IGCSE Biology Theory Unit 3 Full Class Notes

REPRODUCTION IN HUMANS

 Ovulation – release of mature egg cell from ovaries to oviduct
 Menstruation – shedding of endometrium thru vagina from puberty
 Menopause – menstruation stopping at around age 50
 Implantation – fertilized egg cell/zygote/blastocyst gets embedded in uterus wall
 Copulation – placing erect penile urethra in female vagina during sexual intercourse
 Ejaculation – release of semen thru penis
 Erection – enlargement of blood vessels = ^ blood vol = causes erection of penis

FOUR MAIN STAGES OF SEXUAL REPRODUCTION

 Gametes (sperm and eggs) produced
 Male gamete transferred to female gamete
 Fertilization – sperm fuses w egg
 Zygote formed = develops into new individual

MALE GAMETE



 Sperm cell – specialized to swim
 Stream-lined in shape to reduce friction during movement
 Consists of mitochondria for energy for movement
 Head consists of enzymes to digest jelly coating around egg cell during fertilization
 Nucleus is haploid = half no. of chromosomes to maintain genetic stability after fertilization




FEMALE GAMETE

 Egg larger compared to sperm cell = ^ SA for sperm cell to reach
 Large storage of nutrients for energy for developing embryo till placenta forms
 Haploid nucleus for genetic stability
 Cytoplasm w granules of chemicals – released after fertilization into jelly coat
o Forms tough membrane which cant be digested by enzymes = prevents many sperm
cells entering egg

,MALE REPRODUCTIVE SYSTEM




 Testes – many coiled tubes producing sperm + testosterone
 Scrotum – supports testes outside body at lower temp cos ideal temp for production of sperm =
lower than body temp
 Sperm ducts – carries sperm from testes to urethra – also called vas deferens
 Prostate gland + seminal vesicle gland – produces seminal fluid to make up semen
o Nourishes sperm w sugars, proteins, salts + able to swim
 Urethra – tube running thru penis – carries semen or urine outside body
 Penis – allows semen to pass into vagina during intercourse + pass urine out of body

FEMALE REPRODUCTIVE SYSTEM

,  Ovaries – produces ova + female sex hormones – estrogen + progesterone
 Oviduct – fallopian tube, carries ovum to uterus
o Fertilization usually occurs in first one third of oviduct
 Uterus (womb) – where fetus develops + increases in volume during pregnancy
 Cervix – ring of muscle keeping fetus in uterus + relaxes to allow birth of baby
 Vagina – receives penis during intercourse
o Also called birth canal where baby comes out at birth

TRANSFER OF SPERM TO EGG

 Two types of fertilization:
 External fertilization
o Outside body (in most fish species and amphibians)
o Some mating behaviour ensures males and females in same place at same time
o Male fish release sperm into water + females release eggs into water
o Sperm swim thru water to fertilise eggs
 Internal fertilization
o Inside females body (birds + mammals)
o Male animal ejaculate sperm in special fluid into female body
o Fertilization ^ likely to happen cos no external factors preventing sperm reaching

PROCESS OF FERTILISATION IN HUMANS

 Internal fertilization in one of two oviducts (outer 1/3 distance)
 Semen ejaculated into vagina during sexual intercourse – swims towards oviduct
 Sperm cells move from vagina thru uuterus to oviduct
 If ovulation occurs at same time – fertilization can occur
 During fertilization:
o Acrosome (in sperm) releases enzymes
o Digests jelly coat of egg cell
o Sperm cell moves further + contacts w cell membrane + releasing male nucleus to egg
cell
o Male nucleus fuses w female nucleus
 After fertilization
o Diploid zygote formed

, o Tough fertilization membrane formed to prevent entry of more male nulcei
o Zygote formed will have full no. of chromosomes
o Restored diploid chromosome no + genetic variation cos different combo of genes

HUMAN LIFE CYCLE

IMPLANTATION

 Zygote formed begins to develop by mitosis into embryo as it travels down uterus
 Sinks + burrows into uterus lining

Three structures important for development of embryo – umbilical cord, placenta, amniotic sac + fluid

UMBILICAL CORD

 Connects fetus to placenta
 Contains arteries to deliver blood from fetus to placenta for material exchange
 One vein to deliver blood back from placenta to fetus

PLACENTA



 Has villi which burrows into uterus lining rich in blood supply
 Villi – thin, large SA to allow efficient material exchange btwn fetus’ blood + mothers blood via
diffusion
 No mixing of blood cos blood might be different + incompatible = blood clotting
 Function of placenta
o Allows exchange of material between mother
+ fetus w out blood mixing
o O2, water, amino acids, glucose, essential
minerals transferred by umbilical vein to
nourish fetus
o CO2, urea, and other wastes transferred by
umbilical artery to allow excretion and
prevent harmful accumulation
o Allows maternal antibodies to pass into fetus
to give immunity against disease
o Protects fetus by preventing pathogens + toxins crossing it
o Barrier to maternal hormones + other chemicals in mother’s blood
 But nicotine, alcohol, heroin can enter fetus causing lasting damage
o Permits both blood systems to operate at different pressures as not directly connected +
doesn’t harm mother/fetus
o Placenta takes over hormone production as pregnancy progresses = produces
progesterone to prevent ovulation + menstruation
o After implantation developing placenta secretes hormone HCG (Human Chorionic
Gonadotrophin)
 Presence in urine = basis of most pregnancy tests