Download Movement Across the Cell Membrane

Movement Across the Cell
Membrane
Section 4.2-4.5
Inquiry into Life
Permeability of the Plasma Membrane
• The cell membrane is said to be differentially
or selectively permeable.
• This means that some substances can move
across the membrane while others cannot.
Permeability of the Plasma Membrane
• Some things that can normally pass through
the plasma membrane are:
– Small non-charged molecules
– Water
Permeability of the Plasma Membrane
• Some things that cannot normally pass
through the plasma membrane are:
– Macromolecules
• Protein
• Sugar
• etc
– Charged molecules and ions
Permeability of the Plasma Membrane
• Substances that pass through the plasma
membrane can do so by 1 of 2 methods:
Permeability of the Plasma Membrane
• Active transport:
– Transport across a membrane that requires energy
from ATP and/or a carrier protein.
• Exocytosis
• Endocytosis
• Sodium-potassium pump
Permeability of the Plasma Membrane
• Passive transport:
– Transport across a membrane that does not
require energy.
• Diffusion
• Osmosis
• Facilitated transport
Permeability of the Plasma Membrane
• In general, smaller molecules such as CO2, O2,
H2O can diffuse through the cell membrane by
slipping between the hydrophilic heads.
• These molecules follow what is called the
concentration gradient.
Permeability of the Plasma Membrane
• The concentration gradient is the movement
of molecules from an area of high
concentration to an area of lower
concentration.
Permeability of the Plasma Membrane
• Ions (charged particles) and polar molecules
are not able to diffuse along the concentration
gradient to move in and out of cells.
• These substances must be assisted across the
membrane by carrier proteins.
• This can be done by either active transport or
facilitated transport.
Diffusion and Osmosis
• Diffusion is the movement of molecules (other
than water) from high concentration to lower
concentration (down the concentration
gradient) until equilibrium is reached.
• Diffusion does not need a membrane, just a
concentration gradient.
Diffusion and Osmosis
Diffusion and Osmosis
• In living organisms a
good example of
diffusion is oxygen
diffusing through the
alveoli into the
capillaries of lung
tissue.
Diffusion and Osmosis
• Osmosis is a specific type of diffusion.
• It applies to the movement of water down the
concentration gradient through a selectively
permeable membrane.
Diffusion and Osmosis
• As the water moves into the tube the
concentration of salt becomes less (more
dilute).
• The concentration of salt becomes greater due
to the loss of water.
Diffusion and Osmosis
• Osmotic pressure is the pressure that
develops inside a system (cell) due to osmosis.
Diffusion and Osmosis
• When osmosis occurs through the cell
membrane, the strength of the solution the
cell is placed in will determine what happens.
• The term we use to mean the strength of a
given solution is tonicity.
Diffusion and Osmosis
• note the prefixes:
– “iso” means same
– “hypo” means less than
– “hyper” means more than
• The root word, tonic, refers to the soution.
Diffusion and Osmosis
Isotonic
Hypotonic
Solution
Solution
- Equal solute
- Less solute
concentrations
concentration
between the
than the cell.
cell and its
- Water flows
environment.
into the cell
- No net
- Greater water
movement of
concentration
water.
than in the cell.
- Cell may burst.
-
-
-
Hypertonic
Solution
Greater solute
concentration
than the cell.
Water flows
out of the cell.
Lower water
concentration
than the cell.
Cell shrinks.
Diffusion and Osmosis
• A salt solution of 0.9% is isotonic to red blood
cells.
Diffusion and Osmosis
• When plant cells are placed in a hypotonic
solution turgor pressure is created causing
expansion of the vacuole and pushes the
membrane up against the cell wall.
Diffusion and Osmosis
Diffusion and Osmosis
• Crenation and plasmolysis are both the result
of cells placed in hypertonic solutions which
causes the cells to shrink.
• Crenation happens in animal cells and
plasmolysis happens in plant.
Transport by Carrier Proteins
• Special proteins called carrier proteins help to
move substances such as glucose and amino
acids from high concentration to low
concentration through the cell membrane.
This type of movement is called facilitated
transport.
• This type of transport, like diffusion and
osmosis, do not require any energy.
Transport by Carrier Proteins
• Carrier proteins speed the rate at which
substances like water can pass through the
membrane down the concentration gradient
without using ATP.
• Molecules enter the carrier protein, combine
with the carrier and change its shape causing
the molecule to spit out the other side.
Transport by Carrier Proteins
• Another type of transport across membranes
is called active transport.
– This allows molecules to pass up the
concentration gradient (move from lower to
higher concentrations).
– The breakdown of ATP causes the carrier protein
to change shape that drives the molecule across
the membrane.
Transport by Carrier Proteins
• The proteins involved in active transport are
often called pumps because, like pumps that
move water up against the force of gravity,
these molecules move other molecules
against the concentration gradient.
Transport by Carrier Proteins
• The sodium-potassium pump is an important
example:
– It is active in all animal cells, especially nerve cells.
– Na+ ions are moved outside of the cell.
– K+ ions are moved inside of the cell.
Transport by Carrier Proteins
Transport by Carrier Proteins
Transport by Carrier Proteins
Transport by Carrier Proteins
Transport by Carrier Proteins
Transport by Carrier Proteins
Transport by Carrier Proteins
Transport by Carrier Proteins
Transport by Carrier Proteins
Transport by Carrier Proteins
Diffusion and Osmosis
Diffusion and Osmosis
Diffusion and Osmosis