Membrane Transport

Cell Membranes

Cell membranes separate the contents of cellsThe basic structural and functional units of life. which we can consider living from the outside or non-living environment.  Therefore, cells are known as the boundary of life.  We have a picture here of some eggs.  The shell is not the cell membrane but there is a cell membrane inside of the shell.  It’s important to remember that eggs can go bad over a long period of time and that this is because they continue to exchange gases like oxygen and carbon dioxide with the external environment.  Just a note that if you are prepping for the end of the world you can preserve your eggs in mineral oil which will slow the exchange and allow them to last for a very long time. 

The major component of a cell membrane are those phospholipids.  Remember that these look like a balloon with 2 strings.  The headRounded proximal end that fits into the acetabulum of the hip bone. is polar and the tails are nonpolar.  Their orientation in the bilayer has them facing tail to tail, with the polar heads exposed to the waterThe universal solvent essential for life. both inside and outside the cell.  This creates a hydrophobic region of the bilayer across which many polar moleculesGroups of atoms bonded together. cannot cross.  However, nonpolar gases like oxygen and carbon dioxide are freely able to cross the membrane.

Other Items in the Cell Membrane

The lipid bilayer is the primary component of a cell membrane but there are other items inserted in or attached to the lipid bilayer.  Cholesterol is added to the cell membrane for rigidity.  The cell membrane has a sugary coating of glycolipids and glycoproteinsProteins with carbohydrate groups attached, involved in cell signaling. exposed to the extracellular fluid(ECF) Fluid outside cells, including plasma and interstitial fluid..  The extracellular fluid is outside the cell.  And finally there are various proteinsLarge molecules made of amino acids with various functions in the body. bedded in the cell membrane. 

Cell Membrane Protein Functions

Cell membrane proteins have various functions.  Proteins embedded in a cell membrane can function as enzymesProteins that speed up chemical reactions in the body. catalyzing reactions.  Attachment proteins can secure the cell to the extra cellular matrix.  Receptor proteins are important on cells like neuronsThe functional cells of the nervous system that transmit signals. and muscle cells that receive signals from other neurons.  Receptor proteins can connect with hormones such as epinephrineadrenaline): Fight-or-flight hormone from the adrenal medulla..  In factA statement based on direct observation that is repeatedly confirmed., when you have an adrenaline rush, many organs in your body like your trachea and your heart respond to the epinephrine or adrenaline flowing through your bloodstream because they have these receptorA structure that detects stimuli. proteins.  There are transport proteinsBind hormones for transport in the blood. in the cell membrane that can help with facilitated diffusionPassive movement of molecules from areas of high to low concentration. or active transport.  Junction proteins can bind one cell to another cell such as the cells in your skinThe body’s largest organ, providing protection and regulation. which have very tight junctions to prevent entry of microbes and damaging chemicals.  And finally those glycoproteins and glycolipids attached to the outside of the cell are capable of connecting with other cells for recognition.  These are the types of proteins that are used by your immune system to recognize your own cells as well as foreign cells. 

Diffusion

Diffusion is the movementA fundamental property of life involving motion of the body or its parts. from an area of high concentration to an area of low concentration.  This is a passive movement with no energyThe capacity to do work or cause change. required.  You’re familiar with this.  If I were to put a droplet of ink into a glass of water, overtime you would see that the ink molecules disperse or diffuse out to areas of lower concentration.  Ultimately, this will result in the ink molecules being evenly dispersed throughout the water creating a homogeneous mixture.

Concentration
Gradients

Concentration gradients determine the movement of molecules.  In this picture here we have a concentration gradientA difference in the concentration of a substance across a space. with lots of solute molecules on the left and not many solute molecules on the right.  Solute molecules are going to move from the left to the right or from the high to low concentration until there is a homogeneous mixture.  The molecules will not move from right to left or from low to high concentration because energy would be required for that movement against the concentration gradient.

Tonicity

Tonicity is the concentration of solutes (for our purposes in this call solutes are gases or solids) within a solvent (for our purposes in this class solvents are usually liquids).  I have drinks here that are ordered from left to right in their increasing solute concentration or increasing tonicityThe ability of a solution to affect the water balance in a cell..  If we start with water and add carbon dioxide we get seltzer.  If we add salt to our seltzer we get club soda.  And if we add quinine and a whole bunch of other stuff we can finally get tonic water.  We can say that these solutions are ordered from left to right from hypotonic too hyperPrefix meaning “above normal.” tonic.  These words are relative such as seltzer is hypertonicA solution with a higher solute concentration than the inside of a cell, causing water to leave th to water however seltzer is hypotonic to club soda.

Osmosis

Osmosis is the movement of water only through a selectively permeableA membrane that allows some substances to pass while blocking others. membrane.  Therefore, osmosis is the movement of water into and out of cells through the lipid bilayer but also through proteins imbedded in the cell membrane.  I like this cartoon down here because it reminds you that osmosis is the movement of water only.  I don’t know why it have to be Abe Lincoln, but maybe being insulted by him is just really insulting.  So don’t be a fool. 

Osmosis Consequences

We want to learn about osmosis to determine if cells will shrivel or burst in certain types of environments.  The rule to remember is that water follows salt.  This means that water will always flow to a hypertonic environment and try to flood the salty area.   Many diuretics, which make you pee removing water from your body, actually remove salt from your body which draws the water along with it.  These cells here on the left are submerged in a hypertonic environment, therefore water will want to leave these cells and flood the hypertonic environment.  These red blood cells will eventually shrivel called crenation and this plant cell will do something called plasmolysis.  These cells here on the right are submerged in a hypotonic environment.  Therefore, water will be drawn into the cells which would have a hypertonic environment these red blood cells would ultimately burst and the plant cells would become turgid or pressurized with water in the very full vacuole you can see here.  Many students have a common misconception that when there is a homogeneous mixture between the outside and inside of a cell that movement ceases.  This is untrue when there is an isotonicA solution with the same solute concentration as the inside of a cell, maintaining equilibrium. environment water continues to be exchanged freely between the outside and inside environments.  This is what cells strive for and this is essentially homeostasisThe maintenance of a stable internal environment in the body..

Facilitated Diffusion

Facilitated diffusion is movement down a concentration gradient from high to low but requiring a protein to cross the plasma membraneThe outer boundary of a cell that controls what enters and exits..  Water and other polar molecules are not able to cross the lipid bilayer because they get repelled by the hydrophobic region created by the nonpolar tales of the phospholipids.  Therefore, a transport protein acts like a bridge spanning the lipid bilayer and covering the hydro phobic region.  Now the polar molecules are free to move down their concentration gradient.  Water crosses into an out of cell membranes using a protein called an aquaporin.  When you drink a diuretic like coffee or alcohol aquaporins are removed from the urineThe liquid waste excreted by the kidneys. generating tubules in your kidney you are incapable of conserving water.  This is commonly known as breaking the seal which is an undeniable urge to urinate.  Also, the urine and will be incredibly dilute and flooded with the water that you were unable to conserve.

Active Transport

Active transport is essentially the opposite of diffusion and osmosis.  Active transport is movement against the grain or against the concentration gradient, moving a molecule from low concentration too high concentration.  This requires energy in the form of ATPThe energy currency of cells used for muscle contraction..  I sometimes feel as though this is my life moving against the concentration gradient and requiring massive amounts of energy from me.  Anyone else feel that way?  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.  You can see the pump moving sodium from its low concentration to its high concentration.  It will also move potassium from its low to its high concentration.