📚 Movement of Substances: Diffusion, Osmosis and Active Transport | 物质运输：扩散、渗透与主动转运
Every living cell must exchange materials with its environment. Oxygen and nutrients need to enter, while carbon dioxide and waste products must leave. These movements occur across the cell surface membrane, which acts as a selective barrier. Understanding how substances move into and out of cells is essential for IGCSE Biology.
每一个活细胞都必须与环境交换物质。氧气和营养物质需要进入，而二氧化碳和废物必须排出。这些物质跨细胞表面的膜进行运动，细胞膜就像一个选择性的屏障。理解物质如何进出细胞是IGCSE生物学的核心内容。
1. The Cell Surface Membrane: A Gatekeeper | 细胞膜：守门员
The cell surface membrane is made of a phospholipid bilayer with proteins embedded in it. It is described as partially permeable, meaning it allows some molecules to pass through freely but not others. Small, non-polar molecules such as oxygen, carbon dioxide and water can diffuse through the lipid bilayer. Larger molecules like glucose and charged ions require special transport proteins.
细胞膜由磷脂双分子层和镶嵌其中的蛋白质构成。它被描述为部分可渗透的，这意味着它允许一些分子自由通过，但其他分子不能。小的非极性分子，如氧气、二氧化碳和水，可以穿过脂双层。而像葡萄糖这样的大分子以及带电离子则需要特殊的转运蛋白。
Two key types of membrane proteins are involved in transport: channel proteins form pores, and carrier proteins change shape to move substances across. These proteins enable passive or active transport depending on the direction and energy requirements.
在运输中起关键作用的膜蛋白有两种：通道蛋白形成孔道，而载体蛋白通过改变形状来移动物质。这些蛋白质依据移动方向和能量需求，可以实现被动运输或主动运输。
2. Diffusion: The Simple Spreading of Particles | 扩散：粒子的简单散布
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. This process is driven by the random kinetic energy of particles and requires no metabolic energy. Diffusion continues until particles are evenly distributed, a state called equilibrium.
扩散是粒子从高浓度区域到低浓度区域沿浓度梯度的净移动。这个过程由粒子的随机动能驱动，不需要代谢能量。扩散持续到粒子均匀分布为止，这种状态称为平衡。
Rate of diffusion ∝ (concentration gradient × surface area × temperature) / membrane thickness
扩散速率 ∝ (浓度梯度 × 表面积 × 温度) / 膜厚度
In living organisms, the rate of diffusion is increased by a large surface area, a short diffusion distance, a steep concentration gradient and higher temperature. These factors are important in the alveoli of the lungs, villi of the small intestine and root hairs of plants.
在生物体中，大的表面积、短的扩散距离、陡峭的浓度梯度和较高的温度都能提高扩散速率。这些因素在肺泡、小肠绒毛和植物根毛中尤为重要。
3. Examples of Diffusion in Biology | 生物学中扩散的实例
Gas exchange in the lungs: Oxygen diffuses from the air in the alveoli, where its concentration is high, into the blood where it is lower. Carbon dioxide moves in the opposite direction, from the blood into the alveoli.
肺中的气体交换：氧气从浓度较高的肺泡空气扩散进入浓度较低的血液中。二氧化碳则沿相反方向，从血液进入肺泡。
Gas exchange in leaves: Carbon dioxide diffuses into leaves through stomata for photosynthesis, while oxygen produced in photosynthesis diffuses out. The stomatal opening controls the rate of gas movement.
叶片中的气体交换：二氧化碳通过气孔扩散进入叶片用于光合作用，而光合作用产生的氧气则扩散出去。气孔的开闭控制着气体移动的速率。
Oxygen entering single-celled organisms: Amoeba and other unicellular organisms rely on diffusion across their cell surface membrane because their small size gives a large surface area to volume ratio.
单细胞生物获取氧气：变形虫等单细胞生物依靠物质跨细胞膜扩散，因为它们的体积小，表面面积与体积之比较大。
4. Osmosis: A Special Case of Diffusion | 渗透：扩散的特殊情况
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. Water potential is a measure of the tendency of water to move; pure water has the highest water potential, and adding solutes lowers it.
渗透是指水分子通过部分渗透性膜，从水势较高的区域到水势较低区域的净移动。水势是衡量水移动趋势的指标；纯水具有最高的水势，而加入溶质会降低水势。
Osmosis is the net movement of water down a water potential gradient across a partially permeable membrane
渗透是水分子沿水势梯度、通过部分渗透性膜的净移动
Note that osmosis is specific to water; solutes do not move during osmosis. This distinguishes it from simple diffusion of other particles. Water molecules pass through the phospholipid bilayer slowly, but many membranes contain aquaporins, which are channel proteins that speed up water transport.
注意，渗透仅针对水；溶质在渗透过程中不移动。这将其与普通粒子的扩散区分开来。水分子通过磷脂双分子层较慢，但许多膜含有水通道蛋白，即一种可加速水分运输的通道蛋白。
5. Tonicity: Hypertonic, Hypotonic and Isotonic | 张力：高渗、低渗和等渗
These terms describe the relative solute concentrations of two solutions separated by a membrane.
这些术语描述了被膜隔开的两种溶液的相对溶质浓度。
- Isotonic: Equal solute concentration; there is no net movement of water in or out of the cell.
- 等渗：溶质浓度相等；细胞内外水分子没有净移动。
- Hypertonic: The external solution has a higher solute concentration; water moves out of the cell.
- 高渗：外界溶液溶质浓度更高；水从细胞中向外移动。
- Hypotonic: The external solution has a lower solute concentration; water moves into the cell.
- 低渗：外界溶液溶质浓度更低；水向细胞中移动。
These effects are easy to observe if cells are placed in sugar or salt solutions of different concentrations.
如果把细胞放入不同浓度的糖或盐溶液中，就很容易观察这些效应。
6. Osmosis in Plant Cells: Turgor and Plasmolysis | 植物细胞中的渗透：膨压与质壁分离
In a hypotonic solution (e.g. pure water), water enters the plant cell by osmosis. The central vacuole swells and presses the cytoplasm against the cell wall. The cell becomes turgid, and this turgor pressure provides support to non-woody plant tissues.
在低渗溶液（例如纯水中），水通过渗透进入植物细胞。中央液泡膨胀，使细胞质紧贴细胞壁。细胞变得膨大，即具有膨压。这种膨压为草本植物组织提供支撑。
In a hypertonic solution, water leaves the plant cell. The vacuole shrinks, and the cytoplasm pulls away from the cell wall. This process is called plasmolysis, and the cell becomes flaccid. Plasmolysis can cause plants to wilt.
在高渗溶液中，水离开植物细胞。液泡缩小，细胞质脱离细胞壁。这个过程称为质壁分离，细胞变得松软。质壁分离可导致植物萎蔫。
Because plant cells have a rigid cell wall, they do not burst when placed in water, unlike animal cells. The cell wall exerts pressure back, preventing excessive water uptake.
由于植物细胞有坚硬的细胞壁，在置于水中时不会像动物细胞那样破裂。细胞壁产生反压力，防止细胞过度吸水。
7. Osmosis in Animal Cells: Lysis and Crenation | 动物细胞中的渗透：溶血与皱缩
Animal cells have no cell wall, so they are more sensitive to osmotic changes. In a hypotonic solution, water moves into the cell, causing it to swell. Eventually the plasma membrane may burst. The bursting of red blood cells is called haemolysis.
动物细胞没有细胞壁，因此对渗透变化更敏感。在低渗溶液中，水进入细胞，使其膨胀。最终质膜可能破裂。红细胞破裂称为溶血。
In a hypertonic solution, water moves out of an animal cell. The cell shrinks and becomes shrivelled. For red blood cells, this shape change is called crenation. Because of these dangers, animals must tightly control the water balance of their blood and tissue fluids.
在高渗溶液中，水从动物细胞中移出。细胞收缩并变得干瘪。对于红细胞来说，这种形状变化称为皱缩。由于这些危险，动物必须严格控制血液和组织液的水平衡。
One of the main examples in the body is the action of the kidneys, which regulate blood water potential so that cells remain in an isotonic environment.
人体内一个主要的例子是肾脏的作用。肾脏调节血液水势，使细胞保持在等渗环境中。
8. Active Transport: Moving Against the Gradient | 主动转运：逆梯度移动
Sometimes cells must take in substances from regions of low concentration or remove substances to regions of high concentration. Active transport is the movement of molecules or ions across a membrane against their concentration gradient, using energy released by respiration.
有时细胞必须从低浓度区域吸收物质，或向高浓度区域排出物质。主动转运是指分子或离子逆着浓度梯度穿过膜的过程，其中使用呼吸作用释放的能量。
Active transport requires energy (ATP), carrier proteins and specificity
主动转运需要能量（ATP）、载体蛋白和特异性
During active transport, a carrier protein binds to the specific substance. ATP (adenosine triphosphate) is hydrolysed to provide energy, causing the protein to change shape and release the substance on the other side of the membrane.
在主动转运过程中，载体蛋白结合特定物质。ATP（三磷酸腺苷）被水解提供能量，使蛋白改变形状，并将物质释放到膜的另一侧。
9. Examples of Active Transport in Humans and Plants | 主动转运在人类和植物中的实例
In the human kidney: Glucose is reabsorbed from the glomerular filtrate into the blood even when blood sugar is already high. This happens in the proximal convoluted tubule and is a classic example of active transport.
在人类肾脏中：即使血糖已经很高，葡萄糖仍然从肾小球滤液中重吸收回血液。这发生在近曲小管中，是主动转运的典型例子。
In the small intestine: Dietary glucose is absorbed by villi cells. Sometimes the concentration of glucose in the intestine is lower than in the blood. Active transport enables glucose to still be absorbed.
在小肠中：食物中的葡萄糖由绒毛细胞吸收。有时肠道中葡萄糖浓度低于血液中的浓度。主动转运使葡萄糖仍能被吸收。
In plant roots: Mineral ions such as nitrate and magnesium are present at very low concentrations in soil water, but root hair cells actively transport them into the plant against the concentration gradient.
在植物根中：土壤溶液中的硝酸盐和镁离子等矿质离子浓度很低，但根毛细胞通过主动转运逆浓度梯度将它们吸收进植物体内。
10. Comparing the Three Processes | 三种运输方式的比较
Use this table to summarise the key differences between diffusion, osmosis and active transport:
通过下表总结扩散、渗透与主动转运之间的主要区别：
| Feature | Diffusion | Osmosis | Active Transport |
|---|---|---|---|
| Substances moved | Any small particles, gases, solutes | Water only | Ions, glucose, amino acids |
| Direction of movement | Down concentration gradient | Down water potential gradient | Against concentration gradient |
| Energy required? | No | No | Yes, from ATP |
| Membrane proteins needed? | Usually not for lipid-soluble molecules | Aquaporins may help | Yes, carrier proteins |
| Example | O₂ from alveoli to blood | Water from soil into root hairs | Glucose uptake in kidney tubules |
Diffusion and osmosis are passive processes, while active transport is an active process
扩散和渗透是被动过程，而主动转运是主动过程
11. Factors Affecting the Rate of Active Transport | 影响主动转运速率的因素
Since active transport depends on respiration, any condition that affects respiration will affect transport. The main factors are:
由于主动转运依赖于呼吸作用，任何影响呼吸的条件都会影响转运。主要因素有：
- Temperature: Higher temperatures increase enzyme activity in respiration up to an optimum. Above this, enzymes denature and transport stops.
- 温度：在一定范围内，温度升高会提高呼吸作用中的酶活性。超过最适温度后，酶变性，转运停止。
- Oxygen availability: Low oxygen levels reduce aerobic respiration, producing less ATP, so active transport slows down.
- 氧气供应：低氧水平会降低有氧呼吸，产生较少的ATP，因此主动转运减慢。
- Glucose availability: As the main respiratory substrate, a lack of glucose reduces ATP supply.
- 葡萄糖供应：作为主要呼吸底物，葡萄糖缺乏会减少ATP供应。
- Amount of carrier protein: More carrier proteins allow a higher maximum rate of transport.
- 载体蛋白数量：载体蛋白越多，允许的最大转运速率越高。
12. Why This Matters in Exams and Medicine | 为什么这些内容在考试和医学中很重要
Exam questions often ask you to predict what will happen to cells placed in different solutions. You should be able to describe and explain each result using the terms water potential, concentration gradient, partially permeable membrane and ATP.
考试问题常常要求你预测细胞放入不同溶液中会发生什么。你应该能够使用水势、浓度梯度、部分渗透性膜和ATP等术语来描述和解释每种结果。
In medicine, understanding osmosis helps explain why hospital patients are given isotonic saline drips. If pure water were injected into the bloodstream, red blood cells could swell and burst. Similarly, understanding active transport helps explain how certain drugs are absorbed in the intestine or how the kidneys reabsorb essential nutrients.
在医学中，理解渗透有助于解释为什么医院给病人输注等渗盐水。如果直接将纯水注入血液，红细胞可能膨胀破裂。同样，理解主动转运有助于解释某些药物如何在肠道中被吸收，以及肾脏如何重吸收必需营养物质。
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