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7-3 Cell Boundaries
7-3 Cell Boundaries
Photo Credit: © Quest/Science Photo Library/Photo Researchers, Inc.
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7-3 Cell Boundaries
All cells are surrounded by a thin, flexible barrier known as the cell membrane.
Many cells also produce a strong supporting layer around the membrane known as a cell wall.
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Cell Membrane
Cell Membrane
The cell membrane regulates what enters and leaves the cell and also provides protection and support.
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4. Functions of a cell membrane
Isolate the cytoplasm from the external environment
Regulate the exchange of substances
Communicate with other cells
Identification
The membrane is selectively permeable (also called semipermeable ) 
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Cell Membrane
Cell Membrane
Outside of cell
Carbohydrate chains
Proteins
Cell membrane
The cell membrane regulates what enters and leaves the cell.
Inside of cell (cytoplasm)
Protein channel
Lipid bilayer
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Cholesterol - stiffens the membrane by connecting phospholipids Glycolipids - signal molecules Glycoproteins - have an attached chain of sugar (antibodies)
Proteins embedded in membrane serve different functions
Channel Proteins - form small openings for molecules to diffuse through Carrier Proteins- binding site on protein surface "grabs" certain molecules and pulls them into the cell,
Receptor Proteins - molecular triggers that set off cell responses Cell Recognition Proteins - ID tags, to identify cells to the body's immune system Enzymatic Proteins - carry out metabolic reactions
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Cell Walls
What is the main function of the cell wall?
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Cells walls provide support and protection
Most are made from carbohydrates and protein
Cellulose
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Cell Walls
Cell Wall
Cell walls are found in plants, algae, fungi, and many prokaryotes.
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11. Diffusing through Cell Boundaries
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12. Diffusion Through Cell Boundaries
Cells must regulate the movement of molecules from the liquid on the outside of the membrane to the liquid on the other side.
The process of moving molecules or particles is called diffusion (movement of particles from high to low concentration)
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13. Diffusion Through Cell Boundaries
What happens during diffusion?
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14. Diffusion Through Cell Boundaries
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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15. Diffusion Move from HIGH to LOW concentration diffusion of water
passive transport
no energy needed
diffusion of water
diffusion
osmosis

16. Simple Diffusion Move from HIGH to LOW fat fat fat inside cell fat fat
Which way will fat move?
inside cell
fat
fat
fat
LOW
HIGH
fat
outside cell
fat
fat
fat
fat
fat
fat
fat

17. Facilitated Diffusion
Glucose molecules
Facilitated Diffusion
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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18. Facilitated Diffusion
Move from HIGH to LOW through a channel
sugar
sugar
sugar
sugar
inside cell
sugar
sugar
LOW
Which way will sugar move?
HIGH
outside cell
sugar
sugar
sugar
sugar
sugar
sugar
sugar

19. Simple vs. facilitated diffusion
simple diffusion
facilitated diffusion
lipid
inside cell
outside cell
inside cell
outside cell
H2O
protein channel
H2O

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Osmosis
What is osmosis?
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Osmosis
Osmosis
Osmosis is the diffusion of water through a selectively permeable membrane.
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23. 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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Water tends to diffuse from a highly concentrated region to a less concentrated region.
If you compare two solutions, three terms can be used to describe the concentrations:
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Osmosis
hypertonic
a hypotonic solution has a lower concentration of solutes outside the cell than inside the cell.
In an attempt to balance the concentrations of solutes inside and outside the cell, water will rush into the cell, and can cause it to burst
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hypotonic
a hypotonic solution has a lower concentration of solutes outside the cell than inside the cell.
In an attempt to balance the concentrations of solutes inside and outside the cell, water will rush into the cell, and can cause it to burst.
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isotonic
Equal concentrations on the inside and outside of the cell
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Osmosis
Osmotic Pressure 
Osmosis exerts a pressure known as osmotic pressure on the hypertonic side of a selectively permeable membrane.
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Active Transport
Active Transport
Sometimes cells move materials in the opposite direction from which the materials would normally move (Low to High concentration) —that is against a concentration difference. This process is known as active transport.
Active transport requires energy.
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Active Transport
Molecular Transport
In active transport, small molecules and ions are carried across membranes by proteins in the membrane.
Energy use in these systems enables cells to concentrate substances in a particular location, even when diffusion might move them in the opposite direction.
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Active Transport
Molecular Transport
Molecule to be carried
Active Transport
Active transport of particles against a concentration difference requires transport proteins and energy.
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Active Transport
Endocytosis and Exocytosis 
Endocytosis is the process of taking material into the cell.
Two examples of endocytosis are:
Phagocytosis – solid material
pinocytosis-liquid material
During exocytosis, materials are forced out of the cell.
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osmosis
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