The term “stroma” in biology refers to a supportive matrix or framework within various biological structures, serving as the connective component of an organ or tissue, distinct from its functional cells. This framework provides structural integrity and often facilitates essential processes by housing specific components. The presence and composition of stroma vary significantly depending on its location and physiological role.
Stroma within Chloroplasts
Within plant cells, the stroma is a fluid-filled internal space inside chloroplasts, the organelles responsible for photosynthesis. This aqueous matrix encircles the grana, stacks of disc-like thylakoids. The stroma is enclosed by the inner membrane of the chloroplast envelope.
It contains dissolved enzymes, starch granules, ribosomes, and chloroplast DNA. These components are essential for the light-independent reactions of photosynthesis, also known as the Calvin cycle. During this cycle, carbon dioxide from the atmosphere diffuses into the stroma, where enzymes like RuBisCO facilitate its conversion into organic molecules, primarily sugars. The fluid nature of the stroma allows for the efficient diffusion of these enzymes and other molecules, enabling carbohydrate synthesis.
Stroma of the Eye
In the human eye, the stroma is a layer within the cornea, the transparent outer part of the eye that covers the pupil and iris. It is the thickest corneal layer, making up approximately 90% of its total thickness. It is situated between Bowman’s layer anteriorly and Descemet’s membrane posteriorly.
The corneal stroma’s composition is vital for its transparency and structural integrity. It primarily consists of highly organized collagen fibrils, keratocytes, specialized connective tissue cells, and an extracellular ground substance rich in proteoglycans. The collagen fibrils are arranged in flattened layers called lamellae, with fibrils within each lamella running parallel but at different angles to those in adjacent layers. This precise and uniform arrangement of collagen, along with controlled hydration maintained by proteoglycans, minimizes light scattering and allows light to pass through to the retina, enabling clear vision.
Stroma in Other Organs and Tissues
The term “stroma” also applies to supportive frameworks in various other organs and tissues. In the ovary, the stroma is supportive connective tissue that encases and supports ovarian follicles, where eggs develop. This framework helps organize the organ’s functional units.
Beyond specific organs, stroma broadly describes the extracellular matrix (ECM) within connective tissues. This ECM is the non-cellular component that surrounds cells, providing physical scaffolding and mediating cell communication. It is composed of fibrous proteins like collagen and elastin, along with molecules such as proteoglycans and glycosaminoglycans. This network gives connective tissues their ability to withstand mechanical stress, providing both tensile strength and elasticity.
Why Stroma’s Location Matters
Stroma’s specific location is important to the function of the organelle or tissue it supports. In chloroplasts, the stroma’s fluid environment directly facilitates the chemical reactions of the Calvin cycle by allowing enzymes and substrates to move freely and interact. Its placement within the inner membrane ensures necessary products from light-dependent reactions in the thylakoids are readily available for sugar synthesis.
In the cornea, the precise organization and location of the stroma’s collagen fibrils and ground substance are directly responsible for the cornea’s transparency. This structural arrangement ensures minimal light scattering, which is vital for focusing light onto the retina and achieving clear vision. Its location and composition maintain the eye’s shape and mechanical strength. Similarly, in organs like the ovary, the stroma’s supportive framework provides the necessary environment for cellular function and development, anchoring specialized cells and facilitating communication within the tissue.