Cell Transport for Anatomy and Physiology

Lets learn about cellular transport mechanisms.Many substances move in and out of cells. In order to get substances into or out ofthe cell they must get through the cell membrane. Lets take a closer look at the cell membrane.The cell membrane consists of a phospholipid bilayer.Phospholipids consists of a phosphate head that loves water (hydrophyllic) and a lipidtail that hates water hydrophobic. 

The phospholipids arrange themselves intoa bilayer with the head pointing outward and the tails pointing inward.Lipid soluble substances can pass through the phospholipid bilayer.Some examples of lipid soluble substances include oxygen and carbon dioxide. Steroidsare another example. We can say these substances move by diffusion.In diffusion substances move from higher to lower concentration until reaching equilibrium.Here is an example of diffusion. 

A drop of dye in water will move from wherethere is more dye (an area of higher concentration) to where there is less dye (area of lowerconcentration). The molecules will eventually evenly distributethroughout the solution.

 Non-lipid soluble substances cannot pass throughthe membrane so they need a special doorway called a protein channel.In this example, sodium moves into a cell through a protein channel.Other examples of substances that move through protein channels include glucose, chlorideand potassium.

 We can say that these substances move by facilitateddiffusion. In facilitated diffusion substances move fromhigher to lower concentration until reaching equilibrium through a protein channel.Substances can also move against gradients.

 Here we have more sodium outside of the cellthan in. 

If sodium moved by facilitated diffusion it would move into the cell.However, energy can be used to move sodium against its gradient or outside of the cell.Substances can move against their gradients using active transport proteins. These proteinsuse ATP for energy. An example of an active transport proteinis the sodium-potassium pump.

 The sodium-potassium pump moves 3 sodium outside of the cell and2 potassium inside of the cell for every molecule of ATP used.The sodium-potassium pump works to maintain concentration gradients.In osmosis water moves through a semipermeable membrane..which is a membrane that allowswater through but not solute. Water will move toward an area of higher soluteconcentration.

 If a beaker is separated into 2 compartmentsby a semipermeable membrane, and each compartment has a different solute concentration, waterwill move toward the area of higher solute concentration.Tonicity or concentration can be described in terms of isotonic, hypotonic and hypertonicsolutions.

 Isotonic solutions have the same concentration as body fluids.Hypotonic solutions are less concentrated than body fluids.Hypertonic solutions are more concentrated than body fluids.In this experiment a red blood cell is dropped into a hypotonic solution.Water will flow into the red blood cell because the cell contains a higher solute concentration.The cell will swell and burst. 

Likewise if a red blood cell is dropped intoa hypertonic solution, water will flow out of the cell causing it to shrink or crenate.Water again flows toward the higher concentration of solute which this time is outside of thecell. Other methods of moving substances in andout of cells include exocytosis and endocytosis. 

In exocytosis vesicles inside of the celladhere to the cell membrane and release substances neurotransmitters a re released this way.In endocytosis the cell membrane enfolds around substances to bring them into the cell.There are different kinds of endocytosis. In phagocytosis the cell engulfs materialsuch as debris or bacteria.

 In pinocytosis the membrane enfolds aroundextracellular fluid to bring it into the cell. In receptor-mediated endocytosis substancesattach to membrane receptors that signal the cell membrane to enfold around the substance.We hope you have learned something about cell transport and see you next time.