1. Outline 7-3: Cell Boundaries
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7-3 Cell Boundaries
I. Cell Boundaries
A. Cell membrane
1. Thin, flexible barrier around cells
B. Cell Wall
1. Strong supporting layer around the membrane
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7-3 Cell Boundaries
II. Cell Membrane Functions
A. Regulates what enters and leaves cells
B. Provides support
C. Provides protection
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7-3 Cell Boundaries
III. Cell Membrane Structure
A. Cell membranes are fluid
1. This fluidity is caused by lipids which make up the basic structure of the membrane
2. Proteins are embedded in the lipids and are free to move around
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7-3 Cell Boundaries
B. Fluid Mosaic Model of Membrane
1. Main structure is a double layer of special lipids called phospholipids.
a. Phospholipids are made of a:
● phosphate head which is polar
● fatty acid tails which are non-polar
b. Heads are attracted to water (hydrophilic)
c. Tails are repelled by water (hydrophobic)
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d. Phospholipids arrange themselves into a double layer known as the phospholipid bilayer.
● Nonpolar tails are located on inside layer.
● Polar phosphate heads are found on the outside
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7-3 Cell Boundaries
2. Embedded Proteins:
a. Different types of proteins are found embedded throughout the lipid bilayer
● They are free to float around
b. Proteins come in different types & play different roles:
● Channel, or transport, proteins – help to bring substances into & out of cell
● Marker proteins – help to identify cells like chemical ID cards
● Receptor proteins – bind to substances outside of cell
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9. Fluid Mosaic Model of the Cell Membrane
Outside of cell
Marker proteins
Receptor
Carbohydrate chains
Proteins
Cell membrane
The cell membrane regulates what enters and leaves the cell.
Inside of cell (cytoplasm)
Protein channel
Lipid bilayer
Transport protein
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7-3 Cell Boundaries
IV. Cell Wall Structure & Function
A. Structure:
1. Found outside of cell membrane
2. Composed of cellulose in plants
B. Function:
1. Provide support & protection for cell
2. Found in cells of plants, algae, fungi & many prokaryotes
3. Porous enough to allow small molecules to pass through easily (water, O2, CO2, etc.)
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Plant Cell Walls
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12. Diffusion Through Cell Boundaries
V. Diffusion Through Cell Boundaries
A. Every living cell exists in a liquid environment.
1. Cell membrane regulates movement of dissolved molecules from the liquid on one side of the membrane to the liquid on the other side.
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13. Diffusion Through Cell Boundaries
B. Measuring Concentration of Solutions 
1. Concentration is the mass of solute in a given volume of solution, or mass/volume (grams/liter).
2. Frequently shown as % of solute or in units known as molarity (M).
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14. Diffusion Through Cell Boundaries
C. Diffusion
1. Particles tend to move from areas where they are more concentrated to areas where they are less concentrated. This is called diffusion.
2. When the concentration of the solute is the same throughout a system, the system has reached equilibrium.
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15. Diffusion Through Cell Boundaries
Solute concentration higher on one side of membrane
Once equilibrium is reached, solute moves in both directions
Solute will move across membrane towards lower concentration
Diffusion is the process by which molecules of a substance move from areas of higher concentration to areas of lower concentration. Diffusion does not require the cell to use energy.
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16. Diffusion Through Cell Boundaries
3. Diffusion depends upon random particle movements.
a. Therefore, substances diffuse across membranes without requiring the cell to use energy.
b. This is called passive transport.
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17. Diffusion Through Cell Boundaries
D. Membrane Permeability
1. If a substance is able to diffuse across a membrane, the membrane is said to be permeable to it.
2. Membranes are impermeable to many substances.
3. Most membranes are selectively permeable, meaning that only some substances can cross them.
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18. Diffusion Through Cell Boundaries
E. Osmosis
1. Water molecules are small enough to diffuse through cell membranes
2. The diffusion of water through a selectively permeable membrane is called osmosis.
3. Water tends to move from regions with a higher concentration of water to regions with a lower concentration of water.
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19. Selectively permeable membrane
Osmosis
How Osmosis Works
Dilute sugar solution (Water more concentrated)
Concentrated sugar solution (Water less concentrated)
Sugar molecules
Osmosis is the diffusion of water through a selectively permeable membrane. In the first beaker, water is more concentrated on the right side of the membrane. As a result, the water diffuses (as shown in the second beaker) to the area of lower concentration.
Movement of water
Selectively permeable membrane
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Osmosis
4. When you compare two solutions:
a. The region with a higher concentration of solute will have a lower concentration of water. This will be called a hypertonic solution.
b. The region with a lower concentration of a solute will have a higher concentration of water. This will be called a hypotonic solution.
c. When concentrations are the same on both sides of a membrane this is called an isotonic solution.
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21. Selectively permeable membrane
Osmosis
Water will move towards hypertonic side
Hypertonic Side
Hypotonic side
Dilute sugar solution (Water more concentrated)
Concentrated sugar solution (Water less concentrated)
Sugar molecules
How Osmosis Works
Osmosis is the diffusion of water through a selectively permeable membrane. In the first beaker, water is more concentrated on the right side of the membrane. As a result, the water diffuses (as shown in the second beaker) to the area of lower concentration.
Movement of water
Selectively permeable membrane
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Osmosis
5. If you put cells in these 3 kinds of solutions you will get different responses:
a. Hypertonic solutions
• Water will move from inside cell to outside and the cell will shrink
b. Hypotonic solutions
• Water will move from outside of cell to inside and the cell will swell and possibly burst
c. Isotonic solutions
• Water will diffuse in & out of cell equally and cell will stay the same size
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Osmosis
Solutions outside of cells are:
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24. Facilitated Diffusion
Do Osmosis Situations
from transparencies now
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25. Facilitated Diffusion
F. Facilitated Diffusion
1. Cell membranes have protein channels that act as carriers, making it easy for certain molecules to cross.
2. The movement of specific molecules across cell membranes through protein channels, without the use of extra energy, is known as facilitated diffusion.
3. Molecules move from area of higher concentration to lower concentration
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26. Facilitated Diffusion
Glucose molecules
Facilitated Diffusion
This is how glucose enters cells
During facilitated diffusion, molecules, such as glucose, that cannot diffuse across the cell membrane’s lipid bilayer on their own move through protein channels instead.
Protein channel
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Active Transport
G. Active Transport
1. Sometimes cells move materials in the opposite direction from which the materials would normally move—that is against a concentration difference. This process is known as active transport.
2. Active transport requires energy.
3. Small molecules and ions are carried across membranes by proteins in the membrane.
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Active Transport
Molecule to be carried
Active Transport
Active transport of particles against a concentration difference requires transport proteins and energy.
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4. Sodium-potassium pump
a. This transports 3 sodium ions, Na+, OUT of cell
b. Transports 2 potassium ions, K+, INTO cell
c. Both of these are transported AGAINST their concentration gradients
d. Energy is supplied by ATP
e. This pump can use up much of the
ATP in a given cell
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Sodium-potassium pump animation
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Active Transport
H. Using Membranes in Active Transport
1. Endocytosis
a. Process of taking material into the cell by means of infoldings, or pockets, of the cell membrane.
b. The pocket breaks loose from the outer portion of the cell membrane and forms a vacuole, or vesicle, within the cytoplasm.
c. This takes considerable amounts of energy.
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Endocytosis
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Active Transport
d. Two examples of endocytosis are:
Phagocytosis – bringing in solid food
Pinocytosis – bringing in liquid
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PHAGOCYTOSIS
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Active Transport
2. Exocytosis
a. Many cells also release large amounts of material from the cell.
b. During exocytosis, the membrane of the vacuole surrounding the material fuses with the cell membrane, forcing the contents out of the cell.
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