5.1: Introduction

Before You Start

Before beginning this TLP, you should:

• Understand basic concepts including osmosis.
• Be familiar with phase diagrams - take a look at the Phase Diagrams and Solidification TLP.
• Be familiar with the formation and structure of glass

Introduction

Many plants and animals live in extreme climate zones, such as the polar regions or subtropical deserts. One of the major problems associated with living in such regions is that most biological systems are largely composed of water. Water is the universal solvent for biological processes, and most living organisms contain a large amount — humans, for example, are 50-75% water.

In the polar regions temperatures can fall to below –20°C. In deserts, the absence of water can lead to the dehydration of plants and animals. The organisms that live in these regions have become adapted to these conditions.

Plants that live in arctic regions must contend with the formation of ice crystals in their cells. This is fatal to living tissue for a variety of reasons, which are discussed in the next section. Plants and animals living in the desert have a similar problem; dehydration can cause salts and sugars to precipitate out of solution inside their cells.

In this TLP we will consider how biological adaptations to cells allow them to survive such conditions by preventing crystallization of ice or salts within cells. We will look at the basic theory behind nucleation and crystallization, and the details of the water-sucrose system that illustrate key features of the cellular liquid (the cytosol). We will then examine the specific ways in which some plants and animals avoid crystallization, through glass formation, extracellular crystallization (forming the crystals outside the cell) and the use of antifreeze proteins.