Ch10: Living Creatures — Exploring Their Characteristics

All living organisms share seven fundamental characteristics (MRS GREN). These are what distinguish living from non-living matter. However, viruses occupy a grey area — they have genetic material (DNA/RNA) and can reproduce, but only inside a host cell, have no cellular structure, no metabolism of their own, and do not respond to stimuli independently. Most biologists classify viruses as non-living outside host cells, but "alive" when actively replicating inside cells. This philosophical debate appears occasionally in UPSC.

Cell theory (three tenets):

1. All living organisms are composed of one or more cells.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells (Omnis cellula e cellula — Rudolf Virchow, 1855).

Robert Hooke (1665): First observed and named "cells" looking at thin slices of cork under a microscope — the box-like compartments reminded him of monks' cells in a monastery. He was observing dead plant cell walls.

Antonie van Leeuwenhoek: First observed living microorganisms ("animalcules") — bacteria and protozoa — in pond water and dental scrapings using hand-crafted microscopes (1670s).

Prokaryotic vs. Eukaryotic cells:

Prokaryotes (bacteria and archaea) are the oldest life forms (~3.5 billion years old). They have no membrane-bound nucleus — DNA floats in a region called the nucleoid. They lack organelles like mitochondria and chloroplasts (though they carry out equivalent functions in the cell membrane itself).

Eukaryotes evolved later (~2 billion years ago), likely through endosymbiosis — an ancestral cell engulfed a bacterium that became the mitochondrion; a photosynthetic bacterium became the chloroplast. Evidence: mitochondria and chloroplasts have their own circular DNA and ribosomes (like bacteria), and divide by binary fission independently.

Plant vs. Animal cells (key differences):

• Plant cells have: cell wall (cellulose), chloroplasts (for photosynthesis), large central vacuole (maintains turgor pressure).
• Animal cells have: centrioles (for cell division), no cell wall, smaller or no vacuoles.

Photosynthesis equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ (glucose) + 6O₂

Chlorophyll pigment in chloroplasts absorbs sunlight (mainly red and blue wavelengths; reflects green — which is why plants look green). Carbon dioxide is fixed into glucose. Oxygen is released as a by-product — the source of Earth's atmospheric oxygen, which transformed the early atmosphere ~2.4 billion years ago (Great Oxidation Event).

Decomposers — fungi and bacteria — are ecologically crucial: they break down dead organic matter and recycle nutrients (nitrogen, phosphorus, carbon) back into the soil and atmosphere. Without decomposers, nutrients would be locked in dead matter and ecosystems would collapse.

Chemoautotrophs at deep-sea hydrothermal vents derive energy from oxidising inorganic compounds (H₂S, NH₃) — no sunlight needed. These are the basis of chemosynthetic ecosystems, proving that life does not require solar energy — relevant to the search for life on other planets (icy moons like Europa, Enceladus).

Cellular respiration vs. breathing — a critical distinction:

• Breathing (External respiration): Mechanical process of inhaling O₂ and exhaling CO₂; involves lungs, diaphragm, ribcage; an organism-level process.
• Cellular respiration: Chemical process occurring inside cells; breaks down glucose to release ATP energy.

Aerobic respiration (with oxygen): Glucose + 6O₂ → 6CO₂ + 6H₂O + ~36–38 ATP (energy) Occurs in mitochondria. Efficient — releases maximum energy.

Anaerobic respiration (without oxygen):

• In humans/animals (muscle cells during intense exercise): Glucose → Lactic acid + 2 ATP. Lactic acid accumulation causes muscle cramps.
• In yeast/bacteria: Glucose → Ethanol + CO₂ + 2 ATP. This is fermentation — the basis of bread (yeast CO₂ makes dough rise), beer, wine, vinegar production. Also used in biofuel (bioethanol from sugarcane — Brazil model; India's Ethanol Blending Programme targets 20% blending in petrol by 2025-26).

Organs of excretion and their waste products:

• Kidneys: Filter blood; excrete urea (protein metabolism waste), excess water, salts in urine. India's kidney disease burden: ~17% of Indians have chronic kidney disease (CKD) — partly linked to fluoride/arsenic in groundwater, diabetes, hypertension.
• Lungs: Excrete CO₂ and water vapour during exhalation.
• Skin: Sweat glands excrete salts and small amounts of urea; sweating also serves thermoregulation.
• Liver: Converts haemoglobin waste into bilirubin (excreted in bile into intestines — gives faeces its brown colour). Also detoxifies ammonia → urea (urea cycle).

Plants excrete oxygen (by-product of photosynthesis), CO₂ and water vapour (by-product of respiration), excess water through transpiration, and some store waste in resin, gum, latex (rubber), or shed it with leaves.