Cell Transport: Diffusion, Osmosis & Active Transport | 细胞运输：扩散、渗透与主动转运

📚 Cell Transport: Diffusion, Osmosis & Active Transport | 细胞运输：扩散、渗透与主动转运

The cell membrane is the gatekeeper of the cell. Understanding how substances move across this barrier is a fundamental requirement for IGCSE Biology, as nearly every process from respiration to digestion relies on efficient transport.

细胞膜是细胞的守门员。理解物质如何穿过这道屏障是 IGCSE 生物学的一项基本要求，因为从呼吸作用到消化作用，几乎所有过程都依赖于高效的运输。

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1. The Cell Membrane | 细胞膜

The cell membrane is described as partially permeable. It allows small molecules such as water, oxygen and carbon dioxide to pass through freely, but prevents larger molecules like glucose and charged ions from crossing without assistance.

细胞膜被描述为具有部分渗透性。它允许水、氧气和二氧化碳等小分子自由通过，但阻止葡萄糖和带电离子等较大分子在无辅助的情况下穿过。

It is composed of a phospholipid bilayer with embedded proteins. The lipid-soluble substances dissolve through the bilayer, while protein channels and carriers facilitate the movement of specific substances.

它由含有嵌入蛋白质的磷脂双分子层组成。脂溶性物质能溶解并通过双分子层，而蛋白质通道和载体则促进特定物质的移动。

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2. Diffusion | 扩散

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. This movement is driven by the kinetic energy of the particles and continues until they are evenly distributed, a state called dynamic equilibrium.

扩散是粒子从高浓度区域向低浓度区域进行的净移动。这种移动由粒子的动能驱动，并持续到它们均匀分布为止，这种状态称为动态平衡。

A classic example is oxygen diffusing from the alveoli into the blood for respiration, while carbon dioxide diffuses in the opposite direction to be exhaled. No energy is required for diffusion to occur.

一个经典例子是呼吸时氧气从肺泡扩散进入血液，同时二氧化碳朝相反方向扩散并被呼出。扩散的发生不需要能量。

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3. Facilitated Diffusion | 易化扩散

Certain larger molecules, such as glucose, cannot pass through the phospholipid bilayer easily. They rely on specific carrier proteins or channel proteins embedded in the membrane to move across.

某些较大的分子（如葡萄糖）不易穿过磷脂双分子层。它们依靠嵌入膜中的特定载体蛋白或通道蛋白来穿过。

This process still follows the concentration gradient and is therefore passive, meaning it does not require metabolic energy. However, it is limited by the number of available protein channels, which can become saturated.

这个过程仍然顺着浓度梯度进行，因此是被动的，意味着它不需要代谢能量。然而，它受到可用蛋白通道数量的限制，通道可能会饱和。

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4. Osmosis | 渗透作用

Osmosis is a special case of diffusion involving water. It is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) across a partially permeable membrane.

渗透是涉及水的一种特殊扩散。它是水分子通过部分渗透性膜，从水势较高（稀溶液）的区域向水势较低（浓溶液）的区域进行的净移动。

Water potential: Pure water > Dilute solution > Concentrated solution

水势：纯水 > 稀溶液 > 浓溶液

The higher the concentration of solute particles, the lower the water potential. Water always moves towards the region of lower water potential to dilute it.

溶质颗粒的浓度越高，水势就越低。水总是向水势较低的区域移动以稀释它。

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5. Osmosis in Animal Cells | 动物细胞中的渗透作用

Animal cells have no cell wall, making them vulnerable to osmotic changes. In a hypotonic solution (high water potential), water enters the cell, causing it to swell. If too much water enters, the cell may burst, a process known as cytolysis.

动物细胞没有细胞壁，因此容易受到渗透变化的影响。在低渗溶液（高水势）中，水分进入细胞，使其膨胀。如果进入的水过多，细胞可能会破裂，这一过程称为细胞溶解。

In a hypertonic solution (low water potential), water leaves the cell, causing it to shrink and shrivel. This effect is called crenation, and it disrupts normal cell function.

在高渗溶液（低水势）中，水分离开细胞，导致其萎缩、皱缩。这种效应称为皱缩，它会破坏细胞的正常功能。

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6. Osmosis in Plant Cells | 植物细胞中的渗透作用

Plant cells have a rigid cellulose cell wall, which prevents them from bursting. In a hypotonic solution, water enters the cell, pushing the cytoplasm against the cell wall and creating turgor pressure. The cell becomes turgid, which supports the plant.

植物细胞有坚硬的纤维素细胞壁，可防止其破裂。在低渗溶液中，水分进入细胞，将细胞质推向细胞壁并产生膨压。细胞变得硬挺，从而支撑植物体。

In a hypertonic solution, water leaves the vacuole, causing the cytoplasm to pull away from the cell wall. This process is known as plasmolysis, and the plant wilts due to a loss of turgor pressure.

在高渗溶液中，水分离开液泡，导致细胞质与细胞壁分离。这个过程称为质壁分离，植物因失去膨压而枯萎。

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7. Active Transport | 主动转运

Active transport is the movement of particles from a region of lower concentration to a region of higher concentration, meaning it moves against the concentration gradient. This requires energy, which is released from ATP molecules produced during respiration.

主动转运是粒子从低浓度区域向高浓度区域的移动，即逆着浓度梯度移动。这需要能量，能量由呼吸作用过程中产生的 ATP 分子释放。

Active transport involves specific carrier proteins that act as pumps, which bind to the particle and change shape to push it across the membrane. Without ATP, this process would stop entirely.

主动转运涉及充当泵的特定载体蛋白，这些蛋白与粒子结合并改变形状将其推过膜。如果没有 ATP，这个过程将完全停止。

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8. Example: Root Hair Cells & Villi | 示例：根毛细胞与绒毛

Root hair cells absorb minerals like magnesium and nitrate ions from the soil via active transport. The concentration of these minerals is often lower in the soil than inside the root, so energy is required to take them up.

根毛细胞通过主动转运从土壤中吸收镁离子和硝酸根离子等矿物质。土壤中这些矿物质的浓度通常低于根内部，因此需要能量才能将其吸收。

Similarly, the villi in the small intestine absorb glucose and amino acids by active transport. This maintains a high concentration of nutrients inside the blood, allowing efficient diffusion to continue.

同样，小肠中的绒毛通过主动转运吸收葡萄糖和氨基酸。这保持了血液内部高浓度的营养物质，使有效扩散得以持续进行。

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9. Comparison Table | 对比表格

Feature 特征	Diffusion 扩散	Osmosis 渗透作用	Active Transport 主动转运
Direction 方向	High to low 高到低	High to low (water) 高到低（水）	Low to high 低到高
Energy (ATP) 能量（ATP)	Not required 不需要	Not required 不需要	Required 需要
Membrane Protein 膜蛋白	Not needed (channels optional) 不需要（通道可选）	Only the membrane is needed 只需要膜	Carrier pumps required 需要载体泵

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10. Factors Affecting Rate | 影响速率的因素

Several factors determine how quickly substances move across membranes. Understanding these helps explain how cells adapt to their environment.

有几个因素决定了物质穿过膜的速度。理解这些因素有助于解释细胞如何适应环境。

• Temperature – higher temperature increases kinetic energy, so particles move faster.

  温度——温度升高会增加动能，使粒子移动更快。

• Surface area – a larger surface area allows more particles to cross at once.

  表面积——更大的表面积能允许更多粒子同时穿过。

• Concentration gradient – a steeper gradient increases the rate of diffusion and osmosis.

  浓度梯度——梯度越陡，扩散和渗透的速率就越快。

• Distance – a shorter distance across the membrane allows faster transport.

  距离——穿过膜的距离越短，运输速度越快。

Rate = (Surface Area × Concentration Gradient) / Distance

速率 = (表面积 × 浓度梯度) / 距离

A classic example is the villi in the small intestine, which have a huge surface area and a very thin wall, making absorption extremely efficient.

小肠绒毛就是一个经典例子，它有巨大的表面积和非常薄的壁，使吸收极为高效。

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