Q.RuBP carboxylase, PEP carboxylase, Pyruvate dehydrogenase, ATPase, cytochrome oxidase, Hexokinase, Lactate dehydrogenase.
Select/choose enzymes from the list above which are involved in
a. Photosynthesis
b. Respiration
c. Both in photosynthesis and respiration

Cellular Respiration Pathways: From Food to Fuel

Think of your body as a city that never sleeps. Every cell needs electricity to run — to pump ions, build proteins, send signals, divide. That electricity comes in a chemical form: ATP (adenosine triphosphate). Cellular respiration is the city's power plant. It takes fuel (glucose, fats, proteins) and burns it slowly, capturing the released energy as ATP.

The key insight: you don't just set glucose on fire. That would release all the energy as useless heat. Instead, the cell dismantles glucose step by step, like a staircase, so it can pocket the energy at each landing.

The Big Picture: Three Main Stages

Cellular respiration has three major phases, each happening in a different part of the cell:

Stage 1: Glycolysis — The Universal Starter

Glycolysis is ancient. It happens in the cytoplasm, requires no oxygen, and is nearly identical in bacteria, fungi, plants, and animals. It splits one 6-carbon glucose into two 3-carbon pyruvate molecules.

The process has two phases:

1. Energy investment phase — the cell spends 2 ATP to phosphorylate glucose, making it reactive.
2. Energy payoff phase — the split molecules are oxidized, producing 4 ATP and 2 NADH (an electron carrier).

Net per glucose: 2 ATP, 2 NADH, 2 pyruvate.

Stage 2: The Krebs Cycle — Extracting Electrons

Before entering the Krebs cycle, each pyruvate is transported into the mitochondria and converted to acetyl-CoA, releasing one CO₂ and making one NADH per pyruvate. So for one glucose, that's 2 CO₂ and 2 NADH before the cycle even starts.

The Krebs cycle itself is a circular pathway. Acetyl-CoA (2 carbons) combines with oxaloacetate (4 carbons) to form citrate (6 carbons). Through a series of reactions, citrate is gradually dismantled back to oxaloacetate, releasing two CO₂ molecules and capturing energy as:

• 3 NADH
• 1 FADH₂ (another electron carrier)
• 1 ATP (or GTP, depending on the organism)

Since one glucose gives two acetyl-CoA, multiply everything by 2.

Stage 3: The Electron Transport Chain — The Payoff

This is where the vast majority of ATP is made. NADH and FADH₂ carry electrons to protein complexes embedded in the inner mitochondrial membrane. The electrons are passed from complex to complex like a hot potato, losing energy at each step.

That energy is used to pump protons (H⁺) from the matrix into the intermembrane space, creating a concentration gradient — like water behind a dam.

At the end of the chain, the electrons combine with oxygen (the final electron acceptor) and protons to form water. This is why you breathe oxygen: it's the terminal electron acceptor. Without it, the chain stops, and respiration halts.

Chemiosmosis: The Proton Turbine

The proton gradient created by the ETC represents potential energy. Protons flow back into the matrix through a molecular turbine called ATP synthase. As they pass through, the turbine spins, driving the synthesis of ATP from ADP and phosphate. …