How Organisms Reproduce
Reproduction is the process by which living organisms produce new individuals of their own kind. It is not needed for the survival of one organism, but it is necessary for the continuation of a species.
Contents
Why Reproduction Matters
Reproduction is not essential for keeping one organism alive, but it is essential for the survival of the species. Without reproduction, a species would disappear after some time.
New organisms usually look similar to their parents because the basic body design is passed from one generation to the next through DNA.
DNA Copying and Variation
During reproduction, the body makes a copy of DNA. This copying is very accurate, but it is not always perfect.
Because of this, the new organisms are similar to the parent, but they may show small differences. These differences are called variations.
Role of DNA
DNA stores the instructions needed for body design and body functions.
These instructions are passed to the next generation during reproduction.
Why Variation Happens
Small changes may happen while DNA is being copied.
Variation helps the species survive when environmental conditions change.
Asexual Reproduction
In asexual reproduction, only one parent is involved. The new organism usually resembles the parent closely because the genetic material comes from one source.
This method is common in many simple organisms and in some plants.
| Type | Main Idea | Parents Involved | Examples |
|---|---|---|---|
| Asexual reproduction | New organism forms from one parent only | One | Amoeba, Hydra, Yeast, Rhizopus, Spirogyra |
| Sexual reproduction | New organism forms after fusion of male and female gametes | Usually two | Flowering plants, human beings, many animals |
Fission
Fission is a method in which one unicellular organism divides to form new organisms. It is common in bacteria and protozoa.
Binary Fission
One parent cell divides into two daughter cells.
Examples: Amoeba and Leishmania.
Multiple Fission
One parent cell divides into many daughter cells at the same time.
Example: Plasmodium.
Special Point
In Amoeba, division may happen in any plane. In Leishmania, it happens in a fixed orientation because of its body structure.
Budding, Fragmentation and Regeneration
In budding, a small outgrowth called a bud forms on the parent body. The bud grows, matures, and later separates as a new individual.
In fragmentation, the body of a simple multicellular organism breaks into pieces and each piece grows into a new organism. Spirogyra shows this method.
In regeneration, some organisms such as Hydra and Planaria can form a complete new individual from body parts because they have special cells that can divide and develop.
Vegetative Propagation
In vegetative propagation, new plants grow from roots, stems, or leaves. This method is common in plants like potato, Bryophyllum, sugarcane, rose, banana, orange, and jasmine.
Why It Is Useful
- New plants grow faster.
- Flowers and fruits may appear earlier.
- Plants that do not form seeds can still be multiplied.
- Useful characters of the parent plant are preserved.
Examples
- Potato grows from buds present in the eyes.
- Bryophyllum grows from buds on the leaf margin.
- Money plant grows from stem pieces having nodes.
Spore Formation
In spore formation, special structures produce spores that can grow into new individuals under suitable conditions. Bread mould or Rhizopus reproduces in this way.
Spores have thick protective walls, so they can survive until they reach a moist place where they can start growing.
Sexual Reproduction
In sexual reproduction, male and female gametes fuse to form a zygote. This method usually involves two parents.
Sexual reproduction creates more variation because genetic material from two individuals combines in new ways.
Reproduction in Flowering Plants
In flowering plants, reproduction takes place in the flower. The main reproductive parts are the stamen and the pistil.
Male Part
Stamen is the male reproductive part.
It produces pollen grains that carry the male germ-cells.
Female Part
Pistil is the female reproductive part.
It has stigma, style, and ovary. The ovary contains ovules, and each ovule has an egg cell.
Pollination is the transfer of pollen from anther to stigma. It may be self-pollination or cross-pollination.
After pollination, a pollen tube grows through the style to the ovary. Then the male gamete fuses with the female gamete in the ovule. This fusion is called fertilisation.
Reproduction in Human Beings
Human beings reproduce sexually. During adolescence, the body shows changes that prepare it for reproduction. This stage is called puberty.
Common Changes
- Hair grows in armpits and genital area.
- Skin may become oily and pimples may appear.
- Body changes gradually with age.
Different Changes
- In girls: breast size increases and menstruation begins.
- In boys: facial hair appears and voice may crack.
Male Reproductive System
The testes produce sperms and secrete the hormone testosterone. They are placed in the scrotum outside the abdominal cavity because sperm formation needs a temperature slightly lower than normal body temperature.
Sperms move through the vas deferens. The seminal vesicles and prostate gland add fluid that helps in movement and nourishment, and the urethra acts as a common passage through the penis.
Female Reproductive System
The ovaries produce eggs and also secrete hormones. Usually one egg matures and is released about once every month.
The egg moves through the fallopian tube to the uterus. The uterus opens into the vagina through the cervix.
Development and Menstruation
During sexual intercourse, sperms enter the female body through the vagina. They move upward and may meet the egg in the fallopian tube.
If one sperm fuses with the egg, fertilisation occurs and a zygote is formed. The zygote divides to form an embryo, and the embryo gets implanted in the uterus.
The embryo receives food and oxygen from the mother through a special tissue called the placenta. The placenta also helps remove waste materials.
If the egg is not fertilised, it survives only for about one day. The thickened uterine lining then breaks down and comes out through the vagina as blood and mucus. This process is called menstruation.
Reproductive Health and Contraception
Some diseases can spread through sexual contact, including gonorrhoea, syphilis, warts, and HIV-AIDS. Condoms help reduce the spread of many such infections and also help prevent pregnancy.
Methods of Contraception
- Barrier methods such as condoms.
- Hormonal methods such as oral pills.
- Devices such as loop or copper-T placed in the uterus.
- Surgical methods such as blocking vas deferens or fallopian tubes.
Why They Are Used
- To avoid unwanted pregnancy.
- To plan families responsibly.
- To protect the health of the mother.
- To reduce the spread of infections in some cases.
Solved Intext Questions
1. What is the importance of DNA copying in reproduction?
DNA copying passes hereditary information from parents to offspring. It helps produce organisms similar to their parents and also creates small variations.
2. Why is variation beneficial to the species but not necessarily for the individual?
Variation helps a species survive when conditions change. But a particular variation may or may not help one individual directly, so its main value is for the species over time.
3. How does binary fission differ from multiple fission?
In binary fission, one parent cell divides into two daughter cells. In multiple fission, one parent cell divides into many daughter cells at the same time.
4. How will an organism be benefited if it reproduces through spores?
Spores have thick protective walls, so they can survive unfavourable conditions. They also spread easily and grow when they get a suitable moist place.
5. Why can more complex organisms not usually give rise to new individuals through regeneration?
Complex organisms have specialised tissues and organs arranged in a fixed way. Because of this, a broken body part usually cannot grow into a complete new individual.
6. Why is vegetative propagation practised for growing some types of plants?
It is practised because it is quick, preserves useful characters of the parent, allows early flowering and fruiting, and helps grow plants that do not make seeds properly.
7. Why is DNA copying an essential part of the process of reproduction?
DNA copying is essential because it transfers hereditary information from one generation to the next. Without it, the new organism would not get the instructions needed for body design.
8. How is the process of pollination different from fertilisation?
Pollination is the transfer of pollen from anther to stigma. Fertilisation is the fusion of the male gamete with the female gamete inside the ovule.
9. What is the role of the seminal vesicles and the prostate gland?
They add fluids to the sperms. These fluids help in movement and also provide nourishment.
10. What are the changes seen in girls at the time of puberty?
Breast size increases, menstruation begins, and hair grows in the armpits and genital area. Other body changes related to sexual maturity also appear gradually.
11. How does the embryo get nourishment inside the mother's body?
The embryo gets food and oxygen from the mother's blood through the placenta. The placenta also helps remove waste materials.
12. If a woman is using a copper-T, will it help in protecting her from sexually transmitted diseases?
No. Copper-T helps prevent pregnancy, but it does not protect against sexually transmitted diseases.
Solved Exercise Answers
1. Asexual reproduction takes place through budding in:
Answer: (b) Yeast
2. Which of the following is not a part of the female reproductive system in human beings?
Answer: (c) Vas deferens
3. The anther contains:
Answer: (d) Pollen grains
4. What are the advantages of sexual reproduction over asexual reproduction?
Sexual reproduction creates greater variation because genetic material comes from two parents. This improves the chances of survival of the species in changing conditions.
5. What are the functions performed by the testis in human beings?
The testis produces sperms and secretes testosterone. Testosterone controls sperm formation and causes changes in boys during puberty.
6. Why does menstruation occur?
Menstruation occurs when the egg is not fertilised. The thickened uterine lining then breaks down and comes out as blood and mucus.
7. Draw a labelled diagram of the longitudinal section of a flower.
Use the labelled flower diagram given above. It shows petal, sepal, anther, filament, stigma, style, ovary, and ovules.
8. What are the different methods of contraception?
The methods are condoms, oral pills, copper-T or loop, and surgical methods such as blocking the vas deferens or fallopian tubes.
9. How are the modes for reproduction different in unicellular and multicellular organisms?
In unicellular organisms, reproduction often happens by simple cell division such as fission. In multicellular organisms, reproduction is more complex because they have specialised tissues and organs.
10. How does reproduction help in providing stability to populations of species?
Reproduction keeps producing new individuals, so the population continues. DNA copying maintains body design, and variation helps some members survive environmental changes.
11. What could be the reasons for adopting contraceptive methods?
Contraceptive methods are adopted to avoid unwanted pregnancy, to plan families, to protect the health of the mother, and to reduce the spread of sexually transmitted diseases in some cases.