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Transport Across the Cell Membrane. Department of Molecular Biology & Genetics Medical University of Silesia Aleksander L. Sieroń. From Cambpell et al. Fluid Mosaic Model. Mosaic due to many proteins embedded in the fluid matrix of lipid bilayer Two types of Proteins :

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slide1

Transport Across the Cell Membrane

Department of Molecular Biology & Genetics

Medical University of Silesia

Aleksander L. Sieroń

fluid mosaic model
Fluid Mosaic Model
  • Mosaic due to many proteins embedded in the fluid matrix of lipid bilayer
  • Two types of Proteins:
  • 1. Integral - hydrophobic regions span the hydrophobic interior ofmembrane; protein is embedded in membrane
  • 2. Peripheral- not embedded in bilayer; loosely bound to surface of membrane orintegral protein
functions of the proteins
Functions of the proteins:
  • transport or carrier proteins
  • involved in the passage of molecules through the membrane
  • molecules avoid contact with hydrophobic part and
  • can enter the membrane through the protein
  • transport of molecules too large to fit through pores of membrane
  • Highly specific,
    • only specific molecules interact with the protein and
    • then only can that molecule enter the cell.
function s of the plasma membrane
Functionsof the plasma membrane:
  • Transport of materials in and out of the cell
  • Selectively permeable:
    • only certain substances can enter cell and others can not.
  • Two types of Transport:
    • Passive - Does not require Energy
    • Active - Requires Energy (ATP)
permeability of the lipid bilayer
Permeability of the Lipid Bilayer
  • Lipid bilayer of cell membranes serve as a barriers to the passage of most polar molecules
  • Allows the cell to maintain concentrations of solutes in its cytosol
  • Highly impermeable to Ions
  • Easy permeable to hydrophobic (nonpolar) molecules
slide9

The relative permeability of a synthetic lipid bilayer to different classes of molecules

from Alberts et al.

membrane transport proteins
Membrane transport proteins
  • carrier proteins (also called carriers, permeases, or transporters)
  • channel proteins

from Alberts et al., 1994

membrane transport proteins1
Membrane transport proteins

Passive or active transport

Passive transport

from Alberts et al.

slide12

Comparison of passive transport down an electrochemical gradient with active transport against an electrochemical gradient

from Alberts et al.

ionophores
Small hydrophobic molecules – third type of transport molecules

Increase permeability of the cell membrane to specific inorganic ions

Synthesized by microorganisms

MOBILE ION CARRIERS

& CHANNEL FORMERS

Only passive transport

Ex. Valinomycin

Gramidicin A

Ionophores

from Alberts et al.

slide14

Mechanism of Ionophore Action

http://bio.winona.msus.edu/berg/ANIMTNS/Directry.htm

passive transport
Passive Transport:
  • Diffusion
  • Facilitated Diffusion
  • Osmosis
passive transport1
Passive Transport:
  • Diffusion
  • molecules move from an area of high concentration to an areaof low concentration
  • molecules can fit through pores of the membrane
  • continue to move until equilibrium is attained
  • Substances move down its concentration gradient
  • spontaneous, no energy required
passive transport2
Passive transport:
  • Facilitated Diffusion
  • passage of molecules from high to low – down its concentration gradient
  • enter through a carrier protein
  • molecules are too big to fit through pores of the membrane
  • spontaneous process
passive transport3
Passive Transport:
  • Facilitated Diffusion
  • passage of molecules from high to low– down its concentration gradient
  • enter through a carrier protein
  • molecules are too big to fit through pores of the membrane
  • spontaneous process
  • Highly specific
    • only certain substances can enter the cell.
slide23

Passive Diffusion

Facilitated Diffusion

slide24

Passive Transport:

  • Osmosis
  • The passive transport of Water
  • Movement of WATER from high to low concentration
  • Solution - made of solute and solvent
  • Solute - substance being dissolved
  • Solvent -liquid solute is dissolve
  • If solvent is water: Aqueous solution
passive transport4
Passive Transport:
  • Osmosis
  • Three terms used to describe aqueous solutions:
  • Hypertonic
  • Hypotonic
  • Isotonic

solution with a high solute concentration

solution with a low solute concentration

solutions with equal solute concentration

Water always moves from hypotonic to hypertonic solutions!!!

slide26

Osmosis

From Cambpell et al.

slide27

Environment: HIGH SOLUTE

LOW WATER

Cell: LOW SOLUTE

HIGH WATER

Outcome: WATER WILL MOVE OUT OF THE CELL. THE CELL WILL SHRINK!!!

Hypertonic solution

slide28

Environment: LOW SOLUTE

HIGH WATER

Cell: HIGH SOLUTE

LOW WATER

Outcome: WATER WILL MOVE INTO OF THE CELL. THE CELL WILL BURST!!!

Hypotonic solution

slide29

Environment: EQUAL SOLUTE

EQUAL WATER

Cell: EQUAL SOLUTE

EQUAL WATER

Outcome: NO CHANGE IN AMOUNT OF WATER. THE CELL WILL STAY THE SAME!!!

Isotonic solution

slide31

Animal cells

  • Hypertonic - shrinks, shriveled
  • hypotonic - swell, bursts open, lyses
  • isotonic - normal
  • Plant Cells
  • hypertonic - plasmolysis, shrinks
  • hypotonic - normal (healthiest), turgid
  • isotonic - limp, no tendency for water to enter plant wilts, flaccid
aquaporins

http://www.ks.uiuc.edu/Research/aquaporins/

Aquaporins
  • membrane water channels
  • controlling the water contents of cells
  • form tetramers in the cell membrane
  • facilitate the transport of water and, in some cases, other small solutes across the membrane
  • are completely impermeable to charged molecules
  • Diseases:
  • congenital cataracts
  • nephrogenic diabetes insipidus

2003 Nobel prize in Chemistry

slide35

Tajkhorshid, E., Nollert, P., Jensen, M.O., Miercke, L.J., O'Connell, J., Stroud, R.M., and Schulten, K. (2002). Science 296, 525-530

active transport
ActiveTransport
  • opposite todiffusion
  • against the concentration gradient
  • molecules move from a low to high concentration

Requiresenergy

slide41

A hypothetical model showing how a conformational change in a carrier protein could mediate the facilitated diffusion of a solute

From Albertset al.

the na k atpase
The Na+-K+ ATPase
  • Concentration of K+ 10-20 times higher inside the cell
  • Concentration of Na+ 10-20 times higher outside the cell
  • Antiporter
    • actively pumping Na+ out of the cell
    • pumping K+ in to the cell
the na k atpase1
The Na+-K+ ATPase

From Albertset al.

the na k atpase2
The Na+-K+ ATPase
  • Concentration of K+ 10-20 times higher inside the cell
  • Concentration of Na+ 10-20 times higher outside the cell
  • Antiporter
    • actively pumping Na+ out of the cell
    • pumping K+ in to the cell
  • Is electrogenic
the na k atpase3
The Na+-K+ ATPase
  • Function:
  • regulates cell volume (through the osmotic effects of Na+ gradient)
  • drives transport of sugars and amino acids into the cell
  • !!!
  • Almost one-third of the energy requirement of a typical animal cell is consumed in fueling this pump
  • !!!
slide48

Response of a human red blood cell to changes in osmolality (also called tonicity) of the extracellular fluid

From Alberts et al.

transport atpases
Transport ATPases
  • membrane-bound enzymes
  • Hydrolyse the ATP
  • Active transport

The Na+-K+ ATPase

SomeCa2+ pumps

membrane bound enzymes that synthesize atp are transport atpases working in reverse
Membrane-boundenzymesthatsynthesize ATP are transport ATPasesworking in reverse
  • The plasma membrane of bacteria
  • thylakoid membrane of chloroplasts
  • the inner membrane of mitochondria
  • H+ gradients across these membranes drive the synthesis of ATP from ADP and phosphate
  • ATP synthase
  • ATP synthase is responsible for producing nearly all of the ATP in most cells
active transport can be driven by ion gradients
Active Transport Can Be Driven by Ion Gradients
  • The free energy released during the movement of an inorganic ion down an electrochemical gradient is used as the driving force to pump other solutes uphill, against their electrochemical gradient
  • All of these proteins function as coupled transporters - some as symporters, others as antiporters
slide52

The transcellular transport of glucose across an intestinal epithelial cell depends on the asymmetrical distribution of transport proteins in the cell's plasma membrane

From Albertset al.

abc transporter superfamily
ABC transporter superfamily

From Albertset al.

summary
Summary
  • Carrier proteins bind specific solutes and transfer them across the lipid bilayer by undergoing conformational changes that expose the solute binding site sequentially on one side of the membrane and then on the other.
  • Some carrier proteins simply transport a single solute "downhill," whereas others can act as pumps to transport a solute "uphill" against its electrochemical gradient, using energy provided by ATP hydrolysis or by a "downhill" flow of another solute (such as Na+) to drive the requisite series of conformational changes