Define HOMEOSTASIS
 Maintaining a relatively constant or stable internal
environment, even when external conditions change
dramatically
Give the function of each of the
following organelles
 Cell (plasma) membrane
 a flexible boundary between a cell and its environment
 allows nutrients into the cell no matter what the external
conditions are.
Recall the function of each of the
following organelles
 Lysosome
 Clean up crew – break down organelles that have
outlived their usefulness
 Break down lipids, carbs, proteins
 Golgi Apparatus
 Modifies, sorts and packages proteins and other
materials from the ER for storage in cell or release
outside of cell
Homeostasis and Transport Intro
 How does the cell membrane maintain homeostasis?
 Cell membranes help organisms maintain homeostasis
by controlling what substances may enter or leave cells
 Label and describe as many parts/features of the cell
membrane as you can:
Components of Cell Membrane:
Key Terms Associated with
Transport:
 Concentration Gradient - difference in
amount of molecules across space
 Down the concentration gradient:

Higher Concentration  Lower Concentration
 Up the concentration gradient:

Lower Concentration Higher Concentration
 Equilibrium - when the concentration is the
same throughout
Concentration Gradient:
Types of Transport:
 Passive Transport– movement of
molecules down their concentration
gradient without the use of energy
 Active Transport- movement of
molecules up their concentration
gradient with the use of energy
Passive and Active Transport
Passive Transport:
 Determining Factors
 Size
 Type of Molecule
 Chemical Nature of
membrane
 Small, nonpolar,
hydrophobic molecules
 4 Types of Passive
Transport
 Diffusion
 Osmosis
 Facilitated Diffusion
 (Diffusion) Ion channels
Diffusion:
Diffusion:
 Process by which molecules tend to move from an area
where they are more concentrated to an area where
they are less concentrated (passive)
Osmosis:
 Diffusion of water through a selectively permeable
membrane (passive)
 (from an area of higher water concentration to an area
of lower water concentration)
Osmosis:
Diffusion vs. Osmosis
Solutions Can Be:
 Isotonic - solute concentration SAME on each
side of membrane
 No net change in cell size
Solutions Can Be:
 Hypotonic - Solution with a solute
concentration lower than cell
 Water will diffuse INTO cell until equilibrium
 The cell swells and may burst
Solutions Can Be:
 Hypertonic - Solution with a solute
concentration higher than the cell
 Water will diffuse OUT cell until equilibrium
 The cell shrinks
Effects of Osmosis:
 Water diffuses from a hypotonic to a
hypertonic solution
Dealing with Osmosis:
 Plants (root cells) swell in a hypotonic environment
 The swelling stops when the cell membrane is pressed against the
cell wall

The cell wall is strong enough to resist the pressure, called turgor
pressure

(pressure exerted against the cell wall in a hypotonic environment)
Cytolysis and Plasmolysis:
 Cytolysis – In a hypotonic solution, cells can swell
and eventually burst
 Plasmolysis - In a hypertonic environment, water
leaves the cell and the cell shrinks away from the cell
wall as turgor pressure is lost
Contractile Vacuole:
 Unicellular freshwater organisms (Paramecium) live in a
hypotonic environment
 Contractile Vacuole - collects excess water and pumps it
out of cell
Review Questions:
 1. Toward what condition does diffusion eventually lead, in the
absence of other influences?
 2. How is osmosis related to diffusion?
 3. If the concentration of solute molecules outside a cell is lower
than the concentration in the cytoplasm, is the external
solution hypotonic, hypertonic, or isotonic to the cytosol?
 4. Sea water has a higher concentration of solutes than do human
body cells. Why might drinking large amounts of sea water be
dangerous to humans?
Facilitated Diffusion
 Movement of specific molecules across cell
membranes through protein channels (passive)
 (diffusion of materials across a cell membrane assisted
by carrier proteins)
Facilitated Diffusion
 Move molecules, (that cannot diffuse easily), down
their concentration gradient
 Move into or out of cell
 Examples: Glucose, Amino Acids, Ions,
Polar molecules (water)
 Assisted by carrier proteins on the membrane
 Specific to one type molecule
 More protein=faster diffusion
Facilitated Diffusion of Glucose:
Ion Channels—Na , K , Ca
+
 Provide protein channels
for ions to diffuse
 Specific to Ion
 Two types
1.
2.
Open
Gated—open and
close in response to
specific stimuli
+
2+,
Cl-
Active Transport:
 Cells use energy to move up concentration gradient
 ATP supplies energy
 Carrier proteins act as pump
SODIUM-POTASSIUM PUMP:
 In animal cells transport Na+ and K+ up the
Concentration Gradient
 3 Na+ moved outside of cell
 2 K+ moved inside cell
Steps for Transport
3 Na+ bind to carrier
protein in cytoplasm
1.
1.
Carrier protein splits
phosphate group from ATP
P group binds with
carrier protein—shape
changes—releases Na+
3. Now can pick up 2 K+ on
outside
4. Carrier protein changes
shape releases K+
2.
Exocytosis and Endocytosis:
 Transport large molecules across membrane
 Transport a large amount of small molecules
 Energy used
Exocytosis:
 Exporting large molecules
outside of cell
 Process
 Packaged in Golgi Apparatus
 Vesicle transports to cell
membrane
 Vesicle fuses to membrane


Contents released
Vesicle becomes part of
membrane
Endocytosis:
 Cells take in (ingest) substances
 Process
 Depression in cell membrane folds in enclosing material
from outside of the cell
 Pinched off forming a
membrane-bound vesicle


Vesicle fuses with lysosomes
Fuse with other organelles
3 Types of Endocytosis:
 Pinocytosis—”Cell Drinking”
 Ingestion of tiny droplets
 Phagocytosis-”Cell Eating”
 Engulfs large, solid molecule or whole cells
(like bacteria)
 Receptor-mediated—Specific
 Ingestion of specific substances that bind to receptor
proteins on specialized areas of cell membrane
3 Types of Endocytosis:
Review Questions:
 1. Explain the difference between passive and active transport.
 2. What provides the energy that drives the sodium-potassium
pump?
 3. Explain the difference between phagocytosis and pinocytosis.
 4. During intense exercise, potassium tends to accumulate
in the fluid surrounding muscle cells. What membrane
protein helps muscle cells counteract this tendency?
Explain your answer.