iGCSE Biology Notes Reproduction

[q] Asexual reproduction

[a] 

○ Process resulting in the production of genetically identical offspring from one parent
○ Examples are binary fission, budding(using bulbs, tubers and runners)

[a] 

■ Population can be increased rapidly
■ Can exploit suitable environments quickly
■ More time and energy efficient

[a] 

■ Limited genetic variation in population
■ Population is vulnerable to changes in conditions and may only be suited for one habita
■ Disease is likely to affect the whole population as there is no genetic variation

[q] Sexual reproduction

[a] 

○ Sexual reproduction as a process involving the fusion of the nuclei of two gametes to form a zygote and the production of offspring that are genetically different from each other
○ Fertilisation is the fusion of gamete nuclei
○ Nuclei of gametes are haploid
○ The nucleus of a zygote is diploid

[a] 

■ Increases genetic variation
■ The species can adapt to new environments due to variation, giving them a survival advantage
■ Disease is less likely to affect population

[a] 

■ Takes time and energy to find mates
■ Difficult for isolated members of the species to reproduce

[q] Sexual reproduction in plants

[a] 

○ Flowers are the reproductive organ of the plant
○ They usually contain both male and female reproductive parts
○ Plants produce which contains a nucleus inside that is the male gamete
○ Unlike the male gamete in humans, pollen is not capable of locomotion
■ So plants have mechanisms to transfer pollen
○ Transfer of pollen is known as pollination
■ Transferred by insects (left)
■ Transferred by wind (right)

[a]

[a]

[q] Pollen

[a] 

○ Insect pollinated flowers produce smaller amounts of larger, heavier pollen grains that often contain spines or hooks on the outside so that they are able to stick better
○ Wind pollinated produce large amount of small, lightweight pollen grains that are usually smooth

[q] Cross-Pollination

[a] 

Cross-Pollination occurs when the pollen from one plant is transferred to the stigma of another plant of the same species
■ This is the way most plants carry out pollination as it improves genetic variation
■ Bees are common pollinators

[q] Self pollination

[a] 

Self pollination is the transfer of pollen grains from the anther of a flower to the stigma of a flower on a different plant of the same species
■ Reduce genetic variance

[q] Sepal

[a] Protects unopened flower

[q] Petals

[a] Brightly coloured in insect-pollinated flowers to attract insects

[q] Anther

[a] Produces and releases the male sex cell (pollen grain)

[q] Stigma

[a] Top of the female part of the flower which collects pollen grains

[q] Ovary

[a] Produces the female sex cell (Ovum)

[q] Ovule

[a] Contains the female sex cells (found inside the ovary)

[q] Fertilisation

[a] 

○ Occurs when a pollen nucleus fuses with a nucleus in an ovule
○ As the pollen have to grow a pollen tube
■ Only happens if the pollen grain has landed on the same species as the flower
○ The nucleus inside the pollen grain slips down the tube as it grows down the style towards the ovary
○ The ovary contains one or more ovules which each contain an ovum with a female nucleus that a malle pollen nucleus fuses with
○ As soon as the ovule is fertilized an zygote (diploid) is formed
○ The zygote will start to divide and eventually form a seed within the ovule
■ Fruits develop from the ovary, seeds are in the fruit

[q] Germination

[a] 

○ Start of the growth in the seedc
○ Factors that affect germination
■ Water – Allows the seed to swell up and the enzymes in the embryo to start working so that growth can occur
■ Oxygen – So that energy can be released for germination
■ Warmth – Germination improves as temperature rises as the reactions which take place are controlled by enzymes

[a]

[a] Produces fluid called semen that provide sperm cells with nutrients

[a] Sperm passes through the sperm duct to be mixed with fluids produced by the glands before being passed into the urethra for ejaculation

[a] Tube running down the centre of the penis that can carry out urine or semem, a ring of muscle in the urethra prevents the urine and semen from mixing

[a] Produces sperm the male gamete and testosterone

[a] Sac supporting the testes outside the body to ensure sperm are kept at temperature slightly lower than body temperature

[a] Passes urine out of the body from the bladder and allows semen to pass into the vagina of a woman during sexual intercourse

[a]

[a] Connects the ovary to the uterus

[a] Contains ova which will mature and develop when hormones are released

[a] Muscular bag with a soft lining where the fertilised egg will be implanted to develop into a foetus

[a] Ring of muscle at the lower end of the uterus to keep the developing foetus in place during pregnancy

[a] Muscular tube that leads to the inside of the woman’s body, where the male’s penis will enter during sexual intercouse and sperm are deposited

[q] Fertilisation

[a] 

○ Fusion of the nuclei from a male gamete and a female gamete
○ Occurs in the oviduct
○ Gametes have adaptations to increase the chances of fertilisation and successful development of an embryo
○ Sperm cells (45 μm)
■ Produced every day in huge numbers
■ Has a flagellum (tail for swimming)
■ Contains enzymes in the head region (acrosome)
● To digest the outer coating of the egg cell
■ Contain many mitochondria
● Provide respiration so that the flagellum can move back and forth for locomotion
○ Egg cell (0.2mm)
■ Thousands of immature eggs in each ovary, but only one released each month
■ Cytoplasm containing a store of energy
● Provides energy for dividing zygote after fertilisation
● Jelly like coating that changes after fertilisation
○ Forms an impenetrable barrier after fertilisation to prevent other sperm nuclei entering the egg cell

[q] SPERM CELL

[a]

[q] EGG CELL

[a]

[q] Pregnancy & Birth

[a] 

○ After fertilisation in the oviduct, the zygote travels towards the uterus
■ Takes about 3 days
■ During this time, the cells divide several times to form a ball of cells known as a embryo
○ In the uterus, the embryo embeds itself in the thick lining (implantation) and continues to grow and develop
○ Gestation period for humans is 9 months
○ Major organs develop in the first 12 weeks
■ The placenta forms about now
○ The embryo is not called a fetus
○ The fetus is surrounded by an amniotic sac which contains amniotic fluid
■ Protects fetus by cushioning it
○ The umbilical cord joins the fetus’s blood supply to the placenta for exchange of nutrients and removal of waste products

[q] Placenta

[a] 

○ During the gestation period the fetus develops and grows gaining the glucose, amino acids, fats,water and oxygen
○ The bloods run opposite each other, never mixing, in the placenta
○ The mother’s blood absorbs the waste from the fetus’s blood in the placenta
○ Movement of all molecules across the placenta occurs by diffusion
■ Placenta is adapted by having a large a surface area and a thin walls
○ The placenta also acts as a barrier to prevent toxins and pathogens getting into the fetus’s blood
■ Not all toxin molecules and pathogenic organisms are stopped
● Rubella
● Nicotine (should not smoke during pregnancy)
○ The placenta detaches from the uterus wall and is pushed out due to contractions in the muscular wall of the uterus – known as the afterbirth

[q] Stages of Birth

[a] 

○ Amniotic sac brakes
○ Muscles in the uterus wall contract
○ Cervix dilates
○ Baby passes out through the vagina
○ Umbilical cord is tied and cut
○ Afterbirth

[q] Antenatal care (before birth care)

[a] 

○ Diet should consist of folic acid to prevent developmental issues with fetus and the importance of a balanced diet
○ Exercise to stay fit
○ Health precautions such as avoiding infections, tobacco, alcohol and other drugs

[q] Breastfeeding

[a] 

○ Mammary glands enlarge and become prepared to secrete milk
○ Shortly after birth, the mother will be stimulated to release milk due to the sucking action of the baby at the breast
■ Free
■ Helps develop bond between mother and baby
■ Contains antibodies which help prevent infections in babies
■ Contains the exact amounts of nutrients required by the baby at different stages
■ Can cause postnatal depression as it does not come easily to mothers
○ Some mothers use formula milk in a bottle instead
■ Risk of infection increases
■ Expensive
■ Allows more more freedom
■ Allows father to bond with baby

[q] Sex hormones in humans

Secondary sexual characteristics

Changes that occur in puberty

[a] 

■ Controlled by testosterone (in boys) and oestrogen (in girls)
■ Examples body hair growth (occur in both)

[q] Male sexual characteristics

[a] 

■ Growth of penis & testes
■ Growth of facial & body hair
■ Muscles develop
■ Voice breaks
■ Testes start to produce sperm

[q] Female sexual characteristics

[a] 

■ Breasts develop
■ Body hair grows
■ Menstrual cycle begins
■ HIps get wider

[q] Menstrual cycle

[a] 

○ Starts at around age 12 controlled by hormones
○ Average menstrual cycle is 28 days longs
○ Ovulation (the release of an egg) occurs about halfway through the cycle (day 14) and the egg travels down the oviduct to the uterus
○ Failure to fertilise the egg causes menstruation to occur
■ Causes breakdown of 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

[q] Hormonal control of the menstrual cycle

[a] 

○ Controlled by hormones released from ovary and pituitary gland in the brain
○ Oestrogen levels rise on day 1 to peak just before day 14 (before ovulation)
■ Stimulates the uterus to develop a lining
■ Inhibits FSH and lH production post ovulation

[q] Progesterone starts to rise after ovulation has occurred

[a] 

■ Maintains and thickens lining of the uterus
■ Inhibits FSH and LH production
■ If fertilisation does not occur, the levels drop and menstruation occurs

[q] FSH (follicle stimulating hormone) causes an egg to start to mature in the ovary

[a] 

■ Produced in the pituitary gland
■ Stays about the same throughout the menstrual cycle

[q] LH (Luteinizing hormone)

[a] Causes ovulation to occur and stimulates ovary to produce progesterone

[q] How all 4 hormones- Oestrogen, Progesterone, FSH Follicle Stimulating Hormone, LH Luteinizing Hormone, work together?

[a] 

○ The FSH 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 the production of FSH
○ When oestrogen rises to a high enough level it stimulates the release of LH from the pituitary gland which causes ovulation
○ The follicle becomes the corpus luteum and starts producing progesterone
○ If the ovum is not fertilised, the corpus luteum breaks down and stops producing progesterone
○ This is where menstruation begins, the uterus lining breaks down and is removed through the vagina
○ If the ovum is fertilised the corpus luteum keeps producing progesterone until the placenta has developed
■ Placenta takes over the secretion of progesterone

[q] Birth controls

[a] 

○ Important to limit increase in human population
○ Prevent STI (Sexually transmitted infections) and STD
■ Unprotectyed sexual intercouse can lead to the transfer of pathogens via exchange of body fluids
■ Infections passed on in this way are known as STIs
● HIV (Human immunodeficiency virus), the virus that usually leads to the development of AIDS (acquired immunodeficiency virus)
○ HIV can also spread via sharing needles with an infected person
○ From mother to fetus through placenta or breastfeeding
■ Controlled by limiting the number of sexual partners
● Not having unprotected sex
● Getting tested
● Raising awareness

[q] How HIV works

[a] 

■ Immediately after infection people suffer from flu-like symptoms
■ These symptoms pass and for a period of time infected people might not know they are infected
■ The viruses affects lymphocytes
● They are cells of the immune system
■ HIV avoids being recognised and destroyed by lymphocytes by repeatedly changing its protein coat
■ Uses the lymphocytes to increase in number
● Reduces the number of lymphocytes and number of antibodies
■ Making the body immuno-compromised

[q] Natural Birth control

[a] 

■ Abstinence
● avoding of sexual intercouse
■ Rhythm
● Avoiding sexual intercouse during fertile period

[q] Chemical Birth Control

IUD.IUS

[a] 

● Intrauterine device or intrauterine system is a small device fitted inside the uterus by a doctor or nurse
● Release hormones to thicken the mucus produced in the cervix making it difficult for sperm to swim into the uterus

[a] 

● May contain a mixture of progesterone and oestrogen
● Mimic hormone level during pregnancy, the uterus lining is maintained and development of anther egg cell is prevented

[a] Cream that kills sperm

[q] Barrier Birth Control

Barrier

[a] Prevent sperm from reaching egg

[a] 

● Latex sheat worn over the eggs
● Prevents sperm entering the vagina as ejaculate remains in condoms
● Also protects against STIs

[a] 

● Latex sheath inserted into the vagina
● Prevents entry of sperm into the vagina

[a] Rubber caps that fit over the entrance to the cervix

[q] Surgical Birth Control

Vasectomy

[a] 

● Sperm ducts are cut so no sperm present in semen
● Very effective difficult to reverse

[q] Surgical Birth Control

Female sterilisation

[a] 

● The oviducts are cut or tied off
● Difficult to reverse

[q] Fertility treatments

[a] Treatments for people finding it difficult to conceive

[q] Fertility treatments

[a] 

● If the male is not producing healthy sperm, donor sperm can be used
● The sperm are placed into the female’s vagina at the fertile poin in her menstrual cycle

[a] 

● Used when female is not producing enough eggs
● Hormones are given to stimulate egg production

[a] 

● If the female cannot conceive naturally even after taking fertility drugs or if there are issues with both male and female fertility in a couple, IVF is used
● The eggs are inseminated in a petri dish and once embryos have formed, they are placed back into the uterus of the female
● Several embryos are implanted to increase the chance of one developing further