Correct Option: C. Cell membrane Detailed Explanation: The cell membrane, also known as the plasma membrane, is a crucial structure in all living cells. It serves as a barrier that separates the interior of the cell from the external environment. Here’s a step-by-step breakdown of why the cell membrane is primarily responsible for regulating the movement of substances in and out of a living cell:

1. Structure of the Cell Membrane:
2. The cell membrane is composed of a phospholipid bilayer, which consists of two layers of phospholipids. Each phospholipid has a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) "tails."
3. This arrangement creates a semi-permeable membrane that allows certain substances to pass while blocking others.
4. Function of the Cell Membrane:
5. The primary function of the cell membrane is to control the entry and exit of substances. This is essential for maintaining homeostasis, which is the stable internal environment of the cell.
6. The cell membrane contains various proteins that facilitate the transport of molecules. These proteins can act as channels or carriers for specific substances, allowing them to move across the membrane.
7. Transport Mechanisms:
8. Passive Transport: This process does not require energy. Substances move across the membrane along their concentration gradient (from high to low concentration). Examples include diffusion and osmosis.
9. Active Transport: This process requires energy (usually in the form of ATP) to move substances against their concentration gradient (from low to high concentration). This is crucial for maintaining concentration differences of ions and nutrients inside and outside the cell.
10. Signaling and Communication:
11. The cell membrane also plays a role in cell signaling. Receptor proteins on the membrane can bind to signaling molecules (like hormones), triggering responses inside the cell. This is vital for coordinating cellular activities and responses to the environment.

Why Other Options Are Incorrect:

• A. Mitochondria:
• Mitochondria are known as the powerhouses of the cell because they generate ATP through cellular respiration. While they are essential for energy production, they do not regulate the movement of substances in and out of the cell.
• B. Nucleus:
• The nucleus is the control center of the cell, housing the cell's genetic material (DNA). It is responsible for regulating gene expression and cell division, but it does not directly control the movement of substances across the cell membrane.
• D. Ribosome:
• Ribosomes are the sites of protein synthesis in the cell. They translate messenger RNA (mRNA) into proteins. While they are crucial for producing proteins, they do not have a role in regulating the movement of substances in and out of the cell.

Summary of Key Points:

• The cell membrane is a semi-permeable barrier that regulates the movement of substances in and out of the cell.
• It is composed of a phospholipid bilayer with embedded proteins that facilitate transport.
• Transport mechanisms include passive transport (no energy required) and active transport (energy required).
• Other cellular structures like mitochondria, the nucleus, and ribosomes have distinct functions unrelated to the regulation of substance movement across the cell membrane.

This understanding of the cell membrane's role is fundamental in biology, as it underpins many cellular processes and interactions with the environment.