Introduction:
Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy from the sun into chemical energy in the form of organic molecules such as glucose. It involves the conversion of carbon dioxide and water into glucose and oxygen using light energy, which is absorbed by the green pigment chlorophyll present in specialized structures called chloroplasts. This process is essential for the production of food and oxygen, and it plays a critical role in sustaining life on Earth.
Fig: Photosynthesis
Discovery:
The discovery of photosynthesis is attributed to the labor of numerous scientists over a long period of time, including Jan Ingenhousz, whose investigations in the late 18th century revealed that green plants were capable of producing oxygen from sunlight and water. Early in the nineteenth century, Jean Baptiste van Helmont also made a significant discovery when he came to the conclusion that water is the source of the mass of trees. C.B. van Niel postulated in the 20th century that photosynthesis is an electron transfer mechanism, which was later supported by a number of studies and serves as the foundation for our present understanding of photosynthesis. Since photosynthesis serves as the major source of food and oxygen for all living things, it is a necessary process for life to exist on Earth.
Photosynthetic membranes and organelles:
Photosynthetic membranes and organelles are specialized structures in plant and algal cells that are responsible for carrying out the process of photosynthesis. The primary photosynthetic organelle is the chloroplast, which is found in the cytoplasm of plant and algal cells. Chloroplasts contain several types of membranes, including the thylakoid membranes and the inner and outer chloroplast membranes.
The thylakoid membranes are a series of flattened sacs that are arranged in stacks called grana. The pigments responsible for capturing light energy, such as chlorophyll and carotenoids, are embedded in the thylakoid membranes. The light-dependent reactions of photosynthesis, where water is split and oxygen is produced, occur on the thylakoid membranes.
Fig: Structure of the Chloroplast
The inner and outer chloroplast membranes enclose the chloroplast and regulate the movement of materials in and out of the organelle. The inner membrane contains transport proteins that allow for the import and export of molecules, such as sugars and amino acids, while the outer membrane protects the chloroplast from damage and helps maintain its shape.
Photosynthetic membranes and organelles are essential for the process of photosynthesis and play a critical role in the conversion of light energy into chemical energy. The organization and structure of these membranes are optimized for the capture and utilization of light energy, and their specific functions are tightly regulated to ensure efficient and productive photosynthesis.
Photosynthetic Pigments:
Photosynthetic pigments are molecules found in the chloroplasts of plant and algal cells that are responsible for absorbing light energy and transferring it to other molecules within the photosynthetic membrane. These pigments are essential for the process of photosynthesis, as they allow plants and algae to capture and utilize light energy from the sun. The different photosynthetic pigments have different absorption spectra, which allows plants and algae to capture light energy across a wide range of the electromagnetic spectrum. This enables them to maximize their ability to capture energy from the sun and carry out the process of photosynthesis efficiently.
Primary pigments are the pigments directly involved in photosynthesis and are essential for the process to occur. The primary pigments include chlorophyll a, which is found in all photosynthetic organisms and is responsible for absorbing light energy and initiating the electron transport chain, and bacteriochlorophyll, which is found in some photosynthetic bacteria and performs a similar function as chlorophyll a.
Secondary pigments, on the other hand, are pigments that are not directly involved in the photosynthetic process but instead serve as accessory pigments that complement the function of primary pigments. These pigments are not essential for photosynthesis to occur, but they can enhance the efficiency of the process by broadening the range of light wavelengths that can be absorbed. Examples of secondary pigments include chlorophyll b, carotenoids, phycobilins and anthocyanins
Carotenoids: Carotenoids are orange, yellow, and red pigments found in many plants. They serve as accessory pigments and help to expand the range of light that can be absorbed by the plant.
Phycobilins: Phycobilins are water-soluble pigments found in cyanobacteria and some algae. They are responsible for the red, blue, and purple colors seen in these organisms.
Anthocyanins: Anthocyanins are water-soluble pigments that produce red, purple, and blue colors in plants. They are often found in leaves, flowers, and fruits.
Xanthophylls: Xanthophylls are yellow pigments found in leaves and other plant tissues. They serve as accessory pigments and help to protect the plant from excess light.
Types:
There are three main types of photosynthesis.
Oxygenic photosynthesis
The majority of plants, algae, and cyanobacteria use oxygenic photosynthesis, which is the most prevalent type of photosynthesis. In this process, pigments like chlorophyll and carotenoids absorb light energy and utilize it to create ATP and NADPH. Through the Calvin cycle, these energy molecules are subsequently utilized to repair carbon dioxide into organic molecules. Most life on Earth depends on oxygen, which is produced as a byproduct of oxigenic photosynthesis.
Fig: Oxygenic photosynthesis
Anoxygenic photosynthesis
Anoxygenic photosynthesis is a type of photosynthesis that doesn’t result in the production of oxygen as a byproduct. Certain bacteria that survive in situations with little or no oxygen. To absorb light energy and produce ATP, these bacteria utilize pigments that are distinct from those employed in oxygenic photosynthesis. Only specific light wavelengths can be used for anoxygenic photosynthesis, which is also less effective than oxygenic photosynthesis.
C4 photosynthesis
C4 photosynthesis is a type of photosynthesis that has evolved as an adaptation to arid and hot environments with high light intensity. C4 plants are able to achieve a higher photosynthetic efficiency and avoid photorespiration, which can cause energy loss and reduce carbon fixation in other types of plants.
In C4 photosynthesis, carbon dioxide is first fixed into a four-carbon compound, oxaloacetate, in the mesophyll cells of the plant. This process is catalyzed by an enzyme called phosphoenolpyruvate carboxylase (PEPCase). The resulting four-carbon compound is then transported to bundle sheath cells, which are specialized cells that surround the leaf veins and contain the chloroplasts where the Calvin cycle occurs.
Fig: C4 photosynthesis
In the bundle sheath cells, the four-carbon compound is decarboxylated to release CO2, which is then used in the Calvin cycle to produce sugars. The released CO2 is then fixed by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the Calvin cycle. By separating the carbon fixation and Calvin cycle reactions into different cells, C4 photosynthesis can increase the concentration of CO2 in the vicinity of Rubisco, reducing photorespiration and increasing photosynthetic efficiency.
C4 photosynthesis is found in a diverse range of plant species, including many grasses such as maize, sorghum, and sugarcane, as well as some dicots like amaranth and purslane. C4 plants tend to have a characteristic leaf anatomy, with a layer of bundle sheath cells surrounding the veins and a distinct mesophyll layer. Primary productivity is made possible under exceptionally hard conditions by the ability to concentrate CO2 at night and close stomata during the day.
Mechanism of photosynthesis:
The process through which green plants and some other animals transform solar light energy into chemical energy in the form of organic compounds like glucose is known as photosynthesis.
Stages:
The two stages of photosynthesis are the light-dependent reactions and the light-independent reactions, also known as the Calvin cycle.
Light-dependent reactions
Light-dependent photosynthesis processes occur in the thylakoid membranes of chloroplasts and require light energy. They involve a series of chemical processes and electron exchanges that produce ATP and NADPH, which are used in light-independent reactions. Light energy is absorbed by chlorophyll, excited electrons are transferred along a chain of carriers, hydrogen ions are pumped to generate a concentration gradient, NADP+ is reduced to form NADPH, and ATP is synthesized by ATP synthase. These processes supply the energy and reducing power required by light-independent reactions to generate organic compounds like glucose.
Fig: Light dependent reaction of photosynthesis
Light-independent reactions (Calvin cycle)
The Calvin cycle, sometimes referred to as the light-independent reactions, takes place in the stroma of chloroplasts and uses ATP and NADPH generated during the light-dependent processes to fix carbon dioxide into organic compounds. The fundamental processes include the regeneration of RuBP, the production of glucose and other organic molecules, the fixation of CO2 by the enzyme RuBisCO to generate 3-PGA, the conversion of 3-PGA into G3P using ATP and NADPH, and so forth. These processes are essential to the global carbon cycle because they produce the energy and carbon sources required for plant growth and metabolism.
Fig: Calvin cycle
Factors Affecting Photosynthesis:
Photosynthesis can be influenced by several environmental factors that include:
- Light intensity
- Temperature
- Carbon dioxide concentration
- Water availability
- Nutrient availability
- Leaf surface area
- Altitude
- Pollution
Significance:
Photosynthesis is essential for life on Earth. It is the conversion of light energy into chemical energy in the form of organic molecules such as glucose by green plants, algae, and some microorganisms. This process offers a number of significant advantages, including the generation of oxygen, which is the primary source of sustenance for most living things, and the reduction of carbon dioxide in the atmosphere, which aids in mitigating the consequences of climate change. Photosynthesis can also be utilized to create biofuels, and photosynthesizing plants help to avoid soil erosion, maintain healthy soil, and prevent nutrient loss.
References:
- Govindjee, Whitmarsh, J. (Eds.). (2017). Photosynthesis: Plastid Biology, Energy Conversion and Carbon Assimilation (Advances in Photosynthesis and Respiration). Springer.
- Hall, D.O. and Rao, K., 1999. Photosynthesis. Cambridge University Press.
- Blankenship, R. E. (2014). Molecular Mechanisms of Photosynthesis. John Wiley & Sons.