Learning Outcomes
- Understand the process of photosynthesis and its significance for life on earth.
- Grasp the structure of the photosynthetic apparatus in plants.
- Comprehend the role of light in photosynthesis and the conversion of light energy into chemical energy.
- Learn the historical experiments that led to the discovery of photosynthesis.
- Know the differences between C3 and C4 pathways.
Photosynthesis is essential to life on Earth. Green plants, termed autotrophs, can produce their own food through photosynthesis, while other organisms, termed heterotrophs, depend on them for nourishment. This process harnesses light energy to synthesize organic compounds. In addition to providing food, photosynthesis is the primary source of oxygen in the atmosphere. Without it, the planet would be inhospitable to aerobic organisms, including humans.
Several simple experiments in earlier classes have already shown that chlorophyll, light, and CO2 are required for photosynthesis. Experiments on leaves exposed to sunlight and tested for starch formation provided crucial insights.
Early experiments were instrumental in unraveling the mystery of photosynthesis. The most notable contributions include:
Important Note: The empirical equation representing photosynthesis is:
6 CO2 + 12 H2O → C6H12O6 + 6 O2 + 6 H2O.
Photosynthesis predominantly takes place in the green parts of plants, primarily the leaves, though other green parts also contribute. Leaves are equipped with mesophyll cells that house chloroplasts, the photosynthetic machinery. The chloroplast membrane system traps light energy, synthesizing ATP and NADPH, while the stroma is where sugars are synthesized. These processes are divided into two categories:
Plants owe their various shades of green to a range of pigments, separated through paper chromatography. These pigments include:
Conceptual Note: The action spectrum of photosynthesis shows that maximum activity occurs in the blue and red regions of the light spectrum, which corresponds to the absorption spectrum of chlorophyll.
The light reaction encompasses several steps: light absorption, water splitting, oxygen release, and the formation of ATP and NADPH. Photosynthetic pigments are organized into two light-harvesting complexes (LHC) within Photosystem I (PS I) and Photosystem II (PS II). These pigments help absorb light at different wavelengths, increasing efficiency.
Note on Water Splitting: Water molecules are split into O2, protons, and electrons in PS II. This process contributes electrons to PS I and releases oxygen into the atmosphere.
In non-cyclic photophosphorylation, electrons flow from PS II to PS I, producing both ATP and NADPH. However, in cyclic photophosphorylation, only PS I is involved, and electrons are cycled back into the system, leading to the production of ATP alone.
Cyclic Photophosphorylation | Non-Cyclic Photophosphorylation |
---|---|
Involves only PS I. | Involves both PS I and PS II. |
Produces only ATP. | Produces both ATP and NADPH. |
Electrons are recycled. | Electrons are not recycled. |
The chemiosmotic hypothesis explains the mechanism behind ATP synthesis. As in respiration, ATP production in photosynthesis is linked to the development of a proton gradient across the thylakoid membrane. The accumulation of protons inside the thylakoid creates a gradient, which is then broken down by their movement through ATP synthase, resulting in the synthesis of ATP.
Note: The ATP and NADPH produced in the light reactions are immediately used in the biosynthetic phase of photosynthesis, also known as the Calvin Cycle.
The Calvin cycle is the biosynthetic phase of photosynthesis, occurring in the stroma of the chloroplasts. During this cycle, CO2 is fixed into sugars. The cycle can be broken down into three stages:
Six turns of the Calvin cycle are required to produce one molecule of glucose.
Plants can fix CO2 through two different pathways: the C3 pathway and the C4 pathway. The distinction lies in the first stable product of CO2 fixation:
C3 Plants | C4 Plants |
---|---|
First CO2 fixation product: PGA (3-carbon). | First CO2 fixation product: OAA (4-carbon). |
Photosynthesis occurs in all mesophyll cells. | Photosynthesis occurs in bundle sheath cells. |
Have photorespiration. | Lack photorespiration. |
Less tolerant to high temperatures. | More tolerant to high temperatures. |
Important Note: Photorespiration is a wasteful process where RuBisCO binds with O2 instead of CO2, reducing CO2 fixation. C4 plants minimize this by maintaining high CO2 concentrations at RuBisCO’s active site.
The
rate of photosynthesis is influenced by both internal and external factors. Internal factors include the number, size, and age of leaves, chloroplast count, and CO2 concentration. External factors encompass light availability, temperature, CO2 concentration, and water supply.
MCQ: Which pigment primarily absorbs light in the blue and red regions during photosynthesis?
Answer: Chlorophyll a
This understanding of photosynthesis reveals not only the intricate processes by which plants sustain life on earth but also the environmental factors that influence their efficiency.