Question

What is the first step of cellular respiration? In which part of the cell does it occur ? Write the equation for the process of breakdown of glucose in a human cell : (i) in the presence of oxygen (ii) due to lack of oxygen

Answer 1

Cellular Respiration in Human Cells

The first step: Glycolysis (occurs in the cytoplasm)

(i) Aerobic Respiration (in the presence of oxygen)

Glucose is first broken down into pyruvate via glycolysis in the cytoplasm. Then, in the presence of oxygen, pyruvate enters the mitochondria for complete oxidation.

Overall equation:

\[ \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \xrightarrow{\text{mitochondria}} 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy (ATP)} \]

Steps:

• Glycolysis (in cytoplasm): \[ \text{Glucose} \rightarrow 2 \text{Pyruvate} + \text{ATP + NADH} \]
• Krebs Cycle & Link Reaction (in mitochondria): \[ \text{Pyruvate} \rightarrow \text{Acetyl-CoA} \rightarrow \text{CO}_2 + \text{ATP + NADH + FADH}_2 \]
• Oxidative Phosphorylation: \[ \text{NADH + FADH}_2 + O_2 \rightarrow \text{Lots of ATP} \]

(ii) Anaerobic Respiration (in absence of oxygen – in muscle cells)

When oxygen is insufficient, such as during vigorous exercise, muscle cells perform anaerobic respiration. Pyruvate is converted to lactic acid.

Equation:

\[ \text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{cytoplasm, no } O_2} 2\text{C}_3\text{H}_6\text{O}_3 + \text{Energy (2 ATP)} \]

Specifically for pyruvate:

\[ \text{Pyruvic Acid} \xrightarrow{\text{No } O_2} \text{Lactic Acid} + \text{Energy} \]

Summary:

Step	Details
Glycolysis	Breakdown of glucose in cytoplasm
Aerobic Respiration	\( \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP} \)
Anaerobic Respiration	\( \text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_3\text{H}_6\text{O}_3 + 2\text{ATP} \)