Photosynthesis in Higher Plants

1. Chapter at a Glance

All animals, including humans, depend on plants for food. Green plants make their own food by photosynthesis: they use light energy to convert carbon dioxide and water into carbohydrates, releasing oxygen. Photosynthesis is important for two reasons. It is the primary source of all food on Earth, and it releases the oxygen that most living things breathe. In this chapter we follow the story of how scientists discovered photosynthesis, where it happens inside a leaf, which pigments trap light, how light energy becomes chemical energy (ATP and NADPH), how CO₂ is fixed into sugar in the Calvin cycle and the C₄ pathway, why photorespiration wastes energy, and which factors limit the rate. NEET asks about two questions from this chapter every year, very often on the Z-scheme, chemiosmosis, the first stable product of C₃ and C₄ plants, Kranz anatomy and photorespiration.

Photosynthesis is a redox process: water is oxidised (it loses electrons and releases O₂), and carbon dioxide is reduced to carbohydrate. It is also a physico-chemical process — light is trapped by pigments (physical) and used to drive chemical reactions.

2. Early Experiments: How Photosynthesis was Discovered

Our understanding of photosynthesis was built step by step by many scientists. NEET often asks "who showed what", so learn each discovery with its conclusion.

Simple school experiments. Before testing a leaf for starch, the plant is kept in the dark overnight (destarching), so any starch found later must have been made during the experiment. The leaf is boiled in alcohol to remove chlorophyll and then tested with iodine: starch turns blue-black.

• Light is needed: a leaf partly covered with black paper makes starch only in the uncovered part.
• Chlorophyll is needed: a variegated leaf (green and white patches, e.g. Croton or money plant) makes starch only in its green areas.
• CO₂ is needed (Moll's half-leaf experiment): part of a leaf is enclosed in a bottle or tube containing KOH, which absorbs CO₂. That part forms no starch, while the part outside does.

3. Where Does Photosynthesis Take Place?

• Photosynthesis takes place mainly in the green leaves, but also in other green parts such as young stems.
• Inside the leaf, the mesophyll cells (palisade and spongy) contain a large number of chloroplasts. The chloroplasts usually line the cell walls and arrange themselves to receive the best amount of light: they face the light when it is weak and turn edge-on when it is too strong.
• Stomata on the leaf surface let CO₂ in and O₂ out.

The reactions that do not directly need light are often called dark reactions, but this is misleading: they depend on the ATP and NADPH made in light, and they happen during the day. "Biosynthetic phase" or "carbon reactions" is a better name.

4. Pigments Involved in Photosynthesis

Paper chromatography of a leaf extract separates four pigments: chlorophyll a (bright or blue-green), chlorophyll b (yellow-green), xanthophylls (yellow) and carotenes (yellow to yellow-orange).

• Chlorophyll a is the chief pigment of photosynthesis. It is present in the reaction centre of both photosystems.
• Chlorophyll b, xanthophylls and carotenoids are accessory pigments. They absorb light of other wavelengths and pass the energy to chlorophyll a, widening the range of usable light. They also protect chlorophyll a from photo-oxidation.
• An absorption spectrum shows how much light of each wavelength a pigment absorbs. An action spectrum shows the rate of photosynthesis at each wavelength.
• The action spectrum closely follows the absorption spectrum of chlorophyll a, with peaks in the blue and red regions. This shows that chlorophyll a is the main pigment, though the match is not perfect because accessory pigments also contribute.
• Green light is mostly reflected, which is why leaves look green.

5. The Light Reaction (Photochemical Phase)

The light reaction includes light absorption, splitting of water, release of oxygen, and formation of the high-energy molecules ATP and NADPH.

• Pigments are organised into two discrete photochemical light-harvesting complexes (LHC), called Photosystem I (PS I) and Photosystem II (PS II). They are named in the order of their discovery, not the order in which they work.
• Each photosystem has many pigment molecules (an antenna) that pass absorbed energy to a single reaction centre molecule of chlorophyll a.
• In PS I the reaction centre absorbs best at 700 nm and is called P700. In PS II it absorbs best at 680 nm and is called P680.

The electron transport and the Z-scheme

1. PS II absorbs light (680 nm). Electrons in its reaction centre are excited and passed to a primary electron acceptor.
2. The electrons move downhill through an electron transport chain of cytochromes (plastoquinone → cytochrome b₆f complex → plastocyanin) to PS I. On the way, energy is released.
3. Electrons in PS I are excited again by light (700 nm) and passed to another acceptor with a still higher energy level.
4. They then pass downhill (through ferredoxin) to NADP⁺, which is reduced to NADPH + H⁺.

When the carriers are drawn in order of their redox potential, the path looks like the letter Z — hence the name Z-scheme.

Splitting of water (photolysis)

• PS II has lost electrons, so it must get replacements. These come from the splitting of water, which is linked to PS II.
• 2H₂O → 4H⁺ + O₂ + 4e⁻. The splitting happens on the inner side (lumen side) of the thylakoid membrane.
• The water-splitting complex needs manganese (Mn) and chloride (Cl⁻) ions.
• The oxygen released in photosynthesis is the by-product of this reaction — so O₂ comes from water.

Cyclic and non-cyclic photophosphorylation

Making ATP from ADP and inorganic phosphate using light energy is called photophosphorylation.

6. How is ATP Made? The Chemiosmotic Hypothesis

Peter Mitchell's chemiosmotic hypothesis explains how ATP is made in chloroplasts (and mitochondria). ATP synthesis is linked to the development of a proton gradient across the thylakoid membrane. Here the protons build up inside the thylakoid, in the lumen.

1. Splitting of water on the lumen side releases H⁺ into the lumen.
2. As electrons move through the photosystems, the primary acceptor (on the stroma side) passes them to an H carrier, which removes a proton from the stroma and releases it into the lumen.
3. NADP reductase sits on the stroma side; reducing NADP⁺ to NADPH uses up H⁺ from the stroma.

So H⁺ falls in the stroma and rises in the lumen, making the lumen more acidic. This gradient stores energy. The enzyme ATP synthase has two parts: CF₀, embedded in the membrane, forms a channel through which protons diffuse back to the stroma; CF₁ sticks out into the stroma. The flow of protons causes a change in CF₁ that makes ATP. Chemiosmosis therefore needs a membrane, a proton pump, a proton gradient and ATP synthase.

7. The Biosynthetic Phase: Calvin Cycle (C₃ Pathway)

ATP and NADPH from the light reaction are used to fix CO₂ into sugar in the stroma. Melvin Calvin used radioactive ¹⁴C in algae to show that the first stable product of CO₂ fixation is a 3-carbon acid, 3-phosphoglyceric acid (PGA). Plants that fix CO₂ this way are called C₃ plants. Calvin cycle happens in all photosynthetic plants, whether they are C₃ or C₄.

1. Carboxylation: CO₂ combines with the 5-carbon acceptor ribulose-1,5-bisphosphate (RuBP), giving two molecules of 3-PGA. The enzyme is RuBisCO (RuBP carboxylase-oxygenase), the most abundant enzyme on Earth.
2. Reduction: 3-PGA is reduced to a triose sugar, using 2 ATP and 2 NADPH per CO₂ fixed.
3. Regeneration: some triose is used to regenerate RuBP so the cycle can continue. This needs 1 ATP per CO₂.

The cyclic flow of electrons explains why plants need more ATP than NADPH: the Calvin cycle needs ATP and NADPH in a 3 : 2 ratio, and cyclic photophosphorylation supplies the extra ATP.

8. The C₄ Pathway (Hatch and Slack Pathway)

Plants adapted to dry, hot tropical regions, such as maize, sugarcane and sorghum, use a special pathway. Their first stable product is a 4-carbon acid, oxaloacetic acid (OAA), so they are called C₄ plants. They still use the Calvin cycle as their main biosynthetic pathway.

• Kranz anatomy ("Kranz" means wreath): large bundle sheath cells form a ring around the vascular bundles. They have many chloroplasts, thick walls impermeable to gas exchange, and no intercellular spaces.
• In the mesophyll cells, the primary CO₂ acceptor is the 3-carbon phosphoenol pyruvate (PEP), and the enzyme is PEP carboxylase (PEPcase). Mesophyll cells lack RuBisCO.
• OAA is converted into other 4-carbon acids such as malic acid or aspartic acid, which move into the bundle sheath cells.
• In the bundle sheath, the C₄ acid is broken down, releasing CO₂ and a 3-carbon molecule. The CO₂ enters the Calvin cycle there (bundle sheath cells are rich in RuBisCO but lack PEPcase).
• The 3-carbon molecule (pyruvate) returns to the mesophyll and is converted back into PEP, using ATP.

9. Photorespiration

RuBisCO has a much greater affinity for CO₂ than for O₂, but it can bind either — binding is competitive, and the relative concentration of O₂ and CO₂ decides which one wins.

• When O₂ is high (or CO₂ low), RuBisCO acts as an oxygenase: RuBP + O₂ → one molecule of PGA + one molecule of phosphoglycolate (2-carbon).
• The phosphoglycolate is processed through a pathway involving chloroplast, peroxisome and mitochondrion, releasing CO₂.
• In photorespiration there is no synthesis of sugar, ATP or NADPH. Instead, ATP is used and CO₂ is released. It is therefore a wasteful process.
• It happens in C₃ plants, especially on hot, dry, bright days when stomata close and O₂ builds up inside the leaf.
• C₄ plants do not show photorespiration: their bundle sheath cells concentrate CO₂ around RuBisCO, so it works as a carboxylase. This is why C₄ plants are more productive and tolerate higher temperatures.

10. Factors Affecting Photosynthesis

The rate of photosynthesis depends on internal factors (number, size, age and orientation of leaves; mesophyll cells and chloroplasts; internal CO₂ concentration; amount of chlorophyll — all decided by the plant's genes and growth) and external factors (sunlight, temperature, CO₂ concentration and water).

Blackman's law of limiting factors (1905): if a chemical process is affected by more than one factor, its rate is decided by the factor that is nearest to its minimal value — the factor that directly affects the process if its quantity is changed. For example, a plant with plenty of light and water but very low CO₂ cannot photosynthesise faster no matter how bright the light becomes.

• Light: at low intensity, the rate rises in direct proportion to light. At higher intensity, it levels off as other factors become limiting. Very strong light can damage chlorophyll (photo-oxidation). Light is rarely limiting in nature, except for plants in shade or in dense forests.
• CO₂: it is the major limiting factor, because atmospheric CO₂ is only about 0.03–0.04%. Raising CO₂ up to about 0.05% increases the rate; beyond that it can be harmful over long periods. C₄ plants saturate at about 360 μL L⁻¹, while C₃ plants keep responding above 450 μL L⁻¹. Growers use this in greenhouses: tomatoes and bell peppers are grown in CO₂-enriched air for higher yields.
• Temperature: the dark reactions are enzyme-controlled and so depend on temperature, while light reactions are much less sensitive. C₄ plants have a higher optimum temperature than C₃ plants.
• Water: water affects photosynthesis mostly indirectly. Water stress closes stomata, reducing CO₂ supply, and wilting reduces leaf surface area and metabolic activity.

11. Master Summary Table

12. Mind Map

13. Tips and Tricks

14. Important One-liners

• The first action spectrum of photosynthesis was described by Engelmann using Cladophora.
• Chlorophyll a is the primary photosynthetic pigment.
• Accessory pigments protect chlorophyll a from photo-oxidation.
• The reaction centre of PS II is P680 and that of PS I is P700.
• Photolysis of water needs manganese and chloride ions.
• Stroma lamellae lack PS II and NADP reductase, so only cyclic flow occurs there.
• RuBisCO is the most abundant enzyme in the world.
• The CO₂ acceptor in the Calvin cycle is RuBP; in C₄ mesophyll cells it is PEP.
• Kranz anatomy is a feature of C₄ plants.
• Photorespiration does not produce ATP or NADPH.
• Tomatoes and bell peppers are grown in CO₂-enriched greenhouses for higher yield.

15. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

16. Quick Revision Summary