HORTICULTURE

Detailed explanation-1: -Hence, the correct answer is ‘Lumen of thylakoids’.

Detailed explanation-2: -Protons accumulate inside the lumen of thylakoids due to splitting of water. Water splitting complex is associated with PS-II and is located on the inner side of the thylakoid membrane (facing the lumen of thylakoids). It helps in splitting the water.

Detailed explanation-3: -Enclosed by the thylakoid membrane is a narrow soluble space called the chloroplast-or thylakoid lumen. Originally it was believed that the lumen space was more or less devoid of proteins and contained only a few soluble components of the two photosystems, plastocyanin and salts.

Detailed explanation-4: -During photosynthetic electron transport, protons accumulate at high concentration inside the thylakoid space. The concentration gradient between the inside and outside of the thylakoids is the energy source used by the ATP synthase.