BIOLOGY: CHAPTER 9 Photosynthesis and Cellular Respiration
Photosynthesis is a chemical process that takes place in the presence of sunlight, wherein plants make their food and build stores of energy. This phenomenon occurs in chlorophyll containing plant cells. Chlorophyll is a pigment found in plant leaves that gives plants their green color. Chlorophyll absorbs the light energy and utilizes it to produce carbohydrates (sugars) from carbon dioxide (CO2) and water (H2O). This chemical reaction produces oxygen which is released into the atmosphere.

The reaction can be described as follows:
Carbon Dioxide + Water + Sunlight = Glucose + Oxygen
6CO2 + 6H2O + Sunlight --> C6H12O6 + 6O2

The carbohydrates produced by photosynthesis are used by the plants as a source of immediate energy for growth, reproduction, absorption of nutrients, etc. They also act as energy reserves and are stored in the form of starch which enable the plants to survive in drastic conditions, like, extreme winters or droughts. Carbohydrates are also used in the formation of plant tissues to grow leaves, wood, flowers, fruits, roots, etc.

Factors Affecting Photosynthesis
The process of photosynthesis is influenced by various factors which are as follows:
  • Temperature: The optimum temperature for the process is somewhere between 20 and 35°C. The process ceases at temperatures below 0°C due to the slowing of the plant's physiology.
  • Water Availability: Plants have to transpire to absorb carbon dioxide, therefore, if the water becomes scarce, it reduces transpiration and slows down photosynthesis.
  • Carbon Dioxide: High carbon dioxide concentration in the atmosphere promotes the process of photosynthesis.
  • Sunlight: As the intensity of the light increases, chlorophyll becomes more effective, thereby enhancing the process of photosynthesis.

 Next we will do an experiment with ELODEA...



Elodea is a genus of aquatic plants often called the waterweeds. Elodea is native to North America and is also widely used as aquarium vegetation. The introduction of some species of Elodea into waterways in parts of Europe, Australia, Africa, Asia, and New Zealand has created a significant problem and it is now considered a noxious weed in these areas. An older name for this genus is Anacharis, which serves as a common name in North America.

Elodea canadensis, sometimes called American or Canadian water weed or pond weed, is widely known as the generic water weed. The use of these names causes it to be confused with similar-looking plants, like Brazilian elodea (Egeria densa) or hydrilla (Hydrilla verticillata). American water weed is an attractive aquarium plant and is a good substitute for Brazilian elodea. It can be used for science experiments in classrooms demonstrating how plants use carbon dioxide with the usage of bromothymol blue.

The American water weed lives entirely underwater with the exception of small white flowers which bloom at the surface and are attached to the plant by delicate stalks. It produces winter buds from the stem tips that overwinter on the lake bottom. It also often overwinters as an evergreen plant in mild climates. In the fall, leafy stalks will detach from the parent plant, float away, root, and start new plants. This is the American water weed's most important method of spreading, while seed production plays a relatively minor role.

Silty sediments and water rich in nutrients favor the growth of American water weed in nutrient-rich lakes. However, the plants will grow in a wide range of conditions, from very shallow to deep water, and in many sediment types. It can even continue to grow unrooted, as floating fragments. It is found throughout temperate North America, where it is one of the most common aquatic plants.

American water weed is an important part of lake ecosystems. It provides good habitat for many aquatic invertebrates and cover for young fish and amphibians. Waterfowl, especially ducks, as well as beaver, muskrat and aquatic turtles eat this plant. It is also of economic importance as an attractive and easy to keep aquarium plant, although in the states of Oregon, South Carolina, and Washington it has been deemed an invasive species and is illegal to sell.



Now let's look at the concept of cellular respiration to understand the relation between photosynthesis and cellular respiration.
Cellular Respiration
Cellular respiration is the process of oxidizing food molecules or breaking down chemical bonds of glucose into carbon dioxide and water. The energy released during the process is trapped in the form of ATP (Adenosine Tri-Phosphate) and used by different energy consuming activities of the cell. In other words, it's a set of the metabolic reactions that occur in the mitochondria of the cells of organisms to convert biochemical energy from nutrients into ATP, and release byproducts. The complete breakdown of glucose into carbon dioxide and water involves two major steps: glycolysis and aerobic respiration.

Glycolysis: It's an anaerobic catabolism of glucose that occurs in all the cells and produces two molecules of pyruvate and two molecules of ATP.
Aerobic Respiration: It's a process wherein the pyruvate molecules produced in glycolysis, undergoes further breakdown in the presence of oxygen and generates thirty four molecules of ATP.

The reaction can be described as follows:
Glucose + Oxygen = Carbon Dioxide + Water + Energy
C6H12O6 + 6O2 --> 6CO2 + 6H2O + Energy (36 ATP)
Lets look at cell respiration in action using our friend YEAST.

Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described[1] estimated to be only 1% of all fungal species.[2] Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding. Yeasts are unicellular, although some species with yeast forms may become multicellular through the formation of a string of connected budding cells known as pseudohyphae, or false hyphae, as seen in most molds.[3] Yeast size can vary greatly depending on the species, typically measuring 3–4 µm in diameter, although some yeasts can reach over 40 µm.[4]

By fermentation, the yeast species Saccharomyces cerevisiae converts carbohydrates to carbon dioxide and alcohols – for thousands of years the carbon dioxide has been used in baking and the alcohol in alcoholic beverages.[5] It is also extremely important as a model organism in modern cell biology research, and is one of the most thoroughly researched eukaryotic microorganisms. Researchers have used it to gather information about the biology of the eukaryotic cell and ultimately human biology.[6] Other species of yeast, such as Candida albicans, are opportunistic pathogens and can cause infections in humans. Yeasts have recently been used to generate electricity in microbial fuel cells,[7] and produce ethanol for the biofuel industry.

Yeasts do not form a single taxonomic or phylogenetic grouping. The term "yeast" is often taken as a synonym for Saccharomyces cerevisiae,[8] but the phylogenetic diversity of yeasts is shown by their placement in two separate phyla: the Ascomycota and the Basidiomycota. The budding yeasts ("true yeasts") are classified in the order Saccharomycetales.[9]