Download Direction of Osmosis

CH. 5: CELL TRANSPORT
Homeostasis: Maintaining a stable, internal environment
- cell membranes maintain homeostasis by controlling what enters
or leaves the cell.
Permeability - Ability to permeate/ get through
1. If a substance can pass through = permeable
2.
Only lets some substances through = semi permeable
3. Can’t get through = impermeable
How Cell Membranes Determine What Comes In:
- Remember, molecules are in constant motion!
1.
2.
3.
4.
5.
Structure of the membrane
Size of the particles
Chemical make-up of particles (alcohol, lipid, etc)
Particles are/are not soluble in water (can dissolve)
Conditions in or outside the cell (temp, pressure, etc)
Passive Transport:
- Involves the movement of molecules across the cell
membrane without an input of energy by the cell.
NO ENERGY is REQUIRED to move substances across
membrane -- water, lipids, and other lipid soluble
substances
Types:
– Diffusion
– Osmosis
– Facilitated Diffusion
– Filtration
Diffusion: the movement of molecules from an area
of higher concentration to an area of lower concentration,
driven by the molecules’ kinetic energy until a state of
dynamic equilibrium is reached.
Concentration gradient: the difference in concentration
across space.
Dynamic equilibrium –
continued movement of
molecules with no net
change in concentration
Diffusion
Diffusion…
There are factors that affect the rate of diffusion.
- The rate of diffusion refers to how fast it reaches equilibrium:
a. The concentration of molecules
b. The temperature
c. The pressure
Osmosis
Osmosis is the diffusion of
water across a membrane.
Direction of Osmosis The net direction of osmosis
is due to the relative solute
concentrations on the
two sides of the membrane.
Osmosis…
• Direction of Osmosis...
• When the solute concentration outside the cell is higher
than that in the cytosol, the solution outside is hypertonic
(more solute, less water) to the cytosol, and water will
diffuse out of the
cell.
• Plasmolysis =
cell’s shrinking
due to water loss
Osmosis…
• Direction of Osmosis…
• When the solute concentration outside the cell is lower than
that in the cytosol, the solution outside is hypotonic (less
solute, more water) to the cytosol, and water will diffuse into
the cell.
• Cytolysis = cell’s bursting due to water gain
Osmosis…
• Direction of Osmosis…
• When the solute concentrations outside and inside the cell are
equal, the solution outside is isotonic (same solute, same
water), and there will be no net movement of water.
Osmosis…
• How Cells Deal With Osmosis
• To remain alive, cells must compensate for the water that
enters the cell in hypotonic environments and leaves the cell in
hypertonic environments.
• Cells in multicellular organisms respond to hypotonic
environments by pumping solutes out of the cytosol (RBCs
cannot compensate for changes in solute concentration)
• Contractile vacuoles are organelles
that regulate water levels in paramecia.
Facilitated Diffusion:
- Diffusion of molecules across a membrane when they are not
soluble in lipids or are too large (e.g. glucose) to pass through
pores in membrane
• In facilitated diffusion, a molecule binds to a carrier protein on
one side of the cell membrane.
• The carrier protein (specific for one type of molecule) then
changes its shape and transports the molecule down its
concentration gradient to
• Ex – Insulin carries glucose into cells. the other side of the
membrane.
Diffusion through Ion channels
• Ion channels are proteins, or groups of proteins, that provide
small passageways across the cell membrane through which
specific ions can diffuse.
• Ions important in cell function include sodium, potassium,
calcium, and chloride
Active Transport
• Active transport moves molecules across the cell membrane
from an area of lower concentration to an area of higher
concentration.
• Unlike passive transport, active transport requires cells to
expend energy.
• Some types of active transport are performed by carrier proteins
called cell membrane pumps.
Active Transport…
• Cell Membrane Pumps:
• Sodium-Potassium Pump
–The sodium-potassium pump moves three Na+ ions into the
cell’s external environment for every two K+ ions it moves into
the cytosol.
• Animal cells must have a higher concentration of Na+ ions
outside the cell and a higher concentration of K+ ions inside
the cell
–ATP supplies the energy that drives
the pump.
Sodium Potassium Pump…
• The exchange of three Na+ ions for two K+ ions creates an
electrical gradient across the cell membrane
• Outside becomes positively charged relative to the inside,
which becomes negative
• Difference in electrical charge is
important for the conduction of
electrical impulses along nerve cells
Active Transport…
Movement in Vesicles:
• Endocytosis - cells ingest external fluid, macromolecules, and
large particles, including cells by folding around them and
forming a pouch.
–The pouch then pinches off and becomes a membrane-bound
organelle called a vesicle.
• Some vesicles fuse with lysosomes, and their contents are
digested by lysosomal enzymes
Active Transport…
• Endocytosis
–Endocytosis includes pinocytosis, in which the vesicle contains
solutes or fluids, and phagocytosis, in which the vesicle
contains large particles or whole cells.
• Bacteria and viruses are ingested in this way
–Receptor-mediated endocytosis – molecules are brought into
the cell via coated pits (proteins)
ActiveTransport…
• Exocytosis - vesicles made by the cell fuse with the cell
membrane, releasing their contents into the external
environment.
• Used to release large molecules, such as proteins, waste
products, or toxins that would damage the cell if they were
released within the cytosol
Active
Transport…
Endocytosis &
Exocytosis
https://www.youtube.com/watch?v=2icEADP0J4