Membrane TransportMembrane TransportMembrane Transport

By Dr. Carmen Rexach

PhysiologyMount San Antonio College

Passive vs. Active• Passive transport

– simple diffusion– Osmosis– filtration– facilitated diffusion

• Active transport– primary active transport– secondary active transport

Simple diffusion• Net movement of molecules from

an area of high concentration to an area of low concentration by random molecular motion

Diffusion through a cell membrane

• Diffuse rapidly– Nonpolar molecules– small molecules with polar covalent bonds– Gases– Examples: CO2, oxygen, fatty acids, steroid

hormones

• usually do not diffuse– large polar molecules– Ions– Examples: phosphorylated organic molecules

Rate of diffusion• Flux = amount of material crossing a surface per

unit of time• Net flux = difference between one-way flux

• Influenced by– Magnitude of concentration difference– Mass of the molecule– Temperature– Medium (gas, liquid, solid)– surface area of the membrane

Rate of diffusion through membrane

• Net flux (F) across the membrane:– F=kpA(Co-Ci)– Kp=permeability constant

• Dependent on temperature and permeability of membrane

– A = surface area– (Co-Ci) = concentration difference

Remember: The major limiting factor in diffusion across the membrane is its hydrophobic nature!

Ions diffuse through protein channels

• Channels formed by integral membrane proteins

• Selective– Channel diameter– Charged and polar protein surfaces

• Regulation by channel gating• Ligand gated• Voltage gated• Mechanically gated

Diffusion and the electrical gradient• Important consideration in ion diffusion• Membrane separates electrical charges

– Same charges repel– Opposite charges attract

More information on this soon!

There are two driving forces in the movement of molecules acrossthe membrane: the concentration gradient (chemical) and the electrical gradient (electrical). Together they are called the electrochemical gradient.

Osmosis

• The movement of water from an area of high water concentration to an area of low water concentration across a semi-permeable membrane

Osmolality• Mole = molecular weight of an element or compound

measured in grams• Molality = # of moles of solute per kg of solvent• Osmolality = ionic concentration of dissolved

substances per kg of solvent– Osmolality of plasma = 300 mOsm = isosmotic– depends on the number of solute particles

• 1 m of C6H12O6 = 1 osmole (Osm)• 1 m of NaCl = 2 osmoles• 1 m of CO2 + 1 m of KCl = 3 osmoles

Osmolarity vs Osmolality

• Osmolarity refers to the number of solute particles per 1 L of water

• Osmolality refers to the number of solute particles per 1 kg of water

• Since the mass of water is independent of temperature, osmolality is the preferred term for biological systems.

Tonicity

0.85% 0.85% 0.85%

0.85% 10 % 0.02%

Isotonic=300mOsm

Hypertonic = >300mOsm

Hypotonic = <300mOsm

The NaCl concentration in most cells is 0.85%. This corresponds to 300mOsm of nonpenetrating solutes.

Effect of tonicity of solutions on red blood cells

hypotonic isotonic hypertonic

Filtration• Movement of a fluid and solutes down the

pressure gradient (from high pressure to low pressure) across a membrane or filter

• What is filtered depends on the amount of pressure and the size of the pores in the filter– In kidney:

• Normally, protein is not filtered into the urine because of size and charge restrictions

• Glomerulonephritis: Inflammation enlarges the size of “pores” and allows protein to move out into the filtrate

Filtration

Fliltrate forms as substance moves through filterForce: gravity (atm pressure)Restriction: size of filter pores

Filtrate forms as fluid and other substances are forced through capillary fenestrations.Force: blood pressureRestriction: size of the fenestrations

Carrier mediated transport• Protein carriers transport molecules

too large or polar across the plasma membrane

• 4 characteristics of ligand/protein binding– chemical specificity– competition– saturation– affinity

Chemical specificity

• Only molecules with the requisite chemical structure are transported

• Specificity is not absolute

Proteincarrier

A

B Protein carrier

Proteincarrier

Competition

• Structurally related molecules may compete for transport

• Competition decreases transport rate

Proteincarrier

A

B

C

Saturation• Transport system is saturated when

all of the binding sites are occupied • At that point, the rate of transport

can not increase.

Affinity

• The attraction of the carrier protein for the transport substrate

• Transport can be inhibited by compounds or ions which alter the shape of the binding site.

+++ + - - +- - - - - - - - - -

Carrier mediated transport

1) Solute binds to transporter protein (ligand/protein bindingrules)

2) Protein changes shape, moving solute to opposite side of the membrane.

Facilitated diffusion

• Movement of substances with the concentration gradient

• Example– Glucose across plasma membrane– Aquaporins transporting water

Water movement and aquaporins

• Two methods for diffusion of water in and out of cells– Diffusion across lipid bilayer– Aquaporins

• Usually transport solute-free water• About 10 different types of aquaporins• One will transport about 3 billion H2O

molecules per second

Active transport

• Movement against the concentration gradient

• Requires the expenditure of energy• Often called pumps• two types

– primary– secondary

Active transport = against the concentration gradient

Primary active transport

• Steps– molecule or ion binds at recognition site– carrier protein is phosphorylated– protein changes shape– transported molecule flipped to other

side of membrane– phosphate removed enzymatically,

releasing transported molecule

Primary active transport

PO4

PO4

ATP ADP

PO4

1 2

3 4

Major active transport pumps

• Na+K+ATPase pump• Ca++ATPase pump• H+ATPase pump• H+K+ATPase pump

Na+K+ATPase pumps• 3 Na+ out & 2 K+ in• Functions:

– Sets stage for secondary active transport– gradient for electrical impulses in

nerve/muscles

= Na+

= K+

ATPase

ATP ADP

Na+K+ATPase pump

Secondary active transport

• Energy provided by Na+ gradient – not directly by ATP

• co-transport– “hitchhiking”– Uses symport

• counter-transport– “revolving door”– Uses anaport

Na+

Na+

Ca++

K+

ATP

ADP

Symport

Summary of passive and active transport mechanisms

Against concentration gradient

With the concentration gradient

Exocytosis and Endocytosis• Circumvents need to pass through

plasma membrane– Allows membrane impermeable molecules

to pass– Exocytosis: Adds to membrane

• Also energy requiring transport mechanisms

Epithelial transport

• Paracellular pathway• Transcellular pathway

– Luminal and basolateral membranes do not have same permeability or transport characteristics

Alternative functions of endocytosis:

1. Transcellular transport

2. Endosomal processing

3. Recycling the membrane

4. Destroying engulfed materials