Photosynthesis: Chemistry of Food-Making

green plants manufacture food for their development and growth in addition they also provide food for us making available the nutrients essential to our existence the animals that supply us with meat also derive their food from plants we know that it is through the process of photosynthesis that green plants produce the food so vital to us let's review briefly the fundamentals of photosynthesis a plants such as corn absorbs water h2o from the soil it also absorbs dissolved minerals from the soil the plant in addition takes in from the atmosphere carbon dioxide co2 using the energy of sunlight and using chlorophyll the countless molecules of green pigment and leaves that give them their color the leaves manufacture certain chemical products the carbohydrate called glucose a simple sugar is one of the most important some amino acids protein materials are also manufactured photosynthesis produces as a byproduct oxygen which is released into the air now let's consider the carbohydrate product glucose can we prove that green plants produce this simple sugar from this green plant we have taken some bits of leaves the finely cut pieces are in the test tube one test for glucose and other simple sugars uses this blue solution called Benedict's reagent we add Benedict's reagent to the leaves then we heat the two after a few moments the blue solution turns a reddish color this test indicates that a simple sugar is present evidently glucose has been produced in the leaves here's a test to determine if oxygen is given off during photosynthesis a glass funnel and a test tube are arranged so that any gas given off by the green plants in the funnel will be collected in the tube gas indicated by these bubbles is escaping from the plants into the tube after several hours we quart the test tube and remove it from the beaker now we'll use a standard test for oxygen a glowing splint to see if the tube contains this gas the flame indicates the presence of oxygen now let's examine the structure of a leaf and see the part it plays in the process of photosynthesis in the blade of the leaf the veins transport water dissolved minerals and foods between the green cells of the leaf and the stem from the underside of the leaf will strip off a thin layer of leaf tissue the epidermis and prepare a slide under the microscope the openings called stomata and their associated guard cells may be seen it is through these openings the stomata that the carbon dioxide used by the leaf enters and the oxygen produced by the leaf diffuses out here a portion of a leaf has been placed between two sections of pit a cross-section of the leaf can be prepared by slicing through the pit and the leaf the leaf section is then viewed under a microscope both the upper epidermis and the lower epidermis generally consist of single layers of cells between the upper and lower epidermis are the mesophyll cells which include this layer of palisade cells and these spongy cells within the mesophyll cells we find chloroplasts and it is in the chloroplast that photosynthesis occurs here we see the green chloroplasts moving within a living leaf let's see what happens in the chloroplast during photosynthesis you remember that one of the most important products of photosynthesis is glucose in order for glucose to be produced first light energy must be changed to chemical energy the light energy is absorbed by the chlorophyll molecules within the chloroplasts chlorophyll molecules represented by these ovals are large molecules several kinds of chlorophyll are known a molecule of chlorophyll a consists of 55 atoms of carbon 72 atoms of hydrogen 5 atoms of oxygen 4 atoms of nitrogen and one atom of magnesium also complex is chlorophyll b the formula is slightly different chlorophyll molecules become activated as they absorb light energy the energy of the activated chlorophyll molecules is used to rearrange the atoms which make up the carbon dioxide and water molecules here is one interpretation of the way it happens these represent two water molecules each consists of two atoms of hydrogen and one atom of oxygen the light energized chlorophyll splits the water molecules two atoms of oxygen unite to form one molecule of oxygen gas which is released to the air in addition molecules of carbon dioxide co2 are split one oxygen atom from a molecule of carbon dioxide unites with two atoms of hydrogen from the water molecules to form a new molecule of water the remaining atoms of the carbon dioxide unite with the remaining hydrogen atoms to form C h2o a basic unit of the glucose molecule the formation of the glucose unit is perhaps the most important result of photosynthesis until we have further evidence as to the exact manner in which the glucose unit is formed this chemical equation represents the overall reaction however since water is consumed during the reaction the net result is this the ch2oh is a unit of the glucose molecule six ch2oh units join together to form c6 h-12 o-6 the glucose molecule using it as a basis the plant is able to manufacture all of its other constituents this net equation summarizes what the plant accomplishes during photosynthesis from the carbohydrate glucose plants are able to manufacture additional carbohydrates as well as proteins fats and vitamins we have just dealt with the basic steps in photosynthesis the process actually has many intermediate steps more than 30 steps in the overall reaction are known one way plant physiologists seek to learn more about the production of glucose and the other products of photosynthesis is by the use of radioactive tracers during successive stages of photosynthesis plants are fed with carbon dioxide containing radioactive carbon then finally cut leaf samples are prepared and analyzed study of the samples shows that the leaves in the photosynthetic process are capable of absorbing and retaining radioactive carbon supplied in carbon dioxide in a much simpler experiment these students are studying another aspect of photosynthesis the effect of light intensity they are working with artificial light it like sunlight will bring about photosynthesis a light meter is used to measure the intensity under the influence of the light the plants in the inverted funnel are producing bubbles which we know contain oxygen a byproduct of photosynthesis now will increase the intensity of light as the meter indicates with more light there is an increased rate of photosynthesis as indicated by the increased bubble production where more accurate measurements of the rate of photosynthesis are required the bubble count method is replaced by more refined procedures in a photosynthesis laboratory this measuring instrument a manometer apparatus indicates rate of oxygen production changes in the levels of fluid in the tubes can be measured to determine exact amounts of oxygen produced returning to the bubble count method will test the effect of different colors or wavelengths of light using a red filter we find that red light gives rapid oxygen production when we switch from red light to green light keeping the light intensity the same we see that the wavelengths of green light are much less effective in photosynthesis knowing which wavelengths of light are most effective in photosynthesis plant physiologists also study how these wavelengths are absorbed by different pigments in the plant a graph called an absorption spectrum reveals which pigments absorb certain wavelengths analysis of a chlorophyll solution confirms that it is the chlorophyll molecules that most effectively absorb and utilize light energy we know that the process of photosynthesis involves the changing of light energy within the leaf to chemical energy the basic energy transferring chemical compound used by a plant or any other living organism in the processes of food making growth and other life functions is ATP adenosine triphosphate it is believed that within the chloroplast of the leaf ATP is synthesized from two other chemical compounds adp adenosine diphosphate and P assemble for inorganic phosphate we can represent the reaction this way a DP MP making use of light energy form ATP when ATP is broken down ADP and P are formed again and chemical energy is released it is this chemical energy which is the source of energy for the rearrangement of co2 and h2o to form the glucose unit C h2o and oxygen which is released from this glucose unit the plant manufactures of foods that are stored in its various parts it is upon such plant foods that we depend for our existence so within the plant the process that produces the basic food compounds upon which almost all plant and animal life ultimately depends is photosynthesis you