Download Diffusion and Osmosis

Diffusion and Osmosis
Cells have needs too…
• All cells need to:
– Take in food or nutrients
– Take in oxygen and release carbon dioxide
– Remove wastes made inside the cell
– Control their water balance
• All of these activities must happen across the
cell membrane of every single cell
Methods of Transport
1. Passive Transport
2. Active Transport
1. Passive Transport
• Passive transport- the movement of
substances across the membrane without the
use of chemical energy.
– Diffusion and Osmosis
• Passive transport relies on the movement of
substances down a concentration gradient.
Diffusion
• Diffusion- the movement of molecules from
an area of higher concentration to an area of
lower concentration.
– A difference in the concentrations of molecules
across a space is called a concentration gradient.
– Diffusion is driven by the movement of molecules
• The molecules like to “spread out”
Diffusion
• 2nd Law of Thermodynamics
governs biological systems
– universe tends towards disorder (entropy)
Diffusion
movement from high  low concentration
Animation
The process of diffusion is used to deliver
oxygen from your lungs into your blood.
Equilibrium
• Equilibrium occurs
when the concentration
of molecules is the
same throughout a
space.
– Molecular movement
still occurs, but in all
directions  the balance
is maintained.
Membranes
• Some molecules can diffuse across a membrane
(cell membrane). If a molecule can move through
a membrane, that membrane is permeable to
that molecule.
– Ex. Water, oxygen, some ions
• Not all molecules can diffuse across all
membranes. Molecular movement depends on
the size and type of the molecule as well as the
membrane structure.
– Ex. Glucose, large proteins
Membranes again…
Animation
Osmosis…
• Osmosis- special case of diffusion, in which
water molecules diffuse across a cell
membrane from an area of high concentration
of water to areas of low concentration of
water.
– The direction of water movement depends on the
concentrations of water and solute dissolved in
the solution.
• Remember…
• Solution- homogeneous mixture in which one or more
substances are uniformly dissolved in another
substance.
• Solute- substance dissolved in a solution.
Which way will it go?
• In many situations, the solute molecules can’t
actually diffuse across the membrane, which
creates an imbalance.
• Very Simply, the concentrations of water and
solutes “want” to be equal on each side of the
membrane, so the water must move.
Osmosis is the movement
of water from areas of
high concentration of
water to areas of low
concentration of water
Animation
Water flows “downhill”
– If the concentration of water molecules outside
the cell is higher than the concentration in the cell
(solute concentration is lower outside), the
solution is hypotonic to the cell. Water moves into
the cell until equilibrium is reached.
• Hypo means “lower”
Hypotonic- the cell swells
Solute
90% Water
Cell Membrane
Draw arrows
representing the
movement of water.
80%
Water
Water flows “downhill” continued…
– If the concentration of water molecules outside
the cell is lower than the concentration inside the
cell (solute concentration higher), the solution is
hypertonic to the cell. Water moves out of the cell
until equilibrium is reached.
• Hyper mean “higher”
Hypertonic- the cell shrivels
Solute
70% Water
Cell Membrane
Draw arrows
representing the
movement of water.
90%
Water
Water flows “downhill” continued…
• When concentration of water molecules outside the
cell is equal to the concentration inside the cell, the
solution is isotonic to the cell. Water moves into and
out of the cell at an equal rate.
• “iso” means equal
Isotonic- at equilibrium
Solute
80% Water
Cell Membrane
Draw arrows
representing the
movement of water.
80%
Water
Pressure
– If water is moving into the cell, turgor pressure
increases. If the cell takes too much water in it can
burst. This is termed cytolysis.
– If water is moving out of the cell, turgor pressure
drops and the cell wilts. This is termed
plasmolysis.
Managing water balance
• Cell survival depends on balancing water
uptake & loss
freshwater
balanced
saltwater
Passive Movement
• Passive transport- the movement of
substances across the membrane without the
use of chemical energy (No ATP needed)
– Diffusion and Osmosis (see previous)
– Facilitated Diffusion- carrier molecules speed up
the diffusion of molecules that move slowly across
the membrane. No energy is required.
• Larger molecules
• Ex.: Glucose into and out of cells.
Membrane Components
• Carrier molecules- transport molecules across
the cell membrane from high concentration to
low concentration.
Animation
2. Active Transport
• Active Transport- The movement of materials
across a membrane that requires the use of
energy. (Requires ATP)
• Usually moves molecules Uphill, against the
concentration gradient
It takes energy to go uphill
• Cell Membrane pumps- carrier proteins that
move molecules against a concentration gradient.
From areas of low concentration to areas of
higher concentration. This movement requires
the use of chemical energy.
Animation
Cells eat too…
• Endocytosis- the process by which cells engulf
substances that are too large to pass through
the cell membrane. The cell forms a vesicle
around the material.
– “Endo”- means “in” or “internal”
Types of Endocytosis
• Pinocytosis- the
transport of fluids or
solutes into the cell. (the
cell “drinks”)
• Phagocytosismovement of large
molecules or entire
cells/microorganisms
into the cell. (the cell
“eats”)
And get rid of wastes
• Exocytosis- the
passage of large
molecules out of
the cell.
• Vacuoles merge
with the cell
membrane,
expelling their
contents.
Animation
Managing water balance
• Isotonic
– animal cell immersed in
mild salt solution
– Concentration of water
outside=concentration inside
• example:
blood cells in blood plasma
• problem: none
– no net movement of water
» flows across membrane equally, in
both directions
– volume of cell is stable
balanced
Managing water balance
• Hypotonic
– a cell in fresh water
– Concentration of water outside>concentration
inside
• example: Paramecium
• problem: gains water,
swells & can burst
– water continually enters
Paramecium cell
ATP
• solution: contractile vacuole
– pumps water out of cell
– ATP
– plant cells
• turgid
freshwater
Active Transport
• Cells may need to move molecules against
concentration gradient
– shape change transports solute from
one side of membrane to other
– protein “pump”
conformational change
low
– “costs” energy = ATP
ATP
high
“The Doorman”