Download Diffusion, Osmosis, and Movement across a Membrane

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Unit 5, Part 2 Notes – Cell Membrane and Transport
Ms. OK, AP Biology
Structure of the Cell Membrane (the Fluid Mosaic Model)
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Phospholipids- give fluidity / flexibility to the membrane because they have unsaturated fatty acid tails that are
bent to space the phospholipids apart in the membrane
o Phospholipids form a double layer with the hydrophilic (polar / waterloving) heads facing the water on the outside and the inside of the cell
and the hydrophobic (nonpolar / water fearing) tails on the inside of the
membrane so they can separate themselves from the water on the
outside and the inside of the cell
o The membrane tends to be more flexible at higher temperatures, when
the fatty acid tails have more carbon to carbon double bonds (i.e. when
the fatty acid is MORE unsaturated), and when there are fewer
cholesterol (a type of steroid) molecules stuck between the phospholipids
Proteins- each cell membrane has a particular pattern of protein molecules
scattered across the surface of the cell membrane like a mosaic painting, There
are two types of membrane proteins based on their position within the
membrane—integral proteins and peripheral proteins.
o Integral- these membrane proteins are stuck all the way through the
membrane (i.e. they are embedded in the membrane); the portion of the
protein that comes into contact with the phospholipid heads is hydrophilic,
and the portion of the protein that comes into contact with the
phospholipid tails is hydrophobic.
o Peripheral- these membrane proteins are stuck either on the inner or outer
surface of the membrane (i.e. they are not “embedded” all the way
through)
Captions for pictures to the right…
1. This is an enzyme (used to speed up a chemical reaction that changes reactants into products)
that just happens to be on the cell
2. This is a carrier protein (used for facilitative diffusion) or protein pump (used for active transport)
that must bind to the molecule it is transporting before it changes shape to release the molecule on
the other side of the membrane
3. This is a glycoprotein (a protein with a carbohydrate chain attached) that is used to signify a cell’s
identity (ex: a bone cell vs. a blood cell) and helps cells to recognize one another)
4. These proteins are used to attach two cells to one another.
5. This is a protein channel (used in facilitated diffusion) that helps to transport charged particles or large, polar molecules
6. This protein helps to bind to and anchor the cytoskeleton.
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Carbohydrates
o Attach to phospholipid to form a glycolipid or a protein to form a glycoprotein on the outer surface of the
cell membrane
o These carbohydrate chains serve as markers to distinguish one cell from another (i.e. a red blood cell has
different carbohydrate chains on its surface than a bone cell) and assist with recognition between cells
Simple Diffusion
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Spontaneous movement of particles from an area of high concentration to an area of low concentration (i.e.
particles move WITH / DOWN their concentration gradient… note: a concentration gradient means there is a
difference in concentration across an area)
Does not require energy so it is called a passive process
Occurs via random kinetic movement (random movement of particles, which is faster at higher temperatures)
Net diffusion stops when concentration on both sides equal (if crossing a membrane) or when there is a uniform
distribution of particles… this is called equilibrium
o When equilibrium is reached, molecules continue to move, but no net (overall) change in concentration
(hence the phase "net diffusion" above)
o Diffusion of one compound (ex: carbon dioxide) is independent from diffusion of other compounds (ex:
oxygen); both will diffuse down their individual concentration gradients regardless of the concentration of
the other compound
Types of Molecules that can Move through Simple Diffusion Across the Membrane
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Diffusion directly through lipid bilayer
o The greater the lipid solubility of the diffusing particle, the more permeable the membrane will be (in other
words, nonpolar / hydrophobic particles will diffuse more easily through the nonpolar tail region of the
membrane)
o Smaller particles will typically diffuse more rapidly than larger particles
o O2, H2O, CO2 rapidly diffuse across the lipid bilayer
Diffusion of hydrophilic (aka polar) or charged molecules across the membrane
o Plasma membrane is semipermeable (i.e. it only lets certain substances through)
o Water, while polar, is small enough to freely move across the plasma membrane
o Larger hydrophilic molecules, such as sugars, do not freely move through the lipid bilayer
o Charged molecules cannot diffuse through lipid bilayer
o Channel or carrier proteins (see facilitated diffusion section below) are required to transport charged
molecules and larger, hydrophilic molecules through the membrane
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Facilitated Diffusion
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Allows diffusion of large, polar compounds such as sugars and amino acids or charged particles (ex: Na+)
Does not require energy (passive)
Highly Selective – each membrane transport protein transports a specific type of particle
Some transport proteins are channel proteins, which are tubes through the membrane that open when a
particular molecule binds to them
Some proteins are carrier proteins that change shape when a particular molecule binds to them, causing the
protein to release the molecule on the outside of the membrane
Fully reversible - molecules may enter the cell AND leave the cell through the transport protein.
Particles move from areas of high concentration to areas of low concentration.
Movement rate of particle movement depends on the number of membrane transport proteins
o Maximum rate limited by number of protein transporters
o Once all transporters are operating at 100%, an increase in concentration of particles will not increase the
rate of movement
How to Cheat - Glucose Continually Enters the Cell by Facilitated Diffusion
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Glucose binds to transport protein
Transporter changers conformation and
glucose is released into cell
Intracellular glucose is immediately
phosphorylated (i.e. an energized
phosphate group – perhaps from ATP – is
added to the glucose molecule)
o phosphorylated glucose does not
diffuse out (remember that the
transport protein is very specific)
o internal glucose (unphosphorylated)
concentration remains low providing
large concentration difference for
entry (so glucose continues to enter
the cell even though there is a high
concentration of phosphorylated
glucose inside the cell)
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Regulation of Glucose Uptake by Insulin
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Insulin stimulates increase in number of glucose transporters at membrane surface of liver cells as membrane
circles called vesicles bring more glucose transporters to the cell membrane (exocytosis)
o Increase in number of transporters increases glucose diffusion rate
o Driving force (aka the rate of diffusion) remains the same (due to phosphorylation of glucose once inside
the cell )
o Once in liver cells (i.e. removed from the
bloodstream), glucose molecules are joined
together to make a storage polysaccharide
(glycogen)
o This keeps the blood glucose level from getting
too high after a meal. This is one of the ways
that our body maintains homeostasis (stable
internal conditions). Insulin is secreted in
response to high blood glucose levels.
Low insulin levels decrease the number of glucose
transporters at membrane surface
o Portions of membrane with transporters fold
inward (endocytosis), trapping the transport
protein in a vesicle that enters the cell
o Vesicle cannot refuse with membrane until
insulin levels increase
o This keeps the blood glucose levels from
getting too low (additional glucose cannot enter
liver cells from the blood stream
o Another hormone, glucagon, stimulates the breakdown of glycogen in the liver cells into individual
glucose monomers and the secretion of glucose from liver cells into the blood stream. Glucagon is
secreted in response to low blood glucose levels
Diabetes
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Type I - Juvenile Diabetes - cannot make insulin
o Autoimmune disease
o Insulin-secreting pancreatic cells destroyed
Type II - Adult Onset Diabetes - loss of ability to respond to insulin
o Lack of membrane receptors for insulin to stimulate the addition of more transport proteins for bringing
glucose into the cell
Osmosis, the Passive Transport of Water
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Osmosis = the diffusion of water across a semi-permeable membrane (semi-permeable / selectively permeable =
only lets certain substances pass through)
Plasma membrane is permeable to water but not to certain solutes
o Solutes = dissolved particles
o Solvent = liquid in which particles may be dissolved (in this case, water)
Water moves from solution with lower concentration of dissolved particles (i.e. a high concentration of water) to
solution with higher concentration of dissolved particles (i.e. a low concentration of water)
Water moves from dilute solution to concentrated solution
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Solution Types Based on Solute and Water Concentrations (Note: these terms could refer to solutions outside or
inside the cell)
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Hypertonic Solution:
o Hyper = more (think hyperactive); Tonic = dissolved particles
o This solution has a high concentration of solutes and a low concentration of water
Hypotonic Solution:
o Hypo = less, under (think hypodermic, hypothermia); Tonic = dissolved particles
o This solution has a low concentration of solutes and a high concentration of water
Isotonic Solution:
o This solution has the same concentration of solutes and water as the solution to which it is being
compared
Note: During osmosis, water always moves from a hypotonic solution (high water concentration) to a hypertonic solution
(low water concentration)
Let’s say these solutions are on the outside of animal vs. plant cells…
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If an animal cell is in hypotonic solution (high water), the inside of the cell is hypertonic (low water) compared to
the outside solution. Water should move from the outside solution into the cell. If too much water moves in, the
animal cell could swell to the point of bursting the cell membrane. This is called lysis.
If a plant cell is in hypotonic solution, water will move into the plant cell, but it will not burst due to the cell wall.
Pressure of the cell membrane on the cell wall due to water coming in is a good thing for the plant cell. This is
called turgor pressure and keeps plant stems from wilting. A plant cell in this state is considered “turgid.”
If an animal cell is in isotonic solution, the water concentration outside the cell is the same as the inside of the
cell. Water will move into and out of the cell at the same rate, so there is no net (overall) movement of water into
or out of the animal cell. In this situation the animal cell maintains the same size / shape.
If a plant cell is in isotonic solution, water will move into and out of the cell at the same rate. Because there is not
an overall movement of water into the cell, the cell membrane will not push against the cell wall, and the cell is
considered “flaccid.” In this situation, a plant would begin to wilt slightly.
If an animal cell is in hypertonic solution (low water), the inside of the cell is hypotonic (high water) compared to
the outside solution. Water should move from the inside of the cell to the outside solution. When water moves
out of the cell, the cell shrivels or shrinks like a raisin in the sun.
If a plant cell is in hypertonic solution, water will move out of the plant cell, and the cell membrane will pull away
significantly from the cell wall. In this situation, the cell is considered “plasmolyzed,” and the plant as a whole will
wilt considerably.
Using Adaptations to Maintain Water Balance
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Paramecia, single-celled protists that live in freshwater, are constantly in an environment that is hypotonic (high
water) compared to their cells. This means that water is always entering their cells.
Paramecia solve the problem of excess water entering their cells by using an organelle called a contractile
vacuole that can pump water out of the cell.
You can alter the rate of contractile vacuole pumping by placing the Paramecium in increasingly hypotonic (high
water) solutions
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Active Transport
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This is another type of transport across the membrane that
involves movement of solutes UP / AGAINST the concentration
gradient
Movement from low to high concentrations
Requires the use of membrane proteins that act as “pumps” to
move solutes across the membrane after an input of energy
(usually from ATP)
o Can be saturated like facilitated diffusion proteins (in other words, once all the pumps are in use, the rate
of transport will not increase)
o The energy requirement distinguishes active transport from facilitated diffusion
The Na+ / K+ Pump (i.e. the sodium potassium pump): An Example of Active Transport
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An integral membrane protein pumps K+ into the cell and pumps Na+ out of the cell (sometimes from a low to high
concentration, requires energy from ATP)
First, pump binds 3 Na+ inside cell
ATP is hydrolyzed (bond between last two phosphate groups is broken by
adding water, which releases energy) to become a single phosphate group
(called inorganic phosphate or Pi) and ADP (adenosine diphosphate).
The single phosphate group is transferred to the Na+ / K+ pump (this is called
phosphorylation, which energizes the pump and allows it to change shape)
when the pump is phosphorylated, its configuration (aka shape) changes and it
opens up to release the Na+ on the outside of the cell
The Na+ are released (the altered configuration does not favor the binding of more Na+ on the outside of the cell)
Two K+'s from the outside now bind to the altered protein
The binding of the K+ causes the protein to lose its phosphate group
Now that the phosphate group is gone, the altered protein reverts back to its original shape, which was open to
the inside of the cell
The original shape does not favor the binding of K+, so these are released. Na+ then binds to the protein and the
process is repeated
The Na+ / K+ pump is highly important in the nervous system. It helps stop nerve signaling so that muscles are
not perpetually stimulated by motor neurons (nerve cells that stimulate muscle cells)
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Other Active and Transport Mechanisms - The H+ / Sucrose Pump
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H+ is actively pumped out of the cell by a protein pump when
ATP is hydrolyzed (broken by adding water)
H+ accumulates outside the membrane,
H+ binds to another membrane protein, but sucrose must
also bind. When both are bound, the configuration changes,
and the protein opens to the membrane interior.
o This is known as cotransport as two molecules are
moved across a membrane… the movement of H+
down (with) its concentration gradient is used to
power the movement of sucrose up (against) its
concentration gradient
o It is also known as a symport as both molecules are
crossing in the same direction
o If the molecules are moving in opposite directions it
is known as an antiport
The H+ / Sucrose Symporter is used to move sugar from leaf
cells (where they are made during photosynthesis) across
the leaf cell membrane into to the phloem (the tube-like
tissue used to transport sugar from the top of the plant to the roots).
Active Transport to Move Large Quantities across the Cell Membrane
 Endocytosis- requires a vesicle (a membrane-surrounded sac), the cell takes large
quantities of particles by forming new vesicles from the plasma membrane
o Phagocytosis- “cell eating,” the cell membrane engulfs large quantities of
solid particles to take into the cell
o Pinocytosis- “cell drinking”, the cell membrane engulfs large quantities of
liquid with dissolved solutes to take into the cell
o Receptor-Mediated Endocytosis – triggered by a molecule binding to a
receptor on the cell membrane, which initiates folding in of the
membrane…nothing comes in unless the correct molecule binds to the
receptor
 Exocytosis- reverse of endocytosis, cell secretes particles by fusing a vesicle with
the membrane and spilling contents of the vesicle to outside
Top Image: Phagocytosis
Middle Image:
Pinocytosis
Bottom Image: ReceptorMediated Endocytosis
Exocytosis 
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