4.1 GAMETE
FORMATION
4.1.1
THE NECESSITY FOR ORGANISMS TO REPRODUCE
1. All organisms carry out reproduction to:
(a) ensure the
continuity of life
(b) conserve the
continuity of its own species
4.1.2
TYPE OF REPRODUCTION
1. There are two types of
reproduction:
(a) Asexual
reproduction:
-
involves only one parent
-
does not involve gamete
(b) Sexual reproduction:
- involves two
parents from the same species but with different sexes.
- both parents
produce gametes for fertilisation
4.1.3 THE
NECESSITY FOR FORMATION OF GAMATES
1. In human,
§ Male gamete = sperms, reproduced in
testes (male gonads / reproductive organs)
§ Female gamete = ova / ovum, reproduced
in ovaries (female gonads / reproductive organs)
2. Formation of gametes is
important to life because:
(a) It increases the
number of offspring through fertilization
(b) It results in
genetic variation in the offspring
(c)
It maintains the diploid number of zygote (2n) from one generation to another
by fusing one male gamete (n) with one female gamete (n).
(d)
To ensure the survival and continuity of organisms
4.1.4 THE
HUMAN MALE REPRODUCTIVE SYSTEM
1. The human male reproductive system
consists of two testes connected by sperm duct to a penis.
2. Each testis has about a thousand of
seminiferous tubules.
3. All the tubules in a testis are joined
to a single, much-colied tube called the epididymis.
4. The epididymis connects to a sperm
duct called the vas deferens (sperm duct).
5. The two sperm ducts are joined to the
urethra.
6. The urethra leads through the penis to
the exterior.
Sperm
Formation in Humans
1.
Spermatogenesis:
- Spermatogenesis is the formation of sperm
- it occurs in the sminiferous tubules
2. Spermatogenesis in male:
(a) Germinal epithelial cells divide many
times through mitosis to form spermatogonia.
(b)
Spermatogonia undergo growth to form primary spermatocytes.
(c)
Primary spermatocytes divide by first meiotic division to produce haploid
secondary spermocytes.
(d)
Secondary spermocytes undergo a second meiotic division to form spermatids.
(e)
The spermatids develop tails and mature into sperms (spermatozoa)
4.1.6 THE
HUMAN FEMALE REPRODUCTIVE SYSTEM
1. The human female reproductive system
consists of:
§ Two ovaries
§ Two fallopian tubes
§ Uterus
§ Cervix
§ Vagina
§ Vulva (external genital organs)
2. Ovaries produce:
§ Female gametes (ova)
§ Female sex hormones (oestrogen and
progesterone)
Ovum Formation in Humans
1. Oogenesis:
- Oogenesis is the formation of ovum
- It starts in the ovaries of the feotus
before birth
2. Oogenesis in female:
(a) Germinal epithelial cells divide
many times through mitosis to form oogonia.
(b) Oogonia undergo growth to form
primary oocytes.
(c) Primary oocytes are surrounded by a
layer of follicle cells and are called primary follicles. These are formed
before a baby is formed, remain dormant until puberty.
(d) Every month from puberty onwards, one
oocyte undergoes first meiotic division to become a secondary oocyte and a polar
body. Both are haploid..
(e) The secondary oocyte undergoes the second
meiotic division until metaphase II. The secondary oocyte which is lined by
layers of follicle cells is called a secondary follicle.
(f)
The secondary follicle increase in size to form Graafian follicle. At
intervals of approximately 28 days in women, the Graafian follicle merges with
the wall of the ovary, the ovary wall ruptures, the secondary oocyte is
released into the oviduct. This process is known as ovulation.
(g)
When fertilization occurs, the secondary oocyte enters the Fallopian
tube and complete the second meiotic division to form an ovum (n) and a polar body (n).
(h) The haploid polar body formed
during meiosis I also undergoes meriosis II to form another two haploid polar
body.
(i)
All three polar body will eventually degenerate.
4.2 THE ROLE OF HORMONES IN THE MENSTRUAL CYCLE
1. Menstrual cycle = The monthly cycle of changes that occur in
the female body which is associated with the ovulation and the discharge of blood.
2. Menstruation is the discharge of
excess blood and tissues of the endometrium when the uterine lining breaks down
during the menstrual cycle. It normally lasts for about 4 to 5 days.
3. Puberty is the period which marks the
beginning of maturity in both males and females. During this period, there are
various physical, emotional and physiological changes that are associated with
the development of the reproductive system.
4. In female, puberty is characterized by
the onset of menstruation.
5. Menstrual cycle can be divided into three stages:
(a) Menstruation stage (menstruation
occurs, 1st day to 5th day)
(b) Follicle development stage (from the
end of the menstruation to ovulation, 6th day to 14th
day)
(c) Corpus luteum stage (from the
ovulation to the beginning of next menstruation, 15th day to 28th
day)
1. Menstruation stage (1st day
to 5th day):
(a) Progesterone level is at the lowest
(b) The endometrium of uterus breaks down
and sloughs off. The discarded tissue of endometrium together with blood is
discharged. Menstruation begins.
(c) During menstruation, the pituitary
gland is stimulated to secrete FSH
(Follicle-stimulating Hormone)
(d) When FSH reaches the ovary, it will
stimulate the growth of one follicle.
2. Folicle development stage (6th
day to 14th day)
(a) Primary follicle in the ovary develops
into a Graafian follicle.
(b) The development of the follicle in the
ovary stimulates the secretion of oestrogen
hormone.
(c) Ostrogen repairs the uterine wall and
causes the endometrium to thicken in preparation form implantation of zygote.
(d) When the level of oestrogen increases
and reaches a particular level, it stimulates the pituitary gland to secrete LH (luteinising hormone)
(e) The level of LH is at the peak on the
day before ovulation, causing ovalution on the 14th day. A secondary
oocyte is released from the Graafian follicle to the fallopian tube and lastly
to the uterus.
3. Corpus luteum stage (15th
day to 28th day)
(a) After ovulation, the Graafian follicle
that remains in the ovary will form a corpus luteum .
(b) Corpus luteum secretes progesterone.
(c) Progesterone thickens the endometrium
and stimulates the development of blood vessels for implantation of zygote.
(d) If fertilization occurs, the embryonic
tissue will release human chorionic gonadotropin (hCG) to stimulate the corpus
luteum continues to secrete progesterone that will maintain the thickness of
the endometrium.
(e) Progesterone also inhibits the
secretion of FSH and LH, thus preventing further development of follicle and
ovulation during pregnancy.
(f) If fertilization does not occur, the
corpus luteum will degenerate. The progesterone level and oestrogen level will
decrease. The endometrium of the uterus begins to break down and slough off,
resulting menstruation.
4. Effects of hormonal imbalance in the
menstrual cycle on woman:
(a) Premenstrual syndrome (PMS)
- PMS is a combination of physical and
emotional symptoms that are related to the menstrual cycle.
- It occurs in the week before
menstruation
- Symptoms:
(i)
Emotional
and mental symptoms:
§ Irritability
§ Tension
§ Depression
§ low self-esteem
§ lack of concentration
(ii)
Physical
symptoms:
§ Headaches
§ Fatigue
§ Feeling bloated
§ Breast tenderness
§ Abdominal pain
§ Sleep disturbances
§ Appetite change
(b) Miscarriage
- Cause: Failure in the production of
progesterone, causing the thickness of the endometrium to be reduced
- Result: The embryo cannot be embedded
securely in the endometrium and miscarriage occurs.
(c) Menopause
- Cause: The ovary becomes inactive and
stop producing ovum, causing less FSH and LH to be produced. Thus, the ovary
produces less progesterone and oestrogen. Ovulation and the menstrual cycle
become irregular and finally stop.
- Symptoms:
§ Hot flushes
§ Night sweats
§ Sleeping disorder
§ Osteoporosis
§ Mood changes
§ Weight gain
§ Hair loss
- Treatment: Hormone Replacement Therapy
(HRT)
Zygote is formed as a result of
fertilization between an ovum and a sperm.
4.3.1
Fertilisation
a. During sexual intercourse, about 500
million sperms will be ejaculated into the vagina in the form of semen.
b. Through the vagina, the sperms move into
the uterus and towards the Fallopian tube.
c. When the sperms reach the Fallopian
tube, there are only a few hundred of sperms left.
Of all the sperms that managed to move towards the
secondary oocyte, only one sperm will fertilise the secondary oocyte.
a. In the Fallopian tube, the first sperm
that reaches the secondary oocyte will penetrate into the egg membrane with its
head.
b. Upon entry of the sperm, the secondary
oocyte is stimulated to undergo meiosis II. An ovum and a polar body are
formed.
c. The nucleus of the sperm then fuses
with the nucleus of ovum to form a zygote. This process is called
fertilization.
d. The middle piece and the tail of the
sperm will be left behind outside the ovum.
e. The newly fertilized ovum (zygote)
will form a fertilization membrane around itself so that other sperms will not
be able to penetrate it.
4.3.2 Development of zygote
b. The zygote takes about 3 to 4 days to
reach the uterus.
c. As the zygote moves down the Fallopian
tube, it divides repeatedly through mitosis to form a blastocyst.
§ The zygote undergoes mitosis to form a
zygote with two cells.
§ Both cells then divide to form a
zygote with four cells
§ Subsequently, mitosis occurs to form a
zygote with eight cells, sixteen cells, and finally a zygote with a few hundred
cells.
§ When the zygote reaches the uterus, it
has become a solid ball with hundreds of cells, called morula.
§ Eventually, the mass of hundreds of cells
changes into a fluid-filled sphere called blastocyst. The blastocyst consists
of an outer layer of cells and an inner cell mass.
§ The outer layer of cells will later
develop into a placenta and the inner cell mass will develop to form an embryo.
b. About seven days after fertilization,
the blastocyst is fully formed and attached itself to the endometrium of uterus
and is embedded in it. This process is called implanation.
4.3.3
Implantation of Blastocyst
a. During implantation, the outer layer
of blastocyst attaches to the endometrium using its extended projections called
trophoblast viili.
b. The villi secrete enzymes to dissolve
the cells in the uterine wall, forming a cavity that allows the blastocyst to
be embedded into the wall.
c. The villi with rich supply of blood
capillaries extend into the endometrium to implant the blastocyst.
d. In the endometrium, the inner cell
mass in the blastocyst will develop to form the embryo.
e. The villi from the embryonic tissues
and the cells of the uterine wall will form the placenta.
f. The embryo will develop to form a
foetus within two months
g. The foetus will then develop
throughout the whole term and is then born as a baby.
4.3.4
The Formation of Twins
1. Formation of identical twins
- One sperm fertilized one ovum to form
one zygote.
- The zygote divides into two cells to
form two separate embryos.
- Two embryo develop into two foetuses.
- Each foetus has its own umbilical cord
but shares the same placenta.
- Both twins are at the same sex.
- They have same genetic content and
look alike.
2. Formation of fraternal twins
- Two separate sperms fertilized two ova
that released at the same time to form two separate zygotes.
- The two zygotes develop to form two
separate embryos.
- Two embryos develop into two foetuses.
- Each foetus has its own umbilical cord
and individual placenta.
- Both twins are at the same sex or
different.
- They have different genetic content
and appearance.
3. Formation of siamese twins
- One sperm fertilized one ovum to form
one zygote.
- The zygote divides into two cells but
incompletely.
- The two embryos formed are stilled
joined to each other at certain parts.
- The two foetuses share one placenta.
Comparison between identical twins and
fraternal twins
|
SIMILARITIES
|
|
1.
Both
involve the fertilization of sperm with ovum.
2.
Both
involve mitosis in the development of zygote.
|
|
DIFFERENCES
|
|
|
One ovum is fertilized by one sperm.
|
Two ova are fertilized by two
separate sperms.
|
|
The zygote divide after
fertilization
|
The zygote does not divide after
fertilization
|
|
Both foetus share one placenta
|
Each foetus has its own placenta
|
|
Both twins are of the same sex.
|
Both twins may or may not be the
same sex.
|
|
Both twins look alike
|
Both twins have some similar and
some different characteristics as present among siblings.
|
4.3.5 The Structure And The Role Of Placenta
1.
Placenta
= an organ by which the embryo is attached to the uterus wall.
2.
It
is made of:
(a) embryonic tissues (formed by the
chorionic villi)
(b) maternal tissues (formed by the
uterine lining)
3.
During
implantation, trophoblast villi from the blastocyst are extended into the
uterine wall to form placenta. Each villus has a network of blood capillaries
that are separated from the mother.
4.
The
placenta joined to foetus by umbilical cord.
- The umbilical cord has two blood vessels
to transport the substances between the placenta and the foetus
(a) Umbilical vein: transport oxygen and
nutrients that diffuse from the mother’s blood into the placenta to the foetal
blood.
(b) Umbilical artery: transport carbon
dioxide and nitrogenous waste from foetal blood out of the placenta to the
mother’s blood.
5.
The
function of placenta:
(a) Forms a selective barrier between the
mother’s blood and the foetal’s blood.
(b) Allows exchange of substances between
the mother’s blood and the foetal’s blood.
(c) Allows the embryo of foetus to attach
to the uterine wall.
(d) Transports antibodies from the mother
to the foetus.
(e) Secretes hormone progesterone and
oestrogen to maintain the thickness of endometrium so that the embryo is firmly
attached to the uterine wall in the uterus.
6.
Adaptation
of placenta for its function:
(a) Numeous villi in placenta:
- to increase the surface area for
diffusion.
(b) Thin membrane which separate the
foetal’s blood capillaries from the mother’s blood capillaries
- to facilitate diffusion
(c) Both the uterine wall and the placenta
are supplied with a network of blood capillaries
- to increase the chances of diffusion.
4.4 APPRECIATING THE CONTRIBUTION OF
SCIENCE AND TECHNOLOGY TO HUMAN REPRODUCTION
4.4.1
Birth control techniques
There
are 5 ways to prevent pregnancy:
(1)
Natural methods:
(i) The calendar technique / Rhythm method
- The couple should not have sexual
intercourse on the days when the female is fertile.
- This method assumes that ovulation
takes place 14 days before menstruation, so sexual intercourse should not take
place 3 days before and 3 days after ovulation.
(ii) The temperature technique
- This method is based on the fact that
a woman’s body temperature drops a bit before ovulation and becomes higher
after ovulation.
- The body temperature needs to be
recorded everyday at the same time with an ovulation thermometer.
- It is safe to have sexual intercourse
after the body temperature has increased continuously after 3 days.
(iii) The mucous technique
- This method is used to determine the fertile
period by observing and recording the quantity and thickness of mucus that is
discharged from the cervix during the menstrual cycle.
- After menstruation, the mucus is dry
and thick.
- Nearing ovulation, the mucus becomes
thinner, elastic and flow easily.
- A few days after ovulation, the mucus
becomes thick again.
- The sexual intercourse should be
avoided as soon as watery mucus is seen until 4 days after the last day of
watery mucus.
(iv) The withdrawal technique
- In this method, the penis is removed
from the vagina during sexual intercourse before the sperms are ejaculated.
(2)
Physical methods
(i) The condom
- A rubber device that is used on penis
to prevent sperms from entering the uterus during ejaculation.
(ii) The diaphragm
- A rubber device worn by a woman at the
cervix to prevent sperms from entering fallopian tube.
(iii) The intrauterine device (IUD)
- A device that is made from plastic or
metal and is shaped in the form of a T, twisted or rolled.
- The IUD is placed inside the uterus of
the woman by doctor.
- The IUD can prevent the embryo from
attaching itself to the wall of uterus.
(3) Chemical methods
(i) The contraceptive pile
- Contraceptive pills contain hormone
oestrogen and progesterone to prevent formation of follicle and ovulation.
- A contraceptive pill is taken every
day for 21 days. The intake is stopped for seven days. During this period,
menstruation occurs.
- After menstruation, the intake of the
pill is resumed.
(ii) Spermicide
- Spermicides are used to kill sperms
that are ejaculated into the vagina.
- Spermicides can be in the form of
jelly, cream or spray and are applied in the vagina before sexual intercourse.
(4) Sterilisation methods
(i) Tubal ligation
- In woman, both fallopian tubes are
blocked or cut so that the sperm can no longer reach the ova.
(ii) Vasectomy
- The sperm ducts are cut and tied so
that the sperms do not enter the penis.
- Sperms are stilled being produced by
testes but are unable to reach the urethra. They are disintegrate and are
eventually reabsorbed. Seminal fluid is still produced but does not contain
sperms.
(5)
Abortion
-
The elimination of the embryo of the foetus before the 24th week.
4.4.2
Overcoming infertility
1. Causes of sterility problem
(a)
In Women:
(i)
No ovulation due to hormone imbalances.
(ii)
No implantation due to damages or abnormality in the uterus.
(iii) Blacked fallopian tubes
(b)
In men:
(i)
Low sperm count
(ii)
Blocked sperm duct
(iii) Impotence
(iv) Inactive or abnormal sperms
2. Techniques
to overcome sterility
(a) Sperm bank
- This technique is used for a woman whose
husband suffers from infertility, sterility or suffers from a genetic disease.
- Sperms from a donor are frozen and
kept in a sperm bank (in liquid nitrogen at -172 0C).
- The profile of the donor is recorded.
- The sperms from the sperm bank is
injected into the fallopian tube of a woman during avulation.
(b) Artificial insemination
- This technique is used if the man’s
sperms are unable to function or if the sperm count is low.
- Sperms are taken from the husband and
transferred into the vagina of the wife during her ovulation period.
(c) In Vitro fertilization (IVF)
- This technique is used if the
fallopian tubes are blocked or damaged.
- Woman is treated with high dose of FSH
to stimulate the development of a large number of follicles.
- Laparoscope is used to remove a number
of secondary oocytes from ovaries through abdomen before ovulation.
- Sperms from the husband are placed in
a test tube filled with a culture solution that is similar to the fluid in the
fallopian tube.
- The secondary oocytes are then placed
in the laboratory dish that contains the sperms. Fertilization occurs in the
dish.
- The fertilized eggs (zygotes) formed
are allowed to develop in the laboratory dish.
- After 12 hours, zygotes are
transferred into a different culture medium which supplies nutrients for
further development.
- At the eight cells stage, one zygote
is placed inside the uterus via cervix.
- The zygote the gets attached to the
endometrium wall and continue to develop normally in the uterus.
(d) Intrafallopian transfer
- This technique involves in the
transfer of gamete or zygote into the Fallopian tubes.
(i)
Gamete
Intrafallopian Transfer (GIFT)
- transfer of gametes (sperms and
secondary oocytes) into the fallopian tubes.
(ii)
Zygote
Intrafallopian Transfer (ZIFT)
- sperms and secondary oocytes are
placed in the laboratory dish where fertilization occurs.
- The zygote is transferred into the
fallopian tubes.
(e) Surrogate mother
- This technique is used when the wife
cannot conceive.
- The sperms and ova are contributed by
the parents or the sperms may obtain from the sperm bank and the ova from the
surrogate mother.
- Either sperms or an embryo is
transferred to the uterus of the surrogate mother.
The surrogate mother pregnant and once the baby is
born, the baby will be handed over to the couple.
4.5 SEXUAL REPRODUCTION IN FLOWERING
PLANTS
4.5
Sexual
Reproduction in Flowering Plants
The flower is the reproductive organ of a flowering
plant.
1. A flower contains four basic parts:
(a) Stamens: The male reproductive
structures.
-
consists
of:
(i) An
anther: The structure that produces pollen grains that contain the male
gametes.
(ii) A filament: a long stalk which
supports the anther
(b) Carpels: The female reproductive
structures.
-
Several
carpels fuse together to form a pistil. A pistil consists of:
(i) A
stigma: a sticky structure at the apex of the style to which pollen grains
adhere.
(ii) A style: a stalk that joins the stigma
to the ovary
(iii) An ovary: a structure that contains
the ovule.
(c) Sepals: Sepals are modified leaves
which enclose and protect the other parts of the flower in the bud stage.
(d) Petals: Petals are modified leaves which forming the
conspicuous part of a flower. They are bright coloured to attract insects for
pollination.
2. Type of flowers:
(a) Bisexual or hermaphrodite flower
-
Possesses
both the stamens and the pistil
(b) Unisexual flower
-
Has
either the stamens or the pistil
The formation of pollen grains in the anther
·
An
anther has four pollen sacs.
·
Each
pollen sacs contains hundreds of cells called pollen mother cells (2n)
·
Each
pollen mother cell undergoes meiosis to produce four haploid microspores (n)
·
The
nucleus of each microspore divides once by mitosis to produce a tube nucleus
and a generative nucleus.
·
The
microspores develop into pollen grains.
·
The
tube nucleus enables the pollen tube to grow down the style of the flower to
the ovule.
·
The
generative nucleus then divides mitotically to form 2 haploid sperm.
1. The formation of the embryo sac in the
ovule
·
In
the ovule of the ovary, the embryo sac mother cell or megaspore mother cell
(2n) divides meitotically to produce 4 haploid megaspores(n).
·
Three
of these 4 megaspores disintegrate, leaving one in the ovule.
·
The
nucleus of the megaspore undergoes 3 cycles of mitosis to form 8 haploid
nuclei.
·
Three
of the 8 nuclei migrate to one end of the cell to form antipodal cells. Another
two nuclei, called the polar nuclei move to the center.
·
One
of the three nuclei nearest the opening of the ovule develops into an egg cell,
flanked by two synergid cells.
1. Formation of the pollen tube, zygote
and the triploid nucleus
·
During
pollination, a pollen grain lands on the stigma where the pollen will germinate
producing a pollen tube that grows through the style and reaches the ovary. It
penetrates the ovule through the micropyle to the egg in the ovule.
·
One
of the 2 sperm nucleus / male nucleus arising from the generative cell will
fertilize the egg resulting in a diploid zygote. This is fertilization.
·
The
other sperm nucleus / male nucleus fertilizes the 2 polar nuclei, each haploid,
resulting in a triploid (3n) endosperm, a nutritive tissue for the embryo.
·
A
seed coat, which is not a product of fertilization, forms around the endosperm.
The result is a seed composed of the seed coat, the nutritive endosperm, and
the embryo.
Double
fertilization
The
pollen tube enters the ovule through the micropyle and ruptures.
·
One
sperm nucleus fuses with the egg forming the diploid zygote.
·
The
other sperm nucleus fuses with the polar nuclei forming the triploid
endosperm (3n).
·
The
synergid nucleus and the antipodal cells degenerate.
·
The
tube nucleus disintegrates.
The
importance of double fertilisation for the survival of the flowering plants
·
The
formation of a diploid zygote ensures that the genetic information of the
parent plants is passed down to the next generation.
·
The
formation of the gamete and the egg cell occurs through meiosis to ensure that
the variation exists in the nest generation for the survival of species.
·
The
endosperm provides food for the developing embryo during germination to
guarantee the germination of a seed into a new plant.
·
After
double fertilization, the seed is surrounded by the ovary which develops into
fruit. The fruit protects the seeds and aids in the dispersal of the seeds to
ensure the survival of the flowering plants in new habitats.
1. The development of seed and fruit
after fertilization
|
Flower parts |
Post-fertilisation changes |
|
Whole ovule |
Seed |
|
Zygote |
Embryo: consisting of the
developing shoot (plumule), developing root (radicle) and cotyledons |
|
Endosperm nucleus |
Endosperm: to provide
nutrients to the growing embryo |
|
Integuments |
Testa and tegmen /
seed-coat (pericarp): to protect embryo and its endosperm |
|
Ovule stalk |
Seed stalk |
|
Ovary |
Fruit |
|
Ovary wall |
Fruit wall |
|
Stigma and style |
Wither, but in some cases
may persist and be modified to help fruit dispersal |
|
Stamens |
Wither and fall off |
|
Petals |
Wither and fall off |
|
Sepals |
May persist in some cases
and be modified to help fruit dispersal |
4.5
Growth
In Multicellular Organisms
1. Growth = The increase in body size, number of cells
and weight producing changes in the body shape and body functions of an
organism in an irreversible way.
2. Stages in growth:
(i) Cell division
-
involves
mitosis
-
results
in the increase in the number of cell in an organism
-
in plant
cells, the apical meristem contains meristematic cells which divide actively
mitosis to increase the length of the stem.
-
in animal
cells, the increase in the number of the cell through assimilation of food and
water to synthesize protoplasm
(ii) Cell enlargement
-
increases
in the size of each cell
-
in plant
cells, cell enlargement involves vacuolation (the increase in the size of cells
takes place by the absorption of water into the cell and then the water is kept
in the vacuole).
(i) Cell diiferentiation
-
cell
differentiates from a non-specific cell
to a specilised, permanent cell with specific functions.
4.5
THE GROWTH CURVE
1. The parameters to measure growth:
(a) Measurement of dry mass
-
Shows
the accurate measurement of growth but involves the destruction of the organism
(b) Measurement of fresh mass
-
Does not
destroy the organism but does not show the actual growth of the organism as the
water content in the body of the organisms change daily.
(c) Measurement of length and height
-
Does not
destroy the organism but does not show the actual growth of the organism as an
organism which may nit growth in height, may be growing in diameter or sideway.
2. The Sigmoid Growth Curve of an
organism
The Sigmoid Growth Curve can be
divided into five stages:
(a) Lag phase
-
Initial
stage of growth
-
The rate
of growth is slow
-
Mitotic
cell division occurs rapidly
(b) Exponential phase (Rapid growth phase)
-
The rate
of growth is highest
-
Active
cell division and cell elongation occurs
-
The size
of organism increases rapidly
-
Cells
grow due to absorption of water and the increase in the protoplasm
(c) Slow growth phase
-
The
growth rate proceeds at a steady or relatively constant rate
-
The
growth is limited by internal factors (e.g. growth factor) or external factors
(e.g. food storage)
-
A bit of
cell division occur to replace any dead or damage cells
(d) Stationary phase
-
The rate
of growth is zero.
-
The
organism has reached maturity
-
The size
of the organism remain unchanged
-
Cell
division occurs to replace dead or damage cells
(e) Senescence phase
-
The rate
of growth is negative
-
The
organism undergoes the process of aging and eventually death
2. The growth curve of animals with
exoskeletons / insects
·
The
growth curve of animals which have exoskeletons is in the form of steps.
·
The
inflexible external skeleton does not facilitate a smooth growth rate. Hence,
an insect undergoes moulting periodically during growth.
·
Moulting
or ecdysis is the process of removing an old exoskeleton and replacing it with
a new larger one.
·
During
ecdysis, atmospherics air is sucked into the body of an insect to expand it and
replace it with a layer of exoskeleton that is new, soft and still elastic.
·
While
the body of the insect is expanding, the existing exoskeleton will break
·
Before
the new exoskeleton hardens, growth takes place actively to increase the size
of the animal.
4.8 PRIMARY AND SECONDARY GROWTH IN
PLANTS
1. Two type of meristem cells which are
involved in the growth of plant:
(a) Apical meristem
-
Consists
of meristem tissue in the tips of shoot and root.
-
Causes
the increase in length
-
Involves
in the primary growth
(b) Lateral meristem
-
Consists
of vascular cambium and cork cambium.
-
Causes the
increase in diameter
-
Involves
in the secondary growth
2. Primary growth
-
Occurs
at the tips of shoot and root of all plants
-
Importance:
(i)
Increases
the height of the shoot and the length of roots
(ii)
The
primary xylem and phloem involves in transportation in plant
(iii) The primary xylem also provides
mechanical support to the plant
3. Secondary growth
-
Occurs
in woody plants
-
Importance:
(i)
Provide
additional support to large woody plants
(ii)
Produce
more xylem and phloem tissues to sustain the transport of water, mineral salt and
products of photosynthesis in plants
(iii) Produce new xylem and phloem to
replace old and damaged xylem and phloem tissues
4. Secondary growth in dicotyledon stems
·
Activity
of vascular cambium:
(a) The vascular cambium in the vascular
bundle divides actively through mitosis to form a cambium ring.
(b) The inner cells of the cambium ring
differentiate to form secondary xylem, while the outer cells produce the
secondary phloem.
(c) As a result, the primary xylem is
pushed towards the pith while the primary phloem is push towards the epidermis.
·
Activity
of cork cambium
(a) The cells of the cork cambium divide
actively to form an outer layer of cork cells and an inner secondary cortex.
(b) The walls of the cork cells contain a
waxy substance called suberin that makes the cork waterproof to prevent water
loss.
(c) The cork layer forms a protective bark
on the surface of the woody stem.
5. Secondary growth in dicotyledon roots
·
Activity
of vascular cambium:
(a) The vascular cambium in the vascular
bundle divides actively through mitosis to form a cambium ring.
(b) The cells of the cambium ring divide to
produce secondary xylem, on the inside and secondary phloem on the outside.
·
Activity
of cork cambium
(a) Cork cambium produces secondary dermal
tissues.
(b) Old root with secondary growth
function as an anchor for the plant in the soil and a transport for the water
and mineral salts between the younger roots and the shoot system.
6. Annual growth rings:
-
In
spring, vascular cambium is more active and forms larger and thinner-walled
xylem vessels. This allows more water to be transported to the new leaves at
the start of the growing season.
-
In
autumn, the cambium is less active and forms narrower xylem vessels. However,
the xylem vessels becomes more lignified to provide additional support to the
plant.
-
The wood
formed in spring is lighter in colour while the wood formed in autumn is darker
in colour.
-
As a
result, the cross sections of tree trunks shows a pattern of alternating light
and dark regions (annual growth rings).
7. The necessity and importance of
secondary growth to plants:
(a) To increase the diameter of the plant
stems and roots for additional mechanical support.
(b) To produce wood (secondary xylem) to
support and strengthen the growing plant.
(c) To produce more secondary xylem and
phloem to accommodate the increase in demand for water, mineral and organic
nutrients.
(d) To produce new xylem and phloem
tissues to replace the old and damaged ones.
(e) To produce thick and tough bark to
reduce water evaporation from the surface of the stems
(f) To increase the opportunity to produce
seed and propagate as plants that undergo secondary growth live longer.
8. The economic importance of plants that
undergo secondary growth:
(a) Have high commercial value as they
produce valuable timber.
(b) Have woody parts that are strong and hard
that makes them suitable as beams for buildings.
(c) The presence of the medullary rays is
an attractive feature in furniture and decorative items.
(d) Fruit tress that shows secondary
growth bear fruit for several years.
9. Differences between plants that undergo
secondary growth and those that do not.
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Plants that
undergo secondary growth
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Plants that
do not undergo secondary growth
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