Problem 3 - Campbell Biology

Question

Lipid-soluble signaling molecules, such as aldosterone, cross the membranes of all cells but affect only target cells because (A) only target cells retain the appropriate DNA segments. (B) intracellular receptors are present only in target cells. (C) only target cells have enzymes that break down aldosterone. (D) only in target cells is aldosterone able to initiate the phosphorylation cascade that turns genes on.

Step-by-Step Solution

Answer

Option (B): Intracellular receptors are present only in target cells.

Step 1: Understanding the Question

Lipid-soluble signaling molecules like aldosterone can cross cell membranes but only affect specific target cells. The aim is to figure out why only certain cells respond to aldosterone.

Step 2: Identify the Mechanism of Lipid-Soluble Molecules

Lipid-soluble molecules can diffuse through the lipid bilayer of cell membranes due to their solubility in lipids. However, their effect is specific to certain cells.

Step 3: Evaluate Option A

Option (A) suggests that only target cells retain the appropriate DNA segments. This is incorrect because all cells within an organism generally have the same DNA.

Step 4: Evaluate Option B

Option (B) suggests that intracellular receptors are present only in target cells. This is an important consideration for specific cellular responses.

Step 5: Evaluate Option C

Option (C) suggests that only target cells have enzymes that break down aldosterone. This is not typically the mechanism by which specificity is achieved.

Step 6: Evaluate Option D

Option (D) suggests that only in target cells is aldosterone able to initiate the phosphorylation cascade that turns genes on. Though phosphorylation cascades are part of signal transduction, the specificity to aldosterone is due to something more direct.

Step 7: Choosing the Correct Answer

After evaluating the options, option (B) is the best choice. Intracellular receptors specific to aldosterone must be present in target cells to elicit a response.

Key Concepts

Lipid-Soluble Signaling Molecules

Lipid-soluble signaling molecules, such as aldosterone, have the unique ability to pass through cell membranes. This is because they are soluble in the lipid components of the membrane, allowing them to diffuse easily. These molecules include hormones like steroid hormones, thyroid hormones, and vitamin D.

While they can enter every cell, they do not affect every cell. The reason lies in their interaction with specific types of cells known as target cells. Inside these target cells, lipid-soluble molecules bind to their specialized receptors to trigger a response.

Target Cells

Target cells are the specific cells that respond to particular signaling molecules. The key to this specificity is the presence of intracellular receptors that can bind these signaling molecules.

For instance, aldosterone affects only those cells that contain aldosterone receptors inside them. These receptors are usually located in the cytoplasm or nucleus. When aldosterone binds to its receptor, a series of events are triggered, leading to changes in gene expression and cell function.

It's important to note that these receptors are not present in every cell, which explains why only certain cells respond to specific lipid-soluble signaling molecules.

Signal Transduction

Once a lipid-soluble signaling molecule like aldosterone enters a target cell, it must initiate a process to bring about a cellular response. This process is known as signal transduction. Signal transduction involves several steps, beginning with the binding of the signaling molecule to its intracellular receptor.

After binding, this receptor-signal complex usually moves to the cell nucleus, where it can bind directly to specific DNA segments. This binding initiates transcription of certain genes, leading to the production of proteins that alter cell function.

Thus, signal transduction translates the extracellular signal (the hormone) into a specific cellular response, ensuring that cells perform their designated roles efficiently.