Download Membrane Transport 2 - Lynn English Faculty Pages

Thursday November 20
• Objectives You will be able to:
– Explain the concept of diffusion and how it relates to cells
– Describe the different ways that molecules can be
passively transported across the cell membrane
• Agenda
– Reading Quiz
– Start Notes on Passive Transport (Chapter 5-1)
• Homework Due today
– Read section 5-1
– Outline of section
Passive Transport
Section 5-1
Passive Transport
• Doesn’t require energy
• Concentration is the amount of solute
dissolved in a fixed amount of solvent
• Concentration gradient is the difference in the
concentration across space
Diffusion
• Diffusion - the movement of molecules from
high concentrations to low concentrations
– Molecules are in constant motion (kinetic energy)
– This movement drives diffusion
Equilibrium
• Concentration is the same throughout
– Solutions move to equilibrium
• Even at equilibrium, molecules are still moving
– Movement in many directions balances
concentration
Diffusion Across Membranes
• Some molecules can move from high
concentration to low across the cell
membrane
– Depends on size, shape, and nature
• Molecules that dissolve in lipids can move
across the membrane
– Molecules that don’t must move through protein
pores
Osmosis
• Water moving across a membrane
• Hypotonic is when the solute concentration outside is
lower (water moves in- cell expands)
• Hypertonic is when the solute concentration outside is
higher (water moves out- cell shrinks)
• Isotonic is when concentrations are equal
HypOtonic
Inside
Outside
HypERtponic
Inside
Outside
Osmosis in Cells
• Hypotonic environment
– Contractile vacuole- collects water and pumps out
– Plants use cell wall- holds up against turgor
pressure (the pressure water exerts)
– Some pump solutes out of cytosol
– Cytolysis (bursting) (red blood cells)
• Hypertonic environment
– Plant cell membrane shrinks against cell wall
(plasmolysis), plant wilts
– Red blood cells shrink and shrivel
Plants
Hypotonic environment
Hypertonic environment
Red Blood Cells
Facilitated Diffusion
• Carrier proteins help some molecules that
cant diffuse rapidly through the membrane
• Not soluble in lipids, or too large
– Still diffusion because moving from high to low
• Molecule binds to protein, shields it from the
hydrophobic cell membrane, and releases to
other side of the cell
– Carrier molecule is specific to molecule
• Example: glucose
Ion Channels
• Provide passage for ions that can’t diffuse on
their own (not soluble)
• Specific to each ion (Na+ K+ Ca2+ Cl-)
• Some are always open but some have gates
that respond to:
– Stretching of cell membrane
– Electrical signals
– Chemicals in cytosol or environment
Active Transport
Section 5-2
Sodium Potassium Pump
•
•
•
•
Same as passive transport but from low to high
NA+ binds to a carrier protein from inside
Phosphate from ATP changes shape of the protein
New shape allows K+ to bind to protein (outside)
and phosphate leaves
• Loss of phosphate changes shape again and K+ is
released into the cell
• Inside becomes negative while outside becomes
positive– conducts electrical impulses
Endocytosis
• Cells ingest external fluid, macromolecules,
and large cells
• Materials enclosed by cell and pinched off into
a vesicle
– Can fuse with organelles and digested
• Pinocytosis involves fluid
• Phagocytosis involves large particles
– Phagocytes ingest bacteria and viruses
• destroyed by lysozymes
Exocytosis
• The reverse of endocytosis
• Vesicles fuse with cell membrane and
contents are released into the environment
• Used to export large molecules such as
proteins