Gametes: AS Biology

Gametes                                                                                                      

THE OVUM

·         Large cell incapable of movement

·         The cytoplasm contains protein and lipid food reserves for a developing embryo.

·         Surrounding the cell is a jelly-like coating called the zona pellucida, which is then surrounded by follicle cells from the ovary.                                                                                          

SPERM

·         Much smaller than ovum and is motile (can move)

·         To enable it to swim, the sperm cell has a long tail powered by energy released by mitochondria.

·         Sperm are attracted to the ovum by chemicals released from it.

·         To penetrate the ovum, the acrosome (type of lysosome) in the head of the sperm releases digestive enzymes, which break down the zona pellucida of the ovum.

Once a sperm fuses and penetrates the membrane surrounding the egg, chemicals released by the ovum cause the zona pellucida to thicken, preventing any further sperm entering the egg. The sperm nucleus and the egg nucleus fuse to produce a fertilised egg.  

PLANT GAMETES

The male part of the plant is the stamen, made up of the anther attached to the filament. Inside the anther, cells divide to produce pollen grains, which contain the male gamete nuclei.

Inside the female ovary, one or more ovules develop which contain the female gametes, the ova (singular ovum). 

Gametes contain half the full number of chromosomes, 23 in humans, made up of one of each homologous pair and one sex chromosome. When the gametes fuse, the full number of 46 is restored.