Thomas Graham, a 19th-century Scottish physical chemist, fundamentally changed the understanding of how substances move and interact. His foundational work established principles governing particle movement in both gaseous and liquid states. Graham’s research explained the dynamics of diffusion, the specific movement of water known as osmosis, and the physical properties used to classify different types of solutions. These concepts provide the framework for understanding countless physical and chemical processes, forming the basis for many biological functions.
The Movement of Gases
The study of how gases mix or spread spontaneously is governed by Graham’s Law of Diffusion. This law details that the rate at which a gas moves is inversely related to the square root of the mass of its particles. This means that lighter molecules travel faster than heavier molecules under the same conditions. For example, a strong odor quickly permeates a room as lightweight vapor molecules rapidly disperse.
Graham’s original experiments in 1829 measured how quickly different gases escaped through a small opening, a process called effusion. He found that hydrogen gas (2 g/mol) escaped about four times faster than oxygen gas (32 g/mol). Lighter molecules possess a higher average velocity at the same temperature, allowing them to collide with and pass through openings more frequently. The constant, random motion of these particles drives this movement toward a state of uniform concentration.
The relationship between molecular mass and movement rate applies directly to gas exchange within the body. In the lungs, oxygen and carbon dioxide must move across the alveolar and capillary membranes to enter or exit the bloodstream. Oxygen (32 g/mol) is slightly lighter than carbon dioxide (44 g/mol), making its movement rate negligibly faster. However, the efficiency of gas exchange relies more heavily on the concentration gradient and membrane permeability than on this slight difference. The principle ensures that gases continuously move to maintain equilibrium, facilitating respiration and cellular function.
Water Movement Across Membranes
While diffusion describes the movement of any particle from high to low concentration, osmosis is a specific type focusing on the movement of the solvent, typically water. Osmosis requires a semipermeable membrane that permits water passage but restricts larger dissolved particles, or solutes. Water moves across this barrier from a region of low solute concentration to a region of high solute concentration. This movement continues until the concentrations equalize or until pressure stops the net flow.
The behavior of a cell depends on the relative concentration of solutes in the surrounding external solution, a property known as tonicity. Solutions are classified as isotonic, hypotonic, or hypertonic relative to the cell’s internal fluid. An isotonic solution contains the same solute concentration as the cell’s interior, resulting in no net water movement and maintaining the cell’s stable volume.
When an animal cell is placed in a hypotonic solution, the external environment has a lower solute concentration than the cell’s cytoplasm. This causes a net influx of water, which swells the cell and may rupture in a process called hemolysis. Conversely, placing the cell in a hypertonic solution causes water to rush out. The loss of water leads to the cell shriveling and shrinking, a phenomenon known as crenation.
Classifying Solutions
Graham’s investigations into how different substances diffused in water led him to develop a classification system for dissolved substances. He observed that some substances, such as salt or sugar, diffused rapidly and easily passed through parchment paper membranes. He termed these substances “crystalloids” because many could be crystallized from solution. Crystalloids are small ions or molecules that form true solutions when dissolved.
In contrast, other substances he tested, like glue or gelatin, diffused very slowly and were retained by the same membranes. Graham designated these larger particles as “colloids,” a term derived from the Greek word for glue. Colloidal particles are much larger than crystalloids, typically ranging in size from \(10^{-7}\) cm to \(10^{-3}\) cm, making them too large to pass through a semipermeable barrier. Instead of forming a true solution, colloids remain suspended, creating a mixture where the particles are not truly dissolved.
The difference in molecular size and diffusion rate became the basis for a separation technique Graham invented called dialysis. Dialysis uses a semipermeable membrane to separate crystalloids from colloids in a mixed solution. When a solution containing both is placed in a membrane bag immersed in pure water, the small crystalloid particles diffuse out. The large colloid particles are retained inside the membrane, effectively purifying the colloidal substance. This classification provides the foundation for separating and understanding complex biological fluids.
Real-World Biological Relevance
The principles of diffusion, osmosis, and solution classification developed from Graham’s work apply directly to medical and industrial technologies. One significant application is kidney dialysis, a procedure that replicates the natural function of the kidneys to filter blood. This treatment utilizes a semipermeable membrane to separate waste products from the blood, applying the concept of separating crystalloids from colloids.
During hemodialysis, the patient’s blood, which contains large colloidal proteins and small crystalloid waste molecules like urea, flows past a dialysate fluid separated by a synthetic membrane. The small waste products (crystalloids) diffuse out of the blood and into the dialysate, following their concentration gradient. The large blood proteins and cells (colloids) are too large to cross the membrane and remain in the bloodstream.
The classification of intravenous fluids relies on Graham’s distinction between crystalloids and colloids for volume replacement therapy. Crystalloid solutions, such as saline, contain small electrolyte ions and are used to quickly replace lost water and electrolytes. Colloidal solutions, which contain large molecules like albumin, are used when sustained fluid retention in the circulatory system is necessary. Their larger size keeps them from easily escaping the blood vessels. Understanding the movement and retention properties of these solutions is necessary for maintaining fluid balance.