| Free water vs Bound water in life | ||
|---|---|---|
| Free water or unbound water | Bound water | |
| Fluidity | Most of the water in an organism is free to flow. | The minority of water in an organism is bound to organic matter by hydrogen bonds. It can't move freely. |
| Function | 1. Transport of nutrients and waste 2. Excellent solvent 3. Participate in biochemical reactions | It is a component of the cell structure |
| Associations | Free water and bound water can be transformed into each other in the organism. | |
The earliest life on Earth was nurtured in the primordial oceans, so one of the indispensable components in life is water that ranges from 60% to 90%. However, water content varies considerably in different organisms. For instance, it makes up 97% of jellyfish body. While there is very little amount in some living things. The dry seeds only have about 13-15% water. Aquatic organisms generally have more water than terrestrial organisms. Even in an individual organism, water isnāt distributed evenly. Metabolically active tissues and organs contain more, while parts with low metabolic rates contain less.
Free Water or Unbound Water
It's not bound to organic matter in organism and is free to move. Most of content in blood is unbound water. It is also found in cytoplasm and extracellular matrix, especially in the central vacuoles of plant cells. When you squeeze a fruit, the juice that flows out contains abundant unbound water from plant's central vacuole.
The physicochemical properties of unbound water are very similar to pure water. It can evaporate, flow and participate in cellular metabolism easily. In dry climates, it escapes from tissues as vapor. This is why some people's skin becomes dry and even cracks in Northeast Asia due to the dry piercingly Siberian cold wave.
It is also a good solvent where nutrients, wastes, hormones and other substances dissolve to be transported. It serves as the medium for biochemical reactions. Another important characteristic is that it freezes easily. Ice crystals will damage cell structures, so organisms with too much unbound water are less adaptable to harsh environments.
Bound Water
Water inevitably interacts with organic matters in living things. Its movement is limited by electrostatic forces, so itās not prone to evaporate, flow, or participate in biochemical reactions. That is why itās referred to as bound water.
Additional energy is required to overcome electrostatic force to become gaseous. Generally, It is believed that bound water doesnāt evaporate at room temperature. Electrostatic forces not only change their properties but also alter organic substances that water combined with. For instance, hydration shell, one type of bound water, exposes hydrophilic groups on protein surface, while hydrophobic groups are buried inside to keep away from aquatic solution.
The Bound water which is abundant in extracellular matrix combines with polar groups (amino, hydroxyl, carboxyl, etc.) through hydrogen bond to generate spongy hydrocolloids. The volume of spongy hydrocolloids increases several times, and most of the space in extracellular matrix is filled by them. They are one of the components that enable biological tissues to resist pressure and maintain structure.
Another notable characteristic of bound water is its difficulty in freezing. Their freezing point is much lower than 0Ā°C, because it has strong electrostatic interactions with other molecules, such as proteins, polysaccharides, etc. Therefore, it disturbs the regular arrangement of ice crystals. This characteristic is significant for survival under extreme conditions, especially for those living in very cold environments.
Free Water and Bound Water Can Convert to Each Other
The ratio in organisms is not fixed. Organisms can regulate metabolic rate to adapt to environment by changing the ratio. When unbound water increases, reduced viscosity makes metabolism more active in cytoplasm, but life becomes more sensitive to external environment. When bound water increases, the metabolism slowed down. Tissues are less likely to freeze and not prone to dehydration because of evaporation.
The adjustable ratio is beneficial for some animal and plant species to accommodate the rapid changing conditions. Before the coming winter, plants gradually increase bound water in their bodies to prevent tissues freezing and damage. Plants will suffer from frostbite easily during a sudden cold wave, because their bound water isnāt sufficient. Another example is that it prevents cacti losing moisture in hot and arid desert. When the rainy season arrives, these desert plants become extremely active. Temporary roots are grew to take in groundwater. Their mating is also completed in an extremely short time.