TRANSPORT ACROSS MEMBRANE

Basic Membrane Function

¨  Maintain internal cell environment at a steady state regardless of changes in the external environment ¤ Similar to the concept of homeostasis in the human body

but this is at the cellular level

¨  Acts as a selective barrier regulating the movement of substances into and out of the cell

Terminology

¨  Solute vs. Solvent ¨  Concentration vs. Water concentration ¨  Passive vs. Active transport

Types of Transport

¨  Passive transport: ¤  simple diffusion ¤ Facilitated diffusion: channel, carrier ¤ Osmosis

¨  Active transport: ¤ Pump

¨  Bulk membrane transport: ¤ Endocytosis: pinocytosis, phagocytosis, receptor-

mediated ¤ Exocytosis

PASSIVE TRANSPORT

Passive Transport

¨  Does not require energy ¨  Movement of molecules from area of high to low

concentration

Simple Diffusion

¨  Movement of molecules ¤ Across a semi-permeable membrane ¤ Down a concentration gradient ¤ From an area of high to low concentration ¤ No energy required

¨  Continues until equilibrium is achieved ¨  Also known as simple transport

¤ Transport across a membrane without the assistance of membrane proteins

¤ Occurs with small nonpolar molecules (e.g. O2, CO2)

Simple Diffusion

Why can’t polar molecules cross a cell membrane by simple diffusion (unaided by a protein)?

Facilitated Transport

Compare to simple diffusion ¨  Similarities:

¤ Across a semi-permeable membrane ¤ Passive transport ¤ Continues until equilibrium is achieved

¨  Difference: a transport protein is involved in the movement of molecules: ¤ Channel and carrier proteins (integral proteins)

Channel Proteins

¨  move small charged molecules (e.g. ions) down a conc- gradient

¨  Charged particles need help crossing the hydrophobic core

¨  Channel proteins form hydrophilic pathway in the membrane so that certain ions can pass though ¤ Ex. Voltage-gated ion channels for

nerve conduction (K+, Na+…)

Carrier Proteins

¨  moves large uncharged molecules (e.g. glucose) down a conc- gradient

¨  Proteins change shape (conformational change) to allow molecules through

¨  Solute specific Advantage of solute specificity?

Simple vs. Facilitated Transport: Rate of Transport

Simple Diffusion ¨  Rate is determined by:

¤ Concentration gradient

Facilitated Transport ¨  Rate is determined by:

¤ Concentration gradient ¤ Efficiency of transport protein ¤ Number of transport protein

Osmosis

¨  Passive diffusion of water ¨  Water moves from an area of high [water] to an area

of low [water] ¨  Water will always move in the direction to dilute the

solute ¨  Movement of water is in the opposite direction of the

solute

Osmotic Environments

¨  Tonicity: ¤ Osmotic pressure due to the difference in concentration

across a semipermeable membrane ¤  Influenced by solutes that cannot cross a membrane ¤  refers to the concentration of solutes in the environment

surrounding the cell (external)

¨  Isotonic: Iso = same Hypotonic: Hypo = less Hypertonic: Hyper = more

Isotonic Environment

¨  Equal movement of water in and out of cell ¨  Animal cells in equilibrium

Hypotonic Environment

¨  Hypotonic = lower solute concentration outside of cell

¨  Thus higher concentration inside cell (more solutes inside cell)

¨  Water moves into cell ¨  What will happen to the animal cell?

Hypotonic Environment

¨  Result: cell swell and may burst ¨  Osmotic lysis

Hypertonic Environment

¨  Hypertonic: higher [solute] outside of cell ¨  Thus more solutes outside of cell ¨  Water moves out of cell ¨  What happens to the animal cell?

Hypertonic Environment

¨  Result: Cell shrinks

Red Blood Cells

Same effect as animal cells but terminology is different Crenation Equilibrium Hemolysis

Red Blood Cells

Plant Cells

¨  Plant cells have cell walls which changes the effect of the osmotic environments

Plant Cell in Hypotonic Environment

¨  Rigid cell wall prevents plant cell from rupturing ¨  Turgidity: cell swelling

Plant Cell in Hypertonic Environment

¨  As water moves out of cell, cell wall maintains its shape

¨  But cell membrane and cytoplasm shrinks

¨  Plasmolysis: shrinking of plasma membrane and cytoplasm away from cell wall

DIRECTION OF TRANSPORT

Direction of Transport

¨  Uniport: movement of a single molecule in one direction ¤ E.g. all channel proteins

¨  Symport: movement of 2 molecules in the same direction ¤ E.g. Na/glucose symporter

¨  Antiport: movement of 2 molecules in opposite directions ¤ E.g. Na/K pump

Direction of Transport

Direction of Transport

¨  Antiport animation http://faculty.ccbcmd.edu/courses/bio141/lecguide/unit1/prostruct/antiporter_flash.html ¨  Symport and antiport (passive & active) http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/cotransport__symport_and_antiport_.html

ACTIVE TRANSPORT

Active Transport

¨  Movement of molecules against concentration gradient

¨  From low [ ] à high [ ] ¨  Requires energy in the form of ATP ¨  Requires a transport protein: pump ¨  ATP induces a conformational change in the

transporter pump to allow specific molecules to enter/exit cell against its concentration gradient

Pumps

¨  Pumps often function the same as carrier proteins except that they move molecules against the gradient and thus require energy

¨  Pumps can also be coupled so that 2 molecules are moved against the concentration gradient

Example: Na-K Pump

¨  Active transport ¨  Antiport ¨  Pump switches between 2

conformational states: ¤ 3 Na+ out of the cell ¤ 2 K+ into the cell

Animation http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

Compare Passive & Active Transport

EXOCYTOSIS AND ENDOCYTOSIS

Active transport

Bulk Membrane Transport

¨  Transport of molecules too large or too polar to pass through the membrane

¨  Involves the folding of the cell membrane to form a vesicle

¨  Require Energy (Active Transport)

Types of Bulk Membrane Transport

¨  Endocytosis – entry into the cell ¤ Phagocytosis ¤ Pinocytosis ¤ Receptor-mediated

endocytosis

¨  Exocytosis – exiting the cell

Phagocytosis

¨  “Cellular eating” ¨  A process by which living cells ingest other cells

or large particles ¨  Found in simpler forms of life for feeding (e.g.

amoebas) ¨  Used in higher order organisms as a defense

mechanism against invasion by foreign particles ¤ Phagocyte: a cell specialized for protecting the

body by ingesting foreign substances (White Blood Cell)

Mechanisms of Phagocytosis

(2) (1)

1.  Cell engulfs a particle by wrapping around it

2.  Enclosed in a large enough sac to be classified as a vacuole

Mechanisms of Phagocytosis

3.  vacuole fuses with a lysosome 4.  Hydrolytic enzymes in the lysosome digest the particle 5.  Indigestible material will eventually be discharged by

exocytosis (2)

(1) (3) (4)

(5)

Pinocytosis

¨  “Cellular drinking” ¨  Ingestion of dissolved

materials ¨  Occurs in most cell types ¨  Cell folds inwards

(invaginates) to take in fluid containing the desired substance

¨  Process is unspecific in terms of what is being ingested

Pinocytosis

Receptor-Mediated Endocytosis

¨  Intake of molecules that bind specifically to a receptor on the surface of the cell

¨  Molecule specific – mainly protein or molecules carried by protein

¨  Clathrin: coat proteins that help form vesicles

Endocytosis

Animation: (Endocytosis & Exocytosis) http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio02.swf::Endocytosis%20and%20Exocytosis

Exocytosis

¨  Movement of materials from the cell to the cell surface within membrane bound vesicles

¨  Vesicles formed off Golgi body or from endocytosis ¨  Reverse of endocytosis

Exocytosis function

¨  Secretion: release of waste, toxins, signaling molecules

¨  Recycling of membrane proteins (e.g. receptors) ¨  Restoring the cell membrane:

¤ To keep the total surface area of the membrane constant, exocytosis must balance out endocytosis

Quiz 1

If the material the cell takes in is liquid, the process is termed pinocytosis. a.  True b.  False

Quiz 2

Exocytosis is a process by which cells a.  release substances from the cell through pores in

the cell membrane b.  release substances from the cell via vesicles. c.  release substances from the cell via carrier

proteins. d.  bring in substances from the outside via vesicles. e.  bring in substances from the outside via pores in

the cell membrane.

Quiz 3

Certain white blood cells engulf microorganisms and bring them in to digest them. This process is best described as a.  Pinocytosis b.  Receptor-mediated exocytosis c.  Osmosis d.  Phagocytosis

Quiz 4

Which of the following statements about receptor-mediated endocytosis is TRUE? a.  it is used to transport material out of the cell b.  it is not specific c.  it involves clathrin-coated pits d.  it involves transport of material into the cell

through pores in the cell membrane