Correct Option: B. Cell membrane Detailed Explanation: The cell membrane, also known as the plasma membrane, is a crucial structure that surrounds the cell and serves as a barrier between the internal environment of the cell and the external environment. Its primary function is to control the movement of substances into and out of the cell, which is essential for maintaining homeostasis and allowing the cell to function properly. Structure of the Cell Membrane:

• The cell membrane is primarily 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."
• This arrangement creates a semi-permeable membrane that allows certain molecules to pass through while blocking others.
• Embedded within this bilayer are proteins, cholesterol, and carbohydrates, which play various roles in transport, signaling, and structural integrity.

Mechanisms of Transport:

1. Passive Transport: This process does not require energy and includes:
2. Diffusion: Movement of molecules from an area of higher concentration to an area of lower concentration (e.g., oxygen and carbon dioxide).
3. Facilitated Diffusion: Similar to diffusion but involves specific transport proteins to help larger or polar molecules cross the membrane (e.g., glucose).
4. Osmosis: The diffusion of water across a selectively permeable membrane.
5. Active Transport: This process requires energy (usually in the form of ATP) to move substances against their concentration gradient (from lower to higher concentration). An example is the sodium-potassium pump, which maintains the electrochemical gradient in cells.
6. Endocytosis and Exocytosis: These processes involve the membrane engulfing substances to bring them into the cell (endocytosis) or packaging substances in vesicles to expel them from the cell (exocytosis).

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 play a vital role in energy production, they do not control the movement of substances into and out of the cell.
• C. Ribosome:
• Ribosomes are the sites of protein synthesis in the cell. They translate messenger RNA (mRNA) into proteins but do not have any role in regulating the movement of substances across the cell membrane.
• D. Nucleus:
• The nucleus is the control center of the cell, housing the cell's genetic material (DNA) and coordinating activities such as growth, metabolism, and reproduction. However, it does not directly control the movement of substances into and out of the cell.

Summary:

• The cell membrane is responsible for controlling the movement of substances into and out of the cell.
• It is composed of a phospholipid bilayer with embedded proteins that facilitate transport.
• Transport mechanisms include passive transport (diffusion, facilitated diffusion, osmosis) and active transport (requiring energy).
• Other organelles like mitochondria, ribosomes, and the nucleus have distinct functions unrelated to membrane transport.

Revision Summary:

• The cell membrane regulates substance movement in and out of the cell.
• It consists of a phospholipid bilayer with proteins for transport.
• Transport can be passive (no energy) or active (requires energy).
• Other organelles serve different functions and do not control membrane transport.