Movement of substances across cell membranes

Dif­fu­sion

Dif­fu­sion is the spon­ta­neous move­ment of a sub­stance down its con­cen­tra­tion gra­di­ent from higher to lower con­cen­tra­tion.  If you have a bot­tle of per­fume and take the cap off.  The per­fume mol­e­cules will waft out and dif­fuse into the air where there is a lower con­cen­tra­tion of them.  All atoms and mol­e­cules are in motion, the only way you could stop all motion is to cool things down to absolute zero (-273.15 C or 0K).  On the bot­tom of b25 , look at the dif­fu­sion thing and read it.

Fac­tors that affect the RATE of diffusion

1. Dif­fer­ence in con­cen­tra­tion between the inside and out­side of the cell.  The big­ger the dif­fer­ence between con­cen­tra­tions, the dif­fu­sion will be faster.

2. The size of the chem­i­cal sub­stance.  O₂ is two atoms. Glu­cose is 24 atoms big.  Pro­tein is mas­sive.  Oxy­gen can eas­ily dif­fuse across a cell mem­brane.  Sugar can kind of, that’s why it’s assisted by a trans­porter pro­tein to facil­i­tate it.  The pro­teins don’t move at all.

3. Tem­per­a­ture. Higher temps = mol­e­cules move faster. ‘Nuff said.

4. Whether the chem­i­cal sub­stance is water-soluble or lipid sol­u­ble.  The lipid sol­u­ble goes through faster because the cell mem­brane is phos­pho­lipids and can eas­ily dif­fuse through a fatty membrane.

The effect of osmo­sis on cells

Osmo­sis is sim­ply the dif­fu­sion of water.  Any­time water flows in or out, it’s called osmo­sis.  The water flows to wher­ever there is more solute.

1. The cyto­plasm of a cell is 80% water, but not com­pletely.  Any­thing that’s not water are called solutes, such as elec­trolytes.  If we sur­round the cell with a fluid that’s sim­i­larly 80% water and 20% “other stuff” on the out­side of the cell.  The “other stuff” don’t even have to be the same things as the inside of the cell.  With osmo­sis we only care about the con­cen­tra­tion of the water mol­e­cules.  This fluid sur­round­ing the cell is said to be iso­tonic. (Iso– means same and –tonic means tension)

2. Imag­ine another sce­nario with the same cell but this time it’s sur­rounded with 100% dis­tilled water.  Osmo­sis will occur because water will flow inside the cell.  The cell will swell until it rup­tures and when it pops it’s called lysis.  When a red blood cell pops it’s called hemol­y­sis.  It would not be a good idea to inject dis­tilled water into their blood.  This is known as a hypo­tonic solu­tion because it’s low in solute. (Mnemonic: When you place a cell in a hypo­tonic solu­tion, it swells up like a hippo.)

3.  Imag­ine we place the same cell in a con­cen­trated salt solu­tion with 20% water only.  Water will flow out of the cell caus­ing it to shrink and that’s called cre­na­tion.  This solu­tion is known as a hyper­tonic solution.

In any sit­u­a­tion when things will be injected will be an iso­tonic solu­tion.  Always remem­ber, the water flows wher­ever there is more solute, like when some­one puts salt on a snail, the snail dies because the water goes to the salt and the snail gets dehy­drated (its cells cre­nate due to the hyper­tonic solution).

Pro­tein trans­porters and active trans­port “pumps”

Facil­i­tated dif­fu­sion: Some larger mol­e­cules, such as glu­cose, need help by way of a trans­port pro­tein to help carry glu­cose into a cell and this requires no energy.

Pri­mary Active Trans­port: H⁺, Na⁺, Ca⁺² all uti­lize ATP to force these things across the mem­brane.  There is one major dis­tinc­tion between active trans­port and facil­i­tated dif­fu­sion other than one need­ing energy and the other not.  Active trans­port always causes a mol­e­cule to be forced to move from a lower con­cen­tra­tion to higher, which it does not want to nat­u­rally do.  The most impor­tant of all trans­port pumps are the one that uses ATP to simul­ta­ne­ously force K⁺ (potas­sium ions) into the cell and Na⁺  (sodium ions) out of the cell.  This is famously called the Sodium-Potassium pro­tein pump.  Active trans­port processes account for about 40% of the energy used in the body so this is a major thing in case you thought this wasn’t important.

Sec­ondary active trans­port: These is when one chem­i­cal is being forced in or out of the cell and another chem­i­cal just sort-of fol­lows through.  The two terms used for this is SYMPORT and ANTI-PORT.  Sym– means together and two chem­i­cals are going together in the same direc­tion.  Anti-port is when one chem­i­cal is forced in one direc­tion and another is pas­sively going in the other direction.

Anal­ogy: Pic­ture those large turnstile/revolving doors.  Imag­ine you’re push­ing the door to get in and another per­son kind of fol­lows along behind you with­out doing any work.  That’s sym­port.  Again, using the turn­stile exam­ple, imag­ine you’re push­ing the door to get in and another per­son sneaks out with­out doing any work to push them­selves out.  That’s anti-port.

Dif­fu­sion ver­sus Active Transport

Let’s learn how to read this graph below which is a process.  The hor­i­zon­tal x-axis is the inde­pen­dent vari­able.  The ver­ti­cal y-axis is the depen­dent vari­able.  In this par­tic­u­lar exam­ple, the inde­pen­dent vari­able is the “extra­cel­lu­lar solute con­cen­tra­tion.”  The depen­dent vari­able is the “flux into the cell.”  Flux means movement.

You see a lin­ear line labeled dif­fu­sion.  Then we have another line labeled medi­ated trans­port which refers to active trans­port.  To help us visu­al­ize what this graph is try­ing to show: Imag­ine we have a cell and we were to increase the con­cen­tra­tion of a chem­i­cal (solute) out­side a cell (extra­cel­lu­lar).  We want to see what affect this increase in solute affects the move­ment of it into the cell.  We always begin describ­ing a graph with “As the…” and we look at the x-axis first.  In the case of dif­fu­sion, as the extra­cel­lu­lar solute con­cen­tra­tion begins to increase, the rate of dif­fu­sion into the cell increases lin­early.  In the case of active trans­port, you’ll notice that as we increase the con­cen­tra­tion, the rate of trans­porta­tion reaches a max­i­mum flux limit.

Phago­cy­to­sis (Endo­cy­to­sis) and Secre­tion (Exocytosis)

Phago– means to eat and this is when the cell swal­lows some­thing into a food vac­uole and it’s aka endo­cy­to­sis.  ATP energy is used to engulf these substances.

The exact oppo­site is called secre­tion or exo­cy­to­sis (exo– as in some­thing being external/toward the out­side) and this also requires energy in the form of ATP.

On a final note… our needs as organ­isms actu­ally reflect the needs of our cells.

The rea­son why we have lungs is that our cells need oxy­gen for cel­lu­lar res­pi­ra­tion.  We also need lungs to get rid of car­bon diox­ide to get rid of this waste prod­uct.  The rea­son we need kid­neys is to get rid of ammo­nia, urea and uric acids.  If cells weren’t gen­er­at­ing these waste prod­ucts you wouldn’t need kid­neys.  The rea­son you need a diges­tive sys­tem is so that these things will get absorbed by your body when you eat food.  Sounds sim­ple enough but it’s just a reminder that every­thing is there for a rea­son and that’s why you must learn all of this.

Now it’s time for you to read about the prin­ci­ple of homeostasis…

Basic Phys­i­ol­ogy Primers

• The Con­cept of a Ref­er­ence Person
• Review of Cell Mem­brane Structure
• The Pro­teins Embed­ded in Cell Membranes
• Fluid Com­part­ments within the Human Body
• Move­ment of sub­stances across cell membranes
• The Prin­ci­ple of Homeostasis
• Reg­u­la­tion of Body Temperature
• Inflam­ma­tion and the Patho­phys­i­ol­ogy of Fever
• The Men­strual Cycle