Unit 4

• Male reproductive system structures

  • Scrotum
  • Testes
  • Epididymis
  • Vas deferens
  • Seminal vesicle
  • Prostate gland
  • Urethra
  • Penis

  
• Testes


  • Produce gametes (sperm formed by spermatogenesis)
  • Produce testosterone
• Epididymis


  Sperm are stored here and mature to become fully mobile
• Seminal vesicle


  Secretes a fluid into the vas deferens that contains a mixture of chemicals which make up approximately 60% of semen. Seminal fluid provides nutrients for sperm such as fructose for respiration and amino acids. Seminal fluid is alkaline which helps to neutralise the acidity of any urine remaining in the urethra and the acidity of the vaginal tract.
• Prostate gland


  Secretes a fluid into the vas deferens that contains a mixture of chemicals which make up approximately 30% of semen. Prostate fluid contains zinc ions and is also alkaline which helps to neutralise the acidity of any urine remaining in the urethra and the acidity of the vaginal tract.
• Urethra


  • Carries semen (a mixture of spermatozoa, seminal and prostate fluids) through the penis and out of the body
  • Carries urine from the bladder through the penis and out of the body
• Penis


  Specialised organ adapted to transfer semen to the vagina during sexual intercourse
• Female reproductive system structures

  • Ovary
  • Fallopian tubes (oviducts)
  • Uterus
  • Endometrium
  • Cervix
  • Vagina
• Ovary


  • Production of gametes (secondary oocytes formed during oogenesis)
  • Produce oestrogen and progesterone
• Fallopian tubes (oviducts)


  They have a lining of ciliated epithelial cells which move the secondary oocyte to the uterus
• Uterus


  Holds the developing foetus until birth
• Endometrium


  The inner most layer of the uterus wall. It has a good blood supply and builds up every month during the menstrual cycle. If implantation of an embryo does not happen then the endometrium is shed during menstruation.
• Cervix


  A narrow ring of connective tissue and muscle, it acts as a barrier between the uterus and the outside environment during pregnancy. During pregnancy, a mucous plug forms in the cervix which helps prevent entry of pathogens.
• Vagina


  It has muscular walls and opens at the vulva. Semen is deposited in the vagina during sexual intercourse and the foetus is able to pass out from the uterus through the vagina during birth.
• Spermatogenesis


  1. Spermatogonia divide many times by mitosis and increase in size to form primary spermatocytes

  2. Primary spermatocytes undergo meiosis to form secondary spermatocytes

  3. Secondary spermatocytes undergo maturation to form spermatids

  4. Spermatids mature into spermatozoa
• Sertoli cells


  • They provide nourishment for spermatids and protection against the male's immune system
• Interstitial cells (Leydig cells)


  Secrete testosterone which stimulates spermatogenesis
• Oogenesis


  1. Oogonia divide many times by mitosis and enlarge to produce large number of primary oocytes

  2. Primary oocytes undergo meiosis I to form secondary oocytes

  3. Secondary oocytes undergo meiosis II to form ova
• Follicle


  A ball of diploid cells surrounding an oocyte. The cells of the follicle develop and increase in numbers as the oocyte develops.
• Fertilisation


  1. Spermatozoa move into the fallopian tubes

  2. Capacitation increases the permeability of the cell membrane in the head of the sperm above the acrosome

  3. Acrosome reaction releases hydrolase enzymes which digest the zona pellucida

  4. Fusion of sperm and secondary oocyte membranes; genetic material of sperm cell enters the secondary oocyte triggering completion of meiosis II and formation of ovum and second polar body

  5. Cortical reaction modifies the zona pellucida to form the fertilisation membrane and prevent polyspermy

  6. Nuclei of the sperm and ovum fuse to form a zygotic nucleus
• Implantation


  1. The zygote undergoes repeated mitotic divisions as it moves down the oviduct to form a ball of cells called the blastocyst

  2. The blastocyst moves into the uterus where it attaches and sinks into the endometrium

  3. Trophoblast cells form trophoblastic villi that penetrate the endometrium

  4. The placenta begins to develop from the trophoblast cells
• Placenta


  Exchanges gases and nutrients, provides a barrier between maternal and foetal blood, secretes hormones
• Amniotic fluid


  Acts as a shock absorber, protecting the foetus from injury during development
• Menstrual cycle


  1. Anterior pituitary secretes FSH which stimulates follicle maturation

  2. Maturing follicle secretes oestrogen which stimulates LH production and inhibits FSH

  3. LH induces ovulation and stimulates corpus luteum formation

  4. Corpus luteum secretes progesterone which maintains endometrium

  5. Falling FSH and LH cause corpus luteum to degenerate, progesterone levels decrease, endometrium breaks down and is shed
• Pregnancy hormones


  1. Blastocyst secretes hCG which maintains corpus luteum

  2. Corpus luteum and placenta secrete oestrogen and progesterone which inhibit FSH and LH, stimulate uterine growth, and suppress uterine contractions

  3. As corpus luteum degenerates, placenta takes over oestrogen and progesterone secretion
• Birth hormones


  1. Oestrogen increases and progesterone decreases just before birth

  2. Oxytocin secretion stimulates uterine contractions

  3. Prolactin stimulates milk production
• Characteristics of wind-pollinated flowers


  • Small or no petals
  • Located above leaves or produced in early spring
  • Large, feathery stigmas
  • Large, suspended anthers
  • Vast numbers of small, light, smooth-walled pollen grains
• Characteristics of insect-pollinated flowers

  • Large, brightly coloured petals with nectar guides
  • Sticky stigmas inside the flower
  • Anthers on rigid filaments inside the flower
  • Large pollen grains with spiky walls
• Pollen development


  1. Diploid mother cell undergoes meiosis to form a tetrad of 4 immature haploid pollen cells

  2. Each haploid cell can undergo mitosis to form a pollen grain with two haploid nuclei
• Ovule development


  1. Mother cell undergoes meiosis to form 4 haploid megaspores

  2. One megaspore undergoes mitosis to form the embryo sac with 7 haploid nuclei (female gamete, two synergids, two polar nuclei, three antipodal cells)
• The ovule contains a mass of cells called the nucellus
• Pollen grains


  Contain the male gametes and are formed inside the pollen sacs
• Formation of Pollen


  1. Diploid mother cell (2n)

  2. Tetrad formed of 4 immature haploid pollen cells (n)

  3. Each haploid cell from the tetrad can undergo mitosis to form a pollen grain

  4. Pollen grain with two haploid nuclei (n)

  5. Generative nucleus (n) divides by mitosis to form 2 male nuclei (n)

  6. Tube nucleus (n)
• Pollen grain


  • Intine and exine cell wall prevents desiccation (drying out) of the pollen grain
• Development of an Ovule

  1. Nucellus surrounded by two protective integuments

  2. Funicle

  3. Megaspore mother cell (2n)

  4. Meiosis of mother cell produces four haploid megaspores (n)

  5. Three megaspores degenerate

  6. One megaspore develops into the embryo sac

  7. Three mitotic divisions occur

  8. Eight haploid nuclei (n) formed

  9. Two nuclei form polar nuclei (n)

  10. Three antipodal cells, one female gamete (n), two synergids
• Self-pollination


  Transfer of pollen from the anther of a flower to the mature stigma of the same flower or another flower on the same plant of the same species
• Cross-pollination


  Transfer of pollen from the anther of a flower to the mature stigma of another flower on another plant of the same species
• Self-pollination


  Leads to self-fertilisation which leads to inbreeding
• Cross-pollination

  Leads to cross-fertilisation which leads to outbreeding
• Genetic variation in self-pollination

  • Dependent on crossing over, independent assortment, mutation