In the context of photosynthesis, to what does the term 'stroma' refer?

• A. The stack of membrane-bound thylakoids within a chloroplast
• B. The pigment molecules embedded in the thylakoid membrane
• C. The double membrane that encloses the chloroplast
• D. The fluid-filled space surrounding the thylakoids inside a chloroplast

Answer: (D)

Verified step by step guidance

1. Understand the structure of a chloroplast: A chloroplast is an organelle found in plant cells and some algae, responsible for photosynthesis. It has a double membrane, thylakoids (membrane-bound structures), and a fluid-filled space called the stroma.
2. Learn the role of the stroma: The stroma is the fluid-filled space surrounding the thylakoids inside the chloroplast. It contains enzymes, ribosomes, and DNA, and is the site of the Calvin cycle, where carbon fixation occurs during photosynthesis.
3. Differentiate the stroma from other chloroplast components: The thylakoids are stacked into structures called grana, and their membranes contain pigment molecules like chlorophyll for light-dependent reactions. The stroma is distinct from these structures and serves as the matrix for biochemical processes.
4. Review the options provided: The stack of membrane-bound thylakoids refers to grana, pigment molecules are embedded in the thylakoid membrane, and the double membrane encloses the chloroplast. The stroma is the fluid-filled space surrounding the thylakoids.
5. Conclude that the correct answer is: The fluid-filled space surrounding the thylakoids inside a chloroplast, based on the definition and function of the stroma in photosynthesis.