Reproduction

Biology · D Continuity and change. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Asexual reproduction gives genetically identical offspring, which suits a stable environment. Sexual reproduction gives variation: meiosis makes haploid gametes with new combinations of alleles, and fertilization joins two of them. The male gamete is small and mobile. The female gamete is large and has food reserves.

In humans, pituitary hormones (FSH and LH) and ovarian hormones (oestradiol and progesterone) control the ovarian and uterine cycles by feedback. Fertilization happens in the oviduct, and the embryo implants in the endometrium of the uterus.

In flowering plants, pollination carries pollen from an anther to a stigma. Fertilization follows in the ovule, and the seed that forms is then dispersed.

Rules to remember

FSH: stimulates follicles to develop and to secrete oestradiol.
Oestradiol: thickens the endometrium. A high level before ovulation stimulates the LH surge (positive feedback).
LH surge at about day 14: causes ovulation. The empty follicle becomes the corpus luteum.
Progesterone from the corpus luteum: maintains the endometrium and inhibits FSH and LH (negative feedback). When it falls, menstruation begins.
Fertilization: the sperm nucleus enters the egg. The tail and mitochondria of the sperm are destroyed.
IVF: normal hormone secretion is suspended, then FSH is given to cause superovulation. Eggs are collected and fertilized outside the body.
Plants: anther makes pollen, stigma receives it. Self-incompatibility and different ripening times promote cross-pollination. Germination needs water, oxygen and warmth.

Common mistake

Students mix up pollination, fertilization and seed dispersal. Pollination moves pollen to a stigma, fertilization is the fusion of gamete nuclei in the ovule, and dispersal moves the seed away from the parent afterwards.

Worked example

A woman's pituitary gland secretes FSH at the start of a cycle but cannot produce an LH surge. Which events of the cycle still happen, and which do not?

FSH stimulates follicles to grow, and the follicles secrete oestradiol.
Oestradiol makes the endometrium thicken, so this still happens.
Ovulation needs the LH surge, so no egg is released.
With no ovulation there is no corpus luteum, so progesterone stays low.

Answer: Follicles develop and the endometrium thickens, but there is no ovulation and no rise in progesterone.

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D3.1 Reproduction

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