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Cellular Transport
Notes
Plasma (Cell) Membrane
Functions:
a. Controls what enters and exits
the cell to maintain an internal
balance called homeostasis
b. It is selectively permeable
c. Provides protection and support
(maintains integrity of cell)
d. Allows cell to receive and respond
to incoming messages
(signal transduction)
Structure of cell membrane
Phospholipid Bilayer –
2 layers of phospholipids
a.Phosphate head is polar
(water loving)= hydrophilic
b.Fatty acid tails non-polar
(water fearing)=
hydrophobic
Fluid Mosaic Model
The phospholipids are not
attached. They float next to
each other
Phospholipid
Lipid Bilayer
Cell membranes have pores (holes) in it
Selectively permeable: Allows some molecules in
and keeps other molecules out
Some materials can go straight through the phospholipids= lipid soluble
This includes small, non-polar molecules
Others are larger or lipid insoluble and have to use a transport protein to
get through
Lipid
insoluble
Pores
Lipid soluble
Structure of the Plasma Membrane
4 parts of plasma membrane:
1. phospholipids
2. proteins
3. cholesterol
4. carbohydrates
Types of Membrane Proteins
1. Integral- into or all the way through membrane
2. Peripheral- attached to the outside of membrane
Cellular Adhesion Molecules (CAMs)
Enable
cells to
stick to
each
other
•
•
•
•
CAMs direct WBC’s to the site of injury
Help cells of embryos attach to other cells and form the placenta
Help establish connections between nerve cells
In cancers- CAMs aren’t working properly and cancer cells don’t get
slowed as they spread around the body
• Arthritis may occur because WBC’s are captured by the wrong CAMs
Cellular Adhesion Molecules (CAMs)
direct white blood cells (WBC’s) to site of injury
1. CAMs called selectins coat the
WBC, slowing it down as it
passes through the blood vessel
and binds to a carbohydrate on
the inner wall
2. CAMs called adhesion receptor
proteins on the lining of the
blood vessel bind to CAMs called
integrins on the WBC
3. The WBC is directed through a
pore space between the
endothelial cells of the wall of the
blood vessel
Types of Cell Transport
Types of Cellular Transport
•Animations of Active
Transport & Passive
Transport
•
Weeee!!
!
Passive Transport
cell doesn’t use energy
1. Diffusion
2. Facilitated Diffusion
3. Osmosis
4. Filtration
•
Goes down the
concentration
gradient
high
low
Active Transport
cell does use energy
1. Solute (protein) Pumps
2. Endocytosis
3. Exocytosis
4. Transcytosis
This is
gonna
be hard
work!!
Goes against the
concentration
gradient
high
low
Passive Transport
•
•
•
cell uses no energy
molecules move randomly
Molecules spread out from an area of high concentration to an
area of low concentration.
• (HighLow)
Goes down the
concentration gradient, like
skating downhill
Weeee!!!
high
low
3 Types of Passive Transport
1. Simple Diffusion – molecules go through the phospholipid bilayer
2. Facilitative Diffusion – diffusion with the help of transport proteins
3. Osmosis – diffusion of water
4. Filtration- the process that forces molecules through a membrane by
exerting pressure
Diffusion
Animation of
diffusion
random movement of particles from an area of
high concentration to an area of low concentration.
•
Diffusion continues until all molecules are
evenly spaced (equilibrium is reached)-
•
Movement is caused from random
collisions of particles
http://bio.winona.edu/berg/Free.htm
Dissolving sugar cube. The cube slowly disappears as the sugar
dissolves and then diffuses to regions where there is less sugar (H->L)
Eventually the sugar molecules are evenly distributed.
Diffusion of Oxygen and Carbon Dioxide
O2 diffuses in , CO2 diffuses out
An equilibrium in never reached.
Intracellular O2 is always low
because it is constantly used up
in metabolic reactions.
Extracellular o O2 is high because
of the actions of the respiratory
and cardiovascular systems. The
concentration gradient always
allows O2 to diffuse into the cells
The level of the CO2 created as a waste
product is always higher inside the cell than
outside, so CO2 always diffuses out.
Molecules (both solvent and solute) will move across a permeable
membrane until an equilibrium is reached.
Diffusional equilibrium does not normally occur in organisms. Instead, they
tend to reach a physiological steady state, where concentration s of diffusing
substances are unequal, but stable
Factors that influence rate of diffusion
1. Concentration gradient- the higher the concentration, the faster it diffuses.
• But, if the concentration gradient is too high, because of the momentum, the
movement may not be able to stop and the cell will burst.
2. Distance- more rapid diffusion over shorter distances
3. Temperature- higher temps, faster diffusion
Simple Diffusion
Lipid (fat) soluble molecules pass directly through the cell membrane,
between the phospholipid molecules
= small, non-polar molecules
Ex.: fats, CO2, O2, steroids, urea, ethanol, general anesthetics
Facilitated diffusion
Diffusion of specific particles through transport proteins
found in the membrane
a. Transport Proteins are specific – they “select” only certain molecules to cross the
membrane
b. Transports larger or charged molecules
Examples: H2O, amino acids, glucose, ions such as H+, Na+, Cl-, Ca2+
Channel Protein
Small ions
Carrier Protein
Glucose, amino acids
Facilitated diffusion
Larger molecules
like glucose and
amino acids
Small ions
Ion channels can be gated or non-gated
Channel Proteins animations
Types of gates
Ligands= molecules that
bind specifically to
receptors
Osmosis
diffusion of water through a
selectively permeable membrane
• Water moves from high to low
concentrations
• “Water follows salt”
Cell membranes are
generally permeable to
water. So ,water may pass
directly through the
membrane or use protein
channels called aquaporins
Osmosis
animation
Osmosis and Cells
The concentrations in cells:
0.9% NaCl
5% sugar
•
Isotonic Solution
Osmosis
Animations for
isotonic, hypertonic,
and hypotonic
solutions
Isotonic: The concentration of solutes in the solution
is equal to the concentration of solutes inside the cell.
Result: Water moves equally in both directions and the cell remains
same size= Dynamic Equilibrium
•
Hypotonic Solution
Osmosis
Animations for
isotonic, hypertonic,
and hypotonic
solutions
Hypotonic: The solution has a lower concentration of
solutes and a higher concentration of water than
inside the cell. (Low solute; High water)
hemolysis
Result: Water moves from the solution to inside the cell
Cell Swells and bursts open = cytolysis
Hypertonic Solution
•
Osmosis
Animations for
isotonic, hypertonic,
and hypotonic
solutions
Hypertonic: The solution has a higher concentration
of solutes and a lower concentration of water than
inside the cell. (High solute; Low water)
shrivels
Result: Water moves from inside the cell into the solution:
Cell shrinks =Plasmolysis/ Crenates
What type of solution are these red blood cells in?
Isotonic
Hypertonic
Hypotonic
Filtration
• Forces molecules through a membrane by exerting pressure
High  Low pressure
Hydrostatic pressure- will force water molecules through to the other
side of a membrane while large solid particles are left behind
Example: water in blood forced out through capillaries , but blood cells
and large particles in the plasma remain inside.
-------------------------------------------------------------------------Example: Kidneys- pressure forces solute from the
blood across a membrane and into the nephron
Filtration is commonly used to
separate solids from liquids
Blood pressure can force liquid and small particles through
the pore spaces in the walls of capillaries. Larger particles
remain in the blood.
Active Transport
•cell uses energy
Against the
concentration
gradient
•actively moves molecules to
where they are needed
•Movement from an area of
low concentration to an area
of high concentration
This is
gonna be
hard
work!!
high
•(Low  High)
•Three Types:
low
High
Low
Active Transport
1. Protein or Solute Pumps
Sodium
Potassium Pumps
(Active Transport
using proteins)
require energy to
transport in and out of
the cell
Example: Sodium /
Potassium Pumps are
important in nerve
impulses
Protein changes
shape to move
molecules: this
requires energy!
2. Endocytosis:
taking bulky material into a cell
•
•
•
•
Uses energy
Cell membrane in-folds around a particle
Forms a vesicle (a phagosome)
Solid particles are digested as the vesicle
merges with a lysosome
•
This is how white blood cells eat bacteria!
a. phagocytosis- solids – “cell eating”
b. Pinocytosis- liquids
c. Receptor-mediated endocytosis
pinocytosis
phagocytosis
Receptormediated
endocytosis
Moves very
specific types of
particles into the
cell
3. Exocytosis:
Forces material out of cell in bulk
• membrane surrounding the material
fuses with cell membrane
• Cell changes shape – requires
energy
ex: - hormones
-wastes released from cell
-neurotransmitters
- manufactured proteins
secreted by cell
Endocytosis &
Exocytosis
animations
4. Transcytosis
combines endo- and exocytosis to selectively and rapidly transport a
substance or particle from one end of the cell to the other
This process occurs in normal physiology and disease
HIV uses transcytosis to
cross epithelial cells into
the anus, mouth, and
female reproductive
organs.