5.6
扩散
扩散是物质沿着浓度梯度的被动运动，不需要消耗细胞能量。物质，如分子或离子，在胞质或细胞膜上从高浓度扩散到低浓度。最终，浓度会趋于平衡，物质会随机移动，但不会引起浓度的净变化。这种状态被称为动态平衡，它对维持生物体的整体稳态至关重要。
扩散依赖的生物过程
扩散在生物过程中起着不可分割的作用，如呼吸，生…
Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration. In cells, this process usually occurs across the cell membrane, and it requires no cellular energy.
The rate at which molecules cross the membrane depends on several factors, including hydrophobicity, size, and electrical charge.
Small nonpolar molecules, like oxygen and carbon dioxide, can easily slip through the lipid bilayer. Also, many large fat-soluble molecules, like steroids and fatty acids, readily pass through the membrane by simple diffusion.
Small, uncharged but polar molecules, such as water, can cross the membrane, but at a slower rate.
Larger uncharged molecules, such as glucose, cannot pass directly through the membrane. Instead, they move by facilitated diffusion through membrane transport proteins such as channels or carriers.
On the other hand, charged ions, no matter their size, and non-lipid-soluble molecules cannot cross the hydrophobic core of the lipid bilayer and need specialized membrane proteins, like ion channels, to move across.
Even after the concentration stabilizes, particles continue to move randomly between the two sides. In living cells, this bidirectional exchange reaches a dynamic steady state, where there is no net movement, but molecules continue to move in both directions at equal rates.
Q1: What is diffusion and why does it not require cellular energy?
Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration. It requires no cellular energy because molecules move spontaneously down their concentration gradient, driven by random molecular motion. This process continues until concentrations equalize, reaching a state called dynamic equilibrium where molecules continue moving randomly but produce no net change in concentration.
Q2: How do molecule size and polarity affect diffusion across the cell membrane?
Small nonpolar molecules like oxygen and carbon dioxide easily pass through the lipid bilayer via simple diffusion. Small uncharged polar molecules such as water cross more slowly. Larger uncharged molecules like glucose cannot pass directly through the membrane and instead require facilitated transport across cell membrane through specialized transport proteins. Charged ions cannot cross the hydrophobic core regardless of size.
Q3: What role does diffusion play in gas exchange during respiration?
During respiration, oxygen diffuses from the alveoli into the blood because oxygen concentration is higher in the lungs than in blood. Oxygen then diffuses into body tissues where its concentration is lower. Metabolic waste carbon dioxide diffuses from tissues into capillaries, then into alveoli for exhalation. This concentration-gradient-driven exchange enables organisms to obtain oxygen and eliminate carbon dioxide without expending energy.
Q4: What factors influence the rate of diffusion across membranes?
Diffusion rates depend primarily on molecular hydrophobicity, size, and the magnitude of the concentration gradient. Lipid-soluble, nonpolar molecules diffuse faster than polar or charged molecules. A larger concentration difference between compartments accelerates diffusion, while as a system approaches equilibrium, the diffusion rate slows. Temperature, molecular mass, and solvent density also influence how quickly substances move.
Q5: How does diffusion differ from facilitated diffusion in transporting molecules?
Simple diffusion allows substances to move directly across membranes along concentration gradients without assistance or energy. Facilitated diffusion requires membrane-embedded transport proteins to help substances cross membranes, also without expending energy. Both are passive processes, but facilitated diffusion enables larger or charged molecules that cannot pass through the lipid bilayer to move down their concentration gradients.
Q6: Why do charged ions require specialized membrane proteins to cross the cell membrane?
Charged ions cannot cross the hydrophobic core of the lipid bilayer because they are repelled by the nonpolar lipid environment. Regardless of their size, ions require specialized membrane proteins like ion channels to traverse the membrane. These proteins provide a hydrophilic pathway through the membrane, allowing charged particles to move across while maintaining the membrane's barrier function against other charged substances.
Q7: What is dynamic equilibrium and how does it relate to diffusion in living cells?
Dynamic equilibrium is a state where concentrations stabilize and no net movement of molecules occurs, yet particles continue moving randomly between compartments at equal rates in both directions. In living cells, this bidirectional exchange is essential for maintaining homeostasis. Even at equilibrium, molecules remain in constant motion, ensuring that cells can respond to changing conditions and sustain vital biological processes.