Bio F305F. Summer 2006
Name
Test #3
1. (a) Give an example of an easily visible endogenous circadian rhythm
in plants.
Ans.:
Leaves move up and down each day rhythmically reaching the same high or low
point at approximately the same time each day.
Ans.:
Because the rhythm is controlled by the plant itself and will persist with the
same timing in constant environmental conditions, showing that the rhythm does
not require any external cues to persist.
(c) Why is the rhythm called "circadian"?
Ans.:
Circadian = about a day, meaning period between peaks of the rhythm and between
troughs of the rhythm is about 24 h long
2. (a) Angelina Jolie's favorite flowering plant, Bradus pittus, flowers when days are 16 hours long and nights are 8
hours long. She wants to make this plant flower in the middle of the winter,
when days are only 8 hours long. Jolie is on a tight budget,, and wants to
achieve this goal in the most economic way, so that her electric bill does not
go up much. She consults you to learn how best to do this. What advice would you
give her and why?
Ans.:
The photoperiodic signal for Bradus pittus is night length, so if Angelina
shines a white or red light on Bradus pittus briefly in the middle of the long
night in winter, it will think the nights are short and will flower.
(b) Angela has 4 Bradus pittus
plants. You explain to her how she could make all 4 plants flower while treating
only one of the plants in the way described in your answer to 2 (a). What is
this method and why does it work?
Ans.:
If she grafted the other three plants on to the one that gets the light
treatment, then the others will also flower because the light-treated plant
produces a flowering hormone (florigen) which can induce flowering not only in
that plant but also in all the plants that it is connected to by grafting.
3. (a) Draw both layers of a bilayer membrane, and label which portions
are hydrophilic and which are hydrophobic.
(b)
Can scientists predict which portions of a membrane protein will fit
where in a membrane? Explain.
Ans.:
Yes, Hydrophilic amino acids will associate with hydrophilic portions of the
membrane (head groups), hydrophobic amino acids will tend to be clustered in
hydrophobic portions of the membrane.
(c) Describe an experiment that uses mouse and human cells and concludes
membranes are fluid in some circumstances and not fluid in others. Your answer
should describe the results of this experiment and show how the result supports
the conclusion.
Ans.: Use fluorescent tags to label proteins on surfaces of mouse (red) and human (green) cells then induce the two cell types to fuse. If temperature is lowered below ˇ°freezing pointˇ± of membrane right after the fusion, then the red and green labeled proteins will stay separate after the fusion, indicating that the membrane is not so fluid. If the temperature is left above the freezing point of the membrane after the fusion the red- and green-labeled proteins will intermingle, indicating that the membrane is fluid.
4. (a) In mid-summer the membranes of many plants will ˇ°freezeˇ± at
40o F, but in winter a much lower temperature is required to freeze
the membrane. Why?
Ans.:
As the temperature begins to cool in the Fall, plants will begin to put more
double bonds into the fatty acid chains of their phospholipids. These double
bonds put ˇ°kinksˇ± in the chains, making them harder to freeze. In the
summer, without time to put double-bonds into their fatty acid chains, plant
membranes will freeze at the same low temperatures where they will remain fluid
once the double bonds are inserted.
(b) In November, often days are still warm in Austin, yet some plants
are beginning to make their membranes more fluid. What is the environmental
signal that is inducing plants to do this?
Ans.:
Length of night, or photoperiod.
5. (a) Name two kinds of transport proteins, and state how their
functions differ from each other.
Ans.:
Membrane channels allow the passive movement of ions down their concentration
gradient; membrane pumps use the energy of ATP to pump ions from region of low
to region of high concentration.
(b) What would be an example
of a membrane protein that functions in cell-cell recognition, and where is this
protein located?
Ans.:
The female factor in sporophytic self-incompatibility is a membrane protein
kinase, and it is located on the plasma membrane of stigma cells.
(c) There is a membrane enzyme that participates in plant defense
responses. (i) What is this enzyme? (ii) This enzyme breaks up a molecule
commonly found in cells. What is this molecule and where does the enzyme break
it?
Ans.:
(i) Lipase. (ii) It breaks up lipids, separating the head group from the tail
groups, thus releasing fatty acids (like linolenic acid).
6. (a) In the Figure below, label each type of receptor.
(b) (i) What flows across
the type A receptor shown in question 6(a)?
(ii) In principle, the arrow in Figure A of question 6(a) could be
pointing into a cell or out of a cell. What information would you need to know
in order to predict which direction the substance flowing across the receptor
was going?
Ans.:
(i) ions flow across ; (ii) The direction of flow will always be from the side
of the membrane that has the higher concentration of the ion to the side that
has the lower concentration.
(c) Give one specific example of a known Receptor of type A and a known
Receptor of type C above.
Ans.:
Type A: ATP receptor, or
compression (gravity) or stretch-activated receptor; type C: phototropin.
7. (a) In the Figure below, what would be an example of a first
messenger, what would be a relay molecule, and, broadly speaking, what would be
a general function of all effector molecules?
Ans.:
First messenger is hormone; relay molecule is Ga;
general function is to amplify the signal by producing many copies of a product
that can serve as an activator of the next signaling step.
(b) For the relay molecule to move between the receptor and the effector
proteins it must be activated. What activates the relay molecule? What makes the
relay molecule become inactive?
Ans.:
G-protein binding to the activated receptor activates the relay molecule (Ga); GTPase activity of the relay molecule, or the
conversion of bound GTP to GDP inactivates the relay molecule.
(c) Why is having so many steps in a signal transduction chain helpful?
Ans.:
Each step amplifies the response further by increasing the number of active
agents that can induce changes in the cell. The more of these steps linked
together, the greater the amplification.
8. (a) In some (but not all) signal transduction chains that use the
type of receptor shown in Figure 7(a), one downstream response may be
the release of Ca2+ into the cytoplasm. Where is this Ca2+
released from?
Ans.:
From the ER or vacuole.
(b) Why do cells have to keep their calcium concentration so low?
Ans.:
To keep the phosphate in the cell free for important functions. If calcium
remains high in the cytoplasm very long it will form insoluble complexes with
phosphate, making phosphate unavailable for synthesis of critical molecules like
ATP, DNA, RNA.
(c) There is a logical connection between the fact that cells keep their
calcium concentration very low and the fact that cells often use an increase in
calcium as a signaling agent for many different stimuli. What is that
connection?
Ans.: Because the [Ca2+]cyt
is maintained so low, and because calcium enters the cytoplasm via passive
channels, it takes little energy to leak enough calcium into the cytoplasm to
raise the concentration many-fold, so low energy stimuli would be able to induce
an increase in calcium.