📚 Movement of Substances into and out of Cells | 物质进出细胞的方式
All living cells exchange materials with their environment across the plasma membrane. This exchange includes respiratory gases, nutrients, water, ions and waste products. Understanding how substances move into and out of cells is fundamental to explaining processes such as gas exchange, nutrient absorption, nerve impulse transmission and kidney function. In A-Level Biology, the key mechanisms are diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis.
所有活细胞都通过质膜与环境进行物质交换。这种交换包括呼吸气体、营养物质、水、离子和代谢废物。理解物质进出细胞的方式，对于解释气体交换、营养吸收、神经冲动传导和肾脏功能等过程至关重要。在 A-Level 生物课程中，核心机制包括扩散、促进扩散、渗透、主动运输、胞吞和胞吐。
1. The Fluid Mosaic Model of the Cell Membrane | 细胞膜的流动镶嵌模型
The cell surface membrane is described by the fluid mosaic model. A phospholipid bilayer forms the basic structure, with hydrophobic fatty acid tails facing inwards and hydrophilic phosphate heads facing outwards. The membrane also contains proteins, cholesterol, glycoproteins and glycolipids. The term ‘fluid’ refers to the lateral movement of phospholipids and proteins, while ‘mosaic’ describes the patchy arrangement of proteins embedded in the bilayer.
细胞表面膜可以用流动镶嵌模型来描述。磷脂双分子层构成基本结构，疏水的脂肪酸尾部朝内，亲水的磷酸头部朝外。膜中还含有蛋白质、胆固醇、糖蛋白和糖脂。‘流动’指的是磷脂和蛋白质可以侧向移动，‘镶嵌’则描述蛋白质在双分子层中呈斑块状分布。
Membrane permeability depends on the properties of the phospholipid bilayer. Small, non-polar molecules such as O₂ and CO₂ diffuse through rapidly. Small polar molecules such as water and urea pass slowly, while ions and large polar molecules such as glucose cannot pass through the hydrophobic core without transport proteins.
膜的通透性取决于磷脂双分子层的性质。氧气和二氧化碳等小的非极性分子可以快速扩散通过。水和尿素等小的极性分子通过较慢，而离子和葡萄糖等大的极性分子则不能穿过疏水核心，必须借助转运蛋白。
2. Passive and Active Transport: An Overview | 被动运输与主动运输概述
Transport across membranes can be classified into passive and active processes. Passive transport does not require metabolic energy in the form of ATP. Movement occurs down a concentration gradient or electrochemical gradient. Examples include simple diffusion, facilitated diffusion and osmosis.
跨膜运输可分为被动过程和主动过程。被动运输不需要 ATP 形式的代谢能量。物质沿浓度梯度或电化学梯度移动。例子包括简单扩散、促进扩散和渗透。
Active transport requires energy, usually from ATP hydrolysis. It moves substances against their concentration gradient, from a region of lower concentration to a region of higher concentration. Active transport always involves specific carrier proteins called pumps. Bulk transport processes such as endocytosis and exocytosis also require ATP.
主动运输需要能量，通常来自 ATP 的水解。它使物质逆浓度梯度移动，即从较低浓度区域向较高浓度区域运输。主动运输总是涉及称为泵的特异性载体蛋白。胞吞和胞吐等大量运输过程也需要 ATP。
3. Simple Diffusion | 简单扩散
Simple diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is a passive process that does not use ATP or membrane proteins. Diffusion continues until the particles are evenly distributed and equilibrium is reached, although random molecular motion continues.
简单扩散是分子或离子从较高浓度区域向较低浓度区域的净移动，即沿浓度梯度进行。这是一个被动过程，不使用 ATP 或膜蛋白。扩散持续进行，直到粒子均匀分布并达到动态平衡，但分子的随机运动仍然继续。
The rate of simple diffusion across a membrane is influenced by several factors. It is faster when the concentration gradient is steeper, the surface area is larger, the diffusion distance is shorter and the temperature is higher. The size and lipid solubility of the molecule also matter: smaller, non-polar molecules diffuse more easily through the phospholipid bilayer.
简单扩散跨膜的速率受多种因素影响。浓度梯度越陡、表面积越大、扩散距离越短、温度越高，扩散越快。分子的大小和脂溶性也很重要：较小、非极性的分子更容易穿过磷脂双分子层。
Rate of diffusion ∝ (surface area × concentration difference) ÷ diffusion distance
扩散速率 ∝ (表面积 × 浓度差) ÷ 扩散距离
4. Facilitated Diffusion | 促进扩散
Facilitated diffusion allows ions and large polar molecules to cross membranes down their concentration gradient without ATP. It uses two types of transport proteins: channel proteins and carrier proteins. Channel proteins form hydrophilic pores, while carrier proteins change shape to transfer specific molecules across the membrane.
促进扩散使离子和大的极性分子能够沿浓度梯度跨膜运输，且不需要 ATP。它利用两类转运蛋白：通道蛋白和载体蛋白。通道蛋白形成亲水孔道，载体蛋白则通过构象变化将特定分子转运过膜。
Facilitated diffusion shows saturation kinetics. At low substrate concentration, the rate increases with concentration. At high substrate concentration, the rate reaches a maximum because all transport proteins are occupied. Unlike simple diffusion, facilitated diffusion is specific and can be inhibited by molecules that block the transport proteins.
促进扩散表现出饱和动力学特征。在底物浓度较低时，运输速率随浓度增加而加快。当底物浓度较高时，由于所有转运蛋白都被占据，速率达到最大值。与简单扩散不同，促进扩散具有特异性，并可被阻断转运蛋白的分子抑制。
Important examples include glucose uptake into red blood cells via GLUT carrier proteins and the movement of sodium or potassium ions through gated ion channels in nerve cells. Aquaporins are channel proteins that greatly increase the permeability of membranes to water.
重要的例子包括葡萄糖通过 GLUT 载体蛋白进入红细胞，以及钠离子或钾离子通过神经细胞中的门控离子通道进行移动。水通道蛋白是通道蛋白的一种，可大幅提高膜对水的通透性。
5. Osmosis and Water Potential | 渗透作用与水势
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a selectively permeable membrane. Water potential is the tendency of water to move from one place to another and is measured in kilopascals (kPa). Pure water at standard conditions has a water potential of 0 kPa. Adding solutes lowers the water potential, making it more negative.
渗透作用是水分子通过选择透过性膜从水势较高的区域向水势较低的区域的净移动。水势是水从一处向另一处移动的趋势，单位为千帕 (kPa)。标准条件下纯水的水势为 0 kPa。加入溶质会降低水势，使其变为负值。
ψ = ψs + ψp
水势 = 溶质势 + 压力势
In this equation, ψ is the water potential, ψs is the solute potential (always negative or zero) and ψp is the pressure potential. In an animal cell, pressure potential is usually negligible. In a plant cell, the cell wall can exert pressure, making ψp positive and increasing the water potential of the cell contents.
在该方程中，ψ 为水势，ψs 为溶质势（始终为负或零），ψp 为压力势。在动物细胞中，压力势通常可忽略不计。在植物细胞中，细胞壁可产生压力，使 ψp 为正值，并提高细胞内容物的水势。
When an animal cell is placed in a hypotonic solution, water enters by osmosis and the cell may swell and burst, a process called cytolysis. In a hypertonic solution, water leaves and the cell shrinks, a process called crenation. Plant cells in hypotonic solutions become turgid because the cell wall prevents bursting. In hypertonic solutions, plant cells undergo plasmolysis, where the cell membrane pulls away from the cell wall.
当动物细胞置于低渗溶液中时，水通过渗透作用进入细胞，细胞可能膨胀并破裂，这一过程称为溶破。在高渗溶液中，水离开细胞，细胞皱缩，这一过程称为皱缩。植物细胞在低渗溶液中因细胞壁阻止破裂而变得坚挺。在高渗溶液中，植物细胞发生质壁分离，即细胞膜从细胞壁上剥离。
6. Active Transport | 主动运输
Active transport moves substances against their concentration gradient, from lower to higher concentration, and therefore requires energy. The energy is usually supplied by the hydrolysis of ATP to ADP and inorganic phosphate. Active transport is carried out by specific carrier proteins, often called pumps, which undergo conformational changes.
主动运输使物质逆浓度梯度从较低浓度向较高浓度移动，因此需要能量。能量通常由 ATP 水解为 ADP 和无机磷酸提供。主动运输由特定的载体蛋白（常称为泵）完成，这些蛋白发生构象变化。
A key example is the sodium-potassium pump in animal cells. It actively transports three Na⁺ ions out of the cell and two K⁺ ions into the cell for each ATP molecule hydrolysed. This creates concentration gradients that are essential for nerve impulse transmission and secondary active transport.
一个关键例子是动物细胞中的钠钾泵。每水解一个 ATP 分子，它主动将三个 Na⁺ 离子运出细胞，并将两个 K⁺ 离子运入细胞。这产生了对神经冲动传导和次级主动运输至关重要的浓度梯度。
In plant roots, active transport is used to absorb nitrate ions from the soil even when their concentration inside root hair cells is already higher than in the soil. In the mammalian small intestine, glucose is absorbed by co-transport with sodium ions. The sodium gradient is established by active transport, and glucose then enters cells along this gradient via a co-transporter protein. This is an example of secondary active transport.
在植物根部，即使根毛细胞内的硝酸根离子浓度已经高于土壤，主动运输仍用于从土壤中吸收硝酸根离子。在哺乳动物小肠中，葡萄糖通过与钠离子协同运输被吸收。钠离子梯度由主动运输建立，随后葡萄糖通过协同转运蛋白沿该梯度进入细胞。这是次级主动运输的一个例子。
7. Endocytosis | 胞吞作用
Endocytosis is a form of bulk transport that allows cells to take in large molecules, particles or even whole microorganisms. The cell membrane invaginates to surround the material, then pinches off to form a vesicle inside the cell. This process requires ATP and is therefore energy-dependent.
胞吞作用是一种大量运输形式，使细胞能够摄入大分子、颗粒甚至整个微生物。细胞膜内陷包围物质，然后收缩形成细胞内的囊泡。该过程需要 ATP，因此依赖能量。
There are three main types of endocytosis. Phagocytosis involves the uptake of large particles or cells, for example white blood cells engulfing bacteria. Pinocytosis involves the uptake of extracellular fluid and dissolved solutes. Receptor-mediated endocytosis is highly specific and involves the binding of ligands such as cholesterol-carrying low-density lipoproteins to membrane receptors before vesicle formation.
胞吞作用主要有三种类型。吞噬作用涉及摄入大颗粒或细胞，例如白细胞吞噬细菌。胞饮作用涉及摄入细胞外液和溶解的溶质。受体介导的胞吞作用特异性很高，在囊泡形成之前，携带胆固醇的低密度脂蛋白等配体会先与膜受体结合。
8. Exocytosis | 胞吐作用
Exocytosis is the process by which cells release large molecules or waste products to the outside. Vesicles containing the substances move towards the cell surface membrane, fuse with it, and release their contents into the extracellular space. Like endocytosis, exocytosis requires ATP and is a form of active bulk transport.
胞吐作用是细胞向外部释放大分子或废物的过程。含有这些物质的囊泡向细胞表面膜移动，与膜融合，并将其内容物释放到细胞外空间。与胞吞作用一样，胞吐作用需要 ATP，是一种主动的大量运输形式。
Exocytosis is used in many important biological processes. For example, pancreatic cells use exocytosis to secrete digestive enzymes, neurones use it to release neurotransmitters at synapses, and plant cells use it to export cell wall materials such as cellulose and pectin.
胞吐作用参与许多重要的生物学过程。例如，胰腺细胞通过胞吐作用分泌消化酶，神经元在突触处通过胞吐作用释放神经递质，植物细胞则通过胞吐作用输出纤维素和果胶等细胞壁物质。
9. Factors Affecting the Rate of Movement | 影响运输速率的因素
The rate of passive transport generally increases with temperature because molecules gain kinetic energy and move faster. However, very high temperatures can denature transport proteins and disrupt membrane structure, so facilitated diffusion and active transport decline sharply above a certain temperature.
被动运输的速率通常随温度升高而加快，因为分子获得动能，运动更快。然而，过高的温度会使转运蛋白变性并破坏膜结构，因此促进扩散和主动运输在超过一定温度后会急剧下降。
Concentration gradient is a major factor for diffusion and facilitated diffusion until saturation is reached. Surface area also affects the rate of transport, which is why cells specialised for absorption have folded membranes, such as microvilli in the small intestine and root hairs in plants. Diffusion distance is also important: thin exchange surfaces allow faster movement.
浓度梯度是扩散和促进扩散在达到饱和之前的主要影响因素。表面积也影响运输速率，这就是为什么专门负责吸收的细胞具有折叠膜，例如小肠中的微绒毛和植物的根毛。扩散距离也很重要：薄的交换表面使移动更快。
Active transport is sensitive to factors that reduce ATP production. Respiratory inhibitors such as cyanide and low oxygen concentration decrease the rate of active transport. The number of carrier proteins or pumps can also limit the maximum rate of active transport.
主动运输对减少 ATP 生成的因素很敏感。氰化物等呼吸抑制剂和低氧浓度会降低主动运输的速率。载体蛋白或泵的数量也会限制主动运输的最大速率。
10. Comparing Transport Mechanisms | 运输机制比较
Simple diffusion, facilitated diffusion and active transport can be compared using four criteria: direction of movement, use of membrane proteins, requirement for ATP, and tendency to reach saturation. Simple diffusion is down a gradient, uses no proteins and does not saturate. Facilitated diffusion is down a gradient, uses channel or carrier proteins and does saturate.
简单扩散、促进扩散和主动运输可以从四个标准进行比较：运动方向、是否使用膜蛋白、是否需要 ATP，以及是否达到饱和。简单扩散沿梯度进行，不使用蛋白质，不出现饱和。促进扩散沿梯度进行，使用通道蛋白或载体蛋白，并且会饱和。
Active transport moves against a gradient, always uses carrier proteins called pumps, requires ATP directly or indirectly, and also shows saturation. Bulk transport differs from all three because it moves large quantities of material in vesicles, does not rely on concentration gradients, and depends on ATP and cytoskeletal components.
主动运输逆梯度移动，总是使用称为泵的载体蛋白，直接或间接需要 ATP，并且也表现出饱和。大量运输与前三者不同，因为它通过囊泡运输大量物质，不依赖浓度梯度，而依赖 ATP 和细胞骨架成分。
| Feature | Simple diffusion | Facilitated diffusion | Active transport |
|---|---|---|---|
| Gradient | Down | Down | Against |
| Protein | None | Channel or carrier | Carrier/pump |
| ATP | No | No | Yes |
| Saturation | No | Yes | Yes |
The table above summarises the key differences among simple diffusion, facilitated diffusion and active transport. In exam answers, students should always state whether ATP is used and whether movement is down or against a gradient.
上表总结了简单扩散、促进扩散和主动运输之间的主要区别。在考试答案中，学生应始终说明是否使用 ATP，以及物质是沿梯度还是逆梯度移动。
11. Experimental Investigations | 实验探究
Diffusion can be investigated using agar cubes containing a pH indicator or coloured dye. By cutting cubes of different sizes and measuring the distance travelled by the dye or the time taken for colour change, students can show that surface area to volume ratio limits the rate of diffusion. Smaller cubes have a larger surface area to volume ratio and therefore exchange materials faster.
可以用含有 pH 指示剂或有色染料的琼脂块来探究扩散。通过切出不同大小的立方体，并测量染料前进的距离或颜色变化所需的时间，学生可以证明表面积与体积之比限制扩散速率。较小的立方体具有较大的表面积与体积比，因此交换物质更快。
Osmosis is commonly investigated by placing potato strips or cylinders in sucrose solutions of different concentrations. The change in mass or length indicates the direction of water movement. If the solution has a lower water potential than the potato cells, water leaves and the potato loses mass. If the solution has a higher water potential, water enters and the potato gains mass. The water potential of the potato tissue can be estimated by plotting percentage change in mass against sucrose concentration and finding the point where there is no net change.
渗透作用通常通过将马铃薯条或圆柱体放入不同浓度的蔗糖溶液来探究。质量或长度的变化可以指示水分运动的方向。如果溶液的水势低于马铃薯细胞，水分离开，马铃薯质量减少。如果溶液的水势高于细胞，水分进入，马铃薯质量增加。可以绘制质量变化百分比对蔗糖浓度的图，并找到没有净变化的点，从而估算马铃薯组织的水势。
The permeability of cell membranes can be studied using beetroot tissue. When membranes are damaged, the red pigment betalain leaks out and can be measured with a colorimeter. Increasing temperature or treating the tissue with alcohol can increase membrane permeability, showing that temperature and solvents affect membrane structure.
细胞膜的通透性可以用甜菜根组织进行研究。当膜受损时，红色色素甜菜红素会泄漏出来，并可用比色计测量。升高温度或用酒精处理组织可增加膜的通透性，这表明温度和溶剂会影响膜结构。
12. Exam Tips and Common Misconceptions | 考试提示与常见误区
A common misconception is that diffusion only occurs in gases or liquids and stops at equilibrium. In fact, diffusion occurs in solutions and across membranes, and particles continue to move randomly at equilibrium, but there is no net movement. Another common error is saying that active transport uses energy to move substances from high to low concentration; active transport always moves substances against a gradient.
一个常见的误区是认为扩散只发生在气体或液体中，并且在平衡时停止。实际上，扩散发生在溶液中和跨膜过程中，平衡时粒子仍然随机运动，但没有净移动。另一个常见错误是说主动运输利用能量将物质从高浓度移到低浓度；主动运输总是逆梯度移动物质。
When answering questions about osmosis, students should use the term ‘water potential’ precisely. A solution with more solutes has a lower water potential, not just a lower concentration. Osmosis refers specifically to water movement through a selectively permeable membrane. Without a membrane, the process is simply diffusion of water.
在回答关于渗透作用的问题时，学生应准确使用‘水势’这一术语。含有更多溶质的溶液水势较低，而不仅仅是浓度较低。渗透作用专指水通过选择透过性膜的移动。如果没有膜，该过程只是水的扩散。
Finally, when describing facilitated diffusion or active transport, students should mention the role of specific proteins and how saturation occurs. For active transport, linking the process to ATP hydrolysis and the conformational change of the carrier protein is essential for full marks.
最后，在描述促进扩散或主动运输时，学生应提到特异性蛋白质的作用以及饱和如何发生。对于主动运输，将过程与 ATP 水解和载体蛋白的构象变化联系起来是获得满分的关键。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导