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

Step 1: Glycolysis (occurs in the cytoplasm)

(i) Aerobic Respiration (with oxygen)

Glucose first breaks down into pyruvate via glycolysis in the cytoplasm. Then, with oxygen present, pyruvate enters the mitochondria for complete oxidation.

Overall reaction:

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

Stages:

• 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 (without oxygen – in muscle cells)

When oxygen is lacking, like during intense exercise, muscle cells use anaerobic respiration. Pyruvate converts to lactic acid.

Reaction:

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

Specifically for pyruvate:

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

Summary:

Stage	Details
Glycolysis	Glucose breakdown 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	\( \text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_3\text{H}_6\text{O}_3 + 2\text{ATP} \)