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Lesson Four  Photosynthesis

New Words & Expressions

earthenware  n. rough baked clay，pottery 陶器  porcelain 瓷器 china

weigh out  量出, 称出

twig  n. a small and thin woody stem branching off from a branch on a tree 嫩枝

willow twig  嫩柳枝

scrap  v. get rid of 丢弃 

surmise  v. suppose as a reasonable guess 推测

ingredient  n. component or constituent 组分

appreciably  adv. considerably, noticeably or substantially 明显地

catalyst  n. substance which quickens chemical processes without itself changing 催化剂

chromatography  n. a process in which a chemical mixture carried by a liquid or gas is separated into components as a result of differential distribution of the solutes as they flow over a stationary liquid or solid phase层析, 色谱法

stack  v. make into a neat pile 整齐堆放

spinach  n. a vegetable whose soft green leaves can be eaten 菠菜

absorbent a. able to absorb water 吸水的

hemoglobin  n. a red coloring matter in the blood which contains iron and carries oxygen 血红蛋白

photomicrograph  n. micro-photograph显微照相, 显微照片

grana  n. layered substances inside the chloroplast 基粒

radiant  a. sending out light or heat in all directions 放射的

immediate  a. nearest in time, space or degree 最接近的

indefinitely  adv. without limit 无限地

spectrum  n. a set of bands of colored light in the order of their wavelength 光谱

senescence  n. growing old 衰老

illuminance  n. the strength of light 照(明)度  

Chlorophyll and Photosynthesis

The beginning of our knowledge of food manufacture by plants dates back to the early part of the seventeenth century. Previous to this time, it was thought that plants absorbed their nourishment from the soil and used this as food in much the same way as animals use their food. A Belgian biologist, Jan Baptista van Helmont, questioned this assumption and devised an experiment to test it. He dried some soil to remove the water. Then he weighed out (量出, 称出) exactly 200 pounds of this dry soil and placed it in an earthenware container.

叶绿素与光合作用

人类对植物制造食物的了解可追溯到17世纪早期。在此以前人们认为植物从土壤中吸收养分并作为食物来利用的方式同动物十分相似。一位比利时生物学家Jan Baptista Van Helmont对这一假设提出了质疑，并且设计了一个实验进行验证。他将土壤烘干除去水分，然后称取正好200磅烘干土放入一个陶器中。

 In this he planted a willow twig (嫩柳枝) which weighed 5 pounds. The plant was watered with rain water and the soil was covered so its weight would not be altered by possible dust blowing in. After five years he removed the plant, scraped off the soil, and found that it now weighed 164 pounds. The soil was again thoroughly dried and found to weigh only 2 ounces less than 200 pounds. This showed conclusively that the increase in plant substance could not have come from the soil alone. Van Helmont suggested that it must have come from the water. Today we know that water was only a part of the answer. Most of the substance had been derived from the air, that part of the air known as carbon dioxide.

在容器中栽入一棵5磅重的嫩柳枝，用天然雨水浇灌并将土壤封好，以防灰尘落人改变原来土壤的重量。五年后，他将柳树取出，抖落净上面的土壤，发现柳枝重为164磅。将土壤再完全烘干，发现仅比原来的200磅仅少了2盎司。这表明：植物物质的增加不可能单单来源于土壤。VanHelmont认为一定是来自于水。现在我们知道，水仅是答案的一部分，大部分物质来源于空气——空气中那部分被称之为二氧化碳的气体。

We often think of carbon dioxide as an injurious gas because it is given off from the human body as a waste product; yet this gas is absolutely necessary for the continuation of life on the earth. It is the major ingredient (组分) used in the production of food for all forms of life. The normal atmosphere contains only about 0.03% carbon dioxide, yet this is sufficient to supply the plants in photosynthesis. This is not to say that they could not function more efficiently if the concentration of carbon dioxide in the air was higher.

我们常认为二氧化碳是一种有害气体，因为它是以废物的形式从人体排出去的；然而这种气体对于地球上生命的延续是绝对必要的。因为它是为所有生命形式制造食物所用到的主要成分。正常的大气仅含0．03％的二氧化碳，然而这足以供应植物进行光合作用。这并非说空气中二氧化碳浓度再高一些，植物光合作用效率也不可能再提高。

Greenhouse operators have found that it is possible to get increased growth of their plants if the carbon dioxide content of the air is artificially increased. The improved growth continues until the concentration of carbon dioxide reaches about 5%. This is apparently the maximum which can be utilized, for there is no speeding up of growth beyond this level. Ventilation in greenhouses is important because the plants are using the available carbon dioxide, and without proper ventilation the level of this gas in the air will drop appreciably. The corresponding drop in the efficiency of photosynthesis will be reflected in decreased plant growth.

温室的管理者已经发现，人为的增加温室内的二氧化碳量可以促进植物的生长。当二氧化碳的浓度达到5％以前，植物的生长可随之增加而加快。显而易见5％是植物可能利用C02的最大值，因为当超过这一值时，植物生长速度将不再增加。温室通风很重要，因为植物一直在利用可获得的二氧化碳，如果没有适当的通风，空气中的二氧化碳的含量就会明显下降，由此所引起的光合作用效率下降，可以反映为植物生长缓慢。

In addition to carbon dioxide, water is a necessary ingredient in the manufacture of food by a green plant, as Van Helmont surmised. Water contributes the hydrogen which goes into the food. Since hydrogen is a very light element, we find that it makes up only about 1/15 of the weight of the glucose molecule which results from the photosynthetic process. That is why we say that most of the food comes from the air.

正如Van Helmont所推测的那样，除了二氧化碳，在绿色植物制造食物过程中，水也是一个必不可少的成分。水为制造的食物提供了氢。然而，由于氢是非常轻的元素，它在光合作用产生的葡萄糖分子中仅占重量的l／15。这就是为什么我们说食物的组分大部分来自于空气。

A plant needs more than carbon dioxide and water, of course, for photosynthesis to take place. There must be chlorophyll and certain other elements which are necessary for the formation of this catalyst. The two ounces which was lost from the soil in Van Helmont's experiment included such things as nitrogen, phosphorus, potassium, magnesium, manganese (锰), iron, and other elements in smaller quantities. Magnesium and nitrogen have a place in the construction of chlorophyll. Plants grown in soil deficient in either of these elements will show leaves of a pale yellow-green color and will grow poorly.

当然，植物进行光合作用不仅需要二氧化碳和水，还必须有叶绿素和形成这种催化剂所必需的某些其他元素。在Helmont的实验中，土壤重量少了2盎司，这包括诸如氮、磷、钾、镁、锰、铁和其他一些更少量的元素。镁和氮是叶绿素的组成成分，土壤中无论缺少这两种元素中的哪一种，植物都会出现浅黄绿色的叶子，并且植株生长差。

When fertilizer containing these elements is added, there will be an almost overnight change to the bright green color characteristic of normal chlorophyll content. We know also that potassium, iron, and manganese are needed in the formation of chlorophyll even though they are not a part of the chlorophyll molecule in its final form. Phosphorus plays a very important part in the capture of energy.  (stop)

当施入含有这两种元素的肥料时，植株几乎在一夜之间就会转变为含有正常量叶绿素的鲜艳绿色。我们也知道尽管钾、铁、锰不是叶绿素分子最终的组成成分，它们在其合成过程中也是必需的。磷对于获取能量有重要的作用。                                                                                                                                                                                                                                                                                                     

Since chlorophyll plays such a vital role in photosynthesis, it has been studied extensively by plant physiologists in an effort to determine how it accomplishes its transformation of carbon dioxide and water into food. Two slightly different kinds of chlorophyll have been identified in higher plants. These can be separated rather easily by means of paper chromatography. Chlorophyll is first extracted from the leaves of a higher plant; spinach is a favorite of the plant physiologists. Then strips of absorbent paper are immersed in this extract. The chlorophyll moves up the paper and accumulates in two distinct bands. When these two bands are analyzed they are found to have slightly different chemical formulas. One, chlorophyll a, has the formula

C₅₅H₇₂O₅N₄Mg 

由于叶绿素在光合作用中的重要地位，植物生理学家已经对叶绿素作了广泛的研究，旨在确定二氧化碳和水是怎样通过叶绿素合成有机物的。在高等植物中已经鉴别出了两种没有明显区别的叶绿素，它们可通过纸层析方法很容易地分离开。叶绿素首先是从高等植物的叶片中提取出来的。菠菜深受植物生理学家的偏爱。将吸水纸条浸入提取液中，叶绿素沿纸条上移而集中形成两条明显的带。分析了这两条带以后发现它们的化学分子式有细微的差别。叶绿素a的分子式是C₅₅H₇₂O₅N₄Mg；

The other, chlorophyll b, has the slightly different formula

C₅₅H₇₀O₆N₄Mg

The proportion of the two varies in different plants, but the higher plants usually have about three parts of chlorophyll a to one part of chlorophyll b. Chlorophyll a appears somewhat more efficient.  Magnesium lies in a central position in these molecules. Hemoglobin, which is the oxygen-carrying red pigment in the blood of higher animals, has a molecule very similar in structure to that of chlorophyll, except that the central element is iron rather than magnesium.

另一种与其有细微差别的叶绿素b的分子式为C₅₅H₇₀O₆N₄Mg，不同植物两种叶绿素所占比例不同，但高等植物中所含叶绿素a与叶绿素b的比值通常是3：1。叶绿素a在一定程度上表现出较高的效率。镁位于这些分子中心处。高等动物血液中携带氧的血红素——血红蛋白与叶绿素有很相似的分子结构，只是它的中心元素是铁而不是镁而已。

In all but a few of the simplest algae the plant chlorophyll is contained in chloroplasts suspended in the cytoplasm of the cells.  Electron photomicrographs of individual chloroplasts at very high magnification show that they contain large numbers of layered substances, somewhat resembling stacked coins. These layers are known as grana. They seem to have the enzymes necessary for photosynthesis arranged in the proper sequence for the most efficient handling of this process.

除几种最简单的藻类之外，所有植物的叶绿素都存在于悬浮在细胞质中的叶绿体内。从高倍放大的电子显微照片上可看到单个的叶绿体含有大量的一层层的像硬币一样重叠起来的物质，这些层状物被称作基粒。它们似乎含有光合作用所必需的酶，这些酶按一定顺序排列着使光合作用以最大效率发生。

Efficiency of Energy Conversion in Photosynthesis

The production of organic matter by photosynthesis depends on the availability of inorganic nutrients, adequate supplies of water and carbon dioxide, favorable temperature, radiant energy, and the absence of toxic substances from the immediate environment. These factors are part of the environment and may be varied rather widely, giving rise to different levels of plant productivity.

光合作用中的能量转换效率

光合作用产生的有机物依赖于无机养分的可利用性、充足的水分和C02的供应、适宜的温度、辐射能（光照条件）以及环境中没有有毒物质存在等因素。这些因素是环境的组成部分且变化范围很大，从而导致不同水平的植物生产力。

Internal factors such as kinds of pigments, enzyme levels, and the degree of organization of the photosynthetic apparatus also influence productivity. Taken together, the external and internal factors may be evaluated in terms of the efficiency of the plant in converting solar energy into chemical energy. The question asked is, how much of the radiant energy from the sun that falls on a plant is converted into plant organic matter?

内部因素，如色素种类、酶的水平和光合作用器官整体结构水平也影响植物的生产力。综合考虑，影响光合作用的内外部因素可以通过植物将光能转变为化学能的效率来评价。要问的问题是：照射到植物上的太阳辐射能有多少转化成植物有机物呢?

Each hectare of the surface of the earth receives approximately 40 X 10⁶ kcal of energy daily. This energy covers a broad spectrum ranging from short wavelengths of ultraviolet radiation to long-wavelength infrared radiation. Plants, however, can only utilize wavelengths lying between 400 nm and 700 nm in photosynthesis. Therefore only a relatively small portion of the radiant energy reaching the earth's surface is being utilized by plants.

地球表面每公顷面积上每天接受约40X10⁶千卡的能量，这些能量包括从短波的紫外线辐射到长波的红外线辐射很宽的光谱范围。可是，植物光合作用只能利用波长在400nm—700nm的光。因而，到达地面的辐射能量仅有很少的一部分被植物所利用。

To determine the efficiency of the plant in converting solar energy into chemical energy it is necessary to measure how much plant material is produced in unit time (year, month, week) on a unit of land (acre, square meter). The caloric value of the plant material is then determined by combustion and compared to the amount of solar energy which fell on the plants. The efficiency of conversion is calculated as follows:

 为了确定植物将光能转为化学能的效率，测量单位地表面(英亩，平方米)单位时间(年，月，周)内产生的植物物质的多少是必要的。植物物质的热量值可通过燃烧来确定，并将其与照射到植物上的太阳能的量比较。这个能量转换效率可按下式计算：

 Efficiency of energy conversion = Caloric value of plant material / Solar energy available  

Determinations of this kind have been made for a number of plants and plant communities in different parts of the world and the values found to be between 2% and 2.5% for crops being grown under intensive agricultural conditions, such as wheat in the Netherlands and rice in Japan. Under ordinary agricultural practices, the efficiency of energy conversion varies between 0.1% and 1.0%.  Values as high as 6%-10% are found in some crop plants over brief periods of time. Under laboratory conditions, efficiencies as high as 20%-25% have been observed.

能量转换效率：植物物质的热量值／可利用的太阳能

这种计算方法已经被用于测定世界不同地区的许多植物或植物群落。测得的种植在集约农业条件下的作物的能量转换效率为2％-2．5％，例如荷兰的小麦，日本的水稻。在一般农业条件下，能量转换效率在0．1％-1%。在短时期内，一些作物的能量转换效率可高达6％一10％。在实验室条件下，可观察到效率高达20％一25％。

It is seen from the above figures that the efficiency of energy conversion by plants varies widely from values as low as 0.1% to as high as 25%. Several questions can be raised. Is there an upper limit to the efficiency of energy conversion? Why are there such low values? On theoretical grounds, the maximum efficiency of energy conversion is believed to be between 25% and 30%. There is an upper limit, and it has been achieved in certain plants for brief periods of time under optimal conditions.

从上面的数字我们可以知道植物的能量转换效率的变化范围很大，从最低的0．1％到最高的25％。可以提出这样的几个问题：能量转换率有无上限?为什么有如此低的转换效率?从理论上来说，能量最大转换效率被认为能达到25％-30％，有一个上限，并且在适宜环境条件下，某些植物在较短时间内达到了此上限。

The low levels of energy conversion (0.1%-1.0% ) attained by native vegetation or under ordinary agricultural conditions are because of limiting environmental factors---lack of water, low or high temperatures, deficiencies of inorganic nutrients, pests (weeds, fungi, bacteria, rats), faulty seeds, poor cultural practices. By improving farming practices，especially irrigation and fertilization, energy conversion (and crop yield) can be improved 10-fold to 20-fold.

由于一些限制性的环境因素：缺水、低高温、无机营养缺乏、有害动植物(杂草、真菌、细菌、田鼠)、坏种子以及落后的耕作措施, 当地植物或在一般农业条件下，能量转化效率低 (0.1％一1.0%)。通过改进耕作措施，特别是灌溉和施肥，能量转换(及作物产量)能提高10倍--20倍。

Under intensive agricultural conditions, energy conversion efficiencies do not exceed 2%-2.5%. The major reason for such low values is the fact that most crop plants display their maximal photosynthetic efficiency for relatively brief periods of time during the life cycle of the plant. Very young plants do not have sufficient leaf area to carry out high rates of photosynthesis, while older plants are subject to senescence. Still another factor limiting plant productivity is the low level of carbon dioxide in the atmosphere.

在集约农业条件下，能量转换效率不超过2％--2．5%。这样低值的主要原因是，大多数作物只能在整个生命周期中相对短的时期内表现出最大的光合效率。幼小植物没有足够大的叶面积进行高效率的光合作用，而较老植物又趋于衰老。限制植物生产力的另一个因素就是大气中二氧化碳含量低。

Effect of Environmental Factors on Photosynthesis

Despite the early difficulties encountered in studying the chemical events of photosynthesis, considerable information was gained concerning the nature of the photosynthetic processes through studying the effect of various environmental factors on photosynthesis. Some typical results of the influence of temperature, carbon dioxide concentration, and light intensity on the rate of photosynthesis are shown in Figure 1. 

环境因素对光合作用的影响

尽管在研究光合作用化学过程的早期遇到了一些困难，通过研究环境因素对光合作用的影响却得到了关于光合作用过程特性的大量信息。温度、C02浓度和光照强度对光合速率影响的一些代表性结果见图1。