Download Biological Membranes - Fundametnals of Biology - Lecture Notes and more Study notes Biology in PDF only on Docsity!
1
CChhaapptteerr 55 BBiioollooggiiccaall MMeemmbbrraanneess
The beginnings of a cell.
SSuummmmaarryy
ļ½ Structure of cell membrane
ā¦ Lipids
ā¦ Proteins
ļ½ How do solutes move across lipid membranes?
ā¦ Passive transport ā diffusion, facilitated diffusion and osmosis
ā¦ Active transport ā primary and secondary
ļ½ Other functions of membrane proteins
T
T
h
h e
e 1
s
s t
t
C
C
e
e l
l l
l
ļ½ Life began and still exists in a watery environment. The inside and
outside of cells is largely water.
ļ½ What would make a good barrier between these two environments?
ā¦ Must be insoluble in water
ā¦ Must be flexible
BBiioollooggiiccaall MMeemmbbrraanneess
ļ½ Every cell is surrounded by a plasma membrane.
ļ½ Membranes are also used in eukaryotic cells to separate the interior into
compartments ā organelles.
S
S
t
t r
r u
u c
c t
t u
u r
r e
e o
o f
f B
B
i
i o
o l
l o
o g
g i
i c
c a
a l
l M
M
e
e m
m b
b r
r a
a n
n e
e s
s
ļ½ Phospholipids form the bulk of the plasma membrane.
ā¦ Amphipathic molecules
ā¦ Form bilayers in water
ā¦ **Note position of head and tails.
ļ½ Cholesterol, also amphipathic, maintains fluid nature of the membrane.
ļ½ Lipids can move laterally in their own layer of the membrane.
2
ļ½ Contributes to fluid nature and flexibility of plasma membranes.
ļ½ Living membranes have very fluid and less fluid areas. The cause is a
mixture of different phospholipids, fig 6.9.
ā¦ Saturated fatty acid chains make the phospholipid less fluid.
ā¦ Unsaturated fatty acid chains make the phospholipid more fluid.
ļ½ Cholesterol helps maintain the fluidity.
ļ½ Proteins
ā¦ Integral proteins are imbedded in the PL bilayer.
ā¦ Transmembrane proteins span the PL bilayer.
ā¦ Peripheral proteins are located on the inner or outer surface, often
associated with integral proteins.
TThhee PPllaassmmaa MMeemmbbrraannee
Question
ļ½ How do proteins imbed themselves in the plasma membrane?
ā¦ Hint: the phospholipid tails are hydrophobic and the environment
outside and inside the cell is water based.
Functions of Membrane Proteins
ļ½ Membrane proteins and their membranes have many functions:
ā¦ Transport materials
ā¦ Act as enzymes or receptors
ā¦ Recognize cells
ā¦ Structurally link cells
4
ļ½ When the concentration of solute is equal throughout the system it is in
equilibrium.
Simple Diffusion
Diffusion Across Biological Membranes
ļ½ Add a lipid bilayer membrane that is selectively permeable to some
solutes.
ļ½ In cells, if the cell membrane is permeable to a solute, there will be net
movement (=diffusion) of that solute until its concentration is the same
inside and outside the cell = equilibrium.
ļ½ What if the membrane is NOT permeable to the solute?
Facilitated Diffusion
ļ½ Cell membranes contain integral and transmembrane proteins.
ļ½ Membrane proteins form channels (lined with water) or carriers.
ā¦ Channels & carriers are specific for certain solutes.
ā¦ Channels or carriers help solutes move into or out of the celldown
their concentration gradient =facilitated diffusion. The gradient can
be electrical and/or chemical. ***NOTE
THIS IS A PASSIVE PROCESS WITH āHELPā.
ļ½ How are channels and carriers controlled?
ā¦ Channels have gates that are opened by a solute binding to the protein
or by an electrical charge.
ā¦ Carriers have specific binding sites for a certain solute. Diffusion is
also limited by number of carriers.
5
Osmosis
ļ½ Water also moves along its concentration gradient.
ļ½ Water moves in response to the concentration of solutes on each side of a
membrane that is impermeable to the solutes. Special case of diffusion
called osmosis.
Osmotic Pressure
ļ½ Determined by concentration of solutes
ļ½ Cells regulate internal osmotic pressure
ā¦ to prevent shrinking or bursting
Tonicity
ļ½ Tonicity ā ability to attract water.
ļ½ Compare the tonicity of 2 solutions:
ā¦ If they have equal concentrations of solute they are said to be isotonic.
ā¦ If one has a higher concentration of solutes than the other it is
hypertonic. The solution with a lower concentration of solutes is
hypotonic.
Osmosis Across Biological Membranes
ļ½ 2 compartments separated by a membrane that is impermeable to solutes
but permeable to water.
ā¦ Hypertonic solution on one side and hypotonic solution on the other.
ā¦ Water will always move from the hypotonic solution toward the
hypertonic solution until osmotic pressure is the same on both sides.
Osmosis Through a Membrane
Osmosis
ļ½ In the beaker (an open system), the volume increases on the hypertonic
7
OOtthheerr FFuunnccttiioonnss ooff MMeemmbbrraannee PPrrootteeiinnss
ļ½ Cell-cell Attachment
ā¦ Tight junctions
ā¦ Desmosomes
ļ½ Cell-cell communication between adjacent cells.
ā¦ Plasmodesmata in plant cells
ā¦ Gap junctions in animal cells
