wind pollinated flower
Example: grass
Fertilization
It is the fusion of two different gamete i.e male and female gamete to form zygote.
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Pollen grain lands on stigma
Absorbs water and germinate
Form a pollen tube with pollen tube nucleus following generative nucleus
Penetrates stigma by secretion of some enzymes
Enters ovule through micropyle.
Generative nucleus divides to form two male gametes.
Tube nuclear degenerates
One male gamete fuses with the ovum to form zygote.
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Other male gamete fuses with the definitive nucleus to form endosperm nucleus.
Zygote will divide and develop into embryo.
Flower Parts
Ovary
Ovary wall
Ovule
Ovule wall
(integuments)
Ovule stalk
Endosperm nucleus
Zygote
Sepals
Style
Stamen
Stigma
Petals
Post fertilization changes
Fruit
Fruit wall or pericarp
Seed
Seed coat or testa or tegmen
Seed stalk (funicle)
Endosperm
Embryo
● Plumule
● Radicle
● Cotyledons
May persist/ enlarge
Wither and fall off
Dispersal of fruit and seeds
Importance
It is necessary in order to
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Avoid over crowding
Avoid competition for food
Avoid competition for light
Enable plants to colonise new and favourable habitats
Reduce the spread of diseases
Dispersal is carried out by the following methods
1.
2.
3.
4.
By wind
By animals
By water
By explosive mechanism
Dispersal
by
Wind
Animal
Features
Modeofaction
Example
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Small
Light
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Can float in the air
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Orchid seed
Dendlion
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Large
Flattened wing like
Parachute like
Projection
Fine hairs
Surface are a
enlarged
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Carried in the wind
Increases air resistance or buoyanay in
air
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Angsana
fruit
sycamore
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Sweet
Succulent
Brightly coloured
Scented
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Eaten by animals
Undigest seeds are passed out
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Tomato
Apple
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Hooked
Hairy fruits
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Adhere onto the fur or skin of animals
passing by
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Mimosa
Bur
Structure of a seed
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Structures
Seed Stalk
Hilum
Micropyle
Testa
Embryo has
a) Plumule
b) Radicle
c) Cotyledon
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Function
Attaches the seed to the pericarp
The scar left by the seed stalk
A small hole for entry of water
Thick protective seed coat
The first arise shoot
The first arise root
The seed leaves
Food storage tissue
Endosperm
Germination
When suitable environmental conditions are available, the seed will germinate
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Water enters the seed through the micropyle
Testa softens and splits
Enzymes produced by the cotyledons digest the food in endosperm.
Starch → maltose → glucose
Proteins → amino acid
Fats → fatty acids + glycerol
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Simple digested substances translocated to the radical and plumule
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The seeds shrinks and its dry mass decreases at first because of tissue respiration
First foliage leaves develop
Because of photosynthesis dry mass increases
The glucose is used in respiration to provide the energy for the process of growth.
The amino acids are used to build up proteins in the cytoplasm of the new cells.
The fatty acids and glycerol recombine to form fats which are used to make important components
of cell membranes. Fats also provide a considerable amount of energy.
Conditions for germination
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Sufficient water
Suitable temperature
Adequate oxygen supply
Investigating the condition needed for germination
Small seeds such as peas or mustard will grow on wet cotton wool in a test tube. Figure below shows
four tubes set up to investigate the conditions needed for the seeds to germinate.
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Tube A: seeds on wet cotton wool, maintained at 20°C (room temperature)
Tube B: seeds on dry cotton wool, maintained at 20°C
Tube C: seeds in boiled water, with a thin surface layer of oil; maintained at 20°C
The water in tube C has been boiled to remove dissolved oxygen. The layer of oil (e.g. cooking oil)
keeps out oxygen from the air.
Tube D: seeds on wet cotton wool, placed in a refrigerator at 4°C
After a few days the seeds in the control tube (A) will start to germinate.
There will be no germination in tubes B or C. The seeds in tube D may eventually start to germinate, but
much more slowly than in tube A.
Reproduction in Mammals
Sex Hormones
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At puberty, the pituitary gland starts to stimulate the primary sex organs; the testes in males
and the ovaries in females.
Sex hormones – testosterone in males and oestrogen in females are released into the
bloodstream.
They only affect the target organs which have receptors which can recognize them.
Causes secondary sexual characteristics such as the growth of pubic hair and maturation of
sexual organs.
Male reproductive system
The male nuclei involved in the process of human sexual reproduction are located within male gametes
(sex cells) called sperms. The male reproductive system is designed to manufacture sperms and to
deliver them to the place where one of them will be able to fuse with a female nucleus.
Reproductive
organ
Testis
Structure
Function
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Paired
Oval in shape
Suspended in scrotum
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Produce sperms
Produce testosterone
Epididymis
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Coiled tube
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Store sperms temporarily
Scrotum
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Sac formed by the
Skin toenclose testes
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Protect the testes
Sperm duct
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Connects the epididymis to the urethra
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Transport sperm to urethra
Urethra
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Tube that runs through the penis
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Carries both urine and semen at
different times, through the penis
Penis
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An organ of spongy erectile tissue
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Deposits semen into the vagina of
female
Reproductive
glands
Prostate gland
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Seminal vesicle
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Largest among three
Single gland located at the part where
the urethra and two sperm ducts join
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A pair of glands that open directly into
the sperm ducts
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Produces thin white alkaline fluid
containing many enzymes
Helps to activate sperms
Produces thick and clear fluid
containing mucus, amino acids
and
Fructose provides energy
Cowper’s gland
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Open directly into the urethra
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to sperms
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Produces clear viscous fluid
Carries sperms released before
ejaculation.
Sperms along with the nutritional fluid from glands is called semen.
The ejection of semen out of the body is called ejaculation.
During one ejaculation, about 500 million sperms are secreted.
Female reproductive system
The female nuclei which are involved in the process of sexual reproduction are located in the female
gametes called OVA (singular: ‘ovum’). The functions of the female reproductive system are:
(i)
(ii)
To produce ova and ensure that they are fertilised by the male gametes
To protect and nourish the embryo until it is born.
Reproductive organ
Ovary
Structure
● Paired
● Oval in shape
● Attached dorsal wall of abdomen
Function
● Releases ovum every 28 days
● Produces oestrogen and
progesterone
Oviduct (fallopian
tube)
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A narrow tube with a funnel shaped
opening close to ovary
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Uterus
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Hollow
Muscular pear shaped organ
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Transfer ova from ovary into
uterus
Site of fertilization
Protects and nourishes a
developing baby
Cervix
Vagina
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Richly supplied with blood
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Helps keep the baby in the
uterus until it is born
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A ring of muscles at the lower end of
the uterus
Opens into the vagina
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A tube with thinner walls.
The site where sperms are deposited
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Serves as an opening for the
entry of penis
Serves as birth canal
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Comparison of male and female gametes
Characteristics
Chromosome number
Size
Ovum(egg)
23
● Large (0.1) mm
Sperm
23
● Smaller ( 0.6mm)
Shape
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Spherical
Inner plasma membrane
Outer protein coat
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Head with nucleus
Mitochondria in middle
Tail at the end
Mobility
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Immotile
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Motile
Nucleus
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Much larger
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Smaller
Cytoplasm
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A lot of cytoplasm
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Very little
Reserved food
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Cytoplasm as food reserves
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No reserves food
Released
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One at a time about 28 days ●
In millions at each ejaculation
Type of chromosome
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Nucleus contains Only X
chromosome always
Nucleus contains either X or a Y
chromosome
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Release of gametes
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Internal
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Expelled
Gamete Size
Mobility
Number
Sperm
Smaller Very mobile – use its tail
Many more (300,000,000
Egg
Larger Immobile – moved by peristalsis Fewer and limited
Menstrual cycle
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In female occurs after puberty till menopause
Discharge lasts for about 3-5 days
Cycle repeats after 28 days
Regulate by hormones
1. Follicle stimulating hormone (FSH) is secreted by the pituitary gland. It causes an egg to mature
in an ovary and it stimulates ovaries to release hormone oestrogen
2. Oestrogen is secreted by the ovaries. It stops FSH being produced - so that only one egg matures
in a cycle and it stimulates the pituitary gland to release hormone LH.
3. Luteinizing hormone (LH): is also secreted by pituitary gland and causes mature egg to be
released from ovary.
4. Progesterone is a hormone secreted by ovaries. It maintains the lining of the uterus during the
middle part of the menstrual cycle and during pregnancy.
Phases
Menstruation (days 1 – 5)
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Corpus luteum disintegrates
Progesterone secretion reduces
FSH and LH stimulate follicles to ripen
Uterus lining disintegrates
Menstrual flow through vigina which contains
o Dead egg cell
o Blood
o Mucus
o Water
o Disintegrated lining
Pre-ovulation phase (days 6-13)
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Graffin follicle
develops
Other follicles
disintegrate
Increase in LH causes
follicle to mature fully
Uterus lining becomes
thick and vascularised
by oestrogen
Ovulation (days 14)
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Graffin follicle burst
Egg is released from
its follicle from ovary
LH causes ovulation
LH also causes the
formation of corpus
Luteum
Corpus luteum secrets progesterone
Post-ovulation (day 14-28)
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Uterus lining continues to thickness
Egg dies if not fertilized
Corpus luteum begins to shrink
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Decrease in level of estrogen and progesterone
Fertilization
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Is the fusion of sperm with the egg to form a zygote.
Occurs during sexual intercourse
Only one sperm fertilizes the egg and form zygote.
Zygote divide mitotically and form embryo
Implantation
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Embryo moves to the uterus
Embryo implants in uterus lining in 6 days after fertilization
The embryo starts to absorb nutrients from mother
Placenta is formed three months after pregnancy.
Gestation period
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The period between fertilization and birth is called the gestation period
Takes 9 months in humans
Placenta
Is the attachment of fetus to the mother uterus lining by numerous finger like villi.
Functions of placenta
Material
Useful Substances
i.
Glucose
ii.
Amino acid
iii. Vitamins
iv. Minerals
v. Water
vi.
Oxygen
Waste substances
i.
Urea
Function
● Pass from the placentain to the embryo
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Pass from embryo back to mother’s blood.
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Stops reaching the embryo but some viruses can pass
across the placenta and affect the fetus
ii.
Toxins
iii.
Chemicals
Antibodies
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Diffuse from mother’s blood into embryo
Hormones progesterone
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Maintains uterine lining in healthy state.
ii.
CO2
Harmful substances
i.
Pathogens
Two blood systems
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Allows two blood system to function at different blood
pressure since the two blood types do not mix.
In the placenta, the blood capillaries of the feats are separated from the mother’s blood system by
only a thin layer of tissue.
Fetal blood system is not continuous with the mother’s blood system because.
o The blood pressure system of mother would kill the fetus as it is much higherthan of fetus
o Blood group of the fetus may not be the same as the mother, so avoiding agglutination
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Umbilical Cord
“Attaches the fetus to the placenta”
It contains:
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Umbilical arteries
Transports material from the fetus to the
placenta e.g
i. Deoxygenated blood
ii. Metabolic waste products
Urea
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Umbilical vein
Transplants material from the placenta to the
fetus
e.g i. oxygenated blood ii. food Substances
glucose
amino acid
vitamins
water
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minerals
Amniotic Sac (amnion)
“A sac-like membrane around the embryo filled with a watery fluid called amniotic fluid .
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It supports and cushions the fetus
Shock absorber
Protects against physical injury
Allow fetus to move freely.
During birth, it lubricates and reduces friction in vigina.