物质运输
爱德思国际GCSE生物· 2.51–2.69（2017年考纲第3版）· 45 分钟阅读
1. 生物为什么需要运输系统★★☆☆☆⏱ 10 min
所有生物都需要将必需物质（氧气、葡萄糖、矿质离子）运入细胞，同时将代谢废物（二氧化碳、尿素）运出细胞。采用的运输方式取决于生物的体型以及表面积体积比（SA:V）。
表面积体积比（SA:V）
用于衡量生物的总表面积相对于其总体积的比值；体型更小的生物的表面积体积比远大于体型更大的生物。
单细胞生物（例如变形虫）的表面积体积比非常大，最大扩散距离不足1毫米，因此所有物质仅通过扩散作用就可以足够快速地进出细胞，无需专门的运输系统。多细胞生物的表面积体积比小得多，身体深处的细胞的扩散距离可达数厘米，扩散速度太慢无法满足代谢需求，因此需要特化的运输系统。
解释为什么1厘米大小的单细胞生物可以依靠扩散完成物质运输，而10厘米大小的多细胞生物却不能。
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步骤1：计算1厘米生物的表面积体积比：表面积 = 6 × 1² = 6cm²，体积 = 1cm³，因此表面积体积比为6:1。到细胞中心的最大扩散距离为0.5厘米，物质可以快速到达细胞的所有部位。
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步骤2：计算10厘米生物的表面积体积比：表面积 = 6 × 10² = 600cm²，体积 = 1000cm³，因此表面积体积比为0.6:1，比单细胞生物小10倍。
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步骤3：该多细胞生物到内部细胞的最大扩散距离为5厘米，扩散速度太慢，无法输送足够的氧气和葡萄糖以满足能量需求，因此需要专门的运输系统。
Exam tip:
考试题目经常要求你将表面积体积比与运输系统的必要性直接关联；作答时请务必同时提到多细胞生物更小的表面积体积比以及更长的扩散距离，才能拿到全部分数。
2. 植物运输系统★★★☆☆⏱ 12 min
✓ 计算器
开花植物拥有两种特化的运输组织：木质部和韧皮部，二者以维管束的形式分布在根、茎和叶片中。
Xylem: Transports water and mineral ions absorbed from the soil upwards from the roots to all other parts of the plant. Made of dead, hollow lignified cells, transport only occurs in one direction.
Phloem: Transports sucrose (made from glucose during photosynthesis) and amino acids around the plant in a process called translocation, which works in both directions from sources (areas that produce glucose, e.g. leaves) to sinks (areas that use or store glucose, e.g. roots, fruits, developing seeds).
比较木质部和韧皮部在为植株上发育中的草莓果实输送资源时的作用。
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步骤1：木质部将水分和矿质离子从根部向上运输至草莓果实，为细胞膨大提供水分，为健康生长提供矿质离子。
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步骤2：韧皮部通过转运作用，将进行光合作用的叶片（源）合成的蔗糖和氨基酸运输至发育中的果实（库），为果实生长提供能量和构建原料。
水分由根毛细胞吸收，根毛细胞拥有巨大的表面积以最大化渗透作用效率：水分从低浓度的土壤溶液进入根毛细胞浓度更高的细胞质中。蒸腾作用是水分通过开放的气孔从植物叶片表面蒸发的过程，该过程会产生拉力，将连续的水柱从根部沿木质部向上牵引，形成蒸腾流。
蒸腾作用速率受四大关键环境因素影响：
Temperature: Higher temperatures increase the rate of water evaporation, so transpiration is faster.
Wind speed: Higher wind removes water vapour from around leaves, increasing the concentration gradient for diffusion, so transpiration is faster.
Humidity: Higher humidity reduces the concentration gradient between the leaf and air, so transpiration is slower.
Light intensity: Higher light opens stomata for photosynthesis, so more water evaporates, making transpiration faster.
蒸腾计可用于测定蒸腾作用速率：植物吸收水分补充蒸腾作用散失的水分时，测量毛细管内气泡随时间移动的距离即可计算蒸腾速率。
一名学生使用蒸腾计测量带叶枝条的蒸腾速率。当风扇对着枝条吹风时，气泡在5分钟内移动了15毫米，而没有风扇吹风时，气泡5分钟仅移动3毫米。请解释该实验结果。
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步骤1：风扇提高了枝条叶片周围的风速。
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步骤2：风会立即带走从叶片表面蒸发的水蒸气，维持叶片内部高水蒸气浓度和叶片外部低水蒸气浓度之间的高浓度梯度。
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步骤3：这会提升叶片的水分蒸发速率，进而提升蒸腾作用速率。植物会从蒸腾计中吸收更多水分补充散失的水分，因此气泡移动速度更快。
Exam tip:
不要混淆木质部和韧皮部的功能：可以记住Xylem（木质部）对应Water（水），两个字母在字母表中相邻，避免丢失基础分数。
3. 人体血液组分与免疫功能★★★☆☆⏱ 12 min
人体血液由悬浮在血浆中的四大核心组分构成：红细胞、白细胞和血小板。
Plasma: Pale yellow liquid making up ~55% of blood volume. Transports carbon dioxide from respiring cells to lungs, digested food from the small intestine to cells, urea from the liver to kidneys, hormones around the body, and heat energy to regulate body temperature.
Red blood cells: Carry oxygen from the lungs to respiring cells. Three key adaptations: biconcave disc shape (large surface area for oxygen diffusion), no nucleus (more space for haemoglobin), contains haemoglobin (red protein that binds oxygen in the lungs to form oxyhaemoglobin, which releases oxygen at respiring cells).
White blood cells: Part of the immune system. Two main types: phagocytes, which engulf and ingest pathogens via phagocytosis; lymphocytes, which produce antibodies specific to antigens on the surface of a pathogen, clumping pathogens together for destruction by phagocytes.
疫苗接种通过向体内引入减毒或灭活的病原体诱导免疫应答且不引发疾病，从而提供长期免疫力。淋巴细胞会产生针对该病原体的抗体，同时形成存活时间极长的记忆细胞。若后续人体接触到活的病原体，记忆细胞会以快得多的速度产生大量抗体，在病症出现前就将病原体消灭。血小板是小型细胞碎片，可在伤口处触发凝血，避免失血过多，同时阻止微生物进入人体。
说明红细胞具备的两项适应性特征，解释其为何能高效地在人体内运输氧气。
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步骤1：红细胞呈双凹圆盘状，该形态使其相对体积拥有极大的表面积，最大化氧气进出细胞的扩散速率。
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步骤2：红细胞没有细胞核，这为细胞内部腾出了额外空间，可储存更多能结合氧气的血红蛋白，因此每个红细胞单次运输可携带更多氧气。
Exam tip:
作答有关白细胞的题目时，务必明确说明淋巴细胞产生的抗体对某一种病原体具有特异性，这是很多学生容易遗漏的常见得分点。
4. 心脏与循环系统★★★★☆⏱ 15 min
人体拥有双循环系统：血液每完成一次全身循环会经过心脏两次，一次通过肺循环（前往肺部摄取氧气），一次通过体循环（前往身体其余部位输送氧气）。心脏是拥有四个腔室的肌肉器官：
Right atrium: Receives deoxygenated blood from the body via the vena cava
Right ventricle: Pumps deoxygenated blood to the lungs via the pulmonary artery
Left atrium: Receives oxygenated blood from the lungs via the pulmonary vein
Left ventricle: Pumps oxygenated blood to the rest of the body via the aorta; has a thicker muscular wall than the right ventricle to generate higher pressure for long-distance transport around the body.
瓣膜（心房和心室之间的房室瓣、心室出口处的半月瓣）可防止血液倒流，确保血液仅沿单一方向流动。人体共有三大类血管：
Arteries: Carry blood away from the heart at high pressure; thick muscular/elastic walls, narrow lumen, no valves (except semilunar valves at the aorta and pulmonary artery).
Veins: Carry blood back to the heart at low pressure; thin walls, wide lumen, valves to prevent backflow.
Capillaries: Tiny vessels that carry blood through organs for substance exchange; one-cell-thick walls to reduce diffusion distance, permeable to small molecules like oxygen, glucose and carbon dioxide.
当脂肪沉积物使向心肌输送氧气的冠状动脉变窄时，就会引发冠心病（CHD），减少氧气供给，引发胸痛或心脏病发作。可调控的风险因素包括高饱和脂肪/高盐饮食、吸烟、缺乏运动、肥胖和高压状态；不可调控的风险因素包括遗传和年龄。
运动过程中心率会上升，从而向进行呼吸作用的肌肉细胞输送更多氧气和葡萄糖，同时排出多余的二氧化碳。人体在压力或恐惧状态下释放的肾上腺素也会提升心率，让身体为「战或逃」反应做好准备。
解释为什么左心室的肌肉壁比右心室更厚。
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步骤1：右心室仅将脱氧血液泵送至肺部（肺循环），输送距离较短，因此仅需产生相对较低的压力——压力过高会损伤脆弱的肺部毛细血管。
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步骤2：左心室将含氧血液泵送至全身各处（体循环），输送距离远得多且需要克服更大阻力，因此必须产生高得多的压力。
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步骤3：左心室更厚的肌肉壁使其能够以大得多的力量收缩，从而产生上述更高的压力。
Exam tip:
常见的考试陷阱是提问是否所有动脉都运输含氧血：请记住肺动脉将脱氧血输送至肺部，切勿笼统地认为所有动脉都运输含氧血，否则会丢失分数。
5. 常见陷阱
错误做法:
Stating phloem only transports substances up the plant and xylem transports in both directions
原因:
This mixes up core function of the two plant transport tissues, a very common marking point
正确做法:
Remember xylem only transports water/mineral ions upwards from roots; phloem transports sucrose/amino acids in both directions via translocation
错误做法:
Claiming all arteries carry oxygenated blood and all veins carry deoxygenated blood
原因:
The pulmonary artery carries deoxygenated blood to the lungs, and the pulmonary vein carries oxygenated blood to the heart, so this generalisation is incorrect
正确做法:
Define arteries as vessels carrying blood away from the heart, veins as vessels carrying blood to the heart, regardless of oxygen content
错误做法:
Only listing 1 or 2 adaptations of red blood cells when asked for three
原因:
The mark scheme explicitly requires all three adaptations (biconcave shape, no nucleus, haemoglobin) for full marks
正确做法:
Learn all three adaptations and link each directly to its function for oxygen transport
错误做法:
Stating vaccination gives you ready-made antibodies to fight pathogens
原因:
Vaccines trigger your own immune system to make antibodies and memory cells, they do not contain pre-made antibodies for this topic
正确做法:
Explain vaccines contain weakened/dead pathogens that trigger an immune response, producing memory cells for long-term immunity
错误做法:
Forgetting the hepatic portal vein carries blood from the gut to the liver, not from the liver to the heart
原因:
The hepatic portal vein is a unique vessel not connected directly to the heart, and is frequently tested
正确做法:
Memorise the three liver vessels: hepatic artery (to liver), hepatic vein (away to heart), hepatic portal vein (from gut to liver)
6. 速查表
Structure/Component | Key Function | Core Exam Facts |
|---|---|---|
Xylem | Transport water/mineral ions from roots to plant | One direction only (up), dead lignified cells |
Phloem | Transport sucrose/amino acids around plant | Translocation, both directions (source → sink) |
Red blood cell | Carry oxygen to respiring cells | 3 adaptations: biconcave shape, no nucleus, haemoglobin |
Phagocyte | Immune response | Engulfs/ingests pathogens via phagocytosis |
Lymphocyte | Immune response | Produces specific antibodies against pathogen antigens |
Artery | Carry blood away from heart | Thick muscular/elastic wall, narrow lumen, high pressure |
Vein | Carry blood to heart | Thin wall, wide lumen, valves prevent backflow, low pressure |
Capillary | Substance exchange between blood and cells | One-cell-thick wall, short diffusion distance, permeable |
Left ventricle | Pump blood to body | Thicker wall than right ventricle to generate higher pressure |
CHD Risk Factors | Increase coronary artery narrowing risk | High saturated fat/salt diet, smoking, inactivity, obesity, genetics |
7. 常见问题
所有动脉都运输含氧血吗？
不是。肺动脉将心脏右心室的缺氧血运输到肺部以结合氧气，只有体循环动脉（向身体各处输送血液的动脉）才运输含氧血。
木质部与韧皮部的物质运输方向有什么区别？
木质部仅将水分和矿质离子向上运输，从根部输送到植物的其余部分。韧皮部可以双向运输蔗糖和氨基酸，从源端（例如进行光合作用的叶片）运输到库端（例如生长中的根系、果实、储存器官）。
疫苗如何提供长期免疫力？
疫苗包含减毒或灭活的病原体，可在不引发疾病的情况下触发免疫反应。淋巴细胞会产生针对该病原体的抗体，同时生成寿命较长的记忆细胞。如果后续接触到真实的病原体，记忆细胞会以更快的速度产生大量抗体，在人体患病前就将病原体消灭。
深入阅读
下一步
Now that you have mastered transport systems in plants and humans, you are ready to move on to related Edexcel IGCSE Biology topics. Next, learn about excretion in humans, including how kidneys remove urea from the blood and regulate water balance, which builds on your knowledge of plasma function and renal blood vessels. Revise movement of substances into and out of cells to reinforce your understanding of diffusion, osmosis and active transport, which underpin all transport processes in living organisms. Practice transport topic past paper questions to test your knowledge, paying close attention to 'explain' command terms that require you to link structure to function for full marks. Memorise key definitions and labelled diagrams of the heart and plant vascular tissues to maximise your exam marks.