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7-3 Cell Boundaries
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7-3 Cell Boundaries
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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7-3 Cell Boundaries
Cell Membrane
Cell Membrane
The cell membrane regulates what enters
and leaves the cell which maintains
homeostasis and also provides protection
and support for cells without a cell wall.
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7-3 Cell Boundaries
Cell Membrane
Cell Membrane
Outside of
cell
Proteins
Carbohydrate
chains
Cell
membrane
Inside of cell
(cytoplasm)
Protein
channel
Lipid bilayer
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7-3 Cell Boundaries
7-3 Cell Boundaries
Composition of Cell Membrane:
1. Lipid Bilayer: double layered sheet which is a
flexible structure that forms a strong barrier between
the cell and its surroundings
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7-3 Cell Boundaries
Phospholipids contain one phosphate group,
one glycerol, and 2 fatty acids.
Main component of cell
membranes.
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7-3 Cell Boundaries
7-3 Cell Boundaries
Composition of Cell Membrane:
2. Protein molecules are embedded in the lipid
bilayer. Carbohydrates attached to the proteins act
like chemical identification cards allowing cells to
identify each other. Some proteins form channels
and pumps to help move material into and out of the
cell
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7-3 Cell Boundaries
3. Most cells are selectively permeable, meaning that
some substances can pass across them and some
cannot.
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7-3 Cell Boundaries
Cell Walls
Cell Wall
Cell walls are found in plants, algae, fungi, some
protists and many prokaryotes.
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7-3 Cell Boundaries
• Cell walls have openings or pores to
allow water, oxygen, carbon dioxide and
other materials to pass through. They lie
outside the cell membrane. They give
structure and support – the thicker the
cell wall the more stiff or rigid the
structure will be.
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7-3 Cell Boundaries
Cell Walls
What is the main function of the cell wall?
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7-3 Cell Boundaries
Diffusion Through Cell Boundaries
Measuring Concentration
A solution is a mixture of two or more substances.
The substances dissolved in the solution are called
solutes.
The concentration of a solution is the mass of
solute in a given volume of solution, or
mass/volume.
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7-3 Cell Boundaries
Diffusion Through Cell Boundaries
What happens during diffusion?
- Molecules MOVE!!!!
- Molecules are always in motion. They
move around and bump into each other.
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7-3 Cell Boundaries
Diffusion Through Cell Boundaries
Diffusion
Particles in a solution tend to move from an area
where they are more concentrated to an area
where they are less concentrated.
This process is called diffusion.
When the concentration of the solute inside the cell
is the same as the concentration outside the cell,
the cell and its environment has reached
equilibrium.
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7-3 Cell Boundaries
Diffusion Through Cell Boundaries
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7-3 Cell Boundaries
Osmosis
Osmosis
Osmosis is the diffusion of water through a
selectively permeable membrane.
Whenever water moves, into or out of the cell, we
call it osmosis.
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7-3 Cell Boundaries
Osmosis
How Osmosis Works
Dilute sugar
solution
(Water more
concentrated)
Concentrated
sugar solution
(Water less
concentrated)
Sugar
molecules
Selectively
permeable
membrane (only
water can move)
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Movement of
water from
high
concentration Slide
to low
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concentration
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7-3 Cell Boundaries
Osmosis
In osmosis, water tends to diffuse from a highly
concentrated region to a less concentrated region.
If you compare solutions, three terms can be used to
describe the concentrations of the solution due to the
amount of solute dissolved in the water:
- hypertonic (“above strength”)more solute/less
water.
- hypotonic (“below strength”)less solute/more
water.
- isotonic (”same strength”)same amount of solute
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7-3 Cell Boundaries
Osmosis
Osmotic Pressure
Osmosis exerts a pressure known as osmotic
pressure on the cell compared to its environment.
Remember both inside the cell and outside the cell
there is a concentration. It is the water that
moves!!
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7-3 Cell Boundaries
Osmosis
What is osmosis?
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7-3 Cell Boundaries
Facilitated Diffusion
Glucose
molecules
Facilitated Diffusion requires no
energy because molecules still
move from a higher to a lower
High Concentration
concentration. Each protein
channel is specific for the type
of molecule that it allows to
enter the cell.
Low Concentration
Protein
channel
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Active Transport
Active Transport
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.
Active transport
requires energy!
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7-3 Cell Boundaries
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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7-3 Cell Boundaries
Active Transport
Molecular Transport
Molecule to be carried
Active
Transport
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7-3 Cell Boundaries
Active Transport
Endocytosis and Exocytosis
Endocytosis is the process of taking material into
the cell.
Two examples of endocytosis are:
• Phagocytosis (takes in food)
• Pinocytosis (takes in water)
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7-3 Cell Boundaries
• During exocytosis, materials are forced out of the
cell.
• “Exo” = exit
• “Cyto” = cell
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7-3
Unlike a cell wall, a cell membrane
a. is composed of a lipid bilayer.
b. provides rigid support for the surrounding
cell.
c. allows most small molecules and ions to
pass through easily.
d. is found only in plants, fungi, algae, and
many prokaryotes.
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7-3
The concentration of a solution is defined as the
a. volume of solute in a given mass of solution.
b. mass of solute in a given volume of solution.
c. mass of solution in a given volume of solute.
d. volume of solution in a given mass of solute.
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7-3
If a substance is more highly concentrated
outside the cell than inside the cell and the
substance can move through the cell membrane,
the substance will
a. move by diffusion from inside the cell to
outside.
b. remain in high concentration outside the cell.
c. move by diffusion from outside to inside the
cell.
d. cause water to enter the cell by osmosis.
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7-3
The movement of materials in a cell against a
concentration difference is called
a. facilitated diffusion.
b. active transport.
c. osmosis.
d. diffusion.
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7-3
The process by which molecules diffuse across
a membrane through protein channels is called
a. active transport.
b. endocytosis.
c. facilitated diffusion.
d. osmosis.
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