Biology: Cellular Respiration

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Lab 7 – Cellular Respiration

Objectives:
• To be able to define cellular respiration and fermentation. • To give the overall balanced equations for aerobic respiration and alcoholic fermentation. • To distinguish between inputs, products, and efficiency of aerobic respiration and those of fermentation. • Understand the relationship between respiration and photosynthesis.

Note: You should perform experiments as described in this handout, which are adapted from Starr and Taggart’s Lab Manual for Biology (pp.131-142). Answer the questions in this handout as you wait for results to be seen. If this lab is to be used for a lab report, consider these questions when compiling the report.

Introduction:
The first law of thermodynamics states that energy can neither be created nor destroyed, but only converted from one form to another. All living organisms have a constant energy requirement; therefore, they have biochemical mechanisms that allow them to gather, store, and use energy. These mechanisms are collectively known as metabolism. A sequence of reactions must occur with each mechanism of metabolism, forming metabolic pathways.

When we study photosynthesis in next week’s lab, you will investigate the metabolic pathways in which green plants capture light energy to produce the carbohydrate known as glucose. Carbohydrates can be used as temporary stores of energy. They can be broken down more readily than other, more complex molecules for a quick source of usable energy. The process by which usable energy is released from carbohydrates and captured in the bonds of ATP (adenosine triphosphate) for use by cells is known as cellular respiration. Many metabolic processes in cells require energy derived from ATP, so ATP is considered to be the “universal energy currency” of the cell.

All energy pathways can be traced back to sunlight and photosynthesis. For example, the trophic levels of the food chain are integrally linked to metabolic pathways involving light and photosynthesis. Both autotrophs and heterotrophs undergo metabolism, including cellular respiration. For example, photoautotrophs utilize the starchy carbohydrates they produce from photosynthesis to provide themselves with the energy to produce new cells, grow, and reproduce. Heterotrophs utilize food sources as materials for respiration by digesting plants or animals that previously digested photosynthesizing organisms.

Carbon is one of the key components of organic molecules. Thus, it is found in and used by all living organisms. The carbon in CO2 is used during photosynthesis, when energy is stored during the production of glucose. During cellular respiration, glucose is broken down to harvest energy stored in the glucose molecule and the carbon is released as CO2.

Several different cellular respiration pathways have evolved, based on the specific organism and/or environmental conditions in which the organism lives. Oxygen availability has a significant affect on the type of respiration, and two basic pathways exist: aerobic respiration or anaerobic respiration. One type of anaerobic respiration, where the ultimate electron acceptor is an organic molecule, is known as fermentation. For aerobic respiration the general equation is:

C6H1 2O 6 + 6O2 ( 6CO2 + 6H2O + 36 ATP (38 in prokaryotes)
Glucoseoxygen carbon dioxide water energy

For alcohol fermentation (a type of anaerobic respiration) the general equation is:
C6H1 2O 6 ( 2C2H5OH + 2CO2 + 2 ATP
Glucose ethanolcarbon dioxide energy

The process of aerobic respiration yields a much higher amount of ATP than does anaerobic respiration. It involves glycolysis, pyruvate oxidation, Krebs cycle, and electron transport/chemiosmosis. Organisms known as aerobes carry out respiration in the presence of oxygen (oxygen is the final electron acceptor in electron...
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