2. What is fertilisation? Give examples

Fertilization is the process in  reproduction where male and female gametes (reproductive cells) fuse to form a zygote, which eventually develops into a new individual. In most organisms, fertilization involves the fusion of a sperm cell (male gamete) with an egg cell (female gamete), resulting in the formation of a diploid zygote with a complete set of chromosomes. Fertilization typically occurs following the fusion of gametes during  intercourse or reproductive interactions.

Examples of fertilization in different organisms include:

1. Humans: In humans, fertilization occurs when a sperm cell from the male reproductive system (produced in the testes) fuses with an egg cell (ovum) from the female reproductive system (released from the ovary during ovulation) in the fallopian tube. The fusion of sperm and egg forms a zygote, which undergoes further development and implantation in the uterus, leading to pregnancy.

2. Plants: In flowering plants, fertilization involves the fusion of a pollen grain (carrying male gametes) with an ovule (containing female gametes) within the ovary of a flower. Pollination, the transfer of pollen from the male reproductive organ (stamen) to the female reproductive organ (pistil) of a flower, precedes fertilization. Once a pollen grain reaches the stigma of a compatible flower, it germinates and forms a pollen tube, through which sperm cells travel to the ovule for fertilization.

3. Fish: In many fish species, external fertilization occurs where female fish release eggs (ova) into the water, and male fish release sperm (milt) to fertilize the eggs externally. The sperm and eggs meet in the water column, and fertilization occurs externally. This is common in fish species such as salmon and trout.

4. Frogs: Amphibians like frogs typically undergo external fertilization. During mating, a male frog releases sperm (in the form of sperm packets called spermatophores) onto the eggs released by the female frog. Fertilization occurs externally, often in water bodies like ponds or streams where the eggs are laid.

5. Invertebrates: Many invertebrates, including sea urchins, insects, and some marine invertebrates, undergo internal fertilization. In these organisms, sperm is deposited directly into the female's reproductive tract, where it fertilizes the eggs.

These examples demonstrate the diversity of fertilization processes across different organisms, each adapted to suit the reproductive strategies and environmental conditions of the species involved.