Download Cell Membrane Proteins

Cell Membrane Proteins
-- recall that peripheral proteins mainly serve to support the mb’s structure.
-- see fig. 4.3 p. 70.
-- FIVE TYPES (most of these types are integral proteins):
I) Cell-recognition Glycoproteins -- serve as ‘ID tags’ or ‘fingerprints’,
specific to a cell, person, and/or species; recognized by antibodies/white
blood cells. If recognized as ‘foreign’, they might be attacked!
*Organ/Blood donors…
II) Channel Protein -- spans the membrane and is ‘hollow’ (like a tube),
creating a PORE for small, highly polar molecules to pass through (eg. most
ions, along with some water).
III) Carrier Protein -- proteins that also span the membrane.
-- the proteins accept the molecule which is to enter (or exit) the cell,
then morph into a different shape to release the molecule out to the
other side.
-- substances such as glucose, amino acids, glycerol, and ions use
carrier proteins to cross the membrane.
-- sometimes ATP energy is required for carrier protein usage.
** Protein types II and III are generally referred to as Transport Proteins.
IV) Receptor Protein -- molecular ‘triggers’ that set off a sequence of
events within the cell when a certain molecule (hormone, enzyme,
neurotransmitter) binds to it on the exterior of the cell.
-- these proteins are also very specific as to what
molecules they can bind to (ie. lock and key).
V) Enzymatic Protein -- catalyzes a certain reaction at or near the cell
membrane.
Anchoring of Proteins in the Cell Membrane
-- the outside edges of the protein (those edges in contact with the
hydrophobic core of the phospholipid bilayer) are hydrophobic, meaning
that predominantly, amino acids possessing non-polar R-groups line up on
these edges to associate (London Forces) with the hydrophobic core.
-- the ‘ends’ (‘top’/‘bottom’) of the protein are predominantly made up of
amino acids possessing polar R-groups, so that they can form H-bonds with
the water on both the ECF-side and the cytoplasm-side.
-- the interior of a channel protein (the walls lining the pore) is made up of
amino acids with polar R-groups as well, in order to create a hydrophilic
‘tunnel’ for small, polar molecules to pass through.
A simple diagram:
How Molecules Cross the Membrane
1. SIMPLE DIFFUSION (special case: OSMOSIS):
a. Without a Channel Protein -- small, non-polar molecules such as
oxygen, carbon dioxide, and fatty acids. Water as well, even though it
is polar; due to the flow strength.
b. With a Channel Protein -- small, polar molecules such as ions
(water too).
2. TRANSPORT INVOLVING PROTEIN CARRIERS:
-- may or may not involve the expenditure of ATP.
3. ENDOCYTOSIS AND EXOCYTOSIS:
-- involves vesicle formation along with the expenditure of ATP.
A More Specific Look at: TYPES OF TRANSPORT
Class A. PASSIVE TRANSPORT (no ATP required)
I). Simple Diffusion
-- the net movement of a substance (molecules) from a region of
higher concentration to a region of lower concentration.
-- in other words, the net movement of a substance (molecules) down
its concentration gradient until it is distributed evenly (equilibrium is
achieved whereby the rate of movement in either direction is equal).
Def'n: Gradient -- a physical difference between two
regions of space, so that molecules tend to move
from one region to another.
-- there exist concentration gradients,
pressure gradients, and electrical gradients.
-- diffusion is spontaneous.
-- see fig. 4.5 p. 73 and fig. 4.6 p. 73 (O2 in lungs).
-- diffusion can, at times, involve a CHANNEL PROTEIN for
small, polar molecules.
Factors that Affect the Rate of Diffusion:
1. Extent of Concentration Gradient:
-- the existence of a concentration gradient initiates diffusion.
-- the greater the gradient (difference in concentration between
two regions), the faster the diffusion.
2. Temperature:
-- increased temp. increases the KE of molecules and vice versa
-- as temperature increases, rate of diffusion increases.
-- as temperature decreases, rate of diffusion decreases.
3. Ionic/Molecular Size:
-- smaller molecules will diffuse more rapidly than larger ones.
4. Density and Viscosity of the Medium (solvent):
-- the lower the density and viscosity of the medium, the more
quickly the molecules diffuse through it.
eg. diffusion through air is faster than diffusion through water.
5. Movement of the Medium:
-- currents within the medium will aid diffusion.
-- in cells, the phenomenon of cytoplasmic streaming (a
constant movement of cytoplasm) helps with diffusion.
6. Solubility/Polarity:
-- non-polar (hydrophobic, lipid-soluble) molecules will diffuse
across the membrane quite readily (fatty acids, oxygen, CO2).
-- polar (hydrophilic, water-soluble) molecules can also diffuse
but are limited to channel proteins as a means to enter/exit
the cell, thus slowing down their rates of diffusion.
II). Osmosis (special type of diffusion involving water)
-- the movement of water molecules from a region of higher
concentration (lower [solute(s)]) to a region of lower concentration
(higher [solute(s)]) through a selectively permeable membrane.
-- water moves by osmosis in order to ‘even out’ concentrations of
solutions.
-- usually water is the solvent and it always moves UP a solute
concentration gradient.
ie. Water moves to where the [solute(s)] is/are greater.
-- the effect of this is the dilution of the solute and the ‘evening out’
of the concentrations without the solute even having to move
(because sometimes it either cannot move, or it moves more slowly).
-- it is usually easier for water to move instead.
-- see fig. 4.7 p. 74.
A couple of definitions:
OSMOTIC PRESSURE -- the pressure, generated by the amount of
solute in solution, that serves to draw water towards a region.
-- O.P. increases as solute concentration increases.
-- the greater the O.P. of a region, the more water is drawn
towards that region.
TURGOR PRESSURE (a.k.a. Blood or Hydrostatic Pressure) -- the pressure,
generated by the amount of solvent in solution, that serves to push
water to another region (this term is usually reserved for plant cells
with respect to the pressure that cell membranes exert on cell walls,
but we can use it generally as well).
-- turgor pressure increases as water volume increases.
-- the greater the turgor pressure of a region, the more water
is pushed away from that region.
*typically, a solution with a higher osmotic pressure possesses a lower
turgor pressure, and vice versa.
-- The concept of osmosis relies heavily upon the topic of TONICITY
(Solution Strength), which will now be discussed:
TONICITY
-- cells can be subjected to three different types of solutions, all with
different relative tonicities.
** for each of the following scenarios, picture a cell (of course, possessing a
cell membrane) placed in a beaker of solution. The cell contains cytoplasm,
which has a certain tonicity (ie. [solute(s)]), and the beaker will serve as the
ECF with a certain tonicity (ie. [solute(s)]).
a. Isotonic Solutions -- Cells remain stable.
(Iso-- = the same as)
-- where the [solute(s)] (osmotic pressure) is the same on both sides
of the membrane.
-- due to this, there is no NET gain or loss of water by the cell (there
is no gradient).
-- in other words, there are no macroscopic changes to the system (it
is already at equilibrium).
-- see fig. 4.8 p. 75.
b. Hypotonic Solutions -- Cells swell.
(Hypo-- = less than)
-- solutions whose [solute(s)] is less than that of another solution
(region) are said to be hypotonic.
-- let's say that a cell is placed into a hypotonic solution (in a beaker).
-- since the cell has a greater [solute(s)], it therefore has a greater
O.P., and thus, water will flow from the exterior solution into the cell
and the cell will SWELL.
-- this increases the turgor pressure (turgidity) of the cell (more water
= more turgidity).
-- once water enters the cell, the O.P. within the cell decreases (more
water = less O.P.).
* when O.P. increases, turgor pressure decreases and vice versa.
-- water will move back and forth with less and less vigor until
equilibrium is established.
-- see fig. 4.8 p. 75.
Some more Definitions:
Deplasmolysis -- the gaining of water by a cell due to it being exposed to a
hypotonic environment.
Turgidity -- the pressure that water exerts from the cytoplasm toward the
outside of the cell against the cell membrane. Increases as
water volume increases.
Lysis -- the rupturing or bursting of a cell due to extreme deplasmolysis.
-- lysis can occur if the solute concentration difference between the cell
and the surrounding solution is quite large.
Hemolysis -- the bursting of a Red Blood Cell.
c. Hypertonic Solutions -- Cells shrink.
(Hyper-- = greater than)
-- solutions whose [solute(s)] is greater than that of another
solution/region are said to be hypertonic.
-- let's say that a cell is placed into a hypertonic solution (in a beaker).
-- since the surrounding solution has a greater [solute(s)] (greater
O.P.) than the cell, water will flow out of the cell and into the
surrounding solution.
-- the cell loses water volume and may shrivel.
-- along with the loss of water by the cell comes a decrease in turgor
pressure (turgidity), and an increase in O.P.
-- see fig. 4.8 p. 75.
Still more Definitions:
Plasmolysis – the losing of water by a cell due to it being exposed to a
hypertonic environment.
Crenation -- the shrivelling of a cell due to extreme water loss.
-- leads to death of cell.
-- crenation can occur if the solute concentration difference
between the cell and solution is quite large.
III). Facilitated Transport
-- see fig. 4.9 p. 76.
-- passive in nature (no energy (ATP) required), so it follows the law
of diffusion (ie. substances moving DOWN a concentration gradient,
meaning from a higher conc. to a lower conc.).
-- however, a CARRIER PROTEIN is required.
-- glucose, fructose, amino acids, and glycerol are too big to enter the
cell via channel proteins, so they enter the cell via carrier proteins.
-- some ions, as well, utilize carrier proteins.
-- the carrier proteins are very specific for the molecule they can
carry.
-- once the molecule (glucose etc.) binds to the carrier protein, the
protein changes shape so as to ‘carry’ the molecule to the other side of
the membrane.
-- hormones (eg. insulin) have the ability to ‘create’, or make
available, more glucose carriers.
Class B. ENERGY-REQUIRING TRANSPORT (ATP required)
I). Active Transport
-- see fig. 4.10 p. 76.
-- active transport occurs when molecules or ions need to move across
the cell membrane AGAINST (or UP) their concentration gradient (ie.
from a lower conc. to a higher conc)
-- requires ATP energy and a CARRIER PROTEIN.
*cells that perform a lot of active transport (eg. nerve cells)
have a large number of mitochondria near the cell membrane.
-- carrier proteins that are involved with active transport are
commonly called PUMPS.
eg. the sodium/potassium ion pump in nerve cells (see fig. 4.11
p. 77).
-- any molecule can be moved by active transport if it needs to be,
except for macromolecules such as carbohydrates, proteins, and fats,
which are too large.
-- protein pumps have two binding sites:
i. to bind the molecule/ion to be transported.
ii. to bind ATP.
II). Endocytosis/Exocytosis
-- see fig. 4.12 p. 78.
-- some molecules are too large to be transported by protein carriers,
thus they are transported into/out of the cell through VESICLE
formation.
-- ATP is required.
a. Endocytosis (ENTERING by vesicle):
-- the process by which a vesicle is formed at the cell membrane to
bring substances into the cell.
-- molecules may move UP or DOWN their conc. gradients but energy
is ALWAYS required in order for vesicles to form.
-- Three types of endocytosis:
i. Phagocytosis (‘cell-eating’):
-- very large material is engulfed by the cell.
-- this includes bacteria and dead cells.
eg. white blood cells (aka Phagocytes) perform phagocytosis
to engulf bacteria and to recycle worn out red blood cells.
ii. Pinocytosis (‘cell-drinking’):
-- engulfing of large molecules such as proteins, carbohydrates,
fats, and nucleic acids (macromolecules).
-- unlike phagocytosis, this process cannot be seen with a light
microscope.
-- once vesicles enter the cell, the contents must be digested so
they fuse with LYSOSOMES so this can occur.
iii. Receptor-mediated Endocytosis:
-- a form of pinocytosis that is more specific in that it utilizes
receptor proteins shaped for only specific molecules (vitamins,
hormones, lipoproteins, etc.)
-- receptors tend to accumulate in ‘coated pits’, which are
regions of the cell membrane pre-determined for the admittance
of specific substances.
-- see fig. 4.13 p. 79.
b. Exocytosis (Leaving by vesicle):
-- the reverse of endocytosis.
-- required for the secretion of molecules.
-- a vesicle containing materials fuses with the cell membrane and
releases the materials into the ECF.
eg. secretory vesicles from the Golgi.
Cell Size; again…
- for an organism to grow, its cells must divide.
- metabolic requirements impose upper limits on the size that is manageable
for a single cell.
- as a cell (among other things) grows, its volume increases faster than its
surface area (SA) (volume is a cubic function, while area is squared); thus,
its SA to Volume ratio decreases.
- Formulae: V (sphere) = 4/3πr3 ; SA (sphere) = 4πr2
ie. if the radius of a sphere is increased by tenfold, its SA increases by a
hundredfold, and its volume increases by a thousandfold.
- since SA dictates the abilities of cells to import/export materials with
respect to metabolism, if its ratio to cell volume is too small, the cell may
not be able to obtain/export what it needs to survive.
- thus, cells are small to maintain a practical SA:Volume ratio, not only for
import/export issues, but so that the nucleus can control a cell that isn’t too
large.