Question

Which of the following is the reverse of photosynthesis? (a) Respiration (b) Protein synthesis (c) Fat synthesis (d) All of these

Short answer

The correct answer is (a) Respiration.

Step-by-step solution

Understanding Photosynthesis

The first step involves understanding what photosynthesis is. In photosynthesis, plants convert carbon dioxide (CO2) and water (H2O) into glucose (C6H12O6) and oxygen (O2) by using sunlight. So, in the equation form it looks like this: \[ 6CO2 + 12H2O + light \rightarrow C6H12O6 + 6O2 + 6H2O \]. Therefore, the reverse process would involve converting glucose (a sugar) and oxygen back into carbon dioxide and water.

Evaluating Options

To identify which of the provided options is the reverse of photosynthesis, asses what each process does. (a)Respiration, (b)Protein synthesis, (c)Fat synthesis, (d)All of these. Evaluating these, it becomes clear that respiration is the only process that takes glucose and oxygen and converts them back into water and carbon dioxide, which means it's the reverse of photosynthesis. Neither protein synthesis nor fat synthesis involve this conversion. Thus, (d) all of these, is also not correct.

Conclusion

The correct answer is (a) Respiration since it is the process that performs the reverse function of photosynthesis -- it takes the glucose and oxygen created during photosynthesis and converts them back into water and carbon dioxide.

Key concepts

Photosynthesis

Photosynthesis is a remarkable process that fuels life on Earth. It occurs in plant cells inside specialized structures called chloroplasts. During this process, green plants, algae, and some bacteria transform sunlight into chemical energy. They take in carbon dioxide from the air and water from the soil and convert them into glucose, a simple sugar, and oxygen, which is released back into the atmosphere.

Photosynthesis can be simplified into the following chemical equation:

• 6CO₂ + 12H₂O + light → C₆H₁₂O₆ + 6O₂ + 6H₂O

Here is what happens in photosynthesis:

• **Light Absorption**: Plants use chlorophyll, the green pigment in their leaves, to capture light energy.
• **Water Splitting**: The light energy splits water molecules into hydrogen and oxygen.
• **Carbon Fixation**: Carbon dioxide is combined with the hydrogen to form glucose.
• **Oxygen Release**: Oxygen, a by-product, is released into the air.

Without photosynthesis, life as we know it would not exist because it is the source of food and oxygen for nearly all living organisms.

Glucose Oxidation

Glucose oxidation, more commonly known as cellular respiration, is the process that effectively serves as the "reverse" of photosynthesis. In this biochemical pathway, organisms convert glucose and oxygen into energy, water, and carbon dioxide. It's how cells release stored energy from glucose molecules to perform vital functions.

The equation for glucose oxidation is almost the mirror image of photosynthesis:

• C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP)

**Process Overview of Glucose Oxidation:**

• **Glycolysis**: The glucose molecule is split into two three-carbon molecules called pyruvate, generating small amounts of ATP and capturing electrons in high-energy carriers.
• **Citric Acid Cycle (Krebs Cycle)**: Pyruvate enters the mitochondria and is further broken down, producing more energy carrier molecules.
• **Electron Transport Chain**: Electrons pass through a chain of proteins, producing ATP and releasing water as a by-product.

This process is crucial because it provides the energy that powers all cellular activities, making it essential for growth, repair, and maintenance of cells.

Carbon Dioxide Production

Carbon dioxide (CO₂) production is an integral part of cellular respiration, emphasizing its role in energy conversion. As glucose molecules get broken down to release energy, carbon dioxide emerges as a by-product.

**How Carbon Dioxide is Produced in Respiration:**

• **During Glycolysis**: Although CO₂ production starts later, glycolysis initiates the glucose breakdown.
• **Citric Acid Cycle**: Here, carbon atoms in the glucose are oxidized, and CO₂ molecules are released.
• **Decarboxylation Steps**: These are the steps specific to the cycle where carbon atoms are cleaved and converted to CO₂.

The carbon dioxide generated is then transported via the bloodstream to the lungs, where it is exhaled. This continual release of CO₂ helps keep the blood's pH level balanced, ensuring homeostasis in the body.

Understanding CO₂ production is crucial not only for biology but also for tackling environmental issues, as elevated CO₂ levels impact climate change.