ib 1.4 membrane transport

1.4 Membrane Transport

Plan

1 Introduction:

2 Diffusion and Facilitated Diffusion:

2.1 Diffusion

2.2 Facilitated Diffusion

3 Special case of Water: Osmosis:

2

3

1 4

5

4 Active Transport:

5 Transport in bulk

5.1 Endocytosis

5.2 Exocytosis

6 Conclusions and Overviews:

Xavier DANIEL, Ph.D. AS IB

The phospholipid bilayer is a good barrier around cells, especially to water soluble molecules

However, for the cell to survive some materials need to be able to enter and leave the cell

Four basic mechanisms:

Introduction 1

Diffusion and

facilitated diffusion

Active transport Osmosis Bulk transport


Plan

1 Introduction:


2.1 Diffusion



2

3

1 4

5

4 Active Transport:

5 Transport in bulk

5.1 Endocytosis

5.2 Exocytosis




Substances move from HIGH to LOW concentration

“passive transport”

no energy needed

Diffusion 2


Diffusion 2

Diffusion is the net movement of molecules (or ions) from a region of

their high concentration to a region of their lower concentration

The molecules move down a concentration gradient

Molecules have kinetic energy, which makes them move about

randomly

As a result of diffusion molecules reach an equilibrium where they are

evenly spread out

This is when there is no net movement of molecules from either side


Diffusion in liquids

Diffusion 2


Cell (plasma) membrane

1. Separates the components of a cell from its

environment

Surrounds the cell

The cell membrane is the boundary

IN food

- sugars

- proteins

- fats

salts

O2

H2O

OUT waste

- ammonia

- salts

- CO2

- H2O

products

- proteins

Cell needs materials in & products or waste out

Roles of the cell membranes

Controls what goes in

and what goes out of

the cell

2


Cell

membrane

Inside cell Outside cell

Diffusion through a membrane

Diffusion 2

Pore size

Concentration gradient

Concentration inside > Concentration outside Xavier DANIEL, Ph.D. AS IB

diffusion


Diffusion 2

Cell

membrane



EQUILIBRIUM


Diffusion 2

Cell

membrane


Concentration inside = Concentration outside Xavier DANIEL, Ph.D. AS IB

Four factors determine the rate of diffusion

1. The steepness of the concentration gradient. The bigger the difference between the two sides of the membrane the quicker the rate of diffusion.

2. Temperature. Higher temperatures give molecules or ions more kinetic energy. Molecules move around faster, so diffusion is faster.

3. The surface area. The greater the surface area the faster the diffusion can take place. This is because the more molecules or ions can cross the membrane at any one moment.

4. The type of molecule or ion diffusing. Large molecules need more energy to get them to move so they tend to diffuse more slowly. Non-polar molecules diffuse more easily than polar molecules because they are soluble in the non polar phospholipid tails.

Diffusion 2


Diffusion through phospholipid bilayer

What molecules can get through directly?

Hydrophobic molecules

What molecules can NOT get through directly?

polar molecules

Hydrophilic molecules (except water and small hydrophilic molecules)

ions

salts, ammonia

large molecules

starches, proteins

Diffusion 2


Diffusion through phospholipid bilayer

Diffusion 2

Molecules that can diffuse through cell membranes

1. Oxygen – Non-polar so

diffuses very quickly.

1. Carbon dioxide – Polar

but very small so diffuses

quickly.

2. Water – Polar but also

very small so diffuses

quickly.


Plan

1 Introduction:


2.1 Diffusion



2

3

1 4

5

4 Active Transport:

5 Transport in bulk

5.1 Endocytosis

5.2 Exocytosis




Facilitated Diffusion 2

Large polar molecules such as glucose and amino acids, cannot diffuse across the phospholipid bilayer. Also ions such as Na+ or Cl- cannot pass.

These molecules pass through protein channels instead. Diffusion through these channels is called FACILITATED DIFFUSION.

Movement of molecules is still PASSIVE just like ordinary diffusion, the only difference is, the molecules go through a protein channel instead of passing between the phospholipids.


Proteins Channels through cell membrane

Membrane becomes semi-permeable with protein

channels

Specific channels allow specific material across cell

membrane

inside cell

outside cell

sugar aa H2O

salt NH3



Protein channel

Cell

membrane



Pore size

Pore size


Concentration inside > Concentration outside Xavier DANIEL, Ph.D. AS IB

Protein channel

Cell

membrane


diffusion



Protein channel

Cell

membrane



EQUILIBRIUM

Concentration inside = Concentration outside Xavier DANIEL, Ph.D. AS IB

Plan

1 Introduction:


2.1 Diffusion



2

3

1 4

5

4 Active Transport:

5 Transport in bulk

5.1 Endocytosis

5.2 Exocytosis




Osmosis 3

Solution

=

Solvent

+

Solute(s)


Osmosis is diffusion of water

From high concentration of water

To low concentration of water

across a semi-permeable membrane

ONLY water passes through this membrane

Osmosis 3


Water potential

w (Psi-water)

Water always moves from higher w to

lower w

(moves down a water potential gradient)

Equilibrium when both w are equal

Water stops moving

Osmosis 3

“Concentration of water “ ???

w < w w = w


Water potential w (Psi)

Pure water has a w of ZERO

Ψw = Ψs + Ψp

s: Solute potential More osmolytes make Ψw more negative

s is always negative

p: Pressure potential Physical forces exerted on the water by its surroundings

More pressure makes Ψw more positive

p is always positive

Solutes = Osmolytes =

molecules soluble in water

Osmosis 3

Ψw = Ψs + Ψp


Osmosis 3

s: Solute potential More osmolytes make water potential more negative

s is always negative

Ψw = Ψs + Ψp

w < w

w = w p of the cell is

NEGATIVE


Osmosis 3

p: Pressure potential

More pressure makes water more positive

p is always positive

Ψw = Ψs + Ψp


Osmosis 3

Ψw = Ψs + Ψp


Ψw predicts the flow of water

From inside out OR from outside in

Water enters the cell Water goes out of the cell

Ψw = Ψs + Ψp Osmosis 3


Water molecules

Sugar

molecule

DILUTE

SOLUTION

CONCENTRATED

SOLUTION

Osmosis 3

Cell membrane Pore size



Concentration inside > Concentration outside

Cannot diffuse


VERY high conc. of

water molecules.

High water potential

VERY low conc. of

water molecules.

Low water potential

Sugar

molecule

DILUTE

SOLUTION

CONCENTRATED

SOLUTION

Osmosis 3

Cell membrane

Water potential gradient

Water potential inside > Water potential outside


Pore size

Can diffuse


Diffusion

= osmosis

DILUTE

SOLUTION

CONCENTRATED

SOLUTION

Osmosis 3



Osmosis 3

How many Sugar molecules ?

2

How many

Water molecules ?

6

How many Sugar molecules ?

4

How many

Water molecules ?

12


EQUILIBRIUM

Water potential inside = Water potential outside Xavier DANIEL, Ph.D. AS IB

Osmosis 3

To keep water inside cell or organelle

Make concentration of solute(s) high inside

Water will enter by osmosis


Cell (compared to beaker) hypertonic or hypotonic

Beaker (compared to cell) hypertonic or hypotonic

Which way does the water flow? in or out of cell

.05 M .03 M

Osmosis 3


Osmolarity

Direction of osmosis is determined by comparing

total solute concentrations

Hypertonic - more solute, less water

Hypotonic - less solute, more water

Isotonic - equal solute, equal water

hypotonic hypertonic

water

net movement of water

Osmosis 3


Osmosis 3


Osmosis 3


http://upload.wikimedia.org/wikipedia/commons/7/76/Osmotic_pressure_on_blood_cells_diagram.svg

freshwater balanced saltwater

Managing water balance

Cell survival depends on balancing water uptake & loss

Osmosis 3


Isotonic

animal cell immersed in

mild salt solution

example:

blood cells in blood plasma

problem: none

no net movement of water

flows across membrane equally, in

both directions

volume of cell is stable

balanced

Osmosis 3



Hypotonic

a cell in fresh water

example: Paramecium

problem: gains water,

swells & can burst

water continually enters

Paramecium cell

solution: contractile vacuole

pumps water out of cell

ATP

plant cells

turgid freshwater

ATP

Osmosis 3



Hypertonic

a cell in salt water

example: shellfish

problem: lose water & die

solution: take up water or pump

out salt

plant cells

plasmolysis = wilt

saltwater

Osmosis 3



Tissues or organs to be used in medical

procedures must be bathed in a solution with the

same osmolarity as the cytoplasm to prevent

osmosis.

Osmosis 3



Osmosis 3

Estimating the osmolarity of samples


Aquaporins

Water moves rapidly into & out of cells

Evidence that there are water channels

1991 | 2003

Peter Agre John Hopkins

Roderick MacKinnon Rockefeller

Osmosis is not enough ! 3


Plan

1 Introduction:


2.1 Diffusion



2

3

4 1

5

4 Active Transport:

5 Transport in bulk

5.1 Endocytosis

5.2 Exocytosis




Active Transport 4

The need for active transport

Some ions and other molecules have to be transported

AGAINST

their gradient concentration Xavier DANIEL, Ph.D. AS IB

“The Doorman”

conformational change

Specialized proteins = “pump” protein

As their shape change, they transport solute from one side of membrane to the other side

“Costs” energy = ATP

ATP

low

high

Active Transport 4


symport antiport

ATP ATP

Active Transport 4

Several mechanisms


Plan

1 Introduction:


2.1 Diffusion



2

3

4 1

5

4 Active Transport:

5 Transport in bulk

5.1 Endocytosis

5.2 Exocytosis




How about large molecules?

How about many molecules at the

same time ?

Transport in Bulk 5


Moving many/ large molecules into & out of cell

through vesicles & vacuoles

Endocytosis

Exocytosis

Transport in Bulk 5


Exocytosis

Transport in Bulk 5


phagocytosis

pinocytosis

receptor-mediated

endocytosis

fuse with

lysosome for

digestion

non-specific

process

triggered by

molecular signal

Endocytosis

Transport in Bulk 5


Plan

1 Introduction:


2.1 Diffusion



2

3

4 1

5

4 Transport in bulk

4.1 Endocytosis

4.2 Exocytosis

5 Active Transport:




simple diffusion

facilitated

diffusion

active

transport

ATP

Conclusion and Overviews


Getting through cell membrane

Passive Transport

Simple diffusion

diffusion of nonpolar, hydrophobic molecules

lipids

high low concentration gradient

Facilitated transport

diffusion of polar, hydrophilic molecules

through a protein channel

high low concentration gradient

Active transport

diffusion against concentration gradient

low high

uses a protein pump

requires ATP ATP




In axons

Sodium/Potassium Pump

Active Transport

Potassium Channels

Facilitated Transport


Facilitated diffusion and Active Transport in Axons

Xavier DANIEL, Ph.D. IB

Na+/K+ pump: Active transport Na+ and K+ across cell membrane

Na+ OUT Na+ cannot re-enter cell by simple diffusion

K+ IN

K+ diffuses out of the cell

through K+ channels out

in

Na+

Na+ K+

K+

Facilitated diffusion and Active Transport in Neurons


ib 1.4 membrane transport

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