Chapter 12

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 Loosening of cell wall through cell wall through cell wall

acidification in plantsacidification 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

END OF THIS PRESENTATION

THE REMAINING SLIDES PROVIDE ADDITIONAL

INFORMATION – FYI FOR WHICH THE FINAL EXAM

WILL NOT COVER

Voltage-Gated Channel•Example: Na+ channel

•Closed when the intracellular environment is negative

•Open when the intracellular environment is positive - Na+ can enter the cell

Ligand-Gated Channel

Example: Na+-K+ gated channel

Closed when a neurotransmitter is not bound to the extracellular receptor

Open when a neurotransmitter is attached to the receptor - Na+ enters the cell and K+ exits the cell

Resting Membrane Potential

A potential (-70mV) exists across the membrane of a resting neuron – the membrane is polarized

• inside is negative relative to the outside• polarized membrane• due to distribution of ions• Na+/K+ pump

Resting Membrane Potential

Resting Membrane PotentialIonic differences are the consequence of:

•Different membrane permeabilities due to passive ion channels for Na+, K+, and Cl-

•Operation of the sodium-potassium pump

Membrane Potentials: Signals

Membrane potential changes are produced by:

•Changes in membrane permeability to ions

•Alterations of ion concentrations across the membrane

Neurons use changes in membrane potential to receive, integrate, and send information

Two types of signals are produced by a change in membrane potential:

•graded potentials (short-distance)

•action potentials (long-distance)

Levels of Polarization•Depolarization – inside of the membrane becomes less negative (or even reverses) – a reduction in potential

•Repolarization – the membrane returns to its resting membrane potential

•Hyperpolarization – inside of the membrane becomes more negative than the resting potential –an increase in potential

Depolarization increases the probability of producing nerve impulses. Hyperpolarization reduces the probability of producing nerve impulses.

Changes in Membrane Potential

Graded PotentialsShort-lived, local changes in membrane potential (either depolarizations or hyperpolarizations)

Cause currents that decreases in magnitude with distance

Their magnitude varies directly with the strength of the stimulus – the stronger the stimulus the more the voltage changes and the farther the current goes

Sufficiently strong graded potentials can initiate action potentials

Graded Potentials

Voltage changes in graded potentials are decremental, the charge is quickly lost through the permeable plasma membrane

short- distance signal

Action Potentials (APs)An action potential in the axon of a neuron is called a nerve impulse and is the way neurons communicate.

The AP is a brief reversal of membrane potential with a total amplitude of 100 mV (from -70mV to +30mV)

APs do not decrease in strength with distance

The depolarization phase is followed by a repolarization phase and often a short period of hyperpolarization

Events of AP generation and transmission are the same for skeletal muscle cells and neurons

Na+ and K+ channels are closedEach Na+ channel has two voltage-regulated

gates Activation gates –

closed in the resting state

Inactivation gates – open in the resting state

Action Potential: Resting State

Depolarization opens the activation gate (rapid) and closes the inactivation gate (slower) The gate for the K+ is slowly opened with depolarization.

Depolarization PhaseNa+ activation gates open quickly and Na+ enters causing local depolarization which opens more activation gates and cell interior becomes progressively less negative. Rapid depolarization and polarity reversal.

Threshold – a critical level of depolarization (-55 to -50 mV) where depolarization becomes self-generating

Positive Feedback?

Repolarization Phase

Positive intracellular charge opposes further Na+ entry. Sodium inactivation gates of Na+ channels close.

As sodium gates close, the slow voltage-sensitive K+ gates open and K+ leaves the cell following its electrochemical gradient and the internal negativity of the neuron is restored

Hyperpolarization

The slow K+ gates remain open longer than is needed to restore the resting state. This excessive efflux causes hyperpolarization of the membrane

The neuron is insensitive to stimulus and depolarization during this time

Role of the Sodium-Potassium Pump

Repolarization restores the resting electrical conditions of the neuron, but does not restore the resting ionic conditions

Ionic redistribution is accomplished by the sodium-potassium pump following repolarization

• at rest membrane is polarized

• sodium channels open and membrane depolarizes

• potassium leaves cytoplasm and membrane repolarizes

• threshold stimulus reached

Potential Changes

Phases of the Action Potential

Impulse Conduction

Action Potentials

Propagation of an Action Potential

The action potential is self-propagating and moves away from the stimulus (point of origin)

Threshold and Action PotentialsThreshold Voltage– membrane is depolarized by

15 to 20 mV

Subthreshold stimuli produce subthreshold depolarizations and are not translated into APs

Stronger threshold stimuli produce depolarizing currents that are translated into action potentials

All-or-None phenomenon – action potentials either happen completely, or not at all

Stimulus Strength and AP Frequency

Absolute Refractory Period

The absolute refractory period is the time from the opening of the Na+ activation gates until the closing of inactivation gates

When a section of membrane is generating an AP and Na+ channels are open, the neuron cannot respond to another stimulus

Relative Refractory Period

The relative refractory period is the interval following the absolute refractory period when:

Na+ gates are closed

K+ gates are open

Repolarization is occurring

During this period, the threshold level is elevated, allowing only strong stimuli to generate an AP (a strong stimulus can cause more frequent AP generation)

Refractory Periods

SynapseA junction that mediates information transfer from one neuron to another neuron or to an effector cell

Presynaptic neuron – conducts impulses toward the synapse (sender)

Postsynaptic neuron – transmits impulses away from the synapse (receiver)

Chemical SynapsesSpecialized for the release and reception of chemical neurotransmitters

Typically composed of two parts:

Axon terminal of the presynaptic neuron containing membrane-bound synaptic vesicles

Receptor region on the dendrite(s) or soma of the postsynaptic neuron

Synaptic Cleft

Fluid-filled space separating the presynaptic and postsynaptic neurons, prevents nerve impulses from directly passing from one neuron to the next

Transmission across the synaptic cleft:

Is a chemical event (as opposed to an electrical one)

Ensures unidirectional communication between neurons

Synaptic Cleft: Information Transfer

Nerve impulses reach the axon terminal of the presynaptic neuron and open Ca2+ channels

Neurotransmitter is released into the synaptic cleft via exocytosis

Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron

Postsynaptic membrane permeability changes due to opening of ion channels, causing an excitatory or inhibitory effect

Synaptic Cleft: Information Transfer

Termination of Neurotransmitter Effects

Neurotransmitter bound to a postsynaptic neuron produces a continuous postsynaptic effect and also blocks reception of additional “messages”

Terminating Mechanisms:

Degradation by enzymes

Uptake by astrocytes or the presynaptic terminals

Diffusion away from the synaptic cleft

Synaptic Delay

Neurotransmitter must be released, diffuse across the synapse, and bind to receptors (0.3-5.0 ms)

Synaptic delay is the rate-limiting step of neural transmission

Postsynaptic Potentials

Neurotransmitter receptors mediate graded changes in membrane potential according to:

The amount of neurotransmitter released

The amount of time the neurotransmitter is bound to receptors

Inhibitory Postsynaptic Potentials

Neurotransmitter binding to a receptor at inhibitory synapses reduces a postsynaptic neuron’s ability to generate an action potential

Postsynaptic membrane is hyperpolarized due to increased permeability to K+ and/or Cl- ions. Na+ permeability is not affected.

Leaves the charge on the inner membrane face more negative and the neuron becomes less likely to “fire”.

EPSPs and IPSPs

NeurotransmittersChemicals used for neuron communication with the body and the brain

More than 50 different neurotransmitters have been identified

Classified chemically and functionally

Neurotransmitters

Neurotransmitters – Chemical classification

•Acetylcholine (ACh)

•Biogenic amines

•Amino acids

•Peptides

•Novel messengers: ATP and dissolved gases NO and CO

Released at the neuromuscular junction• Enclosed in synaptic vesicles• Degraded by the acetylcholinesterase (AChE)Released by:

– All neurons that stimulate skeletal muscle– Some neurons in the autonomic nervous

system

Neurotransmitters: Acetylcholine

Two classifications: excitatory and inhibitory– Excitatory neurotransmitters cause

depolarizations (e.g., glutamate)

– Inhibitory neurotransmitters cause hyperpolarizations (e.g., GABA and glycine)

Functional Classification of Neurotransmitters

Some neurotransmitters have both excitatory and inhibitory effects (determined by the receptor type of the postsynaptic neuron). ACh is excitatory at neuromuscular junctions with skeletal muscle and Inhibitory in cardiac muscle.

• one neuron sends impulses to several neurons• can amplify an impulse• impulse from a single neuron in CNS may be amplified to activate enough motor units needed for muscle contraction

Divergence

• neuron receives input from several neurons• incoming impulses represent information from different types of sensory receptors• allows nervous system to collect, process, and respond to information• makes it possible for a neuron to sum impulses from different sources

Convergence

Animations on ion flow and signaling in neurons and

muscles• http://highered.mcgraw-hill.com/sit

es/0072437316/student_view0/chapter45/animations.html#