Model Answer
0 min readIntroduction
The plasma membrane, a vital component of all cells, acts as a selective barrier controlling the movement of substances in and out of the cell. This transport can occur through passive or active mechanisms. Passive transport, requiring no cellular energy, includes simple diffusion and facilitated diffusion. Active transport, however, necessitates energy expenditure to move substances against their concentration gradient. Understanding these mechanisms is fundamental to comprehending cellular physiology and various biological processes. This answer will delineate facilitated diffusion and comprehensively describe the mechanism of active transport across the plasma membrane, illustrated with a relevant example.
Facilitated Diffusion
Facilitated diffusion is a type of passive transport where substances move across the plasma membrane down their concentration gradient with the assistance of membrane proteins. Unlike simple diffusion, which doesn't require any protein assistance, facilitated diffusion relies on either channel proteins or carrier proteins.
- Channel Proteins: These form hydrophilic pores through the membrane, allowing specific ions or small polar molecules to pass through.
- Carrier Proteins: These bind to the substance and undergo a conformational change to transport it across the membrane.
Facilitated diffusion is characterized by its specificity (proteins only transport certain molecules), saturation (rate plateaus as protein binding sites become saturated), and dependence on concentration gradient. It does not directly require metabolic energy (ATP).
Active Transport
Active transport is the movement of substances across the plasma membrane against their concentration gradient, requiring the input of cellular energy, typically in the form of ATP. This process is crucial for maintaining cellular homeostasis and performing essential functions like nerve impulse transmission and nutrient absorption.
Types of Active Transport
- Primary Active Transport: This directly utilizes ATP hydrolysis to move ions or molecules. A classic example is the Sodium-Potassium Pump (Na+/K+ ATPase).
- Secondary Active Transport: This utilizes the electrochemical gradient established by primary active transport to move other substances. It doesn't directly use ATP but relies on the potential energy stored in the gradient. This can be further divided into:
- Symport: Both substances move in the same direction.
- Antiport: Substances move in opposite directions.
Mechanism of Active Transport – Sodium-Potassium Pump (Example)
The Na+/K+ ATPase is a transmembrane protein found in the plasma membrane of animal cells. It actively transports 3 sodium ions (Na+) out of the cell and 2 potassium ions (K+) into the cell, against their respective concentration gradients. This process is vital for maintaining the resting membrane potential, regulating cell volume, and driving secondary active transport.
The mechanism involves the following steps:
- Binding: Three Na+ ions bind to the pump inside the cell.
- Phosphorylation: ATP binds to the pump and is hydrolyzed, transferring a phosphate group to the pump protein.
- Conformational Change: Phosphorylation causes the pump to change shape, expelling the Na+ ions outside the cell.
- K+ Binding: Two K+ ions bind to the pump outside the cell.
- Dephosphorylation: The phosphate group is released from the pump.
- Return to Original Shape: The pump returns to its original shape, transporting the K+ ions inside the cell.
This cycle repeats continuously, maintaining the Na+ and K+ gradients across the plasma membrane.
| Feature | Facilitated Diffusion | Active Transport |
|---|---|---|
| Energy Requirement | No ATP required | Requires ATP |
| Concentration Gradient | Down the gradient | Against the gradient |
| Protein Involvement | Required (channel or carrier) | Required (carrier protein) |
| Specificity | High | High |
Conclusion
In conclusion, both facilitated diffusion and active transport are essential mechanisms for regulating the movement of substances across the plasma membrane. While facilitated diffusion relies on proteins to assist passive transport down a concentration gradient, active transport utilizes energy to move substances against their gradient, maintaining cellular homeostasis. The sodium-potassium pump exemplifies the complexity and importance of active transport in cellular function. A thorough understanding of these processes is crucial for comprehending various biological phenomena and their implications in health and disease.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.