LIFE PROCESS- 01 EXPLAINED

While growth, reproduction, and movement are commonly seen in living organisms, they are not universal criteria. For example, mules cannot reproduce, and plants do not show visible movement from place to place. The most fundamental feature of life is the presence of cells and the ability to perform metabolic reactions (breakdown and synthesis of molecules). Non-living things do not have a controlled internal chemical system.

Growth alone can be misleading because non-living things like mountains or crystals can also grow by adding more material. Respiration, which involves the breakdown of glucose to release energy at the cellular level, is a definite sign of life. Even if an organism does not move or reproduce, if its cells are respiring, it is alive.

Metabolism includes two types of reactions: anabolism (building up complex molecules from simpler ones) and catabolism (breaking down complex molecules to release energy). All other life processes like nutrition, respiration, and excretion are actually parts of metabolism. Without metabolism, a cell or organism cannot be considered alive.

Transpiration is the loss of water vapor from the aerial parts of plants, mainly through stomata. It is a life process for plants but does not occur in humans. Humans perform respiration, excretion, and nutrition, but they lose water through sweating and breathing out vapor, which is not called transpiration.

Nutrition is the entire process that includes taking in food, breaking it down, absorbing nutrients, and using them for energy, growth, and repair. Digestion and absorption are only parts of nutrition, while assimilation is the step where absorbed nutrients are incorporated into body tissues.

Autotrophs (auto = self, troph = nourishment) are organisms that can produce their own organic food from carbon dioxide and water using an energy source. Most plants are autotrophs because they use sunlight (photosynthesis). Heterotrophs depend on others for food. Saprotrophs and parasites are types of heterotrophs.

Autotrophic nutrition means making food from inorganic sources. Green algae contain chlorophyll and can perform photosynthesis just like plants, converting carbon dioxide and water into glucose in the presence of sunlight. All other options represent heterotrophic nutrition where organisms depend directly or indirectly on others for food.

Most autotrophs (photoautotrophs) use sunlight as their energy source. They trap solar energy using pigments like chlorophyll and convert it into chemical energy stored in glucose. Some bacteria use chemical energy (chemosynthesis), but for Class 10 level, autotrophic nutrition in green plants is driven by solar energy.

During photosynthesis, plants take in carbon dioxide from the air through stomata and water from the soil through roots. These two raw materials are then converted into glucose and oxygen. Chlorophyll and sunlight are conditions or requirements, not raw materials. Glucose is the product, not a raw material.

The very first event in photosynthesis is the absorption of light energy by chlorophyll molecules present in the chloroplasts. This energy is used to split water molecules (photolysis) and generate energy carriers (ATP and NADPH). The actual fixation of carbon dioxide into glucose happens later in the dark reaction (Calvin cycle).

When light energy strikes chlorophyll, it energizes electrons. To replace these electrons, water molecules are split (photolysis) into oxygen (O2), hydrogen ions (protons, H+), and electrons (e-). Oxygen is released as a byproduct, while protons and electrons are used to form ATP and NADPH, which are then used in the dark reaction.

The dark reaction does not require light directly. It uses the ATP and NADPH generated in the light reaction to fix carbon dioxide from the air. Through a series of enzyme-driven steps, carbon dioxide is reduced (gains hydrogen) to form glucose. This is where inorganic carbon becomes part of an organic molecule.

In C3 plants, carbon dioxide combines with a 5-carbon compound called RuBP (ribulose bisphosphate) using the enzyme RuBisCO. This immediately forms an unstable 6-carbon compound that breaks into two molecules of 3-phosphoglyceric acid (PGA), which is a 3-carbon compound. Glucose is formed much later after several steps.

The oxygen produced during the light reaction (from photolysis of water) is not used by the plant in photosynthesis. It diffuses out of the chloroplast, then out of the leaf through stomata, and is released into the atmosphere. This oxygen is what most living organisms, including humans, use for respiration.

Chloroplasts are specialized organelles found in plant cells, especially in leaves. They contain chlorophyll and other pigments arranged in structures called thylakoids. The light reaction occurs in the thylakoid membranes, and the dark reaction occurs in the stroma (fluid part) of the chloroplast. Mitochondria are for respiration, not photosynthesis.

Glucose is the immediate product of photosynthesis, but it is quickly converted into other forms. Excess glucose is stored as starch (in leaves, roots, seeds), used to build cellulose for cell walls, or converted into oils and proteins (after adding nitrogen and other minerals). Plants do not store large amounts of free glucose because it would disrupt water balance.

When chlorophyll loses electrons after being excited by light, it becomes positively charged. To regain electrons, the chlorophyll molecule pulls electrons from water, causing water to split (photolysis). This reaction is: 2H2O → 4H+ + 4e- + O2. The electrons replace those lost by chlorophyll, allowing photosynthesis to continue.

Autotrophs (plants, algae, some bacteria) have chlorophyll and can convert inorganic carbon dioxide and water into organic glucose using light energy. Heterotrophs (animals, fungi, most bacteria) lack this ability and must obtain organic food by consuming other organisms. This is the fundamental nutritional difference between plants and animals.

Absorbed light energy excites electrons in chlorophyll to a higher energy state. These energized electrons move through an electron transport chain (like electricity), and this flow is used to pump protons and ultimately produce ATP (chemical energy) and NADPH (reducing power). Glucose is formed later using ATP and NADPH, not directly from light.

Carbon dioxide enters the leaf through stomata, moves through intercellular spaces, and enters the mesophyll cells. Inside the chloroplast, it reaches the stroma, where the enzyme RuBisCO attaches it to a 5-carbon compound (RuBP). This fixation step incorporates the inorganic CO2 into an organic molecule (PGA), beginning the formation of glucose.

Autotrophic nutrition (photosynthesis) requires carbon dioxide, water, sunlight, and chlorophyll. Oxygen is actually a byproduct of the process, not a requirement. In fact, high concentrations of oxygen can compete with carbon dioxide for the active site of RuBisCO, leading to photorespiration, which is wasteful for the plant.

ATP provides the energy, and NADPH provides the hydrogen atoms (reducing power) needed to convert carbon dioxide into glucose during the Calvin cycle (dark reaction). After their use, ATP becomes ADP + Pi, and NADPH becomes NADP+, both of which return to the light reaction to be recharged. This creates a cyclic flow of energy carriers.

Stomata are tiny pores mainly on the underside of leaves. Each stoma is surrounded by guard cells that swell with water to open the pore. When open, carbon dioxide from the atmosphere diffuses into the leaf for photosynthesis, and the oxygen produced diffuses out. Stomata also allow water vapor to escape (transpiration).

The light reaction of photosynthesis absolutely requires light to excite chlorophyll and split water. Without light, no ATP or NADPH is produced, so the dark reaction also stops because it depends on these molecules. The plant will live for some time using stored starch, but once reserves are exhausted, it will die due to lack of energy.

Photosynthesis begins when chlorophyll absorbs light. This energy splits water (photolysis), releasing oxygen and providing electrons. The energy is then stored as ATP and NADPH. In the next phase (dark reaction), these energy carriers are used to fix carbon dioxide and eventually build glucose. This is the correct chronological sequence.

While glucose provides carbon skeletons, plants need additional elements. Nitrogen is used to make amino acids (proteins) and nucleic acids. Magnesium is a central atom in the chlorophyll molecule. Phosphorus is a key component of ATP and NADPH. Without these minerals, even with glucose, the plant cannot build complete cells.

Carbon dioxide is a raw material. Initially, increasing CO2 concentration increases the rate of photosynthesis because more substrate is available for RuBisCO. However, after a certain point, other factors like light intensity, temperature, or enzyme capacity become limiting. At very high CO2, the rate plateaus because the plant cannot process it any faster.

Green algae are autotrophs containing chlorophyll. When exposed to sunlight, they perform photosynthesis. As water is split during the light reaction, oxygen gas is produced. Since oxygen is not very soluble in water, it forms visible bubbles. This is a classic experiment to demonstrate that oxygen is released during photosynthesis.

Iodine solution turns blue-black in the presence of starch. During photosynthesis, glucose is produced and then converted into starch for storage. A leaf exposed to sunlight will accumulate starch. This test confirms that photosynthesis happened and that the product (glucose) was polymerized into starch, which is the stored form of food in plants.

Electrons extracted from water are energized by light energy in photosystem II. They then travel through a series of carriers (electron transport chain), losing energy along the way. This energy is used to pump protons and make ATP. At the end of the chain, the electrons reach photosystem I, get re-energized by more light, and finally reduce NADP+ to NADPH. NADPH then carries these electrons (reducing power) to the dark reaction.