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How stuff gets in & out of the cell.
• You learned in section 1 that the membrane
does a lot to control what goes in & out of the
• Today you will learn the specific ways materials
enter & exit the cell.
• You will see that the way a cell gets each
material is unique and specialized.
• Also, that there are ways the cell gets its needed
materials by both using and not using energy.
Objectives: Passive & Active Transport
• Identify what determines the direction in which
passive transport occurs.
• Understand osmosis and why it’s important.
• Illustrate how substances move against a
concentration gradient in active transport.
Concentration gradient
Carrier protein
Sodium-potassium pump
Before we get started…
• You’ll need to know these terms before we
begin. Define these in your notebooks.
• Solution:
• Solute:
• Solvent:
• Concentration: an amount of a substance within
a given volume
Passive Transport: Diffusion &
• In a solution, randomly moving molecules tend
to fill up a space.
• Watch as I drop some food coloring into the
• What happens?
• The process that causes this dispersion of polar
color molecules is diffusion.
• When the space is filled evenly with the
particles, a state called equilibrium is reached.
– A state that exists when the concentration of a
substance is the same through-out a space.
Water temp
– As the ice
melts the
– The low
temperature of
the ice
equalizes with
the warmer
water temp.
Ice Temp
Passive Transport: Concentration
• The amount of a particular substance in a given
volume is called the concentration of the
• When one area has a higher concentration than
another area does, a concentration gradient
– The difference in the concentration of a substance
across a distance.
Visual Concept: Concentration Gradient
Area of High
Area of Low
Passive Transport: Diffusion
• The movement of particles from regions of
higher density to regions of lower density is
• Watch as I drop yellow food coloring into the
• Would you all agree that the concentration of the
food coloring is highest right where it is
• Over time, the particles of color naturally diffuse
through the water, without any need of physical
• One of the main jobs of the cell membrane is to separate
the cytoplasm from the fluid outside the cell.
• But the cell still needs an abundance of materials that
comes from outside the cell.
• Some substances that the cell needs can enter and
leave the cell by diffusing across the cell membrane.
• The direction of movement depends on the
concentration gradient, meaning that the particles will
naturally flow where there is less of them, & usually
where more is needed.
• The greatest part of this is…
Diffusion is Passive Transport
• In cells, diffusion is called
passive transport.
• In passive transport,
substances cross the cell
membrane down their
concentration gradient.
• Some substances diffuse
through the lipid bilayer.
• Other substances diffuse
through transport proteins.
Passive Transport… Simple Diffusion
Diffusion through the membrane is SIMPLE DIFFUSION
Simple Diffusion
• Small, nonpolar molecules can pass directly through the
lipid bilayer. This type of movement is called simple
• Oxygen moves down its concentration gradient into the
• Carbon dioxide diffuses out of the cell.
• Natural steroid hormones, which are nonpolar and fat
soluble, can also diffuse across the lipid bilayer.
Passive Transport & Not So Simple
Facilitated Diffusion
• Many ions and polar molecules that are important for cell
function do not diffuse easily through the nonpolar lipid
• During facilitated diffusion, transport proteins help
these substances diffuse through the cell membrane.
• Two types of transport proteins are channel proteins and
carrier proteins.
Facilitated Diffusion: Passive Transport
Facilitated Diffusion
• Ions, sugars, and amino acids can diffuse through the
cell membrane through channel proteins.
• These proteins, sometimes called pores, serve as
tunnels through the lipid bilayer.
• Each channel allows the diffusion of specific substances
that have the right size and charge.
Visual Concept: Diffusion Through Ion Channels
Passive Transport, continued
Facilitated Diffusion
• Carrier proteins transport substances that fit within their
binding site.
– A protein that transports substances across a membrane
• A carrier protein binds to a specific substance on one
side of the cell membrane. This binding causes the
protein to change shape.
• As the protein’s shape changes, the substance is moved
across the membrane and is released on the other side.
Visual Concept: Passive
Link to McGrawHill online
(a good textbook) for
animations and
explanations of biology
Take the quizzes!
Transport: Facilitated Diffusion
• Water can diffuse across a selectively permeable
membrane in a process called osmosis.
• Osmosis in cells is a form of facilitated diffusion. Polar
water molecules do not diffuse directly through the
bilayer. But the cell membrane contains channel proteins
that only water molecules can pass through.
• Osmosis allows cells to maintain water balance as their
environment changes.
Osmosis/ How the Environment Changes
When ions and polar substances
dissolve in water, they attract and
bind some water molecules. The
remaining water molecules are free
to move around.
If a concentration gradient exists
across a membrane for solutes, a
concentration gradient also exists
across the membrane for free water
Osmosis occurs as free water
molecules move down their
concentration gradient into the
solution that has the lower
concentration of free water
Visual Concept: Osmosis
• The direction of water movement in a cell depends on
the concentration of the cell’s outside environment.
• If the solution is hypertonic, or has a higher solute
concentration than the cytoplasm does, water moves
out of the cell. The cell loses water and shrinks.
• If the solution is hypotonic, or has a lower solute
concentration than the cytoplasm does, water moves
into the cell. The cell gains water and expands in size.
• If the solution is isotonic, or has the same solute
concentration that the cytoplasm does, water diffuses
into and out of the cell at equal rates. The cell stays
the same size.
Hypertonic, Hypotonic, and Isotonic
Another way to say it… with “Free Water
Example of Osmosis
Onion Cells in Hypotonic Solution
Onion Cells in Hypertonic Solution
Cell Swells
Cell Shrinks
Questions on Passive Transport?
What is a concentration gradient?
What is diffusion?
What is passive transport?
What kinds of passive transport are there?
Does passive transport use energy?
Active Transport
Area of High
Area of Low
Active Transport
• The opposite of diffusion is active transport.
• In order to move substances against their
concentration gradients, cells must use energy.
• Active transport requires energy to move substances
against their concentration gradients.
• Most often, the energy needed for active transport is
supplied directly or indirectly by ATP.
Visual Concept: Comparing Active and
Passive Transport
Link on sodium
potassium pump
Active Transport, continued
• Pumps are carrier proteins that require energy to move
substances UP their concentration gradient.
• The sodium-potassium pump is a carrier protein that
actively transports three sodium ions out of the cell and
two potassium ions into the cell.
– This pump is one of the most important carrier proteins in animal
cells. It prevents sodium ions from building up in the cell,
resulting in osmosis into the cell… which could burst the cell.
– The concentration gradients of sodium ions and potassium ions
also help transport other substances, such as glucose, across
the cell membrane.
Sodium-Potassium Pump
Visual Concept: Sodium-Potassium
Active Transport
• Many substances, such as proteins and
polysaccharides, are too large to be transported by
carrier proteins altogether.
• Instead, they cross the cell membrane in vesicles, which
are membrane-bound (lipid bi-layer) sacs.
• The vesicle membrane is a lipid bilayer, like the cell
membrane. Therefore, vesicles can bud off from the
membrane, fuse with it, or fuse with other vesicles.
Active Transport, continued
• The movement of a large substance into a cell by means
of a vesicle is called endocytosis.
• During endocytosis the cell membrane forms a pouch
around the substance.
• The pouch then closes up and pinches off from the
membrane to form a vesicle inside the cell.
• Vesicles that form by endocytosis may fuse with
lysosomes or other organelles.
Active Transport, continued
• Vesicles help the movement of large
molecules two ways:
– Endocytosis
– Exocytosis
Endocytosis = “Into”
Cell ingests large macromolecules or other cells
Exocytosis = “Exits”
Opposite of Endocytosis
Release of contents in the cell to the
external environment
Summary Questions
Does Passive Transport require Energy?
Does Active Transport require Energy?
What is the energy required to Active Transport?
What is the difference between active and passive
• Is diffusion Passive or Active?
• Is Osmosis Passive or Active?
• Is the Sodium-Potassium pump Passive or Active?
• In passive transport, substances cross the cell
membrane down their concentration gradient.
• Osmosis allows cells to maintain water balance as their
environment changes.
• Active transport requires energy to move substances
against their concentration gradients.