Photosynthesis in Higher Plants — Study Notes

1. Introduction

Photosynthesis is the most important biological process on Earth, by which green plants convert light energy into chemical energy in the form of carbohydrates. The word photosynthesis comes from 'photo' (light) and 'synthesis' (to build). Through this process, plants produce food and release oxygen, which is essential for the survival of nearly all living organisms. Photosynthesis is the ultimate source of food and energy for all heterotrophs, and the oxygen released maintains the atmospheric oxygen level. It is estimated that 90 to 95 percent of the total oxygen in the atmosphere comes from photosynthesis carried out by green plants and algae.

The overall equation of photosynthesis is: carbon dioxide plus water, in the presence of sunlight and chlorophyll, gives glucose and oxygen. In plants, photosynthesis occurs mainly in the leaves, inside the chloroplasts, which contain the green pigment chlorophyll. The process occurs in two main phases: the light reaction (which takes place in the thylakoid membranes) and the dark reaction or Calvin cycle (which takes place in the stroma of the chloroplast). This chapter describes the photosynthetic apparatus, the pigments, the mechanisms of the light and dark reactions, and the special pathway of carbon fixation in some plants, called C4 photosynthesis.

2. Early Experiments on Photosynthesis

The understanding of photosynthesis came from a series of landmark experiments. Joseph Priestley (1770) showed that plants restore the air that has been "injured" by burning candles or by breathing animals. He demonstrated that a mint plant could restore the air in a bell jar in which a candle had burnt out, proving that plants release a gas that supports combustion. Jan Ingenhousz (1779) showed that sunlight is essential for the plant to purify the air; only the green parts of plants, in the presence of sunlight, could release oxygen. Von Sachs (1854) proved that glucose is produced during photosynthesis and stored as starch in the leaves. T.W. Engelmann (1881) used the alga Cladophora and aerobic bacteria to show that the oxygen produced during photosynthesis is released by the green part of the cell; the bacteria accumulated where oxygen was released, which corresponded to the region illuminated by red and blue light. Cornelius van Niel (1931) used purple and green bacteria and proved that the oxygen released during photosynthesis comes from water, not from carbon dioxide.

3. Sites of Photosynthesis

Photosynthesis takes place in the chloroplast. The chloroplast is bounded by two membranes. Inside, there are flattened membranous sacs called thylakoids, which may be stacked to form structures called grana. The thylakoid membrane contains the pigments chlorophyll a, chlorophyll b, carotenoids and xanthophylls, along with the electron carriers. The fluid ground substance of the chloroplast, called the stroma, contains the enzymes of the Calvin cycle, along with DNA and ribosomes. The light reaction occurs in the thylakoid membranes, and the dark reaction occurs in the stroma.

3.1 Photosynthetic Pigments

The photosynthetic pigments absorb light and trap its energy. Chlorophyll a is the chief pigment and is directly involved in the light reaction. Chlorophyll b, carotenoids and xanthophylls are accessory pigments. The accessory pigments absorb light of different wavelengths and pass the energy to chlorophyll a; they also protect the chlorophyll from photo-oxidation. Chlorophyll a absorbs light mainly in the red and blue-violet regions of the spectrum and reflects green light, which is why plants appear green.

4. Light Reaction

The light reaction occurs in the thylakoid membranes and requires light. It results in the synthesis of ATP and NADPH, which are then used in the dark reaction. The light reaction involves three major processes: the absorption of light by photosystems, the splitting of water (photolysis), and the synthesis of ATP and NADPH.

4.1 Photosystems

A photosystem consists of a reaction centre and a light-harvesting complex (LHC), also called the antenna complex. The LHC contains hundreds of pigment molecules (chlorophyll a, b, carotenoids and xanthophylls) that absorb light and funnel the energy to the reaction centre. There are two types of photosystems: - Photosystem I (PS I): The reaction centre is chlorophyll a with an absorption peak at 700 nm, called P700. - Photosystem II (PS II): The reaction centre is chlorophyll a with an absorption peak at 680 nm, called P680.

4.2 Non-cyclic Photophosphorylation

In non-cyclic photophosphorylation, both PS I and PS II are involved, and ATP and NADPH are produced. The process is: 1. PS II (P680) absorbs light, and an electron is excited and transferred to an electron acceptor. 2. The electron passes through the electron transport chain consisting of plastoquinone, cytochromes b6f, and plastocyanin, and ultimately reaches PS I. This electron flow drives the pumping of protons, which powers ATP synthesis (photophosphorylation). 3. The electron lost by PS II is replaced by the splitting of water (photolysis): 2H2O gives 4H+ + 4e- + O2. The oxygen released during photosynthesis comes from this photolysis of water. 4. PS I (P700) absorbs light, and its electron is excited and passed to ferredoxin and then to NADP+ reductase, which reduces NADP+ to NADPH. This process is called non-cyclic because the electrons flow from water to NADP+ and do not return to the photosystem. Both ATP and NADPH are produced.

4.3 Cyclic Photophosphorylation

In cyclic photophosphorylation, only PS I is involved, and the electron from PS I cycles back to PS I. When the electron passes through the electron transport chain, ATP is synthesised, but no NADPH is produced and no water is split (no oxygen is released). Cyclic photophosphorylation occurs when the cell needs extra ATP.

5. Dark Reaction (Calvin Cycle)

The dark reaction, also called the Calvin cycle or the biosynthetic phase, occurs in the stroma of the chloroplast and does not require light directly, although it depends on the ATP and NADPH produced by the light reaction. The Calvin cycle, discovered by Melvin Calvin, involves three main phases: - Carboxylation: Carbon dioxide combines with the 5-carbon compound ribulose-1,5-bisphosphate (RuBP), catalysed by the enzyme RuBisCO (ribulose bisphosphate carboxylase-oxygenase). The 6-carbon intermediate immediately splits into two molecules of 3-phosphoglyceric acid (PGA), a 3-carbon compound. - Reduction: PGA is reduced to glyceraldehyde-3-phosphate (G3P) using the ATP and NADPH from the light reaction. For every carbon dioxide fixed, two molecules of G3P are formed. - Regeneration: Some G3P molecules are used to regenerate RuBP, so that the cycle can continue. For every six molecules of carbon dioxide fixed, twelve molecules of G3P are formed; ten are used to regenerate six molecules of RuBP, and two molecules of G3P are used to form one molecule of glucose.

The Calvin cycle is also called the C3 pathway because the first stable product of carbon fixation is a 3-carbon compound, 3-phosphoglyceric acid. Plants that use only this pathway are called C3 plants.

6. C4 Pathway

Some plants, called C4 plants (e.g., maize, sugarcane, sorghum), have a special pathway for carbon fixation. In these plants, the first product of carbon fixation is a 4-carbon compound, oxaloacetic acid (OAA), formed by the carboxylation of phosphoenolpyruvate (PEP) in the mesophyll cells, catalysed by the enzyme PEP carboxylase. This pathway was discovered by Hatch and Slack.

6.1 Kranz Anatomy

C4 plants show a special anatomical feature called Kranz anatomy, in which the vascular bundles are surrounded by a ring of bundle sheath cells, and the mesophyll cells are arranged around them in a radial manner. This arrangement keeps the CO2 concentration high in the bundle sheath cells, where the Calvin cycle operates. The mesophyll cells initially fix CO2 into OAA (a 4-carbon acid, e.g., malate or aspartate), which is transported to the bundle sheath cells, where CO2 is released and enters the Calvin cycle. The released pyruvate returns to the mesophyll cells, where it is converted back to PEP using ATP.

6.2 Advantages of the C4 Pathway

C4 plants have a competitive advantage over C3 plants under conditions of high temperature, high light intensity and drought. Because PEP carboxylase has a very high affinity for CO2 and does not have an oxygenase activity, C4 plants can fix CO2 efficiently even at low CO2 concentrations, avoiding photorespiration. As a result, C4 plants have higher productivity than C3 plants.

7. Photorespiration

Photorespiration is a wasteful process in which RuBisCO, instead of fixing CO2, adds oxygen to RuBP. The RuBisCO enzyme has a dual role: in the presence of high CO2, it acts as a carboxylase (Calvin cycle), but in the presence of high O2 and low CO2, it acts as an oxygenase and initiates photorespiration. In photorespiration, there is no synthesis of ATP, sugar or NADPH; instead, there is a net loss of fixed carbon and energy. Photorespiration occurs in C3 plants and reduces their photosynthetic efficiency. C4 plants do not show photorespiration because the CO2 concentration in the bundle sheath cells is maintained high enough that RuBisCO acts only as a carboxylase.

8. Factors Affecting Photosynthesis

The rate of photosynthesis is affected by both external and internal factors. The external factors include light, temperature, carbon dioxide and water. The internal factors include chlorophyll content, protoplasmic factors and the accumulation of end products. Light is the driving force: the rate of photosynthesis increases with increasing light intensity up to a saturation point. Carbon dioxide is the substrate: an increase in CO2 concentration up to a certain level increases the rate of photosynthesis. Temperature affects the enzymes involved, and most plants show maximum photosynthesis at an optimum temperature (20 to 35 degrees Celsius). Water stress decreases photosynthesis by causing stomatal closure. The rate of photosynthesis is also influenced by the law of limiting factors proposed by Blackman, which states that when several factors affect a process, the rate of the process is limited by the factor that is in the minimum (shortest supply).

Quick Revision Tables

Table 1: Light vs Dark Reaction

Table 2: C3 vs C4 Plants

Table 3: Key Scientists of Photosynthesis

Mind Map

Important Diagrams (SVG)

Common Mistakes

1. Students state that the oxygen released in photosynthesis comes from carbon dioxide; van Niel proved it comes from water.
2. The reaction centre chlorophyll of PS I is written as P680; it is P700, while PS II has P680.
3. Dark reaction is said to occur only at night; it occurs in the stroma and needs the ATP and NADPH from the light reaction, not light itself.
4. RuBisCO is said to be a pure carboxylase; it has dual carboxylase-oxygenase activity, initiating photorespiration in C3 plants.
5. C4 plants are said to show Kranz anatomy in C3 leaves; Kranz anatomy is exclusive to C4 plants.
6. Students think photorespiration produces ATP; photorespiration is wasteful, producing no ATP, NADPH or sugar.
7. The first product of the C3 pathway is said to be glucose; it is 3-PGA, a 3-carbon compound.
8. Photosynthesis is said to occur in all green parts, but in higher plants it mainly occurs in the leaves, specifically in the chloroplasts of mesophyll cells.

Exam Tips

1. Remember the pigment absorption peaks: chlorophyll a absorbs at red and blue-violet, reflected green; PS I = P700, PS II = P680.
2. Non-cyclic photophosphorylation produces both ATP and NADPH and releases oxygen; cyclic produces only ATP.
3. Learn the Calvin cycle in three words: Carboxylation, Reduction, Regeneration.
4. The equation 6CO2 + 12H2O gives C6H12O6 + 6O2 + 6H2O, with oxygen from water, is a sure-shot question.
5. For C4 plants, remember: OAA is the first product, PEP carboxylase is the enzyme, and Kranz anatomy is the structure.
6. Blackman's law of limiting factors is the standard conceptual question for factors affecting photosynthesis.
7. Note the scientist names with their discoveries, especially van Niel and Engelmann.

Conclusion

Photosynthesis is the foundation of life on Earth, converting solar energy into chemical energy stored in carbohydrates and releasing oxygen in the process. The elegant division of labour between the thylakoid membranes, where light is trapped and ATP and NADPH are produced, and the stroma, where carbon dioxide is fixed into sugars, reflects the remarkable sophistication of the chloroplast. The Calvin cycle provides the universal pathway of carbon fixation, while the C4 pathway and Kranz anatomy demonstrate how some plants have evolved to overcome the inefficiencies of photorespiration in hot, dry environments. Understanding the factors that limit photosynthesis and the experiments that revealed its mechanism gives the learner a complete picture of plant energetics, which is essential for agriculture, ecology and the study of respiration in the next chapter.