Maarg Manthan · Topic 4.1

Photosynthesis: Light Reaction and Dark Reaction

Science & Technology › Plant Biology and Nutrition · Topic 4.1

Photosynthesis is the process by which green plants, algae and some bacteria use light energy to make carbohydrates from carbon dioxide and water, and release oxygen. The overall reaction is six molecules of carbon dioxide plus six of water giving one of glucose and six of oxygen, in the presence of light and chlorophyll. It occurs in chloroplasts and has two stages. In the light reaction, in the thylakoid membranes of the grana, light splits water, releases oxygen and makes ATP and NADPH. In the dark reaction, or Calvin cycle, in the stroma, these products are used to fix carbon dioxide into sugar with the enzyme RuBisCO. Photosynthesis is the source of nearly all food and of the oxygen in the atmosphere.

5 min read · UPSC Prelims · Science & Technology · Plant Biology and Nutrition

At a Glance

Site Chloroplast
Light reaction Grana; makes ATP, NADPH, oxygen
Dark reaction Stroma; Calvin cycle
Oxygen comes from Splitting of water
Key enzyme RuBisCO
Exam link UPSC Prelims, Science & Technology: Plant Biology and Nutrition

Where Does It Fit?

This is the first topic of the macro on plant biology and nutrition. It describes how plants make food, which is the starting point for the topics on water, nutrition and respiration in plants. Previous topic: Transport System in Plants and in a Single Cell. Next topic: Plant Water Regulation (Transpiration and Osmosis).

What Happens in the Light and Dark Reactions?

Stage	Details
Light reaction	Occurs in the thylakoids. Chlorophyll absorbs light and excites electrons. Water is split by photolysis, releasing oxygen, protons and electrons. Electron transport makes ATP by photophosphorylation and NADPH
Dark reaction (Calvin cycle)	Occurs in the stroma and does not need light directly. Carbon dioxide combines with ribulose bisphosphate, catalysed by RuBisCO, to form a three-carbon compound, 3-phosphoglycerate, which is reduced to sugar using ATP and NADPH. Melvin Calvin received the Nobel Prize in 1961
Pigments	Chlorophyll a is the main pigment. Chlorophyll b and carotenoids are accessory pigments. Chlorophyll absorbs red and blue light and reflects green
Source of oxygen	Oxygen comes from water, not from carbon dioxide, as shown with labelled oxygen
Products	Glucose, which is stored as starch, and oxygen released through stomata
Energy conversion	Light energy becomes chemical energy. Only about 1 to 2 per cent of incoming sunlight is stored by typical plants

How Do C3, C4 and CAM Plants Differ?

C3 plants: the first product is a three-carbon compound. Wheat, rice, soybean and most trees are C3 plants. They lose some carbon through photorespiration in hot, dry conditions.
C4 plants: the first product is a four-carbon compound, made by the enzyme PEP carboxylase in mesophyll cells and then passed to bundle sheath cells, which have Kranz anatomy. Maize, sugarcane, sorghum and millets are C4 plants, which are more efficient in heat and strong light and have little photorespiration. Hatch and Slack described the pathway.
CAM plants: Crassulacean acid metabolism lets stomata open at night to take in carbon dioxide and close in the day, which saves water. Pineapple, cacti and Kalanchoe are CAM plants.
Photorespiration: RuBisCO takes up oxygen instead of carbon dioxide, which wastes energy, and it is a target of crop research.
Compensation point: the light level at which photosynthesis equals respiration, so there is no net gain of oxygen.

What Else Should You Remember?

History: Joseph Priestley showed in 1774 that plants restore air that a candle has used. Jan Ingenhousz showed that light is needed, Julius von Sachs showed that starch is made in leaves, and Engelmann used algae and bacteria to show that red and blue light work best.
Limiting factors: Blackman’s law says that the rate is limited by the factor in shortest supply, among light, carbon dioxide, temperature and water.
Carbon dioxide level: the atmosphere has about 0.04 per cent carbon dioxide, and many greenhouses raise it to increase yield.
Bacterial photosynthesis: purple and green sulphur bacteria use light but do not release oxygen, and cyanobacteria release oxygen, as in Prokaryotes: Bacteria and Archaea.
Link to respiration: photosynthesis stores energy while respiration releases it, as in Cellular Respiration and Energy Metabolism (ATP Production).
Carbon cycle and climate: forests and oceans take up carbon dioxide through photosynthesis, which makes plants and algae important in climate change.
Algae: phytoplankton in the oceans make about half of the oxygen produced each year.
Artificial photosynthesis: research aims to make fuels such as hydrogen from sunlight and water, as in clean energy work.

Why Does It Matter for the Exam?

Stage and site questions: what happens in grana and stroma, and what the light reaction produces.
Plant-type questions: examples of C3, C4 and CAM plants and why C4 is efficient.
Agriculture link: research on making rice a C4 plant to raise yields.
Environment link: carbon sequestration, blue carbon and the role of phytoplankton.
Common errors: saying that the dark reaction occurs only in the dark or that oxygen comes from carbon dioxide.

As an analytical point, photosynthesis connects sunlight to the food chain. Small improvements in its efficiency could raise crop yields without more land or water.

Why Is This Topic Important?

Facts Stages, sites, pigments asked
Types C3, C4, CAM
Link Crops and climate

What Came Next?

The next topic covers plant water regulation through transpiration and osmosis. Next topic: Plant Water Regulation (Transpiration and Osmosis).

Key People and Terms

Chlorophyll Green pigment that traps light
Photolysis Splitting of water by light
Calvin cycle Carbon fixing reactions
RuBisCO Carbon dioxide fixing enzyme
Kranz anatomy Leaf structure of C4 plants
Photorespiration Oxygen uptake by RuBisCO

Exam Corner

Points to Remember

Light reaction in grana; Calvin cycle in stroma.
Oxygen comes from water.
Maize, sugarcane are C4 plants.
Pineapple, cacti are CAM plants.
Calvin: Nobel 1961.

Do Not Confuse With

C3 and C4 plants 3-carbon versus 4-carbon first product
Photosynthesis and respiration Stores versus releases energy
Light and dark reactions Thylakoid versus stroma

Memory Hook

Grana for the light, stroma for the Calvin cycle; oxygen from water, C4 for maize and cane, CAM at night for cacti’s gain.

Mains Angle

Questions ask about the process and its importance. These points add depth.

Process Two stages
Types C3, C4, CAM
Role Food, oxygen, carbon

A Question You May Face

An original practice question, not a past paper question.

“Explain the light and dark reactions of photosynthesis and discuss the adaptations of C4 and CAM plants.”

How to Answer

Site Chloroplast.
Light reaction ATP, NADPH.
Calvin cycle Sugar.
Types C4, CAM.

GS Relevance

Prelims: stages, sites, pigments, C3, C4 and CAM plants, limiting factors, scientists. GS Paper 3: science and technology, agriculture.

Frequently Asked Questions

What are the two stages of photosynthesis?

Photosynthesis has a light reaction and a dark reaction. In the light reaction, in the thylakoids of the grana, light splits water and makes ATP, NADPH and oxygen. In the dark reaction, the Calvin cycle in the stroma, ATP and NADPH are used to turn carbon dioxide into sugar with the enzyme RuBisCO.

Where does the oxygen released in photosynthesis come from?

The oxygen released in photosynthesis comes from the splitting of water in the light reaction, called photolysis, and not from carbon dioxide. This was shown by experiments with water labelled with a heavy isotope of oxygen, and the oxygen leaves the plant through the stomata.

What is the difference between C3 and C4 plants?

In C3 plants, such as wheat and rice, the first stable product of carbon fixation is a three-carbon compound, and they lose carbon through photorespiration in heat. In C4 plants, such as maize and sugarcane, a four-carbon compound forms first in mesophyll cells and is moved to bundle sheath cells, so they are more efficient in hot and bright conditions.

What are CAM plants?

CAM plants, such as pineapple, cacti and Kalanchoe, use Crassulacean acid metabolism, in which stomata open at night to take in carbon dioxide, which is stored as an organic acid, and close in the day when the carbon dioxide is used for photosynthesis. This saves water and suits dry habitats.

What is RuBisCO?

RuBisCO, or ribulose bisphosphate carboxylase-oxygenase, is the enzyme that fixes carbon dioxide in the Calvin cycle by attaching it to ribulose bisphosphate. It is the most abundant enzyme on Earth, but it can also take up oxygen, which causes photorespiration and reduces the efficiency of photosynthesis in C3 plants.

PYQ Practice — Statement Analysis

1 The Calvin cycle takes place in the stroma of the chloroplast.

True

It fixes carbon dioxide.

2 The oxygen released in photosynthesis comes from carbon dioxide.

False

It comes from water.

3 Maize is a C4 plant.