unit2- membrane structure and function

8/10/2019 Unit2- Membrane Structure and Function

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Membrane Structure and

Function

Chapter 7


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TEM of Phospholipid Bilayer


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Membrane Structure

Basic fabric of membranesis a phospholipid bi-layer

Phospholipids areamphipathic, so the centerof the bi-layer is

hydrophobic and theoutsides are hydrophilic

Proteins are found in thelayerthe hydrophobicregion of proteins are found

in the center of the bi-layer,with the hydrophilic regionsprotruding on both sides

Proteins may be integral orperipheral

Hydrophobic region of proteinHydrophilic regions of protein


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The fluid mosaic model

(Part of cytoskeleton)

(Oligosaccharide added in the Golgi body)

(Protein + Oligosaccharide = Glycoprotein)

Cholesterol

Membranes have

the consistency of

cooking oil!


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The fluidity of membranes

The phospholipids ofmembranes areconstantly drifting -moving laterally

Sometimes thephospholipids flip-flop

The embedded proteinsor surface proteins alsodrift

Some proteins are heldin place by thecytoskeleton


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The fluidity of membranes, contd.

Membranes remain fluid when temperature decreases - up to a certain

critical temperature, after which they solidify The more the concentration of unsaturated hydrocarbons in the

phospholipid tails, the longer the membrane stays fluid (Because of

kinks in the tails, they cannot pack closely)

Cholesterol is a common component of animal membranesit keeps

the membrane fluid at low temperatures, but reduces fluidity atmoderate temperatures.


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Evidence of membrane protein drift

When mouse and human cells were fused, their phospholipid bi-layers,

along with their membrane proteins intermingled within one hourcreating a chimeric plasma membrane.


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Membrane Proteins Integral proteins are either completely embedded

(transmembrane), or partially embedded in the bilayer

Peripheral proteins are not embedded in the membrane,they are attached to the surface of the bilayer or to integralproteins

A transmembrane protein


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Functions of the membrane proteins


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Types of membrane proteinsand their roles

For example: insulin binding to membrane proteins, which starts a signaling

pathway that stimulates cells to take up more glucose from the bloodstream

For example: Enzymes embedded in the inner membrane of mitochondria

play a role in cellular respiration

Passive transport vs. Active transport


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Types of membrane proteinsand their roles, contd.

For example: cells of the immune system need to bind to glycoproteins

on cell surfaces, in order to decide if the cell belongs to the body or is

foreign

Integrins are an example of cell surface receptor proteins that adhere to and

interact with the ECM. Integrins also coordinate activities inside and outside

cells via signal transduction.


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Traffic Across Membranes The phospholipid bilayer is selectively permeableit

allows only certain substances acrossdepending onSIZE and/or POLARITY

Because it is hydrophobic in the center, it does notallow ions and polar molecules acrosseven small

ions like H+, Na+ or OH- cannot cross membranes For the same reason, it does allow nonpolarmolecules like O2, CO2(Diffusion and osmosis)

Large molecules whether polar or nonpolar cannot

cross over (most sugars, proteins, amino acids, lipids,etc.)

Membrane proteins help transport molecules thatcannot cross the bilayer on their own

Selectivepermeability


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Electrostatic Gradient

The interior of cells is negatively charged compared tothe outside

This creates a voltage across the membrane, which iscalled the membrane potential

For this reason, anions will automatically move outsidethe cell (drawn by the + charges) and cations will bedrawn to the inside (drawn by the neg- charges)ionshowever, need to pass through membrane proteins.

This difference in charge is called the electrostaticgradient

The membrane potential of a resting cell is about

-70 mV (It can range from -50 to -200 mV)


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Electrostatic Gradient, contd.


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Concentration Gradient

Molecules introduced to

a new environment, will

move away from their

initial location, creating aconcentration gradient

their concentration

becomes exceedingly

lower as you move awayfrom the introduction site
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Electrostatic gradient + Chemical gradient =

Electrochemical gradient


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PASSIVE TRANSPORT

Passive transport is the movement of

molecules down their electrochemicalgradient

Passive transport requires no energy

expenditure on the part of the cell. Freeenergy is usedthe energy of the system

Examples of passive transport:

Diffusion

Osmosis

Facilitated diffusion (Protein channels

involved)


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Diffusion Molecules have the natural tendency (due to random

molecular motion)of moving from an area where theyare highly concentrated, to an area where their

concentration is lowthey move down their

concentration gradient+

Once the moleculesare evenly dispersedin the environment,they reach a state ofequilibriumtheycontinue to move, butit is equal in everydirectionso no net

change

High free energy

Low free energy

stable system


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Diffusion

Diffusion is passive transport

It is the random movement of molecules

from and area of high concentration to an

area of low concentration

Diffusion requires NO energy

In diffusion, molecules move along theirconcentration gradient


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Osmosis

Osmosis is passive transport

It is the random movement of WATER

molecules from an area of high water

concentration to an area of low water

concentration

Osmosis requires NO energy

In osmosis, molecules move along their

concentration gradient


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Osmosis

The diffusion of water molecules

The tendency of water molecules (due to random

molecular motion)to move from an area wheretheir concentration is high (higher free energy), to

an area where their concentration is lower (lower

free energy)until equilibrium is reached (no net

movement of water)

Movement of water molecules is down their

concentration gradient


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(of water molecules)

As solute concentration

increases, free water

concentration decreases

so water potential

decreases

Water then moves from an area

of high water potential to an

area of low water potential

Low solute High solute

Isotonic Solutionsolute and solvent balanced (Also a form of Passive Transport)

Inside the cell is higher than


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Inside the cell is lower,

because of solutes in the

cytosol

Water molecules always move

from an area of higher water

potential to an area of lowerwater potential, so water rushes

into the cell from the outside

(Net movement is inwards)

Is the cell hypertonic, hypotonic or isotonic with respect to its environment?

Inside the cell is higher than

the outside, because the outside

has more solute particles

Water will therefore move out of

the cell to an area of lower (Net

movement is outwards)

Is equal on both sides,

so no net movement


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Plants & water potential

Plants can use the

potential energy in

water to perform

work. Tomato plant

regains turgor

pressurecell

pushes against walldue to uptake of

water


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Plants & water potential

The combined effects of

1.) solute concentration

2.) physical pressure (cell wall)

can be measured as Water Potential

psi

is measured in megapascals (MPa)

1 Mpa = 10 atmospheres of pressure


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Calculating Water Potential

= P + S

Or

Water = pressure + solutePotential potential potential


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Solute Potential S

Solute potential is also called the osmotic

potential because solutes affect the direction of

osmosis.

S of any solution at atmospheric pressure is

always negativewhy?

Answer = less free water molecules to do work


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Solute Potential S

Solutes bind water

molecules reducing

the number of free

water moleculeslowers waters

ability to do work.


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Pressure Potential P

P is the physical pressure on a solution.

P can be negativetranspiration in the

xylem tissue of a plant (water tension)

Pcan be positivewater in living plant

cells is under positive pressure (turgid)


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Standard for measuring

Pure water is the standard.

Pure water in an open container has awater potential of zero at one

atmosphere of pressure.


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Water Potential: an artificial model

(a) addition ofsolutes on right sidereduces waterpotential. S = -0.23

Water flows fromhypo to hyper

Or from hi on left

to lo on right

W t P t ti l tifi i l d l


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(b) adding +0.23 pressure with plunger no net

flow of water (c) applying +0.30 pressure increases water

potential solution now has of +0.07

Water moves right to left


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(d) negative

pressure or tension

using plunger

decreases waterpotential on the left.

Water moves from

right to left



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Water relations in plant cells

(b) Flaccid cell in pure water Water

potential is into cell cell becomes turgid


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Water relations in plant cells

(a) Flaccid cell placed in hypertonicsolution

Water potential is out of cell plasmolysis


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Calculating Solute potential

Need solute concentration

Use the equation

S = - iCRTi = # particles molecule makes in water

C = Molar concentration

R = pressure constant0.0831 liter barmole oK

T = temperature in degrees Kelvin

= 273 + oC


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Solve for water potential

(literal equation)

Knowing solute potential, waterpotential can be calculated by insertingvalues into the water potential

equation.

= P + S

In an open container, P= 0


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Hints & reminders

1. Remember water always moves from [hi]to [lo].

2. Water moves from hypo hypertonic.

3. [Solute] is related to osmotic pressure.Pressure is related to pressure potential.

4. Pressure raises water potential.

5. When working problems, use zero forpressure potential in animal cells & openbeakers.

6. 1 bar of pressure = 1 atmosphere


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Water and the Bilayer

Although water is a polar molecule, somewater molecules ARE able to sneak pastthe phospholipids via osmosis.

But the majority of the water molecules areprevented from passing the hydrophobictails of the lipid bilayer

So water has to use Aquaporins, a specialclass of integral transmembrane channelproteins


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Aquaporins

More than 10 different mammalianaquaporins have been identified to date,and additional members are suspected toexist.

Some aquaporins transport solute-freewater across cell membranes; they appearto be exclusive water channels and do notpermeate membranes to ions or other

small molecules.

Other aquaporins transport water andother small polar molecules and ions.


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Plasmolysis


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Plasmolysis

When a cell is placed in a hypertonic environmentmore solute outside

than inside:- Water potential is greater inside

- Water will move from where water potential is greater, to where it is

lower

- Water will move out of the cell, causing plasma membrane to collapse

(low pressure potential)- Cell wall will keep cell from losing its shapeanimal cell loses shape


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Facilitated Diffusion

Ions and small polar molecules use facilitated

diffusion

Integral membrane channel proteins

1. open channel(water uses this method -aquaporins)

2. gated channel

3. carrier proteins(glucose uses this method)

Requires no cellular energy (ATP, GTP, etc.)

Specific channel proteins for specific ionslock-key system

Diffusion is down concentration gradient


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Facilitated Diffusion

Ions and small polar (Hydrophilic)

molecules use facilitated diffusion

Membrane channel proteins are used

Requires no cellular energy (ATP, GTP,etc.)

Diffusion is down concentration gradient


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Facilitated

Diffusion, Contd.

F ili d Diff i C d


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Facilitated Diffusion, Contd.

ACTIVE TRANSPORT


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ACTIVE TRANSPORT

Uses cellular energy (ATP, GTP, etc.)

Uses integral membrane proteins

Specific proteins for specific molecules

Molecules can be moved againsttheir

electrochemical gradient Ion pumpslike the Na+ / K+ pumpand the Proton pump (H+)

are an example of active transport

Concentration of Na+ has to be higher outside

the cell whereas that of K+ has to be higherinsidethe cellso active transport is used tomaintain these concentrations (pumping againstelectrochemical gradient)


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Active Transport Contd


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Active Transport Cont d.

Na+ binds to the

transport protein

at specific

binding sites

Na+ binding

causes ATP to

phosphorylate

protein

Phosphorylationcauses

conformational

change in protein,

which moves the

Na+ out of the

cell

When Na+ exits

the binding site,

the binding site

for K+ is madeaccessible and

K+ binds to sites

When K+ binds, it causes another

conformational change, whichmoves K+ into cell

When K+ exits

its binding site, it

causes the

release of theinorganic

phosphate group

1.

2.

3.

4.

5.

6.

A ti T t C td


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Active Transport Contd.

Proton Pumps helps move H+ against their gradient (out of cell)this build-up of

H+ outside the cell is VERY important, because it is a high-energy/ unstables stem that can be used to ener ize other cellular rocesses

Endocytosis = Phagocytosis + Pinocytosis


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Endocytosis = Phagocytosis + Pinocytosis

Endocytosis is

active transport

needs energy

expenditure

Pinocytic vesicle forming

A l h t


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A lymphocyte

attacking E.coli

SEM of stained prep.

TEM of lymphocyteE.coli being ingested


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THE END

unit2- membrane structure and function

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