Human Reproduction

Advantage of sexual reproduction over asexual reproduction :

·       Sexual reproduction produces offspring with genetic variation

Advantage of asexual reproduction over sexual reproduction :

·       Faster

·       Offspring are genetically identical to each other

·       Offspring is suited to a constant environment

·       Offspring retain desired potential character

Disadvantage of asexual reproduction over sexual reproduction :

·       No genetic variation. Hence, offspring inherits disorders of parents

·       Unable to survive if the environment changes

·       Offspring cannot colonisenew environments

·       Overcrowding

·       All susceptible to same disease

Sperm :Large numbers of sperm are released to increase the chances of fertilisation as only a small percentage of sperms reach the point of fertilisation.

Structural differences between sperm and ovum :

·                Sperm have tail for swimming

·                Ovum is larger due to having more cytoplasm and a food store.

·                Sperm have an acrosome to secrete enzymes to digest the membrane and penetrate the ovum. This allows sperm nucleus to enter the ovum.

·                Sperms have mitochondria for energy release for swimming.

Structure of the Male Reproductive System :

·                Testis produces sperms. Testis produces the hormone testosterone thatcontrols male secondary sexual characteristics.

·                Epididymis stores sperms.

·                Sperm duct / Vas deferens transfers sperms to exterior.

·                The scrotum lies outside the body at lower temperature to favour sperm production and storage.

·                Prostate gland, Cowper’s gland and seminal vesicles add fluid to sperm to allow sperm to swim. The fluid has nutrients, such as, sugars, to provide energy for sperm. Chemicals in fluid activate sperm.

·                Erectile tissue fills with blood, allowing penis to be inserted into the vagina during copulation

·                Muscular contractions force semen / sperm along tube

Note :

·                Meiosis occurs in the seminiferous tubules of the testis. 

·                It is essential for the female to have separate reproductive and urinary passages as the urethra is too narrow for the baby, while the vagina (birth canal) is wide enough for the baby.

·                Gametes are haploid to restore the diploid number on fertilisation.

Differences between male and female urethra :

·                Longer in males

·                Male urethra carries both urine and sperm / semen. Female urethra carries urine only.

Prostate Gland :

·                The prostate gland secretes fluid, which is added to sperm.

·                Fluid has chemicals (hormones) that activate sperm and make them swim.

·                Fluid transports sperm.

·                It adds nutrients, such as, sugars, to provide energy for sperm.

·                The fluid is alkaline and neutralisesthe acidity of the vaginal fluid.

Fertilisation :

·                The zygote passes to uterus along the Fallopian Tubes by cilia.

·                The endometrium is rich in blood vessels due to high levels of progesterone.

·                The zygote implants into the uterine lining.

·                It divides to form ball of cells.

·                The cells differentiate.

·                Some cells from placenta and attach to uterine lining.

·                Other cells become embryo and then the fetus.

Pregnancy :

The fetus is protected during pregnancy by:

·                Pelvic girdle cradles fetus

·                Abdominal wall has a muscular layer

·                Amniotic fluid acts as a shock absorber by distributing uneven pressure around the fetus equally. Hence, mechanical damage done to the fetus from bumps and other movements of the mother are less.

·                Mucus plug prevents pathogens from entering the uterus

·                Blood systems are not connected to one another, so no mixing of blood occurs. The fetus is protected from high pressure of mother’s blood

Progesterone maintains the thickness of the uterus lining and increases its vascularisation. It prevents the further production of FSH and thus, stops ovulation. At the end of pregnancy, the hormone, oxytocin initiates the rhythmic contractions of the uterine wall during birth.

PLACENTA

Adaptations for absorption of nutrients: The placenta acts as the site of exchange of substances between mother’s blood and fetal circulation by diffusion down concentration gradient.

·                Interlocking of maternal and fetal tissues and villi provide a large surface area in contact for diffusion to occur.

·                Rich blood supply in both maternal and fetal tissues

·                Very thin barrier between blood supplies allows diffusion to occur readily from one system to another down concentration gradient

·                Blood systems are not connected to each other, so no mixing of blood occurs.The fetus is protected from high pressure of mother’s blood.

·                The placenta is disc like to fit on to uterus wall

Functions :The placenta acts as the site of exchange of substances (especially, nutrients) between mother’s blood and fetal circulation by diffusion down concentration gradient.

·                Oxygen, glucose, amino acids, hormones, water, minerals, like iron and vitamins (A)arein higher concentration in maternal blood and are supplied to the fetus from the mother’s blood by diffusion down concentration gradient. Glucose and amino acids can diffuse through the thin walls as they are small molecules, and are thus, soluble. These are supplied to the fetus via umbilical vein.

·                Oxygen leaves maternal haemoglobin and passes into fetal haemoglobin via umbilical vein.

·                Carbon dioxide, water and ureaare removed from the fetal blood to the mother’s blood for excretion as they are in higher concentration in fetal blood. So, they diffuse down concentration gradient via umbilical artery.

·                Antibodies are supplied to the fetus to provide natural passive immunity to the fetus via umbilical vein.

·                It produces progesterone during pregnancy that maintains the uterine lining.

·                It acts as a barrier to pathogenic bacteria.

The umbilical vein carries oxygenated blood as blood is oxygenated in the placenta and it returns to the fetus from the placenta by the umbilical vein.

Amnion :

·                Amniotic fluid acts as a shock absorber by distributing uneven pressure around the fetus equally. Hence, mechanical damage done to the fetus from bumps and other movements of the mother are less.

·                The fluid allows the fetus to move around more easily.

Process of birth :

·                The head of the fetus lies download just above the cervix.

·                Rhythmic contractions of the uterine wall are initiated by the hormone, oxytocin, cause the amnion to rupture, called the breaking of the waters.

·                The cervix dilates.

·                More powerful uterine contractions expel the fetus head first through the vagina into the outside world.

·                The umbilical cord is then cut. The placenta is expelled as afterbirth.

Testosterone :Testosterone is produced by testis. It promotes sperm production and controls development of malesecondary sexual characteristics when hormone levels in blood increase, for example –

·                Deepening and breakingof the voice

·                Increaseddevelopment of muscles

·                Growth of pubic hair, chest hair and facial hair, like beard

·                Increase in size of reproductive organs (enlargement of penis)

·                Production of active sperm (male gamete) begins

·                Increase in length of arm and leg bones, causing shoulders to broaden and cause Growth Spurts

·                Increased awareness of opposite gender

·                Increased independence / confidence

·                Increased aggression due to a rise in the level of testosterone in the body

During 12 – 16 years of age, there is an increase in size and a change in body proportions, known as a “Growth Spurt”.

Oestrogen :When level of oestrogen in blood increases, the female secondary sexual characteristics are developed, for example –

·                Ovulation begins / Gamete (Ovum) production begins

·                Onset of menstrual cycle

·                Mammary glands (breasts) develop

·                Broadening of the pelvic girdle, causing hips to widen

·                Growth of hair in groins and armpits

·                Thicker layer of fat under the skin

Progesterone :

·                Progesterone maintain the thickness of the uterine lining, called the endometrium, during pregnancy.

·                Itcauses blood vessels to grow into the uterus lining, thus increasing its vascularisation. So, a drop in progesterone secretion stops the uterus wall from growing.

·                It prevents miscarriage / abortion by causing the endometrium from not breaking down and shedding with placenta. The fetus, then, dies.

·                It prevents further ovulation / implantation / by inhibitingFSH production.

·                It stimulates placental growth and allows the embryo to develop.

Time of secretion of Progesterone :

·                Rise in response to peak in oestrogen levels during ovulation by corpus luteum in ovary

·                During pregnancy by placenta

Note :

·                When progesterone levels fall, menstruation is caused. The drop of progesterone stimulates production and release of more FSH.

·                If progesterone is not secretedduring pregnancy, the endometrium breaks down, causing the fetus to abort. This is called a miscarriage.

Menstrual (Oestrous) Cycle :

·                FSH begins maturation of the ovum in the Graafianfollicle and stimulates the production of oestrogen.

·                Oestrogen concentration rises gradually and peaks ovulation.

·                Oestrogen repairs the uterus lining / causes regrowth of endometrium.

·                At ovulation, ovum is released from ovary into oviduct.

·                Rise in oestrogen stimulates pituitary to release (Luteinising Hormone) LH, which causes old follicle cells to form corpus luteum (yellow body) and release progesterone.

·                Progesterone causes blood vessels to grow into the uterus lining and maintain its thickness.

·                When progesterone levels fall, menstruation is caused as the endometrium and some blood are shed.

·                The drop of progesterone stimulates production and release of more FSH.

Note :Implantation occurs in the uterus lining if the ovum is fertilised. The zygote forms a ball of cells, called embryo. The uterus lining becomes more vascular and thicker.

Breast milk :

·                Breast milk, like colostrum contains all the materials needed by the baby in the correct proportions.

·                Milktransfers antibodies from mother to child.

·                Breast milk is normally uncontaminated by pathogens.

·                It forms mother – child bond.

Sex determination :

·                Sex is determined by the sex chromosomes X and Y.

·                The sex chromosomes of females are XX and that of males are XY.

·                Meiosis forms haploid gametes, having 23 chromosomes.

·                All ova have X chromosomes.

·                The sperm fertilisingthe ovum can have X or Y chromosome.

·                Gender of offspring depends on which sperm fertilizes the ovum.

·                So, there is a 1 : 1 chance.

CONTRACEPTION :

Contraceptive Pill :

·                It is a chemical method of contraception

·                The pills contain hormones, oestrogen and progesterone, that prevent ovulation.

·                So, no fertilisation takes place.

·                The hormones increasesviscousity of the mucus at the cervix.

·                The pH changes.

·                These make the endometrium less receptive.

Advantages of contraceptive pill :

·                reliable

·                easy to use

Disadvantages of contraceptive pill :

·                could forget to take it

·                problems occur with blood clotting

Condoms :

·                They prevent the sperms from reaching the ovum.

·                They are placed over the penis before sexual intercourse, so that the body fluids of the male and the female do not mix during sexual intercourse.

·                A femidom is a cap that lines the vagina, covering the end of the uterus.

·                HIV Virus is found in body fluids, like semen.

Advantages of condoms :

·                Reliable

·                Reduced risk of STDs, like AIDS

·                Temporary

Disadvantages of condoms :

·                continuous cost

·                can split and leak

·                affects spontaneity

Sterilisation :Sterilisation / Vasectomy is an operation where the sperm ducts are cut and tied. Sperm cannot pass along the sperm duct and are not deposited in the vagina. Sperm do not reach the ovum. So, fertilisation is impossible.

Advantages of sterilisation:

·                100 % effective

·                Permanent

·                Does not interfere with intercourse

Disadvantages of sterilisation:

·                Irreversible

·                Carries risk of infection

·                It involves an operation, thus, specialist medical expertise is needed

·                Medical intervention is needed

·                Expensive

·                No protection against STDs

GROWTH

Definition :An increase in mass, size, complexity. The organism becomes more efficient.

Growth Patterns

·                First year – very rapid growth

·                Infancy (1 – 4) – steady growth

·                Childhood (5 – 10/12) – slow growth

·                Puberty – very rapid growth

·                Increase in size / maturing of reproductive organs

·                Maximum height by about 20

Changes that take place in the body of a newborn after birth to allow it to survive :

·                Lungs must become functional by inflating. They begin to breathe as there is no blood supply from placenta anymore.

·                The baby must regulate heat loss and control body temperature by beginning to sweat.

·                The baby must learn to suckle to obtain as nutrients as longer available via the placenta.

·                Peristalsis must occur.

·                The digestive system must begin to function.

·                Eye opens to gain information / respond to new stimuli

·                The liver becomes functional as the body excretes waste products of metabolism, like urea

·                Kidneys become functional.

·                Egestion begins.

Growth during 0 to 3 years of age :

·                Teeth appear.

·                The child eats solid food, along with milk. It does not have milk only.

·                The child learns to walk and becomes mobile.

·                It learns to balance.

·                The limbs grow rapidly.

·                The child talks. Skills of speech and communication develop.

·                Rapid growth occurs.

·                The child is more aware of its surroundings as vision becomes more acute.

Factors affecting growth

·                Diet

·                Health / Illness / Disease

·                Hormones

·                Genetic

Babies of same age may be of different mass due to :

·                Different amounts of food / diet of babies

·                Different gender

·                Genetic differences

·                Health

·                Different metabolic rates

·                Different birth masses

·                Mother’s actions during pregnancy

Note :There is a drop in the mass of the baby just after birth as losses by excretion exceed intake by feeding