Transport Across the plasma membrane

Dr Sreekumar A

Transport across the plasma membrane is essential to the life of a cell.
Certain substances must move into the cell to support metabolic reactions.
Certain substances produced by the cell for export and cellular waste materials must move out.

Two types of transport processes are 

Passive transport

• Substances move down its concentration or electrical gradient to cross the membrane using its own kinetic energy.
• Kinetic energy is intrinsic to the particles that are moving.
• No input of energy from the cell.
• Eg: Diffusion ,Osmosis.
• In facilitated diffusion substances must bind to specific proteins to cross a cellular membrane.

Active transport

• Cellular energy is used to drive the substance uphill against its concentration or electrical gradient.
• Cellular energy in the form of ATP is used here.

Passive Transport

Diffusion.

Diffusion is the net movement of material from an area of high concentration of that material to an area with lower concentration. The difference of concentration between the two areas is often termed as the concentration gradient, and diffusion will continue until this gradient has been eliminated. Since diffusion moves material from area of higher concentration to the lower, it is described as moving solutes “down the concentration gradient” (compared with active transport which often moves material from area of low concentration to area of higher concentration, and therefore referred to as moving the material “against the concentration gradient”).

If and when the concentration gradient has been eliminated, no net exchange of material occurs. Although material may move forth from one area to the other, it will be balanced by movement of the same amount of material to the opposite direction.

Diffusion is biologically important because it enables the abolishment of concentration gradients in the body. For example, metabolic activity will consume oxygen  which will reduce its concentration in the bloodstream; diffusion of oxygen in the alveoli of the lungs allows it to be replenished.

Is a passive process in which the random mixing of particles in a solution occurs because of particles own kinetic energy.

Here both the solutes(dissolved substance) and the solvent(Liquid that does the dissolving) undergo diffusion.

If  a particular solute is present in high concentration in one area of a solution and in low concentration in another area,solute molecules will diffuse toward the area of lower concentration.ie they move down their concentration gradient.Later partilcles become evenly distributed throughout the solution and the solution is said  to be at equilibrium.The particles will continue to move about randomly due to their kinetic energy but without change in concentration.

• Substance may also diffuse through a membrane if the membrane is permeable to them.
• Factors which influence the rate of diffusion across plasma membrane
• Steepness of the concentration gradient.
• The greater the difference between the two sides of the membrane,the higher the rate of diffusion.
• Temperature.
• The higher the temperature,the faster the rate of diffusion.
• Mass of the diffusing substance.
• The larger the mass of the diffusing particle,the slower its diffusion rate.
• Surface area : The larger the membrane surface area available for diffusion the faster the diffusion rate.

Eg: Air sacs of the lungs have a large surface area available for diffusion of oxygen from the air into the blood.Some lung diseases,such as emphysema,reduce the surface area. This slows the rate of oxygen diffusion and makes breathing more difficult.

• Diffusion distance : The greater the distance over which diffusion must occur,the longer it takes.

Diffusion across a plasma membrane takes only a fraction of second because the membrane is so thin.In pneumonia fluid collects in lung;the additional fluid increases the diffusion distance because oxygen must move through both the built-up fluid and the membrane to reach the blood stream.

Diffusion through the lipid bilayer

• The basic structural framework of the plasma membrane is the lipid bilayer.
• Non polar,hydrophobic molecules diffuse freely through the lipid bilayer of the plasma membrane of cells without the help of  membrane transport proteins.Eg: O_2,CO _2,Nitrogen gases,fatty acids,steroids,fat soluble vitamins.
• Eg: Movement of oxygen and carbon dioxide between blood and body cells.Movement of oxygen and carbondioxide between blood and air during breathing.

Diffusion through membrane ion channels.

• Most membrane channels are ion channels.
• Each ion can diffuse across the membrane only at certain sites.
• In plasma membrane the most numerous ion channels are selective for Potassium or Chloride ions.Fewer channels are available for Sodium and Calcium ions.
• Diffusion of ions through channels is generally slower than free diffusion through the lipid bilayer.However more than a million potassium ions can flow through a K channel in one second!
• A channel is said to be gated when part of the channel protein acts as a “plug” or “gate”,changing shape in one way to open the pore and in another way to close it.
• When the gates of a channel are open,ions diffuse into or out of cells,down their electrochemical gradients.
• The plasma membranes of different types of cells have diffeent numbers of ion channels and thus display different permeabilities to various ions.

Osmosis
Osmosis is the diffusion of a solvent across a membrane to a region of higher solute concentration. (In biological processes then, it usually is diffusion of water molecules). Most cell membranes are permeable to water, and since the diffusion of water plays such an important role in the biological functioning of any living being, a special term has been coined for it — osmosis.

• Is a net movement of solvent through a selectively permeable membrane.
• In living systems, the solvent is water, wich moves by osmosis across plasma membranes from an area of higher water concentration to lower concentration.
• In other words water move through a selectively permeable membrane from an area of lower solute concentration to an area og higher solute concentration.
• Water molecules pass through a plasma membrane in two ways
• By moving through the lipid bilayer
• By moving through aquaporins(integral membrane proteins that function as water channels)

Osmosis occurs only when a membrane is permeable to water but not to certain solutes.

• Osmotic pressure
• Hydrostatic pressure
• Solution’s tonicity

A solution’s tonicity is a measure of the solution’s ability to change the volume of cells by altering its water content.

• Isotonic
• Hypertonic
• Hypotonic

Medical uses of Isotonic,Hypertonic and Hypotonic solutions.

Isotonic solution

RBC’s and other body cells may be damaged or destroyed if exposed to hypertonic or hypotonic solutions.So most IV fluds are isotonic.

    Eg: isotonic saline (0.9% NaCl)

    D5W (dextrose 5% in water)

Hypertonic solution
Mannitol is useful to treat patients with cerebral oedema where there is excess interstitial fluid in the brain.This causes osmosis of water from interstitial fluid to blood.kidneys excrete the excess of water from blood into urine.

Hypotonic solution
Given either orally or intravenously can be used to treat people who are dehydrated.The water in the hypotonic solution moves from the blood into the interstitial fluid and then into the body cells to rehydrate them.

Facilitated diffusion
Facilitated diffusion is movement of molecules across the cell membrane via special transport proteins that are embedded within the cellular membrane. Many large molecules, such as glucose, are insoluble in lipids and too large to fit through the membrane pores. Therefore, it will bind with its specific carrier proteins, and the complex will then be bonded to a receptor site and moved through the cellular membrane. Bear in mind, however, that facilitated diffusion is a passive process, and the solutes still move down the concentration gradient. The alveoli are tiny grapelike sacs located at the end of the bronchial tubes. This is where oxygen diffuses into the alveoli and is exchanged for carbon dioxide.

• Is done by solutes that are too polar or highly charged to diffuse through the lipid bilayer and are too big to diffuse through membrane channels.
• Here a solute binds to a specific transporter on one side of the membrane and is released to the other side after the transporter undergoes a change in shape.
• The solute binds more often to the transporter on the side of the membrane with a higher concentration of solute.
• The rate of facilitated diffusion is determined by the steepness of the concentration gradient across the membrane.
• The number of transporters available in a plasma membrane places an upper limit known as the transport maximum.Once all the transporters are occupied,the transport maximum is reached, and a further increase in the concentration gradient doesnot increase the rate of facilitated diffusion.
  Eg: Hormone Insulin promotes the insertion of many copies of a specific type of glucose transporter into the plasma membranes of certain cells.Thus the effect of insulin to elevate the transport maximum for facilitated diffusion of glucose into cells.With more transporters available.body cells can pick up glucose from the blood rapidly.

Filtration
Filtration is movement of water and solute molecules across the cell membrane due to hydrostatic pressure generated by the cardiovascular system. Depending on the size of the membrane pores, only solutes of a certain size may pass through it. For example, the membrane pores of the Bowman’s capsule in the kidneys are very small, and only albumin, the smallest of the proteins, have any chance of being filtered through. On the other hand, the membrane pores of liver cells are extremely large, to allow a variety of solutes to pass through and be metabolized

Passive transport – diffusion & osmosis
The fluid inside the cells of the body ( intracellular fluid ) is very different from that outside the cells ( extracellular fluid ) .

The extracellular fluid includes the interstitial fluid which circulates in the spaces between the cells and also  the fluid of the blood plasma that mixes freely with the interstitial fluid through the capillary walls. Extracellular fluid supplies the cells with the nutrients and other substances needed for cellular functions.
ECF contains large quantities of Na+ , Cl- & HCO3- while intracellular fluid contains K+ , Mg 2+ & phosphates. Proteins are found in greater amount in ICF.  These differences between the components of  icf & ecf  are important to the life of the cell which is brought about by different transport mechanisms.

Mainly substances are transported through  the cell membrane by 2 major processes—DIFFUSION ( passive transport ) & active transport.

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

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 ) , where 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.

The importance of area of surface A across which diffusion takes place is illustrated by enormous surface area of capillary bed; the capillary surface area in 1gm of brain tissue is 250 cm2. Individual cells may have their surface area increased by the development of microvilli. These are conspicuous features of epithelial cells in the small intestine and kidney tubules. The rate of diffusion can also be regulated by varyng the area of surface available.

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 —– Simple diffusion &  Facilitated  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.

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

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

1. 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.

2. 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.

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.

What is that limits the rate of facilitated 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. 

Factors that affect the 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—the 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, where 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.

Facilitated diffusion displays considerable 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 DIFFUSION

• 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.

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 ultrafiltration.
• If the two solutions separated by a membrane have same OP, they are called isotonic. One having less OP is called hypotonic.
• 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.

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.

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.

Methods of measurement

• Mechanical methods
• 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