Question

Choose the correct statement

• A. The rate of diffusion increases with increasing size of molecules
• B. Substances that have hydrophilic moiety can easily pass through the plasma membrane
• C. Diffusion is an example of downhill transport
• D. The transport of substances with the help of membrane proteins always require ATP

Answer 1

The correct option is C Diffusion is an example of downhill transport

In diffusion, the movement of molecules occurs along the concentration gradient, that is from higher to lower concentration (downhill transport) until a level of equilibrium is reached.
The rate of diffusion decreases with the increasing size of molecules. Smaller molecules diffuse faster.
Substances having hydrophilic moiety (moiety refers to a part of the molecule and hydrophilic means water-loving) find it difficult to cross the membrane. This is due to the structure of the plasma membrane. The plasma membrane is made up of phospholipids which have hydrophilic heads facing outside and hydrophobic ends facing to the interior. Hence, it is difficult for the hydrophilic substances to pass through the hydrophobic interior core of plasma membrane.
The transport of substances with the help of membrane proteins does not necessarily always require energy (ATP). In facilitated diffusion, the molecules are transported through the membrane by proteins and they do not require energy.
Energy is required only when molecules move against a concentration gradient.