What Is Stroma?

Stroma Definition – Meaning of Stroma

Stroma is the fluid filling up the inner space of the chloroplasts which encircle the grana and the thylakoids. In addition to providing support to the pigment thylakoids, the stroma are now known to contain chloroplast DNA, starch and ribosomes along with enzymes needed for Calvin cycle.

Stroma can also be used to refer to the other support structures namely the connective structures or the fungal tissues carrying spores. It is the part of a tissue or organ having a role, connective or structural in nature.

Structure of Stroma – Stroma in Chloroplast

Stroma is made of an outer membrane and a complex network of inner membranes that goes on to form the grana – disc like structures arranged in a stack. Membranous extensions connect various grana together.

The inner membranes comprise the constituents involved to harvest light energy such as chlorophyll and other photosynthetic pigments. The transparent aqueous matrix or the stroma play a significant role in photosynthesis rather than just merely supporting pigmented substructures.

Chloroplasts evolved derive from free-living prokaryotes which form an endosymbiotic relationship with a few eukaryotic cells. Hence, stroma continues to contain ribosomes and DNA conducting synthesis of proteins. Such proteins are necessary for light-independent reactions of photosynthesis and reactions fixing inorganic materials in organic molecules.

Stroma cells – Function

Chloroplasts perform some important functions of the cells of the plant in addition to comprising their own genome. Many genes required for their functioning are incorporated into the genome of the nucleus. As a result, it has to be capable of modifying its metabolic activity to equilibrate the work of the cell. For this, the stroma is needed as it contains the enzymes required for carbon fixation along with managing the chloroplast response to cellular stresses and signals between different organelles.

Their role is important in both the light-independent and light-dependant reactions of photosynthesis. The stroma in extreme stressful conditions can experience autophagy selectively without having to destroy the pigment molecules and inner membranous structures. Some stroma appear as finger-like projections and do not have thylakoids. They are linked with endoplasmic reticulum and nucleus conducive to crucial mechanisms.

When light energy is captured by the pigment molecules it is converted into chemical energy and the activity of the stroma commences through the electron transport chain. Pigments are found in the Photosystems I and II present on the thylakoid membranes. These pigments channel light energy using it to release high energy electrons. The electrons pass via different proteins where redox reactions take place. The stroma additionally has the products of light-dependent reactions allowing the further processes of photosynthesis to take place. The regeneration of RuBP and reduction of phosphoglycerate occur in the stroma, which are two of the steps taking place in the Calvin cycle.

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