The cell surface membrane is partially permeable: it lets some substances through but not others. How a substance crosses depends on its size, charge and whether it is soluble in lipids.The membrane is a phospholipid bilayer, with proteins, cholesterol and glycoproteins in it (the fluid mosaic model). The fatty acid tails form a hydrophobic core in the middle of the membrane.
1. Through the bilayer. Small, non-polar molecules such as oxygen and carbon dioxide, and lipid-soluble molecules such as steroid hormones, dissolve in the hydrophobic core and pass straight through. Water, although polar, is small enough to cross slowly this way.
2. Through channel proteins. Ions and small polar molecules cannot pass through the hydrophobic core. Channel proteins form water-filled pores that let specific ions through. Aquaporins are channel proteins for water.
3. Using carrier proteins. Larger polar molecules, such as glucose and amino acids, bind to a specific carrier protein, which changes shape to move them across.
4. In vesicles. Very large molecules and particles, such as proteins or bacteria, are moved in or out by the membrane forming vesicles.Because each channel and carrier protein is specific, the cell can control exactly which ions and polar molecules cross its membrane. Whether each route needs energy from the cell is covered in the next lesson.
1. Through the bilayer. Small, non-polar molecules such as oxygen and carbon dioxide, and lipid-soluble molecules such as steroid hormones, dissolve in the hydrophobic core and pass straight through. Water, although polar, is small enough to cross slowly this way.
2. Through channel proteins. Ions and small polar molecules cannot pass through the hydrophobic core. Channel proteins form water-filled pores that let specific ions through. Aquaporins are channel proteins for water.
3. Using carrier proteins. Larger polar molecules, such as glucose and amino acids, bind to a specific carrier protein, which changes shape to move them across.
4. In vesicles. Very large molecules and particles, such as proteins or bacteria, are moved in or out by the membrane forming vesicles.Because each channel and carrier protein is specific, the cell can control exactly which ions and polar molecules cross its membrane. Whether each route needs energy from the cell is covered in the next lesson.