Chapter 12Membrane Transport

Defintions• 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

• Each cell has membrane has a specific set of proteins depending on the cell

Movement of Small Molecules

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

Proteins

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 as well

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

3-D of Carrier Protein

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 of facilitated

diffusion• Movement against a concentration

gradient requires energy– Active transport– Requires the harnessing of some

energy source by the carrier protein

Passive vs Active Transport

Glucose Carrier in Liver Cells

• Glucose carrier crosses the membrane and has at least 2 conformations

• One conformation exposes the binding site on the outside of the cell and the other on the inside of the cell

How it Works (Passive)• 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, 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

Electrochemical Gradient

• Due to concentration gradient and the voltage across the membrane

• This gradient determines the direction of the solute during passive transport

Active Transport

• 3 main methods to move solutes against a gradient– Coupled transporters– ATP-driven pumps– Light-driven pumps

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

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

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

Coupled Transport• The energy in the Na+-K+ pump can be

used to move a second solute• 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)

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

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

• 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

• 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

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

H+ Pumps

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 be maintained on the ion concentrations within the cell

– Channels are gated – open or closed

Ion Channels

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

• Allows for the electrical activity to move across the membrane

Patch Clamp• Technique used to

determine the electrical current in a cell

• Can measure the change in voltage that occurs when the channels are open or closed

3 Types of Channels

• Voltage-gated channels – controlled by membrane potential

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

• Stress activated channel – controlled by mechanical force on the cell

Auditory Hair Cells

• Sound waves cause the stereocilia to tilt and this causes the channels to open and transport signal to the brain