Biology Semester II

Sections:

Introduction |  Section 1 | Section 2 | Section 3

Section Three:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 | Part 10

Biology: Plant Hormones, Nutrition, and Transport: Part Six Mycorrhizae, bacteria, and the uptake of minerals Plants need nitrogen to form and maintain many important biological molecules, including nucleotides and proteins. Yet, the nitrogen in the atmosphere is not in a form that plants can use. To counter this, many plants have a symbiotic relationship with bacteria growing in their roots. The bacteria provide organic nitrogen to the plant in return for space to live. These plants tend to have root nodules in which the nitrogen-fixing bacteria live. The development of a root nodule. The nodule provides a place in the roots of certain plants, most notably legumes (the pea family), where bacteria live symbiotically with the plant. Bacteria of the genus Rhizobium infect a root hair (top image) and develop the root nodule inside the root (bottom image). All the nitrogen in living systems was at one time processed by these bacteria. The bacteria took in atmospheric nitrogen (N2) and modified it to a form that living things could use (such as nitrate, NO3-, or ammonia, NH3). Not all bacteria use the above route to fix nitrogen. Many free-living soil bacteria, such as Nitrobacter, use other chemical pathways. The cyanobacterium Anabaena infects the aquatic fern Azolla. Anabaena fixes a small amount of nitrogen, which is released when the fern dies. Rice farmers in Asia allow Azolla to grow in their rice paddies. This allows farmers to grow rice in nitrogen poor soils without the use of expensive petrochemical fertilizers to increase soil nitrogen. Nitrogen uptake and conversion to organic forms of nitrogen by various soil bacteria. Roots have very thin, fragile extensions, known as root hairs. These root hairs greatly increase the surface area of the root. Symbiotic mycorrhizae fungi also increase the area available for absorbing water and minerals from the soil. Studies have shown that soils sterilized to remove the fungi consistently produce less plant growth than do soils where the fungi are present. The mycorrhizae-plant relationship is an example of the type of symbiosis known as mutualism. Here, both plant and fungus benefit. The fungus dramatically increases the surface area available for mineral and water uptake. It also breaks down organic matter for the plant. In return, the plant provides the mycorrhizae with sugars and amino acids. The relationship is a biological win-win situation! The role of the root hairs is to increase the surface area of roots promoting increased uptake of water and minerals from the soil. Mycorrhizae growing in association with a plant root. Photograph of the fungi (light brown) and the root (dark brown). Now go on to the next part.

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