Class XI · First Year · Sindh / BIEK · Chapter 12

Bioenergetics.

Life runs on energy. Photosynthesis captures the sun's energy into glucose; respiration releases it again as ATP, the energy currency every cell spends.

1 · ATP — the energy currency

Cells don't burn glucose directly to do work; they first transfer its energy into ATP (adenosine triphosphate). When ATP loses a phosphate it becomes ADP + Pi and releases a usable packet of energy; respiration recharges ADP back to ATP. Photosynthesis and respiration are the two processes that manage this flow.

2 · Photosynthesis

Photosynthesis happens in chloroplasts. Using light energy trapped by chlorophyll, the plant turns carbon dioxide and water into glucose, releasing oxygen:

6CO₂ + 6H₂O —(light, chlorophyll)→ C₆H₁₂O₆ + 6O₂

It has two stages:

Light-dependent stage (in the thylakoid membranes): light splits water, releasing O₂ and making ATP and reduced hydrogen carriers.
Light-independent stage (in the stroma): CO₂ is fixed and, using that ATP and hydrogen, built into glucose.

Limiting factors The rate of photosynthesis is limited by whichever factor is in shortest supply — usually light intensity, CO₂ concentration or temperature. You can measure the light-intensity effect in the practical.

3 · Respiration

Respiration releases the energy stored in glucose to make ATP. Aerobic respiration (in mitochondria, using oxygen) releases the most energy:

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

When oxygen is short, cells respire anaerobically (no O₂) — far less energy, and a different product:

In animal muscle: glucose → lactic acid + a little ATP (causes muscle fatigue / oxygen debt).
In yeast & plants (fermentation): glucose → ethanol + CO₂ + a little ATP (used in baking and brewing).

4 · The two processes are linked

Photosynthesis and respiration are almost mirror images:

	Photosynthesis	Aerobic respiration
Where	Chloroplast	Mitochondrion
Energy	Stores light energy in glucose	Releases energy as ATP
Uses	CO₂ + H₂O	Glucose + O₂
Makes	Glucose + O₂	CO₂ + H₂O

Energy flows one way

The oxygen and glucose a plant makes are exactly what respiration consumes; the CO₂ and water respiration makes are what photosynthesis reuses. But the energy doesn't cycle — it flows one way, from the Sun → captured by plants → used by all living things → finally lost as heat.

In one minute

ATP is the energy currency (ATP ⇌ ADP + Pi).
Photosynthesis (chloroplast): 6CO₂ + 6H₂O —light→ glucose + 6O₂; limited by light, CO₂, temperature.
Aerobic respiration (mitochondrion): glucose + 6O₂ → 6CO₂ + 6H₂O + ATP.
Anaerobic: muscle → lactic acid; yeast → ethanol + CO₂ (less ATP).
Matter cycles between the two; energy flows one way from the Sun.

🎬 Watch the walkthrough 🌿 Photosynthesis (light) 💨 Respiration (CO₂) 🍃 Inside a leaf