Legumes, such as peas, beans, and lentils, form symbiotic relationships with bacteria called rhizobia. These bacteria live in nodules on the roots of the legumes and help them to convert atmospheric nitrogen into ammonia, which the plants can then use to make proteins. In return, the legumes provide the bacteria with a home and a source of food.

One of the ways that legumes provide the bacteria with a home is by creating an oxygen-poor environment in the nodules. Rhizobia are able to fix nitrogen only in the absence of oxygen, so the legumes must create a way to remove oxygen from the nodules. They do this by releasing organic acids into the nodules, which react with oxygen to form water and carbon dioxide.

The legumes also provide the bacteria with a source of food in the form of carbohydrates. The bacteria use these carbohydrates to produce energy and to build new cells. In return, the bacteria provide the legumes with fixed nitrogen, which the plants can then use to make proteins.

The symbiotic relationship between legumes and rhizobia is a beneficial one for both parties. The legumes get the nitrogen they need to grow, and the bacteria get a home and a source of food. This relationship is essential for the growth of legumes and for the nitrogen cycle in the environment.