By Janet Rae-Dupree, Pat DuPree
Think of it as a gatekeeper, guardian, or border guard. Despite being only 6 to 10 nanometers thick and visible only through an electron microscope, the cell membrane keeps the cell’s cytoplasm in place and lets only select materials enter and depart the cell as needed.
This semipermeability, or selective permeability, is a result of a double layer (bilayer) of phospholipid molecules interspersed with protein molecules. The outer surface of each layer is made up of tightly packed hydrophilic (or water-loving) polar heads. Inside, between the two layers, you find hydrophobic (or water-fearing) nonpolar tails consisting of fatty acid chains.
Cholesterol molecules between the phospholipid molecules give the otherwise elastic membrane stability and make it less permeable to water-soluble substances. Both cytoplasm and the matrix, the material in which cells lie, are primarily water. The polar heads electrostatically attract polarized water molecules while the nonpolar tails lie between the layers, shielded from water and creating a dry middle layer.
The membrane’s interior is made up of oily fatty acid molecules that are electrostatically symmetric, or nonpolarized. Lipid-soluble molecules can pass through this layer, but water-soluble molecules such as amino acids, sugars, and proteins cannot, instead moving through the membrane via transport channels made by embedded channel proteins. Because phospholipids have both polar and nonpolar regions, they’re also called amphipathic molecules.
Credit: Illustration by Kathryn Born, MA
The cell membrane is designed to hold the cell together and to isolate it as a distinct functional unit of protoplasm. Although it can spontaneously repair minor tears, severe damage to the membrane will cause the cell to disintegrate. The membrane is picky about which molecules it lets in or out. It allows movement across its barrier by diffusion, osmosis, or active transport.
Diffusion is a natural phenomenon with observable effects like Brownian motion. Molecules or other particles spontaneously spread, or migrate, from areas of higher concentration to areas of lower concentration until equilibrium occurs. At equilibrium, diffusion continues, but the net flow balances except for random fluctuations.
This occurs because all molecules possess kinetic energy of random motion. They move at high speeds, colliding with one another, changing directions, and moving away from areas of greater concentration to areas of lower concentration. The diffusion rate depends on the mass and temperature of the molecule; lighter and warmer molecules move faster.
Diffusion is one form of passive transport that doesn’t require the expenditure of cellular energy. A molecule can diffuse passively through the cell membrane if it’s lipid-soluble, uncharged, and very small, or if a carrier molecule can assist it. The unassisted diffusion of very small or lipid-soluble particles is called simple diffusion. The assisted process is known as facilitated diffusion.
The cell membrane allows nonpolar molecules (those that don’t readily bond with water) to flow from an area where they’re highly concentrated to an area where they’re less concentrated. Embedded in the membrane are transmembrane protein molecules called channel proteins that traverse from the outer layer to the inner layer and create diffusion-friendly openings for molecules to move through.
Osmosis is a form of passive transport that’s similar to diffusion and involves a solvent moving through a selectively permeable or semipermeable membrane from an area of higher concentration to an area of lower concentration. Solutions are composed of two parts: a solvent and a solute.
The solvent is the liquid in which a substance is dissolved; water is called the universal solvent because more materials dissolve in it than in any other liquid.
A solute is the substance dissolved in the solvent.
Typically, a cell contains a roughly 1 percent saline solution — in other words, 1 percent salt (solute) and 99 percent water (solvent). Water is a polar molecule that will not pass through the lipid bilayer; however, it’s small enough to move through the pores — formed by protein molecules — of most cell membranes.
Osmosis occurs when there’s a difference in molecular concentration of water on the two sides of the membrane. The membrane allows the solvent (water) to move through but keeps out the solute (the particles dissolved in the water).
Transport by osmosis is affected by the concentration of solute (the number of particles) in the water. One molecule or one ion of solute displaces one molecule of water. Osmolarity is the term used to describe the concentration of solute particles per liter. As water diffuses into a cell, hydrostatic pressure builds within the cell. Eventually, the pressure within the cell becomes equal to, and is balanced by, the osmotic pressure outside.
An isotonic solution has the same concentration of solute and solvent as found inside a cell, so a cell placed in isotonic solution — typically 1 percent saline solution for humans — experiences equal flow of water into and out of the cell, maintaining equilibrium.
A hypotonic solution has less solute and higher water potential than inside the cell. An example is 100 percent distilled water, which has less solute than what is inside the cell. Therefore, if a human cell is placed in a hypotonic solution, molecules diffuse down the concentration gradient until the cell’s membrane bursts.
A hypertonic solution has more solute and lower water potential than inside the cell. So the membrane of a human cell placed in 10 percent saline solution (10 percent salt and 90 percent water) would let water flow out of the cell (from the higher concentration inside to the lower concentration outside), therefore shrinking it.
Active transport occurs across a semipermeable membrane against the normal concentration gradient, moving from the area of lower concentration to the area of higher concentration and requiring an expenditure of energy released from an ATP molecule.
Embedded with the hydrophilic heads in the outer layer of the membrane are transmembrane protein molecules able to detect and move compounds through the membrane. These carrier or transport proteins interact with the passenger molecules and use the ATP-supplied energy to move them against the gradient. The carrier molecules combine with the transport molecules — most importantly amino acids and ions — to pump them against their concentration gradients.
Active transport lets cells obtain nutrients that can’t pass through the membrane by other means. In addition, there are secondary active transport processes that are similar to diffusion but instead use imbalances in electrostatic forces to move molecules across the membrane.
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