The Cell's Membranes


Active Transport

 

Cells need to regulate their internal environment, a process we call homeostasis. To do so requires energy.

Ions and large food particles must be transported into a cell across membranes generally impermeable to them. Waste products must be removed from the cytoplasm, and other molecules or ions are actively pumped in one direction or the other to create concentration gradients which can be used to do other tasks.

Energy stored in ATP molecules is used to accomplish many of these membrane transport processes.

Active Transport is reviewed in a Mini Essay in Kimball's Biology Pages.

 

Two different active transport mechanisms are used by cells.

  1. Special transmembranal proteins, activated by ATP change shape, triggered by the individual molecules they are "designed" to transport.

 

  1. Entire sections of the cell membrane can be altered either infolding--endocytosis or outfolding, usually by fusing with a vesicle from the Golgi Body--exocytosis.

 

Membranal carriers, transmembranal proteins or peripheral proteins, are able to change shape in such a way as to open a passage for a specific molecule, allowing it to diffuse from one side of the membrane to the other. The shape change is generated by energy from ATP and occurs only in the presence of the transported molecule.

In some cases, the substance may be physically carried by the protein to the other side of the membrane before being released.

 

Two types of membrane carrier molecules are known.

 

  1. Uniport carriers move a single substance, either a molecule or ion, across the membrane.

    Important examples include:

    1. proton pumps, carriers which transport hydrogen ions against the concentration gradient and
    2. calcium pumps which sequester Ca2+ ions which can be rapidly released when needed

 

  1. Cotransport carriers are designed to move two different substances simultaneously in the same or opposite directions.
Important examples include:

 

  1. In the digestive tract sodium ions are pumped out in order to bring glucose into the cell.
  2. Neurons use a sodium/potassium ion exchange pump to move 3 Na+ ions out of the cell for every 2 K+ ions brought in. This establishes the electrical potential necessary for nerve impulses.

 

Membrane mediated movement of substances.

Endocytosis allows cells to bring large particles--phagocytosis or lots of smaller particles--pinocytosis into a cell as a membrane enclosed vacuole. The diagram below illustrates this process.

Endocytosis begins as a depression in the plasma membrane (1), eventually forming an "inpouching" (2), and finally a spherical vacuole (3).

Review endocytosis by reading the Mini Essay at Kimball's Biology Pages.


Exocytosis results in the expulsion of materials from the cell and involves the fusion of a vesicle within the cytoplasm to the plasma membrane.

Because cells are constantly losing plasma membrane to various processes such as endocytosis, the replenishing of membrane components must be maintained by exocytosis. This process may also be an important method for adapting membranes to changing conditions.


Modified July 10, 2005