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.
The tails meet making the middle part of the membrane, and the heads go outward making the outer and inner surface of the cell membrane. The glucose molecule is made of carbon atoms connected to many OH groups and H protons. This makes it a polar molecule which is a hydrophilic one. Outside the cell when the glucose molecule tends to get inside down concentration gradient, the polarity of it is accepted by the head of the cell membrane so it can pass, but the middle part of the cell membrane repels it.