8-2: Cell Transport
 What determines the direction a substance will
move during passive transport ?
 Why is osmosis important?
 How do substances move against their
concentration gradients?
The cell’s membrane is a little like a country’s border.
It both barriers regulate who or what enters
(imports/ immigration)
and who or what leaves
(exports/ emigration/ deportation).
Passive Transport
• Random movement of molecules because of
collisions between molecules.
• Requires NO cell energy (doesn’t cost ATP)
• When particles fill a space evenly, and then
maintain that spacing, its called a state of
equilibrium. (for every molecule out, one comes
in/ for every molecule up or right, one goes
down or left, etc)
• Concentration is the amount of substance per
given volume…. “how crowded”. An example
would be a blood sugar reading of 85 mg/dL
• A difference in the concentrations of two areas is
called a concentration gradient
Diffusion
• Diffusion  the random movement of molecules
from an area of higher concentration to an area
of lower concentration. (the smell of cookies diffusing when
you open the oven door, from near the cookies, to the kitchen, to the
rest of the house )
– Directly through CM
– through transport proteins in the CM
• Simple diffusion; small, nonpolar substances
(Oxygen from RBC to muscle)
• Facilitated diffusion; needs help from a protein to
cross the CM (sugar does this)
Facilitated Diffusion
• Channel proteins (imagine a slide on the playground)
– CM has integral proteins that are a specific size and
shape to fit molecule
– Used for sugars, ions and amino acids
– Only sodium gets in the Na channel protein
• Carrier Proteins (imagine a revolving door)
– Molecule must fit into a binding site
– Protein changes shape and
– Drops molecule on inside of CM (could also move
substance out of cell)
Osmosis
• Osmosis  the movement of water, through a
selectively permeable membrane to maintain
homeostasis.
• It is a specialized form of diffusion.
• The amounts of other solute sets up the
concentration gradient that drives the movement
of water… based on overall ratios
• Special channel proteins in CM help regulate
water balance during digestion, for regulation of
body temperature and for water conservation in
the kidneys
Predicting Water Movement
• Water moves out of cells into ….
– Hypertonic solutions have high amounts of solutes.
– If the cell is in a hypertonic solution (like salt water) then water
moves out of the cytoplasm and into the hypertonic solution until
equilibrium is reached
• Water moves into a cell…..
– Solutions with relatively lower concentrations of solutes are
known as hypotonic solutions
– Cells in these solutions tend to gain water from their
environments
• No net change
– Solutions with similar concentrations of solutes are known as
isotonic (equal)
– The cell both loses and gains water, but at the same rates.
Effects of Osmosis
• Plants lose water to things like road salt (wilt)
• Plants water vacuoles gain water and this
creates pressure in the stem – organisms
without cell walls could actually burst
• You need to get out of the pool (fresh water)
frequently because your kidneys are regulating
water balance
• You get really dry skin and feel thirsty when your
cells try to reach equilibrium with ocean water
Active Transport
• Cells must spend energy (ATP) to move some
substances against their concentration gradients
(‘up hill’).
• One example is the sodium-potassium pump.
–
–
–
–
Membrane carrier protein
Bonds to Na which the cell doesn’t want on inside
Spends ATP to change shape of protein
Flips Na to outside and protein is now the right shape
to bond to K (potassium)
– Carrier Protein with K changes back to original shape
(no ATP) and the necessary K is now inside the cell
– Remember direction with “nah” for Na and
“okay” for K
Active Transport Using Vesicles
• Endocytosis is the formation of a vesicle
around something like food and then
bringing it into the CM ( white blood cell
getting bacteria). CM folds in like a pocket.
• Exocytosis is when a vesicle merges with
the CM and dumps something out of the
cell (protista release waste this way).
“turning a plastic bag inside out”
Endo = enter; inside; inner
Exo = exit; outside; outer
*
*
*
*
*
Vesicle
Was outside of CM,
* Was inside of vesicle,
now part of CM
now the inside of the
vesicle
ALL membranes are flexible and inter-changeable