Plant Energy Transport

植物能量运输

Plants are classified as autotrophs because they manufacture their needed nutrients by photosynthesis, converting carbon dioxide and water to sugar fuels with the addition of energy from the Sun. In times of rapid photosynthesis, the main product is glucose, but it is usually converted to the larger sugar sucrose. These sugars that are synthesized in the leaves must be transported to other parts of the plant. Other structures in the plants such as roots and flowers require the energy but cannot manufacture it. Also, sugars may be stored in the roots and stem.

植物被归类为自养生物，因为它们通过光合作用制造所需的营养物质，将二氧化碳和水转化为糖类燃料，并利用来自太阳的能量。在光合作用迅速进行时，主要产物是葡萄糖，但通常会转化为更大的糖类——蔗糖。在叶片中合成的这些糖类必须运输到植物的其他部分。植物的其他结构，如根和花，需要能量但无法自行制造。此外，糖类可能储存在根和茎中。

The sugar and other organic molecules are transported through the plant by means of a special layer of tissue called phloem. Phloem is composed of living cells that transport a water solution of sugars that we commonly call sap. This movement is modeled by the pressure-flow theory, a part of which is that the sugar-containing fluid is moved through sieve tubes by fluid pressure. By this means, nutrients can be moved from the photosynthetic site (the source) to the place where the sugar is being used (the sink) whether it is up or down the stem of the plant.

糖和其他有机分子是通过一种称为韧皮部的特殊组织在植物中运输的。韧皮部由活细胞组成，这些细胞运输一种我们通常称为汁液的含糖水溶液。这种运动由压力流动理论来解释，该理论的一部分是糖含有的液体通过筛管通过液体压力流动。通过这种方式，营养物质可以从光合作用的部位（源）运输到糖被利用的部位（ sink），无论它是向上还是向下运输到植物的茎中。

Pressure-Flow Theory for Nutrient Transfer

营养输送的压-流理论

After sugars are produced in photosynthesis, these sugars must be transported to other parts of the plant for use in the plant's metabolism. Part of the pressure-flow theory is that the sucrose produced is moved by active transport into the companion cells of the phloem in leaf veins. This raises the concentration of sucrose molecules in the companion cells above that in the sieve tubes, so they can then move into the sieve tubes by diffusion. With the concentration of sucrose now greater in the sieve tubes than external to them, water molecules will move into the sieve tubes near those photosynthesis locations by osmosis. With a larger amount of water in the tube, its fluid pressure will be higher than at distant locations in the tube, and the pressure difference will cause flow in those directions.

在光合作用产生糖类之后，这些糖类必须被运输到植物的其他部分，在植物的代谢中发挥作用。压力流动理论的一部分是，产生的蔗糖通过主动运输被移动到叶脉的筛管伴胞细胞中。这会使得伴胞细胞中的蔗糖分子浓度高于筛管中的浓度，因此它们可以通过扩散进入筛管。由于筛管中的蔗糖浓度现在高于筛管外部，水分子会通过渗透作用进入那些进行光合作用区域附近的筛管。由于筛管中水分子的数量更多，其液体压力会比管中远处的部位更高，压力差将导致流体在这些方向上流动。

At some distance from the photosynthetic source, there may be a region, say in a fruit, where sugar is needed. This theory suggests that the sucrose is transported into the fruit by active transport, raising the sugar concentration in the fruit relative to the sieve tube. In response to this concentration difference, water will follow the sugar into the fruit by osmosis.

在光合作用源周围一定距离处，可能存在一个区域，例如在果实中，需要糖分。这一理论认为，蔗糖通过主动运输进入果实，使果实中的糖浓度相对筛管中的浓度升高。在浓度差的作用下，水会通过渗透作用跟随糖分进入果实。

The movement of the water from the sieve tube into the fruit lowers the fluid pressure at that location, continuing to produce the pressure gradient that leads to bulk flow of water to the fruit, carrying the dissolved sugar with it.

水从筛管流入果实的过程降低了该处的流体压力，继续产生导致水分 bulk 流向果实的压力梯度，同时将溶解的糖类携带其中。

Phloem, the Medium for Plant Energy Transfer

植物能量传输的筛管

In the stems of plants is a layer of living tissue called phloem that forms a medium for the movement of a sugar-rich fluid (sap) and which is therefore a key part of the energy transport within vascular plants. Part of the phloem is made up of sieve tubes which are constructed of specialized sieve tube cells with no nuclei. The sieve tubes permit the flow of sugar solution under the influence of a fluid pressure differential according to the pressure-flow theory. Phloem also contains companion cells for the sieve tubes which aid in the transport of sugars to the tubes.

植物茎部有一层活组织称为筛管组织，它形成一种介质，用于糖分丰富的液体（汁液）的流动，因此是维管植物内能量传输的关键部分。筛管组织的一部分由专门的筛管细胞组成，这些细胞没有细胞核。筛管细胞在流体压力差的作用下，根据压力流动理论允许糖分溶液的流动。筛管组织还包含伴随细胞，这些细胞有助于将糖分输送到筛管中。