📚 Translocation in Phloem: Mass Flow and Source-Sink Transport | 韧皮部运输作用：压力流动与源库运输
In A-Level Biology, translocation refers to the transport of soluble organic substances, especially sucrose and amino acids, through the phloem of flowering plants. Unlike xylem transport, translocation carries assimilates from regions of production, known as sources, to regions of storage or use, known as sinks. The main mechanism proposed to explain this process is the mass flow or pressure flow hypothesis, which links osmosis, active loading and hydrostatic pressure gradients.
在 A-Level 生物课程中，运输作用指可溶性有机物质（尤其是蔗糖和氨基酸）通过开花植物韧皮部的转运。与木质部运输不同，运输作用将同化产物从生产部位（源）运往储存或利用部位（库）。解释这一过程的主要机制是压力流动假说，它将渗透作用、主动装载和静水压力梯度联系在一起。
1. Definition and Importance of Translocation | 运输作用的定义与重要性
Translocation is the movement of assimilates, predominantly sucrose and amino acids, through phloem sieve tubes from source to sink. It is an active process at the loading and unloading stages, although the flow of sap through sieve tubes is passive once a pressure gradient is established. Translocation is essential because non-photosynthetic organs, such as roots, flowers, developing fruits and apical buds, cannot synthesise enough carbohydrate to meet their metabolic demands.
运输作用是指同化产物（主要是蔗糖和氨基酸）通过韧皮部筛管从源到库的转运。尽管韧皮部汁液在压力梯度建立后沿筛管的流动是被动的，但装载和卸载阶段是主动过程。运输作用必不可少，因为根、花、发育中的果实和顶芽等非光合器官无法合成足够的碳水化合物来满足自身代谢需求。
2. Source-Sink Relationships | 源库关系
A source is any plant region that produces more assimilates than it consumes. Mature leaves are the main sources during the day, while storage organs such as germinating seeds or tubers can become sources when they mobilise stored starch or proteins. A sink is any region that imports assimilates for growth, respiration or storage; examples include roots, shoot tips, flowers, developing seeds and storage roots.
源是植物中同化产物生产大于消耗的任何部位。白天成熟叶片是主要源，而萌发的种子或块茎等储存器官在动员储存的淀粉或蛋白质时也可以成为源。库是输入同化产物用于生长、呼吸或储存的部位，例如根、茎尖、花、发育中的种子和储藏根。
Source and sink roles are not fixed. A young leaf is a sink because it imports sugars for growth, but after expansion and the onset of photosynthesis it becomes a source. Similarly, a storage root is a sink during summer when it accumulates starch, but acts as a source in the next growing season when starch is hydrolysed to sucrose.
源和库的角色不是固定不变的。幼叶是库，因为它输入糖分用于生长，但当叶片展开并开始光合作用后，就变为源。同样，储藏根在夏季积累淀粉时是库，但在下一个生长季淀粉水解为蔗糖时可作为源。
3. Structure of Phloem: Sieve Tubes and Companion Cells | 韧皮部结构：筛管与伴胞
Phloem tissue contains sieve tube elements, companion cells, phloem parenchyma and phloem fibres. Sieve tube elements are elongated living cells arranged end to end; their end walls are perforated to form sieve plates, allowing cytoplasmic continuity and sap flow. Mature sieve tube elements lack a nucleus, ribosomes and most organelles, so they rely on adjoining companion cells for metabolic support.
韧皮部组织包含筛管分子、伴胞、韧皮薄壁细胞和韧皮纤维。筛管分子是长形活细胞，首尾相连；其端壁穿孔形成筛板，从而保持细胞质连续和汁液流动。成熟筛管分子没有细胞核、核糖体和大部分细胞器，因此依赖相邻伴胞提供代谢支持。
Each sieve tube element is connected to at least one companion cell by numerous plasmodesmata. The companion cell contains a large nucleus, many mitochondria and ribosomes, and supplies ATP and proteins to the sieve element. In many species, transfer cells have wall ingrowths that increase surface area for membrane transport.
每个筛管分子通过大量胞间连丝与至少一个伴胞相连。伴胞含有较大的细胞核、许多线粒体和核糖体，为筛管分子提供 ATP 和蛋白质。许多植物中，传递细胞的细胞壁向内突起，增加了膜运输的表面积。
4. Loading Sucrose at the Source | 源端蔗糖装载
In photosynthesising mesophyll cells, triose phosphate produced by the Calvin cycle is converted to sucrose in the cytosol. Sucrose moves through plasmodesmata or the apoplast to companion cells and then into sieve tube elements. The apoplastic loading pathway is best understood and is an active process.
在光合叶肉细胞中，卡尔文循环产生的磷酸丙糖在细胞质中转化为蔗糖。蔗糖通过胞间连丝或质外体进入伴胞，再进入筛管分子。质外体装载途径研究得最清楚，是一个主动过程。
In the apoplastic pathway, a proton pump (H⁺-ATPase) on the companion cell membrane actively transports H⁺ out into the cell wall, creating a proton gradient. A sucrose-H⁺ symporter then uses the inward movement of H⁺ down its concentration gradient to co-transport sucrose into the companion cell against its concentration gradient. This is an example of secondary active transport.
在质外体途径中，伴胞膜上的质子泵（H⁺-ATP 酶）主动将 H⁺ 运出到细胞壁，形成质子梯度。蔗糖-H⁺ 同向转运蛋白利用 H⁺ 顺浓度梯度内流，将蔗糖逆浓度梯度共同转运进伴胞。这是次级主动运输的一个例子。
Accumulation of sucrose in the sieve tube lowers the water potential of the phloem sap. Water therefore moves from the xylem and surrounding cells into the sieve tube by osmosis, increasing the hydrostatic pressure at the source.
蔗糖在筛管中积累会使韧皮部汁液的水势降低。因此，水通过渗透作用从木质部和周围细胞进入筛管，使源端静水压力升高。
5. Mass Flow Hypothesis | 压力流动假说
The mass flow hypothesis, proposed by Ernst Münch in 1930, states that translocation is driven by a difference in hydrostatic pressure between source and sink. At the source, active sucrose loading lowers water potential, water enters by osmosis, and pressure rises. At the sink, sucrose unloading raises water potential, water leaves, and pressure falls. Phloem sap flows from high pressure to low pressure through sieve pores.
压力流动假说由 Ernst Münch 于 1930 年提出，认为运输作用由源与库之间的静水压力差驱动。在源端，蔗糖主动装载使水势下降，水通过渗透进入，压力升高。在库端，蔗糖卸载使水势上升，水流出，压力下降。韧皮部汁液通过筛孔从高压区流向低压区。
The water potential of phloem sap can be expressed as:
韧皮部汁液的水势可表示为：
Ψ = Ψₛ + Ψₚ
where Ψₛ is the solute potential and Ψₚ is the pressure potential. When sucrose is loaded at the source, Ψₛ becomes more negative but water entry raises Ψₚ, increasing total water potential relative to the sink.
其中 Ψₛ 是溶质势，Ψₚ 是压力势。当源端装载蔗糖时，Ψₛ 变得更负，但水进入使 Ψₚ 升高，从而相对于库端提高总水势。
6. Unloading at the Sink | 库端卸载
At sinks such as growing roots or developing fruits, sucrose is unloaded from sieve tubes. Unloading may be symplastic through plasmodesmata or apoplastic across cell membranes, depending on species and organ. In many sinks, sucrose is converted to hexoses by invertase or to starch, maintaining a low sucrose concentration that favours continued unloading.
在生长中的根或发育果实等库端，蔗糖从筛管卸载。卸载可通过胞间连丝的共质体途径，或跨细胞膜的质外体途径，因物种和器官而异。许多库中，蔗糖被转化酶分解为己糖，或转化为淀粉，从而维持低蔗糖浓度，有利于持续卸载。
Some sinks actively pump sucrose into storage cells using membrane transporters, while others allow passive diffusion if sucrose is rapidly metabolised. In either case, the solute concentration in the sieve tube near the sink decreases, water leaves by osmosis, and hydrostatic pressure falls.
一些库利用膜转运蛋白主动将蔗糖泵入储藏细胞，而另一些库则在蔗糖被快速代谢后允许被动扩散。无论哪种方式，库端筛管内溶质浓度降低，水通过渗透离开，静水压力下降。
7. Evidence for Mass Flow | 压力流动假说的证据
Several lines of evidence support mass flow. When aphid mouthparts (stylets) are severed from their bodies while inserted into phloem, sap continues to exude, showing positive pressure in sieve tubes. The sap contains high sucrose concentrations, and the rate of exudation is highest near sources.
多条证据支持压力流动。当蚜虫口针插入韧皮部后被切断与虫体分离时，汁液继续渗出，说明筛管内存在正压力。汁液中含有高浓度蔗糖，且源附近渗出速率最高。
Radioactive labelling experiments using ¹⁴CO₂ show that labelled sucrose moves from source leaves to sinks at velocities of about 0.1-1 m h⁻¹, which is much faster than diffusion alone could achieve. Metabolic inhibitors and anoxia block loading and unloading, confirming that active transport is required at these steps.
使用 ¹⁴CO₂ 的放射性标记实验表明，标记蔗糖以约 0.1-1 m h⁻¹ 的速度从源叶移动到库，这比单独靠扩散快得多。代谢抑制剂和缺氧会阻断装载和卸载，证实这些步骤需要主动运输。
Ringing experiments, in which a ring of bark (including phloem) is removed, cause sugars to accumulate above the ring and tissues below to starve, whereas removing xylem has no such effect. This shows that phloem, not xylem, is the tissue responsible for downward sugar transport.
环割实验去除一圈树皮（包含韧皮部）后，蔗糖在环割上方积累，下方组织饥饿，而去除木质部则无此效应。这说明负责糖向下运输的是韧皮部而不是木质部。
8. Challenges and Alternative Views | 争议与其他观点
Mass flow has been criticised because sieve plates offer considerable resistance, and many workers doubted whether a passive bulk flow could pass through them fast enough. However, modern observations show that sieve pores are usually open, and that P-protein and callose rarely block them in healthy transporting phloem.
压力流动假说曾受到质疑，因为筛板会产生相当大的阻力，许多研究者怀疑被动集流能否足够快地通过筛板。然而现代观察表明，筛孔通常是开放的，健康的运输韧皮部中 P 蛋白和胼胝质很少堵塞筛孔。
Another objection is that different solutes sometimes move in different directions or at different speeds. This is explained partly by separate sieve tubes within the same vascular bundle operating independently; a single sieve tube generally carries sap in one direction at a time, but adjacent tubes may move solutes in opposite directions to different sinks.
另一个反对意见是不同溶质有时以不同方向或速度移动。这可以部分解释为同一维管束中不同筛管独立运作；单条筛管通常同时只朝一个方向运输汁液，但相邻筛管可将溶质以相反方向运往不同库。
Some researchers propose additional mechanisms such as cytoplasmic streaming, electro-osmosis or peristaltic waves, but current A-Level consensus is that mass flow is the main mechanism, with active loading and unloading controlling the pressure gradient.
一些研究者提出了额外机制，如胞质环流、电渗透或蠕动波，但当前 A-Level 共识认为压力流动是主要机制，由主动装载和卸载控制压力梯度。
9. Comparison with Xylem Transport | 与木质部运输的比较
Xylem transports water and mineral ions from roots to shoots in dead, lignified vessels and tracheids, driven by transpiration pull
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