**16b. Explain how fertilization occurs in humans.**

Fertilization is the process by which a sperm cell fuses with an egg cell to form a zygote, which is the first cell of a new individual. In humans, fertilization occurs in the fallopian tubes, also known as the oviducts.

**Process of Fertilization:**

1. **Sperm Transport:** After ejaculation, millions of sperm cells are released into the vagina. These sperm cells swim through the cervix, uterus, and into the fallopian tubes. The journey may take several hours, and only a small fraction of the sperm cells reach the site of fertilization.

2. **Sperm Activation:** Once in the fallopian tubes, the sperm cells undergo a process called capacitation. This process involves the removal of certain proteins from the sperm's surface, allowing it to swim faster and become capable of fertilizing an egg.

3. **Egg Release:** During ovulation, a mature egg is released from the ovary and enters the fallopian tube. The egg is surrounded by a protective layer called the zona pellucida.

4. **Sperm-Egg Interaction:** When a sperm cell encounters the egg, it binds to the zona pellucida through specific receptor molecules on its surface. This binding triggers a series of biochemical reactions that prepare the sperm for penetration.

5. **Acrosome Reaction:** The acrosome, a structure at the tip of the sperm, releases enzymes that help the sperm penetrate the zona pellucida. The enzymes break down the protective layer, allowing the sperm to reach the surface of the egg.

6. **Fusion of Egg and Sperm:** Once the sperm reaches the surface of the egg, it binds to the egg's plasma membrane. The membranes of the sperm and egg then fuse, allowing the sperm nucleus to enter the egg cytoplasm.

7. **Formation of Zygote:** Inside the egg, the genetic material of the sperm combines with the genetic material of the egg, forming a zygote. The zygote contains the complete set of chromosomes required for the development of a new individual.

8. **Implantation:** After fertilization, the zygote undergoes cell division and travels towards the uterus. It implants itself into the uterine wall, where it further develops into an embryo.

In summary, fertilization in humans involves the transport of sperm to the fallopian tubes, activation of the sperm, interaction and fusion of the sperm and egg, and the formation of a zygote. This process is crucial for sexual reproduction and the creation of new life.