Study Guide for Inorganic Compounds in Living Systems

Classification and Characteristics of Inorganic Molecules

Basic compounds of living organisms are categorized into two main groups: inorganic molecules and organic molecules. Inorganic molecules, which include water, salts, and minerals, are essential substances that cannot be produced by living organisms themselves and must be taken from the external environment. Structurally, these molecules are small enough to pass through the cell membrane without undergoing digestion. Functionally, inorganic molecules serve as structural components, repair materials, and generally act as regulatory molecules within biological systems. Unlike organic molecules, inorganic molecules do not provide energy to the organism, with the notable exception of some chemosynthetic organisms that satisfy their energy requirements by oxidizing inorganic substances.

It is important to note that while many minerals are regulatory, certain substances like lead and mercury are also minerals but exhibit toxic effects rather than regulatory ones. Therefore, these specific toxic minerals are not considered regulatory components of a healthy biological system. The primary types of inorganic molecules discussed in biological contexts include water ($H_2O$), salts, and various minerals.

Water: The Essential Basis of Life

Water is the most abundant substance in nature and within the structure of living organisms, making it vital for life. The water content in the human body varies significantly based on age, gender, and tissue type. In infants, the water ratio is approximately $75-78\%$. For adult males, it is approximately $60\%$, while for adult females, it ranges between $50-55\%$. In the elderly, the water content drops to between $45-50\%$. These variations highlight how water requirements and retention change throughout the human lifecycle.

Water distribution also varies across different organs and tissues within an adult human body. The brain consists of approximately $78-80\%$ water, while muscles contain between $75-78\%$ water. Blood has a very high water concentration of approximately $90\%$. Conversely, denser structures like bones have a much lower water content, at approximately $30-31\%$. Other body structures average between $70-75\%$ water. Structurally, a single water molecule is composed of two positively charged hydrogen ($H$) atoms and one negatively charged oxygen ($O$) atom.

Biological Functions and Physical Properties of Water

Water performs several critical functions including maintaining temperature balance, facilitating the removal of waste, providing cell structure, and acting as a primary medium for transport. One of its most significant physical attributes is its high heat capacity, meaning it can absorb and store a large amount of heat without a significant change in its own temperature. This property is essential for global and local climate regulation. For example, during the summer, oceans absorb massive amounts of solar energy, yet their temperature only changes by $1$ to $2$ degrees. Without this capacity, summer temperatures would become uninhabitable, and winters would become instantly freezing.

This high heat capacity also regulates temperatures in coastal regions. In cities located by the sea, such as Antalya or İzmir, the temperature difference between day and night is very small because the massive body of water balances the heat. In contrast, inland cities located in the middle of steppes, like Ankara or Konya, experience extreme heat during the day and sudden freezing temperatures at night due to the lack of a large water body to stabilize the thermal environment. Furthermore, water is essential for enzymatic activity; enzymes require an environment with at least $15\%$ water content to function. In environments with less than $15\%$ water, enzymes cannot catalyze reactions, and metabolic activities may cease.

Cohesion, Adhesion, and Surface Tension

Water exhibits two primary types of attractive forces: cohesion and adhesion. Cohesion is the attractive force between identical molecules, specifically water molecules pulling on each other. This cohesion results in surface tension, which allows very light insects to move across the surface of water without sinking. Adhesion is the force of attraction between different types of molecules, such as water molecules sticking to the surface of a container or a leaf. For example, water droplets sticking to a glass surface demonstrate adhesion.

In the plant kingdom, the combination of adhesion and cohesion forces is critical for survival. These forces allow water to be transported from the roots all the way up to the leaves, defying gravity. Furthermore, water acts as an excellent solvent. This solvent property allows plants to absorb minerals and necessary nutrients from the soil and enables sugar to dissolve quickly, facilitating necessary metabolic activities within the organism. Water also plays a role in waste removal through processes like perspiration (sweating), which also helps regulate body temperature.

The Unique Freezing Properties of Water

Water possesses a unique property regarding its density when freezing, which protects aquatic life. Most substances contract when they freeze, but water expands. This expansion increases the volume of the water while the mass remains the same, making ice less dense (lighter) than liquid water. Consequently, when a body of water like a lake freezes, the ice forms at the surface rather than the bottom.

The layer of ice on the surface acts as a thermal insulator, preventing the cold air from reaching the water underneath. This keeps the water below the ice at a temperature suitable for the survival of aquatic organisms. If water froze from the bottom up, most aquatic habitats would freeze solid, killing the life within them.

Minerals and Mineral Deficiency

Minerals are inorganic molecules that living beings require but cannot produce themselves. Plants obtain minerals directly from the soil, while animals and humans acquire them through food and water consumption. Like other inorganic molecules, minerals do not provide energy, but they are crucial as regulatory and structural components. They are found naturally in soil, water, and rocks in various forms. A deficiency in specific minerals leads to various health issues and diseases.

• Iron ($Fe$): It is a component of hemoglobin, the substance that gives blood its red color and carries oxygen. Deficiency leads to anemia (kansızlık). Sources include spinach and red meat.
• Phosphorus ($P$): It is part of the structure of bones, teeth, DNA, and ATP. Deficiency causes bone weakness and growth retardation. Sources include meat, milk, and legumes.
• Iodine ($I$): It is a component of the thyroxine hormone produced by the thyroid gland. Deficiency leads to goiter and mental retardation. Sources include seafood.
• Potassium ($K$): It regulates heart rhythm and nerve conduction. Deficiency leads to heart rhythm disorders. Sources include bananas, potatoes, and apricots.
• Sodium ($Na$): It regulates fluid-electrolyte balance and nerve conduction. Deficiency causes appetite loss and cramps. Sources include table salt and mineral water.
• Calcium ($Ca$): It is structural for bones and teeth and is essential for blood clotting. Deficiency leads to osteoporosis (kemik erimesi). Sources include milk, cheese, and yogurt.
• Magnesium ($Mg$): It helps regulate enzyme activity and the muscle/nervous system. Deficiency results in nervous disorders and muscle cramps. Sources include hazelnuts, almonds, and leafy greens.
• Sulfur ($S$): It is found in the structure of some amino acids. Deficiency causes paleness in the skin and hair. Sources include meat, onions, and garlic.
• Zinc ($Zn$): It is effective on the immune system. Deficiency leads to a weakened immune system. Sources include seafood and nuts.
• Fluorine ($F$): It protects dental and bone health. Deficiency leads to tooth decay. Sources include seafood.
• Chlorine ($Cl$): It contributes to the formation of stomach acid ($HCl$). Deficiency leads to digestive problems. Source includes table salt.

Comparative Summary of Inorganic Properties

When comparing water and minerals, both share several fundamental characteristics. Both are inorganic, meaning they are not produced by living organisms and are not used as energy sources. Both are small enough to pass directly through the cell membrane without the need for digestion. Both participate in the structure of living things and play critical roles in the regulation of vital activities. While water specifically serves as a solvent and temperature regulator, minerals often act as cofactors for enzymes or structural building blocks for tissues like bones and teeth. Both are indispensable for maintaining the homeostasis of any living organism.