Transcript Cell Transport notes

CELL TRANSPORT
Passive and Active
Transport
Sources: Starr, C. 1997 Biology: concepts
and applications. Wadsworth Publishing
Company.
Diffusion
Transport process facilitated by:
 Selective permeability of the membrane: no ATP
required
– Ability of cell membranes to allow some substances
but not others to cross them in certain ways, at certain
times.
– O2, CO2 and other small molecules with no net charge
cross the the bilayer itself.
 Following gradients
– Molecules and ions have internal energy that keeps
them in constant motion
– Molecules move down following concentration
gradients
 Factors that affect diffusion rate:
– Each substance diffuses in the direction set
by its own concentration gradient
– Diffusion rates are faster when a gradient is
steep
– Temperature affects diffusion rates (heat
causes molecules to move faster)
– Small molecules move faster than large
ones
– An electric gradient (a difference in charge
between two adjoining region) can modify
the rate and direction of diffusion.
Osmosis
 Movement of H2O across a selectively
permeable membrane
 The number of molecules off all solutes on
either side of the membrane affects the water
concentration.
 Tonicity is the name for the relative
concentration of solutes in two fluids
– isotonic fluids: equal solute concentration
– hypotonic fluid: a fluid with fewer solute
concentration
– hypertonic fluid: a fluid with a high solute
concentration.
Passive Transport:
Facilitated Diffusion
 A solute moves through the interior of a
protein ( facilitated diffusion)
 It is a two-way transport
 The net direction of movement at a given
time depends on how many molecules or
ions of the solute are making random
contact with vacant binding sites in those
proteins
Active Transport
 This mechanism requires an energy boost (ATP).
Mitochondria provides ATP that powers the
molecular motors of cytoskeleton
 Movement is against concentration gradient
 This mechanism can continue until the solute
becomes more concentrated on the side to which
it is being pumped
 When the transport protein interacts with a
particular solute, it changes its shape
 With the change, the bound solute becomes
exposed to the fluid bathing the opposite side of
the membrane.
Active Transport and the Cell Membrane
 The cell membrane is not static.
– It is renewed by the addition of new membrane
vesicles from the Golgi, exocytosis. When the vesicle
membrane and the plasma membrane come into
contact, the lipid molecules of the bilayer rearrange
themselves
– Removal of membrane segments take place in the
form of endocytosis:
 Phagocytic vesicles: Cells engulf a particle by
wrapping pseudopodia around it and packaging it
into a membrane-enclosed sac, a vacuole. Particle
is digested after it fuses with a lysosome
 Pinocytic vesicles: the cell engulfs droplets of
extracellular fluids in tiny vesicles (unspecific)
Receptor-mediated-endocytosis
 Involves proteins with specific receptor sites
exposed to extracellular fluids
 Receptor proteins are usually clustered in regions
called coated pits
 Coated pits are lined up, on their cytoplasmic site
by a fuzzy layer consisting on the protein clathrin
 Extracellular substances that bind to the receptors
are called ligands (general term for any molecule
that binds specifically to a receptor site of another
molecule)
 When appropriate ligands bind to a receptor, they
are carried into the cell by the inward budding of a
coated pit, coated vesicle.
Receptor mediated endocytosis:
An example
 This process enable cells to take in large
quantities of substrates
 Animal cells use this process to take in cholesterol
for use in the synthesis of membranes and
steroids
 In families with hypercholesterolemia, high levels
of cholesterol in blood, the protein receptors are
missing. What is the result of this condition?
Early atherosclerosis
Answer
 Cholesterol is unable to enter cells and
builds up in blood, contributing to early
atherosclerosis (development of fat deposits
on blood vessel linings)
How does the process work? An overview
 Microtubules probably help guide secretory
vesicles from Golgi Complex to plasma
membrane. Below is a possible explanation:
“Specialized proteins function as motor molecules
moving vesicles and other organelles along
microtubules. Motor molecules are proteins (such
as kinesin) that convert chemical energy to
movement. Kinesin attaches to specific receptors
on the vesicles and its “feet”, movable extensions
of the protein, walk along a microtubule. Kinesin
powers it movement by hydrolyzing ATP”
(Campbell, 2002)
Patching and Capping
 “Receptors are brought to the plasma membrane by
vesicles from the trans region of the Golgi complex . How
does the Golgi complex maintain the fluidity of the plasma
membrane , the receptors can move laterally in the
membrane and collect in the specialized regions called
clathrin coated pits.
 When the ligand binds to its specific receptor, the ligand-receptor
complex accumulates in the coated pits. In many cells, these pits and
complexes begin to concentrate in one area of a cell. …this appears as
patches of label on the cell surface (patching) Eventually, the patches
coalesce to form a cap at one pole of the cell (capping). Not all cells
form caps, but most do form patches. Why would this process be an
advantage for the cells? Imagine the large amounts of extracellular
fluid that would be taken up if the cells endocytosed the ligand receptor
complex all over its surface. Thus, the pre-concentration process
minimizes the amount of fluid that is taken up in the vesicle”.
(http://cellbio.utmb.edu/cellbio/recend.htm#process)
What types of ligands enter by receptor
mediated endocytosis?
1. Toxins and lectins
Diptheria Toxin
Pseudomonas toxin
Cholera toxin
Ricin
2. Serum transport proteins and antibodies
Low density lipoprotein
Yolk proteins
IgE
Polymeric IgA
Maternal IgG
IgG, via Fc receptors
3. Hormones and Growth Factors
Insulin
Epidermal Growth Factor
Growth Hormone
Thyroid stimulating hormone
Nerve Growth Factor
Calcitonin
Glucagon
Prolactin
Luteinizing Hormone
Thyroid hormone
Platelet Derived Growth Factor
Interferon
4. Viruses
Rous sarcoma virus
Semliki forest virus
Vesicular stomatitis virus
Adenovirus
Examples of Active Transport
Pumps
 Na-K pump is a major co-transport system
 Ca pumps
– This process helps keep the calcium
concentration inside the cell at least a thousand
times lower than outside
Putting it together
Campbell, N. Biology Sixth Edition. Benjamin Cummnings Publishing 2002
1.“An experiment is designed to study the
mechanism of sucrose uptake.
Cells are
immersed in a sucrose uptake by plants cells and
the pH of the surrounding solution is monitored
with a pH meter. The measurements show that
sucrose uptake by plant cells raises the pH of the
surrrounding solution. The magnitude of the pH
change is proportional to the starting concentration
of sucrose in the extracellular solution.
A
metabolic poison that blocks the ability of the cells
to regenerate ATP also inhibits the pH change in
the surrounding solution. Explain these results.”
2. The rates of sucrose uptake from solutions of different sucrose
concentrations are compared. Explain the shape of the curve in terms
of what is happening at the membranes of plant cells?
Rate of
Sucrose up-take
(umol/g.min)
Sucrose concentration of surrounding solution (mM)