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Homeostasis &
Transport
Chapter 5
Passive Transport
Section 5.1
Homeostasis?
 How do cell membranes help maintain
homeostasis within a cell?
 By controlling what goes into and out of
the cell.
Passive Transport
 The movement of materials across a cell
membrane WITHOUT the use of energy
(ATP).
 Four Types:
 Diffusion
 Osmosis
 Facilitated Diffusion
 Ion Channels
Diffusion
 Movement of molecules from an area of high
concentration to an area of low concentration.
Diffusion through a
Membrane
Cell membrane
Solute moves DOWN concentration gradient
(HIGH to LOW)
Concentration Gradient
 Difference in
concentration of
molecules across
a space (or cell
membrane).
 “Down the
gradient”
Movement from
high to low
concentration
Equilibrium
 When the concentration is EQUAL throughout
the space (or on each side of the cell
membrane)
 End of diffusion
 No NET movement, RANDOM movement still
occurs
Diffusion of Liquids
Diffusion across a membrane
 Depends upon size, shape, and chemical
makeup
 Easy to diffuse:


Small
Nonpolar
Osmosis
 WATER diffusion across a membrane
 “Down the gradient”

Hypotonic solution to hypertonic solution
Osmosis
Hypotonic Solution
 The solute concentration is
higher inside the cell, the
water concentration is higher
outside the cell.
 Net movement = Water into
the cell
 May cause Cytolysis
Hypertonic Solution
 The solute concentration is
higher outside the cell, the
water concentration is higher
inside the cell.
 Net movement = Water out of
the cell
 May cause Plasmolysis
Isotonic Solution
 The solute and water
concentrations are
EQUAL on both sides of
the membrane.
 Equilibrium
Osmosis in Red Blood Cells
Isotonic
Hypotonic
Hypertonic
hypotonic
hypertonic
hypertonic
isotonic
isotonic
hypotonic
Facilitated Diffusion
 Use of carrier proteins the assist diffusion across a
membrane.
 Specific carrier proteins for specific molecules (like
glucose or amino acids)
 “Down the gradient”
Diffusion via ion channels
 Use of membrane proteins to assist
diffusion of ions across a membrane
 Channel is usually specific to 1 type of ion
 Common ions:




Sodium
Potassium
Chlorine
Calcium
Notebook Quiz
1) Osmosis is the diffusion of _____________.
2) During diffusion, molecules tend to move in
what direction?
3) Does the movement of molecules stop once
equilibrium is reached? Explain.
4) What is the function of carrier proteins?
5) If the external solution is hypotonic
compared to the cell, which direction will
water move?
Active Transport
Section 5.2
animation
Active Transport
 Uses energy (ATP) to move materials
against the gradient
 “Up the gradient”
 Low to high concentration
1. Sodium Potassium Pump
 Transports sodium and potassium ions
across the membrane up their gradients
 Low to high concentration
 Uses ATP
2. Endocytosis
 Movement of large amounts of material into
the cell
 Formation of a vesicle from the cell
membrane that will go to the lysosome for
digestion
 Uses ATP
2 types of endocytosis
 1. pinocytosis: “cell drinking”

Transport of a large quantity of liquid into the cell
 2. phagocytosis: “cell eating”

Transport of a large quantity of solids into the
cell

Uses ATP
Moving the “Big Stuff”
Large molecules move materials into the cell by
one of three forms of endocytosis.
3. Exocytosis
 The movement of a large amount of materials
out of the cell
 Vesicle fuses to the cell membrane to release
contents out of the cell
 Uses ATP
Exocytosis)
video
Exocytic
vesicle
immediately
after fusion
with plasma
membrane.
Lab Time!
► Starch
Molecule
► Iodine
Molecule