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.
Molecules that are capable of diffusion across a cellular membrane will eventually reach equal concentrations inside and outside of the cell. At this point the rate at which individual molecules enter and leave will be equal.