Membrane Transport

Definitions

• Solution – mixture of dissolved molecules in a liquid

• Solute – the substance that is dissolved

• Solvent – the liquid

Membrane Transport Proteins

• Many molecules must move back and forth from inside

and outside of the cell

• Most cannot pass through without the assistance of

proteins in the membrane bilayer

– Private passageways for select substances

• Each cell has membrane has a specific set of proteins

depending on the cell

Movement of Small Molecules

Ion Concentrations

• The maintenance of solutes on both sides of the membrane is critical to the cell

– Helps to keep the cell from rupturing

• Concentration of ions on either side varies widely

– Na+ and Cl- are higher outside the cell

– K+ is higher inside the cell

– Must balance the the number of positive and negative

charges, both inside and outside cell

Impermeable Membranes

• Ions and hydrophilic

molecules cannot easily pass

thru the hydrophobic

membrane

• Small and hydrophobic

molecules can

• Must know the list to the left

2 Major Classes

• Carrier proteins – move the solute across the membrane

by binding it on one side and transporting it to the other

side

– Requires a conformation change

• Channel protein – small hydrophilic pores that allow for

solutes to pass through

– Use diffusion to move across

– Also called ion channels when only ions moving

Proteins

Carrier vs Channel

• Channels, if open, will let solutes pass if they have the

right size and charge

– Trapdoor-like

• Carriers require that the solute fit in the binding site

– Turnstile-like

– Why carriers are specific like an enzyme and its

substrate

Mechanisms of Transport

• Provided that there is a pathway, molecules move from a

higher to lower concentration

– Doesn’t require energy

– Passive transport or facilitated diffusion

• Movement against a concentration gradient requires

energy (low to high)

– Active transport

– Requires the harnessing of some energy source by the

carrier protein

• Special types of carriers

Passive vs Active Transport

Carrier Proteins

• Required for almost all small organic molecules

– Exception – fat-soluble molecules and small uncharged molecules that can pass by simple diffusion

• Usually only carry one type of molecule

• Carriers can also be in other membranes of the cell such as the mitochondria

Carriers in the Cell

Passive Transport by Glucose Carrier

• Glucose carrier consists of a protein chain that crosses the

membrane about 12 times and has at least 2 conformations

– switch back and forth

• One conformation exposes the binding site to the outside of

the cell and the other to the inside of the cell

How it Works

• Glucose is high outside the cell so the conformation is open

to take in glucose and move it to the cytosol where the

concentration is low

• When glucose levels are low in the blood, glucagon

(hormone) triggers the breakdown of glycogen (e.g., from

the liver), glucose levels are high in the cell and then the

conformation moves the glucose out of the cell to the blood

stream

• Glucose moves according to the concentration gradient

across the membrane

• Can move only D-glucose, not mirror image L-glucose

Calcium Pumps

• Moves Ca2+ back into the sarcoplasmic reticulum (modified

ER) in skeletal muscle

Voltage Across the Membrane

• Charged molecules have another component – a voltage across the membrane = membrane potential

• Cytoplasm is usually negative relative to the outside, pulls in positive charges and move out negative charges

• Movement across membrane is under 2 forces – electrochemical gradient

– Concentration gradient

– Voltage across the membrane

Electrochemical Gradient

• This gradient determines the direction of the solute during

passive transport

Active Transport

• 3 main methods to move solutes against an

electrochemical gradient

– Coupled transporters – 1 goes down gradient and 1 goes up the

gradient

– ATP-driven pumps – coupled to ATP hydrolysis

– Light-driven pumps – uses light as energy, bacteriorhodopsin

Transporters are Linked

• The active transport proteins are linked together so that

you can establish the electrochemical gradient

• Example

– ATP-driven pump removes Na+ to the outside of the cell

(against the gradient) and then re-enters the cell through

the Na+-coupled transporter which can bring in many

other solutes

– Also seen in bacterial cells to move H+

Na+-K+ ATPase (Na+-K+ Pump)

• Requires ATP hydrolysis to maintain the Na+-K+ equilibrium in the cell

• Transporter is also a ATPase (enzyme)

• This pump keeps the [Na+] 10 to 30 times lower than extracellular levels and the [K+] 10 to 30 times higher than extracellular levels

Na+-K+ Pump

• Moves K+ while moving Na+

• Works constantly to maintain [Na+] inside the cell – Na+ comes in thru other channels or carriers

Na+ and K+ Concentrations

• The [Na+] outside the cell stores a large amount of energy,

like water behind a dam

– Even if the Na+-K+ pump is halted, there is enough

stored energy to conduct other Na+ downhill reactions

• The [K+] inside the cell does not have the same potential

energy

– Electric force pulling K+ into the cell is almost the same

as that pushing it out of the cell

Na+-K+ Pump is a Cycle

Na+-K+ Mechanisms

• Pump adds a PO4+ group so that it can pick up 3 Na+

• When 3 Na+ are in place, change shape and pump Na+ out

• Opens site for 2 K+ to bind, when in place, PO4+ group is

removed and it changes to original shape

• Dumps K+ to inside, reforming the site for 3 more Na+

• Visit http://highered.mcgraw-

hill.com/sites/0072437316/student_view0/chapter6/animations.html

– See animation at Sodium-Potassium Exchange Pump (682.0K)

Coupled Transporters

• The energy in the Na+-K+ pump can be used to move a second solute

– Energy trapped in the Na+ gradient to move down its gradient and another molecule against its gradient

• Couple the movement of 2 molecules in several ways

– Symport – move both in the same direction

– Antiport – move in opposite direction

• Carrier proteins that only carry one molecule is called uniport (not coupled)

• Visit http://highered.mcgraw-

hill.com/sites/0072437316/student_view0/chapter6/animations.html

– See animation at Cotransport

Coupled Transporters

Na+-Driven Symport

• If one molecule of the transport pair is missing, the transport of the second does not occur

2 Methods of Glucose Transport

• 2 mechanisms are separate

– Passive transport at the

apical surface

– Active transport at the basal

surface

• Caused by the tight junctions

Na+-Driven Transport

• Na+ driven symport

– Used to move other sugars and amino acids

• Na+ driven antiport

– Also very important in cells

– Na+-H+ exchanger is used to move Na+ into the cell

and then moves the H+ out of the cell

• Regulates the pH of the cytosol

Osmosis

• The movement of water from region of low solute

concentration (high water concentration) to an area of

high solute concentration (low water concentration)

• Driving force is the osmotic pressure caused by the

difference in water pressure

Osmotic Solutions – Tonicity

(tonos = tension) • Isotonic – equal solute on each side of the membrane

• Hypotonic – less solute outside cell, water rushes into cell and

cell bursts

• Hypertonic – more solute outside cell, water rushes out of cell

and cell shrivels

Osmotic Swelling

• Animal cells maintain normal cell structure with Na+-K+ pump

(moves out Na+ and prevents Cl- from moving in)

• Plants have cell walls – turgor pressure is the effect of osmosis

and active transport of ions into the cell – keeps leaves and

stems upright

• Protozoans have special water collecting vacuoles to remove

excess water

Human Red Blood Cells or Erythrocytes

Tonicity in Action

• An isotonic solution has an equal amount of dissolved solute in it compared to the things around it.

• Typically in humans and most other mammals, the isotonic solution is 0.9 weight percent (9 g/L) salt in aqueous solution, this is also known as saline, which is generally administered via an intra-venous drip.

• Red blood cells normally exist in a 0.9 percent salt solution (saline) with the same concentration of salt in the outside solution.

• Source: http://en.wikipedia.org/wiki/Isotonic.

Water, water, everwhere… • “Water, water, everywhere,

Nor any drop to drink” (pt. II, st. 9. from the “The Rhyme of Ancient Mariner ” by Samuel Taylor Coleridge [1772-1834])

• Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of ~3.5%. This means that for every 1 liter of seawater there are 35 grams of salts (mostly, but not entirely, sodium chloride) dissolved in it. Source: http://en.wikipedia.org/wiki/Sea_water

• A person who drinks undiluted sea water will actually become more dehydrated & may salt in the intestine may cause diarrhea. To could potentially extend your drinking supply though; it can be diluted with potable water by a factor of 4 or greater to bring it below a concentration of 0.9% solute, rendering it safer for consumption.

Calcium Pumps

• Calcium is kept at low concentration in the cell by ATP-

driven calcium pump similar to Na+-K+ pump with the

exception that it does not transport a second solute

• Tightly regulated as it can influence many other molecules

in the cytoplasm

• Influx of calcium is usually the trigger of cell signaling

H+ Gradients

• Drive the movement of molecule across the membranes of plants, fungi and bacteria

• Similar to animal Na+-K+ pump but moves H+

H+ Pumps

Several reasons for moving H+ through membranes in plants

• Cell wall acidification (H+) helps to loosen the cellulose fibers so that plant cells can increase in size and elongate.

• Cation ion exchange by means of secreting H+

allows roots to harvest positively charged mineral nutrients (e.g., Mg++, Ca++, K+, Na+) that are attached to negatively charged clay particles in the soil.

• The relative concentrations of H+ in vacuoles varies. With anthocyanins (a natural pH indicator) in the cell sap of a vacuole, this imparts the color seen in some flowers and other plant tissues (e.g. hydrangea, violets, ornamental maize, purple cabbage).

Loosening of cell wall through cell wall acidification in plants

CATION EXHANGE IN PLANTS

Anthocyanins, pH, and color in plants

Channel Proteins

• Channel proteins create a hydrophilic opening in which

small water-soluble molecules can pass into or out of the

cell

– Gap junctions and porins make very large openings

• Ion channels are very specific with regards to pore size

and the charge on the molecule to be moved

– Move mainly Na, K, Cl and Ca

Ion Channels

• Have ion selectivity – allows some ions to pass and

restricts others

– Based on pore size and the charges on the inner ‘wall’ of

the channel

• Ion channels are not always open

– Have the ability to regulate the movement of ions so that

control can maintain the ion concentrations within the cell

– Channels are gated – open or closed

• Specific stimuli triggers the change in shape and

opening or closing of channel

Ion Channels

Channels Are Either Open or Closed

Membrane Potential

• Basis of all electrical activity in cells

• Active transport can keep ion concentration far from

equilibrium in the cell

• Channels open and the ions rush in because of the

gradient difference – changes the voltage across the

membrane

– As voltage changes, other ion channels open and other

ions rush in

• Allows for the electrical activity to move across the

membrane

Variety of Channels

• Ion channels vary with respect to

– Ion selectivity – which ions can go thru

– Gating – conditions that influence opening and closing

Membrane Ion Channels

Passive, or leakage, channels – always open

Chemically (or ligand)-gated channels – open

with binding of a specific neurotransmitter (the

ligand)

Voltage-gated channels – open and close in

response to changes in the membrane potential

Mechanically-gated channels – open and close in

response to physical deformation of receptors

Types of plasma membrane ion channels

3 Types of Channels

• Voltage-gated channels – controlled by membrane potential

• Ligand-gated channels – controlled by binding of a ligand to a

membrane protein (either on the outside or the inside)

• Stress activated channel – controlled by mechanical force on the

cell

Auditory Hair Cells

• Stress activated

• Sound waves cause the stereocilia to tilt and this causes the

channels to open and transport signal to the brain

• Hair cells to auditory nerve to brain

Voltage-Gated Channels

• Move impulses along the nerve

• Have voltage sensors that are sensitive to changes in

membrane potential

– Allows for changes in the charge across the membrane

• Distribution of ions gives rise to membrane potential

– Usually negative inside and positive outside