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 and 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.
•
•
•
•
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 concentration gradient.
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.