A cell’s well-being depends on its ability to control the passage of molecules across the cell membrane. Some molecules can diffuse through the cell membrane without any assistance from the cell. Others require the help of transmembrane proteins to move into or out of the cell. Three primary factors determine whether a molecule will diffuse across a cell membrane: concentration, charge and size.
The Cell Membrane
A cell membrane is composed of two layers of phospholipids. Each phospholipid molecule has a hydrophilic phosphate head and two hydrophobic lipid tails. The heads line up along the inner and outer surfaces of the cell membrane, while the tails fill the middle space. Various types of transmembrane proteins provide facilitated diffusion or active transport for molecules that can’t passively diffuse through the cell membrane. Primary active transport requires the cell to expend energy to move molecules through the cell membrane. Diffusion requires no energy from the cell to do so.
Diffusion occurs because molecules like to spread out from areas of high concentration to areas of lower concentration. Electrochemical and kinetic energy are used to power diffusion. The primary determinant of whether a molecule will diffuse across a cell membrane is the concentration of the molecule on each side of the cell membrane. For example, the extracellular concentration of oxygen is higher than the intracellular concentration, which is why oxygen diffuses into the cell. Carbon dioxide diffuses out for similar reasons.
Charge and Polarity
An ion is an atom or molecule that has an outright charge, due to an imbalance between the number of protons and electrons. Polarity is an uneven distribution of charge across a molecule, with some partially positive and negative regions. Charged and polarized molecules dissolve in water, whereas uncharged molecules dissolve in lipids. The lipid tails in the cell membrane prevent charged and polarized molecules from diffusing through the cell membrane. However, some cells actively maintain an electric potential on either side of the cell membrane that can attract or repel ions and polarized molecules.
Some molecules are small enough to slip past the lipid tails, even if the molecules are polarized. For example, water is polarized, but small enough to freely diffuse across the cell membrane. This is also true of carbon dioxide, the byproduct of cellular metabolism. Oxygen molecules have no polarity and are also small enough to easily diffuse into the cell. Sugar molecules, which contain five or more carbon atoms, are both polar and too large to diffuse through the cell membrane and must travel through transmembrane proteins.