Cambridge O Level Biology PDF

Head to www.savemyexams.com for more awesome resources Cambridge O Level Biology Your notes 16.4 Sexual Reproduction in Humans Contents The Reproductive Systems Gametes & Fertilisation Development of the Zygote Sex...

Head to www.savemyexams.com for more awesome resources Cambridge O Level Biology Your notes 16.4 Sexual Reproduction in Humans Contents The Reproductive Systems Gametes & Fertilisation Development of the Zygote Sexual Hormones in Humans Page 1 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The Reproductive Systems Your notes The Male Reproductive System The male reproductive system Male reproductive structures and their function table Page 2 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Structure Function Your notes Produces fluid called semen that provides sperm cells with Prostate gland nutrients Sperm passes through the sperm duct to be mixed with fluids Sperm Duct produced by the glands before being passed into the urethra for ejaculation Tube running down the centre of the penis that can carry out urine Urethra or semen, a ring of muscle in the urethra prevents the urine and semen from mixing Testis Contained in a bag of skin (scrotum) and produces sperm (male gamete) and testosterone (hormone) Sac supporting the testes outside the body to ensure sperm are Scrotum kept at temperature slightly lower than body temperature Passes urine out of the body from the bladder and allows semen Penis to pass into the vagina of a woman during sexual intercourse Page 3 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The Female Reproductive System Your notes The female reproductive system Female reproductive structures and their function table Structure Function Connects the ovary to the uterus and is lined with ciliated cells to push Oviduct the released ovum down it. Fertilisation occurs here Contains ova (female gametes) which will mature and develop when Ovary hormones are released Muscular bag with a soft lining where the fertilised egg (zygote) will be Uterus implanted to develop into a foetus Ring of muscle at the lower end of the uterus to keep the developing Cervix foetus in place during pregnancy Muscular tube that leads to the inside of the woman’s body, where Vagina the male’s penis will enter during sexual intercourse and sperm are deposited Page 4 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Gametes & Fertilisation Your notes Gametes & Fertilisation Fertilisation is the fusion of the nuclei from a male gamete (sperm cell) and a female gamete (egg cell) It occurs in the oviducts This results in the formation of a zygote which will undergo cell division and grow to form an embryo Gametes have adaptations to increase the chances of fertilisation and successful development of an embryo Page 5 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Adaptations of Gametes Your notes Gamete Adaptive feature Reason Has a flagellum (tail) Enables it to swim to the egg To digest through the jelly coat and Contains enzymes in the head Sperm cell membrane of an egg cell when it region (acrosome) meets one Provide energy from respiration so Contains many mitochondria that the flagellum can move back and forth for locomotion Cytoplasm containing a store of Provides energy for the dividing energy zygote after fertilisation Egg Forms an impenetrable barrier after Jelly-like coating that changes fertilisation to prevent other sperm after fertilisation nuclei entering the egg cell Comparison of Male & Female Gametes Table Sperm Egg Size Very small (45µm) Large (0.2mm) Round cell with few structural Head region and flagellum, many Structure adaptations, covered in a jelly structural adaptations coating Motility Capable of locomotion Not capable of locomotion Produced every day in huge Thousands of immature eggs in Numbers numbers (around 100 million per each ovary, but only one released day) each month Page 6 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Comparing sperm and egg cells Page 7 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Development of the Zygote Your notes Development of the Zygote After fertilisation in the oviduct, the zygote travels towards the uterus This takes about 3 days, during which time the zygote will divide several times to form a ball of cells known as an embryo In the uterus, the embryo embeds itself in the thick lining (implantation) and continues to grow and develop The gestation period for humans is 9 months Major development of organs takes place within the first 12 weeks, during which time the embryo gets nutrients from the mother by diffusion through the uterus lining After this point, the organs are all in place, the placenta has formed and the embryo is now called a fetus The remaining gestation time is used by the fetus to grow bigger in size Page 8 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The foetus in the uterus The fetus is surrounded by an amniotic sac which contains amniotic fluid (made from the mother’s blood plasma) This protects the fetus during development by cushioning it from bumps to the mother’s abdomen The umbilical cord joins the fetus’s blood supply to the placenta for exchange of nutrients and removal of waste products Page 9 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Virus Transmission Across the Placenta During the gestation period the fetus develops and grows by gaining the glucose, amino acids, fats, Your notes water and oxygen it needs from the mother’s blood The blood runs opposite each other, never mixing, in the placenta The fetus’s blood connects to and from the placenta by the umbilical cord The mother’s blood also absorbs the waste from the fetus’s blood in the placenta; substances like carbon dioxide and urea are removed from the fetus’s blood so that they do not build up to dangerous levels Movement of all molecules across the placenta occurs by diffusion due to differences in concentration gradients The placenta is adapted for this diffusion by having a large surface area and a thin wall for efficient diffusion The placenta acts as a barrier to prevent toxins and pathogens from getting into the fetus’s blood Not all toxin molecules or pathogenic organisms are stopped from passing through the placenta (this usually depends on the size of the molecule) This is why pregnant women are advised not to smoke during pregnancy, as molecules like nicotine can pass across the placenta Certain viruses are small enough to cross the placenta and reach the bloodstream of the fetus Examples of such viruses include rubella and HIV Once circulating in the blood of the fetus, these viruses can cause very serious health complications Rubella may lead to problems such as deafness, heart disease and eye problems HIV could be fatal to a foetus The structure of the placenta diagram Page 10 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Certain molecules and viruses are small enough to cross the placental barrier and reach the bloodstream of the fetus Examiner Tip You are not expected to know the structural details of the placenta but it is worth learning at least two specific substances that move in either direction across the placenta – this is a common exam question and non-specific answers such as ‘waste products’ and ‘nutrients’ will not get any marks! Page 11 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Sexual Hormones in Humans Your notes Secondary Sexual Characteristics Primary sexual characteristics are present during development in the uterus and are the differences in reproductive organs, etc between males and females Secondary sexual characteristics are the changes that occur during puberty as children become adolescents They are controlled by the release of hormones - oestrogen in girls and testosterone in boys Human secondary sexual characteristics Female secondary sexual characteristics table Female Effects of Oestrogen Development of breasts Body hair grows Menstrual cycle begins Hips get wider Male secondary sexual characteristics table Male Effects of Testosterone Page 12 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Growth of penis and testes Your notes Growth of facial and body hair Muscles develop Voice breaks Testes start to produce sperm Some changes occur to both boys and girls, including growth of sexual organs and growth of body hair Emotional changes also occur due to the increased levels of hormones in the body These include more interest in sex and increased mood swings Page 13 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The Menstrual Cycle Starts in early adolescence in girls (around age 12) and is controlled by hormones Your notes The average menstrual cycle is 28 days long Ovulation (the release of an egg) occurs about halfway through the cycle (day 14) and the egg then travels down the oviduct to the uterus Failure to fertilise the egg causes menstruation (commonly called a period) to occur - this is caused by the breakdown of the thickened lining of the uterus Menstruation lasts around 5 - 7 days and signals the beginning of the next cycle After menstruation finishes, the lining of the uterus starts to thicken again in preparation for possible implantation in the next cycle Changes in the lining of the uterus during the menstrual cycle The menstrual cycle is controlled by hormones released from the ovary and the pituitary gland in the brain Oestrogen levels rise from day 1 to peak just before day 14 This causes the uterine wall to start thickening and the egg to mature The peak in oestrogen occurs just before the egg is released Progesterone stays low from day 1 – 14 and starts to rise once ovulation has occurred Page 14 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The increasing levels cause the uterine lining to thicken further; a fall in progesterone levels causes the uterine lining to break down (menstruation / ‘period’) Your notes The roles of FSH and LH Changes in the levels of the pituitary hormones FSH and LH in the blood during the menstrual cycle Page 15 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources FSH (follicle-stimulating hormone) is released by the pituitary gland and causes an egg to start maturing in the ovary It also stimulates the ovaries to start releasing oestrogen Your notes The pituitary gland is stimulated to release luteinising hormone (LH) when oestrogen levels have reached their peak LH causes ovulation to occur and also stimulates the ovary to produce progesterone Page 16 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The roles of oestrogen and progesterone Your notes Changes in the levels of oestrogen and progesterone in the blood during the menstrual cycle Oestrogen levels rise from day 1 to peak just before day 14 This causes the uterine wall to start thickening and the egg to mature The peak in oestrogen occurs just before the egg is released Page 17 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Progesterone stays low from day 1 – 14 and starts to rise once ovulation has occurred The increasing levels cause the uterine lining to thicken further; a fall in progesterone levels causes the uterine lining to break down (menstruation / ‘period’) Your notes Interaction between all four of the menstrual cycle hormones The pituitary gland produces FSH which stimulates the development of a follicle in the ovary An egg develops inside the follicle and the follicle produces the hormone oestrogen Oestrogen causes growth and repair of the lining of the uterus wall and inhibits production of FSH When oestrogen rises to a high enough level it stimulates the release of LH from the pituitary gland which causes ovulation (usually around day 14 of the cycle) The follicle becomes the corpus luteum and starts producing progesterone Progesterone maintains the uterus lining (the thickness of the uterus wall) If the ovum is not fertilised, the corpus luteum breaks down and progesterone levels drop This causes menstruation, where the uterus lining breaks down and is removed through the vagina - commonly known as having a period If pregnancy does occur the corpus luteum continues to produce progesterone, preventing the uterus lining from breaking down and aborting the pregnancy It does this until the placenta has developed, at which point it starts secreting progesterone and continues to do so throughout the pregnancy Page 18 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Diagram showing where hormones involved in the menstrual cycle are made and how they interact Page 19 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers