DR NILESH KATE

MBBS,MD

ASSOCIATE PROF

DEPT. OF PHYSIOLOGY

TRANSPORT ACROSS CELL MEMBRANE.

At the end of the class, you must know

1. Importance of cell membrane2. Types of Transport mechanisms3. Active transport in detail4. Primary active transport5. Secondary active transport

- Co-transport and Counter

transport

About Cell Membranes

1. All cells have a cell membrane

2. Functions: a. Controls what enters

and exits the cell to maintain an internal balance called homeostasis

b. Provides protection and support for the cell

Structure of Cell membraneStructure of Cell membrane

It is a double layer of It is a double layer of phospholipids – lipid phospholipids – lipid bilayer.bilayer.

It is an elastic (7.5-10 It is an elastic (7.5-10 nm)nm)

It contains almost It contains almost proteins called proteins called membrane proteinsmembrane proteins

3. Structure of cell membrane

Lipid Bilayer -2 layers of phospholipids (Gorter & Gorter & Grendel (1925)Grendel (1925)

a. Phosphate head is polar (water loving)

b. Fatty acid tails non-polar (water fearing)

c. Proteins embedded in membrane

About Cell Membranes (continued)

Phospholipid

Lipid Bilayer

Lipid bi-layer

Proteins

Polar heads love water & dissolve.

Non-polar tails hide from water.

Carbohydrate cell markers

Fluid Mosaic

Model of the cell

membrane

4. Cell membranes have pores (holes) in it

a. Selectively permeable: Allows some molecules in and keeps other molecules out

b. The structure helps it be selective!

About Cell Membranes (continued)

Pores

Outside of cell

Inside of cell (cytoplasm)

Lipid Bilayer

Proteins

Transport Protein Phospholipids

Carbohydratechains

Structure of the Cell MembraneStructure of the Cell Membrane

Membrane ProteinsIntegral Proteins: (70% of Cell membrane proteins part and parcel of membrane structure

Pumps: They transper substances against Concentration / Electrical gradients

Channel Proteins: Opened and closed by gates

Carrier Proteins: Involved in transport of substances

Enzyme Proteins: Takes place in membrane reaction

Receptor Proteins: They bear appropriate sites for recognition of Specific Ligands.

Functions of Cell Membrane:

Protective Function

Selective permeability

Absorptive function

Excretory function

Exchange of gases

Maintenance of shape and

size of the cell.

Transport – What it means? Its highly selective filter, Its highly selective filter, permits nutrients and permits nutrients and leaves the waste products leaves the waste products from the cell.from the cell.

Maintain Homeostasis.Maintain Homeostasis.

Makes Cytosol Makes Cytosol environment to differentenvironment to different

Play an important role in Play an important role in cell to cell communication.cell to cell communication.

Its detects Chemical Its detects Chemical messengers arriving at the messengers arriving at the cell surface. cell surface.

TRANSPORT MECHANISMSTRANSPORT MECHANISMS

TRANSPORTTRANSPORT

Passive processPassive processActive processActive process

Primary Transport Primary Transport

Secondary TransportSecondary Transport

Simple diffusionSimple diffusion

Facilitated diffusionFacilitated diffusion

OsmosisOsmosis

Bulk flowBulk flow

FiltrationFiltration

FACTORS AFFECTING NET RATE OF DIFFUSION

FICK’S LAW OF DIFFUSION:

J = - DA X ( C1-C2 ) at particular temperature. T

D = Diffusion coefficient. A = Surface area. C1&C2 = Concentrations on either sides. (Lipid solubility – It is the major determinant in the pharmacokinetics of a drug)

Factors that Influence Diffusion Rates Distance -

The shorter the distance, the more quickly [ ] gradients are eliminated

Few cells are father than 125 microns from a blood vessel

Molecular Size Ions and small molecules diffuse more rapidly

Temperature - temp., motion of particles

Steepness of concentrated gradient - The larger the [ ] gradient, the faster diffusion proceeds

Membrane surface area - The larger the area, the faster diffusion proceed

Diffusion Across Membranes Simple Diffusion

Lipophilic substances can enter cells easily because they diffuse through the lipid portion of the membrane

Examples are fatty acids, steroids, alcohol, oxygen, carbon dioxide, and urea,

Channel-Mediated Diffusion Membrane channels are transmembrane

proteins Only 0.8 nm in diameter

Used by ions, very small water-soluble compounds

Much more complex than simple diffusion Are there enough channels available? Size and charge of the ion affects which channels it

can pass through

Diffusion Through the Plasma Membrane

Figure 3.7

OSMOSISOsmosis is the process of moving water across a semi permeable membrane towards ion or solute rich region in a solution

OSMOTIC PRESSUREThe amount of pressure

that can prevent the movement of water from another region which is partitioned by the permeable membrane

colloidal osmotic pressure of plasma is 25 mmHg

Osmolarity and Tonicity Mole - the gram molecular weight of a substance

1 mole of Glucose =180; 1 mole of NaCl = 58.5 Molarity - the number of moles of solute per liter of solution

1.0 M glucose contains 180 g/L; 1.0 M NaCl contains 58.5 g/L Most body fluids are less concentrated than 1 M; use mM

(millimolar) or µM (micromolar) concentrations --10-3 and 10-6, respectively.

Osmolarity = the total solute concentration in an aqueous solution Osmolarity = molarity (mol/L) x # of particles in solutions

A 1 M Glucose solution = 1 Osmolar (Osm) But a 1 M NaCl soln = 2 Osmolar because NaCl dissociates

into 2 particles (Na and Cl) whereas Glucose does not A 1 M MgCl2 solution = what osmolarity???? __________

Physiological solutions are expressed in milliosmoles per liter (mOsm/L) blood plasma = 300 mOsm/L or 0.3 Osm/L

Tonicity Tonicity - ability of a solution to affect fluid volume

and pressure within a cell depends on concentration and permeability of solute

Isotonic solution solution with the same solute concentration as that of the

cytosol; normal saline Hypotonic solution

lower concentration of nonpermeating solutes than that of the cytosol (high water concentration)

cells absorb water, swell and may burst (lyse) Hypertonic solution

has higher concentration of nonpermeating solutes than that of the cytosol (low water concentration)

cells lose water + shrivel (crenate)

Osmosis and Cells Important because large volume changes caused

by water movement disrupt normal cell function Cell shrinkage or swelling

Isotonic: cell neither shrinks nor swells Hypertonic: cell shrinks (crenation) Hypotonic: cell swells (lysis)

Effects of Tonicity on RBCs

Hypotonic, isotonic and hypertonic solutions affect the fluid volume of a red blood cell. Notice the crenated and swollen cells.

VESICULAR TRANSPORTIt is the transport of membrane bounded substances moving across plasma membrane

It is classified into:

1. Endocytosis 2. Exocytosis.

Endocytosis It is a process by which the large number of particles are taken with forming the vesicle into the cell

It is classified into: 1. Phagocytosis It is a process by which the large number of particles are engulfed into the cell. 2. Pinocytosis It is a process by which the large number of particles which are soluble in water are taken into the cell

Endocytosis

Receptor Mediated Endocytosis A selective process Involves formation of vesicles at

surface of membrane Vesicles contain receptors on their membrane Vesicles contain specific target molecule in

high concentration Clathrin-coated vesicle in cytoplasm

uptake of LDL from bloodstream If receptors are lacking, LDL’s accumulate

and hypercholesterolemia develops

Receptor Mediated Endocytosis

Mechanism of PhagocytosisThe cell membrane invaginates the material from ECF.

It is pinched off from the membrane and takes the material into ICF

The phagocytic cell such as a macrophage may be attracted to a particle like a bacteria or virus by chemical attractant.

Pinocytosis In the process of pinocytosis the cell membrane forms an

invagination.

Whatever substance (Proteins) is found within the area of invagination is brought into the cell.

In general this material will be dissolved in water and thus this process is also refered to as "cellular drinking"

This is opposed to the ingestion of large particle like bacteria or other cells or cell debris.

ExocytosisExocytosis is a process in which an intracellular

vesicle (membrane bounded sphere) moves to the plasma membrane and fused the substance into the Extra cellular fluids

For example a few of the processes that use Exocytosis are:

1. Secretion of proteins like enzymes and antibodies from cells.

2. Release of neurotransmitter from presynaptic neurons

3. Arosome reaction during fertilization 4. Recycling of plasma membrane

ExocytosisExocytosis The opposite of endocytosis is exocytosis. Large molecules that are The opposite of endocytosis is exocytosis. Large molecules that are manufactured in the cell are released through the cell membrane.manufactured in the cell are released through the cell membrane.

CMT: Facilitated Diffusion Glucose and amino acids are insoluble in lipids and too large

to fit through membrane channels Passive process, i.e. no ATP used Solute binds to receptor on carrier protein

Latter changes shape then releases solute on other side of membrane Substance moved down its concentration gradient

Saturation of a Carrier Protein

1. When the concentration of x molecules outside the cell is low, the transport rate is low because it is limited by the number of molecules available to be transported.

2. When more molecules are present outside the cell, as long as enough carrier proteins are available, more molecules can be transported; thus, the transport rate increases.

3. The transport rate is limited by the number of carrier proteins and the rate at which each carrier protein can transport solutes. When the number of molecules outside the cell is so large that the carrier proteins are all occupied, the system is saturated and the transport rate cannot increase.

BULK Transport The movement of large number of ions, molecules or particles that are dissolved or carried in a medium such as a fluid or air is called bulk flow.

Rate of Bulk transport is determined by the differences in hydrostatic pressure or air pressure.

Eg: 1. Flow of blood within the vessels. 2.Movement of air into and out of the lungs.

Active transport Passive transportEnergy is utilisedEnergy is utilised No Energy is utilisedNo Energy is utilised

Movement of ions takes place Movement of ions takes place against conc. gradientagainst conc. gradient

Movement of ions takes place Movement of ions takes place favouring conc. gradientfavouring conc. gradient

Specific carrier is requiredSpecific carrier is required No carrier is requiredNo carrier is required

Cellular respiratory rate is Cellular respiratory rate is No changeNo change

Enzymes are involvedEnzymes are involved No Enzymes are involvedNo Enzymes are involved

Types of Cellular Transport Passive Transport

cell doesn’t use energy1. Diffusion

2. Facilitated Diffusion

3. Osmosis

Active Transportcell does use energy

1. Protein Pumps

2. Endocytosis

3. Exocytosishigh

low

This is gonna be

hard work!!

high

low

Weeee!!!

What is active transport? What is active transport? Active transport is the Active transport is the transport of substances from transport of substances from a region of lower a region of lower concentration to higher concentration to higher concentration using energy, concentration using energy, usually in the form of usually in the form of ATP. .

Examples: Na, K and Ca Examples: Na, K and Ca active transport. active transport.

1.1.sodium-potassium pump

2.Calcium pump2.Calcium pump

3.Potassium hydrogen pump 3.Potassium hydrogen pump

Active Transport needed for,

1. Maintaining the 1. Maintaining the Chemical and Electrical Chemical and Electrical Charge at rest.Charge at rest.

2. Intake of Substances through gated Channels.

3. Collecting of ions with more concentration.

ACTIVE TRANSPORT - WHY ? Cells cannot rely solely on

passive movement of substances across their membranes.

In many instances, it is necessary to move substances against their electrical or chemical gradient to maintain the appropriate concentrations inside of the cell or organelle.

Pumps involved in ACTIVE TRANSPORT

1.Sodium-potassium pump Found in many cells

2.Calcium pump Found in membrane of Sarcoplasmic reticulum 3.Potassium hydrogen pump Found in Gastrointestine cell membrane

Working of Na-K pumpWorking of Na-K pump

Primary active transport

Primary active transport is Primary active transport is the transport of sustances the transport of sustances uphill using energy (uphill using energy (ATP hydrolysis) hydrolysis)

It cause a conformational It cause a conformational change that results in the change that results in the transport of the molecule transport of the molecule through the protein.through the protein.

Eg. Na+-K+ pump. Eg. Na+-K+ pump.

Functions of Na+K pump Functions of Na+K pump

1. 1. It is responsible for maintaining the high K+ and low Na+ concentration inside the cell.

2. It maintains intracellular negativity.

3. Maintains red blood cell volume.

4. Activate the Carrier protein.

Inhibition of the pump

1. The pump requires binding of Na and K and ATP for its operation.

therefore ,if the concentration of any of these substances is low,the pump does function.

2. Decrease in temperature3. Oxygen lack.4. Metabolic poisons Eg.2,4 dinitrophenol prevents the formation of

ATP.

Calcium pump Ca+ ATPase pump 1.Present in the sarcoplasmic

reticulum of muscle cells , which maintains intracellular ionic Ca2+ concentration below 0.1mmol/l.

The direction is from cytoplasm to ECF.That is why cytoplasm of most cells have low Ca2+ concentration.

Secondary active transport The transport of substances against a

concentration gradient involving energy to establish a gradient across the cell membrane, utilizes the gradient to transport a molecule of interest up its concentration gradient .

THE TRANSPORT MAY BE In the same direction (SYMPORT) In the opposite direction (ANTIPORT)

Mechanisms of Secondary Active Transport

Carriers type processes

Carriers are transport proteins that binds ions and other molecules and then change their configuration moving the bound molecules from one side of cell membrane to the other.

Types of carriers :1.Uniporters2.Symporters 3.Antiporters

UNIPORT The movement of

a single Substance.

It requires no energy from the cell.

Examples. Simple diffusion. Facilitated

diffusion.

Mechanism of Uniport

Lower concentration regionLower concentration region

Symport (Co-transport) Transport of two

substances using the energy produced by concentration difference developed by primary active transport

Substances are moving in the same direction.

Example: transport of amino acids, Glucose,

Mechanism of Co-transportECFECF

ICFICF

ECFECF

ICFICF

ECFECF

ICFICF

sodiumsodium

glucoseglucose

Antiport (Counter-transport) In this process, the two

substances move across the membrane in opposite directions.

Example:Exchange of H+ and Na+ in Renal tubule.

MECHANISM OF COUNTER-TRANSPORT

Higher conc. of H+Higher conc. of H+

Lower conc. of H+Lower conc. of H+

The Na+, glucose Secondary Transport

Sodium co-transport of glucose occurs during absorption of glucose from the intestine and reabsorption of glucose from renal tubule.

Transport of substances in GIT

Transport of substances in NephronGG

H+H+

CaCa

CaCa

ClCl

Passive Membrane Transport – Review -

Process Energy Source Example

Simple diffusion Kinetic energy Movement of O2 through membrane

Facilitated diffusion Kinetic energy Movement of glucose into

cells

Osmosis Kinetic energy Movement of H2O in & out of cells

Filtration Hydrostatic pressure Formation of kidney filtrate

Active Membrane Transport – Review

Process Energy Source ExampleActive transport of

solutes ATP Movement of ions across membranes

Exocytosis ATP Neurotransmitter secretion

Endocytosis ATP White blood cell phagocytosis

Fluid-phase endocytosis ATP Absorption by intestinal cells

Receptor-mediated endocytosis ATP Hormone and cholesterol

uptakeEndocytosis via caveoli ATP Cholesterol regulation

Endocytosis via coatomer vesicles ATP Intracellular trafficking

of molecules

OTHER TRANSPORT PROCESSES

Across epithelia. Through cell proper Through tight

junctions

Wednesday, May 3, 2023

OTHER TRANSPORT PROCESSES

Ultra filtration.

STARLING’S FORCES- at the capillary level

Plasma osmotic pressure 25 mm Hg.

Hydrostatic pressure

Wednesday, May 3, 2023

Objectives:Objectives:

1. Importance of cell membrane1. Importance of cell membrane

2. Types of Transport mechanisms2. Types of Transport mechanisms

3. Active transport in detail3. Active transport in detail

4. Primary active transport4. Primary active transport

5. Secondary active transport5. Secondary active transport

- Co-transport and Counter transport- Co-transport and Counter transport

Thank You