Introduction to Bioenergetics: ATP, Photosynthesis & Cellular Respiration Explained

Bioenergetics | ATP Structure, Photosynthesis, Respiration & Energy Flow (2026)

1. Introduction to Bioenergetics

Bioenergetics is the study of how energy flows through living systems. It looks at the ways cells capture, store, transfer and use energy for growth, movement, reproduction and every other life process.

Nearly all the energy used by living organisms on Earth ultimately comes from the Sun through photosynthesis. Photosynthesis is the great energy-capturing process; cellular respiration is the energy-releasing process. Together they form the foundation of bioenergetics.

2. ATP – The Universal Energy Currency

2.1 Structure of ATP

ATP (Adenosine Triphosphate) is made of three parts:

Adenine (a nitrogenous base)
Ribose (a 5-carbon sugar)
Three phosphate groups linked by high-energy bonds

(Image suggestion: Chemical structure of ATP – Public Domain / GFDL, Wikimedia Commons, original by cacycle)

2.2 Discovery

ATP was discovered in 1929 by the German biochemist Karl Lohmann. In 1941 Fritz Lipmann proposed that ATP is the main energy-transfer molecule in the cell — work that contributed to his Nobel recognition.

2.3 Why ATP Is Ideal as Energy Currency

It is small and water-soluble, so it diffuses easily inside the cell.
Hydrolysis releases a useful amount of energy (~7.3 kcal/mol or 30.5 kJ/mol).
It can be regenerated rapidly.
It couples energy-releasing (exergonic) reactions perfectly with energy-requiring (endergonic) reactions.

2.4 Breakdown (Hydrolysis) of ATP

ATP + H₂O → ADP + Pi + Energy (~7.3 kcal/mol)
DP + H₂O → AMP + Pi + Energy (~7.3 kcal/mol)

2.5 Synthesis of ATP

ADP + Pi + Energy → ATP + H₂O

This happens during photosynthesis (photophosphorylation) and during respiration (oxidative phosphorylation and substrate-level phosphorylation).

Key principle: ATP is made in energy-releasing processes and broken down in energy-consuming processes.

3. Photosynthesis – Capturing Light Energy

Overall equation:

$6 𝐶 𝑂_{2} + 6 𝐻_{2} 𝑂 + Light Energy \to 𝐶_{6} 𝐻_{12} 𝑂_{6} + 6 𝑂_{2}$

3.1 Roles of Light, CO₂ and Water

Light is the energy source.
CO₂ provides the carbon atoms for sugars.
Water supplies electrons and protons; oxygen is released as a by-product.

3.2 Photosynthetic Pigments

Two main groups:

Chlorophylls (primary pigments)
- Chlorophyll a (blue-green)
- Chlorophyll b (yellow-green)
Carotenoids (accessory pigments)
- Carotenes (orange)
- Xanthophylls (yellow)

These pigments absorb light, pass the energy to reaction centres, and also protect the photosynthetic machinery from photo-oxidation.

3.3 Absorption Spectra of Chlorophyll a & b

(Image suggestion: Absorption spectrum of chlorophyll a and b – CC BY-SA 4.0, Serge Helfrich, Wikimedia Commons)

Chlorophyll a peaks at ~430 nm (blue-violet) and ~662 nm (red).
Chlorophyll b peaks at ~453 nm (blue) and ~642 nm (orange-red).
Both reflect green light, which is why leaves look green.

3.4 Photosystems

Pigments are organised into Photosystem II (P680) and Photosystem I (P700) in the thylakoid membrane. Each photosystem has an antenna complex that gathers light and a reaction centre that starts the electron flow.

3.5 Light-Dependent Reactions

Non-Cyclic Photophosphorylation (the Z-scheme)

Light hits PSII (P680) → electrons are excited and passed to a primary acceptor.
Water is split (photolysis): $2 𝐻_{2} 𝑂 \to 4 𝐻^{+} + 4 𝑒^{-} + 𝑂_{2}$ .
Electrons travel down the electron-transport chain → ATP is made by chemiosmosis.
Electrons reach PSI (P700).
Light re-excites the electrons → they reduce NADP⁺ to NADPH.

Products: ATP + NADPH + O₂

Cyclic Photophosphorylation

Only PSI is involved. It produces extra ATP but no NADPH and no oxygen.

(Image suggestion: Z-scheme of the light reactions – Public Domain, Lanzi, Wikimedia Commons)

3.6 Calvin Cycle (Light-Independent Reactions)

Takes place in the stroma and has three stages:

Carbon fixation – CO₂ is added to RuBP by the enzyme Rubisco → 3-PGA.
Reduction – 3-PGA is converted to G3P using ATP and NADPH.
Regeneration – most of the G3P is used to regenerate RuBP (also requires ATP).

(Image suggestion: Diagram of the Calvin cycle – CC BY-SA 3.0, based on Mike Jones / Adenosine, Wikimedia Commons)

4. Cellular Respiration – Releasing Energy

4.1 Aerobic Respiration – Four Stages

Glycolysis (cytoplasm)
Link reaction (mitochondrial matrix)
Krebs (citric acid) cycle (mitochondrial matrix)
Electron transport chain + chemiosmosis (inner mitochondrial membrane)

4.2 Glycolysis (Outline)

Glucose (6C) is split into two molecules of pyruvate (3C). Net products per glucose: 2 ATP + 2 NADH + 2 pyruvate

4.3 Link Reaction

Pyruvate + CoA + NAD⁺ → Acetyl-CoA + CO₂ + NADH

4.4 Krebs Cycle

Per acetyl-CoA: 3 NADH + 1 FADH₂ + 1 ATP + 2 CO₂ (Double these numbers for one glucose molecule.)

4.5 Electron Transport Chain & Chemiosmosis

NADH and FADH₂ donate high-energy electrons.
Electrons pass along a series of carriers (oxidation-reduction reactions).
A proton gradient is built up across the inner membrane.
Protons flow back through ATP synthase → ATP is produced.
Oxygen is the final electron acceptor and is reduced to water.

(Image suggestion: Mitochondrial electron transport chain – Public Domain, Fvasconcellos, Wikimedia Commons)

4.6 Role of NAD and FAD

These coenzymes carry high-energy electrons from glycolysis and the Krebs cycle to the electron transport chain.

4.7 Energy Yield

Aerobic respiration: approximately 30–32 ATP per glucose.
Anaerobic respiration: only 2 ATP per glucose.

The much higher yield in aerobic respiration comes from complete oxidation of glucose plus the powerful chemiosmotic mechanism.

5. Anaerobic Respiration

After glycolysis, if oxygen is absent, pyruvate is converted to:

Lactic acid (in animals and some bacteria), or
Ethanol + CO₂ (in yeast and some plants)

The purpose is simply to regenerate NAD⁺ so that glycolysis can keep producing its small amount of ATP.

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Bioenergetics | ATP Structure, Photosynthesis, Respiration & Energy Flow (2026)