Download Cells and Their Environment

Cells and Their
Environment
Section 1: Passive Transport
• Your body responds to
external conditions to
maintain stable
INTERNAL
CONDITIONS. This is
called HOMEOSTASIS.
• One way cells maintain
homeostasis is by
CONTROLLING THE
MOVEMENT OF
SUBSTANCES INTO
AND ACROSS THEIR
CELL MEMBRANE.
Random Motion and Concentration
• Movement across the cell membrane
that does NOT require energy is
called PASSIVE TRANSPORT.
• A DIFFERENCE in the concentration
of a substance across a space is
called a CONCENTRATION
GRADIENT.
• As a substance enters a second
area the concentration of the
second area INCREASES while
the concentration of a substance
DECREASES IN THE FIRST
AREA.
• Eventually the concentration of
the substance in both areas will
be EQUAL- this means the
amount of the substance moving
from the FIRST AREA TO THE
SECOND AREA IS EQUAL to
the amount of the substance
moving from the SECOND
AREA TO THE FIRST AREA.
• EQUILIBRIUM is a condition in which the
concentration of a substance is equal
throughout a space.
Movement of Substances
• Particles of a substance in a solution move
around RANDOMLY.
• If there is a CONCENTRATION
GRADIENT in the solution, the solution will
move from an area of HIGH
CONCENTRATION to an area of LOWER
CONCENTRATION in a process called
DIFFUSION.
• If DIFFUSION is allowed to
continue EQUILIBRIUM
eventually results.
• MOLECULES and IONS
dissolved in the CYTOPLASM
and in the FLUID OUTSIDE
THE CELLS enter and leave
cells by diffusing across the
CELL MEMBRANE.
• The NONPOLAR INTERIOR
of the cell membrane
REPELS IONS AND MOST
POLAR MOLECULES and are
thus PREVENTED from
diffusing across the cell
membrane.
• Molecules that are VERY SMALL or
NONPOLAR can diffuse across the
cell membrane down their
CONCENTRATION GRADIENT.
• Diffusion of such molecules across
the cell membrane is the
SIMPLEST type of PASSIVE
TRANSPORT.
Osmosis
• The diffusion of water through a
SELECTIVELY PERMEABLE
membrane is called OSMOSIS.
• Osmosis is a type of PASSIVE
TRANSPORT.
• A solution is a SUBSTANCE dissolved
in ANOTHER SUBSTANCE.
• On either side of the membrane many
IONS and POLAR MOLECULES are
dissolved in water.
• If solutions on either side of the cell
membrane have DIFFERENT
concentrations of DISSOLVED
PARTICLES they will also have
DIFFERENT concentrations of FREE
WATER molecules.
• OSMOSIS will occur as FREE
WATER MOLECULES move
INTO the solution with the
LOWER CONCENTRATION of
free water molecules.
Osmosis
3 possible directions for water movement:
•
water moves out: cell SHRINKS – HYPERTONIC
SOLUTION
1.
• 2. water moves in: cell
SWELLS – HYPOTONIC
SOLUTION
• 3. No net movement:
NO CHANGE in cell
VOLUME – ISOTONIC
solution
Table 1 Page 77
If the fluid
outside the cell
has…
Then outside
fluid is…
Water
diffuses…
Effect on cell
…lower free
water molecule
concentration
than cytoplasm
…hypertonic
… out of cell
H2O moves out
of the cell
(shrinks)
Cell shrinks
(becomes
smaller)
…higher free
water molecule
concentration
than cytoplasm
…hypotonic
…into cell.
H2O moves into
the cell (swells)
Cell swells
(becomes
larger)
…same free
…isotonic
water molecule
concentration as
cytoplasm
…into and out of Cell stays same
cell at equal
size (no change)
rates
Real Life Example
Isotonic
or
Hypotonic
Hypertonic
• Swelling caused by HYPOTONIC
solution could cause a cell to
BURST.
• When the cell loses water
caused by a HYPERTONIC
solution the cell will SHRINK.
• When there is no change and
water flow evenly across a cell,
ISOTONIC solution is
occurring.
Crossing the cell membrane
• IONS and polar molecules can cross
the cell membrane when they are
aided by TRANSPORT PROTEINS
CALLED CHANNELS.
• Each channel allows only a SPECIFIC
SUBSTANCE to pass through the
cell membrane-this SELECTIVITY is
one of the most important
properties of the CELL MEMBRANE.
Diffusion Through Ion Channel
• Example: NERVE CELLS sending
electrical signals throughout the
body; MUSCLE CELLS in your heart
making your heart beat.
• Diffusion through ion pores is a form
of PASSIVE TRANSPORT.
Electron Charge and Ion Transport
• The inside of a typical cell is NEGATIVELY
CHARGED with respect to the OUTSIDE of the
cell.
• OPPOSITE charges ATTRACT and LIKE charges
REPEL.
• The direction of movement caused by the
CONCENTRATION GRADIENT may oppose the
direction of movement caused by the ION’S
ELECTRICAL CHARGE. This is very important to
the functioning of NERVE CELLS in animals.
Facilitated Diffusion
• TRANSPORT PROTEINS that can BIND
to a specific substance on one side of the
cell membrane and RELEASE it on the
other side are called a CARRIER
PROTEIN.
• When carrier proteins are used to
transport specific substances down their
concentration gradient the process is
called FACILITATED DIFFUSION.
Active Transport
• The transport of a substance across the
cell membrane against its concentration
gradient is called ACTIVE TRANSPORT.
– Unlike PASSIVE TRANSPORT, active
transport requires ENERGY which is
supplied by ATP
– Some active transport processes
involve CARRIER PROTEINS.
– Carrier proteins in active transport
function as “PUMPS” that move
substance AGAINST their
CONCENTRATION GRADIENT.
– Carrier proteins are often called
MEMBRANE PUMPS.
Sodium-Potassium Pump
• The SODIUM-POTASSIUM PUMP transport
3 sodium ions, Na+, OUT of a cell and 2
potassium ions, K+, INTO the cell.
– Sodium ions are usually more concentrated
INSIDE the cell than OUTSIDE the cell
and potassium ions are usually more
concentrated OUTSIDE the cell than
INSIDE the cell. Thus, the sodiumpotassium pump transport these ions
AGAINST their CONCENTRATION
GRADIENT.
– Energy needed to power the Sodium
potassium pump is supplied by ATP.
2 reasons that the sodium
potassium pump is important
1 – It keeps the cell from accumulating
too many sodium ions which would cause
the cell to fill with water by osmosis
and swell or burst.
2- It helps maintain the concentration
gradient of sodium ions and potassium
ions across the cell membrane.
Movement in Vesicles
• Some substances are too LARGE to
be transported by carrier proteins.
• Vesicles move these large
substances INTO a cell by
ENDOCYTOSIS and move
substances OUT of the cells by
EXOCYTOSIS.
Endocytosis
Exocytosis
Membrane Receptor Proteins
• How do your cells communicate ??????
– They communicate through SIGNAL
MOLECULES. One familiar example of
signal molecules are HORMONES.
• Cells can RECEIVE the MESSAGES carried
by certain signal molecules because the
cell membrane contains specialized
PROTEINS that BIND these signal
molecules.
• Examples: Muscle cells could
NOT CONTRACT or RELAX
without receptor proteins and
signal molecules.
• Where are the receptor
proteins???
– EMBEDDED IN THE LIPID
BILAYER OF THE CELL
MEMBRANE
• Many drugs affect the
BINDING of signal molecules to
receptor proteins.
–Example: In heart-muscle
cells, signal molecules
STIMULATE the cells, causing
the HEART RATE TO
INCREASE.
• BETA BLOCKERS are
prescribed to patients with
a RAPID heartbeat. BetaBlockers interfere with the
binding of signal molecules
to the receptor, preventing
the heart rate from
increasing too rapidly.