Life Processes in Living Organisms Part 1 — Maharashtra (SSC) Class 10 Science Solutions (Free)

Nutrition is the process by which an organism obtains and processes the raw materials and energy it needs for growth, repair, and every other life activity. Autotrophic nutrition — the organism makes its own food, rather than consuming other organisms: - Photosynthetic autotrophs (green plants, algae) use sunlight, water and carbon dioxide to synthesise glucose, releasing oxygen as a by-product. - Chemosynthetic autotrophs (certain bacteria, often found in deep-sea vents with no sunlight) instead obtain energy by oxidising inorganic chemicals like hydrogen sulphide. Heterotrophic nutrition — the organism cannot make its own food and must consume organic matter from other organisms: - Herbivores feed on plants (e.g. a cow eating grass). - Carnivores feed on other animals (e.g. a lion eating prey). - Omnivores feed on both (e.g. humans, bears). - Decomposers (bacteria and fungi) break down dead organic matter, recycling nutrients back into the ecosystem. Regardless of the mode of nutrition, every organism ultimately requires the same core building-block elements: carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, along with smaller amounts of various minerals — nutrition is simply the different strategies organisms use to acquire these elements in usable form.

Photosynthesis happens in two linked stages, both occurring inside the chloroplast, but in different compartments. Light reactions (occur in the thylakoid membrane): Chlorophyll pigments absorb light energy (photons), which excites electrons to a higher energy state. This triggers the splitting of water molecules (photolysis), releasing oxygen as a by-product: 2H₂O + light energy → 4H⁺ + O₂ + 4e⁻ The excited electrons pass along an electron transport chain, pumping H⁺ ions across the membrane to build a concentration gradient. This gradient drives the production of ATP and NADPH — energy-carrying molecules that the second stage needs. Dark reactions / Calvin cycle (occur in the stroma, and do not directly need light, though they depend entirely on the ATP and NADPH the light reactions supplied): Carbon dioxide is "fixed" (captured) by the enzyme RuBisCO onto a 5-carbon molecule, forming an unstable 6-carbon intermediate that immediately splits into two 3-carbon molecules (3-PGA). Using the ATP and NADPH from the light reactions, these are reduced into G3P, some of which is used to build glucose, while the rest regenerates the original 5-carbon starting molecule (RuBP) so the cycle can continue. Why the two stages depend on each other: the light reactions cannot make glucose themselves — they only capture light energy into ATP and NADPH. The Calvin cycle cannot run without those two molecules to supply energy and reducing power. Overall, the combined result is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂.

Respiration is the biochemical process of oxidising glucose (or other food molecules) to release the chemical energy stored in it, captured mainly in the form of ATP. Aerobic respiration (requires oxygen, occurs mainly in the mitochondria): C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (≈2,880 kJ per mole of glucose) It proceeds through three stages: glycolysis (in the cytoplasm, net yield 2 ATP), the Krebs cycle (in the mitochondrial matrix, generating NADH and FADH₂ as electron carriers), and the electron transport chain (on the inner mitochondrial membrane, generating the bulk of the ATP, roughly 32 more). Anaerobic respiration (occurs without oxygen, entirely in the cytoplasm): Glucose is only partially broken down — for example, into ethanol and CO₂ in yeast (alcoholic fermentation), or into lactate in oxygen-starved muscle cells — yielding a net of only 2 ATP. Why anaerobic respiration is far less efficient: since glucose is not fully oxidised all the way to CO₂ and H₂O, most of its stored chemical energy remains locked in the leftover ethanol or lactate rather than being captured as usable ATP — this is exactly why anaerobic respiration yields roughly 16 times less ATP per glucose molecule than the complete aerobic pathway.

Aerobic respiration proceeds through three connected stages, each in a different part of the cell: 1. Glycolysis — location: cytoplasm. One molecule of glucose (6 carbons) is broken down into two molecules of pyruvate (3 carbons each), with a net production of 2 ATP and 2 NADH. This stage does NOT require oxygen, which is why it also runs during anaerobic respiration. 2. Krebs cycle (citric acid cycle) — location: mitochondrial matrix. Each pyruvate is first converted into acetyl-CoA, which then enters the cycle. Across roughly 8 enzyme-catalysed steps, the carbon skeleton is completely oxidised, releasing CO₂ and generating (per acetyl-CoA, so twice per original glucose) 3 NADH, 1 FADH₂, and 1 ATP. 3. Electron transport chain — location: the cristae (folds of the inner mitochondrial membrane). The NADH and FADH₂ generated in the earlier stages release their stored electrons here. These electrons pass through a series of protein complexes (labelled I, III, IV), and the energy released pumps H⁺ ions across the membrane, building a concentration gradient. This gradient then drives ATP synthase to generate the majority of the cell's ATP — roughly 32 more molecules. Oxygen's essential role is to act as the FINAL electron acceptor at the end of this chain, combining with the spent electrons and H⁺ ions to form water — without oxygen to accept these electrons, the whole chain backs up and stops.

The mitochondrion is often called the "powerhouse of the cell" because it is the primary site where aerobic respiration's energy-yielding later stages occur. Structure: - Outer membrane — smooth and freely permeable to small molecules. - Inner membrane — highly impermeable, and folded into finger-like projections called cristae, which greatly increase its surface area. - Matrix — the fluid-filled interior space, where the Krebs cycle takes place. Why the structure matters functionally: The cristae fold of the inner membrane houses the electron transport chain protein complexes and the ATP synthase enzyme, packed densely along its greatly increased surface area — more folded membrane surface means more room for these complexes, and therefore a greater capacity to generate ATP. The proton (H⁺) gradient built up across this inner membrane during the electron transport chain is what physically drives ATP synthase to produce ATP, in a process called chemiosmosis. Overall energy yield: each glucose molecule yields roughly 30–32 ATP from the mitochondrial stages (Krebs cycle plus electron transport chain) combined, dwarfing the mere 2 ATP that glycolysis alone produces in the cytoplasm — which is exactly why cells with very high energy demands, like muscle and liver cells, are packed with unusually large numbers of mitochondria.

Photosynthesis (occurs only in plants, algae, and photosynthetic bacteria): builds glucose FROM carbon dioxide and water, using light energy as the power source. It is an endergonic process — it stores energy rather than releasing it. 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂ Respiration (occurs in ALL living organisms, all the time): breaks glucose DOWN into carbon dioxide and water, releasing the stored energy as ATP. It is an exergonic process. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP) How they are complementary: Notice that the equations are exact mirror images of each other. The oxygen and glucose that photosynthesis PRODUCES are precisely what respiration CONSUMES; and the carbon dioxide and water that respiration PRODUCES are precisely what photosynthesis CONSUMES. Together, these two processes form a closed cycle that keeps Earth's atmospheric oxygen and carbon dioxide levels in a long-term balance — plants (which both photosynthesise and respire) are typically net producers of oxygen and net consumers of carbon dioxide over a full day, while animals, capable only of respiration, are entirely dependent on this cycle continuing.

The Respiratory Quotient (RQ) is the ratio of the volume of carbon dioxide released to the volume of oxygen consumed during respiration: RQ = CO₂ released / O₂ consumed RQ for carbohydrates: from the equation C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, exactly 6 molecules of CO₂ are produced for every 6 molecules of O₂ consumed, so RQ = 6/6 = 1.0. RQ for fats: fat molecules contain proportionally far less oxygen within their own structure than carbohydrates do, so more external O₂ must be consumed to fully oxidise them relative to the CO₂ produced, giving a typical RQ of around 0.7. RQ for proteins: protein metabolism is more complex, since it also involves excreting nitrogen-containing waste, giving an intermediate typical RQ of around 0.8–0.9. Why RQ is useful: by measuring the ratio of CO₂ released to O₂ consumed in an organism's breath (expired gas), physiologists can estimate WHICH substrate — carbohydrate, fat, or protein — the body is predominantly burning for energy at that time, without needing to directly sample blood or tissue. An RQ close to 1.0 during exercise, for instance, suggests the body is relying heavily on carbohydrate stores, while a lower RQ during rest or fasting suggests a shift toward burning fat reserves.