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Maddie Hughes
Period 6
10/4/09
Membranes
1. What does selective permeability mean and why is that important to cells?
Selective permeability means that the plasma membrane allows some
substances to cross into or out of the cell through the membrane more easily
than others. This is important because it allows the cell to regulate transport
across cellular boundaries, for example by allowing nutrients to enter and waste
to exit the cell, while at the same time regulating the concentration of materials
within the cell such as inorganic ions, like Na+ or Cl.-2. What is an amphipathic molecule?
This is a molecule that has both a hydrophilic and hydrophobic region.
Phospholipids are an example of amphipatchic molecules; these phospholipids
are the most abundant in cell membranes, because the ability to form
membranes is part of their molecular structure. Most of the proteins within the
cell membrane also have both hydrophobic and hydrophilic regions.
3. How is the fluidity of cell’s membrane maintained?
In order to remain fluid the temperature of the membrane must remain above a
certain temperature, because if the membrane becomes to cool, it will solidify,
thus inhibiting the necessary functions of the membrane. In order to remain fluid
to a lower temperature membranes are often rich in phospholipids with
unsaturated hydrocarbon tails, because the kinks from the double bonds in these
tails cause the phospholipis to stay farther apart and more fluid then if they were
closely packed together, as is the case with saturated hydrocarbon tails. Also,
cholesterol is used in the membrane as a temperature buffer which keeps the
membrane fluid by hindering the close packing of phospholipids, while at the
same time keeping the membrane at a good fluidity level by restraining the
movement of phospholipids so the membrane doesn’t heat up too much.
4. Label the diagram below – for each structure – briefly list it’s function:
Fibers of extracellular matrix: purple tube like structures with green and purple
fibers branching off. They function in embedding the animal cells and contain
polysaccharides and proteins. Integral protein: The upright purple crosssections. They function as channels for facilitated diffusion and also help to
structure the membrane. Cholesterol: attached yellow hexagons. They function
as temperature buffers and help to maintain membrane fluidity. Peripheral
protein: The purple cross-sections that lay alongside the membrane. They
attach to the fibers of the extracellular matrix and the microfilaments of the
cytoskeleton (serve a variety of transport and mechanical function) to help give
structure to the membrane. Glycoprotein: purple “bubble” which bonds to green
hexagons. These are proteins that bond with carbohydrates (sugars) to form the
cells identification tag. Glycolipid: green, strip bonded to a phospholipid. The
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lipid bonds with the sugar, and also functions in cell to cell recognition.
5. List the six broad functions of membrane proteins.
Transport, enzymatic activity, signal transduction, cell-cell recognition,
intercellular joining, and attachment to the cytoskeleton and extracellular matrix.
6. How do glycolipids and glycoprotein help in cell to cell recognition?
On each cell’s surface the carbohydrates vary, even from one cell type to another
in an individual, so this diversity of molecules and location enables membrane
carbohydrates such as those found in glycolipids and glycoprotein to function as
markers to help identify each cell.
7. Why is membrane sidedness an important concept in cell biology?
Membrane sidedness is an important concept in cell biology because the
molecules which come from the inside of the endoplasmic reticulum end up on
the outside of the cell membrane when vesicles release proteins in exocytosis,
so the composition must match up. The outside layer of membrane is distinctly
different from the inside layer, and if the two were confused, the functions of the
membrane would be greatly disturbed.
8. What is diffusion and how does a concentration gradient relate to passive
transport?
Diffusion is when a substance moves down its concentration gradient from a
more concentrated to less concentrated area. The concentration gradient relates
to passive transport because in passive transport no energy is used to push
materials across the membrane, instead, the tendency for the substance to move
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to a lower area of concentration drives the diffusion.
9. Why is free water concentration the “driving” force in osmosis?
Free water concentration is the driving force in osmosis because similar to
diffusion, there is a tendency for water to travel to the area of lower concentration
from the area of higher concentration until equilibrium is established. “Free”
water is key in this situation though, because although one solution may contain
more solute, it does not necessarily contain more water to the fact that water
clusters around hydrophilic solute molecules, making that water unavailable for
travel.
10. Why is water balance different for cells that have walls as compared to cells
without walls?
Cells with walls will not burst when placed in a hypotonic solution. The cell will
absorb water, but before it can burst the wall will exert a force back on the cell so
that no more water can enter, therefore, it can absorb more water than an animal
cell, which bursts if too much water is taken in. In fact, plant cells are healthier in
a hypotonic environment, where their cells are turgid (firm), while animal cells
fare much better in an isotonic environment.
11.
Label the diagram below:
In this diagram the first row shows an animal cell, in this case a red blood cell,
when placed in three different solutions. In the hypotonic solution the cell has
burst because more water was taken in then the cell could hold. In the isotonic
solution the cell is normal because it is at equilibrium with the system and takes
in as much water as necessary to stay healthy. In the hypertonic solution the cell
has shriveled and dried because it lost too much water to the outside solution,
which contains more nonpenetrating solutes. In the second row a plant cell is
sown. In the first column the cell is at a normal water level as shown by the
turgid state of the cell and the tautness of the cell wall (dark brown). In the
second column the cell is flaccid, or limp, because there is not enough water to
fully extend the cell wall. In the third column the cell has been plasmolyzed. This
means the plant cell has shriveled and the plasma membrane has pulled away
from the cell wall; this will most likely kill the cell.
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12. What is the relationship between ion channels, gated channels and facilitated
diffusion.
In facilitated diffusion, molecules and ions need to be assisted in order to make it
across the cell membrane. They are aided by transport proteins that span the
membrane. One example of these channel proteins is the ion channels, which
allow ions to pass into or out of the cell. Oftentimes these ion channels function
as gated channels, which means they must have an electrical or chemical
stimulus which causes them to open or close and allow the ion to pass and
diffusion to occur.
13. How is ATP specifically used in active transport?
ATP is used in active transport by serving as the energy which causes diffusion
to occur in the opposite direction of its concentration gradient. One way ATP is
specifically used is that it can transfer its terminal phosphate group directly to the
transport protein, which causes the protein to change its conformation in a way
which carries a solute bound to the protein across the desired membrane.
14. Define and contrast the following terms: membrane potential, electrochemical
gradient, electrogenic pump and proton pump.
Membrane Potential: The voltage across a membrane. It serves as a battery
which affects the travel of all charged substances across a membrane.
Electrochemical Gradient: The combination of the chemical and electrical forces
acting on an ion.
Electrogenic Pump: A transport protein that generates voltage across a
membrane.
Proton Pump: The main electrogenic pump of plants (and bacteria), which
actively transports hydrogen ions out of the cell. These terms are different
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because the first refers to the general set up of the voltage on a membrane, while
the other terms fall into this general category. The second term the direction of
travel of the ions across the membrane, while the third and fourth terms are
similar because they describe how the ions travel across the membranes, though
different in that the third term refers to travel in animal cells, while the third term
refers to transport of ions in animal and bacteria cells.
15. What is cotransport and why is it an advantage in living systems?
Cotransport is when a substance being diffused down its concentration gradient
assists in the uphill transport of another substance. This is an advantage in living
systems because one use of energy can cause multiple occurrences, which
lowers the necessary energy needed and produced while at the same time
speeding up diffusion. This is good for living systems because they do not have
to work as hard while still accomplishing a large amount of work.
16. What is a ligand?
A ligand is a molecule that binds specifically to the receptor site of another
molecule.
17. Contrast the following terms: phagocytosis, pinocytosis and receptor-mediated
endocytosis. Phagocytocis is where a particle is engulfed in a cell and wrapped
in pseudopodia and packaged in a membrane formed vacuole. The cell then
digests the material using hydrolytic enzymes. This is different than pinocytosis,
where simply gulps droplets of extracellular fluid in order to obtain the molecules
dissolved within the droplets. This process is very non-specific and absorbs
fluids instead of the solid substances of phagocytosis. Receptor mediated
endocytosis is extremely different from pinocytosis because only specific
substances are absorbed by the cell, instead of whatever happens to be in the
droplets.
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