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7-3 Cell Boundaries &
Cell Transport
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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Homeostasis – Maintaining a Balance
Cells must keep the proper concentration of nutrients and water
and eliminate wastes. The plasma membrane is selectively
permeable – it will allow some things to pass through, while
blocking other things.
Cell Membrane
The cell membrane regulates what enters and leaves the cell and
also provides protection and support.
Outside of
cell
Proteins
Carbohydrate
chains
Cell
membrane
Inside of cell
(cytoplasm)
Protein
channel
Lipid bilayer
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Cell Wall
Cell walls are found in plants, algae,
fungi, and many prokaryotes. The cell
wall is used for support and to
maintain structure.
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Discuss the function of the cell
membrane and the cell wall
Write your answer in box #1
What Does Selective Permeability mean?
• It is a membrane that only certain substances can
cross.
• Ex: A window screen is selectively permeable because
it lets dust and air in, but not flies
• A coffee filter is selectively permeable because it lets
water through but not the coffee grounds
The cell membrane is said to be selectively permeable or
semipermeable.
This means that some substances are allowed to move freely
across the cell membrane whereas other are not.
Discuss…
why is the cell membrane is
semipermeable?
Write your answer in box #2
Cell Membrane Structure
A phospholipid molecule consists of a
phosphate head with two lipid tails, which
are long chains of carbon and hydrogen
atoms. The phosphate head is polar,
which means it has a slight charge. The
lipid tails are nonpolar, which means they
have no charge.
inside cell
“attracted to water”
phosphate
lipid
“repelled by water”
outside cell
Important Point: The lipid bilayer is not solid. The individual phospholipid
molecules float within the bilayer. Because of this semi-fluid state, very tiny
molecules like water, carbon dioxide, and oxygen can freely pass across the lipid
bilayer. Lipid Mosaic Model.
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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Two Methods of Transport Across Cell
Membrane:
I.
Passive Transport
a. Diffusion (simple)
b. Osmosis
c. Facilitated diffusion
II. Active Transport
a. Transport proteins
b. Endocytosis
c. Exocytosis
Discuss…
Two methods of transport
across a cell membrane.
Passive Transport
• Does not require the use of energy.
• The most common method of passive
transport is called diffusion.
• Movement of that substance across the
cell membrane will continue until
equilibrium is established.
Diffusion Through Cell Boundaries
Particles in a solution tend to move from an
area of high concentration to an area of low
concentration
This process is called diffusion.
When the concentration of the solute is the same
throughout a system, the system has reached
equilibrium.
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Concentration Gradient
• when a substances flows from a
high concentration to a low
concentration they flow with the
concentration gradient
Diffusion Through Cell Boundaries
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Discuss…
How are diffusion through the cell
membrane and the concentration
gradient related?
Write your answer in box #3
Osmosis
Osmosis is the diffusion of water
through a selectively permeable
membrane.
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How Osmosis Works
Dilute sugar
solution
(Water more
concentrated)
Concentrated
sugar solution
(Water less
concentrated)
Sugar
molecules
Selectively
permeable
membrane
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Movement of
water
Osmosis – A type of simple diffusion that refers specifically to the
movement of water across the cell membrane.
Important Point: Water has a tendency to move from areas of lower
solute concentration (more pure water) to areas of higher solute
concentration (less pure water).
Discuss…
What is osmosis?
Is osmosis flowing with the concentration
gradient or against the concentration
gradient? Explain.
Write your answer in box #4
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:
hypertonic (“above strength”).
hypotonic (“below strength”).
isotonic (”same strength”)
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Osmotic Pressure
Osmosis exerts a pressure known as osmotic
pressure on the hypertonic side of a selectively
permeable membrane.
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Three Types of solutions:
- Hypertonic solutions
- Hypotonic solutions
- Isotonic solutions
Osmosis in Red Blood Cells
Isotonic
Hypotonic
Hypertonic
Discuss…
How does osmotic pressure
(tonicity) affects cells?
Isotonic Solution
NO NET
MOVEMENT OF
H2O (equal amounts
entering & leaving)
Hypotonic
Solution
Cell Bursts
Hypertonic
Solution
Wilting
Facilitated Diffusion
Polar molecules or molecules
that may be too large to pass
through the lipid bilayer may
require the use of channel
proteins which provide a
pathway across the cell
membrane.
Discuss…
Compare and contrast diffusion
(simple) and facilitated diffusion.
Write your answer in box # 5
Active Transport
- Requires the use of energy.
- Active transport (energy) is required
whenever a substance needs to move
against the concentration gradient.
The direction of movement of the
substance is from areas of low
concentration to areas of high
concentration.
- Active transport usually involves the
use of specialized carrier proteins
embedded in the cell membrane that
function as pumps.
Discuss…
The differences between passive
transport and active transport.
Write your answer on box #6
Channel Proteins
• Need to make “doors” through membrane
• protein channels allow substances in & out
• specific channels allow specific material in & out
• H2O channel, salt channel, sugar channel, etc.
inside cell
outside
waste cellsalt
H2O
aa
sugar
Active Transport
Endocytosis and Exocytosis
Endocytosis is the process of taking material into the
cell.
Two examples of endocytosis are:
• phagocytosis
• pinocytosis
During Exocytosis, materials are forced out of the cell.
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- Two other types of active transport:
Exocytosis – The exporting of large molecules or large
amounts of a substance out of the cell.
Endocytosis – The importing of large molecules or large
amounts of a substance into the cell.
Endocytosis
Exocytosis
Large molecules that are manufactured in the cell are
released through the cell membrane.
Inside Cell
Cell environment
Discuss…
The 3 types of active transport.
Write your answer on box #7
Three Forms of Transport Across the Membrane
40
7-3
Unlike a cell wall, a cell membrane
•
•
•
•
is composed of a lipid bilayer.
provides rigid support for the surrounding cell.
allows most small molecules and ions to pass through easily.
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
•
•
•
•
volume of solute in a given mass of solution.
mass of solute in a given volume of solution.
mass of solution in a given volume of solute.
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
•
move by diffusion from inside the cell to outside.
•
remain in high concentration outside the cell.
•
move by diffusion from outside to inside the cell.
•
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
•
•
•
•
facilitated diffusion.
active transport.
osmosis.
diffusion.
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7-3
The process by which molecules diffuse across a membrane through protein
channels is called
•
•
•
•
active transport.
endocytosis.
facilitated diffusion.
osmosis.
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