Inorganic Substances in Cells

Section 2: Inorganic Substances in Cells

Water in Cells

It is widely believed that life on Earth originated in the oceans, where life has always been dependent on water from the very beginning. Ancient environmental conditions have permanently influenced the chemical composition and characteristics of organisms. Even organisms living on land are inseparable from water. Whenever you feel thirsty, you experience the dependence of life on water. The water content in organisms varies with different species, generally ranging from 60% to 95%, with jellyfish having a water content of up to 97%. Water is a crucial component of cells and is the most abundant compound in living cells.

Water serves as a good solvent within cells, allowing many substances to dissolve in it. Many biochemical reactions within cells also require the participation of water. The vast majority of cells in multicellular organisms must be immersed in a liquid environment based on water. The flow of water within organisms facilitates the transport of nutrients to various cells and removes waste products generated by cellular metabolism to excretory organs or directly out of the body.

Why does water serve as a good solvent within cells? What unique properties does it possess that support life? These qualities stem from its molecular structure. A water molecule consists of two hydrogen atoms and one oxygen atom, with hydrogen atoms bonding to oxygen through shared electron pairs. Oxygen's stronger attraction to shared electrons compared to hydrogen results in a slightly negative charge at one end (oxygen) and a slightly positive charge at the other end (hydrogen) of the water molecule. This asymmetrical distribution of electrons makes water a polar molecule. Molecules (or ions) carrying positive or negative charges easily bond with water, making water an excellent solvent.

Due to the polarity of water molecules, when the negative end (oxygen) of one water molecule approaches the positive end (hydrogen) of another water molecule, they are attracted by a weak electrostatic force known as hydrogen bonding. Each water molecule can interact with surrounding water molecules through hydrogen bonds. These bonds are weak and easily broken, constantly forming and breaking, allowing water to remain in a liquid state at normal temperatures with high fluidity. Additionally, the presence of hydrogen bonds gives water a high specific heat capacity, meaning it resists changes in temperature, a critical property for maintaining stability in biological systems.

Water exists in cells in two forms: the majority as free water, which can flow freely (called free water), and a portion bound to other substances within cells (called bound water). Free water acts as a good solvent within cells, while bound water is an essential structural component of cells, comprising approximately 4.5% of all water in cells. Bound water in cells primarily combines with substances like proteins and polysaccharides, losing its fluidity and solubility to become a structural component of organisms. In normal conditions, the higher the proportion of free water in cells, the more vigorous their metabolism; conversely, a higher proportion of bound water enhances cells' resistance to adverse conditions like drought and cold. For instance, drying seeds reduces their free water content, lowering their metabolic rate and facilitating storage. Before the onset of winter, northern wheat gradually decreases its proportion of free water while increasing its proportion of bound water to prevent damage from excessive freezing.

Inorganic Salts in Cells

When you burn a wheat seed, you can observe some grayish-white ashes left behind, which are the inorganic salts within the wheat seed. Human and animal bodies also contain inorganic salts.

Most inorganic salts in cells exist in the form of ions. Common cations include Na+, K+, Ca2+, Mg2+, Fe2+, Fe3+, while common anions include Cl-, SO42-, PO42-, HCO3-. Unlike water, inorganic salts are present in cells in small quantities, accounting for only 1% to 1.5% of cell fresh weight. What roles do they play in cells?

Mg is essential for chlorophyll formation, Fe for hemoglobin. P is a vital component of cell membranes, nuclei, and many essential compounds. Ions like Na+ and Ca2+ are also indispensable for life activities. For example, Na+ deficiency in the body can reduce the excitability of nerve and muscle cells, leading to symptoms like muscle soreness and weakness. Therefore, after excessive sweating that results in the loss of inorganic salts, drinking diluted saltwater is advisable. Mammalian blood must contain a certain amount of Ca2+; if Ca2+ levels are too low, animals may experience seizures. Moreover, maintaining a certain quantity of certain inorganic salt ions is crucial for maintaining cellular acid-base balance. It is evident that many types of inorganic salts play vital roles in maintaining cellular and organismal life activities.

ological tissues.