Human Biochemistry, Volume 2 - Murray R. 1993

Biochemistry of Intra- and Intercellular Communication
Membranes: Structure, Assembly, and Function
Maintenance of the Normal Composition of the Environment

Maintaining the proper COMPOSITION OF THE environment inside and around The Cell is fundamental to life. Because life originated in an aqueous medium, enzymatic reactions, as well as cellular, subcellular, and other biological processes, evolved specifically within this environment. Today, most mammals inhabit a gaseous environment. How, then, is their Water balance maintained? Water is retained within the Organism through membranes that pervade the aqueous environment and form distinct compartments.

Internal Aqueous Compartments

Water accounts for approximately 56% of total human body Muscle mass and is distributed between two major compartments.

A. Intracellular fluid: this compartment contains two-thirds of the total fluid volume and provides the conditions necessary for 1) energy conversion, storage, and utilization; 2) Homeostasis; 3) Replication; and 4) specialized cellular Functions.

B. Extracellular fluid: this compartment contains roughly one-third of total body water, which is distributed between the plasma and the interstitial space. The extracellular fluid serves as a delivery system. It supplies Cells with nutrients (such as glucose, Fatty acids, and Amino Acids), oxygen, various ions and Trace Elements, and numerous regulatory molecules (Hormones) that coordinate the activities of spatially separated cells. The extracellular fluid also removes СО₂, Metabolic waste products, and toxic or detoxified substances from the immediate cellular microenvironment.

Composition of Cells and Extracellular Fluid

As shown in Table 42.1, the ionic composition of intracellular fluid differs markedly from that of the extracellular environment. The intracellular milieu is rich in K⁺ and Mg²⁺ ions, with phosphate serving as the primary anion. In contrast, extracellular fluid contains high concentrations of Na⁺ and Са²⁺ ions, with Cl^-1) as the principal anion. It is also worth noting that glucose concentration is higher in the extracellular fluid than within the cell, whereas Proteins exhibit the exact opposite distribution. What underlies this difference? It is believed that the primordial ocean, where life first emerged, had high concentrations of K⁺ and Mg²⁺ ions; consequently, enzymatic reactions and other biological processes evolved to function optimally in this specific environment. Over time, the composition of the primordial ocean gradually changed, becoming dominated by Na⁺ and Са²⁺ ions, which subjected cells to intense Selection pressure. Under these conditions, developing an entirely new suite of biochemical and physiological processes would have required colossal changes. Instead, a barrier evolved between the cell and its environment—a membrane embedded with transport pumps that ensures the Stability of the internal microenvironment.

¹⁾ The specified anion ratio is characteristic primarily of animal cells.— Transl.

Class="center">Table 42.1. Average concentrations of various substances outside and inside mammalian cells