Summary
Highlights
Cells transport materials across their lipid bilayer membrane, which integrates protein and carbohydrate components. Some molecules diffuse through, others use membrane proteins. Large cargo transport involves endocytosis and exocytosis.
Five categories of molecules attempt to cross the cell membrane: small non-polar molecules (like oxygen) diffuse quickly, small polar molecules (like water) cross slowly, large non-polar molecules (like vitamin A) cross very slowly, and large polar molecules (like glucose) or charged ions (like Na+, K+) have difficulty crossing without assistance. Many common molecules like water, glucose, and ions use transport proteins.
When cells need to transport large quantities or very large molecules, they employ bulk transport: endocytosis (taking material in) and exocytosis (expelling material). Both processes require energy in the form of ATP.
There are three types of endocytosis. Phagocytosis (cell eating) is used by white blood cells like macrophages. It involves the cell extending pseudopods to engulf material, forming a phagosome, which then fuses with a lysosome to digest the material. The remnants are expelled via exocytosis.
Pinocytosis (cell drinking) is a non-specific process where the plasma membrane invaginates to form a small cup around extracellular fluid and dissolved solutes, creating a pinocytotic vesicle. These vesicles are much smaller than phagosomes.
Receptor-mediated endocytosis uses special receptor proteins on the cell membrane, often in clathrin-coated pits, to selectively internalize specific molecules like transferrin or LDL. Once internalized, the clathrin detaches, and the vesicle fuses with an endosome. Endosomes separate the cargo from receptors, allowing for receptor recycling back to the cell surface.
Exocytosis begins in the Golgi apparatus, where proteins, lipids, and hormones are packaged into vesicles. These vesicles are transported along the cytoskeleton (microfilaments, microtubules, intermediate filaments) using motor proteins (kinesin, dynein) powered by ATP. The vesicle then fuses with the cell membrane, releasing its contents into the extracellular space.
Endocytosis is the process of cells taking in extracellular material, with forms including phagocytosis, pinocytosis, and receptor-mediated endocytosis. Exocytosis is the process of cells expelling material into the extracellular space. Both processes are energy-dependent, requiring ATP.