Introduction

Understanding the cell membrane is vital for grasping many core concepts in biology. As you prepare for the AP® Biology exam, being able to answer the question “What does the cell membrane do?” can help you tackle questions about cellular processes, transport mechanisms, and more. In this post, we’ll explore the structure of the plasma membrane, its selective permeability, additional functional roles, and how it compares to the cell wall.

The Structure of the Cell Membrane

A. Overview of the Cell Membrane

• The plasma membrane is a thin, flexible barrier that surrounds the cytoplasm of the cell.
• It provides a boundary separating the cell’s internal environment from the external environment.

B. The Fluid Mosaic Model

• The fluid mosaic model describes how proteins, lipids, and carbohydrates float within the membrane’s phospholipid bilayer.
• Phospholipids have hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails, forming a double layer where the tails face inward.
• Embedded proteins carry out key functions like transport and communication, while carbohydrates aid in cell recognition.

Selective Permeability of the Cell Membrane

A. What Is Selective Permeability?

• The plasma membrane is selectively permeable, meaning it allows certain substances to pass while restricting others.
• This selective barrier function helps maintain internal conditions (homeostasis) crucial for cell survival.

B. Factors Influencing Selective Permeability

• The hydrophobic core of the phospholipid bilayer restricts the entry of many substances. 
• Transport proteins and channel proteins help overcome this barrier for molecules that cannot easily pass through.

C. Small Nonpolar Molecules

• Molecules like N2, O2, and CO2 are small and nonpolar, enabling them to diffuse freely across the membrane without any additional help.

D. Hydrophilic Substances

• Large polar molecules (e.g., glucose) and charged ions (e.g., Na+, Cl–) struggle to cross the membrane on their own.
• Transport proteins (carrier proteins, channel proteins) facilitate their movement, either passively (without energy) or actively (with energy input).

E. Polar Uncharged Molecules 

• Water (H2O) is polar but uncharged. Though small amounts can diffuse directly through the lipid bilayer, aquaporins (special channel proteins) greatly expedite its transport.

Additional Functions of the Cell Membrane

A. Communication and Signal Transduction

• The plasma membrane helps cells “talk” to each other and respond to external cues through receptor proteins.
• Once a signaling molecule (like a hormone) binds to a receptor, a cascade of cellular events can lead to changes in gene expression or metabolic activity.

B. Transport Mechanisms

• Passive Transport: Diffusion and osmosis move substances down their concentration gradient without using energy.
• Active Transport: Cells expend ATP to move molecules against their concentration gradient (e.g., the sodium-potassium pump).

Comparison with the Cell Wall

A. Structure and Function of the Cell Wall

• Found in plants, prokaryotes, and fungi, the cell wall provides structural support, protection, and shape to cells.
• Plant cell walls are chiefly composed of cellulose, which offers adjustable rigidity for plant growth and protection.

B. Differences Between the Cell Membrane and Cell Wall

• The cell membrane is a flexible, selectively permeable membrane, while the cell wall is more rigid and provides extra support.
• Unlike the cell wall, the plasma membrane is responsible for regulating what enters and leaves the cell in a more dynamic manner.

Practice Questions

1. Explain why small nonpolar molecules can cross the membrane freely while larger polar molecules require the help of transport proteins.
2. Define selective permeability and discuss two factors that influence it.
3. Compare and contrast the plasma membrane’s function with that of the cell wall in plant cells.

Conclusion

From controlling traffic in and out of the cell to facilitating communication through signal receptors, the cell membrane is central to cellular function. By mastering these concepts, you’ll be better prepared for questions on the AP® Biology exam that involve membrane structure, function, and transport mechanisms.

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