Photosynthesis in Higher Plants

Learning Outcomes

1. Understand the process of photosynthesis and its significance for life on earth.
2. Grasp the structure of the photosynthetic apparatus in plants.
3. Comprehend the role of light in photosynthesis and the conversion of light energy into chemical energy.
4. Learn the historical experiments that led to the discovery of photosynthesis.
5. 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.

Early Understanding of Photosynthesis

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.

1. Chlorophyll’s Role: Only green parts of the plant, where chlorophyll is present, engage in photosynthesis.
2. CO2 Necessity: CO2 is indispensable, as demonstrated by experiments where leaves deprived of CO2 could not form starch.

Historical Experiments in Photosynthesis

Early experiments were instrumental in unraveling the mystery of photosynthesis. The most notable contributions include:

1. Joseph Priestley’s Experiment: In 1770, Priestley observed that a burning candle or a breathing animal in a sealed jar extinguished the air. However, when a mint plant was added to the setup, the candle burned longer, and the mouse survived. He concluded that plants restore what animals and candles remove from the air.
2. Jan Ingenhousz’s Discovery: Ingenhousz later demonstrated that sunlight is essential for this restoration process. In his experiments with aquatic plants, he observed oxygen bubbles only in sunlight, proving that green parts of plants release oxygen.
3. Julius von Sachs: In the mid-1800s, Sachs discovered that plants produce glucose, which is stored as starch, and that chlorophyll, located in chloroplasts, is involved in photosynthesis.
4. T.W. Engelmann’s Experiment: Engelmann used a prism to show that blue and red light wavelengths are most effective in promoting photosynthesis.
5. Cornelius van Niel: Van Niel’s studies on purple and green bacteria showed that photosynthesis is a light-dependent reaction in which hydrogen from an oxidizable compound reduces CO2 to form carbohydrates.

Important Note: The empirical equation representing photosynthesis is:
6 CO2 + 12 H2O → C6H12O6 + 6 O2 + 6 H2O.

Where Does Photosynthesis Occur?

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:

1. Light Reactions: Occur in the membrane and are driven directly by light. These reactions produce ATP and NADPH.
2. Dark Reactions: Occur in the stroma and rely on the products of light reactions to synthesize sugar. Despite the name, these reactions are light-dependent.

Types of Pigments Involved in Photosynthesis

Plants owe their various shades of green to a range of pigments, separated through paper chromatography. These pigments include:

1. Chlorophyll a: The chief pigment, bright or blue-green.
2. Chlorophyll b: Yellow-green in color.
3. Xanthophylls: Yellow pigments.
4. Carotenoids: Yellow to yellow-orange.

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.

Light Reaction

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.

1. Photosystem II: Chlorophyll in PS II absorbs 680 nm light, causing electrons to become excited and move to a higher orbit. These electrons are passed to an electron transport chain, eventually reaching PS I.
2. Photosystem I: In PS I, electrons absorb 700 nm light and are transferred to NADP+, reducing it to NADPH.
3. Z Scheme: The flow of electrons from PS II to PS I and ultimately to NADP+ forms a Z-shaped pathway known as the Z scheme.

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.

Cyclic and Non-Cyclic Photophosphorylation

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.

Chemiosmotic Hypothesis

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.

1. Water Splitting: Protons are produced when water is split on the inner side of the thylakoid membrane.
2. Electron Transport: Protons are transported across the membrane as electrons pass through the transport chain.
3. NADP Reductase: Located on the stroma side, this enzyme helps convert NADP+ to NADPH using electrons and protons from the stroma.

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

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:

1. Carboxylation: CO2 is fixed into a 5-carbon compound, RuBP, by the enzyme RuBisCO, producing two molecules of PGA.
2. Reduction: ATP and NADPH are used to convert PGA into sugars.
3. Regeneration: RuBP is regenerated to continue the cycle, using ATP for phosphorylation.

Six turns of the Calvin cycle are required to produce one molecule of glucose.

C3 and C4 Pathways

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:

1. C3 Plants: The first product is a 3-carbon compound called PGA. These plants carry out the Calvin cycle in all mesophyll cells.
2. C4 Plants: The first product is a 4-carbon compound, OAA. These plants have a unique Kranz anatomy, with large bundle sheath cells where the Calvin cycle takes place.

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.

Factors Affecting Photosynthesis

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.

1. Light: Photosynthesis increases with light intensity until light saturation is reached. Beyond this, other factors limit the rate. In most plants, 10% of full sunlight is sufficient for light saturation.
2. CO2 Concentration: CO2 is a major limiting factor. Increasing CO2 levels can boost photosynthesis, especially in C3 plants, which respond more to CO2 enrichment than C4 plants.
3. Temperature: The dark reactions of photosynthesis are temperature-dependent. C4 plants thrive in higher temperatures, while C3 plants have lower temperature optima.
4. Water: Water indirectly affects photosynthesis by causing stomatal closure and reducing CO2 availability. Water stress also causes leaves to wilt, decreasing photosynthetic activity.

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.