1. CH T R 5
AP E
Homeostasis and Cell Transport

2. Homeostasis
• Homeostasis: the biological balance
between a cell or an organism and its
environment.
− Cell membranes help organisms maintain
homeostasis by controlling what
substances may enter or leave cells.

3. Structure of the Cell
Membrane

4. Membrane Components
• Phospholipids • Proteins
• Cholesterol (peripheral and integral)
• Carbohydrates (glucose)

5. Phospholipids
• Pictured here is a phospholipid, which is the
lipid that makes up the cell membrane and the
membranes of cell organelles.
• It consists of a polar head and two non-polar
tails. Phospholipids differ from triglycerides
because they have 2 fatty acids instead of 3.
=

6. Phospholipids
• Phospholipid
heads are
hydrophilic, or
water-loving, so
they want to be
near the water.
The fatty acids
are hydrophobic,
or water-fearing,
so they orient
themselves away
from water.

7. FLUID MOSAIC MODEL
• FLUID- because individual phospholipids and proteins can
move side-to-side within the layer, like it’s a liquid.
• MOSAIC- because of the pattern produced by the
scattered protein molecules when the membrane is
viewed from above.

8. Selectively Permeable Membrane
• The cell membrane is selectively permeable,
because it allows some things but not all things
to pass through it. Molecules like O2, CO2 and
H2O move easily across the membrane.
• Ions, hydrophilic molecules larger than
water, and large molecules such as proteins do
not move through the membrane on their own.

10. Passive Transport
• In Passive Transport, substances
cross the cell membrane with NO
energy input from the cell.
• The simplest type of passive
transport is diffusion.

11. Diffusion
• Diffusion is the movement of
molecules from an area of
higher concentration to an area
of lower concentration.
− Concentration Gradient - the
difference in the
concentration of molecules
across a distance.
− Molecules tend to move from
where they are more
concentrated to where they
are less concentrated
(“down” their concentration
gradient).
− Concentration Gradient

12. Diffusion
• Diffusion is driven entirely by the molecules’
kinetic energy.
− Molecules are in constant motion.
• Diffusion will eventually cause the molecules
to be in equilibrium – the concentration of
molecules will be the same throughout the
space the molecules occupy.
− At equilibrium, molecules continue to move,
but their movements are in different
directions and “cancel” each other out!
• How Diffusion Works

13. Diffusion

14. Diffusion

15. Diffusion

16. Diffusion Across Membranes
• Diffusion across a membrane is also called
simple diffusion.
• Remember though that cell membranes are
selectively permeable - some substances can
move in and out easily, but others cannot.
− The diffusion of a molecule across the cell
membrane depends on:
 Size of molecule
 Type of molecule
 Chemical nature of the membrane

17. Diffusion Across Membranes
Diffusion across a membrane
Selectively
Permeable
Membrane

18. Diffusion Across Membranes

19. Osmosis
• Osmosis is a special type
of diffusion. Osmosis is the
diffusion of water across a
membrane.
• In osmosis, ONLY water is
moving.
• In order to understand
osmosis, we need to have a
little review of solutions:
− solute = substance
dissolved in the solution
ex. salt
− solvent = substance that
dissolves another
substance ex. water

20. Osmosis
Water molecules
diffuse across a
cell membrane
from an area of
higher
concentration to
an area of lower
concentration.

21. Direction of Osmosis
• The overall movement of water is
determined by the concentration of
solutes on either side of the membrane.
High H2O Low H2O
concentration concentration
Low solute High solute
concentration concentration
• There are 3 types of “environments” a cell
can be in based on solute concentration. They
are called hypertonic, hypotonic, and isotonic.

22. • A simple rule to remember
is:
Salt Steals!!!
• Salt is a solute. When it is
concentrated inside or
outside of the cell, it will
draw the water in its
direction. This is also why
you get thirsty after eating
something salty.
However, this works for
any solute, not just salt…it
could be sugar, for
example.

23. Direction of Osmosis
• In a Hypertonic solution, the
concentration of solute (ex.
salt) outside the cell is higher
than the concentration inside
the cell.
− The word "HYPER" means
MORE. Just think, you are
HYPER when you have MORE
energy. In this case, there
are more solute (ex. salt)
molecules outside the cell,
and because “salt steals” it
will “steal” or draw the
water in its direction. In
other words, water diffuses
out of the cell.

24. Direction of Osmosis
• In a Hypotonic solution,
the concentration of solute
(ex. salt) outside the cell
is lower than the
concentration inside the
cell.
− The word "HYPO" means
LESS. In this case,
there are less solute
(ex. salt) molecules
outside the cell, and
since “salt steals”,
water diffuses into the

25. Direction of Osmosis
• In an Isotonic solution,
the concentration of
solutes is equal outside
and inside of the cell.
− “ISO" means the
SAME. If the
concentration of solute
(ex. salt) is equal on
both sides, water
diffuses into and out
of the cell at equal
rates but it won't have
any effect on the
overall amount of

26. = solute(like salt) = water membrane
high concentration of solute lower concentration of solute
HYPERTONIC HYPOTONIC
ENVIRONMENT ENVIRONMENT

27. = solute(like salt) = water
high concentration lower concentration
of solute of solute

28. = solute(like salt) = water
high concentration lower concentration
of solute of solute

29. = solute(like salt) = water
high concentration lower concentration
of solute of solute

30. The net flow of water is towards the higher solute
concentration.
high concentration lower concentration
of solute of solute
• How Osmosis Works

31. How Cells Deal With Osmosis
• How Freshwater Cells Deal with Osmosis
− When organisms live in freshwater
environments (like a pond), they are living in
a hypotonic environment.
− Therefore, water is constantly diffusing into
them.
− Some of them, like a Paramecium, have a
special structure called a contractile vacuole
which collects water then pumps in out of the
cell.

34. How Cells Deal With Osmosis
• How Plant Cells Deal with Osmosis
in a Hypotonic Environment
– Plants have roots surrounded by water
and usually live in a HYPOtonic
environment.
1) Water diffuses INTO the cells through
osmosis.
2) Water may be stored in the vacuole.
3) The water molecules exert pressure
against the cell wall.
– This pressure is known as turgor
pressure.
1) The cell wall is strong enough to prevent
the cell from bursting open.

35. How Cells Deal With Osmosis
• How Plant Cells Deal with Osmosis in a
Hypertonic Environment
– If a Plant is in a HYPERtonic environment:
1) Water diffuses OUT OF the cells through
osmosis.
2) Cells shrink away from the cell walls and
turgor pressure is lost.
a) This condition is called plasmolysis, and is the
reason that plants wilt. (lysis means to die in Latin)

36. Turgor Pressure and
Plasmolysis
Turgor Pressure Plasmolysis
• Movement of Water by Osmosis in Plant Cell

37. How Cells Deal With Osmosis
• How Animal Cells Deal with Osmosis in
a HYPOtonic environment
– If animal cells like red blood cells are in
a hypotonic environment:
1) Water diffuses INTO the cells through
osmosis.
2) No cell wall is present to prevent the cell
from bursting.
3) When a cell bursts it is called cytolysis.
(again lysis means “to die”)

39. How Cells Deal With Osmosis
• How Animal Cells Deal with Osmosis in
a HYPERtonic environment
– If animal cells like red blood cells are in
a hypertonic environment:
1) Water diffuses OUT OF the cells through
osmosis.
2) The cells shrink and shrivel. This process is
known as crenation.
• Animal Cell in Different Solutions

41. Example of Cell Crenation
• One way that people try to
remove slugs from their
garden is by pouring salt on
them.
• When you pour salt on a
slug, it appears to “melt”.
• What is actually happening is
that the slug is in a
hypertonic environment.
• Through osmosis, water and
fluids will move out of the
slugs body causing it to
shrivel, an example of cell
crenation.

42. Three Types of Solutions
Cytolysis
Crenation
Turgor Pressure
Plasmolysis

43. Facilitated Diffusion
• Another type of Passive Transport is called Facilitated Diffusion
– Facilitated diffusion occurs for molecules that cannot diffuse
through cell membranes, even when there is a concentration
gradient.
• Example: Molecules that are just too BIG to pass directly through the membrane.
– Again, no energy is required.
– Diffusion through the membrane is facilitated, or helped, by
proteins called carrier proteins.
– Carrier Protein: A special type of integral protein inside the
membrane that acts as a “tube” to let larger molecules through the
membrane.
– Only occurs when molecules are going DOWN their concentration
gradient—must be going from high concentration to low
concentration.
– Carrier proteins involved in facilitated diffusion are each specific
for one type of molecule.
Carrier Protein
Cell
Membrane

44. Facilitated Diffusion
• Example of Facilitated Diffusion: Molecules of
glucose, which are the cell’s source of energy, are
too large to pass through the membrane and must
move into cells by facilitated diffusion.
• How Facilitated Diffusion Works

48. Diffusion Through Ion Channels
• Ion channels – transport ions such as sodium
(Na+), potassium (K+), calcium (Ca2+), and
chloride (Cl-)
• When ion channels transport ions from higher to lower
concentrations they are a form of passive transport.
• Each type of ion channel is usually specific for one
type of ion.
• Some ion channels are always open.
• Some have “gates” that open and close in
response to:
− Stretching of the cell membrane
− Electrical signals
− Chemicals in the cell or external environment

50. Active Transport
• Movement of materials
across the cell membrane
from an area of lower
concentration to an area of
higher concentration (“up”
or “against” their
concentration gradient).
• Requires energy from the
cell.
− It is like riding a bike
uphill.

51. Types of Active Transport
• Cell Membrane Pumps
• Endocytosis
• Exocytosis

52. Cell Membrane Pumps
• Some types of active transport are performed by
carrier proteins called cell membrane pumps.
• These carrier proteins function in the same way as
the carrier proteins used in facilitated diffusion.
– The molecule to be transported binds to the carrier
protein on one side of the cell membrane.
– The carrier protein changes shape, shielding the
molecule from the hydrophobic interior of the
phospholipid bilayer.
– The carrier protein then transports the molecule
through the membrane and releases it on the other
side.
– Unlike the carrier proteins used in facilitated
diffusion, cell membrane pumps require energy.
• Example: Sodium-Potassium Pump

53. Sodium-Potassium Pump
• The sodium-potassium pump moves 3 Na+ ions
out of the cell for every 2 K+ ions it moves into
the cell. Both ions move up or against their
concentration gradients.
− To function normally, some animal cells must
have a higher concentration of Na+ ions
outside the cell and a higher concentration of
K+ ions inside the cell.
− The sodium-potassium pump maintains these
concentration differences.
• ATP supplies the energy that drives the pump.

54. Sodium-Potassium Pump

55. Steps of the Sodium-Potassium
Pump
1. Three Na+
ions from
the inside of
the cell bind
to the
carrier
protein.

56. Steps of the Sodium-Potassium
Pump
2. A phosphate
group is
removed
from ATP
and bound to
the carrier
protein.

57. Steps of the Sodium-Potassium
Pump
3. The carrier
protein changes
shape, allowing
three Na+ ions
to be released
to the outside
of the cell.

58. Steps of the Sodium-Potassium
Pump
4. Two K+ ions
from the
outside of
the cell bind
to the
carrier
protein.

59. Steps of the Sodium-Potassium
Pump
5. The phosphate
group is
released and
the carrier
protein goes
back to its
original shape.

60. Steps of the Sodium-Potassium
Pump
6. The two K+
ions are
released to
the inside of
the cell and
the cycle is
ready to
repeat.
• How the Sodium-
Potassium Pump Works

61. Importance of the Sodium-Potassium
Pump
• The ion exchange creates an electrical gradient
across the cell membrane.
– Outside becomes positively charged.
– Inside becomes negatively charged.
• This difference in charge is important for the
conduction of electrical impulses along nerve cells.
• Helps muscle cells contract.
– People drink sports drinks when they exercise to
replace some of the sodium and potassium ions
that are important for this pump. Without
these ions you would get muscle cramps.

62. Movement in Vesicles
• Some substances, such as macromolecules,
solid clumps of food, and whole cells are
too large to pass through the cell
membrane by the transport processes
studied so far.
• Cells employ two other transport
mechanisms– endocytosis and exocytosis–to
move such substances into or out of cells.
– Both of these mechanisms require cells to
expend energy. Therefore, they are
types of active transport.

63. Endocytosis
• “Endo” means inside and “cytosis” refers to the
cytoplasm. So, endocytosis brings bulky material into
the cytoplasm.
• In endocytosis, cells ingest external materials by the
cell membrane folding around them and forming a
pouch.
• The pouch then pinches off and becomes a
membrane-bound organelle called a vesicle that holds
the materials.
• Some of the vesicles fuse with lysosomes, and their
contents are digested by lysosomal enzymes.
• Other vesicles fuse with other membrane-bound
organelles.

64. Endocytosis

65. Endocytosis
• Two Types:
(1) Phagocytosis – “cell eating”; solid particles are
engulfed by the cell
Example: The white blood cells known as
phagocytes engulf invading microorganisms by
this process.
(1) Pinocytosis – “cell drinking”; liquid particles are
engulfed by the cell
• Endocytosis

66. Exocytosis
• The opposite of endocytosis is called
exocytosis.
• “Exo” means “out”.
• Process by which bulky substances are
released from the cell through a vesicle
that transports the substances to the
cell surface and then fuses with the
membrane to let the substances out of
the cell.

67. Exocytosis
• Example:
Hormones or
wastes released
from cell

68. Exocytosis
• Exocytosis

69. Summary Weeee!!!
• Passive Transport-cell does NOT use energy
 Diffusion
 Osmosis
 Facilitated Diffusion
 Diffusion Through Ion Channels high
low
• Active Transport-cell does use energy
 Cell Membrane Pumps This is
gonna
 Endocytosis be hard
1. Phagocytosis – “cell eating” work!!!
2. Pinocytosis – “cell drinking” high
 Exocytosis
low