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Biology · 术语表

什么是 Nitrogen fixation and nodules？

Definition 23.11 University Biology — Year 1 · 第 23 章 — Plant Water and Mineral Nutrition

No eukaryote can use the $\mathrm{N_2}$ of the air. Some bacteria can: nitrogenase breaks the triple bond of $\mathrm{N_2}$ and reduces it to two ammonia, at a cost of sixteen ATP and eight electrons per molecule, and only where oxygen is excluded. Legumes (peas, beans, clover) house such bacteria (Rhizobium) in root nodules: the plant builds a chamber, feeds the bacteria sugar, and keeps the oxygen low with a red protein, leghaemoglobin, that delivers it to the bacteria’s respiration at a concentration too low to harm the enzyme; the bacteria give back ammonium, which the plant turns into amino acids. A field of clover fixes a hundred kilograms of nitrogen per hectare a year, for which the plant pays a tenth of its photosynthesis. The nitrogen cycle as a whole belongs to the Year 2 volume.

Nodules on the roots of a legume: each is a chamber of plant tissue filled with bacteria, pink with leghaemoglobin, where atmospheric nitrogen is fixed.

示例

Example 23.12 (The price of nitrogen)

A maize plant needs about $30\,\mathrm{mg}$ of nitrogen a day; from nitrate at $2\,\mathrm{mmol}/\mathrm{L}$ it gets that in the litre of water it transpires, and spends a few hundred millimoles of electrons reducing it. A clover plant fixing the same amount pays $16\,$ ATP per $\mathrm{N_2}$ plus the reductant — roughly $6\,\mathrm{g}$ of sugar per gram of nitrogen, some $0.2\,\mathrm{g}$ a day, a tenth of its production — and can grow on soil with no nitrate at all. Farmers have rotated legumes with cereals for two thousand years to move that nitrogen from one field to the next.

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