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IGCSE Biology Theory Unit 3 Full Class Notes
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Full class notes for IGCSE Biology Unit 3 from the Pearson Edexcel syllabus. Unit 3: Reproduction and Inheritance Sub-topics: a) Reproduction b) Inheritance
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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
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