151 transport

Transport Processes

AP 151

Membrane Transport• Plasma membrane is selectively permeable

– Impermeable membrane - membrane though which nothing can pass

– Freely permeable membrane - any substance can pass through it

– Selectively permeable membrane - permits free passage of some materials and restricts passage of others

• Distinction may be based on size, electrical charge, molecular shape, lipid solubility

• Cells differ in their permeabilities; depending on – what lipids and proteins are present in the membrane and

– how these components are arranged.

• Passage across the membrane is either passive or active

– Passive transport requires no ATP

• movement down concentration gradient

• filtration and simple diffusion

–Active transport requires ATP

• movement against concentration gradient

• carrier mediated

• vesicular transport

Membrane Permeability

Types of Transport Processes

• Diffusion– results from random motion of particles (ions,

molec.)– is a passive process

• Carrier-mediated transport– Requires the presence of specialized integral

proteins– Can be passive or active

• Vesicular transport– Movment of materials with small membranous

sacs, or vesicles– Always an active process

Membrane Permeability

• Diffusion through lipid bilayer– Nonpolar, hydrophobic substances diffuse

through lipid layer; these are “lipid soluble” or lipophilic (fat-loving) substances

• Diffusion through channel proteins– water and charged hydrophilic solutes

diffuse through channel proteins; these are lipid insoluble or lipophobic (fat fearing) substances

• Cells control permeability by regulating number of channel proteins

Simple Diffusion• Net movement of particles from area

of high concentration to area of low concentration– due to their constant, random motion– Difference between the high and low

concentrations is a concentration gradient

– Diffusion tends to eliminate the gradient– Also known as movement “down the

concentra- tion gradient”

Diffusion• Examples:

– Scent of fresh flowers, drop of ink coloring a glass of water, movement of oxygen and CO2 through cell membranes

• Simple diffusion – nonpolar and lipid-soluble substances – Diffuse directly through the lipid bilayer– Diffuse through channel proteins

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

Effect of Membrane Permeability on Diffusion

Figure 3.8a

Osmosis: A Special Case of Diffusion

• Each solute in the intra- and extracellular fluids diffuses as if it were the only material in solution.– From more to less, i.e., down the [ ] gradient– Some into the cytosol, others out of the cytosol– Yet, total concentration of ions and molecules on

either side of the membrane stays the same– This equilibrium persists because a typical cell

membrane is freely permeable to water.

• Whenever a solute concentration gradient exist, a concentration gradient for water also exists.– Thus, the higher the solute concentration, the

lower the water concentration.

Osmosis - By Definition• Movement of water • Across a selectively permeable membrane• Down its concentration gradient (from high

to low concentration)• Toward the solution containing the higher

solute concentration – This solution has a lower water concentration– Continues until water concentrations and solute

concen-trations are the same on either side of the membrane

Effect of Membrane Permeability on Diffusion and Osmosis

Figure 3.8b

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.

Filtration• Cell membrane works like a sieve• Depends on pressure difference on either

side of a partition• Moves from side of greater pressure to

lower• Water and small molecules move through

the pores of the membrane while large molecules don’t.

• Example: urine formation in the kidneys.

Carrier Mediated Transport

• Many molecules cannot enter or leave cell by diffusion

• CMT utilizes proteins to carry solutes across cell membrane

• Characteristics of mediated transport:1. Specificity - each transport protein binds to

and transports only a single type of molecule or ion

2. Competition - results from similar molecules binding to the same protein.

3. Saturation - rate of movement of molecules is limited by the number of available transport proteins

Membrane Carriers• Uniporter

– carries only one solute at a time• Symport

– carries 2 or more solutes simultaneously in same direction (cotransport)

• Antiport– carries 2 or more solutes in opposite

directions (countertransport)• sodium-potassium pump brings in K+ and removes

Na+ from cell

• Any carrier type can use either facilitated diffusion or active transport

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.

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

CMT: Active Transport• Uses ATP to move solutes across a

membrane• It is not dependent on a [ ] gradient

– Can move substances against their [ ] gradients - i.e. from lower to higher concentrations! Wow!

– Allows for greater accumulation of a substance on one side of the membrane than on the other.

• Carrier proteins utilized called ion or exchange pumps.– Ion pumps: actively transport Na+, K+, Ca++, Cl-

– Exchange pumps: Na+-K+ pump

Types of Active Transport

Figure 3.11

Binding of cytoplasmic Na+ to the pump protein stimulates phosphorylation by ATP.

1

2

3

4

Phosphorylation causes the protein to change its shape.

The shape change expels Na+ to the outside, and extracellular K+ binds.

5

Loss of phosphate restores the original conformation of the pump protein.

K+ binding triggers release of the phosphate group.

6K+ is released and Na+ sites are ready to bind Na+ again; the cycle repeats.

Concentration gradients of K+ and Na+

Extracellular fluid

Cytoplasm

Sodium-Potassium Pump

Figure 3.10

Functions of Na+ -K+ Pump• Regulation of cell volume

– “fixed anions” attract cations causing osmosis– cell swelling stimulates the Na+- K+ pump to

ion concentration, osmolarity and cell swelling

• Heat production (thyroid hormone increase # of pumps; heat a by-product)

• Maintenance of a membrane potential in all cells– pump keeps inside negative, outside positive

• Secondary active transport (No ATP used)– steep concentration gradient of Na+ and K+ maintained

across the cell membrane– carriers move Na+ with 2nd solute easily into cell

• SGLT saves glucose in kidney

Secondary Active

Transport

• Ions or molecules move in same (symport) or different (antiport) direction.

• Is the movement of glucose a symporter example or an antiporter example?

• This example shows cotransport of Na+ and glucose. 1. A sodium-potassium

exchange pump maintains a concentration of Na that is higher outside the cell than inside. Active transport.

2. Na moves back into the cell by a carrier protein that also moves glucose. The concentration gradient for Na provides the energy required to move glucose against its concentration gradient.

Vesicular Transport

• Transport large particles or fluid droplets through membrane in vesicles– uses ATP

• Exocytosis –transport out of cell • Endocytosis –transport into cell

– phagocytosis – engulfing large particles– pinocytosis – taking in fluid droplets– receptor mediated endocytosis – taking in

specific molecules bound to receptors

Vesicular TransportEndocytosis

• Packaging of extracellular materials in vesicles at the cell surface

• Involves relatively large volumes of extracellular material

• Requires energy in the form of ATP• Three major types

1. Receptor-mediated endocytosis 2. Pinocytosis3. Phagocytosis

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

Vesicular TransportPinocytosis or “Cell-

Drinking”• Taking in droplets of ECF

– occurs in all human cells• Not as selective as ‘receptor-

mediated endocytosis’• Membrane caves in, then pinches

off into the cytoplasm as pinocytotic vesicle

Vesicular TransportPhagocytosis or “Cell-

Eating”

Keeps tissues free of debris and infectious microorganisms.

Vesicular Transport: Exocytosis

• Secreting material or replacement of plasma membrane

Passive Membrane Transport – Review -

Process Energy Source Example

Simple diffusion Kinetic energyMovement of O2 through

membrane

Facilitated diffusion

Kinetic energyMovement of glucose into

cells

Osmosis Kinetic energyMovement of H2O in & out of

cells

FiltrationHydrostatic

pressureFormation of kidney filtrate

Active Membrane Transport – Review

Process Energy Source Example

Active transport of solutes

ATPMovement of ions across

membranes

Exocytosis ATPNeurotransmitter

secretion

Endocytosis ATPWhite blood cell

phagocytosis

Fluid-phase endocytosis ATPAbsorption by intestinal

cells

Receptor-mediated endocytosis

ATPHormone and cholesterol

uptake

Endocytosis via caveoli ATP Cholesterol regulation

Endocytosis via coatomer vesicles

ATPIntracellular trafficking

of molecules