A sound with a low frequency will have a low pitch, such as a human's heartbeat. A sound with a high frequency will have a high pitch, such as a dog whistle. Humans cannot hear sounds of every frequency. The range of hearing for a healthy young person is 20 to 20,000 hertz. The hearing range of humans gets worse with age. People lose the ability to hear sounds of high frequency as they get older. The highest frequency that a normal middle-aged adult can hear is only 12-14 kilohertz.
Keypoints 1  : The plasma membrane acts as a partially permeable membrane and selectively controls the movement of substances in and out of a cell. 2  Factors that determine whether a molecule can pass through the plasma membrane are the size and polarity of the molecule.
Endocytosis is a cellular process where cells absorb molecules or substances from outside the cell by engulfing it with the cell membrane.This technique is critical to the survival of the cell, since most molecules important to the cell cannot normally pass through the cellâs hydrophobic plasma membrane.inocytosis. In this process, the cell membrane folds around the molecule or substance, and it is dissolved into the interior of the cell. Molecules such as proteins and other water-soluble substances are thus injested this way, which could not otherwise breach the cell membrane.