Problem 7 - Biology: The Dynamic Science

Question

The nod genes in the bacteria in soybean nodules allow the bacteria to fix nitrogen. Which of the following is not a step in this process? a. The products of nod genes cause cells of the root cortex to divide and become the root nodule in which bacteroids fix nitrogen for the plant. b. In the cortex cells bacteria enlarge and become immobile, forming bacteroids. c. Bacteria enter the root hair cell and multiply, causing the cell plasma membrane to form an infection thread that extends into the root cortex. d. Roots release flavonoid, which turns on the expression of bacterial nod genes. Products of nod genes cause the tip of the root hair to curl toward the bacteria. e. Root hairs trigger release of bacterial enzymes that break down root hair cell walls. f. All of the above are steps in the process.

Step-by-Step Solution

Answer

The statement that is not a step in the nitrogen fixation process by nod genes in soybean nodules is e. Bacterial enzymes do not degrade the root hair cell walls.

Step 1: Evaluate statement a

Statement a describes that the products of nod genes stimulate root cortex cells to divide and form root nodules. This is a correct description of the process as nod genes do indeed play a key role in nodule formation and nitrogen fixation.

Step 2: Evaluate statement b

Statement b explains that in cortex cells, bacteria evolve into larger, immobile bacteroids. This is true as during the formation of nodules, bacteria differentiate into bacteroids which are responsible for nitrogen fixation.

Step 3: Evaluate statement c

Statement c mentions that bacteria enter the root hair cell, multiply and trigger the formation of infection thread. This is also accurate as this is part of the process of nodule formation where bacteria enter the plant cells via infection threads.

Step 4: Evaluate statement d

Statement d indicates that root releases flavonoid which triggers the expression of bacterial nod genes, leading to curling of root hair tip towards bacteria. This is in line with the biological process, as flavonoids released by roots do activate nod genes in bacteria.

Step 5: Evaluate statement e

Statement e posits that root hairs stimulates release of bacterial enzymes that degrade root hair cell walls. This is not consistent with the biology of nodule formation and nitrogen fixation process. Bacterial enzymes are not typically involved in degrading the root hair cell walls.