Cell Transport
Diffusion and Osmosis
Solutions
• Solution-a liquid mixture of two or more
substances in which the molecules of the
substances are evenly mixed
• Solute-the substance that is dissolved in the
liquid
• Solvent-the substance in which the solute
dissolves
• Water + Sugar
– Solute?
– Solvent?
Solutions
• To find the concentration of the solution:
– Mass of solute divided by the volume of the
solution
• Example: You dissolve 12 grams of salt in
3 liters of water. What is the
concentration?
Solutions
• In a solution, molecules move constantly.
Molecules tend to move from an area of
high concentration to an area of low
concentration
Passive Transport
• The movement of molecules across a cell
membrane without energy input from the
cell – it does not require energy to be
used!!
– Examples: Diffusion and Osmosis
Diffusion
• The net movement of particles
(liquid or gas) from an area of
higher concentration to an
area of lower concentration.
Diffusion
• Concentration-the number of molecules of a
substance in a given volume
• Concentration Gradient-the difference in the
concentration of a substance from one location
to another
– Molecules move from an area with a higher
concentration to an area with a lower concentration
– Eventually, they molecules will be equally spread
out!
Diffusion
An obnoxious individual with a lit cigar gets
on an elevator on the 3rd floor with the
cigar still burning. You are unfortunate to
be in a tall building and he comments,
“Hey we are both going to the 62nd floor!”
Disliking smoke, you move to the farthest
corner you can. Eventually you are unable
to escape the smoke! The nearer you are
to the substance, the stronger the
concentration.
Diffusion
Osmosis
• The movement of water across
a selectively permeable (semi
permeable) membrane.
–Selectively permeable-some
things can cross and some
things cannot!
Osmosis
• Water moves from an area of higher water
concentration to an area of lower water
concentration
• Water will move to balance out the
concentration gradient that exists
– Water will continue to move until equilibrium is
reached. At this point, water molecules will
continue to move in both directions
Facilitated Diffusion
• Definition: the diffusion of molecules
across a membrane through transport
proteins
– Transport Protein-openings formed by
proteins that pierce the cell membrane
• Make it easier for molecules to enter or exit a cell
• Does not require energy=passive transport
Osmosis
• Solutions can be described in one of three
ways when compared to another solution:
– Isotonic-solution that has an equal concentration
of dissolved particles compared with another
solution
Osmosis
– Hypertonic-solution that has a higher
concentration of dissolved particles compared
with another solution
Osmosis
• Hypotonic-solution that has a lower
concentration of dissolved particles compared
with another solution
• DIFFUSION IS WHEN PARTICLES
MOVE
• OSMOSIS IS WHEN WATER MOVES
Active Transport
• The cell membrane controls passage of
materials into and out of the cell.
• Sometimes the cell will need materials that
cannot simply diffuse through the cell
membrane, and these processes require the
cell to use energy!
Active Transport
• Active Transport-drives molecules across
a membrane from a region of lower
concentration to a region of higher
concentration (this is going against a
concentration gradient!)
– This is backwards from diffusion!!
• REQUIRES ENERGY!!!
Endocytosis
• Definition: the process of taking liquids or
fairly large molecules into a cell by
engulfing them in a membrane
• A lysosome breaks down the membrane
surrounding the new vesicle to allow the
contents to be released into the cell
Endocytosis
Phagocytosis
• Definition: a type of endocytosis in which
the cell membrane engulfs large particles
• “Cell Eating”
• Plays a key role in your immune system
Exocytosis
• Definition: the release of substances out
of a cell by the fusion of a vesicle with the
membrane
Exocytosis
Questions…
1. How do transport proteins that are pumps differ
from those that are channels?
2. How do endocytosis and exocytosis differ from
diffusion?
3. Small lipid molecules are in high concentration
outside a cell. They slowly cross the membrane
into the cell. What term describes this action?
Does it require energy?
4. Ions are in low concentration outside a cell. They
move rapidly into the cell via protein molecules.
What term describes this action? Does it require
energy?