細胞の化学

Cells are the basic building blocks of living organisms. They are composed mostly of water, organic molecules, and inorganic ions. Water makes up around 70% of the cell. This aqueous environment in the cytoplasm is essential for the cell’s structure and for many of the reactions that occur within it. Water is a highly polar molecule, so it will interact with other polar molecules and ions in the cell but not with the nonpolar ones. The hydrophilic environment of the cytoplasm promotes the formation of proteins’ three-dimensional structures, with nonpolar amino acids at the core and polar ones at the surface. In the case of membrane formation, polar head groups of phospholipids interact with water. In contrast, the nonpolar tails interact with each other creating a hydrophobic barrier to the outside of the cell. The pH of biological fluids in the cell and its compartments is precisely buffered. The pH of the cytoplasm is around 7.2 and is regulated by molecules such as phosphate ions. In contrast, the pH in the lysosomes, specialized cellular compartments, is around 5. This is because lysosomes contain enzymes that function optimally in acidic environments. Organic molecules in the cell include carbohydrates, proteins, lipids, and nucleotides. All of these various macromolecules have a variety of roles. Carbohydrates are a primary source of energy to power various metabolic processes; however, the cell can break down proteins and lipids for fuel as well. Proteins can be enzymes that catalyze reactions, or they can contribute to cell structure. Lipids are a significant component of cell membranes. Additionally, these macromolecules can covalently bond to each other to form conjugates such as glycoproteins and glycolipids, often found in cell membranes. Nucleotides serve as the genetic material and also associate with proteins to form nucleoproteins that tightly package the DNA in a cell. The nucleotide ATP is also the source of energy for many cellular processes. Inorganic ions critical to cell function include sodium, potassium, magnesium, calcium, phosphate, and chloride. Although these ions constitute less than 1% of the cell mass, they have diverse roles within the cell. For example, ions such as calcium are used to relay biological signals throughout a cell, while magnesium is essential for many enzymes’ catalytic activity.