Final answer:
Phytohemagglutinin is added to a cell culture in karyotyping to promote cell growth, allowing cells to reach metaphase where chromosomes are visible for examination.
Step-by-step explanation:
The purpose of adding phytohemagglutinin to a cell culture when preparing cells for karyotyping is to promote cell growth. Phytohemagglutinin is a mitogen that stimulates cell division. In karyotyping, it is essential to have cells in metaphase of cell division, where the chromosomes are most visible under a microscope.
After the cells have divided, other chemicals like colchicine are used to stop cell division so the chromosomes can be examined.