Download 1.3 and 1.4 Cell Membrane and Transport

Phospholipids
 PHOSPHOLIPIDS: are the foundations of the cell
membrane.
 The are amphipathic (are both hydrophilic and
hydrophobic)
 Hydrophilic: polar, water loving head
 Hydrophobic: nonpolar, water fearing tails, made of 2
fatty acid chains
Lipid Bilayer
 The phospholipids for a lipid bilayer (2 layers of
phospholipids, with the adjacent hydrophobic tails)
 This prevents the tail from having contact with “water”
environments
Fluid Mosaic Model of The Cell
Membrane
 The cell membrane is fluid (ie not rigid)
 Thus, it can change shape
 Cholesterol molecules in the bilayer allow for this
fluidity
 (At moderate temperatures, cholesterol makes the
bilayer less fluid. At colder temperatures, it makes it
slightly more liquid. This helps maintain functionality
and strength of the membrane over a wide range of
temperatures)
 The fluid mosaic model proposes that proteins float in
the fluid lipid bilayer (like boats on a pond)
 Some are anchored and cannot move
 These tend to be Integral Proteins, which span the
bilayer
 Some can float freely and can move within the
membrane
 These tend to be Peripheral or Surface Proteins, which
are found only on one side of the bilayer
Proteins found in the Plasma
Membrane
Transport Protein
 These are protein pores which allow ions and
molecules to pass into or out of the cell.
 They can be:
 Protein Channels (which do not require energy to
transport material)
 Protein Pumps (require energy to transport material)
Enzymes
 Biological Catalysts (speed up chemicals reactions)
 Membranes provide a convenient surface for enzymes
to be embedded.
 Enzymes for related reactions are organized next to
each other in order to organize the reactions for
greater efficiency.
Cell Surface Receptors
 Many proteins have a “lock and key” surface to which
only specific substances can fit.
 When the appropriate substance binds to the proteins,
it may initiate a reaction in the cell or cause the
substance to be brought into the cell (receptor
mediated transport)
 This is how some large food molecules, chemical
messengers, or even viruses enter cells
Cell Surface Identity Markers
 Each cell carries its own ID markers (glycoproteins –
proteins with carbohydrate molecules attached to
them)
 These molecules identify the cell as belonging to that
individual organism
Cell Adhesion Proteins
 Allow adjacent cells to stick together via interlocking
proteins on their membranes
DOUBLE BILAYER
 Note: the cell membrane
is actually a DOUBLE
BILYAER.
 This means there are 2
layers of lipid bilyaers.
 However, most diagrams
will just show one bilayer
because 2 takes up a lot
of space!
Membrane Permeability
 Permeable: all molecules can pass though (ie: pores in
the membrane)
 Non-permeable: nothing can pass through (ie: no
pores in the membrane)
 Semi-permeable: allows only small molecules to pass
through (because there are small pores in the
membrane)
 The cell membrane is semi-permeable
Cell Transport
 How do molecules move through the cell membrane?
 PASSIVE TRANSPORT
 Movement of molecules from high to low concentration
 Does not require energy
 ACTIVE TRANSPORT
 Movement of molecules from low to high concentration
 Requires energy (ATP energy)
Passive Transport
Diffusion
 Random movement of any molecule from high to low
concentration (with the concentration gradient)
1.
Animations
 http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/F
lash/Osmosis_Animation.htm
 http://highered.mcgraw-
hill.com/sites/0072495855/student_view0/chapter2/a
nimation__how_diffusion_works.html
Passive Transport
2. Osmosis
 Diffusion of water across a membrane
 Water moves from high concentrations of water to low
concentrations of water
OR
 Water move from low solute concentration to high
solute concentration
Osmosis
Terms Associated with Osmosis
 ISOTONIC
 (iso = equal / tonic = solute)
 2 solutions (inside and outside the cell) with
equal concentrations of solute and water.
 Net flow of water between the 2 is equal.
 We say both are in equilibrium.
 H2O diffusion still occurs in both directions at
equilibrium
 Cells want to reach equilibrium (have an equal
concentration of solutes inside and outside the cell)
 If the concentration of solutes is too high inside the
cell or too low, the cell will either take in or release
water
Terms Associated with Osmosis
 HYPERTONIC
 (hyper = more / tonic = solute)
 Any solution having more solute (less H2O)
than inside cell.
 Water will flow out of cell so cell shrinks.
 Ex: If a cell was put into a salt water
solution, water would move out of the cell
to “dilute” the extracellular fluid
Terms Associated with Osmosis
 HYPOTONIC
 (hypo = less / tonic = solute)
 Any solution having less solute (more H2O)
than the cell contents.
 Water moves into the cell (which has more
solute, less H2O) so cell expands.
 (Think of a hippopotamus with bloated cheeks)
Animations
 http://highered.mcgraw-
hill.com/sites/0072495855/student_view0/chapter2/a
nimation__how_osmosis_works.html
Passive Transport
3. Facilitated Diffusion
 Passage of molecules from high to low concentration
(diffusion) through a protein channel
 The protein channels are unique in size and shape and
only allow particular molecules to move through them.
 Used when the molecules are too large to diffuse
directly through the phospholipid bilayer or do not
have the proper charge

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html
ACTIVE TRANSPORT
 Movement of materials AGAINST the concentration
gradient (from low to high concentration)
 Requires ATP enery
 1. Endocytosis
 Movement of molecules INTO the cell
Types of Endocytosis
a) Phagocytosis
 Movement of large particles into the cell by the
formation of vacuoles
 Cell “eating”
 This is how some unicellular organisms (ie amoeba)
eat
 Involves the formation of pseudopods (false feet)
 Usually receptor mediated endocytosis (the molecule
entering the cell activates endocytosis by binding to
receptors)


http://academic.brooklyn.cuny.edu/biology/bio4fv/page/rectpr.htm
http://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000098&ptid=17
Types of Endocytosis
b) Pinocytosis
 Cell “drinking”
 The movement of liquid material or small particles
into the cell by vacuole formation

http://academic.brooklyn.cuny.edu/biology/bio4fv/page/endocytb.htm
Active Transport
2. Exocytosis
 Movement of materials out of the cell
 Vacuole passes plasma membrane to dump contents
out of the cell
Active Transport
3. Protein Pump
 Transport protein that moves materials in or out of the
cell (against the concentration gradient) but requires
energy (ATP) to be activated
 Ex: Sodium-Potassium Pump
 Moves 3 Na+ out of the cell and 2 K+ into the cell

http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html