Photosynthesis

Chapter in a nutshell: Photosynthesis is the process by which green plant cells containing chlorophyll manufacture food (glucose and starch) from carbon dioxide and water, using light energy, and release oxygen as a by-product. It occurs inside chloroplasts in two linked phases — a light-dependent (photochemical) phase in the thylakoids and a light-independent (biosynthetic / dark) phase in the stroma. As the original source of all food and free atmospheric oxygen, it supports virtually all life on Earth.

1. What is Photosynthesis? Definition & Importance

Photosynthesis is the process by which living plant cells, containing chlorophyll, produce food substances (glucose and starch) from carbon dioxide and water, by using light energy. Plants release oxygen as a waste product.

The balanced overall equation is:

6CO₂ + 12H₂O → (light energy / chlorophyll) → C₆H₁₂O₆ + 6H₂O + 6O₂↑

Note that 12 water molecules are used and 6 are re-formed during the process, so water appears both as a raw material and a product. The first stable food product is the simple sugar glucose (C₆H₁₂O₆).

Importance of photosynthesis:

• Food for all — it is the ultimate source of energy and food for every living organism: directly for plants, indirectly (via food chains) for all animals and humans.
• Oxygen to breathe — it is the only biological process that releases free oxygen into the atmosphere, supporting respiration in nearly all organisms.
• Up to 90% of all photosynthesis on Earth takes place in the oceans (by phytoplankton and aquatic plants).

"If there were no green plants, all life on the earth would come to an end."

2. The Chloroplast — Site of Photosynthesis

The entire photosynthetic machinery lies in chloroplasts — green, oval, double-membrane-bound cell organelles discovered by Robert Hill. There may be 40–50 chloroplasts per cell and over 500,000 per sq. mm of leaf surface.

Internal structure (Fig. 6.1):

3. Chlorophyll — The Vital Plant Pigment

Chlorophyll (chloro = green, phyll = leaf) is the green colouring matter contained in the walls of the thylakoids. It is a complex substance made of carbon, hydrogen, oxygen, nitrogen and magnesium.

• There are nine types; chlorophyll-a and chlorophyll-b are the most abundant and best known.
• Chlorophyll absorbs light at both ends of the visible spectrum — blue and red light — which are most effective for photosynthesis, and reflects green light, which is why plants appear green. Green light is least effective.
• Too much light destroys chlorophyll, yet its formation depends on light exposure (grass under a stone turns yellowish).

Carotenoids (carotenes – red; xanthophylls – yellow) are accessory pigments that absorb and transfer light energy to chlorophyll and protect chlorophyll from photo-oxidation. They are usually masked by the green chlorophyll.

4. Stomata — Letting in CO₂

Stomata (sing. stoma, meaning "mouth") are minute openings, present in large numbers on the lower surface of the leaf, that allow CO₂ in and O₂ out. Each stoma is bordered by two guard cells with a thick inner wall (facing the pore) and a thin outer wall; guard cell cytoplasm contains chloroplasts.

When stomata open, gas exchange occurs but water is also lost — hence "transpiration is the price the plant pays for photosynthesis."

Theories of stomatal opening/closing:

5. Mechanism — The Two Phases of Photosynthesis

Photosynthesis is essentially an oxidation–reduction process: water is oxidised to oxygen, and carbon dioxide is reduced to carbohydrate. It occurs in two distinct but linked phases.

A. Light-Dependent (Photochemical / Light) Phase

• Occurs in the thylakoids (grana) of the chloroplast; light plays the key role.
• Step I — Activation of chlorophyll: chlorophyll absorbs photons (smallest unit of light energy) and becomes activated.
• Step II — Photolysis of water: the absorbed energy splits water into hydrogen ions, electrons and oxygen.

2H₂O → (energy of 4 photons) → 4H⁺ + 4e⁻ + O₂

Photolysis = splitting of H₂O molecules into hydrogen ions and oxygen in the presence of light and grana.

End results of photolysis:

B. Light-Independent (Biosynthetic / Dark) Phase

• Occurs in the stroma; does not require light (occurs simultaneously with the light reaction, the gap being less than one-thousandth of a second).
• The new term light-independent is preferred because the "dark" reaction does not occur only at night — it merely does not depend on light.
• The hydrogen of NADPH combines with CO₂, using ATP energy, to form glucose (C₆H₁₂O₆).
• CO₂ is fixed by the acceptor RuBP (Ribulose bisphosphate), catalysed by the enzyme Rubisco; RuBP is regenerated at the cycle's end. This is the Calvin cycle.

Polymerisation: several glucose molecules join to form starch (insoluble), for temporary storage; some glucose becomes sucrose, oils, etc.

6. End Products & Fate of Carbohydrates

There are three end products: glucose, water and oxygen.

Photorespiration is the uptake of oxygen and release of CO₂ in the light by photosynthesising tissue, by which a part of the assimilated carbon is lost.

7. Adaptations in a Leaf for Photosynthesis

1. Large surface area for maximum light absorption.
2. Leaf arrangement at right angles to light for maximum exposure.
3. Transparent cuticle & upper epidermis to let light through.
4. Numerous stomata for rapid gas exchange.
5. Thinness of the leaf reduces transport distance between cells.
6. Chloroplasts concentrated in the upper layers of the leaf.
7. Extensive vein system for rapid transport to and from mesophyll.

8. Factors Affecting Photosynthesis

External factors: (i) Light intensity — rate rises with light up to a limit, then stabilises at the level fixed by CO₂ (e.g. stabilises at 0.02% CO₂; raising CO₂ to 0.05% raises the ceiling). (ii) CO₂ concentration — higher CO₂ allows a higher maximum rate. (iii) Temperature — rate rises to an optimum of 35°C (a 10°C rise up to 35°C doubles the rate), falls beyond, and enzymes are destroyed above 40°C. (iv) Water content — scarcity closes stomata, reducing CO₂ intake; only ~1% of absorbed water is used in photosynthesis.

Internal factors: (i) Chlorophyll — mineral/nutrient deficiency reduces it. (ii) Protoplasm — dehydration or accumulation of carbohydrates lowers the rate. (iii) Structure of leaf — cuticle thickness, stomatal distribution and leaf size affect light and CO₂ entry.

9. The Carbon Cycle

The carbon cycle is the series of reactions by which carbon (as CO₂, ~0.03% of air) is removed from the air, used by organisms, and returned. Steps: (i) Photosynthesis — producers fix CO₂ into carbohydrates; (ii) Food chains — carbon passes to consumers; (iii) Respiration — plants/animals oxidise food, returning CO₂; (iv) Decay — decomposers (bacteria, fungi) break down dead remains; (v) Combustion — burning wood/fossil fuels; (vi) Burning of limestone in lime kilns also releases CO₂.

Worked / Structured Examples

Example 1 — Trace the path of hydrogen. Water → photolysis (light + grana) → H⁺ ions → picked up by NADP → NADPH → carried to stroma → combines with CO₂ (using ATP) → glucose. Water is the only source of hydrogen ions.

Example 2 — Correct logical sequence (common exam item). (i) photons → (ii) grana → (iii) water molecules → (iv) hydrogen and hydroxyl ions → (v) oxygen.

Example 3 — Iodine (starch) test for a leaf. (1) Dip leaf in boiling water ~1 min to kill cells; (2) boil in methylated spirit over a water bath to remove chlorophyll (leaf turns pale-white, hard/brittle); (3) soften in hot water; (4) spread in a dish and add iodine solution. Starch-containing regions turn blue-black; starchless regions stay brown.

Example 4 — Variegated leaf (Coleus) experiment. Destarch a variegated plant, expose to sun, then do the iodine test. Only the green parts turn blue-black, the non-green (white) parts stay brown → proves chlorophyll is necessary for photosynthesis.

Example 5 — Half-leaf KOH experiment (Fig. 6.7). Insert half a destarched leaf into a flask containing potassium hydroxide (absorbs CO₂); keep in sunlight; test for starch. The half exposed to air turns blue-black; the half inside the CO₂-free flask does not → proves CO₂ is necessary.

Example 6 — Oxygen evolution (Fig. 6.8). Place Hydrilla/Elodea under an inverted funnel and test-tube in pond water in sunlight. Gas bubbles collect; a glowing splinter bursts into flame, showing the gas is oxygen.

Key Terms — Quick Glossary

Common Mistakes to Avoid

• "Dark reaction occurs at night." Wrong — it is light-independent; it occurs in daytime alongside the light reaction, simply not needing light directly.
• "Oxygen in glucose comes from CO₂." The oxygen released comes from water (photolysis), not from CO₂.
• "Chloroplast and chlorophyll are the same." No — chlorophyll is the pigment; the chloroplast is the organelle containing it.
• "Oxygen is a requirement for photosynthesis." No — oxygen is a by-product; the requirements are light, chlorophyll, CO₂ and water.
• "Immediate product is starch." The first product is glucose; starch forms later by polymerisation.
• Confusing the sites. Light reaction → grana/thylakoids; dark reaction → stroma. Photolysis needs light and grana (not temperature).

Likely Exam Questions (with crisp answers)

1. Define photosynthesis. Process by which chlorophyll-containing plant cells make glucose/starch from CO₂ and water using light energy, releasing oxygen.
2. Write the balanced equation. 6CO₂ + 12H₂O → (light/chlorophyll) → C₆H₁₂O₆ + 6H₂O + 6O₂↑.
3. What is the source of oxygen released? Water, via photolysis.
4. Where do the light and dark reactions occur? Light → grana (thylakoids); dark → stroma.
5. Define photolysis. Splitting of water into H⁺ ions and oxygen in the presence of light and grana.
6. What is photophosphorylation? Formation of ATP from ADP + Pi using light energy during the light reaction.
7. Name the CO₂ acceptor and the enzyme. RuBP (Ribulose bisphosphate); enzyme Rubisco.
8. Why is starch (not glucose) used to measure photosynthesis? Glucose formed soon gets converted into starch, which is easily detected by the iodine test.
9. Why destarch a plant before an experiment? To remove existing starch so that any starch formed must be due to the experiment.
10. Which colours of light are most/least effective? Most — blue and red; least — green (reflected by chlorophyll).
11. State the optimum temperature. About 35°C; enzymes are destroyed above 40°C.
12. What is the role of light energy? To activate chlorophyll (and split water).
13. Why is "transpiration the price plant pays for photosynthesis"? Open stomata that admit CO₂ also let water escape as transpiration.
14. What are the three end products and their fates? Glucose (used/stored as starch/converted), water (reused), oxygen (some respired, most released).
15. Function of carotenoids? Absorb light and transfer energy to chlorophyll; protect chlorophyll from photo-oxidation.
16. What is the functional light-absorbing unit of the chloroplast? The quantasome.
17. Name the recent theory of stomatal opening. K⁺ ion concentration theory.