Cell Membranes
and Methods of Transport
How Do Molecules Move?
• Stand with your classmates in
locations that are evenly spaced
throughout the classroom.
• Your teacher will spray an air
freshener into the room. When
your first smell the air
freshener, raise your hand.
• Note how long it takes for other
students to smell the scent.
How Do Molecules Move?
Developing a Hypothesis
How was each student’s distance from the
teacher related to when he or she smelled
the air freshener?
Develop a hypothesis about why this pattern
occurred.
Key Concepts
• How do most small molecules cross the cell
membrane?
• Why is osmosis important in cells?
• What is the difference between passive
transport and active transport?
Functions of Membranes
1.
2.
3.
4.
Protect cell
Control incoming and outgoing substances
Maintain ion concentrations of various substances
Selectively permeable - allows some molecules in,
others are kept out
Phospholipid Bi-layer
Selectively Permeable
Membrane
• Oxygen, food molecules
and waste products all
must pass through the
cell membrane.
• Substances that can
move into or out of a
cell do so by one of four
methods:
Diffusion
Osmosis
Facilitated Transport
Active Transport
Fluid Mosaic Model
Methods of Transport
Across Membranes
1. Diffusion -passive transport of solutes - no
energy expended
2. Osmosis - Passive transport of water
across membrane – no energy expended
3. Facilitated Diffusion - Use of proteins to
carry polar molecules or ions across
4. Active Transport- requires energy to transport
molecules against a concentration
gradient – energy is in the form of
ATP
Types of Transport
Diffusion
• Diffusion is the main method by
which small molecules move
across the cell membrane.
• It is the process by which
molecules tend to move from an
area of high concentration to an
area of low concentration.
• Diffusion is caused by molecules
moving and colliding.
• Molecules move from one side of
a membrane to another, unfacilitated, depending on the
concentration.
Diffusion of liquids
Diffusion
• Diffusion is the net movement of
molecules (or ions) from a region of
their high concentration to a region
of their lower concentration.
The molecules move down a
concentration gradient.
Molecules have kinetic energy, which
makes them move about randomly.
As a result of diffusion molecules reach
an equilibrium where they are evenly
spread out.
This is when there is no net movement of
molecules from either side.
Diffusion of Bromine
Diffusion through a membrane
Cell membrane
Inside cell
Outside cell
Diffusion through a membrane
Cell membrane
diffusion
Inside cell
Outside cell
Diffusion through a membrane
Cell membrane
Inside cell
Outside cell
EQUILIBRIUM
What determines the rate of diffusion?
There 4 factors:
1. The steepness of the concentration gradient. The bigger
the difference between the two sides of the membrane
the quicker the rate of diffusion.
2. Temperature. Higher temperatures give molecules or ions
more kinetic energy. Molecules move around faster, so
diffusion is faster.
3. The surface area. The greater the surface area the faster
the diffusion can take place. This is because the more
molecules or ions can cross the membrane at any one
moment.
4. The type of molecule or ion diffusing. Large molecules
need more energy to get them to move so they tend to
diffuse more slowly. Non-polar molecules diffuse more
easily than polar molecules because they are soluble in the
non polar phospholipid tails.
Molecules that diffuse through cell
membranes
Oxygen – Non-polar
so diffuses very
quickly.
Carbon dioxide –
Polar but very small
so diffuses quickly.
Water – Polar but
also very small so
diffuses quickly.
Osmosis
• The diffusion of water molecules through a selectively
permeable membrane is called osmosis.
• Because cells cannot function properly without adequate
water, many cellular processes depend on osmosis.
DILUTE SOLUTION
Osmosis
CONCENTRATED SOLUTION
Cell membrane
partially
permeable.
Sugar molecule
VERY Low conc.
of water
molecules. High
water potential.
VERY High conc.
of water
molecules. High
water potential.
Inside cell
Outside cell
Osmosis
Cell membrane
partially
permeable.
OSMOSIS
High conc. of
water molecules.
High water
potential.
Inside cell
Low conc. of
water molecules.
High water
potential.
Outside cell
Osmosis
Cell membrane
partially
permeable.
OSMOSIS
Inside cell
Outside cell
EQUILIBRIUM. Equal water concentration on each side.
Equal water potential has been reached. There is no net
movement of water
Salt Sucks
• Water will move in the direction where there is a
high concentration of solute (and hence a lower
concentration of water.
• A simple rule to remember is:
Salt Sucks
• Salt is a solute, when it is concentrated inside or
outside the cell, it will draw the water in its
direction. This is also why you get thirsty after
eating something salty.
Passive Transport
• The movement of
dissolved materials
through a cell
membrane without
using cellular energy
is called passive
transport.
• Diffusion and
osmosis are both
types of passive
transport.
DIFUSSION LAB
• Open Lab that was emailed to you.
• Lets discuss this. Link
Facilitated Difussion
Facilitated diffusion
• Large polar molecules such as
glucose and amino acids, cannot
diffuse across the phospholipid
bilayer. Also ions such as Na+ or Clcannot pass.
• These molecules pass through
protein channels instead. Diffusion
through these channels is called
FACILITATED DIFFUSION.
• Movement of molecules is still
PASSIVE just like ordinary
diffusion, the only difference is,
the molecules go through a protein
channel instead of passing between
the phospholipids.
Facilitated Diffusion through a
membrane
Cell membrane
Protein channel
Inside cell
Outside cell
Facilitated Diffusion through a
membrane
Cell membrane
diffusion
Protein channel
Inside cell
Outside cell
Facilitated Diffusion through a
membrane
Cell membrane
diffusion
Protein channel
Inside cell
Outside cell
EQUILIBRIUM
Facilitated Diffusion:
Molecules will randomly move through the
opening like pore, by diffusion. This requires no
energy, it is a PASSIVE process. Molecules
move from an area of high concentration to an
area of low conc.
Lactose intolerance
Active Transport
Active Transport
• When a cell needs to take in
materials that are in higher
concentration inside the cell than
outside the cell, the movement
of the materials requires energy.
• Active transport is the
movement of materials through a
cell membrane using cellular
energy (ATP).
• The main difference between
passive and active transport is
that active transport requires the
cell to use its own energy and
passive transport does not.
Active Transport
• Cells have several ways of moving materials by active
transport.
• In one method, transport proteins in the cell membrane
‘pick up’ molecules outside the cell and carry them in.
• Another method of active transport is engulfing, in which
the cell membrane wraps around, or engulfs, a particle
and forms a vacuole within the cell.
Vesicles and the Cell Membrane
•
•
•
•
Endocytosis is the case when a
molecule causes the cell membrane
to bulge inward, forming a vesicle.
Phagocytosis is the type of
endocytosis where an entire cell is
engulfed.
Pinocytosis is when the external fluid
is engulfed.
Receptor-mediated endocytosis
occurs when the material to be
transported binds to certain specific
molecules in the membrane.
Examples include the transport of
insulin and cholesterol into animal
cells.
The opposite of endocytosis is
exocytosis. Large molecules that are
manufactured in the cell are released
through the cell membrane.
Receptor Proteins
These proteins are
used in intercellular
communication. In
this animation you
can see the a
hormone binding to
the receptor.
This causes the
receptor protein
release a signal to
perform some action.
Carrier Proteins
These are carrier
proteins. They do not
extend through the
membrane.
They bond and drag
molecules through the
bi-lipid layer and
release them on the
opposite side.
Vesicle-mediated transport
• Vesicles and vacuoles
that fuse with the cell
membrane may be
utilized to release or
transport chemicals out
of the cell or to allow
them to enter a cell.
• Exocytosis is the term
applied when transport
is out of the cell.
Overview of Transport Systems
Most Cells are Small
• One reason is related to
the fact that all materials
move into and out of the
cell membrane.
• Once a molecule enters a
cell, it is carried to its
destination by a stream
of moving cytoplasm.
• In very large, streams of
cytoplasm must travel
farther to carry materials
from the cell membrane
to all parts of the cell.
Solutions
• Solutions are made of
solute and a solvent
• Solvent - the liquid into
which the solute is
poured and dissolved.
We will use water as
our solvent today.
• Solute - substance that
is dissolved or put into
the solvent. Salt and
sucrose are solutes.
Tonicity is a relative term
• Hypotonic Solution
• Hypertonic Solution
• Isotonic Solution
Isotonic
• If the concentration
of solute (salt) is
equal on both sides,
the water will move
back in forth but it
won't have any result
on the overall
amount of water on
either side.
• "ISO" means the
same
Hypotonic Solution
• The word "HYPO" means less, in this
case there are less solute (salt)
molecules outside the cell, since salt
sucks, water will move into the cell.
• The cell will gain water and grow
larger. In plant cells, the central
vacuoles will fill and the plant becomes
stiff and rigid, the cell wall keeps the
plant from bursting
• In animal cells, the cell may be in
danger of bursting, organelles called
CONTRACTILE VACUOLES will
pump water out of the cell to prevent
this.
Hypertonic Solutions
• The word "HYPER" means more, in this
case there are more solute (salt) molecules
outside the cell, which causes the water to
be sucked in that direction.
• In plant cells, the central vacuole loses
water and the cells shrink, causing wilting.
• In animal cells, the cells also shrink.
• In both cases, the cell may die.
• This is why it is dangerous to drink sea
water - its a myth that drinking sea water
will cause you to go insane, but people
marooned at sea will speed up dehydration
(and death) by drinking sea water.
• This is also why "salting fields" was a
common tactic during war, it would kill
the crops in the field, thus causing food
shortages.
Plant and Animal Cells put into
various solutions
Effects of Plant Cells in Different
Solutions
RBC’s put in Various Solutions
Tonicity is a relative term
• Hypotonic Solution –
One solution has a lower concentration of
solute than another.
• Hypertonic Solution –
One solution has a higher concentration of
solute than another.
• Isotonic Solution
Both solutions have same concentrations of
solute.
Math Skills: Ratios
The concentration of a solution can be expressed
as a ratio. A ratio compares two numbers. It tells
you how much you have of one item in
comparison to another.
For example, suppose you dissolve 5 g of sugar in
1 L of water. You can express the concentration
of the solution in ratio form as 5g : 1L, or 5g/L.
Practice
Suppose you dissolve 7g of salt in 1L of water.
Express the concentration of the solution as a
ratio.