Photosynthesis is the process by which green plants use light to create organic matter (glucose) from carbon dioxide and water, releasing oxygen in the process. It takes place in chloroplasts and consists of two stages: the light-dependent and dark reactions. On this page, we explain in simple terms: what photosynthesis is, where it occurs, how the phases differ, what the equation looks like, and what is produced. At the end, there is an interactive quiz to test your knowledge.
What is photosynthesis and where does it happen
Photosynthesis (from the Greek phōs — light and synthesis — putting together) is the formation of organic substances from inorganic ones using light. Simply put, a plant "makes its own food" from air and water using solar energy.
Photosynthesis occurs in chloroplasts — green plastids found in abundance in leaf cells. Inside the chloroplasts is a green pigment called chlorophyll: it captures light energy and triggers the entire process. Plants appear green precisely because of chlorophyll.
For photosynthesis, a plant needs three things:
- light (sunlight or artificial);
- water — absorbed by roots from the soil;
- carbon dioxide (CO₂) — enters from the air through leaf stomata.
Photosynthesis is divided into two sequential phases — the light-dependent and dark reactions. Their names are often confused, so let's look at each one separately.
The process of photosynthesis: what is made from what
This diagram helps visualize the key points: on the left, what enters the leaf (light, water, and carbon dioxide); on the right, what is produced (glucose and oxygen). The plant releases oxygen into the air and uses glucose as food, storing it as starch.
Photosynthesis equation
The entire process can be written as one summary equation. Six molecules of carbon dioxide and six molecules of water, under the influence of light and with the help of chlorophyll, turn into one molecule of glucose and six molecules of oxygen.
Light-dependent and dark reactions of photosynthesis
Photosynthesis occurs in two stages. The light-dependent reaction takes place on membranes inside the chloroplast and is only possible in the presence of light: light energy splits water, releasing oxygen and storing energy (in ATP and NADPH molecules). The dark reaction occurs in the stroma of the chloroplast and does not directly require light: here, glucose is synthesized from carbon dioxide using the stored energy. Important: "dark" does not mean "nocturnal" — this phase uses the products of the light-dependent phase, so in the dark, it will soon stop. Let's compare both phases in the table.
| Feature | Light-dependent reaction | Dark reaction |
|---|---|---|
| Where it occurs | on thylakoid membranes (chloroplast grana) | in the chloroplast stroma |
| Light required | yes, only in light | no, but depends on light-dependent products |
| What is needed | light, water (H2O) | carbon dioxide (CO2), ATP and NADPH energy |
| What is produced | oxygen (O2), ATP, NADPH | glucose (C6H12O6) |
Importance of photosynthesis for life on Earth
Photosynthesis is one of the most important processes in nature. Thanks to it:
- the air is supplied with oxygen, which humans, animals, and plants themselves breathe;
- carbon dioxide is removed from the air;
- organic matter is created — the foundation of food for all living beings. Plants feed herbivores, herbivores feed predators, so the entire food chain begins with photosynthesis;
- light energy is stored in organic substances. Coal, oil, and peat are also "preserved" energy from ancient photosynthesis.
That is why green plants are often called the "lungs of the planet": they continuously supply the biosphere with oxygen and food.
Breakdown: what is made from what
Example 1. Where does glucose come from
Question: from which substances does a plant make glucose?
Breakdown. Glucose (C₆H₁₂O₆) consists of carbon, hydrogen, and oxygen. The plant takes carbon from atmospheric carbon dioxide (CO₂), and hydrogen from water (H₂O). Energy for the "assembly" is provided by light, and glucose is assembled in the dark reaction, in the chloroplast stroma.
Answer: glucose is formed from carbon dioxide and water using light energy.
Example 2. Where does oxygen come from
Question: from which substance is the oxygen released by the plant produced?
Breakdown. Many think that oxygen comes from carbon dioxide, but that is not true. In the light-dependent reaction, light energy splits water (H₂O) molecules — this is when oxygen (O₂) is released. Carbon dioxide is used only to build glucose.
Answer: oxygen is released when water is split during the light-dependent reaction.
Example 3. Checking the equation by atom count
Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Check carbon (C): 6 on the left (from 6·CO₂), 6 on the right (in glucose C₆…) — it matches.
Check oxygen (O): 6·2 + 6·1 = 18 on the left, 6 (in glucose) + 6·2 = 18 on the right — it matches.
Check hydrogen (H): 6·2 = 12 on the left, 12 on the right (in glucose C₆H₁₂O₆) — it matches.
Conclusion: the equation is balanced, all atoms are accounted for.
Common misconceptions about photosynthesis
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People think that plants only release oxygen and do not breathe at all.
Plants breathe just like animals: they constantly absorb oxygen and release carbon dioxide (respiration). It is just that during the day in the light, photosynthesis is more active than respiration, so more oxygen is released than is absorbed.
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Confusing light-dependent and dark reactions: thinking glucose is formed in the light-dependent phase and oxygen in the dark phase.
It is the opposite. In the light-dependent phase, oxygen is released and energy is stored (ATP, NADPH). Glucose is assembled in the dark phase from carbon dioxide using this stored energy.
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Interpreting "dark reaction" as "a phase that happens at night, in the dark."
"Dark" only means "does not directly require light." But it uses ATP and NADPH from the light-dependent phase, so without light, these reserves run out and the dark reaction soon stops as well. In reality, both phases occur during the day, one after the other.
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Thinking that oxygen is formed from carbon dioxide.
Oxygen is released from water (H₂O) when it is split in the light-dependent phase. Carbon dioxide (CO₂) is used to build glucose, not to produce oxygen.
Questions and answers
How does photosynthesis differ from respiration?
These are opposite processes. During photosynthesis, a plant absorbs carbon dioxide and water in the light, creates glucose, and releases oxygen — meaning it stores energy. During respiration, conversely, glucose and oxygen are consumed, and carbon dioxide, water, and energy are released. All living cells breathe constantly, while photosynthesis occurs only in green cells and only in the light.
Is light needed for the dark reaction of photosynthesis?
Directly, light is not needed for the dark reaction — the reactions occur in the chloroplast stroma without light. But the dark reaction uses ATP and NADPH, which are formed in the light-dependent phase. Therefore, without light, these reserves quickly run out, and the dark reaction also stops. The name "dark" does not mean "nocturnal," but rather "not requiring light directly."
Where exactly does photosynthesis occur?
Photosynthesis occurs in chloroplasts — green plastids in leaf cells. The light-dependent reaction takes place on thylakoid membranes (in grana), and the dark reaction in the chloroplast stroma. The green color and the ability to capture light are provided to chloroplasts by the pigment chlorophyll.
What is produced during photosynthesis?
The main products of photosynthesis are glucose (organic matter, plant food) and oxygen, which is released into the air. During the process, intermediate substances are also formed — ATP and NADPH, which transfer energy from the light-dependent phase to the dark phase.
At what time of day does photosynthesis occur?
Photosynthesis occurs during the day, as long as there is light. At night, in the dark, it stops because the light-dependent phase cannot work without light, and without its products, the dark reaction also stops. Meanwhile, plant respiration continues around the clock.
