Organisms reproduce to ensure the continuation of the species and to pass down their genes to the next generation. There are two types of reproduction: sexual reproduction and asexual reproduction.
Sexual reproduction Asexual reproductionNew individual produced as New individual produceda result of the fertilisation without involving thebetween a male and female formation of gametegameteTwo individuals are involved Only one individual or parentas parents is involvedThe new individual The new individual is exactlyInherits characteristics of like its parentboth parents
Inhuman beings, the male gamete is the sperm while the female gamete is the ovum. Sperms are produced in the testes while the ovum is produced in the ovary. The formation of sperms is called spermatogenesis. Spermatogenesis occurs in the germinal epithelium of the seminiferous tubules.
A spermatozoon or sperm cell consist of head, the middle piece and the tail. Figure 4.4 shows a spermatozoon. The head contains enzymes to help it penetrate the ovum. The middle piece contains a large number of mitochondria to provide energy for the movement of the tail.
The formation of ovum is called oogenesis. Oogenesis is regulated by hormones and occurs in the germinal epithelium of the ovary of a female even before birth. A cross section of the ovary is shown in Figure 4.5.
The meaning of menstruation The relationship between menstruation and the menstrual cycle The importance of the menstrual cycle Hormones involved in the menstrual cycle Premenstrual syndrome and menopause
Menstruation is the breakdown of the lining of the uterus wall and its discharge through the vagina with a small amount of blood and some cells.
Menstrual cycle consisting of follicle development, ovulation, thickening of the endometrium and menstruation. Thus, menstruation occurs during the menstrual cycle
Firstlyprepares the inner lining of the uterus to receive a fertilised ovum. Secondly, it controls the development of mature ova and ovulation, and ensures that they occur simultaneously with the thickening of the endometrium. In enabling the production of the female gamete, the ovum, the menstrual cycle propagation of the species.
Two groups of hormones involved in the menstrual cycleHormones that control Hormones that controlchanges in the ovaries changes in the uterus• Gonadotrophin-releasing • Progesteronehormone (GnRH) • Oestrogen•Follicle-stimulatinghormone (FSH)•Luteinising hormone (LH)
Hormone Site of secretionGonadotrophin-releasing hormone Hypothalamus(GnRH)Follicle-stimulating hormone (FSH) Anterior lobe of the pituitary glandLuteinising hormone (LH) Anterior lobe of the pituitary glandOestrogen Follicle cells of the ovaryProgesterone Corpus luteum
1 day before menstruation begins, the hypothalamus produces GnRH. GnRH stimulates the anterior lobe of the pituitary gland to secrete FSH and LH. FSH stimulates the growth of a follicle in the ovary. The follicle produces the hormone, oestrogen.
At the beginning of its development, the follicle is small and produces a small amount of oestrogen. As the follicle grows, its oestrogen production increases. Although rising, the oestrogen level is still relatively low. The low oestrogen level exerts negative feedback on the pituitary gland. This feedback keeps FSH and LH low during development of the follicle ( days 1-11).
As the time of ovulation draws near, the maturing follicle increases its oestrogen secretion and oestrogen levels begin to rise steeply, and peak about day 12. This exerts positive feedback on the hypothalamus, resulting in increased GnRH secretion. High level of GnRH in turn cause FSH and LH secterion to rise. Thus, the peak in oestrogen level is followed by an FSH and LH peak that occurs just prior to ovulation.
The high LH level has many effect: • Stimulates the completion of meiosis I by changing the primary oocyte in the follicle into secondary oocyte. • It then stimulates ovulation at about the 14th day of a 28 day menstrual cycle. • It does this by enabling enzymes to break down the follicle so that ovulation can take place. • After ovulation, it promotes the development of the corpus luteum. • LH also stimulates the corpus leteum to secrete progesterone and oestrogen.
Oestrogen and progesterone are the two female sex hormones that induce the changes to the endometrium. Between 1 – 5 of the menstrual cycle: • Low level of oestrogen and progresterone • Causes the endometrium to break down and its blood vessels to burst. • Menstruation occurs
Between days 5 – 13, higher oestrogen secretion by a growing follicle in the ovary causes repair and growth of the endometrium. Increasing level of progesterone secretion by the corpus luteum between days 15 – 21 causes the endometrium to further vascularised and thicken to 2 – 3 mm. The endometrium is now ready for implantation of the embryo. If no implantation occurs, the corpus luteum begins to degenerate after day 21 and the resulting of low level of progesterone causes breakdown of the endometrium and menstruation. If implantation occurs, the progesterone level continues to rise and endometrium is further developed to support the growing embryo.
The menstrual cycle refers to the changes in the uterus wall of a woman that take place every 28 days, beginning from puberty (11 years old) until menopause (45-50 years old). The menstrual cycle is regulated by hormones. The hormones involved in the menstrual cycle are Follicle stimulating hormone (FSH), oestrogen, luteinising hormone (LH) and progesterone.
The menstrual cycle occurs every 28 days. A girl starts to menstruate when she reaches puberty. Menstruation is the blood discharged from the vagina when the uterine lining breaks down. Menstruation occurs when there is no fertilisation, and it lasts from 3 to 5 days. During this period, FSH stimulates the development of the follicle and the production of oestrogen to repair the uterus lining. From day 5 to 13, the FSH and oestrogen level continue to increase and a Graafian follicle is formed.
On day 13, the LH level increases rapidly causing ovulation to take place on the day 14. From day 15 to 25, the corpus luteum formed secretes progesterone which stimulates the endometrium of the uterus to become even thicker and filled with blood vessels. If the ovum is not fertilised, the uterus lining breaks down and the corpus luteum degenerates. This causes the progesterone level to drop even further, followed by more breakdown of the uterine lining, which is discharged as menstrual flow.
If the ovum is fertilised, the zygote develops into an embryo, which implants itself into the endometrium of the uterus. The corpus luteum continues to secrete until the placenta is formed. Progestrone inhibits the production of FSH and LH causing the menstrual cycle to stop during pregnancy.
Some woman suffer from PMS a few days before menstruation, due to the imbalance in the oestrogen and progesterone level. During this period, they are often depressed, irritable and tired. Some suffer from headaches and backaches while others gain weight easily.
Menopause occurs when the menstrual cycle stops. It occurs usually when a woman reaches 45 to 50 years of age. During the time, very little FSH is produced, resulting in lower levels of oestrogen. The follicles and the uterus walls do not develop; there is no ovulation and the menstrual cycle is disrupted. The menstruation eventually stop.
Millions of sperms are ejaculated into the vagina during sexual intercourse. The sperms swim up pass the cervix into the uterus towards the fallopian tubes. Once a sperm penetrates the ovum, it discards its tail, and its nucleus moves towards the nucleus of the ovum, fertilisation occurs. Fertilisation is the fusion of the sperm nucleus with the ovum nucleus to produce a zygote. It occurs in the Fallopian tube.
The development of the zygote begins after two days, the zygote divides several times by mitosis to form a solid mass of cells called the morula followed by a blastula which is a fluid-filled sphere with hundreds of cells around it. One end of the blastula is made up of a mass of cells which continue to develop into embryo, while the remaining cells surrounding the fluid becomes the trophoblast which later develop into the placenta. Once the blastula reaches the uterus, implantation occurs.
Implantation is the attaching of the blastula onto the endometrium of the uterus. The embryo becomes the foetus after eight weeks. The foetus is enclosed in an amniotic sac filled with amniotic fluid to protect it from any physical shock.
There are three kinds of twins which are identical twins, fraternal twins and Siamese twins.
Formation of identical twins Formation of fraternal twins
Formation of identical twins Formation of fraternal twinsFormed when a sperm fertilises an Formed when two ova are released atovum to produce a zygote. The zygote the same time, and fertilised by twoformed divides into two zygotes which different spermsdevelop into two foetuses in the uterusThe foetuse are identical as they were The foetuses are not identical as theyfrom the same zygote are formed from different sperms and ovaThe two foetuses share the same The two foetuses do not share theplacenta but have their own umbilical same placenta. They each have theircord own placenta and their own umbilical cordsThey are genetically identical. They are genetically not identical.Therefore they are always the same Therefore they can be the same orsex different sex
Siamese twins are formed when the mitotic division that occurs during the formation of identical twins is incomplete. As a result, the twins are joined at the parts of the body where the incomplete division occurs.
During foetal development, the foetus depends on the mother for food and oxygen, and to get rid of its waste products. The placenta helps in the exchange of substances between the foetus and the mother. The foetus is connected to the placenta by the umbilical cord.
The placenta arises from the chorion (trophoblast at the embryo stage) which extends fingerlike projection called villi, into the endometrium to make contact with the uterine blood. The finger-like villi help to increase the surface area for the exchange of nutrients. The membrane of the chorionic villi separates the foetal blood from the mother’s blood.
Diffusion of nutrients occur across the membrane of the chorion. The umbilical arteries carry deoxygenated blood, carbon dioxide and waste products from the foetus to the placenta while the umbilical vein carries oxygenated blood, nutrients, antibodies and hormones from the mother’s artery to the placenta.
Newly married couples should start family planning to prepare themselves for parenthood. Family planning helps them decide when to start a family and how many children they want to have. If they do not want to start a family yet, they can prevent pregnancy by using the various method of contraception available today, made possible by the advancement in science and technology.
Avoid sex during the fertile period Insert an intrauterine device (IUD) into the uterus (by a doctor) Place diaphragm (domestic shaped structure that is made of rubber) at the cervix Use a condom Take oral contraceptives pills (prescribed by a doctor) Apply spermicides to the vagina to kill the sperms Vasectomy for men where the vas deferens in a man’s body are cut surgically. Tubal ligation in woman, where the Fallopian tubes are surgically cut off to pevent pregnancy.
Blocked Fallopian tubes Difficulty in implantation of zygote Inability to produce an ovum due to hormonal imbalance Low sperm count
Sperm bank a sperm bank is a place where sperms are frozen and stored in liquid nitrogen under -2000C. Artificial insemination sperms are collected and injected into the uterus of a female during her fertile period. In vitro fertilisation This method is used for females with blocked Fallopian tubes. The ovum is extracted from the ovary during ovulation and placed in a Petri dish containing the husband’s sperms. After fertilisation, the embryo is implanted into the uterus where it continues its development. The baby is called a test tube baby.
Surrogate mother a surrogate mother is a woman who gives birth to a baby for another woman who is unable to conceive due to abnormalities in the uterus. In vitro fertilisation is carried out using the ovum and sperms of the parents but the embryo is implanted into the uterus of the surrogate mother.
Sexually-transmitted diseases are transmitted by sexual contact. They are also known as venereal diseases. Example of sexually transmitted diseases are syphilis, gonorrhea and AIDS.
Inflowering plants or angiosperms, both male and female gametes are produced in flowers.
The male reproductive structures in aflower are the stamens. Each stamen consists of two parts that is: •The filament: a little stalk which holds the anther •The anther: the place where the pollen grains are produced
The female reproductive structures are the carpelwhich consists of three parts, that is• A stigma a sticky surface at the apex of the style• A style a stalk that join the stigma to the ovary• An ovary the structure that carries the ovule
The anther has four pollen sacs Each pollen sac contains hundred of cells called pollen mother cell. Pollen mother cell (2n) meiosis 4 Microspores, (n) mitosis Tube nucleus and generative nucleus develop Pollen grains * Microspores male gametophyte
(a) Embryo sac mother cell in the ovule undergoes meiosis and forms four haploid megaspores.(b) Three of the four megaspores die, leaving one megaspore in the ovule.(c) The functional megaspores enlarges to many times its original size, filling up most of the ovule.(d) The nucleus of the megaspores then undergoes three mitotic division and form eight haploid nuclei. * Megaspores female gametophyte
Three of the eight nuclei migrate to one end of the cell forming antipodal cells. Another two move to the centre to form polar nuclei. The last three nuclei at the other end, nearest the ovule opening, form an egg cell clamped by two synergid cells. The structure thus formed is known as the embryo sac.
Pollination: the process in which ripe pollen from an anther are transferred into a stigma. The landing of a pollen grain on a stigma starts the fertilisation process. Sugar in the stigma stimulate the pollen grain to germinate and form a tube known as a pollen tube. The pollen tube grows into the style towards the ovule.
At the same time, the generative nucleus divides by mitosis into two male gamete nuclei. Led by the nucleus tube, the male nuclei move down the pollen tube. When the pollen tube reaches the ovary, it penetrate the ovule through the micropyle. The nucleus tube disintegrates leaving a clear passage for the male nuclei to enter the embryo sac.
One of the male nuclei fuses with the egg cell to form a diploid zygote (2n). The other male nucleus moves further in to fuse with the two polar nuclei forming a triploid nucleus (3n). The triploid nucleus then divides to form a special nutritive tissue called the endosperm. The zygote itself divides and grows into an embryo. The outer layer of the ovule loses moisture and develops a hard seed coat. The seed coat protects the embryo and its endosperm.
The process of fertilisation in the ovule mostly happen twice in flowering plants. The first time it happens, a diploid zygote is formed. The second time it happens, a triploid nucleus is formed. Thus, the process is known as double fertilisation.
After fertilisation, the ovary and its constituents develops as shown below: • Ovule seed • Wall of ovule seed coat • Zygote embryo • Ovary wall fruit wall • Ovary fruit
Double fertilisation helps flowering plants to survive because each new generation carries its own temporary food store in the endosperm of the seed. The endosperm provides the nutrition and energy that is needed by the embryo for its growth.
Learning outcomes: • The parameter for measuring growth • The sigmoid growth curve of an organism • The growth curve of humans • The growth curve of an insects
The meaning of growth: • It can be described as the irreversible increase in body size, mass and number of cells in an organisms. • The latter are quantitative changes that occurs during growth. • However, growth also involves qualitative changes to cells.
Growth in multicellular organism can be estimated by measuring some parameters of organism like length, height, volume, fresh mass and dry mass over a certain period of time Height or length, is convenient but the problem is inaccurate. A tree for example while not increasing in height, may continue to grow in diameter by growing sideways. Volume: more accurate but is frequently not practical to measure. Fresh mass: the measurement is inaccurate because involve temporary changes in water content. Dry mass: mass of organism after all moisture has been removed by heating.
When a growth parameter such as height or mass of an organism is plotted against time, a growth curve is obtained. The pattern of growth is about the same for most organisms. The curve is S-shaped and known as a sigmoid curve. Link
• When the length of insects is plotted against the time, it shows very unusual, intermittent growth.• The graph has a staircase shape.• The horizontal part indicate zero growth. It correspond to the time when the insect stops growing.• The five vertical lines of curve are sudden growth spurts. This correspond to the time when the nymphs cast away their old external skeleton and new, larger ones. This process is called moulting or ecdysis.• Each young stage between moults• is called an instar.
Learning outcomes: • The meaning of growth • The types of growth in plants • Primary growth tissue • Secondary growth tissue • The importance of primary and secondary growth • The economic importance of plants that undergo secondary growth
Growth can be described as the irreversible increase in body size, mass and number of cells in an organism. Quantitative changes : changes in the number of cells. Qualitative changes : changes in shape, function and complexity specialized with specialisation. The process of growth can be divided into three stages: • Cell division stage • Cell enlargement stage • Cell differentiation stage
There are two types of growth in plants, namely primary growth and secondary growth. Primary growth : the growth that occurs after germination Secondary growth the growth in diameter of the stem, root and trunk of plants the growth pattern is an annual series of sigmoid curves such plants show unlimited growth, that is, they grow continuously throughout their lives, and thus, their growth curves never flatten out.
Perennials plants that continue their growth from year to year Annuals plant that complete their whole life cycle in a year Biennials plants that have two-year life cycle
There are some plants that do not show secondary growth and live as annuals and biennials, for example, the pea plant, Pisum sativum.
Primary growth begins by rapid cell division in a group of cells at the extreme tip of the stem and root. This group of cells divides by mitosis and forms the apical meristem. Growth at apical meristem leads to the formation of mature primary tissue of the stem and root such as the cortex and primary phloem and primary xilem in the vascular bundle. This results in an increase in length of the stem and root.
After primary growth complete, secondary growth continues in woody plants like trees and shrubs. In non-woody plants like herbs, no secondary growth occurs. Secondary growth is the result of cell divisions in the lateral meristems of both stem and the root. The lateral meristem are the vascular cambium and cork cambium.
The vascular cambium located between primary phloem and primary xilem in vascular bundles The cork cambium located immediately below the epidermis.
Parenchyma cells divide actively The vascular cambium join up to form ring The cell of cambium ring divideOuter cells become the secondary phloem tissue and secondary xylem tissueSmaller ring in the rings form medullary rays connecting the cortex with the pith
Secondary xylem develops thick walls with deposition lignin. It then becomes wood. Similarly, the cork cambium beneath the epidermis divides actively to form the outer cork and the inner secondary cortex. The external cork layer forms a protective bark at the surface of the woody stem.
In trees of temperate climates, the activity of the vascular cambium is not uniform throughout the year. In spring the cambium is more active and form larger and thinner-walled xylem vessels (spring wood) In autumn the cambium is less active and forms narrower vessels and fibers (autumn wood) These two kind of wood are seen together in transverse sections of the stems, as concentric rings called annual growth rings.
Each annual ring indicates one year’s growth. Hence, by counting the total number of annual rings, the age of the tree can be determined. Trees in tropical countries like Malaysia, however, do dot show clear annual rings because the climate is not distinctly seasonal.
Primary growth: • Responsible for the increase in height of a plant • It anchors the plant firmly to the soil Secondary growth: • Enables the plants to live longer and grow throughout their life • They are able to support their increasing weight • The thickened cork and bark reduce the evaporation of water from the surface of the stem • Protect the plants attack by insects and parasitic fungi • They also guard the plant body against variation in external temperature.
The plant have commercial value and produce the most valuable timber, eg: meranti, kapur, keriung and cengal. The timber may be used as beams for buildings, for durable fencing and high-quality furniture. The wood and bark of plants can be used to produce useful resins and oils. Hibiscus, yellow bells and bougainvillea are valuable as ornamental plants in gardens and parks. Durian tree, mango tree and rambutan tree produce a substantial quantity of fruits for local consumption and for export.