6 Endocytic pathways
In the final two sections of this course, we shall look at some examples of endocytosis and exocytosis, in different types of cell. The molecules that are taken up or released by a cell and the triggers for secretion depend greatly on the type of cell, although the underlying transport processes are similar for many cell types, and relate to the systems described above.
The plasma membrane not only separates the cell interior from the extracellular environment, it also regulates and coordinates the entry and exit of different types of molecule. Many small molecules, such as amino acids, water, sugars and ions, can cross the plasma membrane.
Give three mechanisms by which small molecules can cross the plasma membrane, with examples.
Water and most gases cross the membrane by simple diffusion. Glucose and amino acids are taken up by facilitated diffusion on carrier molecules. Ions cross through pores or specific transporters, which may require energy, depending on the ion, the membrane potential and the concentration gradient across the membrane.
Macromolecules, however, must be carried into the cell by endocytosis.