Lets learn about cellular transport mechanisms.
Many substances move in and out of cells.
In order to get substances into or out of the 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 lipid
tail that hates water hydrophobic.
The phospholipids arrange themselves into a 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.
Steroids are 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 where there is more dye (an area of higher concentration)
to where there is less dye (area of lower concentration).
The molecules will eventually evenly distribute throughout the solution.
Non-lipid soluble substances cannot pass through the 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, chloride
and potassium.
We can say that these substances move by facilitated diffusion.
In facilitated diffusion substances move from higher to lower concentration until reaching
equilibrium through a protein channel.
Substances can also move against gradients.
Here we have more sodium outside of the cell than 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 proteins use ATP for energy.
An example of an active transport protein is the sodium-potassium pump.
The sodium-potassium pump moves 3 sodium outside of the cell and 2 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 allows
water through but not solute.
Water will move toward an area of higher solute concentration.
If a beaker is separated into 2 compartments by a semipermeable membrane, and each compartment
has a different solute concentration, water will move toward the area of higher solute
concentration.
Tonicity or concentration can be described in terms of isotonic, hypotonic and hypertonic
solutions.
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 into a 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 the
cell.
Other methods of moving substances in and out of cells include exocytosis and endocytosis.
In exocytosis vesicles inside of the cell adhere 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 material such as debris or bacteria.
In pinocytosis the membrane enfolds around extracellular fluid to bring it into the cell.
In receptor-mediated endocytosis substances attach 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.