Photosynthesis - Definition, Formula, Process, Diagram

Photosynthesis - Definition, Formula, Process, Diagram


Photosynthesis converts light energy by phototrophs into chemical energy, then used for cellular power activities. Sugars, formed from water and carbon dioxide, are used to store chemical energy.

Plants, algae, and certain bacteria use photosynthesis to convert sunlight, carbon dioxide (CO2), and water into food (sugars) and oxygen.

Plants require food to breathe, grow, and replicate. Plants, unlike animals, can produce their food through the process of photosynthesis. Plants need carbon dioxide (from the air), water (from the ground), and light to perform photosynthesis (usually from the sun).

Plants have pigments that absorb the light energy required for photosynthesis. Different pigments react to visible light at different wavelengths. Chlorophyll, the primary pigment used in photosynthesis, reflects green light while strongly absorbing red and blue light. Chlorophyll is also responsible for the plant's green color. Photosynthesis occurs in chloroplasts, which contain chlorophyll, in plants.

Plants absorb carbon dioxide (CO2) and water (H2O) from the air and soil during photosynthesis. Water is oxidized within the plant cell, which means it loses electrons, whereas carbon dioxide is reduced, which means it gains electrons. This converts water to oxygen and carbon dioxide to glucose. The plant then returns the oxygen to the atmosphere while storing energy within the glucose molecules.

The photosynthesis equation is:- 

6CO2 + 6H2O → C6H12O6 + 6O2.

Types of Photosynthetic processes:-

Photosynthetic energy transfer is the most important energy transfer on the planet. Carbon fixation by photosynthetic organisms provides the high-energy molecules required to support nearly all living things.
Oxygenic photosynthesis and anoxygenic photosynthesis are the two types of photosynthetic processes. They both work on the same principles, but oxygenic photosynthesis is the most common in plants, algae, and cyanobacteria.

Oxygenic photosynthesis is how certain photoautotrophs convert light energy to chemical energy by producing molecular oxygen.

The oxygenic photosynthesis equation is:- 

6CO2 + 12H2O + Light Energy → C6H12O6 + 6O2 + 6H2O

On the other hand, Anoxygenic photosynthesis is how certain bacteria convert light energy to chemical energy without producing molecular oxygen.

The oxygenic photosynthesis equation is:- 

CO2 + 2H2A + Light Energy → (CH2O) + 2A + H2O

Types of Photosynthesis:-

Plants use three different processes to fix carbon during the photosynthesis process - C3, C4, and CAM. Fixing carbon is how plants remove carbon from atmospheric carbon dioxide and convert it into organic molecules.

C3, C4, and CAM are the three main types of photosynthetic pathways. They all use the Calvin cycle to produce sugars from CO2, but each pathway is slightly different.

Because the three photosynthetic pathways differ biochemically and morphologically, each has its advantages and disadvantages. These distinctions result in varying levels of performance in various environments.

C3 Photosynthesis

Plants that use only the Calvin cycle for fixing the CO2 from the air are known as C3 plants.

C3 plants account for approximately 85 percent of all plant species. Cereal grains include wheat, rice, barley, and oats. C3 plants include peanuts, cotton, sugar beets, tobacco, spinach, soybeans, and most trees. Most lawn grasses, such as rye and fescue, are C3 plants.

Carbon dioxide enters a plant through its stomata (microscopic pores on plant leaves), where the enzyme Rubisco fixes carbon into sugar via the Calvin-Benson cycle.

In this process, the enzyme RuBisCO is used relatively inefficiently to fix CO2 from the air and produce the 3-carbon organic intermediate molecule 3-phosphoglycerate. C3 photosynthetic plants have a distinct leaf structure and are not adapted to suboptimal conditions.

C3 plants have the disadvantage that their photosynthetic efficiency suffers in hot, dry conditions. Rubisco is designed to fix carbon dioxide, but it can also fix oxygen molecules. When the concentration of CO2 in the chloroplasts falls below about 50 ppm, the catalyst rubisco, which aids in carbon fixation, begins to fix oxygen instead. Rubisco fixes oxygen about 20% of the time, triggering a photorespiration process that recycles the toxic.

Photorespiration consumes plant energy that could have been used to photosynthesize. This is a significant waste of the energy collected from the light and causes the rubisco to operate at only about a quarter of its maximum rate.

In C3 plants, when stomata open to allow carbon dioxide in, they also allow water vapor to escape, putting C3 plants at a disadvantage in drought and high-temperature environments.

However, plants have evolved another form of photosynthesis to help reduce these losses in hot, dry environments.

C4 Photosynthesis

In C4 plants, photorespiration is overcome by a two-stage strategy that maintains high CO2 levels and low oxygen levels in the chloroplast, where the Calvin cycle operates.

The light-dependent reactions and the Calvin cycle are physically separated in C4 plants. C4 plants have specialized leaf anatomy with two types of photosynthetic cells: mesophyll cells (on the leaf's exterior, near the stomata) and bundle sheath cells (in the interior of the leaf, far away from stomata). Rubisco is only found in bundle sheath cells, where the Calvin cycle operates.

In C4 photosynthesis, where a four-carbon compound is produced, the unique leaf anatomy allows carbon dioxide to concentrate in 'bundle sheath' cells around Rubisco. This structure delivers carbon dioxide directly to Rubisco, effectively eliminating its contact with oxygen and the need for photorespiration.

C4 plants use a different enzyme for the first step of carbon fixation. This enzyme is known as phosphoenolpyruvate (PEP) carboxylase, and it lacks oxygenase activity but has a much higher affinity for CO2 than Rubisco. As the name 'PEP carboxylase' suggests, the enzyme attaches CO2 to a compound known as phosphoenolpyruvate (PEP).

The C4 pathway is used by about 3% of all vascular plants, including crabgrass, sugarcane, and corn. C4 plants are common in hot habitats but are less common in cooler habitats.

CAM Photosynthesis

Cacti and pineapples, for example, are adapted to dry environments and use the crassulacean acid metabolism (CAM) pathway to reduce photorespiration. This name is derived from the Crassulaceae plant family, which scientists discovered the pathway in the first place.

Stomata open at night in this pathway, allowing CO2 to diffuse into the leaf and combine with PEP to form malate. This acid is then stored in large central vacuoles until the following day. Malate is released from the vacuoles and decarboxylated during the day. In the C3 pathway, Rubisco then combines the released CO2 with RuBP. Because photosynthesis in CAM plants is proportional to vacuolar storage capacity, CAM plants typically have thick and fleshy water-storing leaves or stems.

CAM plants do not open their stomata during the day, but they can still photosynthesize. This is due to the organic acids being transported out of the vacuole and broken down, releasing CO2, entering the Calvin cycle. This controlled release keeps a high concentration of CO2 in the vicinity of Rubisco.

Calvin cycle:-

The Calvin cycle is how plants and algae convert carbon dioxide from the atmosphere into three-carbon sugars. Plants and animals can later convert these three-carbon compounds into amino acids, nucleotides, and more complex sugars like starches.

These plant sugars can also be used as a source of energy by animals that eat plants and predators that eat herbivores.

The Calvin cycle is essential to all living things on Earth. The Calvin cycle provides energy and food to plants.

Most new organic matter is formed through this 'carbon fixation' process. Plants use the sugars produced by the Calvin cycle for long-term energy storage, as opposed to ATP, which is rapidly depleted after it is produced.

The Calvin cycle is also known as the C3 cycle, photosynthesis's light-independent or dark reaction of photosynthesis because photons from the Sun do not directly power it. Instead, the Calvin cycle is powered by ATP and NADPH, which are produced by capturing photon energy in light-dependent reactions.

C3 plants solely rely on the Calvin cycle for carbon fixation. 

In the Calvin cycle, Carbon dioxide diffuses into the stroma of chloroplasts and combines with ribulose1,5-biphosphate, a five-carbon sugar (RuBP). RuBisCo, the enzyme that catalyzes this reaction, is a large molecule that may be the most abundant organic molecule on Earth. 

This catalyzed reaction generates a 6-carbon intermediate, which rapidly decays to form two molecules of the 3-carbon compound 3-phosphoglyceric acid (3PGA). Because this 3-carbon molecule is the first stable product of photosynthesis, this cycle is commonly referred to as the C3 cycle.

Calvin cycle is most active during the day when NADPH and ATP are plentiful.

Importance of photosynthesis:-

The importance of photosynthesis in the survival of life on Earth cannot be overstated. Photosynthesis is the most basic life process for nearly all plants and animals. It provides the energy that powers all their metabolic functions and the oxygen required for respiration. If photosynthesis ceased, the Earth would soon be devoid of food and other organic matter. Most organisms would die, and the Earth's atmosphere would eventually be nearly empty of gaseous oxygen. The only organisms that can survive in such conditions are chemosynthetic bacteria, which can use the chemical energy of certain inorganic compounds and thus are not dependent on light energy conversion.

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