LifeProcess-D Explained

Before any chemical reaction can occur, the pigment chlorophyll present in the chloroplasts must capture or absorb light energy from the sun. This absorption of light energy is the very first step that triggers all subsequent events in photosynthesis. Without this absorption, no energy would be available to drive the reactions.

Chlorophyll is the green pigment found in the chloroplasts of leaves. It is specially designed to absorb light energy, mainly in the blue and red regions of the spectrum, while reflecting green light, which is why plants appear green. Other pigments like carotene also absorb light but chlorophyll is the main one.

When chlorophyll absorbs light energy, the energy level of electrons in the chlorophyll molecule increases. These electrons become excited or energized. This light energy is now converted into chemical energy in the form of these high-energy electrons. This converted chemical energy is then used to carry out further reactions like splitting of water.

The light energy absorbed by chlorophyll is converted into chemical energy and stored in two special molecules called ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate hydrogen). ATP is the energy currency of the cell, and NADPH is a reducing agent that carries hydrogen atoms. Both are used in the next stage to reduce carbon dioxide.

The chemical energy from excited electrons is used to split water molecules (H2O) into their components: hydrogen (H+) and oxygen (O2). This process is called photolysis, meaning splitting by light. The oxygen is released as a byproduct into the atmosphere, while the hydrogen is used later to reduce carbon dioxide. This reaction is: 2H2O → 4H+ + 4e- + O2.

The oxygen gas produced from the photolysis or splitting of water is not needed by the plant for photosynthesis. It is a waste product of the light reaction. This oxygen diffuses out of the chloroplast, then out of the leaf cell, and finally exits the leaf through the stomatal pores into the atmosphere. This oxygen is what we and other animals breathe.

The hydrogen (in the form of protons and electrons carried by NADPH) produced from the splitting of water is not wasted. It is carried to the next stage of photosynthesis called the dark reaction or Calvin cycle. Here, this hydrogen is used to reduce carbon dioxide (CO2) by adding hydrogen atoms to it, ultimately forming carbohydrates like glucose. This is where the actual food is made.

In chemistry, reduction means gain of hydrogen or loss of oxygen. During photosynthesis, carbon dioxide (CO2) is reduced, meaning hydrogen atoms (obtained from the splitting of water and carried by NADPH) are added to it. This reduction converts the inorganic molecule CO2 into an organic carbohydrate like glucose (C6H12O6). This is the step where actual food is synthesized.

The chloroplast has two main parts: the thylakoids (stacked discs) where light absorption and water splitting occur, and the fluid-filled stroma around them. The reduction of carbon dioxide to carbohydrates does not require light directly, so it happens in the stroma. The ATP and NADPH produced in the thylakoids move into the stroma and are used to fix and reduce CO2 into glucose.

RuBisCO is the most abundant enzyme on Earth. It is present in the stroma of chloroplasts. This enzyme catalyses the first step of carbon dioxide reduction, where CO2 combines with a 5-carbon compound called RuBP (ribulose bisphosphate). This is called carbon fixation. Without RuBisCO, carbon dioxide cannot be converted into carbohydrates.

When RuBisCO attaches carbon dioxide to RuBP, a very unstable 6-carbon compound is formed. This immediately breaks into two molecules of a 3-carbon compound called 3-phosphoglyceric acid (PGA). PGA is the first stable product of carbon dioxide reduction in C3 plants (like wheat, rice, and most trees). Glucose is formed much later after several steps using energy from ATP and NADPH.

The reduction of carbon dioxide is an energy-requiring process. The ATP that was produced during the light reaction (conversion of light energy to chemical energy) is used in the stroma to provide the necessary energy to convert the initial 3-carbon compound (PGA) into the final carbohydrate (glucose). ATP is broken down to ADP + phosphate, releasing energy that powers the reaction.

The term “reduction of carbon dioxide” specifically means adding hydrogen to CO2. NADPH, which was produced during the light reaction, carries hydrogen atoms (along with high-energy electrons). When NADPH donates these hydrogen atoms to the intermediates of the Calvin cycle, carbon dioxide is gradually converted into glucose. NADPH itself becomes NADP+ and goes back to the light reaction to be recharged.

This equation summarizes the entire process of photosynthesis, but specifically the reduction part: six molecules of carbon dioxide (inorganic) are reduced using hydrogen from water to form one molecule of glucose (carbohydrate, C6H12O6) and six molecules of oxygen are released as a byproduct. The energy for this reduction comes from light, absorbed by chlorophyll, and is temporarily stored in ATP.

Stomata (singular: stoma) are small openings or pores found mainly on the underside of leaves, and sometimes on stems. Each stoma is surrounded by two specialized cells called guard cells. These pores are the main gateway for the exchange of gases (carbon dioxide in, oxygen out) and the loss of water vapor (transpiration) between the plant and the atmosphere.

For the reduction of carbon dioxide to carbohydrates, the plant needs a constant supply of CO2 from the air. When stomatal pores are open, carbon dioxide diffuses into the leaf, reaches the mesophyll cells, and then enters the chloroplasts where it is reduced to glucose. At the same time, the oxygen produced from splitting of water diffuses out of the leaf through these same open pores.

Each stoma is surrounded by two bean-shaped (in dicots) or dumbbell-shaped (in grasses) cells called guard cells. These guard cells have chlorophyll and are sensitive to light, water, and carbon dioxide concentration. By changing their shape through absorbing or losing water (turgor pressure), they control whether the stomatal pore is open or closed.

When guard cells take up water (due to high concentration of solutes like potassium ions), they become swollen or turgid. Because their inner walls (towards the pore) are thicker and less elastic than their outer walls, they bulge outward. This bulging pulls the pore open, creating a gap through which gases can pass. So, turgid guard cells = open stoma.

When guard cells lose water (due to loss of solutes or high transpiration), they lose their turgor pressure and become flaccid or limp. The thin outer walls collapse inward, and the thick inner walls come closer together, causing the stomatal pore to close. Closed stomata prevent further water loss but also stop the entry of carbon dioxide needed for reduction.

During daytime, sunlight is available. Guard cells have chlorophyll, so they perform photosynthesis, producing ATP and sugar. This leads to active transport of potassium ions into the guard cells, increasing their solute concentration. Water enters by osmosis, guard cells become turgid, and the stoma opens. This allows carbon dioxide to enter for the reduction process to make carbohydrates.

At night, there is no sunlight, so the light absorption and conversion steps do not occur. Without these, the reduction of carbon dioxide cannot happen because it requires the ATP and NADPH produced in the light reactions. Therefore, the plant does not need CO2 at night. Closing stomata at night also prevents unnecessary water loss (transpiration) when photosynthesis is not occurring.

Chlorophyll absorbs light energy regardless of whether stomata are open or closed. However, if stomata are closed, carbon dioxide cannot enter the leaf. Without CO2, the reduction of carbon dioxide to carbohydrates cannot happen, even though the light reactions (absorption and water splitting) might continue for a short time. Eventually, the plant may face photorespiration or damage.

The thylakoid membranes contain the chlorophyll molecules and the electron transport chain. When light energy is absorbed by chlorophyll, the energy is converted into chemical energy as electrons move through this membrane. This process creates a proton gradient that drives the synthesis of ATP (photophosphorylation). NADPH is also formed on the stroma side of the thylakoid membrane. The stroma is where these molecules are later used to reduce CO2.

The reduction of carbon dioxide requires adding electrons along with hydrogen. NADPH is the carrier that brings these electrons (and hydrogen) from the light reaction to the Calvin cycle. The electrons originally come from the splitting of water molecules. NADPH donates these electrons to the carbon intermediates, turning CO2 into carbohydrates. Without NADPH, reduction cannot happen.

Light absorption and water splitting (light reactions) do not require carbon dioxide directly. They will proceed as long as light and water are available, producing ATP and NADPH. However, the Calvin cycle (reduction of CO2 to carbohydrates) requires CO2 as a substrate. Without CO2, this step cannot occur. The accumulated ATP and NADPH will eventually inhibit further light reactions, and the plant will be unable to make food.

The chemical equation for photosynthesis shows that six molecules of carbon dioxide (6CO2) are required to produce one molecule of glucose (C6H12O6). Each CO2 molecule contains one carbon atom. Glucose contains six carbon atoms. Therefore, six CO2 molecules must be fixed and reduced to build one glucose molecule. This happens through a cycle (Calvin cycle) that turns six times to produce one glucose.

The immediate carbohydrate produced is a 3-carbon sugar, which is then converted into glucose. Glucose is highly reactive and affects water balance, so plants quickly convert it into starch for storage (in leaves, roots, seeds), cellulose for building cell walls, or oils and proteins (after adding nitrogen and minerals). These stored carbohydrates provide energy for the plant and, directly or indirectly, for animals that eat plants.

The splitting of water produces hydrogen (carried by NADPH) and oxygen. The hydrogen is used to reduce carbon dioxide. If stomata are open, CO2 enters, and the reduction of CO2 consumes NADPH, which in turn creates a demand for more water splitting to regenerate NADPH. Thus, open stomata indirectly keep the light reactions (including water splitting) running by supplying CO2 for the dark reaction.

To prevent excessive water loss (transpiration) in hot, dry conditions, plants close their stomata. However, this closure also blocks the entry of carbon dioxide. Without a supply of CO2, the reduction of carbon dioxide to carbohydrates cannot take place, regardless of how much light is available. The plant may then undergo photorespiration, which is wasteful, and its growth slows down.

The correct sequence is: First, chlorophyll absorbs light energy (2). This absorbed energy is then converted into chemical energy (ATP and NADPH) (4). This chemical energy is used to split water molecules into hydrogen and oxygen (3) – though splitting actually occurs alongside ATP formation. Finally, the hydrogen and chemical energy are used for the reduction of carbon dioxide to carbohydrates (1). This order reflects how energy flows from absorption to final food synthesis.