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Cell Transport
• How things move in and out of the cell
• General Biology (Chapter 7)
• Honors Biology (Chapter 5)
Cell Transport Overview
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Homeostasis
Understanding concentrations
Structure of the cell membrane
Types of Transport
Types of Passive Transport
Types of Active Transport
Regulating Transport in Cells
Homeostasis
• Homeostasis – maintaining a stable internal
environment
• Applies to organisms as a whole
• Also applies to individual cells
• Transport is an important part of a cell’s
ability to maintain homeostasis
Concentration of Solutions
• Solution - a homogenous mixture
throughout which two or more substances
are uniformly dispersed
• solute - substance that is being dissolved in
a solution, Example: Kool-Aid, sugar
• solvent - the substance that is being
dissolved, Example: water
Concentration of Solutions
• Concentration gradient - the difference in
concentration of molecules across a distance
• Concentration - refers to the amount of
solute that is present in a certain amount of
solvent
• Particles naturally move from areas of high
concentration to areas of low concentration
Concentration of Solutions
• Particles that move from areas of HIGH TO
LOW concentrations are said to move
WITH the concentration gradient
• Particles that move in the opposite
directions, from areas of LOW TO HIGH
concentration, are said to move AGAINST
the concentration gradient
The cell membrane
• The cell membrane
controls what enters and
leaves the cell (cell
transport)
• The unique combination
of molecules that make
up the cell membrane
allow it to “choose”
what enters and leaves
the cell
Cell membrane – con’t
• The fluid mosaic model of
the cell membrane – says
that all of the different
molecules in the cell
membrane work together
to do one function
(regulate transport), just as
the different tiles in a
mosaic come together to
make a picture.
Cell membrane – con’t
• The cell membrane
is primarily made
up of a double layer
of phospholipids
• For this reason it is
often called a lipid
bilayer
Cell membrane – con’t
• A phospholipid is a
molecule that is made up of
two regions.
• The “head” of the
phospholipid is polar and,
therefore, hydrophilic
(water-loving)
• The “tails” of the
phospholipid are nonpolar
and, therefore, hydrophobic
(water-fearing)
Cell membrane – con’t
• Water is found in the
cytoplasm inside the cell
and in the extracellular
fluid outside the cell
• So phospholipids arrange
themselves so that the
hydrophilic heads are
touching the water and the
hydrophobic tails are
tucked safely inside away
from the water
Cell membrane – con’t
• Only small, charged particles can move
through the lipid bilayer
• Examples include oxygen and carbon
dioxide
• Nonpolar and large molecules must enter or
leave the cell through proteins.
Cell membrane – con’t
• The proteins in the cell
serve as “doorways”
for substances that
cannot enter through
the lipid bilayer.
• They can be used for
facilitated diffusion or
active transport
Cell membrane – con’t
• Transport proteins have a certain shape and only
bind with molecules that fit their shape.
• Therefore, if a molecule cannot enter or leave the
cell through the lipid bilayer and it doesn’t fit any
of the transport proteins, it cannot pass through the
membrane
• This is what makes the cell membrane semipermeable, or selectively permeable; allowing
some things to pass, put not others.
Cell membrane – con’t
• Cholesterol molecules in animal cells help
stabilize the cell membrane by anchoring the
phospholipids in place, which keeps the cell
membrane from falling apart ( the cell wall
does this in bacterial, plant, and fungal cells)
• Cholesterol also keeps phospholipids away
from one another enough to keep them from
solidifying when temperatures get cold.
Cell membrane – con’t
• Carbohydrate chains that are attached to
different phospholipids and proteins in the
cell membrane act as identification
molecules.
• This allows molecules that need to enter the
cell to find their “doorways” (proteins) into
the cell
Types of Cell Transport
Passive transport - transport of substances
across the cell membrane that does not
require energy (moves with the
concentration gradient)
Active Transport - transport of substances
across the cell membrane that does require
energy (moves against the concentration
gradient)
3 Types of Passive Transport
• Diffusion - the movement of
molecules/atoms from areas of high
concentration to low concentrations
• Osmosis - the diffusion of water across a
membrane
• Facilitated diffusion - diffusion of large or
charged molecules through the cell
membrane by transport proteins
Diffusion
• Occurs naturally without energy
• Example: If you place a drop of food
coloring in a glass of water, the molecules
of food coloring will naturally move away
from each other to other parts of the glass
where there is no food coloring (they are
moving from areas of high concentration of
food coloring to areas of low concentration)
Osmosis
• Water moves from where there is a more
water to where there is less water.
• Requires no energy (passive transport)
• Movement of water has to do with
concentrations of solutions involved
• Concentration terms are relative (two or
more solutions must be compared to say
which one is more or less concentrated
Types of Solutions involved in
Osmosis
• Hypotonic
• Hypertonic
• Isotonic
Hypotonic Solution
10% solute
90% water
30% solute
70% water
• A solution that has
less solute than the
cell
• When a cell is placed
in a hypotonic
solution, water moves
into the cell
• lysis - when a cell
takes in water until it
bursts
Hypertonic Solution
60% solute
40% water
30% solute
70% water
• A solution that has
more solute than the
cell
• When a cell is placed
in a hypertonic
solution, water moves
out of the cell
• crenation - the cell
shrinks and becomes
prickly as it loses
water
Isotonic Solution
30% solute
70% water
30% solute
70% water
• A solution that has the
same amount of solute
as the cell
• When a cell is placed
in an istonic solution,
water moves in and
out of the cell at the
same rate
• there is no net
movement of water
Facilitated Diffusion
• Particles move from high to low
concentration
• they are too large to squeeze between
phospholipids, so they must go through
carrier (transport, or channel) proteins
• the proteins are very specific. Only one type
of molecule can enter each kind
• therefore, there are hundreds of different
carrier proteins
Types of Active Transport
• Active transport
• sodium-potassium pump
• endocytosis (pinocytosis and phagocytosis)
• exocytosis
Steps of Active Transport
• A molecule/atom approaches the cell
membrane
• the molecule enters a carrier protein as far
as it can go
• the carrier protein changes shape around the
molecule and pushes it into the cell
• the carrier protein returns to its original
shape in order to repeat the process again
with another molecule
Sodium-Potassium Pump
• A common example of an active transport
pump
• 3 sodium ions are pumped out of the cell,
• 2 potassium ions are pumped into the cell
• Next of Kin (mnemonic to help you
remember)
Endocytosis
• When the cell takes in large molecules from
outside the cell by moving the cell
membrane around the particle and forming
a vessicle around the particle as it brings it
into the cell
• phagocytosis - endocytosis of a solid
• pinocytosis - endocytosis of a liquid
Exocytosis
• When the cell expels a molecule from inside
the cell by moving a vessicle that contains
the molecule to the edge of the cell
membrane
• Then, the membrane of the vessicle joins
the cell membrane and pushes the particle
out of the cell