Download TRANSPORT ACROSS MEMBRANE

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:
mediated
¤  Exocytosis
pinocytosis, phagocytosis, receptor-
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.
¨ 
Symport: movement of 2 molecules in the same
direction
¤  E.g.
¨ 
all channel proteins
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_wo
rks.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
1. 
2. 
Cell engulfs a particle by
wrapping around it
Enclosed in a large enough sac
to be classified as a vacuole
(1)
(2)
Mechanisms of Phagocytosis
3. 
4. 
5. 
vacuole fuses with a lysosome
Hydrolytic enzymes in the lysosome digest the particle
Indigestible material will eventually be discharged by
exocytosis
(1)
(2)
(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 receptormediated 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