Transcript Document

AP Bio November 12-13
• CHPs 2-5 Test debrief
• P 5: finish Microscopy
• P5: Inner Life of Cell
• Microscopy Quiz
• Transport
Coming up………………
CHPs 7, 8, 11, 12 quizzes
Pro No Eu Do
Plasma Membranes Why should we care?
 Problems with the cell membrane are involved
in many diseases
 Type 2 Diabetes, organ transplant rejection,
Cholera, Cystic fibrosis, cancer
Membrane Structure & Function
A plasma membrane encloses every
It protects a cell by acting as a
barrier between it’s living contents &
the surrounding environment.
It is selectively permeable thus it
regulates what goes into & out of the
It receives and produces signals to
and from other cells.
It identifies the cell as belonging to a
particular organism and tissue
It maintains connections between cells
in organs and tissue
1.At the turn of the last century,
researchers noted lipid-soluble
molecules entered cells more rapidly
than water soluble molecules,
suggesting lipids are component of
plasma membrane.
1.At the turn of the last century,
researchers noted lipid-soluble
molecules entered cells more rapidly
than water soluble molecules,
suggesting lipids are component of
plasma membrane.
2. Later, chemical analysis
revealed the membrane contained
3. Gorter & Grendel (1925)
found amount of phospholipid
extracted from a red blood cell
was just enough to form one
bilayer; suggested nonpolar tails
(hydrophobic) directed inward,
polar heads (hydrophilic)
4. To account for permeability of
membrane to nonlipid substances,
Danielli & Davson proposed
sandwich model (later proved
wrong) with phospholipid bilayer
between layers of protein.
5. With electron microscopy,
Robertson proposed proteins
were embedded in outer
membrane and all membranes in
cells had similar compositionsthe unit membrane model.
6. In 1972, Singer & Nicolson
introduced the currently
accepted fluid-mosaic model of
membrane structure.
Fluid-Mosaic Plasma Membrane
• The main biological molecules in membranes are
lipids and proteins, but include some
• The most abundant lipids are phospholipids.
•The molecules in the bilayer are arranged such
that the hydrophobic fatty acid tails are sheltered
from water while the
hydrophilic phosphate
groups interact
with water.
Membranes are fluid
• Membrane molecules are held in place by
relatively weak hydrophobic interactions.
• Most of the lipids and some proteins can drift
from side to side in the plane of the membrane,
but rarely flip-flop from one layer to the other.
• Cholesterol is wedged between phospholipid
molecules in the plasma membrane of animals
• At warm temperatures, it restricts the
movement of phospholipids and reduces
• At cool temperatures, it maintains fluidity by
preventing tight packing.
Membranes are mosaics of structure
and function
• A membrane is a collage of different proteins
embedded in the lipid bilayer.
• The proteins in the plasma membrane may
provide a variety of major cell functions.
• Many transport proteins simply provide
corridors allowing a specific molecule or ion to
cross the membrane.
– These channel proteins allow fast transport.
• Some transport proteins do not provide
channels but appear to actually move the solute
across the membrane as the protein changes
Membrane carbohydrates are
important for cell to cell recognition
• The membrane plays the key role in cell to cell
– Cell to cell recognition is the ability of a cell to
distinguish one type of neighboring cell from another.
– This attribute is important in cell sorting and
organization as tissues and organs in development.
– It is also the basis for rejection of foreign cells by the
immune system.
A membrane’s structure and composition
results in selective permeability.
• A steady traffic of small molecules and ions
moves across the plasma membrane in both
• However, substances do not move across the
barrier indiscriminately; membranes are
selectively permeable.
• Specific ions and polar molecules can cross the
lipid bilayer by passing through transport
proteins that span the membrane.
• Each transport protein is specific as to the
substances that it will move.
Passive transport is diffusion across a
• Diffusion is the tendency of molecules of any
substance to spread out in the available space
• Movements of individual molecules are random.
• The diffusion of a substance across a biological
membrane is passive transport because it requires
no energy from the cell to make it happen.
• For example, if we start with a permeable
membrane separating a solution with dye
molecules from pure water, dye molecules will
cross the barrier randomly.
• The dye will cross the membrane until both
solutions have equal concentrations of the dye.
• At this dynamic equilibrium as many molecules
pass one way as cross the other direction.
• In the absence of other forces, a substance will
diffuse from where it is more concentrated to
where it is less concentrated, down its
concentration gradient.
• Each substance diffuses down its own
concentration gradient, independent of the
concentration gradients of other substances.
Osmosis is the passive transport of water
• Differences in the relative concentration
of dissolved materials in two solutions can
lead to the movement of ions from one to
the other.
– The solution with the higher concentration of
solutes is hypertonic.
– The solution with the lower concentration of
solutes is hypotonic.
– Solutions with equal solute concentrations are
• Free (unbound) water molecules will move from
the hypotonic solution where they are abundant
to the hypertonic solution where they are rarer.
• This diffusion of water across a selectively
permeable membrane is a special case of passive
transport called osmosis.
• Osmosis continues
until the solutions
are isotonic.
• The same cell in a hypertonic environment will
loose water, shrivel, and probably die.
• A cell in a hypotonic solution will gain water,
swell, and burst.
• For example, Paramecium, a protist, is
hypertonic when compared to the pond water in
which it lives.
– In spite of a cell membrane that is less permeable to
water than other cells, water still continually enters
the Paramecium cell.
– To solve this problem,
Paramecium have a
specialized organelle,
the contractile vacuole,
that functions as a bilge
pump to force water out
of the cell.
As a plant cell looses water, its volume shrinks.
• Eventually, the plasma membrane pulls away
from the wall.
• This plasmolysis is usually lethal.
Facilitated Diffusion
• Many polar molecules and ions that are normally
impeded by the lipid bilayer of the membrane
diffuse passively with the help of transport
proteins that span the membrane.
• The passive movement of molecules down its
concentration gradient via a transport protein is
called facilitated diffusion.
Active transport is the pumping of
solutes against their gradients
• Some proteins can move solutes against their
concentration gradient, from the side where they
are less concentrated to the side where they are
more concentrated.
• This active transport requires the cell to expend
its own metabolic energy.
•During endocytosis, a cell brings in macromolecules
and particulate matter by forming new vesicles from
the plasma membrane.
•One type of endocytosis is phagocytosis, “cellular
• In pinocytosis, “cellular drinking”, a cell
creates a vesicle around a droplet of extracellular fluid.