5.2 Selective Permeability and Solute Gradients
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Evaluate how the components of membranes that impact the selective permeability of those membranes to different types of solutes (small/large, hydrophobic/hydrophilic, neutral/charged).
- Compare and contrast how channel proteins and carrier proteins facilitate solute movement across membranes.
- Explain how concentration gradients and electrochemical gradients impact the movement of different types of solutes across membranes.
All cells and organelles need some kind of physical barrier to keep their contents in and external materials out. They also need a way of exchanging materials between their internal and external environments. This section explores how selectively permeable membranes accomplish this, and how solute gradients influence the movement of materials across these membranes.
Selective Permeability
Membranes must allow certain substances to enter or leave while preventing other substances from passing though. In other words, plasma membranes are selectively permeable. If plasma membranes were to lose this selectivity, the cell would no longer be able to sustain itself and would die. In addition to the plasma membrane, most organelles are surrounded by their own membranes, and these membranes are also selectively permeable. Whether a solute can cross a membrane depends on at least two things: the properties of the solute and the properties of the membrane itself.
Solute Properties
A solute is a substance that can be dissolved in a solvent. Recall that membrane lipids are amphiphilic: they have hydrophilic and hydrophobic regions, resulting in the lipid bilayer having a hydrophobic core. This characteristic helps move some materials through the membrane and hinders the movement of others. Gasses, hydrophobic molecules, and small polar (hydrophilic) molecules like water can easily slip through the membrane’s hydrophobic lipid core. Some examples of solutes that easily enter or exit human cells include lipid-soluble (hydrophobic) vitamins A, D, E, and K lipid-soluble drugs and hormones, and oxygen and carbon dioxide molecules. Water is not lipid-soluble, but its small size allows it to diffuse across membranes anyway (Figure 5.14).
Some solutes cannot readily pass through membranes. For instance, polar substances cannot easily diffuse through the membrane’s non-polar lipid core (except for very small molecules like water). Ions are charged, which prevents them from directly crossing membranes. Ions of hydrogen (H+), sodium (Na+), potassium (K+), calcium (Ca2+), chloride (Cl–), and many others must enter or exit cells with the help of transmembrane proteins (channels/carriers). Larger polar molecules, such as simple sugars and amino acids, also need the help of various transmembrane proteins (channels/carriers) to transport themselves across membranes. Some molecules, such as proteins and polysaccharides like g