Question

Nerve growth factor (NGF) is a water-soluble signaling molecule. Would you expect the receptor for NGF to be intracellular or in the plasma membrane? Why?

Short answer

The receptor for NGF (nerve growth factor) must be in the plasma membrane. This is because NGF is a water-soluble molecule that cannot pass through the lipid bilayer of the plasma membrane to reach and bind to the intracellular receptors.

Step-by-step solution

Lipid bilayer of plasma membrane

The plasma membrane of cells is a phospholipid bilayer which means it comprises two layers of phospholipid molecules. The phospholipid molecule contains a head (hydrophilic) and a tail (hydrophobic) region.

In the plasma membrane, the hydrophobic tail is present towards the interior of the membrane, whereas the hydrophilic head is arranged outwards towards the cytoplasm.

Due to the bilayer nature of the plasma membrane, non-polar molecules (hydrophobic) can easily pass through the membrane, whereas the polar (hydrophilic) molecules cannot pass through the membrane.

Intracellular receptors

Some receptors are present inside the cell. In the cell, they are either found in the cytoplasm or the nucleus. Signaling molecules like steroid hormones and thyroid hormones can bind to intracellular receptors.

This is because these are hydrophobic molecules that can pass through the hydrophobic interior of the plasma membrane. They cross the membrane and reach the receptors inside the cell, and binds to it.

This exhibits that only hydrophobic molecules passing through the plasma membrane can bind to the intracellular receptors.

Binding of NGF to the plasma membrane receptor

Polar molecules are hydrophilic (water-loving), so they are prohibited from entering the interior of the membrane. As a result, they cannot pass through the lipid membrane.

Similarly, NGF being a polar molecule, cannot pass through the lipid membrane to reach the intracellular receptors. This means the receptors for NGF are present in the plasma membrane to which the NGF can bind and initiate signal transduction cascade.

Thus, the receptor for the NGF signaling molecule is found on the surface of the plasma membrane.