35. Soil

The inorganic nutrient most often lacking in crops is a. carbon. b. nitrogen. c. phosphorus. d. potassium.

Consider the following statements regarding limiting nutrients. Select True or False for each statement. T/F Nitrogen (N), phosphorus (P), and potassium (K) are common examples. T/F Their presence limits the availability of micronutrients. T/F Their availability tends to limit plant growth. T/F Certain macronutrients and micronutrients can be considered limiting nutrients.

Micronutrients are needed in very small amounts because a. most of them are mobile in the plant. b. most serve mainly as cofactors of enzymes. c. most are supplied in large enough quantities in seeds. d. they play only a minor role in the growth and health of the plant.

Where is the start codon located? a. at the start (5′ end) of the mRNA b. in the DNA just upstream of where transcription starts c. at the downstream end of the 5′ untranslated region (UTR) d. at the upstream end of the 3′ untranslated region (UTR)

Where does most nutrient uptake occur in roots? a. at the root cap, where root tissue first encounters soil away from the zone of nutrient depletion b. at the Casparian strip, where ions must enter the symplast before entering xylem cells c. in the symplastic and apoplastic pathways d. in root hairs, in the zone of maturation

Why are proton pumps in root-hair plasma membranes important? a. They pump protons into cells, generating a membrane potential (voltage). b. They allow toxins to be concentrated in vacuoles, so the toxins do not poison enzymes in the cytoplasm. c. They set up an electrochemical gradient that makes it possible for roots to absorb cations and anions. d. They set up the membrane voltage required for action potentials to occur.

Why is the presence of clay particles important in soil? a. They provide macronutrients—particularly nitrogen, phosphorus, and potassium. b. They bind metal ions, which would be toxic if absorbed by plants. c.They allow water to percolate through the soil, making oxygen-rich air pockets available. d. The negative charges on clay bind to positively charged ions and prevent them from being leached out of the soil.

Some of the problems associated with intensive irrigation include all of the following except a. soil salinization. b. overfertilization. c. land subsidence. d. aquifer depletion.

Suppose that certain root cells have an overall charge that is more negative than normal. What impact would this likely have on the uptake of anions such as NO3−? a., Anions would be less likely to enter roots. b. Anions would be more likely to enter roots. c. This would have no impact on the ability of anions to enter roots. d. This would make anions and cations equally likely to enter roots.

A mineral deficiency is likely to affect older leaves more than younger leaves if a. the mineral is a micronutrient. b. the mineral is very mobile within the plant. c. the mineral is required for chlorophyll synthesis. d. the mineral is a macronutrient.

In a semester-long experiment tracking growth in plants, your lab partner—who often skips class—carefully records the mass of water added to a potted plant with the expectation that this addition will be fully accounted for in the mass gained by the plant. Is your lab partner right or wrong? Explain.

Your friend claims that all plants are autotrophs because they perform photosynthesis. Is that a correct statement? Explain.

Two groups of tomatoes were grown under laboratory conditions, one with humus added to the soil and one a control without humus. The leaves of the plants grown without humus were yellowish (less green) compared with those of the plants grown in humus-enriched soil. The best explanation is that a. the healthy plants used the food in the decomposing leaves of the humus for energy to make chlorophyll. b. the humus made the soil more loosely packed, so water penetrated more easily to the roots. c. the humus contained minerals such as magnesium and iron needed for the synthesis of chlorophyll. d. the heat released by the decomposing leaves of the humus caused more rapid growth and chlorophyll synthesis.

In what ways are a promoter and a start codon similar? In what ways are they different?

Why is it important for plants to exclude certain ions? Summarize the difference between active and passive exclusion mechanisms.

There is a conflict between van Helmont's data on willow tree growth and the data on essential nutrients listed in Table 36.1. According to the table, nutrients other than C, H, and O should make up about 4 percent of a willow tree's mass. Most or all of these nutrients should come from soil. But van Helmont claimed that the soil in his experiment lost just 60 g, while the tree gained 74,000 g. Calculate the percentage of the added mass accounted for by soil, and compare it to the predicted 4 percent. State at least one hypothesis to explain the conflict between expected and observed results. How would you test this hypothesis?

DRAW IT Draw a simple sketch of cation exchange, showing a root hair, a soil particle with anions, and a hydrogen ion displacing a mineral cation.

Toxins like αα-amanitin are used for research in much the same way as null mutants (Chapter 16)—to disrupt a process and see what happens when it no longer works. Researchers examined the ability of αα-amanitin to inhibit different RNA polymerases. They purified RNA polymerases I, II, and III from rat liver, incubated the enzymes with different concentrations of αα-amanitin, and then tested their activity. The results of this experiment are shown here. These findings suggest that cells treated with αα-amanitin will have a reduced level of: a. tRNAs b. rRNAs c. snRNAs d. mRNAs

If you wanted to use αα-amanitin to shut down 95 percent of transcription by RNA polymerase II, roughly what concentration of αα-amanitin would you use? Note that the scale on the x-axis of the graph in Question 13 is logarithmic rather than linear, so that each tick mark shows a tenfold higher concentration.