Water and mineral salts in the cell

Content and forms of intracellular water

On average, water accounts for about 80% of the total cell mass, but this indicator is extremely unstable and depends on both the tissue type and the age of the organism. Jellyfish cells (up to 95%) and human embryonic tissues (90%) are the most hydrated. In actively functioning cardiac muscle cells, this figure is 79%. As the organism ages, tissues lose moisture; in old cells, its share drops to 60%.

Intracellular fluid exists in two main states:

• Free water (about 95%) is a mobile fraction available for dissolving substances and carrying out biochemical reactions.
• Bound water (about 5%) is structurally bound to various macromolecules, ensuring their stability.

Physicochemical basics and functions of water

The unique biological role of water in the cell is strictly determined by its physicochemical nature: the molecules are small, highly polar, and capable of binding to each other via hydrogen bonds. Due to this, water performs a number of critically important functions:

1. Medium for biochemical reactions and universal solvent.
2. Transport function — ensures the movement of substances both inside the cell itself and throughout the entire organism (in the circulatory system).
3. Structural function — maintains cell volume and turgor by creating intracellular osmotic and turgor pressure.
4. Thermoregulation — protects the cell from temperature fluctuations due to high heat capacity and thermal conductivity.
5. Participation in chemical reactions — water molecules are directly involved in hydrolysis processes and act as a source of hydrogen ions (H⁺) during photosynthesis.
6. Reproductive role — creates the necessary fluid environment for external fertilization.
7. Physiological role in plants — makes transpiration (evaporation) and seed germination possible.

Classification of substances by solubility

Depending on their ability to interact with water molecules, all cellular compounds are divided into two broad groups:

• Hydrophilic (highly soluble substances). These include acids, alkalis, inorganic mineral salts, as well as monosaccharides and proteins.
• Hydrophobic (insoluble substances). This group includes polysaccharides, fats, and similar compounds.

Mineral salts and cellular homeostasis

Inorganic salts make up a small but vital fraction of the cell — 1–1.5% of its mass. Most often, they are represented by derivatives of phosphoric, hydrochloric, sulfuric, and other acids. In the cytoplasm, salts can exist either in solid form or as solutions, where they dissociate into charged particles: anions and cations.

Their key task is to ensure basic homeostasis. Any sharp fluctuations in ion concentration (both deficiency and excess) are fraught with cell death.

Biological significance of salts:

• Generation of osmotic pressure. It arises due to the difference in ion concentrations on both sides of the cell membrane.
• Regulation of acid-base balance (pH). Salts form phosphate and carbonate buffer systems. They maintain the tissue fluid pH within strict slightly alkaline limits (pH 7.2–7.4).
• Enzymatic activity. Many ions act as cofactors, which are mandatory components required for the function of a number of enzymes.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of hydrogen bond formation between water molecules
• Turgor and osmotic pressure: physics and biology of the process
• Mechanism of action of phosphate and carbonate buffer systems
• Free and bound water: biochemical differences between fractions
• The role of mineral ions as cofactors in enzymatic reactions

Other topics in the section "Chemical composition of the cell"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.