"How do organisms reproduce?" Sexual reproduction - notes

Sexual Reproduction

When reproduction takes place as a result of fusion of two gametes, one

from each parent, it is called sexual reproduction

The process of fusion of male and female gametes is called fertilization.

˜ The formation of gametes involves exchange of chromosomal (genetic)

fragments between homologous chromosomes causing genetic

recombination which leads to variation.

Gametes: Special Reproductive cells are different from normal cells as they have half the  

               number of chromosomes.

e.g Human cells have 46 chromosomes but sperm and egg have 46/2=23 chromosomes.

So when sperm and egg fuse, zygote will have 23+23= 46 chromosome.

Male gamete : small in size and motile (has tail)

Female gamete: big in size ( because of stored food reserve) and non motile.

Sexual Reproduction in Plants

It occurs mostly in flowering plants. In fact flowers are the reproductive organ of plants.

FLOWERS

Bisexual Flowers	Unisexual Flowers
Both male and female reproductive part i.e., stamen & carpel are present.	Either male or female reproductive part is present.
Eg. Hibiscus, mustard	Eg. Papaya, Watermelon

A typical flower consists of four main whorls namely 1. Calyx (Sepals),

                                                                                      2. Corolla (Petals),

                                                                                      3. Androceium (Stamens) and

                                                                                      4. Gynoecium( Carpels).

                        Reproductive Part of Flower

1.      STAMENà Anther (2n)   +    Filament

           (male part)          

                                Pollen grains

                             (Male gamete)

                                      (n)

2.       CARPEL-à Stigma    +     Style    +    Ovary(2n)

         (female part)                                              

                                                                         Egg cell (ovule)

                                                                              (n) 

 Pollination: Transfer of pollen grains from anther to stigma of carpel.

(Self-Pollination) Pollen grains transferred to the stigma of the carpel of the same flower (Cross-Pollination) Pollen grains transferred  to the stigma of carpel of the another flower ˜ This transfer of pollens is achieved by agents like wind, water or animals.

˜ After Pollination, a pollen tube grows out of pollen grains, through which male

      germ cell reaches the ovary and fusses with the  female germ cell.

˜ Fertilization : The fusion between the pollen grain and female egg cell.                                    

      - It occurs inside the ovary. Zygote is produced in this process.

      -Zygote divides several times to form an embryo within the ovule.                                               

      -The ovule develops a tough coat and is converted into a seed.

     - Ovary grows rapidly and ripens to forms a fruit, while the seed contains the future                

      plant or embryo which develops into a seedling under suitable condition. This

      process is known as Germination.

Reproduction in Human Beings

˜ Humans use Sexual Mode of reproduction.

˜ It needs sexual maturation which includes creation of the germ cells

     i.e., egg (ova) in the female and sperm in the male .

This period of attainment of sexual maturation is called Puberty.

Pubertal changes/Secondary sexual characteristics:

    (Common)         Hair growth in armpits, genital region,

                              Acne/ pimples(excessive oil production by sebaceous glands in skin)

   (Girls)              :  Breast development, Menstruation

  ( Boys)             : Facial hair i.e. beards, moustaches, Adam’s apple (change in voice)

                             Penis enlargement..

˜ Human beings have a well developed male and female reproductive system.

Male Reproductive System- Male reproductive organ (Testis)

              + Other glands (Prostate Gland and seminal vesicles)

              + Ducts (Vas deferens, Ejaculatory duct…)

.

 The Testes (male reproductive organ)

Function:  a) Formation of male germ cell (sperms) takes place.

             Location: A pair of testes are located inside scrotum situated outside the

                     abdominal cavity. It is meant to keep relatively a low temperature

                     needed for the production of sperms by testes.

Function:   b) Testes release a male sex hormone called testosterone whose

          function is to:

        1. Regulate the production of sperm

        2. Brings about changes in appearance seen in boys at the time of puberty.

 The sperms along with the secretion of prostate gland and seminal vesicle,

      together constitute semen. 

     ( Sperms + Secretions of Prostate gland and seminal vesicles = Semen)

Secretions of Prostate gland and seminal vesicles- 1.Provide nourishment to sperms.

                                        2. Help sperms in their movement by lubricating the ducts.

 The semen is released and made to enter into the female genital tract during Copulation.

Female Reproductive System- Female reproductive organ (Ovary)

                                           + Ducts (Fallopian tube, Uterus, Vagina)

Ovary-Location: One ovary each on both sides of abdomen.

         Function: 1.The female germ cells or eggs are produced in the ovaries.

          (When a girl is born, the ovaries already contain thousands of immature eggs.

           At the puberty, some of these eggs start maturing. One egg is produced

           every month by one of the ovaries.

    2. Female sex hormones (oestrogen and progesterone) are produced in ovary.

Fallopian tube: Carries egg from ovary to the uterus (also known as womb)

 Uterus: is an elastic bag like structure in which development of embryo takes place.

˜ The Uterus opens into the vagina through the cervix (birth canal).

Fertilisation : (Fusion of sperm and egg)

˜ Fertilization occurs in the fallopian tube of female genital tract. (Internal fertilization)

˜ The fertilized egg also called zygote (2n) starts dividing, reaches uterus and gets embedded

       in the thick lining of the uterus, (which is richly supplied with blood to nourish the growing

     embryo) . This process is known as Implantation.

˜ Placenta. The Embryo gets nutrition from the mother's blood with the help of a special tissue

                      called Placenta.

                (Function) - It provides a large surface area for glucose and oxygen to pass from   

                                           the mother to the embryo.

                                    -  Similarly the wastes from developing embryo are removed to               

                                        mother's blood through placenta.

                                     Placenta also produces hormones to support pregnancy.

˜ The child is born as a result of rhythmic contractions of the muscles in the uterus. After nine months (36 weeks) of development inside mother's womb. This is called Gestation Period.

If no fertisation occurs (i.e. sperms are not present in the female genital tract) –

1.      Zygote is not formed,

2.      The inner wall of uterus breaks which causes bleeding through vagina. This process

is called MENSTRUATION. It occurs at a regular interval of 28 days.

˜ The sexual cycle in a woman continues upto the age of 45 to 50 years.

 After that the ovary do not release egg. This stage is called Menopause.

 It  also marks the end of menstruation in the woman.

Reproductive Health

˜ Reproductive Health means a total well-being in all aspects of reproduction, i.e.,

      physical emotional, social and behavioural.

˜ Contraception : It is the avoidance of pregnancy.

                             It can be achieved by preventing the fertilization of ova.

                              

                                           Methods of contraception

1.      Physical Barrier:  To prevent union of sperm & egg.

Use of condoms, Diaphragm &  cervical caps.

2.      Surgical Method: Also called sterilization.

   In vasectomy, the vas deferens of male is blocked to prevent sperm transfer.

   In tubectomy, the fallopian tube of female is blocked to prevent egg to reach uterus.

3. Chemical Method : Oral contraceptive (OCs) - changes the hormonal balance to

                                     check the egg release in females. OCs cause side effect.

4. IUDs: Intrauterine contraceptive device- Copper-T or loop is placed in uterus to

          prevent pregnancy.

˜ Healthy society needs a balanced sex ratio that can be achieved by educating the              

      people to avoid malpractices like female foeticide & prenatal sex determination.

Sexually Transmitted diseases (STDs)

VIRAL STDs:  Eg. H.I.V. – AIDS, Warts

Bacterial STDs:  Eg. Syphilis and Gonorrhoea

STDs are communicated during unsafe sexual contact.

These diseases (STDs) can be prevented by using physical barrier i.e. condom.