Homeostasis & Transport
Ch.5
(5-1) Homeostasis
• Cell membranes control what goes in &
out
Passive Transport
• Movement of materials across a cell
membrane without the use of E (ATP)
• 4 Types:
– Diffusion
– Osmosis
– Facilitated Diffusion
– Ion Channels
Diffusion
• Movement of molecules from an area of
high concentration to low conc.
Concentration Gradient
• Difference in conc. of molecules across
a membrane
• “Down the gradient”
– Move from high to low conc.
Equilibrium
• Conc. is equal on both sides of the
membrane
• Diffusion ends
Diffusion Across a Membrane
• Depends upon size, shape, & chemical
makeup
• Easy to diffuse:
– Small
– Nonpolar
(dissolve in
lipids)
Osmosis
• Water diffusion across a membrane
Hypotonic Solution
• Solute conc. is higher inside & water
conc. is higher outside the cell
• Water moves into the cell
• May cause cytolysis
(cells burst)
Hypertonic Solution
• Solute conc. is higher outside & water
conc. is higher inside the cell
• Water moves out of the cell
• May cause plasmolysis
(cells shrink)
Isotonic Solution
• Solute & water conc.’s are equal on
both sides of the membrane
• Equilibrium
Isotonic
Hypertonic
Hypotonic
How Cells Deal with Osmosis
• Contractile vacuoles: collect
excess water & pump it out of
the cell
– Requires E
• Turgor pressure: P that water exerts
against the cell wall
Facilitated Diffusion
• Carrier proteins assist diffusion across a
membrane
• Specific to certain molecules (glucose)
Diffusion via Ion Channels
• Ion channels assist diffusion across a
membrane
– Some always open, others gated
• Common ions:
– Na+
– K+
– Cl– Ca2+
(5-2) Active Transport
• Movement of a substance across a cell
membrane against a conc. gradient
– Requires E
Na-K Pump
• Carrier protein that transports K+ into &
Na+ out of the cells
• Helps conduction of electrical impulses
in nerve cells
Endocytosis
• Cell surrounds & engulfs substances
• 2 Types:
– Pinocytosis: fluids or solutes
– Phagocytosis: large particles
Exocytosis
• Vesicle inside a cell fuses w/ the cell
membrane & releases contents outside
of cell
– Proteins, waste, toxins