Freezing presents a paradox in biology, capable of both destruction and preservation. Sub-zero temperatures can damage cellular structures, yet many organisms have adapted to endure them. Scientists use this understanding to devise methods for long-term biological storage. This highlights the complex interplay of temperature, water, and life.
The Effects of Freezing on Living Systems
When biological systems freeze, ice crystal formation threatens cells. Water solidifies, forming extracellular ice. This draws water out of cells via osmosis, causing dehydration and increased solute concentration. Osmotic stress disrupts cellular functions and damages proteins.
If cooling is too rapid or cells don’t dehydrate, ice crystals form inside cells. Intracellular ice is lethal, puncturing organelles and membranes. Freezing and thawing also damage cell membranes. Changes in lipid packing and protein denaturation compromise membrane fluidity and permeability, leading to cell death.
Biological Protection Against Freezing
Many organisms have evolved mechanisms to survive freezing temperatures, often involving proteins. Antifreeze proteins (AFPs), found in polar fish, insects, and plants, are an example. AFPs bind to nascent ice crystals. This inhibits ice crystal growth and prevents smaller crystals from recrystallizing into larger, damaging ones.
Freeze-tolerant organisms also use natural cryoprotectants. Compounds like glycerol, trehalose, and glucose accumulate in cells. These molecules lower the freezing point of intracellular water, reducing ice formation. They also protect cellular components like proteins and membranes, stabilizing them against dehydration and high solute concentrations.
Some organisms avoid freezing, for example, through supercooling, where body fluids remain liquid below their freezing point. Others, like some insects, undergo controlled dehydration before winter. They shed body water, reducing ice formation and increasing tolerance to low temperatures. These strategies highlight life’s adaptability in cold environments.
Cryopreservation Techniques and Applications
Scientists developed cryopreservation techniques, inspired by natural freeze protection, to preserve biological materials. Slow freezing is a widely used method, involving controlled cooling rates (0.3-1°C/minute). This approach uses cryoprotective agents (CPAs) like dimethyl sulfoxide (DMSO) or glycerol, which permeate cells to mitigate ice crystal formation and osmotic stress. The goal is to encourage extracellular ice formation, allowing controlled cellular dehydration.
Vitrification, an alternative, avoids ice crystal formation through ultra-rapid cooling. It cools samples at thousands of degrees Celsius per minute with high CPA concentrations, transforming cellular water into a glass-like solid. Vitrification eliminates ice crystal damage, offering superior preservation for some cell types.
Cryopreservation methods have applications in medicine and research. They are used for long-term storage of human sperm, eggs, and embryos in reproductive medicine. It is also standard for blood cells (red blood cells, platelets) and stem cells (e.g., hematopoietic stem cells for bone marrow transplants). While effective for individual cells and small tissues, challenges remain for larger organs due to difficulties in uniform CPA distribution and cooling.