Key Concepts: Diffusion, Osmosis, Active Transport, and Vesicle Transport 

The structure of the plasma membrane allows nonpolar or uncharged molecules to pass through the nonpolar, fat-soluble tails of the lipid bilayer.¹ The plasma membrane transports uncharged, nonpolar molecules such as oxygen, carbon dioxide, and steroids more easily than polar, charged molecules. Some small polar molecules, such as water, are permeable.¹

Diffusion

Diffusion: The principle of diffusion demonstrates that when mixing a solution, the solutes (dissolved substances) will move from an area of high concentration to low concentration until equally dispersed through the solvent (dissolving fluid).¹The solutes’ movement from high to low concentration is termed going down the concentration gradient (separate areas that contain differing amounts of solute). The ability of a solute to disperse is dependent on multiple factors. Diffusion is applied to human biology as molecules move in and out of cells to reach and maintain an equilibrium or homeostasis. It is important to note that the overall charge of the concentration gradient is negative inside the cell due to the greater loss of potassium cations.¹ Therefore, the outside of the cell is more positively charged. This is called the electrochemical gradient. Diffusion is considered passive as it does not require more than its own kinetic energy to move across the plasma membrane. 

Simple Diffusion: Movement of molecules through the plasma membrane into the cell using its own kinetic energy without plasma proteins.¹ Molecules include fat-soluble vitamins (A, D, E, and K), oxygen, carbon dioxide, water, urea, fatty acids, steroids, etc. Simple diffusion favors nonpolar, uncharged molecules due to the lipid bilayer. 

Facilitated Diffusion: Polar, highly charged molecules require the help of plasma proteins to move through plasma membranes into cells. Facilitated diffusion can be either:

• Channel-Mediated: Substance passes through channel protein into the cell. Channel proteins are used for ions, especially potassium and chloride ions to go into the cell (less for sodium and calcium).¹

• Carrier-Mediated: Substance binds to a carrier protein and passes into the cell as the protein changes shape.¹

Osmosis

Osmosis: Osmosis, similar to diffusion, is a passive process in that movement depends on concentration. Water moves from an area of high water concentration to a low water concentration.¹ In another sense, water moves from an area of low solute concentration to a high solute concentration. Water moves through simple diffusion or aquaporins (integral plasma membrane channel proteins) that allow for water permeability. Hydrostatic pressure measures the pressure exerted by water against the membrane, and osmotic pressure measures the pressure applied by the solute that is impermeable to the membrane.¹

When a solution is isotonic, the osmotic pressure is at an equilibrium. A hypotonic solution contains more water than solute, so water moves from high water concentration to low water concentration inside of the cell which can cause swelling and bursting.¹ A hypertonic solution contains less water compared to solute, and water from cells exits into extracellular fluid, causing shrinkage (crenation) of cells.¹ 

Active Transport

Unlike diffusion, active transport requires energy either in the form of adenosine triphosphate or energy provided by the electrical charge of the concentration gradient. Active transport moves monosaccharides, amino acids, and ions such as sodium, potassium, hydrogen, calcium, iodide, and chloride against the concentration gradient or uphill; however, they are also transported by facilitated diffusion at times.¹ There are two types of active transport:

• Primary Active Transport: The action of hydrolysis of ATP releases phosphate which changes the carrier protein shape once attached, allowing one substance to travel against the concentration gradient. The other substance crosses once the phosphate is removed from the carrier protein.¹

  • The sodium-potassium pump is one of the most common forms of primary active transport found within the body with each bringing two potassium cations into the cytoplasm and three sodium cations into the extracellular fluid.¹ Therefore, the outside of the cell is more positively charged than the inside of the cell. 

• Secondary Active Transport: Solutes are transported against the concentration gradient at the same time either in the same direction (symporter) or opposite direction (antiporter) due to the leakage of sodium back into the cell.¹ The constant motion of expelling sodium maintains the concentration gradient and allows for sodium-calcium and sodium-hydrogen antiporters and sodium-glucose and sodium-glucose symporters to function. 

Vesicles

Vesicles are sacs formed within the plasma membrane that transport substances into cells (endocytosis) and out of cells (exocytosis). There are three types of endocytosis:

Endocytosis

• Receptor-mediated endocytosis: Once a substance (ligand) binds to the extracellular side of the plasma membrane via receptor proteins, a vesicle forms and transports the substance to organelles. Substances transported via endocytosis include low-density lipoprotein (LDL) cholesterol, vitamins, and certain hormones.¹

• Phagocytosis: Phagocyte cells, such as macrophages and neutrophils, engulf microbes and other substances once bonded to a receptor protein, forming the vesicle termed a phagosome that undergoes processes for degradation within the lysosome and excretion of residue.¹ 

• Bulk-phase endocytosis: Solutes within the extracellular fluid are transported into the cell without the use of a receptor protein when vesicles form within the plasma membrane and pinch off to combine with a lysosome.¹

Exocytosis

Secretory vesicles form within the cell on the innermost part of the plasma membrane and release substances outside the cell into the extracellular fluid.¹ This is very important for cells that release secretions and neurotransmitters to get rid of waste.