Chapter 4 ( Reproduction and Growth )

All organism carry out reproduction growth and development to ensure the continuity of life.

 The process of producing offspring or new individuals is called reproduction.

 There are two types of reproduction : a) asexual reproduction which involves only one parent (does not involve gametes)                                                                b) sexual reproduction which involves two parents from the same species but with different sexes. Both gametes                   produce gametes for fertilisation

Sexual reproduction involves the production and fusion of male and femele gametes. In human, male gametes called Sperms are produces in the male gomard (reproductive organs) called testes while the famale gametes called ova (singular: ovum) are gametes in the female gonards called ovaries.

 Sexual reproduction involves meiosis

                   

 Meiosis occurs in the testes to produce sperms and in the ovary to produce ovum

 The process of sperm formation in human is called spermatogenesis while the process of ovum formation in human is called oogenesis.

 Spermatogenesis is a process a sperm formation in testes.

 Each testis consists of many seminiferous tubules.Spermatogenesis occurs on the walls of seminiferous tubules.

 Each seminiferous tubule is lined with a layer of germinal epithelium which consists of primordial germ cells

                    

                                                                                                                                      

Stages of a sperm formation

 Each testis consists of many seminiferous tubules.Spermatogenesis occurs on the walls of seminiferous tubules.

 Each seminiferous tubule is lined with a layer of germinal epithelium which consists of primordial germ cells

 Oogenesis is the process of ovum formation in the ovary

 Formation of ovum begins in the ovary of a foetus

 The ovary wall consists of a layer of germinal epithelium.This layer of cell is made up of primordial 

    

                                  

                                                                                                   The process of oogenesis

 In a menstrual cycle, there are changes in the hormone level that involve hormones such as progesterone, oestrogen, luteinising hormone (LH) and follicle stimulating hormone (FSH). These charges in hormonal level result in the development of follicles and charges in the thickness of uterine wall.

 FSH  stimulates the development of primary follicle in the ovary.LH cause ovulation.Oestrogen inhibits the production and the release of FSH and also rebuilds the endometrium.Progesterone released by corpus luteum, maintains the development and the thickness of endometrium.

 Sexual reproduction in human involves a few stages: a) fertilisation between a sperm and an ovum                                                                                         b) development of a zygote to form an embryo                                                                                         c) implantation ofthe embryo                                                                                         d) development of the embruo to form a foetus                                                                                          e) development of the foetus to become a baby

 Exchange of nutrients and waste products takes place through the placenta. The placenta supplies oxygen, nutrients (glucoce, amino acid), water and antibodies from the mother's blood to the foetus through the veins in the umbilical cord.The placenta removes nitrogenous waste products such as urea and carbon dioxide from the foetal blood to the mother's blood via the artery in the umbilical cord. 

Identical twins originate from one fertilised egg. Fraternal twins are formed when two eggs are released simultaneouusly from the ovaries during ovulation. Both are fertilised by two different sperms to form two zygotes. Both zygotes develop independently to form two embryos with different genetic content and each having its own placenta

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Technology in human reproduction is used to : a) prevent pregnancy                                                                              b) overcome sterility

Sexual reproduction in flowering plants involve pollen grain and ovule. Pollen grains are produced in the anther through meiosis.Ovule is formed in the ovary.

Each pollen grain contains a tube nucleus and two male nuclei. Each ovule contains an ovum and two polar nuclei that are involved in fertilisation.

 Flowering plants carry out double fertilisation. The first male nucleus fuses with the ovum to form a diploid zygote. The second male nucleus fuses with two polar nuclei to form a triploid nucleus.

 After fertilisation, the avule becomes the seed, the ovary becomes the fruit, diploid zygote becomes the embryo, the integument layers becomes the seed coat or testa ant the triploid nucleus becomes the endosperm.

 Growth is an irreversible process,which involves : a) increase in the numberof cells                                                                                  b) increase in the size of the cells                                                                                  c) cell differentiation

 Increase in the number of cells is due to cell division through mitosis.

 Increase in cell size is due to cell enlargement ( elongation ) process while cell differentiation forms specialised cell.

 Parameters for measuring growth are height, dry mass, fresh mass and lenght of body

 The growth curve for insects or an organism with an organism with an exoskeleton is a series of stries of steps.

 The rate of growth can be determined by measuring the changes in the parameter.The growth rate is the gradient of the growth curve over a period of time.

 Insects undergo a few ecdysis before it can achieve the adult stage.

 There are two types of growth in plants : a) primary growth                                                                      b) secondary growth

 Primary growth occurs at the tip of the shoots and roots. It increases the length os shoots and roots.

 Primary growth involves the apical meristerms at the tip of the shoots and roots that divide actively through mitosis.

 Secondary growth increases the diameter or circumference of the stem and root.

 Secondary growth occurs in the roots and stems due to the activity of  lateral meristems such as vascular cambium and cork cambium

 Importance of secondary growth in plants : a) increases the circumference of dicotyledon stems to provide support for the plant                                                                         b) produces more xylem and phloem tissue to transport additional subtances such as water, mineral salt                         and food needed by tha plant as it increases in size                                                                         c) produces xylem and phloem tissues to replace the damaged or old conducting vessels.