Introduction:
Dimethyl sulfoxide (DMSO) is a commonly used cryopreservant for mammalian cell cultures. It is a highly polar organic solvent that has several properties that make it suitable for cryopreservation.

Explanation:
DMSO is an effective cryopreservant for mammalian cell cultures primarily because it protects cells by preventing crystallization of water. When cells are frozen, the water inside them can form ice crystals, which can cause damage to the cells. DMSO acts as a cryoprotectant by lowering the freezing point of water, thus preventing the formation of ice crystals.

Key Points:
- Organic Solvent: DMSO is an organic solvent that is highly polar and has the ability to dissolve both polar and nonpolar compounds. This property allows it to easily penetrate cell membranes and enter the cells.

- Penetrates Cells: DMSO has a low molecular weight and is highly soluble in both water and lipids. These properties enable it to easily penetrate cell membranes and enter the cells. Once inside the cells, it can protect them from the damaging effects of freezing.

- Prevents Crystallization of Water: DMSO acts as a cryoprotectant by preventing the crystallization of water during freezing. It does this by forming hydrogen bonds with water molecules, which disrupts the formation of ice crystals. This helps to maintain the integrity of the cells during the freezing process.

- Cryopreservation: Cryopreservation is the process of freezing cells or tissues at extremely low temperatures to preserve them for long-term storage. DMSO is commonly used as a cryopreservant for mammalian cell cultures because of its ability to protect cells from damage during freezing.

Conclusion:
DMSO is used as a cryopreservant for mammalian cell cultures primarily because it protects cells by preventing the crystallization of water. Its properties as an organic solvent and its ability to easily penetrate cells contribute to its effectiveness as a cryoprotectant. By understanding the role of DMSO in cryopreservation, researchers can ensure the successful preservation of mammalian cell cultures for various applications in the field of biotechnology and medicine.