Bulk and Active Transport

Exocytosis

Exocytosis is cell puking.  Seriously.  This is when a cell decides to remove contents in bulk.  They will create a vesicle of all the waste, kind of like collecting all the garbage in a garbage bag.  The cell uses proteinsLarge molecules made of amino acids with various functions in the body. on the vesicle. It also uses proteins on the cell membrane. These proteins bind the vesicle to the inside of the cell membrane.  Then, a pore is created to the outside. This allows the vesicle to spill its contents into the interstitial fluidThe fluid surrounding cells within tissues. surrounding the cell. 

Endocytosis

Endocytosis is cell eating.  This type of bulk transport allows a cell to take in a large amount of substances.  In your tissues there are cellsThe basic structural and functional units of life. called phagocytes.  The prefix phago- means “to eat.”  These phagocytes roam around your tissues, eating anything that they don’t recognize.  Sometimes these cells can also be called macrophages.  There are also cells that drink the ECF.  These are called pinocytes and these cells love to drink pinot noir wine.  Kidding.  The cells that usually drink the interstitial fluids in a tissue are called dendritic cellsImmune cells in the epidermis that help fight infections..  Both pinocytes and phagocytes will present their findings to leukocytes that live in lymph nodes.  These white blood cells are then able to mount an immune response if needed.  So, phagocytes and pinocytes are like the first line of defense.

Active Transport

Active transport is essentially the opposite of diffusionPassive movement of molecules from areas of high to low concentration. and osmosis.  Instead of moving a gradient to a homogenous mixture, active transport creates a gradient.  Active transport involves movementA fundamental property of life involving motion of the body or its parts. against the grain or concentration gradientA difference in the concentration of a substance across a space.. It moves a molecule from low concentration to high concentration.  This requires energyThe capacity to do work or cause change. in the form of ATPThe energy currency of cells used for muscle contraction..  I sometimes feel like this is my life moving against the concentration gradient. It requires massive amounts of energy from me.  Anyone else feel that way? 

The Na/K Pump

I have a picture here of an interesting little protein called the sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. potassium(K⁺): Major ICF cation; essential for muscle and nerve function. pump.  It uses ATP to maintain the sodium and potassium gradients in your cells. 

Sodium is in high concentrations outside your cells and potassium is in high concentration inside your cells.  Unfortunately, because of this gradient, these ionsCharged atoms or molecules. will leak out of and into cells.  The pump basically puts the ions back into their correct location.  So, they leak, the pump puts them back.

Let’s label these pictures to tell a story.  Here’s a cell.  Let’s put some sodium cations in the IF and not as many in the cytosol.  These cations will leak into the cell.  The Na/K pump will load three sodium cations. It will use ATP to move them back to the outside of the cell.

The bottom pictures here also tell a story, but right to left.  Potassium is in high concentration inside the cell.  These potassium cations leak out of the cell. The Na/K pump loads up two of them. It then uses ATP to move them against their gradient, back into the cell.  We have 6 separate pictures here. The pump does this all in one action. The 5 ions all move at the same time.

You can see from this picture that it looks like the pump moves 3 sodium cations. It moves only 2 potassium cations. Why the disproportion?  Because sodium leaks in faster than potassium leaks out.