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Membrane Structure and Function

Transport of Substances through the cell membrane

Free interactive Physio toolshttp://www.winona.edu/biology/adam_ip/home/

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Lecture outline

I. Membrane Function and StructureA. PhospholipidsB. ProteinsC. CarbohydratesD. Cholesterol

II. Transport across the membraneA. Passive

i. Osmosis and osmotic pressure

ii. Simple a. Factors that

influence b. Examples

iii. Facilitated a. characteristics

iv. Rates of simple vs. facilitatedB. Active

i. Primaryii. Secondary

Cell Membrane Proteins

• In the cell membrane are phospholipids, proteins, sugars, etc., that separate intracellular and extracellular fluid, and limit what can travel through it.

• Proteins embedded in the cell membrane create channels or pores.

• In autoimmune disorders, these proteins may be perceived as antigens (foreign object) that can be attacked and destroyed by our white blood cells.

• Proteins are either on the outside of the cell membrane, in the middle of the membrane, on the inside, or a combination of these.

• The proteins that are only on the inside of the cell membrane can turn on activities in the cell.

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Phospholipids and proteins make cell membranes semipermeable

• Cell membranes are made from phospholipids, which are amphipathic (both water loving and water hating).

• The phosphate heads love water (hydrophilic), and can attach to other water loving molecules.

• The two fatty acid (FA) tails dislike water (hydrophobic), so they bind with other hydrophobic molecules (lipids).

• Substances that love lipids can get to the middle of the membrane and go into the cell, but water loving substances have a hard time crossing.

• Therefore, only hydrophobic molecules can cross a cell membrane easily, as well as small hydrophilic molecules such as gasses. Water can also easily cross.

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Membrane Function• The cell membrane organizes

chemical activities of cell– separates cells from outside

environment– controls passage of molecules

across membranes– provides surface for enzyme

reactions– Proteins embedded in the

membrane contribute to cell function, too!

• Form selective channels• Serve as carrier proteins• Serve as receptors

Phosphate heads6

Membrane Structure– phospholipids have polar “head”

(hydrophilic) and nonpolar “tail” (hydrophobic)

– form stable bilayer in water with heads out and tails in

– hydrophobic interior from fatty acid tails forms barrier to hydrophilic molecules

Phosphate heads

Fatty Acid tails

Fatty Acid tails

Cell Membrane Proteins

• Proteins can be integral (throughout the membrane) or peripheral (one side or the other).

• Integral proteins can create a pore (stays open all the time), or channel with a gate that can open and close.

• A peripheral protein on the surface of the membrane can bind a chemical (it can be a glucose receptor).

• A peripheral protein on the inside of the membrane can start a series of enzymatic reactions within the cell.

• Some proteins can bind substances on the outside of the membrane and transport them into cell (facilitative diffusion).

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•Provide function to a membrane•Can move laterally•Membrane also shows “sidedness” of electrical charges

•interior – has more negative charges•exterior – has more positive charges

• Proteins are defined by how deep they are in the membrane– integral proteins: form channels, pores, carriers– peripheral proteins: binds chemicals; inner membrane peripheral proteins start enzymatic reactions in cells

K+

Proteins:

Sugars Affect the Charges• Sugars outside of the cell can attach to the phosphate

heads or to the proteins (that will now be called a glycoprotein).

• If there are many glucose molecules on the outside of the cell, it will make the outside of the membrane more negatively charged.

• Every cell is set up like a battery, with a separation of charges across the cell membrane. This is called potential; one area is more negative than another area.

• There is storage of electricity, like a battery. The inside of the cell should be more negative than the outside of the cell. But if there is a glycocalyx (sugar bundle) on the outside of the cell, it makes it a negative charge.

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•glycoproteins (majority of integral proteins)• proteoglycans are a type of glycoprotein that has more

carbohydrates than usual.•glycolipids (approx. 10%)• involved in cell-cell attachments/interactions• play a role in immune reactions

GLYCOCALYX

(-) (-) (-) (-)(-)

(-)(-)

Carbohydrates

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• present in membranes in varying amounts• increases membrane FLEXIBILITY and STABILITY during temperature changes• helps to increase hydrophobicity of membrane

(-) (-) (-) (-)(-)

(-)(-)

Cholesterol:

Cholesterol

• In addition to proteins, cell membranes contain cholesterol. It is a lipid, so it’s located in the middle of the membrane.

• Cholesterol maintains the fluidity of cell membrane so the lipids are not frozen in place, but not so much that there are gaps in the cell membrane. There needs to be a balance of flexibility and stability.

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Cell Membranes• What can get through a cell membrane? Hydrophobic

molecules, gases like CO2, O2, small hydrophilic molecules like ethanol, and water can get through.

• Large hydrophilic molecules like glucose, and substances with a charge (like K+, Na+, and Cl-) can also pass, but can only cross the lipid center by active and passive transport. This requires the assistance of proteins in the cell membrane.

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Transport across a membrane: Understand this!

• A barrier to water-soluble substances• Allow lipid soluble substances to cross through membrane

hydrophilic“head”

hydrophobicFA “tail”

ions H2O

CO2

LIPIDS by themselves are:

O2

N2

glucoseLipids

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Movement across the cell Membrane

and ions

glucose

H2O

… but, in a living cell, hydrophilic molecules still get across! How?

CO2O2N2

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Passive Transport Active Transport• occurs down a concentration gradient• no energy is required• Two types of passive transport:• Simple (no mediator)• Facilitative (needs carrier protein)

• occurs against a concentration gradient• Requires energy (ATP)• Involves a “pump”

Figure 4-2; Guyton & Hall Passive transport

Osmosis &

Active transport

Passive vs. Active Transport• Passive transport means no cellular energy

required, no ATP used.• Active transport means ATP is used, either

directly or indirectly.• Passive transport makes substances move

from high to low concentration, down their gradient.

• Active transport is when at least one solute is moved against its concentration gradient.

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Osmosis

• Osmosis is when water passes through a membrane. Osmosis is passive diffusion, no ATP is used.

• Water moves from high to low concentration.

• That is, water moves from an area with few particles dissolved in it to an area with many particles dissolved in it.

• If you have two sides of a membrane, and the particles can’t move, water will move.

• How does it get through? There are proteins called aquaporins imbedded in the cell membrane that only allow water to pass.

• Aquaporins are made when a gene turns on, and they are taken back out of the membrane when the cell has enough water. Genetic problems can cause the wrong amount of aquaporins to be made, which causes water imbalances.

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Osmosis:Osmosis causes a net flow of water across a semipermeable membrane

(permeable to water but not solute)

Osmosis when water molecules move from an area of pure water toward an area that is a water/salt solution. Water moves down its concentration gradient.

Figure 4-9; Guyton & Hall

Sel

ectiv

ely

perm

eabl

e m

embr

ane

This movement is affected by the solute concentration (osmotic force) and hydrostatic forces (more on this later)

Simple Diffusion

• Simple diffusion of a solute (the particles dissolved in the water) is also passive. Rate of diffusion depends on two things:

• 1) How big is the gradient? – If the concentration on one side of the membrane

is greatly different than the other side, it is a steep gradient. The steeper the gradient, the faster the rate of diffusion

• 2) Is the solute permeable?

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Simple Diffusion

• Simple diffusion of small molecules can occur without any protein assistance by using a pore.– A pore is a passageway that is always open.

• Simple diffusion of larger molecules needs a protein channel.– A channel is a passageway that has a gate which is not always

open. – There are two main types of channels

• Ligand Gated Channels (LGC) open when a special chemical (ligand; such as a hormone or neurotransmitter) binds to it.

• Voltage Gated Channels (VGC) open only by receiving an electrical charge.

• All forms of simple diffusion are passive transport because they do not require ATP.

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Simple Diffusion (also called Non-carrier mediated transport)

1. Simple Diffusion (passive) • is tendency of molecules to

spread out spontaneously from area of high concentration to area of low concentration

• At equilibrium, there is no net gain nor loss of cell fluid.

• It is passive; molecule diffuses down concentration gradient without input of cellular energy

• Need permeability • Need concentration gradient

(chemical/ electrical) Can a molecule move from side A to side B? Yes, if it is permeable to the membrane and if there is a concentration gradient.

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(a) lipid-soluble molecules move readily across the membrane(rate depends on lipid solubility)

(b) water-soluble molecules cross via channels or pores (made out of proteins!).

• Ungated channels• Gated channels- Chemical and Electrical gated channels

(c) Different molecules diffuse independently of each other

(a) (b)

Simple Diffusion

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Ungated channels• determined by size, shape, distribution of charge, etc.

Characteristics:

Na+ Na+ and other ions

Gated channels• voltage (e.g. voltage-dependent Na+ channels)• ligand activated (an example of a ligand is a hormone or a neurotransmitter such as ACh)

Ion Channels- allow simple diffusion

Ungated Voltage gated Ligand gated

Acetylcholine (Ach)

• Ach is a neurotransmitter that is stored in a neuron (nerve cell). When released onto skeletal muscle, it causes it to contract.

• But when Ach is released onto cardiac muscle, it inhibits contraction.

• The difference is because of what is attached to the cell membrane protein on the inside of the cell. The cell membrane proteins on skeletal muscle are attached to things that cause contraction, but the cell membrane proteins on cardiac muscle are attached to things that inhibit contraction.

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Facilitated Diffusion

• Help me get into the house….bring the wheelchair!• Facilitated diffusion is still passive, no ATP is used. It is the same

end result as simple diffusion. The difference is that it requires a protein to physically bind to it and move it across the cell membrane. Therefore, it can be saturated.

• The rate at which solute is moved is limited by the number of carriers you have. When drunken people in a bar want to go home when the bar closes, and there is only one taxi, it would take a long time for all the people to get home. To get home faster, need more carriers.

• If each carrier moves one molecule, the rest of molecules have to wait their turn.

• We will talk about this again in the kidney lecture: when there are too many glucose molecules in the kidney, the receptors become saturated and glucose spills out in the urine.

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Facilitated Diffusion (also called carrier mediated diffusion)

Figure 4-7; Guyton & Hall

• Specific proteins facilitate diffusion across membranes

– no cellular energy required – Carrier protein interacts

with the solute (particle)– Specificity – carrier only

acts upon specific substrates.

– Saturation – the rate of transport will reach a maximum based on the number of carriers available in the membrane.

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1

2

3

4

5

=solute= transporter

Ex. Pass-through rate is 1 each minute

Transport maximum is reached when carriers are saturated (called Vmax)

1/min

2/min

3/min

4/min

5/min

Rate of Simple vs. Facilitated Diffusion

• If you increase concentration gradient, rate increases as well.

• Facilitative will reach velocity maximum. When it is saturated, it levels off.

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rate of diffusion

Concentration of substance

simple diffusion

Simple vs. Facilitated

Tm

facilitated diffusion

What limits maximum rate (Vmax) of facilitated diffusion? Number of carriers

Vmax

rate of diffusion (Co-Ci)

Active Transport

• Active transport is the movement of a substance against its concentration gradient (from low to high concentration). It requires ATP.

• There are two types of Active Transport:• Primary Active Transport

– Uses ATP directly• Secondary Active Transport

– Uses ATP indirectly because it uses an electrochemical gradient

– As one molecule crosses the membrane, it helps another molecule enter against its concentration gradient. 31

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Active Transport

Primary Active Transport– molecules are “pumped”

against a concentration – gradient at the expense

of energy (ATP) – direct use of ATP

Secondary Active Transport– transport is driven by the

energy stored in the concentration gradient of another molecule (Na+)

– One molecule down gradient

– One molecule against gradient

– indirect use of ATP

Primary Active Transport

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This shows the Sodium/Potassium ATPase pump. When sodium enters a cell, it has to be actively pumped back out by primary active transport

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Primary Active Transport• Cells expend energy for

active transport– transport protein involved in

moving solute against concentration gradient

– energy from ATP– rate limited by Vmax of the

transporters

• up to 90% of cell energy expended for active transport!– active transport of two

solutes in opposite directions

Na+/K+ ATPaseplays an important role in regulating osmotic balance by maintaining Na+ and K+ balance requires one to two thirds of cell’s energy!Others exist- calcium ATPase and H+ ATPase

Secondary Active Transport

• Secondary active transport (also called co-transport), also uses ATP, but one substance uses ATP to cross the membrane while another sneaks in without directly using ATP, like a revolving door.

• There are two main forms of secondary active transport:• Antiport

– The two different molecules are pumped in opposite directions

• Symport– Uses the downhill movement of one solute species from high to

low concentration to move another molecule uphill from low concentration to high concentration.

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http://www.sumanasinc.com/webcontent/animations/content/carrier_proteins.html

Secondary Active Transport

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As amino acids use ATP to exit a cell, it opens a gate for sodium ions to get in. The sodium ions are using secondary active transport.

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Secondary Active Transport

1. Symport: substance is transported in the same

direction as the “driver” ion (Na+)

Examples:

inside

outside

Na+ AA Na+ gluc 2 HCO3-Na+

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2. Antiport: substance is transported in the opposite

direction as the “driver” ion (Na+)

Examples:

Na+

Ca2+

Na+

H+ Cl-/H+

Na+/HCO3-

outside

inside

• Sample test questions: given the following list, answer the questions below.

• Simple Diffusion

• Facilitative Transport

• Primary active Transport

• Secondary active Transport

• Which has net movement of water? – Simple diffusion

• Select all that apply: This type of transport moves solutes down the concentration gradient. – Simple, facilitative, secondary active

• Which ones have a solute moved against its gradient: – Primary and secondary

• Which is moved against its gradient and ATP is directly used: – Primary

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Membrane Potentials and Action Potentials

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Lecture outlineI. Review

A. PermeabilityB. Concentration gradientsC. Sidedness of the membrane

II. Electrical gradientsA. PotentialB. ElectrolytesC. Conductance (permeability)

III. Resting membrane potentialA. Caused by

i. Proteinsii. Na+/K+ ATPaseiii. K+ “leak” channels (pores)

IV. Excitable cells

Diffusion Down Electrical Gradients• We talked about solutes diffusing down their concentration gradient.• There is another gradient that solutes diffuse down….their electrical gradient. The

insides of a cell are more negatively charged than the outside of a cell. That’s because there are a lot of proteins inside of cells, and most proteins are negatively charged.

• K+ is in high concentration inside the cell, and Na + is in high concentration outside of the cell. Although K+ channels are gated, they are leaky, so K+ can leak in and out of the cell whenever it wants to. Sodium’s channels are gated, so Na+ mostly has to stay outside of the cell until its gate opens, and then it will rush in. After it is inside the cell, it does not want to leave, but there is a Na+ pump that forces Na+ to leave. It is like having a boat that is filling up with water. Sodium is the water, and you have to bail it out. The more water that leaks in, the faster you have to bail.

• Since K+ can freely leave and enter the cell, its positive charge is attracted to the negative charges of the proteins on the inside of the cell. Therefore, it is in higher concentration on the inside of the cell, but Na+ is in higher concentration on the outside of the cell.

• Positively charged ions on the outside of the cell will want to diffuse down their electrical gradient to get to the inside of the cell, but only K+ can get in freely. The others have to wait until their gate opens.

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Membrane Potential• How negative is the inside of the cell membrane? • At rest, the inside of most cell membranes is minus 70 mV

(milivolts). • At rest, the outside of most cell membranes is +30 mV.• That means there is “sidedness” of cell membranes…the

inside of the membrane has a negative charge and the outside has a positive charge.

• This separation of charges is called the membrane potential.

• If the charges on a battery reach equilibrium on both sides, the battery will be dead. That can happen to our cells, too….if the charges are no longer different (if they reach equillibrium), the cells will die.

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Membrane Potential

• Some of the proteins embedded in the cell membrane (integral proteins) form selective channels that allow particular substances to cross the membrane and get into the cell.

• The integral proteins form because of gene expression. If something goes wrong with a gene, the proteins might stop working properly.

• Charged ions such as K+, Na+, Ca++ are called electrolytes. When they move, they carry their electrical charge with them.

• The primary intracellular ions are potassium, phosphate, and proteins ions

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Speed of Conduction

• Conductivity means permeability. If conductivity of an ion increases, it means that the permeability of that ion increased.

• Ions diffuse at a faster rate when there is less resistance. The more resistance there is, the less conductivity, and less resistance will cause more conductivity.

• Myelin is a fat shealth wrapped around the axons of some neurons. If a neuron is myelinated, the resistance is decreased because it makes electrical charges move faster.

• Another thing that affects speed of electrical transmission is the size of neuron: bigger neurons carry current faster (expand the freeway to add extra lanes, you will get home faster).

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Electricity• Current: the flow of charge

• Voltage: separation of opposite charges (mV)– Voltage = Potential– Voltage difference = Potential difference

• Resistance: opposition to charge movement (friction)

• Conductance: allowing a charge to move (permeability)

+++ ++

- - - - - - -

- +

What are the charged things that run through our body fluids? Electrolytes! Ions: Na+ K+ Cl- Ca++

+ -

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Unlike simple concentration gradients, when dealing with things that are charged ….

You must ask a third question!1. Is the membrane permeable to it?2. Is there a chemical gradient for it?

– Things tend to move from high to low concentration

3. Is there an electrical gradient for it?– Things tend to move to regions of

opposite charge

Only then, can you predict if the substance will move across the membrane!

+++ ++

- - - - - - -

-+

= Na+Sometimes, the chemical gradient is favors one ion to go in one direction, and the electrical gradient favors it to go in the other direction. The stronger pull will win.

Summary

• Every cell has a separation of charge.

• K+ is in higher concentration on the inside of the cell, so it constantly leaks out of the cell to diffuse down its concentration gradient.

• But when its positive charges join with the positive charges of Na+ on the outside of the cell, both K+ and Na+ want toget into the cell because their charges are attracted to the negative charges on the inside of the cell.

• K+ can get back in because its channel is always open, but Na+ has to stay out because its channel is always closed unless something opens it.

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Because of this separation of chemicals and electrical charges, every cell has a Resting

Membrane “Potential”• There is a difference in electrical

charge across the membrane (a potential difference)

• The cell membrane is more negative inside; more positive outside

• What causes this?– Mainly, ion concentration gradients and

differences in membrane permeability (leaky to K+ but not to Na+ or protein)

• At rest, the overall charge of the inside of the cell membrane is

-70 to -90 mV

Separation of Charges

• The membrane potential is how negative or positive the overall charges are.

• The inside of the cell membrane is usually minus 70 mV.• If K+ diffuses out of the cell, down its concentration

gradient, it takes its positive charges with it, leaving the inside of the cell more negative.

• What if the cell suddenly became permeable to Na+? • Sodium would rush into the cell, down its concentration

gradient, taking its positive charges with it, making the inside of the cell more positive.

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• Cells with resting membrane potential are at minus 70mV. They are not at their resting K+ potential.

• If you open more K+ channels to get rid of more K+ from the cell, the overall cell membrane voltage will get closer to minus 94 mV (at which time, the cell will reach K+ equilibrium, and the cell will die; but the body does not let it get that far).

• You also cannot let sodium continue on into the cell until it reaches equilibrium, or the cell will not be able to metabolize, and it will die.

• To prevent too much sodium from entering the cell, the Na+ channels remain closed most of the time.

• When there is an action potential (nerve stimulation), the sodium gate opens first and the potassium gate opens second.

• Sodium rushes into the cell, changing the voltage on the inside of the cell membrane from negative to positive. This causes the outside of the cell membrane to also flip, from positive to negative.

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• Then the K+ does not want to stay inside the cell any more. It wants to leave for two reasons: to diffuse down its concentration gradient, and to diffuse down its electrical gradient. When it leaves the cell, taking its positive charges with it, the outside of the cell membrane flips from negative back to positive again.

• Sodium cannot leave a cell once it is inside. It has to be pumped out, by sodium-potassium ATPase (the mother protein, or housekeeping protein). Mom organizes the house and the kids mess it up again. She directs the kids to take their toys and put that here, and put that there.

• Whether the cell is at rest or during an action potential, Na-K ATPase is active all the time, constantly trying to reestablish the gradients. The housekeeper protein never stops working!

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mV0

ENa+61RMP -74 mVEK -94 mV

Normal conditionsNormal conditions

20 mV 135 mV

Resting membrane potential is -74 mV Equilibrium of K+ (and cell death!) would occur at -94 mVEquilibrium of Na+ (and cell death!) would occur at +61 mV

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ATP

3 Na+

2 K+

ADP

K+ Na+

Na+ K+

inside outside

Remember: sodium is pumped out of the cell, potassium is pumped in...

What keeps the ion gradients from running down? The sodium/potassium ATPase (“the housekeeper”) Do we want our cells to be like a “dead battery?”

Integral membrane protein found in all cells which “pumps” (against their gradients across the membrane) Na+ and K+.Fueled by ATP

ATP ADP + Pi + energy

Excitable Cells

• Excitable cells (neurons and muscles) are those that want this large electrical current to use for work.

• They have proteins that form sodium channels. Not all cells have these proteins.

• All cells have the genes to make these proteins, but only the excitable cells EXPRESS these genes, and actually make the proteins that fuse with the cell membrane and form a sodium channel.

• Muscle cells use the electrical force to contract, and neurons use it to excite the neurons touching them.

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Excitable Cells• Cells that can experience a momentary

change in membrane voltage are “excitable” cells

• That temporary change in voltage is due to a momentary change in permeability

• The membrane, for only a moment, becomes more permeable to Na+ than to K+

• When this happens, there is a reversal of charges on the inside and the outside of the cell membrane.

• Cell becomes positive inside!!!• This is called an Action Potential

GENE EXPRESSION !

Definitions:

• There is a potential difference (pd) across the cell membrane

• (minus 70 mV) is called the “Resting Membrane Potential”

• Because a charge is present (it is not zero), we say the membrane is “polarized”

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• If it becomes less negative, it is called depolarization (happens when sodium is entering the cell).

• If it becomes more negative than minus 70, it is hyperpolarization. (happens when K+ leaves the cell)

• In either case, when you go back towards minus 70, it is repolarization.

• Threshold is the point at which the charges begin to be positive on the inside of the cell membrane and negative on the outside of the cell membrane.

Question• To depolarize a cell, what kind of charge must be put

into the cell, positive or negative? Positive

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0 mV

hyperpolarization

depolarization

repolarization

threshold

resting potential

-90 mV

+

-

excitabilityexcitability

Excitability

Depolarization-a current entering the cell that decreases the polarity (voltage) across the membrane (that is, bring voltage closer to 0 mV).

To depolarize, what kind of charge must be put into the cell, positive or negative? Positive

Hyperpolarization-a current that increases the voltage across the membrane (brings it farther from 0 mV; makes it more negative)

Repolarization

Cell voltage returns towards resting potential

Voltmeter- - - -

+- 70 mV

“Resting Membrane Potential”(70 mV)

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time

Voltage (mV)

Stim Elec

REC 1 REC 2 REC 3

++++

First try: a small depolarizing stimulus (-65 mV)

-70 --60 --50 --40 --30 --20 --10 - 0 -+10 -+20 -+30 -+40 -

+++ ++ +

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time

Voltage (mV)

Stim Elec

REC 1 REC 2 REC 3

++++++

Next try: a slightly larger depolarizing stimulus (-60 mV)

-70 --60 --50 --40 --30 --20 --10 - 0 -+10 -+20 -+30 -+40 -

++++++ +++ ++

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time

Voltage (mV)

Stim Elec

REC 1 REC 2 REC 3

+++++++++

-70 --60 --50 --40 --30 --20 --10 - 0 -+10 -+20 -+30 -+40 -

++++++++++++++++++ +++++++++

Next try: a slightly larger depolarizing stimulus (-55 mV)

RMP

Local Potentials

Action Potentials

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time

Voltage (mV)

Stim Elec

REC 1 REC 2 REC 3

- - - - -

-70 --60 --50 --40 --30 --20 --10 - 0 -+10 -+20 -+30 -+40 -

Local Potentials

RMP

Action Potentials

Can we get even larger Action Potentials?

Try an even larger depolarizing stimulus (-50 mV)

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time

Voltage (mV)

Stim Elec

REC 1 REC 2 REC 3

- - - - -

-70 --60 --50 --40 --30 --20 --10 - 0 -+10 -+20 -+30 -+40 -

Local Potentials

RMP

Action Potentials

Can we get even larger Action Potentials?

Try an even larger depolarizing stimulus (-50 mV)

No higher, no larger, Identical!

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Note the timeframe for one AP

Definition:

Threshold voltage is the minimum voltage needed to trigger an AP. not a number, rather the “trigger” to open voltage operated channels

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Action Potentials

• They don’t reflect the shape, size of the stimulus, rather they are uniform in size, shape; always identical

• They are “all-or-none” (ie. either you trigger an AP if you reach threshold – or if subthreshold, you don’t get an AP – get local potential.)

• They do not diminish in size no matter how far from the stimulus; regenerate anew at each point along the axon

Sodium Channels

• When a small change in voltage occurs but it is not large enough to cause an action potential, it is called a local potential. These are graded; you can apply a stronger stimulus to make them larger peaks.

• Once you have a large enough voltage change to open the sodium channels, it is called an action potential.

• Once one sodium gate opens, another and another will open. Thus, action potentials are propagated down the cell membrane.

• The neuron that is responding to a small amount of tissue damage will send fewer action potentials than a neuron that is responding to a large amount of tissue damage, but the amount of voltage is the same. The brain senses the greater number of action potentials and understands that there is a greater amount of damage.

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Action Potentials• AP’s are uniform in size, shape; always identical• They are “all-or-none” (i.e. either you trigger an AP if

you reach threshold – or if sub-threshold, you don’t get an AP – just get a local potential.)

• They do not diminish in size no matter how far from the stimulus; regenerate anew at each point along the axon

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http://www.blackwellpublishing.com/matthews/channel.html

http://icarus.med.utoronto.ca/neurons/index.swf

Ion channels cause the action potential

• Thus, permeability of axon membrane to ions is determined by the number of open channels.

• Ion channels are usually selectively permeable

• some pass only Na+ ions and are generally called ‘Na+ channels’

• some pass only K+ ions = ‘K+ channels’

• some pass only Ca++ ions = ‘Ca++ channels’

• some pass only Cl- ions = ‘Cl- channels’69

Interactive Physio

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Free interactive Physio toolshttp://www.winona.edu/biology/adam_ip/home/

Click On:Fluids and ElectrolytesIntro to Body Fluids

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