Question

Describe how cell metabolism affects the processing of a pear as it moves through the process of cellular respiration.  Be sure to list each step of cellular respiration and account for the energy released from the pear at each step.

Answer 1

The goals of respiration are: 1. to transport the gases (Oxygen and Carbon dioxide), 2. To generate the energy for cellular processes.

On the basis of broader goals of respiration, it could be categorized into two types:

A. Organismal respiration

B. Cellular respiration

In cellular respiration, the six-carbon sugar molecule glucose in converted to Carbon dioxide and water in the presence of Oxygen to yield 30 or 32 ATP (the energy currency of the cells). This cellular processes could be divided into four steps:

1. Glycolysis (Glucose to Pyruvate)

2. The bridge process (Pyruvate to Acetyl-CoA)

3. Krebs cycle (Acetyl-CoA + oxaloacetate = CO2)

4. Electron transport system (O₂ to H₂O and ATPs)

In the cellular respiration, several steps in the path of oxygen reduction in mitochondria have the potential to produce highly reactive Oxygen species (ROS) (e.g. Superoxide, hydrogen peroxide, etc.) that can damage cells, is known as oxidative stress. The pear (rich source of antioxidants and fibre), when moves through the process of cellular respiration, reduces oxidative state through its antioxidant systems that consist of enzymatic antioxidants such as superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidases (GPxs), thioredoxin (Trx) as well as the non-enzymatic antioxidants. Along with these, this pear also possesses enough carbohydrates required for energy production.

The carbohydrates ultimately converted into glucose which broken down into pyruvic acid (three carbon molecule) via glycolysis (first step) then this pyruvic acid converted into acetyl-coA through the bridge reaction (second step), which enters into Kreb`s cycle (third step). Products of all three steps (glycolysis, bridge step and Kreb`s cycle), i.e. NADH/FADH enter into Electron transport system (fourth step), where ATP synthesis takes place.

Account for the energy released from the pear at each steps are as follows:

Step 1: Glycolysis: 2 NADH

2 ATP

Step 2: The bridge reaction (Pyruvate oxidation): 2 NADH

Step 3: Kreb`s cycle: 6 NADH (mitochondrial matrix)

2 FADH₂
2 ATP or 2 GTP

Step 4: Electron transport system: This is the step of cellular respiration. The electrons carried by NADH and FADH₂ are pulled from these molecules by enzymes in the mitochondrial membrane and used to drive a process called oxidative phosphorylation, wherein an electrochemical gradient driven by the released of the power that involved in the addition of phosphate molecules to ADP to produce ATP, here Oxygen acts as the final electron acceptor in the chain and creates H₂O.

Conclusion:

Fruits like pear rich in antioxidants and many other molecules required for energy production could be considered as Honeydew. Overall, 32 to 34 ATP molecules are generated in cellular respiration, depending on how the energy yield is summed. Thus cellular respiration yields a total of 32 to 34 ATP.