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.
As you learned in the chapter on cells, the cell membrane is primarily responsible for regulating what can cross the membrane and what stays on only one side. The cell membrane is a phospholipid bilayer, so only substances that can pass directly through the hydrophobic core can diffuse through unaided.