ALA Exam 3 Questions & Answers 2024/2025
What role might the statoliths (small, free-floating starch grains) play in gravitropism? - ANSWERSIt looks like they always tend to settle at the bottom of the cells. Therefore, they may press against the cell membrane, perhaps triggering a receptor in...
What role might the statoliths (small, free-floating starch grains) play in gravitropism? - ANSWERSIt looks
like they always tend to settle at the bottom of the cells. Therefore, they may press against the cell
membrane, perhaps triggering a receptor in the membrane, indicating the relative orientation of the
root.
Do stems grow "up" in response to light or gravity? How would a stem respond if it were suddenly placed
on its side (and illuminated with diffuse light from all around)? Go to the website below and click
"Arabidopsis Shoot Gravitropism" on the left. How does the growth response compare with what you
predicted? - ANSWERSStems grow both AGAINST the pull of gravity and TOWARD light. If placed on its
side, and illuminated from all directions, it would tend to grow up, against gravity
If stem growth is influenced by light AND gravity, which might have more of an impact? Go to the
website below and click "Cool Corn Phototropism" on the left. Explain the observed growth response. -
ANSWERSThe plants will tend to grow toward the light, but gravity will tend to counteract that influence
to some extent. In general, light has more of an impact on stem growth than gravity.
Plants tend to close their stomata when conditions are hot and dry in order to minimize the amount of
water lost through evaporation. Is it possible that somehow the water conditions in the soil trigger the
roots to control the opening and closing of stomata in the leaves? Consider the experimental
observations below. What conclusion can be drawn? Propose a mechanism by which roots can
communicate with the leaves - ANSWERSResults Panel—Notice the stomates in the plant on the left are
closed---- even though the leaves have the same water content as those on the plant to the right (whose
stomates are open). This suggests that the dry roots in the plant on the left are somehow controlling
what happens in the leaves—perhaps by sending a hormone to the leaves that triggers stomates to close
when roots are dry.
What is the function of root hairs? - ANSWERSIt helps to protect the root as it grows through soil. Also
plays a role in gravitropism.
Consider region "C". (root cap) In general, which direction does water tend to move through xylem? -
ANSWERSup
, 15. Consider region "C" (root cap) . In general, which direction does sugar tend to move through
phloem? - ANSWERSd
8. What trends do you observe regarding the relationship between the diameter of each of the cell types
and the time of day? How does the change in diameter correlate with relative amount of sunlight and
amount of water likely evaporating from the leaves - ANSWERSXylem gets narrower during midday
(when it is sunniest/warmest) while the phloem cells get wider at midday
Do the results above support the hypothesis that water is pulled through xylem under negative
pressure? Explain - ANSWERSWater is pulled up the tree under negative pressure (it is literally sucked up
as water evaporates from the leaves). This causes the xylem cells to shrink, but only when water is lost
through the leaves (not at night when it is cool and there is no sun to drive the evaporation).
22. Why do trees that grow in tropical rain forest often lack growth rings? - ANSWERSThe vascular
cambium undergoes fairly continuous cell division without becoming dormant during the winter months.
However, other seasonal variation (e.g., amount of water) can certainly have an impact on the formation
of rings
What is the record for the longest length of time that a vascular cambium has lived and produced new
wood (in other words—how old is the oldest tree)? - ANSWERSAround 5,000 years! This is roughly the
age of the oldest non-clonal tree (and, therefore, the age of the vascular cambium).
24. The tree below is likely dead by now. Why? - ANSWERSPhloem cells have been removed. As a result,
sugars produced in leaves won't be able to make it to the roots. Once roots die, the entire tree will also
die
4 types of tissue - ANSWERSepithelial, connective, muscle, nervous
epithelial tissue - ANSWERSimage
nervous tissue - ANSWERSimage
muscle tissue - ANSWERSimage
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