Types of movement across the cell membrane

Substances need to move around the body of an organism. To do this they must cross Biological Membranes. There are a number of ways in which this may happen.

Passive Transport

Passive transport is a movement of biochemical and other atomic or molecular substances across cell membranes without need of energy input.

Kinds of Passive Transport

There are three main kinds of passive transport.

1- Osmosis2- Diffusion3- Facilitated diffusion

Osmosis

Osmosis is defined as the diffusion of water across a semi-permeable membrane.

Osmosis involves movement of water from regions in which there are relatively more water molecules per volume, i.e. "high water potential", to regions where there are fewer water molecules per volume "lower water potential". 

Plasma membrane is permeable to water but not to soluteo Solute = dissolved particleo Solvent = liquid medium in which particles may be dissolved

Water moves from solution with lower concentration of dissolved particles to solution with higher concentration of dissolved particles

Water moves from dilute solution to concentrated solution Osmotic potential is the total of all dissolved particles

Osmosis Produces a Physical Force

Movement of water into a cell can put pressure on plasma membrane Animal cells will expand and may burst

o Some cells, such as Paramecium have organelles called contractile vacuoles which are basically little pumps which pump excess water out of cell

o You can alter the rate of contractile vacuole pumping by placing it in increasingly hypotonic solutions

Organisms with a cell wall, such as plants, do not bursto Cell membrane pushes against cell wallo The rigid cell wall resists due to its own structural integrityo These opposing forces create turgidity, which keeps plants uprighto If you don't water a plant, it wilts (this is called plasmolysis). Water it, the leaves

will come back up do to the reestablishment of turgidity.

Solution Types Relative to Cell

Hypertonic Solution: Solute concentration higher than cell

o More dissolved particles outside of cell than inside of cello Hyper = more (think hyperactive); o Tonic = dissolved particleso Water moves out of cell into solutiono Cell shrinks

Hypotonic Solution: Solute concentration lower than cell

o Less dissolved particles outside of cell than inside of cello Hypo = less, under (think hypodermic, hypothermia); o Tonic = dissolved particleso Water moves into cell from solutiono Cell expands (and may burst)

Isotonic Solution: Solute concentration equal to that of cell

o No net water movement

Diffusion

Spontaneous movement of particles from an area of high concentration to an area of low concentration

Does not require energy Occurs via random kinetic movement Net diffusion stops when concentration on both sides equal (if crossing a membrane) or

when there is a uniform distribution of particleso Equilibrium is reachedo Molecules continue to move, but no net change in concentration (hence the phase

"net diffusion" aboveo Diffusion of one compound is independent to diffusion of other compounds

Factors Affecting Diffusion across a Plasma Membrane

Several factors that affect the rate of diffusion include temperature, density of the diffusing substance, medium of diffusion and concentration gradient.

Diffusion directly through lipid bilayero The greater the lipid solubility of the diffusing particle, the more permeable the

membrane will beo All else being equal, smaller particles will diffuse more rapidly than larger

particles.o O2, H2O, CO2 rapidly diffuse across lipid bilayer.

Diffusion of Hydrophilic Molecules Across a Plasma Membraneo Plasma membrane is semipermeable.o Water, while polar, is small enough to freely move across the plasma membrane.o Larger hydrophilic uncharged molecules, such as sugars, do not freely diffuse.o Charged molecules cannot diffuse through lipid bilayer.o Ion channels and specific transporters are required for charged molecules and

larger, uncharged molecules.

Temperature

As temperature increases the average kinetic energy of particles increases. Greater kinetic energies lead to increased velocities. The increased velocity means that there is a greater chance of collisions between particles, resulting in an increased rate of diffusion. Generally, the rate of diffusion increases with temperature.

Density of Diffusing Substance

Density is defined as the amount of material that exists within a given volume. Regions of high density contain a greater number of particles per unit volume than regions of lower density. An increased number of particles leads to a greater chance of collisions, and this leads to an increased rate of diffusion. A lower number of particles leads to a reduced chance of collisions and this lowers the rate of diffusion. Therefore, high-density regions have a greater rate of diffusion than low-density regions.

Medium of Diffusion

Diffusion also depends upon the medium in which it takes place. Physically, the particles within the medium act as a barrier to diffusion. This means that the greater the number of molecules or larger particles within the medium, the lower the rate of diffusion.

Concentration Gradient

The concentration of a substance is defined as the number of solute molecules that can be found within a given volume. A large difference in concentration leads to a greater probability of molecular collisions over the region and therefore increases the rate of diffusion. Generally, the greater the concentration gradient, the greater the rate of diffusion.

Facilitated Diffusion

Allows diffusion of large, membrane insoluble compounds such as sugars and amino acids

Does not require energy (passive) Highly Selective Substance binds to membrane-spanning transport protein Fully reversible - molecules may enter the cell and leave the cell through the transport

protein. Particles move from areas of high concentration to areas of low concentration.

How to Cheat - Glucose Enters the Cell by Facilitated Diffusion

Glucose binds to transport protein

Transporter changers conformation and glucose is released into cell Intracellular glucose is immediately phosphorylated

o phosphorylated glucose does not diffuse out (remember that the transport protein is very specific)

o internal glucose (unphosphorylated) concentration remains low providing large concentration difference.

Regulation of Glucose Uptake by Insulin

Insulin stimulates increase in number of glucose transporters at membrane surface

o Increase number of transporters increases diffusion rateo Driving force (phosphorylation) remains the same

Low insulin levels decrease the number of glucose transporters at membrane surface

o Portions of membrane with transporters endocytose, trapping the transport protein in a vesicle

o Vesicle cannot refuse with membrane until insulin levels increase

Diabetes

Type I - Juvenile Diabetes - cannot make insulino Autoimmune diseaseo Insulin-secreting pancreatic cells destroyed

Type II - Adult Onset Diabetes - loss of ability to respond to insulino Lack of membrane receptors for insulino Therefore, cannot mobilize enough facilitative transport proteins to

surface

References:

https://en.wikipedia.org/wiki

www.ivyroses.com

http://www.uic.edu

https://www.boundless.com/biology/

http://alevelnotes.com/