Attributes of Living organisms

Every living thing shares a bundle of properties that, together, mark it out from non-living matter - no single property is by itself a perfect test. Cellular structure: every organism is built of cells, which are either prokaryotic (no true nucleus or membrane-bound organelles - bacteria, blue-green algae) or eukaryotic (a definite nucleus plus membrane-bound organelles - eukaryotes can be unicellular, like Amoeba, or multicellular, like Oedogonium). Growth in a living thing is intrinsic - it happens by the addition of new protoplasm and by cell division from within the organism itself. Plants keep growing indefinitely through life; animals grow only for a defined period and then stop, although cell division continues afterwards for tissue repair. Non-living things such as mountains or sand dunes can also 'grow', but only extrinsically, by piling up material on their surface - nothing is being manufactured internally. Reproduction is the tendency of an organism to perpetuate its own kind, and it comes in two flavours: asexual reproduction (offspring closely resembling the parent, produced by mechanisms such as conidia formation in Aspergillus/Penicillium, budding in Hydra/Yeast, binary fission in Bacteria/Amoeba, fragmentation in Spirogyra, or regeneration in Planaria) and sexual reproduction, which introduces variation through recombination. Sterile worker bees and mules are the classic exceptions - individually incapable of reproducing, yet still unquestionably alive. Because growth in unicellular organisms happens through cell division, and that same division is also how their population grows, growth and reproduction end up tightly linked in such organisms. Metabolism is the sum of every chemical reaction running inside a cell, and it splits into anabolism (building larger molecules from smaller ones, storing chemical energy - e.g. synthesising proteins from amino acids) and catabolism (breaking larger molecules down into smaller ones, releasing stored energy - e.g. breaking glucose down to CO2 and water). Response to stimuli: every organism senses and reacts to its physical, chemical or biological surroundings. In animals, sensing through dedicated sense organs is called consciousness; in plants, which lack sense organs, the equivalent responsiveness - such as a sunflower bending toward light or a touch-me-not plant folding its leaflets on contact - is called irritability. Homeostasis is the property of self-regulation that lets an organism hold a stable internal state even while its external environment keeps changing; it is essential for survival. Movement, nutrition, respiration and excretion round out the list of properties associated with living things. …