DIFFUSION AND OSMOSIS

A BIOLOGICAL OUTLOOK

ANUPAMA CN

I M.Sc, BT

MCC

DIFFUSION

Diffusion means free movement of substances in a random fashion caused by the normal kinetic motion of substances. Motion of these particles is called heat, the greater the motion higher the temperature, and motion never ceases at any condition, except absolute zero temperature. When a moving molecule A approaches at a stationary molecule B, the electrostatic and internuclear forces of A repel B , adding some of the energy of motion to molecule B. Consequently molecule B gains kinetic energy of motion while molecule A slows down, losing some of its kinetic energy.

This continual movement of molecules among each other in liquid or in gas is

called DIFFUSION.

KINETICS OF DIFFUSION- The Concentration difference

When a large amount of dissolved substance is placed in a sovent at one end of a chamber, it immediately begins to diffuse towards the opposite end of the chamber. If the same amount of substance is placed in the opposite end of the chamber it begins to diffuse towards the first end, the same amount diffusing in each direction. As a result the net rate of diffusion from one end to other is zero. If the concentration of the substance is greater at one end of the chamber than at the other end the net rate of diffusion from the area of higher concentration to the lower concentration is directly proportional to the larger concentration minus the lower concentration. The total concentration change along the axis of chamber is called concentration difference and the

concentration difference divided by the distance is called concentration or diffusion gradient. When the molecular size is greater, rapidity with which molecule diffuse from one point to another is less. The rate of diffusion is approximately inversely proportional to square root of molecular weight but is affected by shape of the molecule as well. Different factors which affect the rate of diffusion are:

The greater the concentration difference, greater is the rate of diffusion.

The less the square root of molecular weight, the grater is the rate of diffusion.

The shorter the distance, the greater the rate.

The greater the cross section of the chamber in which diffusion takes place, the greater is the rate of diffusion.

The greater the temperature, greater the molecular motion and greater the diffusion.

FICKE’S EQUATION

The time for equilibriation by diffusion is proportional to square of diffusion distance. This aspect has biological importance. It limits the size of individual cells since cellular metabolism depends upon the rapid diffusion of O2 & substrates from membrane to metabolic sites. In human body no metabolically active cell is more than 20 micro meters from a capillary. The rate of diffusion of molecules down a concentration gradient is given by the FICKE’S EQUATION.

dv/dt =J= - DA (dc/dx )

dv/dt= rate of diffusion in moles /sec

dc/dx is the concentration gradient down which diffusion is occurring in mol cm-3 cm-1.

‘-‘sign indicates that diffusion is occurring in direction of decreasing concentration, A is the area of plane of solution at right angles to movement. D is the diffusion coefficient . Value of D depends on size of molecule & viscosity of solution. Eg: In lung tissue , at rest only a fraction of total lung surface is used for gas exchange and while at exercise total surface area of 75 m2 becomes available for diffusion.

DIFFUSION ACROSS CELL MEMBRANE

It is divided into 2 types: Fecilitated diffusion and Simple diffusion

DIAGRAMATIC REPRESENTATION OF SIMPLE AND FACILITATED DIFFUSION

SIMPLE DIFFUSION In simple diffusion molecular kinetic movement occurs while in fecilitated diffusion , there is interaction of carrier protein with molecules and ions. The carrier protein aids passage of molecules or ions through them by binding chemically with them. Simple diffusion can take place in 2 ways

through the interstices of lipid bilayer through water channels that penetrate all the way through some of the

large transport proteins.

FACILITATED DIFFUSION Also called a carrier mediated diffusion because a substance transported in this manner diffuses through the membrane with a specific carrier protein helping it to do so. Facilitated diffusion differs from simple diffusion through an open channel in the foll important way; When the rate of diffusion approaches a maximum called Vmax, as the concentration of substance increases, in simple diffusion rate increases with the concentration of diffusing substance. Limits of Rate of Fecilitated Diffusion: The carrier protein has a channel large enough to transport a specific molecule pathway through the membrane. Also there is a binding receptor on the inside of the protein carrier, the molecule to be transported enters channel and gets bound. Then in a fraction of second, a conformational change occurs in the carrier protein, so that the channel now opens on the opposite side of the membrane. Because of the binding force of the receptor is weak, the thermal motion of the attached molecules causes it to break away and to be released to the opposite side. The most important substance that cross cell membrane by facilitated diffusion is glucose and most of aminoacids. In glucose the carrier molecule has a molecular weight of 45, 000.

DIFFUSION THROUGH PROTEIN CHANNELS

Protein channels provide watery pathway through the interstices of protein molecule. They have 2 important properties:

They are selectively permeable

Many of the channels can be opened and closed by gates, which are a means of controlling the ion permeability of the channel.

DIFFUSION VIA CARRIER PROTEIN

The opening and closing of gates are controlled in 2 principal ways:

VOLTAGE GATING:Here the molecular confirmation of the gate respond to electrical potential across the cell membrane. This gate is the basic cause of action potential in the nerves that are responsible for signals.

CHEMICAL GATING ( LIGAND GATING ): Here a chemical substance binds with the protein which helps in opening and closing of a gate. Eg : Acetyl choline in acetyl choline channel. This gating is important for the

transmission of nerve signals from one nerve to another.

FACTORS EFFECTING NET RATE OF DIFFUSION

Effects of concentration difference on net diffusion through a membrane is proportional to the concentration on outside to concentration on inside.

Effect of membrane electric potential on diffusion of ions— NEST EQUATION: At normal body temperature the electric difference that will balance a given concentration difference of univalent ions is determined by NEST EQUATION:

EMF(mV) = + or – 61 log c1/c2 EMF is electromotive force between sides 1 & 2 C1 is the concentration on side 1

C2 is concentration on side 2 The polarity of the voltage on side 1 in equation above is + for –ve ions & -ve for +ve ions. This equation is helpful for understanding nerve impulses.

Effect of pressure difference across the membrane :Pressure effect is mostly seen in blood capillary membrane in all tissues of the body. The pressure is 20 mm Hg greater inside the capillary than outside. Fecilitated diffusion is believed to depend upon the pressure in the cell membrane of a relatively small number of carrier molecules. These ferry the glucose across the membrane by first binding to the sugar at the border of the membrane at which the glucose concentration is higher. The sugar carrier complex then transfers the glucose to the other border of the membrane when it dissociates to deliver the sugar into the fluid on that side.

Substrate Specificity Eg : Glucose entry is a stereospecific process. Physiological isomer D-glucose is rapidly transported into the cell, the optical isomer L glucose is not rapidly transported.

PHYSIOLOGICAL IMPORTANCE OF DIFFUSON

Admixture of food stuff with digestive juices.

Absorption from the intestine.

Exchange between plasma and red cells.

Exchange in the capillary bed. Eg : Food stuff, oxygen etc. pass out from the blood stream to the tissue fluids and then to the tissue cells where they are used up. The metabolites including CO2 comes out of the cell, to the tissue fluids and to the blood stream.

Exchange in lung capillaries

Admixture of gases in the lungs.

OSMOSIS

The diffusion of water through a semipermeable membrane is called osmosis. If a layer of water be separated from sugar solution by a semipermeable membrane, it will be seen that the sugar solution gradually increases in volume for some time and then there will be no further change. Here the sugar molecules being impermeable the water molecules will pass from the water molecules to the sugar solution, than will pass from the latter to the former. Due to this the volume and level of sugar solution will rise. This will raise the hydrostatic pressure of the sugar solution and this increased pressure will force more and more water molecules to pass out of sugar solution. Thus a time will come when the movement of water molecules on either side will be the same , so that no further alteration in volume takes place . At this stage the hydrostatic pressure of the sugar solution exactly neutralizes the attractive force of the solution for water molecules.

OSMOSIS

This force under which a solvent moves from a solution of lower solute concentration to a solution of higher solute concentration when a selectively

permeable membrane separates these solutions is called OSMOTIC PRESSURE.

Osmotic pressure does not depend on the size of the molecules but the total number of discrete particles per unit volume. If the solution be ionizable, the osmotic pressure will be proportionally more. The passage of water against concentration gradient is called ULTRAFILTERATION. If the two solutions separated by a membrane have same OP, they are called isotonic. One having less OP is called hypotonic. Ex: 0.9% NaCl solution is isotonic with blood plasma commonly known as normal saline or physiological saline. Two solutions having same number of particles per unit volume are called isosmotic. Osmosis is important in biological systems, as many biological membranes are semipermeable. In general, these membranes are impermeable to organic solutes with large molecules, such as polysaccharides, while permeable to water and small, uncharged solutes. Permeability may depend on solubility properties, charge, or chemistry, as well as solute size. Water molecules travel through the plasma cell wall, tonoplast (vacuole) or protoplast in two ways, either by diffusing across the phospholipid bilayer directly, or via aquaporins (small transmembrane proteins similar to those in facilitated diffusion and in creating ion channels). Osmosis provides the primary means by which water is transported into and out of cells. The turgor pressure of a cell is largely maintained by osmosis, across the cell membrane, between the cell interior and its relatively hypotonic environment.

METHODS OF MEASUREMENT MECHANICAL METHOD

By putting weights: The simplest way is to apply adequate pressure upon the stronger solution to prevent any rise of pressure

BIOLOGICAL METHODS

HAMBURGER’S RED CORPUSCLE METHOD: Red cells are kept in unknown solution for sometime after which the cell volume is noted. If the cell volume be reduced the solution is hypertonic than plasma & if the cell swells up then the solution is hypotonic.

DE VRI’S PLANT CELL METHOD: Here plant cells are used instead of red cells.

PHYSICAL METHOD

By noting the depression of freezing point --- higher the concentration lower will be the freezing point and higher the OP.

By noting the vapour tension – higher the concentration lower will be the rate of evaporation.

HILL’S METHOD: Using the thermopile—higher the rate of evaporation more will be the fall of temperature and less will be the OP.

PHYSIOLOGICAL IMPORTANCE OF OSMOSIS

Absorption from intestine.

Exchange in capillary bed—continuous osmotic exchange is going on between blood, tissue fluids, tissue cells and lymph.

Regulation of urine formation.

Reabsorption of CSF.

Continuous osmotic exchange between plasma and red cells.

Using osmometry, physicians can measure dissolved salts and sugars in blood and urine samples, diagnosing a variety of diseases. Scientists also use it to measure molecular weight of an unknown compound.

CLINICAL USE : Injection of hypertonic, hypotonic and isotonic saline and other solutions are given in suitable cases by this method. Saline purgatives and saline diuretics also work on osmotic principle. OSMOMETRY is the measurement of the osmotic strength of a solution, colloid, or compound. There are a wide variety of osmometers that may be used for this purpose: vapor pressure, freezing point depression, or membrane osmometry are the most common.

Vapor Pressure Osmometry: the osmometer determines the concentration of osmotically active particles that may reduce a solution’s vapor pressure. Large numbers of particles depress pressure. Similarly, large quantities of

solute depress the freezing point of a solvent; in the case of water, salt depresses its freezing point to zero degrees Fahrenheit.

OSMOMETER

Membrane Osmometry: It separates the solvent from the polymer solution with a semipermeable membrane, held tightly between two chambers. The flow from solvent to polymer is measured to determine the osmotic strength. When pressure is reduced on the solvent side, this tendency is reduce; when the two pressures are in equilibrium, the pressure on the solvent side can be measure to determine osmotic pressure.

REFERENCE

http://www.iscid.org/encyclopedia/Osmometry

http://www.biologycorner.com/APbiology/cellular/%28notes%29cell_membrane.

html

http://en.wikipedia.org/wiki/Fick%27s_laws_of_diffusion

http://www.similima.com/phy28.html

http://en.wikipedia.org/wiki/Osmosis