Question

Mice are administered a drug that inhibits endocytosis. Then, they are intravenously infused with a substance that is found to accumulate rapidly inside cells. This substance is most likely a A. steroid hormone. B. polypeptide. C. second messenger. D. glucose analog.

Short answer

A. steroid hormone.

Step-by-step solution

Understanding Endocytosis

Endocytosis is a cellular process in which substances are brought into the cell by engulfing them in a vesicle. If endocytosis is inhibited, substances that typically enter cells through this process would not be able to do so.

Identifying How Each Substance Enters Cells

Analyze how each option gets into a cell: A. Steroid hormone can freely diffuse through the plasma membrane. B. Polypeptide generally requires receptors and endocytosis to enter. C. Second messenger is typically produced within a cell. D. Glucose analog, like glucose, enters cells through specific transporter proteins.

Linking Substance Accumulation to Transport Mechanism

Given that endocytosis is inhibited and the substance still accumulates inside the cell, the substance must enter the cell through a method other than endocytosis. Since steroid hormones can diffuse through membranes, they would still enter the cell despite the inhibition.

Selecting the Correct Answer

Based on understanding how each substance enters the cell, the substance most likely to accumulate inside the cell, despite inhibition of endocytosis, is the steroid hormone (A).

Key concepts

cellular transport

Cells constantly need to transport molecules in and out to survive and function. This process of transport can occur via different mechanisms depending on the type of molecule and the needs of the cell.
There are two main types of cellular transport:

• **Passive Transport:** Moves substances across the cell membrane without the use of cellular energy (ATP). This includes simple diffusion, facilitated diffusion, and osmosis. An example is oxygen moving into cells.
• **Active Transport:** Requires cellular energy to move substances against their concentration gradient. This process includes direct use of ATP or harnessing energy from an electrochemical gradient. Examples include the sodium-potassium pump and endocytosis.

Understanding these mechanisms is crucial for grasping how cells control their internal environment and respond to external changes.

steroid hormone diffusion

Steroid hormones, such as cortisol and estrogen, are lipophilic (fat-loving) molecules. These hormones can freely diffuse across the cell membrane because the membrane is made up of a lipid bilayer.
The process works like this:

• Steroids are produced by endocrine glands and released into the bloodstream.
• They passively diffuse through the lipid bilayer of the cell membrane due to their compatibility with lipids.
• Inside the cell, they bind to specific intracellular receptors.
• The hormone-receptor complex then moves to the cell nucleus where it can influence gene transcription, affecting protein synthesis and cellular function.

Steroid hormones' ability to diffuse freely through cell membranes explains why in our exercise scenario, a steroid hormone could still enter cells despite endocytosis being inhibited.

cell membrane permeability

Cell membrane permeability refers to the ease with which substances can pass through the cell membrane. This is crucial for maintaining homeostasis and allowing the cell to function properly.
Several factors influence cell membrane permeability:

• **Lipid Solubility:** Lipid-soluble (nonpolar) substances like steroid hormones can easily pass through the membrane.
• **Size and Charge:** Small and nonpolar molecules pass through more easily than large or polar ones.
• **Transport Proteins:** Specific proteins help in the transport of certain molecules, like glucose and ions, across the membrane.

Changes in membrane permeability can significantly impact how substances enter or exit the cell, affecting cellular activities and overall health.

biological processes

Biological processes are the vital processes occurring within living organisms to sustain life. Cellular transport is a fundamental biological process that maintains cells' internal environments and supports growth, communication, and reproduction.
Some key biological processes include:

• **Metabolism:** The sum of all chemical reactions in the body, including catabolism (breaking down molecules) and anabolism (building up molecules).
• **Cell Division and Growth:** Mechanisms like mitosis and meiosis allow cells to replicate and grow.
• **Signal Transduction:** Cells communicate through pathways that enable them to respond to signals like hormones or nutrients.
• **Homeostasis:** Maintaining a stable internal environment despite external changes, crucial for survival.

Understanding these processes helps in comprehending how cellular transport and other mechanisms fit into the larger picture of life.