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Lecture 11

BIOLOGY 2EE3 Lecture Notes - Lecture 11: Ileum, Clostridiaceae, Archaea

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Turlough Finan

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Microbial Symbiont
Animals & microbes have evolved together. Most associations formed are healthy + beneficial.
Ex: catabolism of cellulose by bacteria in animal’s guts: herbivores can’t synthesize cellulose
A symbiont has developed a long-lasting relationship with another organism (symbiosis)
Types of relationships:
oParasitism – one species does obvious harm to another
oCommensalism – one species benefits but nothing happens to other species
oMutualism – both species benefit from the relationship
Location: Can live on or in host
oEctosymbionts – live on surface, do not have to be single-celled organisms
oEndosymbionts – live within cells/tissues of the host
Consortium of microbes — stable community where growth/survival of members is interdependent, 1+
species of Symbiont may be present. Exist in digestive tracts of many animal species - provides nutritional
benefits & are needed for maintaining homeostasis in host.
Symbionts of Plants
Fungi colonize the roots of most of the plants that grow in the forest – the fungi help with transport of
nutrients from the soil to the plant.
Root Nodules also sometimes are filled with nitrogen fixing bacteria, these bacteria allow plants to grow
even if soil nitrogen is absent
Alfalfa Root Nodule formation
oThe flavone, luteolin, is released by alfalfa roots and induces the expression of nod genes in the
bacterium S.meliloti – essentially synthesizing the structure of nodules
oThe structure then induces the roots to curl, through the tips of the roots, the plants itself creates a tube
within itself that allows for the bacteria to grow - the bacteria then grows this nodule type house and
stays within it
Nodule formation
Bacterial nod genes - expressed in presence of plant roots
Infection thread and then a nodule form, providing a low-O2 environment; Root hairs in presence of
Rhizobial cells release flavonoids which induce nod factor synthesis in cells, and then the Rhizobial cells
secrete nod factors which cause curling of the roots
oThe plant provides leghemoglobin (hemoprotein, provides the red color in nodules) – which binds
oxygen. This protein prevents O2 from damaging N2-fixing enzymes while still providing oxygen to the
bacteria as they are aerobic and need it for ETC and TCA - to generate energy from carbon nutrients
provided by the plant. Amount of free oxygen is very low
oN2-fixing bacteria provide ammonium to plant, they reduce N2  NH3
-Both plants were grown in nitrog e n- deficient soil. Plant grown in the abs enc e of
Sinorhizobium meliloti and a plant grown in the presence of Sinorhizobium meliloti. Minimal growth
in bacteria-free plant because there was no nitrogen in soil which it needs, and there was enhanced
growth is a result of the ability of S. meliloti to fix nitrogen and convert it to nitrogenous compounds
for plant use.
Nodule Metabolism
Plants synthesize from a sugar (sucrose, glucose) - C4-dicarboxylic acids (malate, succinate, fumarate) then
this C4 is taken up by bacteroids. The C4-sugar now enters the TCA cycle. The e- + ATP generated are used
by nitrogenase to convert N2  NH3 (nitrogen fixation).
Plant Symbionts
Frankia filamentous bacteria & Actinorhizal plants (most advantage): Infection of the root. Host provides
fixed carbon to support N2 fixation by endosymbiont
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