The "dark current" of photoreceptors refers to the exit of k+ out of the cell when light is not hitting the cell. the exit of na+ and ca2+ out of the cell when light is not hitting the cell. the entry of k+ into the cell when light is not hitting the cell. the entry of na+ and ca2+ into the cell when light is not hitting the cell.

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.

answer: ice crystals in air

the water from the water body or another source of water gets evaporated as water vapor as an effect of heat on water. these water vapor rises in the upper layers of the earth atmosphere under high atmospheric pressure. the atmospheric pressure of these water molecules starts to decrease due to the effect of cold air. the invisible water vapor condenses into visible water droplets and ice crystals.these ice crystals and water droplets being less denser than the surrounding air floats on air as clouds.