Cell
Membrane
Structure
and
Cellular
Transport
Accel Bio 2014
Biological Membranes are
Phospholipid Bilayers
Phospholipid
Structure
• hydrophobic “tails”
of fatty acids
• hydrophilic “head”
with a glycerol & a
phosphate group (PO4)
Hydrophobic =
“afraid” of water
Hydrophilic =
friend of water
“_______”
Biological membranes are
selectively permeable barriers
Lipid bilayer membranes are permeable to:
• hydrophobic lipids
• small, uncharged molecules such O2 and CO2
• small molecules such as water
Lipid bilayers are not permeable to:
• ions (charged atoms)
• large hydrophilic molecules (like glucose)
These substances cannot pass through the lipid bilayer easily.
They must find another (more favorable) way across…
A more complete picture of
a Cell Membrane
• Embedded proteins “float” in fluid phospholipid “sea”
A model of the FLUID membrane: http://www.susanahalpine.com/anim/Life/memb.htm
Components
of the Cell
Membrane
• Phospholipids: make up selectivelypermeable barrier
• Proteins: *many functions*, including transport
channels - selectively allow some larger
molecules and charged atoms (ions) through
• Carbohydrates: chemical ID tags, found on
outside of cell
• Cholesterol: keeps membrane fluid
Cell Membrane Proteins:
a slightly different representation
http://www2.warwick.ac.uk/fac/sci/chemistry/research/dixon/dixongroup/members/msrhar/research/background/
Membrane Protein Functions are Varied
Transport Across the Cell
Membrane Occurs Various Ways
• Passive Processes
1. Diffusion
2. Osmosis
3. Facilitated Diffusion
• Active Processes
4. Active Transport (protein pump)
5. Endocytosis
•
•
•
Phagocytosis
Pinocytosis
Receptor-Mediated Endocytosis
6. Exocytosis
Movement across a membrane
can occur by diffusion
Molecules are in constant motion. Due to their
random movement, molecules have a natural
tendency to exhibit net movement from areas of
_______
high concentration to areas of _____
low
concentration. This movement is called diffusion.
Molecular motion never ceases,
but we can reach equilibrium
Eventually, diffusion leads to a state where there are
equal concentrations of molecules across a
membrane. This state is called equilibrium.
In equilibrium, the movement of molecules in a given direction
(left or right) across a membrane is equally probable. Thus,
unless disturbed, a system in equilibrium will tend to stay in
equilibrium.
DYNAMIC
Diffusion occurs down a
concentration gradient
A difference in concentrations of an atom/molecule
across a membrane is called a concentration
gradient.
Diffusion of molecules to an area of low concentration
is described as movement down the concentration
gradient.
(Active transport to an area of higher concentration is
described as movement against the conc gradient)
Osmosis
Osmosis is the diffusion of water across a
selectively permeable membrane,
low
high concentration to areas of ____
from areas of ____
concentration.
Cells contain and are bathed in solutions that contain
dissolved substances like ions, sugars, proteins, and
vitamins in water.
A solution is a mixture of a liquid and at least one type of
dissolved solid substance. The substance that is dissolved
is called the solute. The liquid that it is dissolved in is called
the solvent.
What
drives
osmosis?
Setup: U-tube
with sel. perm.
membrane, pour
two different
concentration salt
solutions on either
side of tube.
Membrane is
permeable to H2O
but NOT to salt.
What happens???
WHY?
What
drives
osmosis?
Setup: U-tube
with sel. perm.
membrane, pour
two different
concentration salt
solutions on either
side of tube.
Membrane is
permeable to H2O
but NOT to salt.
What happens???
WHY?
A little solution vocabulary
Hypo = below
Hyper = above / over
Iso = same
Hypotonic:
refers to a solution that has a lower solute
content than the solution you are comparing it to
Hypertonic:
refers to a solution that has a higher solute
content than the solution you are comparing it to
Isotonic: Guess what this means!
Movement of Water Across a
Membrane: Which way net flow?
Movement of Water Across a
Membrane: Which way net flow?
Red Blood Cells in Different Solutions
Hypotonic
Isotonic
http://arbl.cvmbs.colostate.edu/hbooks/cmb/cells/pmemb/osmosis.html
Hypertonic
Facilitated Diffusion: Movement across
a membrane, from high to low
concentration, using a transport protein
Water also crosses membrane through
Aquaporin protein channels
http://en.wikipedia.org/wiki/File:AQP-channel.png
http://plantphys.info/plant_physiology/osmosis.shtml
Not all cell transport is passive!
Passive Processes
• Do not require the use of cellular energy.
• Always involve net movement from areas of high
concentration to areas of low concentration.
Ex) Diffusion, Osmosis, & Facilitated Diffusion
Active Processes
• Require use of cellular energy.
• Allow net movement from areas of low to areas
of high concentration.
Ex) Active Transport, Endocytosis, & Exocytosis
How do cells “use” energy?
ATP (Adenosine Tri-phosphate)
Energy “currency” of the cell. Transfers energy by transferring PO4- groups.
Adenine
http://scienceaid.co.uk/biology/biochemistry/images/atp.jpg
The bonds between
the phosphate (PO4-)
groups are less stable
and have high
(potential) energy.
Hydrolysis of ATP
• releases energy and produces ADP + Pi
Where does the energy in ATP come from?
Cell Respiration
C6H12O6 + O2 + ADP + Pi  ATP + CO2 + H2O
http://gleesonbiology.pbworks.com/f/1221960382/03_32_ATP_and_ADP_cycle.jpg
How does ATP transfer energy to do work?
Phosphorylation: transfer of a phosphate group
Active
Transport:
Movement
across a
membrane
from low to
high
concentration
which requires
the use of
energy (ATP)
and a protein
“pump”
Another Active Process: Exocytosis
• a transport vesicle fuses with cell membrane,
releasing vesicle contents to the outside
Yet another Active Process: Endocytosis
Again, motor proteins use ATP, as does the process by which the cell
cytoskeleton proteins are re-arranged to re-shape the membrane.
http://bioap.wikispaces.com/file/view/endocytosis_types.png/177216843/735x365/endocytosis_types.png
Receptor-Mediated Endocytosis
Rec-Med Endocytosis of LDL particles (Low-Density Lipoproteins):
http://www.susanahalpine.com/anim/Life/endo.htm
Specific Examples of Cell Transport
Passive Processes
• Diffusion: movement of oxygen into cells
• Osmosis: movement of water into cells placed in
pure water
• Facilitated Diffusion: movement of glucose into cells
through specialized glucose protein channels
Active Processes
• Active Transport: sodium-potassium (Na+/K+) pump
• Endocytosis
•
•
•
Phagocytosis: ingestion of bacteria by white blood cells
Pinocytosis: “nursing” of human egg cells
Receptor-Mediated Endocytosis: uptake of LDL particles
• Exocytosis: Ex1) secretion of mucus
Ex2) secretion of insulin protein
Transport Animations
Fluid Mosaic Model of the Membrane
http://www.susanahalpine.com/anim/Life/memb.htm
Sodium-Potassium Pump (narrated)
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.
html
Endo/Exo-cytosis (narrated)
http://local.brookings.k12.sd.us/biology/PHVideos/Chapter%2007E.mpg
Receptor-Mediated Endocytosis of an LDL particle
http://www.susanahalpine.com/anim/Life/endo.htm
Overview of Membranes & Transport, including Action Potential
http://www.wiley.com/legacy/college/boyer/0470003790/animations/membrane_transport/membrane_transp
ort.htm
Microscope Videos of various types of Cell Transport
http://www.linkpublishing.com/video-transport.htm