Nutrition and Photosynthesis Lecture Notes

Introduction to Nutrition and Organismal Energy Requirements

Food is an essential requirement for all living organisms, primarily providing the necessary materials for growth and repair of body tissues. In addition to these structural functions, several organisms require food to maintain their internal body temperature. The substances consumed as food vary significantly across the biological spectrum, ranging from single-cellular organisms such as basic amoebae to highly complex multicellular organisms like human beings. Even within the human body, individual cells require a wide variety of substances as food to carry out their specific physiological functions. The mode of acquiring this food varies from one organism to another.

Organisms are generally categorized into two groups based on how they obtain nutrition. Autotrophs are organisms capable of using light energy to synthesize chemical compounds. They acquire vital nutrients like mineral salts and water from the soil, as well as specific gases from the atmosphere. These organisms are capable of producing complex compounds, including carbohydrates, proteins, and lipids, from these very simple substances. The compounds produced by autotrophic plants are utilized for providing energy to the vast majority of living organisms on Earth. Heterotrophs, by contrast, must depend on other organisms for their nutritional needs.

The Scientific Definition and Process of Photosynthesis

Photosynthesis is the process by which photosynthetic plants, containing the green pigment called chlorophyll, build up complex organic molecules from simple inorganic ones using sunlight as an primary energy source. This process makes plants the "Universal food provider" for all living organisms. While humans eat many plant products directly, even a dependence on animal products reveals a secondary reliance on plants, as those animals typically depend on plants for their own food.

Photosynthesis is characterized as a very complex process involving several sequential reactions and the formation of numerous intermediate compounds. Scientists spent centuries working to formulate a simple equation to represent this process. The equation most widely used today is related to the one proposed by C.B. Van Niel in 1931. His mathematical and chemical assessment was that for each molecule of carbohydrate formation, one molecule of carbon dioxide and two molecules of water are required. This process also results in the production of one molecule each of oxygen and water as byproducts. The standard balanced equation used to show the synthesis of glucose ($C_6H_{12}O_6$) is:

$6CO_2 + 12H_2O \xrightarrow[\text{Chlorophyll}]{\text{Light}} C_6H_{12}O_6 + 6H_2O + 6O_2$

Plants synthesize simple carbohydrates first, eventually building more complex substances like starch and cellulose. They are also capable of synthesizing proteins and fats. Animals are incapable of synthesizing carbohydrates and must depend entirely on plants for these compounds.

Historical Milestones in the Study of Plant Nutrition

Scientific investigations into plant life processes began in earnest in the 17th century. A scientist named Van Helmont (1577-1644) established the role of water in plant growth through a meticulous experiment conducted over a period of five years. He found that the increase in plant mass was largely due to water, although he did not understand the process of photosynthesis at the time. It took another 300 years of scientific advancement to arrive at the current modern definition of photosynthesis.

C.B. Van Niel provided a major breakthrough in 1931 while working on purple sulfur bacteria. He discovered that light plays a specific role in photosynthesis but found that bacteria use hydrogen sulfide ($H_2S$) as a starting material instead of water ($H_2O$). In bacterial photosynthesis, no oxygen is liberated; instead, sulfur is produced. Van Niel later observed a similar process in green plants where oxygen is released.

Joseph Priestley and the Discovery of Oxygen and Gas Exchange

In 1770, Joseph Priestley (1733–1804) performed a series of experiments that revealed the essential role of air in the growth of green plants. At that time, photosynthesis was still unknown. Priestley discovered oxygen in 1774, which was later named by Lavoisier in 1775. Through his "bell jar" experiments, Priestley observed that a candle burning in a closed container would quickly extinguish, and a mouse kept in the same closed jar would suffocate. He concluded that both the burning candle and the breathing mouse "damage" the air.

However, when Priestley placed a mint plant inside the same bell jar, he discovered that the mouse stayed alive and the candle continued to burn if relighted from the outside. He hypothesized that plants restore the air that breathing animals and burning candles remove. This confirmed that gaseous exchange occurs between the plants and their environment, and that plants release a gas that aids in combustion and the survival of living organisms.

Experimental Evidence for Starch and Carbon Dioxide

Activity-1 provides a method to test for the presence of starch (a carbohydrate) in leaves. A thin leaf well-exposed to sunlight is boiled in water and then transferred to a test tube containing methylated spirit. This tube is boiled in a water bath until the chlorophyll dissolves in the spirit, leaving the leaf pale. The leaf is then treated with iodine solution. The appearance of a bluish-black color indicates the presence of starch, proving that carbohydrates were produced.

To prove that carbon dioxide is necessary for photosynthesis, Mohl's half-leaf experiment is used. A plant is first "destarched" by being kept in the dark for three days. One leaf is then partially inserted into a wide-mouthed transparent bottle containing potassium hydroxide ($KOH$), which has the property of absorbing carbon dioxide ($CO_2$). Half of the leaf remains inside the bottle (without $CO_2$) and the other half remains outside (with $CO_2$). After exposure to sunlight, only the part of the leaf outside the bottle turns bluish-black when tested with iodine. This demonstrates that photosynthesis cannot occur in the absence of carbon dioxide.

The Role of Light and Pigments

Jan Ingenhousz (1730–1799), a Dutch scientist, discovered in 1779 that plants only form oxygen in the presence of light. In his experiment with the aquatic plant Hydrilla (a submerged, rootless water plant), he observed that small oxygen bubbles formed around the green parts of the plant in bright sunlight, but stopped forming in the dark.

In the early 20th century, T.W. Engelman further refined this by detecting the maximum rate of photosynthesis using different colors of light. He exposed algae to a rainbow spectrum and used oxygen-sensitive bacteria to find where the most oxygen was being produced. He found that the bacteria crowded around the areas of the algae illuminated by bright red and blue rays of light. This established the effect of light color on photosynthesis and led to the study of colored pigments and their role in utilizing light energy.

Physiological Mechanisms for Gaseous Exchange

Plants must take in air for photosynthesis and respiration. Massive amounts of gaseous exchange occur through the stomata, which are tiny openings usually present in the leaves. These remain functional as long as the stomata are open. However, plants are not limited to leaves for gas exchange; they also utilize lenticels located on the stems and aerial roots to facilitate the movement of gases into and out of the plant body.