Cell Transport
Section 7-3 in the textbook
Structure of the
Cell Membrane
What type of organic
compound is shown?
Label each of the following:
• Glycerol
• Phosphate
• Saturated Fatty Acid
• Unsaturated Fatty Acid
Structure of the Cell Membrane
Phospholipid molecules
make up the cell membrane.
The head of the phospholipid
is polar
• What is a polar molecule?
• The polar head is
hydrophilic (they can mix
with water.
Structure of the Cell Membrane
The tails of the
phospholipid is nonpolar
• The nonpolar tail is
hydrophobic (they can’t
mix with water.
Phospholipid Bilayer:
the tails face inward to
avoid contact with water,
the heads face outward to
be in contact with water.
Structure of the Cell Membrane
The phospholipid bilayer is selectively permeable: it
only allows certain materials to pass through.
• Only small, nonpolar, hydrophobic molecules can
pass through the phospholipid bilayer.
Can it pass through
the bilayer:
– H2O
– O2
– Proteins
Structure of the Cell Membrane
The cell membrane made is made many different
molecules, not just phospholipids.
• Fluid Mosaic Model: the cell membrane is made
of many different molecules that are free-floating
through the membrane.
• The membrane
also contains:
– Proteins
– Carbohydrates
– Cholesterol
Structure of the Cell Membrane
Proteins
• Act as channels or pumps that help move materials
across the membrane.
Carbohydrates
• Attach to the outside of the cell membrane and help
your body recognize cells.
Cholesterol
• Stabilizes the cell
membrane and keeps it
fluid at low temperatures
Surface Area to Volume Ratio
Why are cells so small?
• The smaller the cell, the higher the
surface area to volume ratio (SA/V).
– The surface area of the cell (cell
membrane) determines how fast
materials can move in and out.
– The volume determines how
much material can fit inside of the
cell
• The higher the surface area to
volume ratio, the faster materials
can move in and out of the cell.
Surface Area to Volume Ratio
Surface Area to Volume Ratio
A high surface area to volume ratio is important for
maintaining homeostasis
• Example
– Lungs are filled with small sacs called alveoli.
– Alveoli increase the surface area
of your lungs which allows
oxygen to enter your body more
quickly.
– Smoking can destroy the alveoli,
decreasing the surface area of
your lungs, preventing your
body from absorbing oxygen.
Movement Through the
Membrane
Passive Transport: Any type of
movement of molecules that does
not require the use of energy.
3 types of passive transport move
molecules in and out of the cell
membrane.
1. Diffusion
2. Facilitated Diffusion
3. Osmosis
Movement Through the Membrane
Diffusion: The movement of molecules from an area of
higher concentration to an area of lower
concentration, until an equilibrium is reached.
What causes the molecules to move?
– Heat causes the molecules to moves randomly
– Molecules collide with each other and spread out.
– The greater the temperature or the greater the
concentration of molecules the faster the rate of
diffusion.
Movement Through the Membrane
Diffusion in cells:
– Only small and nonpolar molecules can diffuse
through the phospholipid bilayer.
– Equilibrium: Diffusion will stop when the
concentration of molecules inside the cell is equal
to the concentration of molecules outside the cell
Can it pass through the
phospholipid bilayer:
O2, H2O, Proteins, H+,
Fatty Acids
Movement Through the Membrane
Facilitated Diffusion: Molecules diffuse through the
cell membrane through proteins, not through the
phospholipid bilayer.
• Carrier Proteins will bind to molecule,
change shape and release molecule
on other side of the membrane.
• Channel Proteins – tunnel shaped
proteins in the membrane.
– Aquaporins are proteins in the cell
membrane that allow water
molecules to move in and out of the
cell.
Osmosis
Osmosis: the diffusion of water molecules
through a selectively permeable membrane.
• Water moves
from high to low
concentration.
• Is the membrane
permeable to the
water or the
solute? How
can you tell?
Osmosis
Tonicity: Determines the direction of
water movement due to osmosis.
• Hypertonic: the solution with a
greater concentration of the solute.
– A cell would lose water and shrink
if it is place in a hypertonic
solution.
– Plasmolysis: when the cell
membrane of a plant cell pulls
away from the cell wall.
– Causes the plant to wilt due to a
lack of osmotic pressure.
Osmosis
• Hypotonic: the solution with the
lesser concentration of the solute.
– A cell would gain water and
swell if it is place in a hypotonic
solution.
– Cytolysis: when a cell bursts
because of osmotic pressure.
– Turgor Pressure: the pressure
created when the cell
membrane pushes against a cell
wall.
Osmosis
• Isotonic: concentration of
the solute is the same on
both sides of the membrane
(equilibrium).
– Cells will stay the same size
in a isotonic solution, there is
no net movement of water
molecules.
Active Transport
Which diagram represents passive
transport and which one
represents active transport?
How is active transport different
then passive transport?
Active Transport
Active Transport: is the movement of materials
through a cell membrane using energy.
• Molecules move against a concentration gradient;
from low concentration to high concentration.
• Proteins molecules
in the cell membrane
use energy to pump
ions and small
molecules from low
concentration to high
concentration.
Active Transport
Exocytosis: An energy using process by which large
molecules move outside the cell without passing
through the cell membrane
– Molecule inside vesicles inside the cell move
towards the membrane.
– The vesicle fuses
with the cell
membrane and the
molecules are
expelled outside of
the cell.
Active Transport
Endocytosis: an energy-using process by which cells
absorb molecules (such as proteins) by engulfing them.
• Incoming particles are surrounded by a piece of the
cell membrane that forms a vesicle.
– Phagocytosis: The incoming substance is a solid
particle.
– Pinocytosis: The incoming substance is a liquid.
Homeostasis and Cells
• Homeostasis: relatively
constant internal physical and
chemical conditions.
• To maintain homeostasis,
unicellular organisms grow,
respond to environment,
transform energy and
reproduce.
• Explain how homeostasis and
cell transport (diffusion, osmosis
and active transport) are
related?
Specialized Cells
• Cells with a specific
structure and function and
are found in multi-celled
organisms.
• We have blood cells, brain
cells, bone cells, liver cell,
skin cells, etc.
• Why can specialized cells
only be found in
mulitcellular organisms?
The cell membrane
Function:
• Regulates what enters
and leaves the cell.
Selective permeability
• Some substances can
pass across the cell
membrane and others
can’t
Levels of Organization
Organ
System
Organs
Tissues
Cells
Levels of Organization
Tissue: A group of similar cells
working together.
• What are some examples?
Organ: A groups of tissues working
together to perform a specific job.
• What are some examples?
Organ System: Many organs working
together to do a specific job.
• Can you name all 10 organ
systems?
Cell Transport Problems
• In terms of glucose
concentration, which side
is a hypotonic solution?
• What could you say about
the water concentration
on side A relative to side
B?
• Which molecule(s) will
move across the
membrane and in which
net direction(s)?
What will happen to the water levels when the
system reaches equilibrium?
Cell Transport Problems
• What is going to
happen to solute A
(both direction and
percentages)?
• What is going to
happen to solute Z
(both direction and
percentages)?
• What is going to
happen to the water
levels, specifically?
Cellular Communication
• Cellular Junctions –
connections from cell
to cell to send
chemical signals
• Ex: Electrical signals
are passed through a
gap junction to make
a heartbeat.
• Receptors – Areas on
the cell that the
signaling molecule
can bind
How is the structure of
the nerve related to
its function?
7.3-7.4 Wrap up Questions
1.
2.
3.
4.
5.
Explain Diffusion
Explain Osmosis
What does selectively permeable mean?
What is facilitated diffusion?
Are phospholipids the only molecules in a
cell membrane?
6. What is active transport?
7. What are tissues, organs, and organ
systems? Give Examples
Structure
Phospholipid Bilayer: a double-layer sheet that
makes up nearly all of membranes.
• Polar Head
– Hydrophilic
“Water Loving”
• Lipid Tails
– Hydrophobic
“Water Fearing”
Building A Membrane
1. Using the materials provided to you, build a
model of a cell membrane.
2. Make a key explaining what pieces represent
the Polar Head, Lipid Tails, Protein Molecules,
and Carbohydrate Molecules
3. Have your teacher check your model before
you eat it!
Other membrane structures
• Protein molecules
– Form channels and pumps
that help to move material
across the membrane
• Carbohydrate molecules
– Act like ID markers for the
cell
– Basis for blood typing: the
antigens on the blood cells
(A, B, AB) are carbohydrate
chains
Osmosis Practice
The cytoplasm of Elodea cells is composed of
about 70% water molecules and 30% other
kinds of molecules.
What happens when the Elodea cells are put into
a liquid that is
a. 50 percent water.
b. 70 percent water.
c. 100 percent water .