-protons/H+ move down concentration gradient ..

Photophosphorylation is the production of ATP using the energy of sunlight. Photophosphorylation is made possible as a result of chemiosmosis. Chemiosmosis is the movement of ions across a selectively permeable membrane, down their concentration gradient. During photosynthesis, light is absorbed by chlorophyll molecules. Electrons within these molecules are then raised to a higher energy state. These electrons then travel through Photosystem II, a chain of electron carriers and Photosystem I. As the electrons travel through the chain of electron carriers, they release energy. This energy is used to pump hydrogen ions across the thylakoid membrane and into the space within the thylakoid. A concentration gradient of hydrogen ions forms within this space. These then move back across the thylakoid membrane, down their concentration gradient through ATP synthase. ATP synthase uses the energy released from the movement of hydrogen ions down their concentration gradient to synthesise ATP from ADP and inorganic phosphate.

The net force driving molecules down a concentration gradient is the chemical driving force.

For example, if a cell is placed in a 300 milliosmolar solution of urea, because the cell membrane is permeable to urea, the urea will flow down its concentration gradient and enter the cell.

How does the concentration gradient affect the process …

Molecules in this situation are described as going  down their concentration gradient.

The Na+ gradient created by the Na+/K+ pump again provides the energy that moves the glucose up its concentration gradient.

LabBench Activity Key Concepts Diffusion

The structure of the chloroplast helps improve the production of NADPH and ATP which are both used in the light-independent reaction in the chloroplast stroma. The fluid stroma contains the enzymes for the Calvin cycle, the close association of the key molecules (e.g., PSII, PSI and ATP synthase) in the thylakoid membrane allows the efficient flow of electrons through the memebrane and the movement of protons (H+) across the membrane using carrier molecules and ATP synthase. The large number of thylakoids increases the total surface area for the absorption of sunlight and the number of key molecules used for the production of NADPH and ATP. The small size of the thylakoid lumen allows for the production of a H+ concentration gradient between lumen and stroma where the energy is used to produce ATP. The step numbers in the Table 2 correspond to the numbers in Figure 1.