Do we make energy from air? Plants hate the color green? How does the sun power our planet?

Ever wondered how plants seem to thrive effortlessly, basking in sunlight and producing oxygen? The magic happens through two fundamental processes: Photosynthesis and Cellular Respiration. How can we make energy just by breathing? Cellular Respiration is the answer to that. Let's break it down.

Image Source - Shows process of photosynthesis

Photosynthesis: A Sunlit Symphony

Step 1: Light Absorption

The key player in light absorption is chlorophyll, a pigment found in the chloroplasts of plant cells. Chlorophyll is like a molecular powerhouse that enables plants to harness the energy from sunlight.

Chlorophyll Structure

Chlorophyll molecules consist of a porphyrin ring, which is a large, planar, and cyclic structure containing a central magnesium ion. This structure is crucial for the absorption of light.

Photon Capture

When sunlight, which is composed of photons (particles of light), strikes a leaf, chlorophyll molecules absorb these photons. The energy from the photons is used to excite electrons within the chlorophyll molecule.

Energy Transfer

The excited electrons now possess higher energy levels and move to a higher energy state. This energy transfer is crucial for initiating the process of photosynthesis.


Within the chloroplasts, chlorophyll molecules are organized into clusters called photosystems. The two main types are Photosystem I and Photosystem II. These photosystems work in harmony to capture light energy and convert it into chemical energy.

Reaction Centers

Each photosystem contains a reaction center, where the light energy is converted into chemical energy. In the reaction center, the excited electrons are passed through a series of proteins, creating an electron transport chain.

Water Splitting (Photosystem II)

In Photosystem II, light energy is used to split water molecules into oxygen, protons, and electrons. This is a crucial step that releases oxygen as a byproduct, contributing to the oxygen we breathe.

Electron Transport Chain: The excited electrons move through the electron transport chain, generating energy that will be used in the next stages of photosynthesis.

Step 2: Carbon Dioxide Intake

External Intake

Plants absorb carbon dioxide from the external environment through small pores on their leaves called stomata. These openings allow for the exchange of gases, including the uptake of carbon dioxide.

Transport through Airspaces

Once absorbed, carbon dioxide moves through the airspaces in the leaf to reach the cells where photosynthesis occurs.

Entrance into Chloroplasts

Carbon dioxide enters the chloroplasts, the cellular organelles responsible for photosynthesis. Here, it becomes a key player in the synthesis of glucose.

Carbon Fixation

During the Calvin cycle, an essential part of photosynthesis, carbon dioxide molecules undergo a series of chemical reactions that result in the fixation of carbon into organic compounds.

Glucose Formation

The carbon atoms from carbon dioxide, along with hydrogen and oxygen from water, contribute to the formation of glucose—a sugar that serves as an energy source for the plant.

Oxygen Release

As a byproduct of photosynthesis, oxygen is released back into the atmosphere through the stomata. This process contributes to the oxygen supply in our environment.

Continuous Cycle

Carbon dioxide is a critical component in the cyclic process of photosynthesis. It is constantly taken in by plants, contributing to the synthesis of organic molecules that sustain life.

Step 3: Glucose Production

In the chloroplasts, sunlight powers the conversion of carbon dioxide and water into glucose—a plant's energy currency—and releases oxygen as a byproduct.

Step 4: Oxygen Release

Photosynthesis Kickstart

The journey towards glucose production begins with the capture of sunlight by chlorophyll, initiating the photosynthetic process.

Carbon Dioxide Integration

Carbon dioxide, absorbed through stomata, enters chloroplasts, where it combines with hydrogen and oxygen from water.

Chlorophyll's Role

Within the chloroplasts, chlorophyll molecules facilitate the conversion of light energy into chemical energy, providing the power needed for the subsequent reactions.

Calvin Cycle

The Calvin cycle, a series of intricate chemical reactions, takes place in the stroma of chloroplasts. During this cycle, carbon dioxide is fixed, and glucose is gradually synthesized.

Chemical Transformations

Carbon atoms from carbon dioxide are rearranged and combined with other elements to form glucose molecules—a type of sugar and a primary source of energy for the plant.

Energy Storage

The synthesized glucose serves as an energy storage molecule. Plants use this stored energy for various metabolic processes, growth, and reproduction.

Oxygen Release

As a byproduct of glucose production, oxygen is released into the atmosphere, contributing to the oxygen content in the environment.

End Result

The end result of this intricate process is the creation of glucose, a vital component that fuels the plant's activities and provides energy for other organisms within the ecosystem.

Image Source - Shows ETC diagram

Electron Transport Chain

Electron Flow

The Electron Transport Chain (ETC) is a crucial stage in cellular respiration, occurring in the inner mitochondrial membrane. It involves a series of protein complexes through which electrons flow.


NAD+ (Nicotinamide Adenine Dinucleotide) is a coenzyme that plays a pivotal role in cellular respiration. During glycolysis and the citric acid cycle, NAD+ accepts electrons and becomes NADH.

Electron Carrying

NADH acts as an electron carrier, shuttling high-energy electrons to the ETC. These electrons enter the chain at the first protein complex.

Energy Release

As electrons move through the ETC, they release energy. This energy is used by protein complexes in the chain to actively pump protons (H+) across the inner mitochondrial membrane, creating a proton gradient.

ATP Synthesis

The proton gradient sets the stage for ATP synthesis. Protons flow back into the mitochondrial matrix through ATP synthase, driving the production of ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi).

Oxygen's Role

Oxygen serves as the final electron acceptor at the end of the ETC. It combines with electrons and protons to form water. This step ensures the continuous flow of electrons through the chain.

NADH Regeneration

The electrons delivered by NADH to the ETC are eventually passed to oxygen. This process regenerates NAD+, allowing it to participate in glycolysis and the citric acid cycle again.

Image Source - Shows a diagram for cellular respiration

Cellular processes for Cellular Respiration


  • Location: Cytoplasm
  • Overview: Glycolysis is the initial stage of cellular respiration, where one molecule of glucose is broken down into two molecules of pyruvate.
  • Cellular Level: In the cytoplasm, a series of enzymatic reactions split glucose into two three-carbon molecules, generating a small amount of ATP and NADH.

Pyruvate Decarboxylation

  • Location: Mitochondrial Matrix
  • Overview: Each pyruvate molecule produced in glycolysis enters the mitochondria, and a carbon dioxide molecule is removed.
  • Cellular Level: Pyruvate is actively transported into the mitochondrial matrix, where it undergoes decarboxylation, resulting in the release of carbon dioxide.

Citric Acid Cycle (Krebs Cycle)

  • Location: Mitochondrial Matrix
  • Overview: The remaining acetyl group from pyruvate is further oxidized, producing NADH and FADH2, and releasing carbon dioxide.
  • Cellular Level: In the mitochondrial matrix, the acetyl group combines with oxaloacetate to form citric acid, kicking off a series of reactions that yield energy-rich molecules.

Electron Transport Chain (ETC)

  • Location: Inner Mitochondrial Membrane
  • Overview: NADH and FADH2 generated in previous stages donate electrons to the ETC, driving the synthesis of ATP.
  • Cellular Level: Electrons move through protein complexes in the inner mitochondrial membrane, actively pumping protons. This creates a proton gradient used to generate ATP through ATP synthase.

Oxidative Phosphorylation

  • Location: Inner Mitochondrial Membrane
  • Overview: ATP synthesis driven by the flow of protons back into the mitochondrial matrix.
  • Cellular Level: Protons flowing through ATP synthase provide the energy needed to phosphorylate ADP, forming ATP.

Regeneration of NAD+

  • Location: Mitochondrial Matrix
  • Overview: NAD+ is crucial for glycolysis and the citric acid cycle. It is regenerated as electrons pass through the ETC.
  • Cellular Level: Electrons from NADH are ultimately passed to oxygen, forming water and regenerating NAD+ for further use in glycolysis and the citric acid cycle.

The Dance of Life

These two processes, photosynthesis, and cellular respiration, form a beautiful cycle. Photosynthesis captures sunlight to produce energy, while cellular respiration releases that stored energy, ensuring the continuous flow of vitality.

So, the next time you marvel at the greenery around you, remember the intricate ballet happening within every leaf and stem—nature's way of breathing and feasting on sunlight. It's the symphony of life, conducted by the remarkable processes of photosynthesis and cellular respiration.

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