IB Biology Topic B2.3.10: Adaptations of Sperm and Egg Cells Notes

Sperm and Egg Cells Are Shaped by Their Jobs: Delivery Versus Provisioning

1. Sperm and egg cells are gametes, the specialised haploid reproductive cells that fuse at fertilisation.
2. The sperm is built to travel and deliver DNA, while the egg is built to provision and protect the new life.

Sperm Adaptations: A Stripped-Down Cell Built to Swim

1. Flagellum (Tail) for Motility
   1. The long flagellum beats with a whip-like motion to propel the sperm forward.
   2. This lets the sperm swim through the female reproductive tract toward the egg.
2. Midpiece Packed with Mitochondria
   1. The midpiece is packed with mitochondria.
   2. These generate the ATP that powers the flagellum over the long swim.
3. Acrosome Filled with Hydrolytic Enzymes
   1. The acrosome is a cap over the head that holds hydrolytic (digestive) enzymes.
   2. These enzymes digest the egg's coatings so the sperm can reach the membrane.
4. Haploid Nucleus in a Streamlined Head
   1. The head carries a haploid nucleus with the father's single set of chromosomes.
   2. The cell has little cytoplasm, giving a light, streamlined shape that reduces drag.

Egg Adaptations: A Large Cell Built to Provision and Protect

1. Large Cell with Cytoplasm and Nutrient Stores
   1. The egg is a large cell with abundant cytoplasm and nutrient stores (yolk).
   2. These stores feed the early embryo before it can draw nutrients from the mother.
2. Many Mitochondria
   1. The egg holds many mitochondria in its large cytoplasm.
   2. These supply the ATP needed for the demands of early development.
3. Zona Pellucida (Glycoprotein Coat)
   1. The zona pellucida is a glycoprotein coat surrounding the egg.
   2. It carries the receptors a sperm must bind to before it can enter.
4. Cortical Granules That Block Polyspermy
   1. The cytoplasm holds cortical granules just beneath the membrane.
   2. When the first sperm enters, they release their contents in the cortical reaction.
   3. This hardens the zona pellucida so no further sperm can bind, blocking polyspermy.
5. Haploid Nucleus
   1. The egg carries a haploid nucleus with the mother's single set of chromosomes.
   2. It combines with the sperm's nucleus to restore the diploid number.

Sperm Versus Egg: The Same Genes, Opposite Designs

1. Size and shape: sperm are small and streamlined for swimming, while eggs are large with cytoplasm and nutrient stores.
2. Mitochondria: sperm concentrate them in the midpiece to power the flagellum, while eggs hold many to fuel early development.
3. Genetic material: both are haploid, each supplying one set of chromosomes to the zygote.
4. Specialisation: sperm are built for delivery, while eggs are built for provisioning and protection.

Quick Recap Before You Test Yourself

1. The sperm is a delivery vehicle: flagellum, midpiece mitochondria, acrosome enzymes, haploid nucleus.
2. The egg is a provisioner: large size, nutrient stores, many mitochondria, zona pellucida, cortical granules, haploid nucleus.