CHAPTER 8
CELLS & THEIR ENVIRONMENT
• Review: Cell (or plasma) membrane – is composed of a
lipid bilayer with embedded proteins that float. The cell
membrane is a selectively permeable membrane, which
means it controls what enters and what leaves the cell.
Remember that the cell membrane allows to cell to
remain separate from the environment.
• The currently accepted model is the
Fluid Mosaic Model.
• According to this model, the cell membrane
consists of a double layer of phospholipids in
which large protein molecules float.
Cellular Transport
• The cell membrane helps organisms maintain
homeostasis.
• Homeostasis is the maintenance of the internal
environment despite changes in the external
environment.
• Smaller molecules like H20, CO2, and 02 pass easily
through the cell membrane.
• Larger molecules like glucose, amino acids, ions, and
most polar molecules cannot pass through. Ions (which
have a charge—Na+, Cl-) and most polar molecules
cannot diffuse across the cell membrane because they
cannot pass through the nonpolar interior of the lipid
bilayer.
Cellular Two Types of Cellular Transport:
1. Passive Transport
2. Active Transport
• 1. Passive Transport (going down the
concentration gradient)
• The passage of substances through the membrane
from regions of higher concentration into regions
of lower concentrations.
• Movement across the cell membrane that does
NOT require energy from the cell.
Three Types of Passive Transport:
1. Diffusion 2. Osmosis
3. Facilitated Diffusion
In all 3, the movement of a substance is from a
region of higher concentration to regions of
lower concentration
High
Low
Three Types of Passive Transport:
1. Diffusion 2. Osmosis
3. Facilitated Diffusion
• 1. Diffusion
• Simplest type of passive transport.
• If there is a concentration gradient in the solution,
the substance will move from an area of higher
concentration to an
area of lower concentration.
• This movement
results because of
the random movement of particles.
https://www.youtube.com/watch?v=VY0mZUDvbH4
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Three Types of Passive Transport:
1. Diffusion 2. Osmosis
3. Facilitated Diffusion
Diffusion depends on a concentration gradient, which is
the difference in concentration of a substance across a
space.
Equilibrium is a state that exists when the concentration
of a substance is the same throughout a space.
In order for diffusion to occur, there must be a
concentration gradient. Why? If there is no
concentration gradient, then the substance would be in
equilibrium.
Ion channel is a doughnut-shaped transport
protein with a polar pore through which ions
can pass. An ion that enters the pore can
cross the cell membrane without contacting
the nonpolar interior of the lipid bilayer.
Three Types of Passive Transport:
1. Diffusion 2. Osmosis
3. Facilitated Diffusion
• 2. Osmosis
• The diffusion of water molecules through a
selectively permeable membrane from an area of
higher concentration to an area of lower water
concentration.
• In living
organisms,
water enters
and leaves
cells by
osmosis.
Osmosis Cont.
Three Types of Solutions Describing Water Balance in
Regions Surrounding a Cell:
1) Isotonic solution
•
•
No net water movement.
The concentration of dissolved substance or particles inside the
cell is equal to the concentration outside the cell.
•
Example: if a cell is placed in an isotonic solution, water still
moves into and out of a cell at random but there is not new
movement of water.
**The cell stays the same size.**
•
Osmosis
Three Types of Solutions Describing Water Balance in Regions
Surrounding a Cell:
2) Hypotonic solution
•
Water moves into the cell.
•
The concentration of dissolved substance or particles in
water outside the cell is lower than the concentration
inside the cell; therefore, the concentration of water is
higher outside the cell than inside the cell.
•
**The cell swells.**
•
Turgor pressure is the pressure that exists in a cell. As
water diffuses into the cell, the cell swells and its internal
pressure increases.
Osmosis Cont.
Three Types of Solutions Describing Water Balance in
Regions Surrounding a Cell:
3) Hypertonic solution
•
Water moves out of the cell.
•
The concentration of the dissolved substance or
particles is greater in the water outside the cell than
the water inside the cell.
•
**The cells shrinks.**
Plasmolysis is the loss of water from a cell resulting in
a drop in turgor pressure. This causes a plant to wilt.
Three Types of Passive Transport:
1. Diffusion 2. Osmosis
3. Facilitated Diffusion
• 3. Facilitated Diffusion
• The passage of substances through the cell
membrane by means of transport proteins, called
a carrier protein, from regions of higher
concentration into regions of lower concentration.
• A carrier protein is a type of transport protein,
which binds to a specific substance on one side of
the cell membrane, carries the substance across
the cell membrane, and releases it on the other
side. Substances such as amino acids and sugars
(glucose) cross the cell membrane by facilitated
diffusion.
• This process is used for molecules that cannot
diffuse through the cell membrane.
Cellular Two Types of Cellular Transport:
1. Passive Transport and 2.Active Transport
1.
Active Transport **Requires energy from the cell**
• Some substances move in and out of a cell against a
concentration gradient.
-The passage of substances across a membrane using
energy—ATP. Substances are moved from regions of
lower concentration into regions of higher
concentrations.
-Some active-transport processes involve carrier proteins.
Like the carrier proteins used in facilitated diffusion, the
carrier protein used in active transport bind the specific
substances on one side of the cell membrane. But in
active transport, the substances bind to carrier
(channel) proteins where they are in low concentration
and are released where they are higher in concentration.
Thus, carrier proteins in active transport function as
“pumps” the move substances against their
1)
•
Two Types of Active Transport:
1)Endocytosis and 2)Exocytosis
Endocytosis
The process in which cells TAKE IN large materials that
cannot pass through the cell membrane, like proteins
and polysaccharides.
•
The movement of a substance INTO a cell by a vesicle
in called endocytosis.
•
Large molecules, groups of molecules, or even whole
cells are engulfed and enclosed by a portion of the
cell’s membrane. That portion of the membrane then breaks away, and the resulting vesicle with its contents moves into
the inside of the cell.
Endocytosis Cont.
1)
•
Endocytosis
Two Types of Endocytosis:
1) Pinocytosis
o Involves the transport of solutes or fluids.
2) Phagocytosis
o Involves the transport of large particles or whole
cells.
Example: An amoeba engulfs food particles by
phagocytosis.
Phagocytosis
Pinocytosis
Two Types of Active Transport:
1)Endocytosis and 2)Exocytosis
1) Exocytosis
-The process where certain substances are moved
OUT of the cell.
-Vesicles travel to
the cell membrane
and substances inside vesicles are
ejected from the cell.
An example would be
waste products.