Difference Between Light Reaction And Dark Reaction: Definition, Difference

Edited By Irshad Anwar | Updated on Jul 02, 2025 07:00 PM IST

The Basics Of Photosynthesis

Photosynthesis is a biological process by which plants, algae, and some bacteria convert light energy from the sun into chemical energy in the form of glucose. This process is driven by the absorption of light by chlorophyll, the splitting of water molecules, and the fixation of carbon dioxide to form organic molecules.

The net chemical equation for photosynthesis is:

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

This entire process happens in the chloroplasts of plant cells. There, the chlorophyll pigment captures the light energy needed to drive the whole mechanism.

Light Reaction

The details are given below:

Definition And Location

Happens in Thylakoid Membranes: The light reactions occur inside the thylakoid membranes of chloroplasts.
Role of Chlorophyll: Chlorophyll absorbs light energy, which will later be transposed into the driving of photochemical reactions.

Key Processes

Photolysis of Water: The water molecule undergoes splitting into oxygen, protons, and electrons.
Product formation: Light energy gets converted into chemical energy in the form of ATP and NADPH.
Photosystems I and II: These are the two complexes involved in capturing the light energy and yielding a chemical energy product.

Steps Involved

Absorption of Light: Absorbance of photons by chlorophyll excites the electron to higher energy levels.
Water Splitting: Water is split to replace the excited electrons, releasing oxygen.
Electron Transport Chain: Electron flow along the transport chain liberates energy to pump protons and create a proton gradient.
Formation of ATP (Photophosphorylation): The gradient of protons drives the activity of ATP synthase to make ATP.

NEET Highest Scoring Chapters & Topics

Know Most Scoring Concepts in NEET 2024 Based on Previous Year Analysis.

Know More

Dark Reaction

The details are given below:

Definition And Location

The dark reactions, also known as the Calvin cycle, occur in the stroma of the chloroplast.
RuBisCO enzyme is responsible for catalysing carbon dioxide fixation.

Important Processes

Carbon Fixation: CO₂ is fixed into an organic molecule.
Reduction Phase: 3-phosphoglycerate is reduced to G3P using ATP and NADPH.
Regeneration of RuBP: RuBP is regenerated to allow the continuation of the cycle.

Steps Involved

Role of RuBisCO: Catalyses carbon fixation, the first step of the process.
Formation of G3P: G3P is produced at the reduction phase.
ATP and NADPH Utilisation: They provide energy and reduce power respectively.

Table: Differences Between Light And Dark Reactions

Feature	Light Reactions	Dark Reactions
Location	Thylakoid membranes	Stroma of chloroplasts
Primary Function	Convert light energy to chemical energy	Fix CO2 and synthesise glucose
Energy Requirements	Light energy (photons)	ATP and NADPH (produced in light reactions)
Products Formed	ATP, NADPH, O2	G3P (eventually glucose)
Enzymes Involved	Photosystems I and II, ATP synthase	RuBisCO, various Calvin cycle enzymes

Significance Of Both Reactions In Photosynthesis

The significance of both reactions is given below:

Interdependence Of Light And Dark Reactions

Light Reactions Provide ATP and NADPH: Used in the Calvin cycle.
Dark Reactions are Dependent on Light Reactions: They consume the products to fix carbon and produce glucose.

Contribution To Glucose Formation

Light Reactions Produce the Energy Carriers: ATP and NADPH.
Dark Reactions Synthesise Glucose: Using the energy carriers produced in light reactions.

Importance In Plant Growth And Survival

Energy Production: ATP and glucose are vital in plant metabolism.
Growth and Development: Glucose provides the carbon skeletons for other organic molecules necessary for growth.

Recommended Video On 'Difference Between Light Reaction And Dark Reaction'

Frequently Asked Questions (FAQs)

1. What are the main differences between light and dark reactions in photosynthesis?

Light reactions convert light energy into chemical energy in the form of ATP and NADPH. Dark reactions use this chemical energy for fixing CO2 into glucose.

2. Why is the Calvin cycle also known as the dark reaction?

This is called the dark reaction because it doesn't require direct light and can take place in darkness if the energy carriers produced by the light reactions are available.

3. How do light and dark reactions work together in photosynthesis?

Light reactions produce ATP and NADPH that dark reactions then use to fix carbon dioxide and produce glucose.

4. What role does ATP play in the dark reaction?

In the cycle of Calvin, ATP is the energy to drive chemical reactions from carbon fixation through reduction.

5. Can dark reactions occur during the day?

Yes, dark reactions can occur during the day if ATP and NADPH are available from light reactions.

6. What is the primary function of dark reactions?

The primary function of dark reactions, also known as the Calvin cycle, is to use the energy from ATP and NADPH (produced in light reactions) to fix atmospheric carbon dioxide into glucose. This process is also called carbon fixation or carbon assimilation.

7. Where do dark reactions occur in the chloroplast?

Dark reactions occur in the stroma of the chloroplast. The stroma is the fluid-filled space surrounding the thylakoids, containing enzymes necessary for the Calvin cycle and other metabolic processes.

8. What is the main product of dark reactions?

The main product of dark reactions is glucose (C6H12O6). This simple sugar is formed through the fixation of carbon dioxide using the energy and reducing power from ATP and NADPH produced in light reactions. Glucose can then be used for immediate energy or stored as starch.

9. Why is RuBisCO considered a key enzyme in dark reactions?

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is considered a key enzyme in dark reactions because it catalyzes the first major step of carbon fixation. It's responsible for attaching CO2 to a 5-carbon sugar (ribulose bisphosphate) to initiate the Calvin cycle. RuBisCO is also the most abundant protein on Earth due to its relatively low efficiency and the plant's need to compensate.

10. How does the Calvin cycle in dark reactions regenerate its starting compound?

The Calvin cycle regenerates its starting compound, ribulose bisphosphate (RuBP), through a series of reactions in its final stage. After carbon fixation and reduction steps, some of the 3-carbon products are used to regenerate RuBP through phosphorylation and isomerization reactions. This regeneration allows the cycle to continue without constant input of new starting materials.

11. What is the primary function of light reactions?

The primary function of light reactions is to convert light energy into chemical energy. This is done by splitting water molecules to release electrons, which are then used to produce ATP and NADPH. These energy-rich molecules are essential for powering the dark reactions.

12. Where do light reactions take place in the chloroplast?

Light reactions take place in the thylakoid membranes of the chloroplast. These membranes form stacked structures called grana, which increase the surface area for light absorption and house the photosystems necessary for light reactions.

13. What are the key products of light reactions?

The key products of light reactions are ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). ATP provides energy, while NADPH provides reducing power for the dark reactions. Oxygen is also produced as a byproduct of water splitting during light reactions.

14. What role does water play in light reactions?

Water plays a critical role in light reactions as the source of electrons and protons. In a process called photolysis, water molecules are split into oxygen, protons (H+ ions), and electrons. The electrons replace those excited by light in the photosystems, the protons contribute to the gradient for ATP synthesis, and oxygen is released as a byproduct.

15. How do light reactions create a proton gradient, and why is this important?

Light reactions create a proton gradient across the thylakoid membrane through the electron transport chain. As electrons move through the chain, protons (H+ ions) are pumped from the stroma into the thylakoid space. This gradient is crucial as it drives ATP synthesis through chemiosmosis, providing the energy currency for dark reactions.

16. Why are dark reactions called "dark" if they can occur during the day?

Dark reactions are called "dark" because they don't directly require light energy to proceed. They can occur both day and night as long as the products of light reactions (ATP and NADPH) are available. The term "dark" is somewhat misleading, as these reactions typically happen during daylight hours when light reactions are also occurring.

17. How do light reactions and dark reactions work together in photosynthesis?

Light reactions and dark reactions work together in a cycle. Light reactions capture light energy to produce ATP and NADPH, which are then used by dark reactions to fix carbon dioxide into glucose. The ADP and NADP+ produced in dark reactions are then recycled back to light reactions, continuing the cycle.

18. Can dark reactions occur without light reactions?

No, dark reactions cannot occur without light reactions. Dark reactions depend on the ATP and NADPH produced by light reactions to power the carbon fixation process. Without these energy-rich molecules from light reactions, dark reactions cannot proceed.

19. How does the location of light and dark reactions contribute to their efficiency?

The separation of light and dark reactions in different parts of the chloroplast contributes to their efficiency by allowing each process to occur in an optimized environment. Light reactions in the thylakoids can maximize light absorption, while dark reactions in the stroma have access to carbon dioxide and the enzymes needed for carbon fixation.

20. How does the rate of light reactions compare to dark reactions?

Light reactions generally occur much faster than dark reactions. Light reactions can take place in a matter of nanoseconds to milliseconds, while the Calvin cycle (dark reactions) takes several seconds to minutes to complete one turn. This difference in speed is one reason why dark reactions are often the limiting factor in photosynthesis rates.

21. What is the main difference between light reactions and dark reactions in photosynthesis?

The main difference is that light reactions require light energy and occur in the thylakoid membranes, while dark reactions don't require direct light and take place in the stroma of chloroplasts. Light reactions produce ATP and NADPH, which are then used in dark reactions to fix carbon dioxide into glucose.

22. Why are light reactions considered "endergonic" while dark reactions are "exergonic"?

Light reactions are considered endergonic because they require an input of energy (from light) to proceed. They store this energy in chemical bonds of ATP and NADPH. Dark reactions, on the other hand, are exergonic because they release energy stored in ATP and NADPH to drive the synthesis of glucose, which is energetically favorable.

23. How do environmental factors differently affect light and dark reactions?

Environmental factors affect light and dark reactions differently. Light intensity directly impacts light reactions, while temperature has a more significant effect on dark reactions. CO2 concentration primarily affects dark reactions, as it's the substrate for carbon fixation. Water availability affects both, but its splitting in light reactions is particularly crucial.

24. What is the relationship between light intensity and the rate of light and dark reactions?

Light intensity directly affects the rate of light reactions - as intensity increases, so does the rate of light reactions, up to a saturation point. Dark reactions, while not directly light-dependent, are indirectly affected as they rely on products from light reactions. However, dark reactions have a maximum rate limited by enzyme activity, so they may become the limiting factor at high light intensities.

25. What happens to excess energy from light reactions when dark reactions can't keep up?

When dark reactions can't keep up with the energy production from light reactions, plants have several mechanisms to dissipate excess energy. These include non-photochemical quenching, which releases energy as heat, and the xanthophyll cycle, which helps protect the photosystems from damage due to excess light energy.

26. How do plants adjust the balance between light and dark reactions?

Plants adjust the balance between light and dark reactions through various regulatory mechanisms. These include adjusting the number and activity of enzymes involved in each process, regulating the opening and closing of stomata to control CO2 intake, and modifying the distribution of light energy between photosystems. Long-term adaptations can also involve changes in leaf structure and chloroplast density.

27. What is the role of photosystems in light reactions?

Photosystems are protein complexes in the thylakoid membrane that are crucial for light reactions. They contain chlorophyll and other pigments that capture light energy. There are two types: Photosystem II, which splits water and initiates electron flow, and Photosystem I, which generates NADPH. Together, they drive the light-dependent production of ATP and NADPH.

28. What happens to the light and dark reactions during prolonged darkness?

During prolonged darkness, light reactions cease immediately as they require light energy. Dark reactions can continue for a short time using stored ATP and NADPH, but will eventually stop when these resources are depleted. Plants may then switch to respiration to maintain essential functions, using stored glucose until light becomes available again.

29. How do light and dark reactions contribute to the overall energy efficiency of photosynthesis?

Light reactions are relatively efficient at capturing light energy, converting about 27% of absorbed light into chemical energy. However, dark reactions are less efficient, with only about 3-6% of the original light energy ending up fixed in glucose. The overall low efficiency is due to various factors including reflection of light, heat loss, and limitations in the Calvin cycle.

30. What role do electron carriers play in connecting light and dark reactions?

Electron carriers, primarily NADP+/NADPH, play a crucial role in connecting light and dark reactions. In light reactions, NADP+ accepts electrons and protons to form NADPH. This NADPH then travels to the stroma where it provides reducing power for the dark reactions, converting back to NADP+ in the process. This cyclic transfer of electrons links the two stages of photosynthesis.

31. How do photorespiration and the dark reactions compete?

Photorespiration competes with dark reactions (Calvin cycle) for the enzyme RuBisCO. In high oxygen conditions, RuBisCO can fix oxygen instead of carbon dioxide, leading to photorespiration. This process produces no useful products and actually consumes energy, reducing the efficiency of photosynthesis. Plants have evolved various mechanisms to minimize photorespiration and favor the dark reactions.

32. What is the significance of the light-independent nature of dark reactions?

The light-independent nature of dark reactions allows plants to continue fixing carbon and producing glucose even when light is not directly available. This is crucial for maintaining metabolic processes during brief periods of shade or darkness. However, dark reactions still indirectly depend on light reactions for the supply of ATP and NADPH.

33. How do C4 plants modify their dark reactions compared to C3 plants?

C4 plants modify their dark reactions by separating initial carbon fixation from the Calvin cycle. They first fix CO2 into a 4-carbon compound in mesophyll cells, then transport this compound to bundle sheath cells where the Calvin cycle occurs. This spatial separation helps concentrate CO2 around RuBisCO, reducing photorespiration and improving efficiency in high-temperature, high-light environments.

34. How do light and dark reactions differ in their use of water?

Light reactions actively split water molecules (photolysis) to obtain electrons and protons, releasing oxygen as a byproduct. Dark reactions don't directly use water, but water molecules are incorporated into glucose during carbon fixation. The hydrogen in glucose comes from NADPH (ultimately from water), while the oxygen comes from CO2.

35. How do the energy requirements differ between light and dark reactions?

Light reactions require light energy to proceed and result in the production of energy-rich compounds (ATP and NADPH). Dark reactions, while not directly requiring light, use the chemical energy stored in ATP and NADPH to drive the endergonic process of carbon fixation. In essence, light reactions capture and convert energy, while dark reactions use this captured energy.

36. What is the role of carbon dioxide in light reactions versus dark reactions?

Carbon dioxide plays no direct role in light reactions. It is, however, the key substrate in dark reactions. In the Calvin cycle, CO2 is fixed (attached) to a 5-carbon sugar (ribulose bisphosphate) by the enzyme RuBisCO, initiating the process of glucose synthesis. The concentration of CO2 can be a limiting factor for the rate of dark reactions.

37. How do light and dark reactions contribute to the production of different types of sugars?

Light reactions don't directly produce sugars, but provide the energy (ATP) and reducing power (NADPH) necessary for sugar production. Dark reactions use these products to fix carbon dioxide into 3-carbon sugars, which are then combined to form glucose (a 6-carbon sugar). Further metabolic processes can convert glucose into other types of sugars like fructose or sucrose.

38. What is the significance of the cyclic nature of dark reactions (Calvin cycle)?

The cyclic nature of dark reactions (Calvin cycle) is significant because it allows for continuous carbon fixation without the need for constant input of new starting materials. The regeneration of the initial acceptor molecule (ribulose bisphosphate) ensures that the cycle can continue as long as ATP, NADPH, and CO2 are available, maximizing the efficiency of carbon fixation.

39. How do light and dark reactions differ in their sensitivity to temperature?

Light reactions are less sensitive to temperature changes as they are primarily driven by light energy. Dark reactions, being enzyme-catalyzed, are more temperature-sensitive. As temperature increases, the rate of dark reactions increases up to an optimum, after which enzyme denaturation can occur, rapidly decreasing the rate. This makes dark reactions often the limiting factor in photosynthesis at high temperatures.

40. What is the relationship between chlorophyll and the light and dark reactions?

Chlorophyll is crucial for light reactions, as it's the primary pigment that absorbs light energy to initiate the process. It's found in the photosystems of the thylakoid membranes. Dark reactions don't directly involve chlorophyll, but they depend on the products of light reactions. The amount and efficiency of chlorophyll can indirectly affect dark reactions by influencing the supply of ATP and NADPH.

41. How do light and dark reactions contribute to the production of oxygen in photosynthesis?

Oxygen production occurs exclusively during the light reactions of photosynthesis. Specifically, it's a byproduct of the splitting of water molecules (photolysis) in Photosystem II. Dark reactions don't produce oxygen; instead, they consume the products of light reactions (ATP and NADPH) to fix carbon dioxide into glucose.

42. What is the role of ATP in light reactions compared to dark reactions?

In light reactions, ATP is produced through chemiosmosis, driven by the proton gradient created by the electron transport chain. In dark reactions, this ATP is consumed to provide energy for the carbon fixation process. The continuous cycling of ATP production in light reactions and consumption in dark reactions links these two stages of photosynthesis.

43. How do plants balance the rates of light and dark reactions?

Plants balance the rates of light and dark reactions through various regulatory mechanisms. These include adjusting enzyme activities, controlling the distribution of light energy between photosystems, regulating CO2 intake through stomatal opening, and long-term adaptations in leaf structure and chloroplast density. This balance ensures efficient use of resources and prevents the accumulation of excess intermediates.

44. What happens when there's an imbalance between light and dark reactions?

When there's an imbalance between light and dark reactions, several issues can arise. If light reactions outpace dark reactions, excess energy can damage the photosystems (photoinhibition). Plants may dissipate this energy as heat or through other protective mechanisms. If dark reactions are faster, they may be limited by the supply of ATP and NADPH, reducing overall photosynthetic efficiency.

45. How do the products of light reactions drive the dark reactions?

The products of light reactions, ATP and NADPH, drive the dark reactions by providing the necessary energy and reducing power. ATP supplies the energy needed for carbon fixation and regeneration of the CO2 acceptor in the Calvin cycle. NADPH provides the electrons (reducing power) needed to convert fixed carbon into glucose. Without these inputs from light reactions, dark reactions cannot proceed.

46. What is the significance of the spatial separation of light and dark reactions in the chloroplast?

The spatial separation of light and dark reactions in the chloroplast allows for optimization of each process. Light reactions occur in the thylakoid membranes, where light can be efficiently captured by chlorophyll in the photosystems. Dark reactions occur in the stroma, where enzymes for the Calvin cycle are located and have access to CO2. This separation also allows for the generation of the proton gradient necessary for ATP synthesis.

47. How do light and dark reactions differ in their use of enzymes?

Light reactions primarily involve protein complexes (photosystems, electron transport chain components) rather than traditional enzymes. The main enzyme involved is ATP synthase. Dark reactions, on the other hand, are a series of enzyme-catalyzed reactions, with RuBisCO being the key enzyme. The reliance on enzymes makes dark reactions more sensitive to factors like temperature and pH than light reactions.

48. What is the role of the electron transport chain in connecting light and dark reactions?

The electron transport chain in light reactions connects to dark reactions by facilitating the production of ATP and NADPH. As electrons move through the chain, they drive proton pumping, creating a gradient that powers ATP synthesis. The chain also reduces NADP+ to NADPH. Both ATP and NADPH are then used in dark reactions for carbon fixation, thus linking the two processes.