Photosynthesis is the process by which green plants use light energy to make organic food from carbon dioxide and water. It supplies food for plants and, directly or indirectly, for most other organisms. Oxygen is released during the process.
A simplified overall equation is:
6CO₂ + 12H₂O + light energy → C₆H₁₂O₆ + 6O₂ + 6H₂O
This equation summarises the process; photosynthesis takes place through many linked reactions rather than one step. The oxygen released comes from water.
Table of Contents
Where Does Photosynthesis Take Place?
Photosynthesis occurs mainly in the chloroplasts of green plant cells. Inside a chloroplast:
- Thylakoid membranes contain the pigments and systems needed to capture light energy.
- Grana are stacks of thylakoids.
- Stroma is the fluid region where the Calvin cycle takes place.
Photosynthetic Pigments
Pigments absorb light of different wavelengths.
- Chlorophyll a is the main reaction-centre pigment.
- Chlorophyll b and carotenoids act as accessory pigments. They absorb additional light and help transfer energy to the reaction centre.
A photosystem contains many pigment molecules associated with proteins. Photosystem II has a reaction centre called P680, while Photosystem I has P700.
Two Major Phases of Photosynthesis
| Phase | Main location | Main result |
|---|---|---|
| Light reactions | Thylakoid membranes | Light energy is used to make ATP and NADPH; oxygen is released |
| Calvin cycle | Stroma | CO₂ is used to make carbohydrate with the help of ATP and NADPH |
The Calvin cycle does not directly require light, but it depends on ATP and NADPH supplied by the light reactions.
Light Reactions
In the light reactions, pigments absorb light and excite electrons. Water is split to replace electrons lost from Photosystem II. This splitting of water releases oxygen, electrons and hydrogen ions.
Electrons move through an electron transport pathway. Their energy helps build a hydrogen-ion gradient across the thylakoid membrane. The resulting movement of hydrogen ions through ATP synthase drives ATP formation. This is the chemiosmotic mechanism.
Non-cyclic Photophosphorylation
Electrons move from water through Photosystem II and Photosystem I and ultimately help reduce NADP⁺ to NADPH.
Main products: ATP, NADPH and O₂.
Cyclic Photophosphorylation
Electrons excited from Photosystem I return through an electron transport pathway to the same photosystem.
Main product: ATP. This route does not produce NADPH or release oxygen.
Calvin Cycle: The C₃ Pathway
The Calvin cycle uses CO₂, ATP and NADPH in the chloroplast stroma. It has three broad stages:
- Carboxylation: CO₂ combines with the five-carbon acceptor RuBP. The enzyme RuBisCO catalyses this step. The first stable product is a three-carbon compound, 3-PGA.
- Reduction: ATP and NADPH help convert the fixed carbon into three-carbon sugars.
- Regeneration: Some of the products are used to regenerate RuBP so the cycle can continue.
It is called the C₃ pathway because its first stable product has three carbon atoms.
C₄ Pathway
Some plants, such as maize and sugarcane, use a CO₂-concentrating pathway before the Calvin cycle. They are called C₄ plants because their first stable carbon-fixation product has four carbon atoms.
In the mesophyll cells, the enzyme PEP carboxylase first fixes CO₂ into a four-carbon compound. Carbon is then transported to the bundle sheath cells, where CO₂ is released for use in the Calvin cycle. This arrangement helps maintain a high CO₂ concentration around RuBisCO.
Many C₄ leaves show Kranz anatomy, with prominent bundle sheath cells surrounding the vascular bundles.
| Feature | C₃ plants | C₄ plants |
|---|---|---|
| First stable product of initial CO₂ fixation | Three-carbon 3-PGA | Four-carbon compound |
| Initial CO₂-fixing enzyme | RuBisCO | PEP carboxylase |
| Distinct CO₂-concentrating step | Absent | Present |
| Photorespiration | Can be significant | Greatly reduced |
Remember: C₄ plants still use the Calvin cycle to make carbohydrates. Their C₄ pathway concentrates CO₂ before it reaches that cycle.
Photorespiration
RuBisCO can bind oxygen instead of CO₂. This starts photorespiration, a process that consumes energy and reduces the efficiency of carbon fixation. It becomes more likely when CO₂ availability is low relative to oxygen.
The CO₂-concentrating mechanism in C₄ plants greatly limits photorespiration.
Factors Affecting Photosynthesis
The rate of photosynthesis depends on several factors:
- Light: Intensity and wavelength affect how much useful energy is available.
- Carbon dioxide: A low CO₂ supply can restrict carbon fixation.
- Temperature: Photosynthetic reactions depend on enzymes, whose activity changes with temperature.
- Water: Shortage of water can cause stomata to close, reducing CO₂ entry.
When several factors act together, the factor in shortest effective supply may limit the overall rate. This is the law of limiting factors.
Quick Revision
- Chloroplast: Main site of photosynthesis in green plant cells.
- Thylakoid membrane: Site of light reactions.
- Stroma: Site of the Calvin cycle.
- ATP and NADPH: Products of light reactions used for carbon fixation.
- RuBisCO: Catalyses CO₂ fixation in the Calvin cycle.
- PEP carboxylase: Initially fixes CO₂ in C₄ mesophyll cells.
- Photorespiration: Begins when RuBisCO acts on oxygen rather than CO₂.
Check Your Understanding
- From which substance does the oxygen released in photosynthesis come?
- Where do the light reactions take place?
- Which two energy-rich products of the light reactions are used in the Calvin cycle?
- Why is the Calvin cycle called the C₃ pathway?
- How do C₄ plants reduce photorespiration?
Answers: (1) Water. (2) Thylakoid membranes. (3) ATP and NADPH. (4) The first stable product of CO₂ fixation has three carbon atoms. (5) They concentrate CO₂ around RuBisCO in bundle sheath cells.
For all chapters, visit the CBSE Class 11 Biology – Complete Study Guide.
Frequently Asked Questions
What is photosynthesis?
It is the process by which plants use light energy to make organic compounds from carbon dioxide and water.
Where does photosynthesis occur?
It takes place mainly in chloroplasts, especially in green leaf cells.
What is the role of chlorophyll?
Chlorophyll absorbs light energy used to drive photosynthesis.
Where do the light reactions occur?
They occur in the thylakoid membranes of chloroplasts.
What are the main products of the light reactions?
ATP, NADPH and oxygen are produced.
Where does the oxygen released in photosynthesis come from?
It comes from the splitting of water.
What is photophosphorylation?
It is the formation of ATP using energy captured from light.
How does cyclic photophosphorylation differ from non-cyclic?
Cyclic electron flow mainly produces ATP; non-cyclic flow produces ATP and NADPH and releases oxygen.
What is the Calvin cycle?
It is a series of reactions in the chloroplast stroma that fixes CO₂ and uses ATP and NADPH to make carbohydrates.
What is RuBisCO?
It is the enzyme that catalyses CO₂ fixation to RuBP in the Calvin cycle.
What is the first stable product of the C3 pathway?
It is 3-phosphoglycerate, a three-carbon compound.
What is the first stable product of the C4 pathway?
It is oxaloacetate, a four-carbon compound.
What is Kranz anatomy?
It is the arrangement of mesophyll and prominent bundle-sheath cells characteristic of many C4 leaves.
What is photorespiration?
It is a process initiated when RuBisCO binds oxygen rather than carbon dioxide, reducing the efficiency of carbon fixation.
Which factors influence photosynthetic rate?
Light, carbon dioxide concentration, temperature and water availability can influence it.