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Lecture Outline

Homeostasis Negative Feedback Systems

Body Fluid Compartments and ionic compositions

Cell Membrane

Membrane Transport Mechanisms Simple diffusion

Diffusion through lipid bilayer

Diffusion through protein lined pore/channel

Facilitated diffusion

Active transport

Tom Stavraky 661-3474

MS 206

Tom.Stavraky@Schulich.uwo.ca

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Terry Fox

Terry Fox Run

for Cancer Research

Sunday, September 18th

at

UWO and Springbank

Gardens

Terryfox.org

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Internal and External Environments

External Environment

Internal Environment

Its important tomaintain this internal

environment so the

cells can function

properly .

Homeostasis

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Body Fluid Compartments

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Chemical Composition Inside and

Outside a Cell

Substance Plasma Interstitial Intracellular

Sodium ions 144 140 14

Potassium 4.8 5.0 150

Calcium 2.5 2.0 10-4

Chloride 102 125 10

Bicarbonate 22 24 12

Proteins 1.2 0.2 4

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Cell Membrane

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Membrane Transport Mechanisms

1. Endocytosis/Exocytosis

2. Diffusion

Simple diffusion

Through cell membrane/lipid bilayer

Through protein-lined pore

3. Facilitated diffusion

4. Active transport

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Simple Diffusion

The movement of molecules due to theirrandom thermalmotion

- Water moves at 2,500 kph

- Glucose moves at 850 kph

Molecules move from an area ofhigh concentration to lowconcentration (down theirconcentration gradient)

Until chemical (dynamic) equilibrium is reached

(no net movement)

High

Conc.

Low

Conc.

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Diffusion Distance

Diffusion is not efficientover long distances

Glucose traveling 10

takes 3.5 sec at 38oC

Glucose traveling 10 cm

takes 11 years

The time for diffusion

increases with the

square of the distance

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Diffusion Through Cell Membrane

Substances must be non polar (lipid soluble) inorder to penetrate the lipid (fatty acid tail) region of

the lipid bilayer.

non polar molecules include: O2, CO2, fatty acids, steroids

and some alcohols

Driving force is the concentration gradient

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Ficks First Law of Diffusion

J = - kT

x

dc

x

A

6r dxWhere:

J = net rate of diffusion in moles or grams per unit time

k = Boltzmann constant

T = absolute temperature

r= molecular radius = viscosity of the medium

The 4 values above (K, T, r, ) are sometimes all summarized into thediffusion coefficient, D.

A = total surface area of the membrane for diffusiondc = concentration gradient of the solute

dx

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Ficks First Law of Diffusion

J = - kT x dc x A

6r dx

Questions:

1. How would you decrease the rate of diffusion?

2.Why is there a - before the kT?

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Membrane Transport Mechanisms

Diffusion Simple diffusion

Through cell membrane/lipid bilayer

Through protein-lined channel/pore

Facilitated diffusion

Active transport

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Diffusion Through Protein

Channels/Pores (pg EC-16)

Polarmolecules (ions andother water-solublemolecules) cannot diffusedirectly through the cellmembrane due to thehydrophobicfatty acidregion.

They require a proteinpore/channel (leak channel)

Water moves through porescalled aquaporins

Driving force is theconcentration gradient

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Factors affecting the rate of diffusion

through protein channel (EC-17)

1. Size of the molecule (< 0.8 nm)

2. Charge on the molecule and

channel (+, -)

3. Electrochemical gradient

4. Pressure gradient

( kinetic energy)

5. Hydration energy (water

shell)

- Hilles theory of closest fit

-Water shell must be removed by the

channel without the water

knowing it so ion remains

energetically comfortable

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Some Ion Charges and Diameters

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Factors affecting the rate of movement

through protein leak channel

1. Size*

2. Charge*

3. Electrochemical

4. Pressure gradient5. Hydration energy (water shell)*

Some of these factors act as *filters

preventing one ion from passingthrough another ions channel this is

NOT chemical specificity, as we will

see in a moment.

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Membrane Transport Mechanisms

Diffusion Simple diffusion

Through cell membrane/lipid bilayer

Through protein-lined channel/pore

Facilitated Diffusion

Active transport

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Facilitated Diffusion

Some large molecules (>0.8 nm) cross the membrane fasterthan Ficks law predicts

Attachment of a molecule to an integral membrane protein

will cause a conformational change in that protein moving the

molecule across the membrane

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Facilitated Diffusion

This is a form ofcarrier

mediated transport

Driving force is the

concentration gradient

Characteristics:

Chemical Specificity

Competitively inhibited

Saturation Kinetics

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Facilitated Diffusion

Saturation Kinetics Transport rate is

limited by the number

of carriers and thespeed of the

conformational

change..

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Membrane Transport Mechanisms

Diffusion Simple diffusion

Through cell membrane/lipid bilayer

Through protein-lined channel/pore

Facilitated Diffusion

Active transport

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Active Transport

A form ofcarrier mediated transport Chemical Specificity

Competitively inhibited

Saturation Kinetics

*Moves substances against the conc. grad.

- Requires energy (ATP)

Eg. Na+/K+ pump..

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Active Transport

Sodium/Potassium

Pump (ATPase)

Pumps 3 Na+ out and

2 K+ in (against their

conc. gradients) for

every ATP split

Extracellular

fluid

Intracellular

fluid

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Sequence of events in Na+/K+ pump

At rest all binding sites empty, ATP attached but not split

Step 1. Three Na+ bind to their intracellular binding sites

Step 1 to 2. ATP is hydrolyzed, ADP is released causing conformation

change of protein

Steps 2 and 3. Spontaneous conf. change Na+ is shuttled across

membrane (protein now has low affinity for Na+)

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Sequence of events in Na+/K+ pump

Step 4. K+ ions attach to

extracellular binding sites

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Sequence of events in Na+/K+ pump

Step 4 to 5. Inorganic Phosphate (Pi) is released from protein protein changes shape

Step 5 to 6. New ATP binds to protein causing conformation change

K+ is shuttled across membrane into the cell

Step 6. Protein returns to resting configuration and sequence

repeats

Metabolic inhibitors such as ouabain and digoxin can inhibit the pump by

binding to extracellular side (usually between steps 3 and 4)

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Functions of the Na+/K+ pump

Helps maintain the concentration gradients forNa+ and K+ across the cell membrane

Causes slight increased negativity inside the cell(more positive charge is being removed thanreplaced)

Keeps the cell from swelling and bursting due toosmosis

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Secondary Active TransportEC-24

The Na+/K+ pumpmaintains largeconcentration gradientfor Na+ high conc.outside and low inside

These concentrationgradients can be usedto power other

transport mechanisms

Indirectly requiresATP