Cell Membranes are Selectively Permeable

What can pass?

Water and other smaller moleculesWater and other smaller molecules Gases like oxygen, carbon dioxide, and Gases like oxygen, carbon dioxide, and

nitrogennitrogen Small polar moleculesSmall polar molecules Larger non-polar molecules pass Larger non-polar molecules pass

through more slowlythrough more slowly

Diffusion

Random motion of particles powers Random motion of particles powers diffusion- kinetic energydiffusion- kinetic energy

Particles move from an area of higher Particles move from an area of higher concentration to an area of lower concentration to an area of lower concentration ultimately reaching concentration ultimately reaching equilibriumequilibrium

http://www.biosci.ohiou.edu/introbioslab/Bios170/diffusion/Diffusion.html

Rate of Diffusion

Depends uponDepends uponTemperatureTemperatureSize of moleculesSize of moleculesElectrical chargesElectrical chargesConcentration gradientConcentration gradient

Dialysis

Diffusion of a solute across a Diffusion of a solute across a selectively permeable membraneselectively permeable membrane

Useful when kidneys do not function Useful when kidneys do not function properly; blood can be cleaned by properly; blood can be cleaned by kidney dialysiskidney dialysis

Kidney dialysis machines were once not very common. Now, patients can find one in most every community.

Osmosis

Diffusion of water across a Diffusion of water across a selectively permeable selectively permeable membranemembrane

Osmotic pressure is the Osmotic pressure is the tendency of water to move into tendency of water to move into that solution. that solution.

Isotonic solution Isotonic- when the Isotonic- when the

solution outside the solution outside the cell is of equal cell is of equal concentration of concentration of solute as that inside solute as that inside the cellthe cell

Solute concentration Solute concentration of solution equal to of solution equal to that of cell that of cell

No net water No net water movement movement

Hypotonic solution

Solute concentration of Solute concentration of solution lower than cell solution lower than cell

Less dissolved particles Less dissolved particles outside of cell than inside of outside of cell than inside of cell cell

Hypo = less, under (think Hypo = less, under (think hypodermic, hypothermia); hypodermic, hypothermia); Tonic = dissolved particles Tonic = dissolved particles

Water moves into cell from Water moves into cell from solution solution

Cell expands (and may burst) Cell expands (and may burst)

Hypertonic solution

Solute concentration of Solute concentration of solution higher than solution higher than cell cell

More dissolved More dissolved particles outside of cell particles outside of cell than inside of cell than inside of cell

Higher water Higher water concentration inside concentration inside cellcell

Water moves out of Water moves out of cellcell

What about human cells?

Human cells are Human cells are isotonic with a 0.9% isotonic with a 0.9% sodium chloride sodium chloride solution.solution.

Human cells are Human cells are isotonic with a 0.9% isotonic with a 0.9% sodium chloride sodium chloride solution.solution.

Osmotic pressure

Solutions that are hypertonic have a higher Solutions that are hypertonic have a higher osmotic pressure than the cellsosmotic pressure than the cells

Solutions that are hypotonic have a lower Solutions that are hypotonic have a lower osmotic pressure than the cellsosmotic pressure than the cells

Remember that osmosis is the movement of Remember that osmosis is the movement of waterwater

Who is this man?

Osmotic Pressure changes Cell Size

PlasmolysisPlasmolysis- cell contents shrink away from - cell contents shrink away from its cell wall caused by a hypertonic solutionits cell wall caused by a hypertonic solution

CrenationCrenation- cell shrinks caused by a - cell shrinks caused by a hypertonic solutionhypertonic solution

CytolysisCytolysis- cell contents swell and burst the - cell contents swell and burst the cell caused by a hypotonic solutioncell caused by a hypotonic solution

plasmolysis

crenation

Turgor Pressure

The pressure that a plant cell places on The pressure that a plant cell places on its cell wallits cell wall

Caused by the plant’s cellular Caused by the plant’s cellular environment being almost always environment being almost always hypotonichypotonic

Turgor provides structural support in Turgor provides structural support in non-woody plantsnon-woody plants

Aquaporin-1

Water sometimes seems to come into Water sometimes seems to come into the cell faster than osmosis can account the cell faster than osmosis can account forfor

Aquaporin-1 is an integral protein that Aquaporin-1 is an integral protein that acts as a gated water channelacts as a gated water channel

Aquaporin-1 allows water to pass Aquaporin-1 allows water to pass through but not protonsthrough but not protons

Question for Experts- (that’s you)

Large molecules cannot move Large molecules cannot move easily through the cell’s selectively easily through the cell’s selectively permeable membrane. Ions cannot permeable membrane. Ions cannot cross a lipid bilayer. Why is this cross a lipid bilayer. Why is this advantageous to the cell?advantageous to the cell?

Carrier Mediated Transport

Two formsTwo formsFacilitated diffusionFacilitated diffusion

Still diffusion- down Still diffusion- down the gradientthe gradient

Active transportActive transportEnergy requirementEnergy requirement

Facilitated Diffusion

Occurs down a concentration gradientOccurs down a concentration gradient Membrane can be made permeable to Membrane can be made permeable to

certain molecules by carrier or transport certain molecules by carrier or transport proteinsproteins

Carrier Mediated Active Transport

Against diffusion flow- from low Against diffusion flow- from low concentration to higherconcentration to higher

Must use an energy source like ATPMust use an energy source like ATP

Sodium Potassium Pumps Found in most animal Found in most animal

cellscells Exchange sodium ions Exchange sodium ions

(Na(Na++) on inside of cell for ) on inside of cell for potassium ions (Kpotassium ions (K++) on ) on outside of celloutside of cell

Usually 2 potassiums for Usually 2 potassiums for three sodiums three sodiums

Sod

ium

Pota

ssiu

m

Pu

mp

Electrical Gradient When the When the

concentration of concentration of net charges in net charges in one area is one area is different than different than anotheranother

The difference in The difference in the concentration the concentration of electrical of electrical charges drives charges drives ions across the ions across the membranemembrane

Net “+”

Net “-”

Cotransport

Electrochemical gradient- higher Electrochemical gradient- higher concentration of electrical charges and concentration of electrical charges and molecules in one place than in anothermolecules in one place than in another

Moving down the gradient can supply Moving down the gradient can supply enough energy to push a molecule enough energy to push a molecule through a protein transport back through a protein transport back against the gradientagainst the gradient

Energy Requirement for Transport

Generally speaking, Facilitated Diffusion is Generally speaking, Facilitated Diffusion is said to require no energy to move molecules said to require no energy to move molecules across the membraneacross the membrane

In reality, the energy of diffusion is what In reality, the energy of diffusion is what powers this transportpowers this transport

In active transport, some other type of In active transport, some other type of energy is required to make the transport energy is required to make the transport work against the concentration gradientwork against the concentration gradient

Endocytosis

Three typesThree typesPinocytosis- pino “to drink”; Pinocytosis- pino “to drink”;

cyto “cell”cyto “cell”Phagocytosis- phago “to eat”Phagocytosis- phago “to eat”Receptor-mediated endocytosisReceptor-mediated endocytosis

Pinocytosis

Cell takes in dissolved materials in the Cell takes in dissolved materials in the form of fluids; so you could say fluids form of fluids; so you could say fluids or very small particlesor very small particles

PIN

OC

YTO

SIS

PINOCYTIC VESICLES FORMING

Phagocytosis

Cell can ingest large particles by wrapping Cell can ingest large particles by wrapping the cell membrane around the particle and the cell membrane around the particle and taking it into the celltaking it into the cell

Cell takes in food and bacteriaCell takes in food and bacteria

Receptor Mediated Endocytosis

Involves bonding of receptor proteins to the Involves bonding of receptor proteins to the material to be ingestedmaterial to be ingested

Molecules bind to Molecules bind to receptors called receptors called ligandsligands concentrated in coated concentrated in coated pitspits Coated pits are coated Coated pits are coated

by the protein by the protein ClathrinClathrin The pits move inwards The pits move inwards

via endocytosis and are via endocytosis and are then termed coated then termed coated vesiclesvesicles

Receptor Mediated Endocytosis

1

2

1. Coated pit- coated with clathrin

2. Coated vesicle

The coated vesicle ultimately becomes an The coated vesicle ultimately becomes an endosome when the coating is lostendosome when the coating is lost

Cholesterol is taken into cells by this Cholesterol is taken into cells by this pathwaypathway

Receptor Mediated Endocytosis

Brown and Goldstein- 1985 Nobel PrizeBrown and Goldstein- 1985 Nobel Prize

Receptor Mediated Endocytosis

Michael S. Brown (right) and Joseph L. Goldstein (left)

are currently Regental Professors in the

Department of Molecular Genetics at the University of

Texas Southwestern Medical Center in Dallas where they have worked

together since 1972

                                          

Drs. Brown and Goldstein personify the team dynamic at the heart of UT Southwestern’s approach to research. They shared the 1985 Nobel Prize for their discovery of the underlying mechanisms of cholesterol metabolism. Their findings led to the development of statin drugs, the cholesterol-lowering compounds that today are used by 16 million Americans and are the most widely prescribed medications in the United States. And their discovery is improving more lives every year. New federal cholesterol guidelines will triple the number of Americans taking statin drugs to lower their cholesterol, reducing the risk of heart disease and stroke for countless people.