Chapter 13. Chloroplasts The MOUSE'S PETITION, * Found in the TRAP where he had been confin'd all Night.Parcere subjectis, & debellare superbos. -VIRGIL.OH! hear a pensive prisoner's prayer,For liberty that sighs;And never let thine heart be shutAgainst the wretch's cries.For here forlorn and sad I sit,Within the wiry grate;And tremble at th' approaching morn,Which brings impending fate.If e'er thy breast with freedom glow'd,And spurn'd a tyrant's chain,Let not thy strong oppressive forceA free-born mouse detain.Oh! do not stain with guiltless bloodThy hospitable hearth;Nor triumph that thy wiles betray'dA prize so little worth.The scatter'd gleanings of a feastMy frugal meals supply;But if thine unrelenting heartThat slender boon deny,The cheerful light, the vital air,Are blessings widely given;Let nature's commoners enjoyThe common gifts of heaven.The well-taught philosophic mindTo all compassion gives;481Casts round the world an equal eye,And feels for all that lives.If mind, as ancient sages taught,A never dying flame,Still shifts through matter's varying forms,In every form the same,Beware, lest in the worm you crushA brother's soul you find;And tremble lest thy luckless handDislodge a kindred mind.Or, if this transient gleam of dayBe all of life we share,Let pity plead within thy breastThat little all to spare.So may thy hospitable boardWith health and peace be crown'd;And every charm of heartfelt easeBeneath thy roof be found.So, when destruction lurks unseen,Which men like mice may share,May some kind angel clear thy path,And break the hidden snare.* To Doctor PRIESTLEY.The Author is concerned to find, that what was intended as the petition of mercy against justice, has been construed as the plea of humanity against cruelty. She is certain that cruelty could never be apprehended from the Gentleman to whom this is addressed; and the poor animal would have suffered more as the victim of domestic economy, than of philosophical curiosity. From Poems. By Anna Lætitia Aikin. London: Printed for Joseph Johnson, in St. Paul's Church-Yard, 1773. pp. 37-40.13.1. Discovery of chloroplasts and photosynthesisUp until now I have been discussing cells that are surrounded by a plasma membrane and contain ER, Golgi apparati and their associated membranes, coated and naked vesicles, vacuoles and peroxisomes. These cells could have been from either a plant or an animal. In this chapter, I will discuss an organelle that is found exclusively in plant cells: the chloroplast. The chloroplast is involved in photosynthesis and cells that contain chloroplasts 482are autotrophic, that is, able to make their own food. It must be remembered that most plants contain both photosynthetic and colorless cells. In most plants, the majority of cells do not perform photosynthesis, and thus must be considered to be heterotrophic. Even so, each year, all the chloroplasts in the world fix about 10-100 billion tons of carbon dioxide, which is approximately equal to the mass of metropolitan New York City (Kamen, 1963).13.1.1. Discovery of photosynthesisExperimental studies of plant assimilation began when the alchemist, Jean Baptiste van Helmont (1683) used his balance, one of the only tools available to scientists at the time, to study plant growth. He believed that all matter was built up from the single essence water, an idea that can be traced back to Thales. Van Helmont used the growth of plants to test this thesis. According tovan Helmont, “I have learned from the following clear experiments that all plants make up their matter completely from the element of water. I conclude this because, I have taken an earthen-ware container and placed in it 200 pounds of earth, that I have placed in a baking oven and allowed to dry. I moistened this earth with rain-water and placed in it a willow stem which weighed five pounds. To ensure that the dust from the air didn't add to the weight, I threw a screen over the soil. I watered it when necessary with only rainwater or distilled water. The tree grew and set into the ground. After five years, the willow became a tree that weighed 169 pounds and about 3 ounces and I did not even take into consideration the weight of the leaves, which fell off every Autumn for four years. Eventually I took the earth out of the container and found that it weighed only two ounces less than the original twohundred pounds. Thus the 164 pounds of wood, bark and knots had grown alone from the water.”In performing experiments in other realms of alchemy, van Helmont found that when he burned 62 pounds of coal he was left with only one pound of ashes. He called the material that made up the escaping 61 pounds, the spiritis sylvestres, wild spirit or “Gas”, and although van Helmont was the discovererof gases, he did not realize that the tree that grew in the pot was created from gas as well as from water. The idea that the leaves of plants assimilated air was originally proposed by Empedocles (Lambridis, 1976) but dismissed by Aristotle (Barnes, 1984) and his disciple Theophrastus (1916), who believed that all the nourishment came in through the roots. The proposal that plants assimilated air was not taken seriously until 1727. At this time Stephen Hales 483(1727) burned plants and quantified the amount of gas given off and the amount of ash that remained. From the results of these experiments, Hales suggested that plants may assimilate air, just as animals do (Boyle, 1662; chapter 14). Experiments on the role of gases in plant growth were renewed atthe end of the eighteenth century when Joseph Priestley decided to take up where Stephan Hales left off.Like a burning candle, living beings require “clean air”; and a test for the presence of clean (or dephlogistonated) air is to see if a mouse can live, or a candle can burn, when placed in a container of the sample air (Faraday, 1860).Using such a test, Joseph Priestley (1774) accidently found that a sprig of mint could purify the air that had been previously fouled by the breathing of an animal, or the burning of a candle. Perhaps Priestley was lucky to find the oxygenic (or dephlogistonating) property of plants, and just chose mint to purify the air because of its refreshing smell. However he also found that groundsel, a bad-smelling weed, and spinach also had the ability to purify the air, and this ability was a general property of plants.Priestley later found his experiments to be irreproducible, perhaps becausehe did not