The stroma is a chloroplast structure that is equivalent to the matrix of the mitochondria (Figure 15.2a). It consists of the space surrounding the grana in the organelle (Figure 17.4c). In photosynthesis, protons are pumped out of the stroma into the thylakoid lumen of the grana to form a proton gradient. The gradient is used as an energy source by the CF0-CF1 complex to make ATP.

ATP and NADPH produced by the photosystems on the thylakoids during photosynthesis are released into the surrounding stroma, where all of the synthetic dark reactions (Calvin cycle) occur.