Download Biology 123 Dr. Raut`s Class Session 6

Biology 123 Dr. Raut’s Class
Session 6- 2/2/2015
1. What does amphipathic mean? Why is it important that the plasma membrane is amphipathic?
Amphipathic means that the molecule- a phospholipid, in this case- has both a hydrophobic and a
hydrophilic region. In a phospholipid, the hydrophobic area is the fatty acid tails, and the
hydrophilic area is the glycerol head. It is important for the plasma membrane to be amphipathic,
because it helps it arrange itself and helps with selective permeability.
2. How is the plasma membrane arranged? How does this change with changing temperatures?
How is this affected by cholesterol?
The plasma membrane is a phospholipid bilayer. This means that the hydrophobic fatty acid tails
arrange themselves inside the membrane, while the glycerol heads face out towards the aqueous
solutions of the cytoplasm and the solution outside the cell. These phospholipids can either be
saturated or unsaturated. Saturated phospholipids make the plasma membrane more viscous,
which is helpful in hot temperatures. Unsaturated phospholipids make the plasma membrane
more fluid, which is helpful in cold temperatures. Cholesterol is helpful in both hot and cold
temperatures, because it can make the membrane either more viscous or fluid.
3. What are the different types of proteins that can be found on the membrane? What are the
functions of these different proteins?
Proteins on the membrane can either be peripheral proteins-only attach to the surface of the
membrane- and integral proteins-penetrate into the membrane, often spanning through the entire
membrane. Plasma membranes can function in transporting molecules across the membrane,
enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment
to the cytoskeleton and the extracellular membrane.
4. How do molecules get to the plasma membrane?
Molecules that are going to the membrane are sent from the golgi in vesicles. Any materials that
will be attached to the plasma membrane will be attached to the vesicle in the same way. Then
the vesicle will fuse with the plasma membrane, making the outside face of the vesicle
continuous with the inside of the plasma membrane and the inside of the vesicle continuous with
the outside of the membrane.
5. Why is it important that the plasma membrane is selectively permeable?
Selectively permeable means that the plasma membrane has the ability to pick and choose what
come inside of the cell. This is incredibly important! Otherwise, anything could just diffuse into
your cells without the cell having any control.
6. What are the two basic kinds of transport across the plasma membrane? What is the difference
in the two basic methods of transport?
The two basic types of transport are passive transport and active transport. The difference
between these two methods is that active transport uses ATP and passive transport does not.
Therefore, if something can diffuse into or out of the cell by following its concentration gradient,
then that is preferable for the cell because it means the cell does not have to spend the energy to
move that molecule. However, if the cell ever wants to move a molecule against its concentration
gradient, it will have to use active transport.
7. Describe the three types of passive transport.
Simple diffusion: small, nonpolar molecules are able to diffuse across the plasma membrane’s
hydrophobic region with no problem. They simply follow their concentration gradient and
diffuse across the membrane. Examples: oxygen and CO2
Osmosis: defined as the movement of water from an area of high free water concentration to an
area of low free water molecule concentration across a selectively permeable membrane. Even
though water is polar, it is small enough that it can diffuse across the membrane on its own;
however, this is too slow for the cell to rely on alone, and they often are aided by aquaporins,
which are an example of a channel protein.
Facilitated diffusion: molecules diffuse passively across the membrane with the help of transport
proteins. This allows ions and polar molecules to cross the plasma membrane. These transport
proteins include both channel proteins and carrier proteins. Channel proteins are lined with
hydrophilic regions and can be either open, in which the molecule flows right through, or gated,
in which the “gate” will open or close in response to a stimulus. Carrier proteins undergo a subtle
change in shape when the molecule binds to the protein causing the molecule to be put on the
other side of the membrane.
8. A cell has a glucose concentration of .75 M and is in a solution with a glucose concentration
of .3 M. Is this solution hypotonic, isotonic, or hypertonic? What will water do?
This solution is hypotonic. The water will rush into the cell.
9. Compare and contrast animal and plant cells in hypotonic, isotonic, and hypertonic solutions.
In a hypotonic solution, the water will rush into the cell. In an animal cell, this will cause the cell
to eventually lyse. Due to the plant cell’s cell wall, this water will simply cause the cell to be
turgid, which means that the cell is pushing out on the cell wall. Being turgid helps a plant to
stand up.
In isotonic solution, the same amount of water that is coming into the cell is leaving the cell. In
an animal cell, this will make the cell normal and happy. In a plant cell, the cell will become
flaccid. This will cause both the plant cell and the entire plant to be limp.
In hypertonic solutions, the water will rush out of the cell. In an animal cell, this will cause the
cell to shrivel. In plant cells, this will cause the cell to plasmolyze. When a cell plasmolyzes, the
entire cytoplasm is pulled back from the cell wall. This will make the plant appear very wilted
and can cause plant death.
10. Which of the following is the most probable description of an integral, transmembrane
protein?
A) amphipathic with a hydrophilic head and a hydrophobic tail
B) a globular protein with hydrophobic amino acids in the interior and hydrophilic amino acids
arranged around the outside
C) a fibrous protein coated with hydrophobic fatty acids
D) a glycolipid attached to the portion of the protein facing the exterior of the cell and
cytoskeletal elements attached to the interior portion
E) a middle region composed of alpha helical stretches of hydrophobic amino acids, with
hydrophilic regions at both ends of the proteins
11. You observe plant cells under a microscope as they are placed in an unknown solution. First
the cells plasmolyze; after a minute, the plasmolysis reverses and the cells appear normal. What
would you conclude about the unknown solution?
A) It is hypertonic to the plant cells, and its solute cannot cross the plant cell membranes.
B) It is hypotonic to the plant cells, and its solute cannot cross the plant cell membranes.
C) It is isotonic to the plant cells, but its solute can cross the plant cell membranes.
D) It is hypertonic to the plant cells, but its solute can cross the plant cell membranes.
E) It is hypotonic to the plant cells, but its solute can cross the plant cell membranes.
12. Which of the following is not true of carrier molecules involved in facilitated diffusion?
A) They increase the speed of transport across a membrane.
B) They can concentrate solute molecules on one side of the membrane.
C) They may have specific binding sites for the molecules they transport.
D) They may undergo a change in shape upon binding of solute.
E) They do not require an energy investment from the cell to operate.