Chapter 7 How Do Organisms Reproduce

Asexual reproduction involves only one parent, produces offspring that are genetically identical to the parent (clones), and does not involve fusion of gametes. It is fast, simple, and energy-efficient. Examples include binary fission in Amoeba, budding in Hydra and yeast, fragmentation in Planaria, and spore formation in Rhizopus (bread mould). Sexual reproduction involves two parents, requires formation and fusion of male and female gametes, and produces genetically diverse offspring. This diversity is important for adaptation and evolution. Sexual reproduction is more complex and energy-intensive. Examples include human reproduction, flowering plants, and most animals. The key advantage of sexual reproduction is genetic variation in the offspring, which increases the species' ability to survive changing environments.

Flowering plants reproduce sexually through their flowers. The flower contains male reproductive parts (stamens with pollen grains containing male gametes) and female reproductive parts (pistil with ovary containing ovules with female gametes). Pollination is the transfer of pollen from the anther to the stigma of the same or another flower, carried by wind, insects, water, or birds. After pollination, pollen germinates and a pollen tube grows down through the style to the ovary. The male gamete travels through the tube and fuses with the female gamete (egg) in the ovule — this is fertilisation. After fertilisation, the ovule develops into a seed containing the embryo, and the ovary wall develops into the fruit. The seed can germinate under suitable conditions to produce a new plant.

In animals, asexual reproduction occurs through binary fission (Amoeba and Paramecium split into two equal halves), budding (Hydra forms a small outgrowth that breaks off to form a new organism), and regeneration (Planaria can regenerate a whole organism from a cut piece due to cells called neoblasts). In plants, vegetative propagation is common — new plants form from roots (sweet potato), stems (potato tubers, ginger), or leaves (Bryophyllum). Budding also occurs in yeast. Spore formation is seen in fungi and lower plants — spores are light, can travel long distances, and germinate in suitable conditions. Tissue culture is a modern artificial technique where small pieces of plant tissue are grown in nutrient media under sterile conditions to produce many identical plants quickly.

Puberty is the stage when the body undergoes significant physical and hormonal changes that mark the beginning of reproductive maturity. In boys, testosterone secreted by the testes causes the voice to become deeper, growth of facial and body hair, increase in height, broadening of shoulders, and development of the testes and penis. Sperm production begins. In girls, oestrogen from the ovaries causes breast development, widening of hips, growth of pubic and underarm hair, and the beginning of the menstrual cycle. In both sexes, there is increased growth of body hair, development of sweat glands, and acne due to oil gland activity. Emotional changes also occur. Puberty typically begins earlier in girls (10–12 years) than in boys (12–14 years) and marks sexual maturation.

The menstrual cycle is a monthly cycle of hormonal and physical changes in females of reproductive age, lasting about 28 days on average. It prepares the body for possible pregnancy. During the first phase, the uterine lining (endometrium) breaks down and is shed as menstrual flow (menstruation), lasting 3–5 days. In the follicular phase, a follicle in the ovary matures and oestrogen causes the endometrium to rebuild. Around day 14, ovulation occurs — the mature egg is released from the ovary. During the luteal phase, the ruptured follicle forms the corpus luteum which secretes progesterone to maintain the endometrium. If the egg is not fertilised, the corpus luteum degenerates, progesterone levels fall, and the endometrium sheds again, beginning the next cycle. Pregnancy occurs if fertilisation takes place.