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Reproduction: Class 12 Biology
Asexual and sexual reproduction, the male and female reproductive systems, gametogenesis, the menstrual cycle, fertilisation, pregnancy, birth, the placenta, lactation, birth control, and reproduction in flowering plants.
Key points
Asexual reproduction involves one parent and no gametes, and the offspring are genetically identical clones. Examples are binary fission in bacteria, budding in yeast and Hydra, and vegetative propagation in plants. It is fast but gives no variation. Sexual reproduction joins two gametes from two parents, which gives variation, and this helps a species to adapt.
The human male system: the testes lie in the scrotum, which keeps them about 2 to 3 °C below body temperature, needed for sperm production. Sperm form in seminiferous tubules, mature in the epididymis and pass along the sperm duct (vas deferens). The seminal vesicles, prostate gland and Cowper's glands add fluid that nourishes sperm. The mixture is semen, and it leaves through the urethra in the penis. Interstitial cells between the tubules make testosterone.
Spermatogenesis takes place in the seminiferous tubules. Germ cells divide by mitosis, then a primary spermatocyte (2n) divides by meiosis I into two secondary spermatocytes (n), and meiosis II gives four spermatids (n). Each spermatid changes into a sperm with a head containing the nucleus and an acrosome with enzymes, a middle piece with mitochondria and a tail for swimming.
The female system: the ovaries make eggs and hormones. The oviducts (Fallopian tubes) carry the egg with cilia, and fertilisation takes place in the upper oviduct. The uterus has a muscular wall and an inner lining called the endometrium, where the embryo implants. The cervix leads to the vagina, which receives sperm and forms part of the birth canal.
Oogenesis starts before birth. A primary oocyte (2n) begins meiosis I but stops. At puberty, in each cycle, one completes meiosis I to give one large secondary oocyte and a small polar body. Meiosis II is completed only if a sperm enters, giving an ovum and a second polar body. Unequal division of the cytoplasm keeps food reserves in one large cell.
The menstrual cycle (about 28 days):
- Days 1 to 5, menstruation: the lining of the uterus breaks down and leaves with blood.
- Days 6 to 13, follicular phase: FSH from the pituitary makes a follicle grow and secrete oestrogen. Oestrogen repairs and thickens the endometrium.
- Day 14, ovulation: a surge of LH, triggered by high oestrogen, makes the follicle release the egg.
- Days 15 to 28, luteal phase: the empty follicle becomes the corpus luteum and secretes progesterone, which keeps the lining thick. If no pregnancy occurs the corpus luteum breaks down, hormones fall and menstruation begins again.
Fertilisation is the fusion of a sperm nucleus with the egg nucleus to form a diploid zygote. The acrosome enzymes digest the egg coat, and once one sperm enters, the egg membrane changes to block others. The zygote divides by mitosis (cleavage) while it moves down the oviduct, forms a ball of cells, and about a week after fertilisation it implants in the endometrium.
The placenta forms from embryonic and maternal tissue and is joined to the embryo by the umbilical cord. Oxygen, glucose, amino acids, antibodies and water pass to the fetus by diffusion, facilitated diffusion and active transport. Carbon dioxide and urea pass back. The bloods do not mix. The placenta also secretes progesterone and oestrogen after the first months. Pregnancy lasts about 38 weeks from fertilisation (about 40 weeks from the last period).
At birth the pituitary releases oxytocin, which causes strong contractions of the uterus. The cervix widens, the amniotic sac breaks, the baby is pushed out, and the placenta follows as the afterbirth. Prolactin then causes milk production, and suckling causes release of oxytocin for milk ejection. Early milk (colostrum) is rich in antibodies.
Birth control methods include barrier methods such as the condom, the contraceptive pill (oestrogen and progesterone that stop ovulation), the intrauterine device, natural methods based on the fertile period, and surgery such as vasectomy and tubal ligation. In vitro fertilisation (IVF) helps couples who cannot conceive, as eggs are fertilised in a dish and embryos are placed in the uterus.
In flowering plants the flower is the reproductive organ. The stamen (anther and filament) makes pollen grains, and the carpel (stigma, style and ovary) holds ovules. Pollen is transferred by wind or insects (pollination). The pollen grain grows a tube to the ovule. One male nucleus fuses with the egg to form the zygote, and the other fuses with the two polar nuclei to form the endosperm. This is double fertilisation. The ovule becomes the seed and the ovary becomes the fruit.
Study tip: Learn the menstrual cycle as a graph with four lines (FSH, LH, oestrogen, progesterone) and practise explaining each peak in order. Draw neat labelled diagrams of the male and female systems and a flower, since labelling marks are easy to earn.
Definitions and flashcards
Say the meaning first, then open the card to check.
Gamete
A haploid sex cell, such as a sperm or an egg.
Zygote
The diploid cell formed when a sperm and an egg fuse.
Spermatogenesis
The formation of sperm from germ cells in the testis.
Ovulation
The release of a secondary oocyte from the ovary.
Corpus luteum
The structure formed from the empty follicle that secretes progesterone.
Implantation
The attachment of the early embryo to the lining of the uterus.
Placenta
The organ in the uterus where materials are exchanged between mother and fetus.
Double fertilisation
The fusion of one male nucleus with the egg and another with the polar nuclei in a flowering plant.
Pollination
The transfer of pollen from an anther to a stigma.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
Why are the testes found outside the body? (2 marks)
Sperm production needs a temperature about 2 to 3 °C lower than the core body temperature. The scrotum hangs outside the abdomen and keeps the testes cooler.
State two differences between spermatogenesis and oogenesis. (2 marks)
Spermatogenesis gives four equal sperm from one cell and continues from puberty throughout life. Oogenesis gives one large ovum and polar bodies from one cell, starts before birth and each cycle releases only one egg.
Explain the role of hormones in the menstrual cycle. (3 marks)
FSH makes a follicle grow in the ovary and secrete oestrogen, which rebuilds the uterine lining. A peak of oestrogen triggers a surge of LH, which causes ovulation. LH then turns the follicle into the corpus luteum, which secretes progesterone to maintain the lining. If there is no pregnancy, progesterone and oestrogen fall, the lining breaks down and menstruation starts.
What are the functions of the placenta? (3 marks)
It supplies oxygen, glucose, amino acids, water and antibodies from the mother's blood to the fetus. It removes carbon dioxide and urea from the fetus. It acts as a barrier that keeps the two bloods separate and filters out many harmful substances, and it secretes progesterone and oestrogen to maintain the pregnancy.
What is double fertilisation in flowering plants? (3 marks)
The pollen tube carries two male nuclei into the embryo sac. One fuses with the egg nucleus to form the diploid zygote, which becomes the embryo. The other fuses with the two polar nuclei to form the triploid endosperm nucleus, which develops into the food store of the seed.
Describe how the contraceptive pill works and name one other method of birth control. (3 marks)
The pill contains oestrogen and progesterone. These hormones keep the blood level high enough to stop the pituitary releasing FSH and LH, so follicles do not develop and no egg is ovulated. Another method is the condom, a barrier that stops sperm reaching the egg and also reduces the spread of infections.
Common mistakes
- Saying fertilisation takes place in the uterus. It takes place in the upper oviduct.
- Writing that the fetal and maternal blood mix in the placenta. They stay separate and exchange by diffusion across membranes.
- Giving the wrong hormone roles, for example saying LH is released by the ovary. FSH and LH come from the anterior pituitary.
- Forgetting that meiosis II of the egg is completed only after sperm entry.
- Mixing up pollination (transfer of pollen) with fertilisation (fusion of nuclei).