Download HB Unit 3 Homeostasis and Cell Transport

Unit 3: Homeostasis and Cell
Transport
Sect 1 Passive Transport
Sect 2 Active Transport
Objectives
• Explain how an equilibrium is established as a result of
diffusion.
• Distinguish between diffusion and osmosis.
• Explain how substances cross the cell membrane through
facilitated diffusion.
• Explain how ion channels assist the diffusion of ions across
the cell membrane.
Passive Transport: Diffusion
• Movement of molecules from high to low concentrations
across a concentration gradient.
• Simplest type of cell transport.
• Due to molecule’s kinetic energy.
• Results in equilibrium (even distribution of molecules).
• Molecules can diffuse across a cell membrane by dissolving
in the phospholipid bilayer or by passing through membrane
pores.
Passive Transport: Osmosis
• Diffusion of water from high to low concentrations across a
membrane.
Direction of Osmosis
• Net direction is determined by the relative solute
concentrations on the two sides of the membrane.
Direction of Diffusion-Hypotonic Solution
• when the solute concentration outside the cell is
lower than in the cytosol, the solution outside is
hypotonic to the cytosol; water will diffuse into the
cell.
Direction of Osmosis- Isotonic Solution
• When the solute concentrations outside and inside the cell
are equal, the solution outside is isotonic, and there will be
no net movement of water.
Direction of Osmosis-Hypertonic Solution
• When the solute concentration outside the cell is higher
than that in the cytosol, the solution outside is hypertonic
to the cytosol, and water will diffuse out of the cell.
How Cells Deal with Osmosis
• Contractile vacuoles pump excess water from unicellular,
freshwater organisms.
• Cell walls in plants resist turgor pressure in hypotonic
conditions.
• Plasmolysis (wilting) occurs in plant cells in hypertonic
conditions.
• Cytolysis (bursting) occurs in animal cells in hypertonic
conditions.
Facilitated Diffusion
• Large molecule binds to a carrier protein on one side of the
cell membrane.
• Carrier protein changes shape and transports the molecule
down its concentration gradient to the other side of the
membrane.
ex. glucose transport
Diffusion Through Ion Channels
• Ion channels: proteins, groups of proteins, that provide
passageways across the cell membrane through which
specific ions can diffuse.
Active Transport
•Active transport: moves molecules from an area of lower
concentration to an area of higher concentration across the
cell membrane.
•Requires energy.
•May be done by a carrier proteins.
Active Transport: Sodium-Potassium Pump
•Sodium-Potassium Pump
–Moves three Na+ ions out of the cell for every two K+ ions
it moves into the cytosol.
–ATP supplies the energy to drive the pump.
Active Transport: Endocytosis
• Cells ingest materials by enfolding them into a pouch.
• Pouch pinches off and becomes a membrane-bound
vesicle.
• Includes:
pinocytosis: vesicle contains solutes or fluids
phagocytosis: vesicle contains large particles or cells.
Active Transport: Exocytosis
• Vesicles within the cell fuse with the cell membrane.
• Contents are released outside of cell.