Direction of Osmosis

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Transcript Direction of Osmosis

Part 3: Homeostasis and Cell Transport
(Chapter 5)
Passive Transport
•Involves the movement of molecules across the cell
membrane without an input of energy by the cell.
• NO ENERGY REQUIRED to move substances
across membrane -- water, lipids, and other
lipid soluble substances.
• Types:
1. Diffusion
2. Osmosis
3. Facilitated Diffusion
4. Diffusion through Ion Channels
1. Diffusion
• Diffusion is the movement of molecules from
an area of higher concentration to an area of
lower concentration (with the concentration
gradient*) until equilibrium** is reached
• *Concentration gradient: the difference in
concentration across space.
• **Equilibrium: molecules are eventually
evenly dispersed OR the concentration of
molecules is the same throughout the entire
Simple Diffusion: diffusion across
a membrane
• Cell Membrane = semi-permeable
• Some materials can pass through, some cannot
• Molecules will move from high to low
• The membrane determine what molecules
can pass through freely and what molecules
need “assistance”
2. Osmosis
Osmosis is the
diffusion of
water across a
Direction of Osmosis
The net direction
of osmosis is
determined by the
relative solute
concentrations on
the two sides of
the membrane.
• Direction of Osmosis
– When the solute concentration outside the cell
is higher than that in the cytosol, the solution
outside is hypertonic (more solute, less water)
to the cytosol, and water will diffuse out of the
– Plasmolysis = cell’s shrinking due to water loss
• Direction of Osmosis
– When the solute concentration outside the cell is
lower than that in the cytosol, the solution outside is
hypotonic (less solute, more water) to the cytosol,
and water will diffuse into the cell.
– Cytolysis = cell’s bursting due to water gain
• Direction of Osmosis
– When the solute concentrations outside and inside
the cell are equal, the solution outside is isotonic
(same solute, same water), and there will be no net
movement of water.
Review of Osmosis terms
Hypertonic, Hypotonic, Isotonic
• How Cells Deal With Osmosis
– Cells must compensate for the water that enters
the cell in hypotonic environments and leaves the
cell in hypertonic environments.
– Multicellular organisms
– cells respond to hypotonic environments by pumping
solutes out of the cytosol
– *RBCs cannot compensate for changes in solute
– Unicellular organisms
– May have Contractile vacuoles: organelles that regulate
water levels
Real cells
3. Facilitated Diffusion
• Diffusion of molecules across a membrane
when they are not soluble in lipids or are too
large (e.g. glucose) to pass through pores in
• A molecule binds to a carrier protein on one
side of the cell membrane.
• Carrier Protein
• specific for one type of molecule
• changes its shape and transports the molecule
Facilitated Diffusion
4. Diffusion Through Ion Channels
• Ion channels: proteins, or groups of proteins, that
provide small passageways across the cell
membrane through which specific ions can
• Some are always open
• Some are “gated”
• Ions:
• Charged particles like Na+, Ca2+, Cl• important in cell function
• include sodium, potassium, calcium, and chloride
Ion Channels
Active Transport
Cell Membrane Pumps and Vesicles
• Active transport
• moves molecules across the cell
membrane from an area of lower
concentration to an area of higher
concentration (against concentration
• requires cells to expend ENERGY
• Some types of active transport are
performed by carrier proteins called cell
membrane pumps.
Cell Membrane Pumps
• Sodium-Potassium Pump
• moves three Na+ ions into the cell’s external
environment for every two K+ ions it moves into
the cytosol.
• most animal cells must have a higher
concentration of Na+ ions outside the cell and
a higher concentration of K+ ions inside the
• The Na+/K- pump allows for the gradient
• ATP supplies the energy that drives the pump.
More on the Sodium-Potassium
The exchange of three
Na+ ions for two K+
ions creates an electrical gradient across the
cell membrane
• Outside becomes positively charged relative
to the inside, which becomes negative
• Difference in electrical charge is important for
the conduction of electrical impulses along
nerve cells
Sodium-Potassium Pump
Sodium-Potassium Pump
Movement in Vesicles
• Endocytosis
cells ingest external fluid, macromolecules, and large
particles, including cells by folding around them and
forming a pouch.
The pouch then pinches off and becomes a membranebound organelle called a vesicle.
• Some vesicles fuse with lysosomes, and their
contents are digested by lysosomal enzymes
Two main types
• Pinocytosis: vesicle contains solutes or fluids
• Phagocytosis: vesicle contains large particles or
whole cells.
Movement in Vesicles
• Exocytosis
• Vesicles made by the cell fuse with the cell
membrane, releasing their contents into the
external environment.
• Used to release large molecules, such as
proteins, waste products, or toxins that would
damage the cell if they were released within
the cytosol