Photosynthesis in higher plants.
PHOTOSYNTHESIS IN HIGHER PLANTS
The Basics: Where and How?
Site: Mesophyll cells in leaves contain Chloroplasts.
Pigments: Chlorophyll a (the main pigment), Chlorophyll b, Xanthophylls, and Carotenoids.
Equation:
$$6CO_2 + 12H_2O \xrightarrow{\text{Light}} C_6H_{12}O_6 + 6H_2O + 6O_2$$
1. The Light-Dependent Reaction (Photochemical Phase)
This happens in the Thylakoids (Grana). It requires direct sunlight to produce high-energy intermediates: ATP and NADPH.
Key Steps:
Light Absorption: Chlorophyll absorbs light at specific wavelengths (Absorption Spectrum).
Water Splitting (Photolysis): Water is broken down into protons, electrons, and oxygen. This is the source of the oxygen we breathe!
Photophosphorylation: The process of creating ATP using light.
Non-Cyclic (Z-Scheme): Involves both PS II and PS I. It produces both ATP and NADPH.
Cyclic: Involves only PS I. It produces only ATP.
2. The Light-Independent Reaction (Biosynthetic Phase)
Commonly called the Dark Reaction, but don't let the name fool you—it doesn't need "darkness," it just doesn't need direct light. It happens in the Stroma.
The Calvin Cycle ($C_3$ Pathway)
Every photosynthetic plant uses this cycle. It has three main stages:
Carboxylation: $CO_2$ is fixed by the enzyme RuBisCO (the most abundant protein on Earth).
Reduction: ATP and NADPH are used to convert the fixed carbon into sugar (Glucose).
Regeneration: The starting molecule (RuBP) is recreated so the cycle can continue.
Note: To make one molecule of glucose, the Calvin cycle must turn 6 times.
3. The $C_4$ Pathway (Hatch-Slack Pathway)
Some plants (like Maize and Sorghum) live in dry, tropical areas. They’ve evolved a "specialized" anatomy called Kranz Anatomy to avoid a wasteful process called Photorespiration.
They use two types of cells: Mesophyll cells and Bundle sheath cells.
They are much more efficient at high temperatures and low $CO_2$ levels than $C_3$ plants.
Factors Affecting Photosynthesis
Light Intensity: Higher intensity increases the rate until another factor becomes limiting.
$CO_2$ Concentration: This is usually the major limiting factor in nature.
Temperature: Dark reactions are enzymatic and thus highly temperature-sensitive.
Water: Stress causes stomata to close, reducing $CO_2$ availability.
Quick Comparison: $C_3$ vs $C_4$ Plants
| Feature | C3 Plants | C4 Plants |
| First Stable Product | 3-PGA (3-carbon) | OAA (4-carbon) |
| Optimum Temp | $20\text{--}25^\circ\text{C}$ | $30\text{--}45^\circ\text{C}$ |
| Photorespiration | High (Wasteful) | Negligible (Efficient) |
| Examples | Wheat, Rice, Soybeans | Maize, Sugarcane |
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