5.3.1 Study Notes Active Transport

A. Active transport

• transport of a specific solute across plasma membranes “up” or “against” (from low to high) 

• its concentration gradient through use of cellular energy (ATP)

  • Examples:

    • iodine is concentrated in cells of thyroid gland

    • glucose is completely absorbed into lining of digestive tract

    • sodium is mostly reabsorbed by kidney tubule lining

• requires both carrier proteins and ATP

  • therefore cells must have high number of mitochondria near membranes where active transport occurs

  • proteins are called “protein pumps”

    • the sodium potassium pump is an important carrier system in nerve and muscle cells

      • Salt (NaCl) crosses a plasma membrane because Na+  are pumped across, and the Cl-  is attracted to Na+  and simply diffuses across specific channels in the membrane.

        • Cystic fibrosis results from a faulty chloride channel protein, causing this mucus in bronchial tubes and pancreatic ducts.

B. Bulk Transport

• Exocytosis 

  • a vesicle formed by the Golgi body fuses with the plasma membrane as secretion occurs

    • insulin leaves insulin-secreting cells by this method

• Endocytosis 

  • cells take in substances by vesicle formation as plasma membrane pinches off

  • can be different forms: 

    • phagocytosis

    • pinocytosis

    • receptor-mediated endocytosis

  • phagocytosis

    • cells engulf large particles (e.g., bacteria, food particles),

    • forming an endocytic vesicle

    • example: performed by ameboid type cells (e.g., amoebas and macrophages)

      • endocytic vesicle fuses with a lysosome, digestion of the internalized substance occurs

  • Pinocytosis 

    • occurs when vesicles form around a liquid or very small particles

    • this is only visible with electron microscopy

      • Cells that use pinocytosis to ingest substances such as 

        • blood cells

        • cells that line the kidney tubules 

        • intestinal wall, and plant root cells

  • Receptor-mediated endocytosis 

    • a form of pinocytosis

    • occurs when specific macromolecules bind to plasma membrane receptors

      • The receptor proteins are shaped to fit with specific substances (vitamin, hormone, lipoprotein molecule, etc.)

      • found at one location in the plasma membrane

        • This location is a coated pit with a layer of fibrous protein on the cytoplasmic side

        • when the vesicle is uncoated, it may fuse with a lysosome

• Pits are associated with exchange of substances between cells (e.g., maternal and fetal blood)

  • this system is selective and more efficient than pinocytosis

  • it is important in moving substances from maternal to fetal blood

  • Cholesterol (transported in a molecule called a low-density lipoprotein, LDL) enters a cell from the bloodstream via receptors in coated pits

    • in familial hypocholesterolemia, the LDL receptor cannot bind to the coated pit and the excess cholesterol accumulates in the circulatory system