Question

You have been growing some animal cells in a test tube. The cells grow for several days, then do not appear to grow as well. You conduct some tests and determine there is a lot of lactic acid in the test tube. Which of the following is the most likely explanation for this condition

Answer 1

Answer:

D. There is no enough oxygen in the culture fluid

Explanation:

The cells in the test tube need to carryout one of the important metabolic processes of living organisms - respiration. The process involves the breakdown of glucose to generate energy for other metabolic processes in the cell.

Respiration can be carried out in the presence of oxygen (aerobic) or in the absence of oxygen (anaerobic). Human cells are primarily aerobic but can carryout respiration anaerobically in the presence of inadequate oxygen.

For aerobic respiration, glucose is broken down in the presence of oxygen to produce carbon-dioxide, water and energy in the form of ATP:

$C_6H_1_2O_6 + 6O_2 --> 6CO_2 + 6H_2O + ATP$

In the absence of oxygen, the glucose becomes converted to lactic acid and a smaller amount of ATP is produced as compared to aerobic respiration.

Hence, lactic acid gradually builds up in the absence of oxygen due to anaerobic respiration.

The correct option is D.

Answer 2

Answer: There is not enough oxygen in the culture fluid

Explanation:

 

The animal cell is the basic building unit of animal organisms. It is a type of eukaryotic cell like plant cells but unlike plant cells, animal cells cannot generate their own food. They must consume it and then extract its energy through a process called cellular respiration.

This consists of a series of chemical reactions in which organic molecules and oxygen are consumed to release energy (called ATP) and occurs in organelles (parts of the cell) called mitochondria. This type of cellular respiration is called aerobic respiration, because it uses oxygen.

The most used fuel in cellular respiration is glucose (a sugar). Glucose is a relatively large molecule and contains more energy in its bonds than smaller molecules. Therefore, when it is broken, energy is released that can be used by the body to perform its vital functions. When glucose or other sugars are not available for use in cellular respiration, cells can use other molecules as fuel, such as fats (known as lipids) or proteins.

During cellular respiration, a glucose molecule is gradually degraded into carbon dioxide and water. At the same time, some ATP is directly produced in the reactions that transform glucose. However, much more ATP is produced later in a process called oxidative phosphorylation.

The steps of cellular respiration are:

• Glycolysis: Glucose is converted into two molecules of pyruvate, an organic three-carbon molecule. In these reactions ATP is generated

• Pyruvate oxidation: Each pyruvate from glycolysis travels to the mitochondrial matrix. There, the pyruvate is converted into a two-carbon molecule bound to coenzyme A, known as acetyl-CoA.

• Citric acid cycle: The acetyl-CoA obtained in the previous step combines with a four-carbon molecule and goes through a cycle of reactions to finally regenerate the initial four-carbon molecule. In the process ATP is generated, and carbon dioxide is released.

• Oxidative phosphorylation: The produced in previous steps deposit their electrons in the electron transport chain and return to their "empty" forms. The movement of the electrons through the chain releases energy that is used to pump protons out of the matrix and form a gradient. The protons flow back into the matrix, through an enzyme called ATP synthase, to generate ATP. At the end of the electron transport chain, oxygen receives the electrons and collects protons from the medium to form water.

Glycolysis can occur in the absence of oxygen in a process called fermentation. The other three stages of cellular respiration (pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation) require the presence of oxygen to occur.

Then, in the absence of oxygen, fermentation occurs and lactic acid comes from the breakdown of glucose. This process has a lower yield, produces very little energy (ATP), so the cells will not grow properly.

Thereby, fermentation is less efficient at using the energy from glucose, because only 2 ATP are produced per glucose. While, 38 ATP per glucose are produced by aerobic respiration.