Question

Which hormones can cross cell membranes? a. peptide hormones b. steroid hormones c. Both a and b are correct. d. Neither a nor b is correct.

Short answer

The correct answer is b. steroid hormones, as they are hydrophobic and can easily cross the cell membrane. Peptide hormones, being hydrophilic, cannot freely cross the hydrophobic core of the cell membrane.

Step-by-step solution

Understand the two types of hormones

Peptide hormones are made of amino acids and have a hydrophilic (water-soluble) structure. Steroid hormones, on the other hand, are derived from cholesterol and have a hydrophobic (lipid-soluble) structure.

Understand cell membrane composition

Cell membranes are composed mainly of a phospholipid bilayer, which has a hydrophobic core. This means that a molecule has to be hydrophobic to cross the cell membrane easily.

Determine which hormones can cross cell membranes

As we know that peptide hormones are hydrophilic, they cannot freely cross the hydrophobic core of the cell membrane. Steroid hormones, being hydrophobic, can easily cross the cell membrane.

Conclude the answer

Given this information, we can conclude that the correct answer is b. steroid hormones, as they are the only ones that can easily cross cell membranes due to their hydrophobic nature.

Key concepts

Peptide Hormones

Peptide hormones, as the name suggests, are made up of chains of amino acids and are generally hydrophilic, meaning they are soluble in water. This quality makes them incompatible with the hydrophobic interior of the phospholipid bilayer of cell membranes.

Due to their water-soluble nature, peptide hormones cannot freely pass through the lipid bilayer. Instead, they must bind to specific receptors on the exterior surface of the target cell. This binding triggers a series of events within the cell, often involving second messengers, to transmit the hormone's message.

Common examples of peptide hormones include insulin, which regulates glucose levels in the blood, and glucagon, which has the opposite effect. Understanding how these hormones interact with receptors is crucial for grasping their role in regulating various physiological processes.

Steroid Hormones

Steroid hormones are lipophilic (or hydrophobic) molecules derived from cholesterol. Their solubility in lipids allows them to easily cross the hydrophobic core of cell membranes.

This class of hormones includes estrogen, testosterone, and cortisol, among others. Their ability to pass through the cell membrane is a key feature that distinguishes steroid hormones from peptide hormones. Once inside the cell, steroid hormones typically bind to specific receptor proteins in the cytoplasm or nucleus, influencing gene expression and subsequent protein synthesis.

The direct interaction between steroid hormones and the cell's genetic machinery is a critical aspect of their mechanism of action and forms the basis for their powerful and long-lasting effects on the body.

Phospholipid Bilayer

The phospholipid bilayer is a crucial component of all cell membranes, providing a semi-permeable barrier that regulates the passage of substances in and out of cells. It is composed of two layers of phospholipids, which are molecules with a hydrophilic 'head' and two hydrophobic 'tails'.

The hydrophilic heads face outward towards the water-rich environments inside and outside the cell, while the hydrophobic tails form the core of the bilayer, away from water. This structure creates a hydrophobic barrier that selectively allows only certain molecules to pass through, largely by passive or facilitated diffusion.

Understanding the structure and function of the phospholipid bilayer is paramount in comprehending how hormones interact with and enter cells, which is essential for many cellular processes and signaling pathways.

Hydrophilic and Hydrophobic Molecules

In the context of hormones and cell membrane interactions, the terms hydrophilic and hydrophobic are frequently used. Hydrophilic molecules, like peptide hormones, are attracted to water and dissolve readily in aqueous environments. On the other hand, hydrophobic molecules resist interaction with water but mix well with oils and fats.

These characteristics have significant implications for how different substances interact with the phospholipid bilayer of cell membranes. While hydrophilic substances have trouble crossing the bilayer on their own, hydrophobic substances, such as steroid hormones, can pass through more easily due to the lipophilic nature of the bilayer's core.

Understanding these concepts is fundamental when studying the mechanisms of hormone action and their physiological effects, as they determine the route and mode of hormone signaling within the body.