Movement of Bulky Substances Across the Cell Membranes

Bulky Material Cross Cell Membranes Within Vesicles 

The lipid nature of a cell’s plasma membranes creates an interesting problem for the cell. The substances that cells require for growth are mostly large polar molecules that cannot cross the hydrophobic barrier a lipid bilayer creates. How do these substances get into cells? Two processes are involved in this bulk transport: endocytosis and exocytosis.

Endocytosis

In endocytosis, the plasma membrane envelops food particles and fluids. Cells use three major types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis.Like active transport, these processes also require energy expenditure.

Phagocytosis 

If the material,the cell takes in is particulate (made up of  discrete particles), such as an organism or some other fragment  of organic matter, the process is called phagocytosis (Greek phagein, “to eat,” and cytos, “cell”). 

 Figure:Phagocytosis of a bacterium, Rickettsia tsutsugamushi, by a mouse peritoneal mesothelial cell. The bacterium enters the host cell by phagocytosis and replicates in the cytoplasm.

Pinocytosis 

If the material,the cell takes in is liquid, the process is called pinocytosis (Greek pinein, “to drink”). Pinocytosis is common among animal cells. Mammalian egg cells, for example, “nurse” from surrounding cells; the nearby cells secrete nutrients, which the maturing egg cell takes up by pinocytosis.

Figure:Pinocytosis in a smooth muscle cell. 

Virtually all eukaryotic cells constantly carry out these kinds of endocytotic processes, trapping particles and extracellular fluid in vesicles and ingesting them. Endocytosis rates vary from one cell type to another. They can be surprisingly high; some types of white blood cells ingest up to 25% of their cell volume each hour.

Receptor-Mediated Endocytosis

Sometimes endocytosis is targeted at specific molecules. In these instances the targeted molecules are transported into cells by receptor-mediated endocytosis. These molecules first bind to specific receptors in the plasma membrane—the binding is quite specific, with the target molecule shape fitting snugly into its receptor. Different cell types contain a characteristic battery of receptor types, each targeted at a different kind of molecule.

The portion of the receptor molecule that protrudes into the membrane is locked in place within an indented pit coated on the cytoplasmic side with the protein clathrin. Each pit acts as a molecular mousetrap, closing over to form an internal vesicle when the right molecule enters the pit.

Figure:A coated pit appears in the plasma membrane of a developing egg cell, covered with a layer of proteins. When an appropriate collection of molecules gathers in the coated pit, the pit deepens and will eventually seal off to form a vesicle.

The trigger that releases the trap is the binding of the properly fitted target molecule to the embedded receptor. When binding occurs, the  cell reacts by initiating endocytosis; the process is highly specific and very fast. The vesicle is now inside the cell carrying its cargo.

Exocytosis

The reverse of endocytosis is exocytosis, the discharge of material from vesicles at the cell surface. In plant cells, exocytosis is an important means of exporting the materials needed to construct the cell wall through the plasma membrane. Among protists, contractile vacuole discharge is considered a form of exocytosis. In animal cells, exocytosis provides a mechanism for secreting many hormones, neurotransmitters, digestive enzymes, and other substances.

Figure:Proteins and other molecules are secreted from cells in small packets called vesicles, whose membranes fuse with the plasma membrane, releasing their contents outside the cell. A false color transmission electron micrograph showing exocytosis(right side).

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