5.5 Vesicular Transport Across Membranes
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Contrast the mechanisms of vesicular transport and protein-mediated transmembrane transport such as facilitated diffusion and active transport
- Evaluate the connections between exocytosis, endocytosis, and the endomembrane system (Chapter 4.3)
- Describe the differences between phagocytosis, pinocytosis, and receptor-mediated endocytosis
In addition to moving small ions and molecules through the membrane, cells also need to remove and take in larger molecules and substances like proteins or polysaccharides. Some cells are even capable of engulfing entire unicellular microorganisms. When a cell uptakes and releases large substances, this process requires energy. However, these substances are too large to cross the membrane via carrier or channel proteins. Vesicular, or bulk, transport is the movement of large substances such as polysaccharides and proteins, from one side of a membrane to another via membrane-bound vesicles. Vesicular transport also occurs between organelles within a cell, but this chapter will focus on transport across the plasma membrane. This transport mechanism can be subdivided into two main categories: endocytosis and exocytosis.
Exocytosis
Exocytosis (“exo” = outside) is the cellular process that involves transporting materials from the inside of a cell into the extracellular fluid via the fusion of a vesicle with the membrane. Material within the cell’s endomembrane system (Chapter 4.3) become enveloped in a vesicle, and that vesicle then fuses with the plasma membrane’s interior. This fusion opens the membranous envelope on the cell’s exterior, and the material is expelled into the extracellular space (Figure 5.29). The vesicle membrane then becomes integrated into the plasma membrane, with the vesicle’s luminal (inside) surface becoming the plasma membrane’s extracellular surface.
An example of animal cells releasing molecules via exocytosis include extracellular matrix protein secretion, neurotransmitter secretion (by neurons), peptide hormone secretion (by endocrine cells), the release of digestive enzymes into the gut, and the elimination of waste products. In plant cells, exocytosis is used to secrete enzymes and structural proteins for the cell wall. Exocytosis is regulated by a complex interplay of factors, including changes in the cytosolic calcium concentration, enzyme activity and interactions with proteins in the membrane. These regulations are critical for ensuring that the proper amount of material is secreted at the appropriate time and place, to maintain cellular homeostasis.
Endocytosis
Endocytosis (“endo” = inside/within) is a type of transport in which a cell takes in materials from its surrounding environment by forming a vesicle or vacuole around those materials. The cell’s membrane invaginates, forming a pocket around the target substance(s). The pocket pinches off, resulting in a