Kerala SSLC Class 9 Solutions for biology – The Continuity of Life (English Medium)
Page No. 67:
Along with the change in voice, what other changes may take place in the body on becoming an adult?
On becoming an adult, the following changes take place in the body:
- Development of sexual organs
- Growth of body and facial hair
- Growth in height and body
- Development of acne
We know that it is sexual reproduction that takes place in man. What is sexual reproduction? What are the different stages in it?
Sexual reproduction is a process of production of new living organisms by the union of gametes from both parents.
Different stages of sexual reproduction:
- Development of male and female gametes
- Transfer of male gametes to the female body
- Formation of zygote
- Development of embryo
Page No. 68:
On attaining sexual maturity (puberty), the male and female sex organs undergo a number of changes. What are these changes?
Changes in sex organs in males and females on attaining puberty:
- Development of breasts in females
- Maturation of ovaries, uterus and vagina in females
- Development of pubic and underarm hair in females
- Enlargement of scrotum and testes in males
- Lengthening of the penis in males
- Enlargement of seminal vesicles and the prostate gland in males
- Appearance of pubic and facial hair in males
How are the positions of the male and female sex organs different?
The male sex organs are located on the outside of the body. On the other hand, the female sex organs are located in the abdominal cavity of the body.
Why is the testis seen inside the scrotum?
The main function of the testes is sperm formation. For this, the temperature of the testes needs to be lower than that of the body temperature. The scrotum helps the testes to maintain a temperature which is 2-2.5℃ lesser than that of the body temperature.
Where is the sperm deposited in the female sex organ?
The sperms are deposited in the vagina of a female.
Page No. 69:
Where does fertilization take place?
Ovum is present in the fallopian tube. Thus, fertilisation takes place in the fallopian tube inside the female’s body.
Which is the path of the sperms?
- In males, sperms are produced in each testis present in the scrotum.
- From the testis, the vas deferens carries the sperm in the semen to the urinary tract.
- Semen is discharged to the exterior of the male body through the penis.
- In females, the sperms get deposited to the vagina.
- From the vagina, the sperms swim through the uterus and reach the fallopian tube for fertilisation with the ovum.
When does the formation of gametes begin?
The formation of gametes begins in human beings on attaining puberty, i.e. at about 12-13 years of age.
What are the factors that influence this (gamete formation)?
Factors which influence gamete formation:
- Quantity of sex hormones produced
- Age of secretion of sex hormones
Page No. 70:
How many chromosomes are there in the germinal cells?
Germinal cells have a complete set of chromosomes, i.e. germinal cells in humans have 46 chromosomes.
In the figure, what is the type of cell division that took place prior to the growth phase? What is its significance?
Cells undergo mitosis before the growth phase.
Significance of mitosis:
- Mitosis helps increase the number of cells.
- An exact replica of parent cells is produced during mitosis.
- Genetic constitution of parent cells is restored in the daughter cells.
- Mitosis helps in the repair of wear and tear in body tissues.
What is the significance of the cell division that takes place after the growth phase?
Meiotic cell division takes place after the growth phase.
Significance of meiosis:
- Helps maintain the number of chromosomes in sexually reproducing organisms
- Helps maintain the stability of species by restricting the multiplication of chromosomes
- Helps introduce variation in organisms
How many chromosomes are there in the daughter cells after division?
After mitotic cell division, daughter cells have ‘2n’ number of chromosomes. After meiotic cell division, daughter cells have ‘n’ number of chromosomes.
What is the reason for the difference in the number of chromosomes in the daughter cells?
Mitosis is an equational division which maintains the genetic composition of cells. A cell which undergoes mitosis has the same number of chromosomes as that of the parent cell.
On the other hand, meiosis is a reduction division, which helps in variation. A cell which undergoes meiosis has half the number of chromosomes as that of the parent cell.
Hence, there is a difference in the number of chromosomes in the daughter cells.
Page No. 71:
Is there any difference between the number of the sperms and ova produced after gametogenesis?
What is the reason for this?
- After gametogenesis, one ovum is produced in females and four sperm cells are produced in males.
- During gametogenesis, both male and female cells undergo meiosis to form four daughter cells.
- However, during gametogenesis in females, only one ovum and three polar bodies are formed.
Are the sperms and ova similar in structure and characteristics? Complete the table with the help of given figures.
What happens to the ovum if fertilization does not take place?
If fertilisation does not take place, then the ovum dissolves and gets destroyed, the uterine lining which was prepared to receive the embryo is also shed off. This uterine lining along with the destroyed ovum is discharged from the body.
Page No. 72:
How does the zygote formed in the fallopian tube pass through the embryonic stages to become a fully grown child?
The embryonic stages which a zygote passes through to become a fully grown child:
- The zygote is formed as a result of fertilisation of male and female gametes.
- Fertilisation takes place in the fallopian tube.
The zygote starts dividing and simultaneously starts moving towards the uterus.
- The zygote reaches the uterus after about seven days of fertilisation.
- It gets implanted in the uterus.
- The embryo develops and different germinal layers are formed.
- The germinal layers differentiate to form their respective tissues and structures.
- Development continues for about nine months till the baby is fully grown and ready to see the light of the outside world.
Page No. 73:
Where does the zygote formed in the fallopian tube grow and develop?
The zygote formed in the fallopian tube grows and develops in the uterus.
How long does the zygote take to reach the uterus?
The zygote takes about 7 days to reach the uterus.
Where does the growing embryo get the necessary oxygen and nutrients from?
The growing embryo gets the necessary oxygen and nutrients from the mother’s body through the placenta. An umbilical cord connects the embryo to the placenta.
How is the waste matter in the embryo excreted?
Waste matter in the embryo is excreted through the placenta.
How is the embryo in the uterus protected from external shock?
Amniotic fluid present in the amniotic sac protects the embryo in the uterus from external shock.
How long does the embryo take to attain full growth?
The embryo takes about nine months to attain full growth.
Do all adult men and women have the ability to reproduce?
All healthy adult men and women have the ability to reproduce, unless there is some disease or disorder.
Sterility is a state in which a man or woman is unable to reproduce. What could be the reason for this?
Reasons for sterility in females:
- Imbalance of hormonal secretion essential for ovulation
- Damage to ovaries
- Damage to fallopian tubes
- Production of immature follicles
Reasons for sterility in males:
- Hormone imbalance
- Low count of sperms
- Damage to sperm ducts
- Infection or disease causing damage to gonads
- Stress and other physical conditions
Page No. 76:
Are the male and female sex organs present in the same flower in the pea plant?
The flowers of pea plant are bisexual. They possess both male and female sex organs.
Page No. 77:
Explain the following terms:
- Unisexual flowers
- Bisexual flowers
- Monoecious and dioecious plants
- Unisexual flowers: Either male or female reproductive organ is present.
- Bisexual flowers: Flowers which have both male and female reproductive systems.
- Monoecious Plants: These plants have male and female flowers on the same plant.Dioecious Plants: These plants have male and female flowers on different plants.
Explain the following terms:
- Self Pollination
- Cross Pollination
- Self-pollination: The transfer of pollen grains from the anther of a flower to the stigma of the same flower or another flower on the same plant is called self-pollination.Examples: Wheat, tobacco, pea
- Cross-pollination: The transfer of pollen grains from the anther of a flower on one plant to the stigma of a flower on another plant of the same species is called cross-pollination.Examples: Hibiscus, China rose, brinjal, tomato, lady finger
- Pollinators: Agents which help to bring about cross-pollination are called pollinators.Examples:
EXAMPLES OF PLANTS
Sweet pea, Dahlia
Asclepias tuberosa, Echinacea purpurea
Liatris spicata, Camassia scillioides
Habenaria obtusata, Trichopoda sp.
Pond lily, Magnolia
Page No. 77:
What are the changes that occur in the pollen grain after pollination?
After pollination, a tube called a pollen tube grows from the pollen grain.
The pollen tube starts growing towards the ovary.
It carries two male gametes from the pollen grain to the ovary.
Explain the formation of gametes and their function.
The generative nucleus of the pollen grain divides mitotically to form two male gametes. One male gamete fuses with the egg cell to form the zygote and the other male gametes help in the formation of nutritive tissue called the endosperm.
The female gamete, i.e. the egg cell, is formed by a cell in the ovule. This cell undergoes meiosis and then mitosis to form the egg cell. The egg cell takes part in the formation of the zygote.
Describe the changes that occur in the flower after fertilization.
After fertilisation, the ovary develops into a fruit and the ovules develop into seeds.
When the seed matures, it germinates and gives rise to a new plant.
Do plants reproduce only through seeds? Do the vegetative parts of the plant like root, stem, leaf etc., help in reproduction?
Seeds are the most important means by which the plants propagate. However, many plants can propagate by means other than seeds as well.
- Bryophyllum can propagate vegetatively by plantlets present on mature leaves of the plant.
- Potato and onion propagate by means of their underground stem.
- Dahlia and sweet potato can also propagate by means of their roots.