Biology | Answers to Review Questions

Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Answers to Review Questions

Chapter 39 (p. 792)

1. Macronutrients are those required in large quantities, whereas micronutrients are those that are required only in small quantities. Both kinds of nutrients are required to the same degree but not in the same quantities. Carbon, oxygen, and hydrogen are major components of organic molecules. Nitrogen is essential to the formation of amino acids, proteins, nucleotides, nucleic acids, chlorophyll, enzymes, and coenzymes. Magnesium is a component of chlorophyll and activates enzymes. Phosphorous is found in ADP and ATP, nucleic acids, phospholipids, and coenzymes. Sulfur is in some amino acids, proteins, and coenzymes.

2. Plants cannot utilize atmospheric or dissolved nitrogen. They can only use nitrogen that is chemically combined. Legumes have formed an association with nitrogen-fixing bacteria. The legumes house the bacteria in nodules on their roots and the bacteria provide chemically combined (fixed) nitrogen as a byproduct of their metabolism.

3. Pressure potential is pressure exerted by the atmosphere on the plant. Solute potential refers to the higher concentration of solutes within the root contributing to the increase of diffusion of water from the surrounding soil into the plant. Pressure potential at the top of the plant is less than the pressure coming from the roots, so water rises up the plant (positive pressure from the atmosphere and negative pressure, or pull, via transpiration from the leaves).

4. Of the water that enters a plant, 90% leaves it via transpiration.

5. Root hairs are turgid as a result of their greater solute potential. Energy is expended to accumulate minerals in a plant root because their concentration is much greater in the root than in soil, so ions must be pumped in at a cost of ATP and against a concentration gradient.

6. Water transport is reduced to near zero when the relative humidity is 100%, i.e., at night. Almost no transpiration occurs under these circumstances.

7. Stomatal control requires energy; ATP is used to pump ions into guard cells, water follows passively, and cells stay turgid.

8. Translocation is the movement of sugars from where they are produced to where they are used; they are stored as starch and transported as sucrose. The driving force behind translocation is hydrostatic pressure.

9. Carbohydrates move through a plant as follows: sucrose is produced at the source, which is then actively loaded into the veinlet sieve tubes, which increases their solute concentration. Then water flows into the tubes via osmosis, sucrose is unloaded at the sink, water follows it and moves toward the sink where it is pulled out, and sucrose follows it. The process has an active source and sink and is passive in between.