Download Chapter 5 Cell Membrane

Chapter 5 Cell Membrane
Plasma Membrane
• --the fluid mosaic model (S.J Singer)
• -- semi-permeable
• --fluid portion is a double layer of
phospholipids, called the phospholipid bilayer
Jobs of the cell membrane
• Isolate the cytoplasm from the external
environment
• Regulate the exchange of substances
• Communicate with other cells
• Identification
Phospholipid Bilayer
• Phospholipids contain a hydrophilic head and a
nonpolar hydrophobic tail
• Hydrogen bonds form between the phospholipid
"heads" and the watery environment inside and
outside of the cell
Hydrophobic interactions force the "tails" to face
inward
Phospholipids are not bonded to each other,
which makes the double layer fluid
• Cholesterol embedded in the membrane makes it
stronger and less fluid
Proteins embedded in membrane
• 1. Channel Proteins - form small openings for
molecules to difuse through
2. Carrier Proteins- binding site on protein
surface "grabs" certain molecules and pulls them
into the cell
3. Receptor Proteins - molecular triggers that set
off cell responses (such as release of hormones or
opening of channel proteins)
4. Cell Recognition Proteins - ID tags, to idenitfy
cells to the body's immune system
5. Enzymatic Proteins - carry out metabolic
reactions
Transport Across Membrane
• Passive Transport
• Simple Diffusion - water, oxygen and other
molecules move from areas of high
concentration to areas of low concentration,
down a concentration gradient
• Facilitation Diffusion - diffusion that is
assisted by proteins (channel or carrier
proteins)
Osmosis
• diffusion of water. Salt Sucks
Osmosis affects the turgidity of cells, different
solution can affect the cells internal water
amounts
• Contractiles Vacuoles are found in freshwater
microorganisms - they pump out excess water
• Turgor pressure occurs in plants cells as their
central vacuoles fill with water.
Types of Osmosis Solution
• Isotonic: equal amount solute inside and
outside of the cell.
• Hypertonic: More solute on the outside of the
cell
– Water will move out of the cell
– Cell will swivel up
• Hypotonic: More solute inside the cell
– Water will move in the cell
– Cell will burst
Active Transport
• involves moving molecules "uphill" against the
concentration gradient, which requires energy
• Endocytosis - taking substances into the cell
(pinocytosis for water, phagocytosis for solids)
Exocytosis - pushing substances out of the cell,
such as the removal of waste
Sodium-Potassium Pump - pumps out 3 sodiums
for ever 2 potassium's taken in against gradient
•
Observation of elodea cells in salt
water. What happens and why?
Modifications of Cell Surfaces
• Tight junctions are composed of protein fibers
that seal adjacent cells to prevent leakage,
something which can be useful in organs such as
the bladder and the lining of the digestive tract.
Tight junctions literally fuse the cells together
forming a sheet of cells restricting molecules to
one side of the sheet or the other.
• Tight junctions can also partition the cells in which
they are found. Certain membrane proteins can be
restricted to one side of the junction, as well, since
the tight junction prevents protein migration
within the membrane.
Continue…
• Desmosomes anchor adjacent cells together by making
connections that work like staples or rivets that attach
to components of the cytoskeleton. Many epithelial
cells must adhere to adjacent membranes to prevent
free passage or free movement, and to not break apart
under stress.
• Desmosome filaments are composed of specialized
glycoproteins proteins. Intermediate filaments of
keratin in the desmosomes help strengthen the
junction.
• Actin microfilaments can also attach to desmosomes,
but have less strength
Continue…
Plants have plasmodesmata - channels
between the cell wall that cytosol can
pass through
Continue…
• Gap junctions are protein channels, called
connexons, between adjacent cells that
permit the transfer of small molecules, such as
nutrient monomers, between the cells. They
are common in brain cells, forming the
synapse, in many glands, and in cells in the
heart muscle that coordinate contraction for
heartbeat. Gap junctions can be gated.