Glycolysis Process Explanation Essay

All You Need to Know About Photosynthesis and Cellular Respiration

The processes of photosynthesis and cellular respiration are linked to each other. It is important to understand the differences between the two.
Photosynthesis and cellular respiration are the life processes performed by most living organisms to obtain usable energy from nature. While photosynthesis is performed by most plants which can prepare their own food, most animals fulfill their energy requirements through cellular respiration.

Photosynthesis is the process by which plant cells convert light energy from the sun into chemical energy, so as to create energy-rich carbohydrate molecules like glucose. Cellular respiration is the process of breaking down food molecules to obtain energy and store it in the form of adenosine triphosphate (ATP) molecules.

Plant cells, after creating sugar molecules through photosynthesis, undergo cellular respiration to create ATP molecules. Animals obtain food molecules from plants and other organisms, and then undergo cellular respiration to obtain ATP molecules. All living organisms utilize these stored ATP molecules to carry out their metabolic processes.
Photosynthesis takes place in the cells of plant leaves. It occurs in structures called chloroplasts, which contain chlorophyll. The plant cells absorb light from the sun through the pigment chlorophyll, and using water and carbon dioxide obtained from the environment. They undergo a series of chemical reactions to produce carbohydrate molecules.
Chemical reaction in Photosynthesis
Carbon Dioxide + Water + Sunlight → Glucose + Oxygen
Stages in the Process of Photosynthesis
Light-dependent Reactions
This is the first stage of the photosynthetic process. These reactions take place in the presence of sunlight, and use light energy from the sun to produce ATP molecules and other molecules known as NADPH. These molecules are used as the energy source to carry out the chemical changes in the next stage of photosynthesis.
Light-independent Reactions (Calvin Cycle)
In this stage of photosynthesis, energy-containing sugar molecules are synthesized. The ATP and NADPH produced are used to fuel the reactions in this stage. Here, CO2 molecules are broken down and converted into sugars and other compounds. The Calvin Cycle is repeated twice in order to yield one molecule of glucose.
Cellular respiration takes place in the same way in both plants and animals. Living cells obtain the products of photosynthesis (sugar molecules) and undergo cellular respiration to produce ATP molecules. Some cells respire aerobically, using oxygen, while others undergo anaerobic respiration, without using oxygen. The process involves a set of chemical reactions to convert chemical energy from the glucose molecules into ATP molecules.
Chemical reaction in Cellular Respiration
Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)
Stages in the Process of Cellular Respiration
Cellular respiration begins at this stage in the cytoplasm of the cells, and yields 2 carbon-based molecules called pyruvate, and 2 molecules of ATP. Oxygen plays no part during this stage, so it is called anaerobic respiration.
This process takes place in specialized structures within the cell called mitochondria, and uses the products of glycolysis, the pyruvate molecules, to release energy, along with CO2 and water as the by-products of the reaction. The energy released is stored in the form of ATP molecules. Usually, a total of 38 ATP molecules is produced.
Differences between Photosynthesis and Cellular Respiration
  • Photosynthesis takes place in two stages of the light reactions and the dark reactions. Cellular respiration involves aerobic (glycolysis) and anaerobic respiration.
  • Photosynthesis takes place only when there is sunlight. Cellular respiration occurs at all times.
  • Photosynthesis takes place in plant leaves containing the chlorophyll pigment. Cellular respiration takes place in the cytoplasm and mitochondria of the cell.
  • Photosynthesis utilizes sunlight to produce food molecules. Cellular respiration utilizes glucose molecules to obtain energy-storing ATP molecules.
  • Photosynthesis uses water, sunlight, and carbon dioxide from the atmosphere to create glucose molecules, and releases oxygen as a by-product. Cellular respiration uses glucose molecules and oxygen to produce ATP molecules and carbon dioxide as the by-product.
  • Photosynthesis involves conversion of one type of energy into another: light energy into chemical energy. Cellular respiration involves using that chemical energy and breaking it down to release energy.
  • Photosynthesis occurs only in plants and some bacteria. Cellular respiration takes place in all types of living organisms.


Figure %: The Glycolytic Pathway.

Now that we have a general understanding of the broad topics of metabolism and respiration, we will turn our discussion to more specific metabolic pathways that lead to the derivation of ATP. In this SparkNote we will look at glycolysis, the metabolism of glucose, a digestive product of carbohydrates found in many food products that we ingest.

Taking place in the cell cytoplasm, glycolysis actually comprises a series of nine steps involving a number of intermediate structures and specific enzymes that help catalyze each reaction. In this section, we will go through each of these reactions, learning the roles of the associated intermediates and enzymes. (Note: specific knowledge of the nine steps of glycolysis is not necessary for the AP Biology test. In regard to that test, this summary presents all information necessary about glycolysis and the first two sections of this SparkNote can be skipped. The third section on fermentation, however, will be covered on the AP test).

Over the course of glycolysis' nine steps, the 6-carbon molecule glucose is broken down to two 3-carbon pyruvate molecules. The reaction does not occur spontaneously: 2 ATP molecules must be broken down to drive the splitting of glucose into the 2 pyruvates. However, in the course of the breakdown of glucose, the glycolysis reaction produces four ATP, resulting in a net gain of two ATP for the entire process. Glycolysis also results in the production of 2 NADH molecules, which eventually play an important role in the production of additional ATP in the electron transport chain. Glycolysis itself is an anaerobic process. After a cell has completed glycolysis, and depending on the circumstances in which the cell finds itself, that cell can either move into the process of aerobic respiration and commence the citric acid cycle or continue with less efficient aneorobic respiration in a process called fermentation, covered in the third section of this SparkNote on glycolysis.

In the first two sections of this SparkNote, we will look at glycolysis in two major stages. The first involves the phosphorylation of the glucose ring in preparation for an eventual breakdown into two 3-carbon molecules. In the second stage, the two 3-carbon molecules are converted into pyruvate.

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