= Solution
\b[The main differences concern cellular construction and the usual modes of obtaining food, movement and development; the main interdependence concerns transfers of matter and <energy> through <ecosystems>.] The nineteenth-century animal–vegetable division is useful as a comparison of familiar forms, but it is not a complete modern classification of life.
A typical <plant cell> has a <cell wall>, a large central <vacuole> and, in photosynthetic tissue, <chloroplasts>. An <animal cell> lacks a cellulose wall and chloroplasts and has a different extracellular and tissue organization. Both possess a <cell membrane>, <cytoplasm>, a <cell nucleus> and, in their usual aerobic cells, <mitochondria>. Roots and other non-green parts of a plant need not contain functional <chloroplasts>; an erythrocyte's loss of its <cell nucleus> is likewise not a definition of all animal cells.<Biology 2e by Mary Ann Clark and others/4.3 Eukaryotic Cells>[Animal Cells versus Plant Cells; plant cell walls, vacuoles and chloroplasts]{ref}
Most green <plants> acquire carbon through <photosynthesis>, taking <carbon dioxide> from their surroundings and mineral nutrients and <water> chiefly through roots. Familiar <animals> obtain preformed organic material by feeding and digesting it or absorbing material derived from a host. Animals generally have conspicuous coordinated movement, and many have <nervous systems> and <muscles>; higher plants regulate growth and movement without such organs. These are useful generalizations with exceptions. Some plants depend on other organisms for carbon; some animals are permanently attached; and neither lack of locomotion nor a green appearance alone settles every microscopic classification. <Fungi> and <bacteria> should not be forced into one of these two kingdoms in a modern account.
One major interdependence is nutritional. <Photosynthesis> converts light input into chemical <energy> stored in organic material. Plant material feeds herbivores and, through them, carnivores; it also enters detrital food webs. An animal that eats an animal can therefore remain indirectly dependent on primary production by <plants>, algae or other photosynthetic organisms. Chemical elements pass between organisms, but <energy> is progressively dissipated as <heat>; it is not recycled in the same closed manner as an element.<Biology 2e by Mary Ann Clark and others/8.1 Overview of Photosynthesis>[Overview and food-chain examples: light capture, organic products and heterotrophic dependence]{ref}
A second relation is gas exchange. In its simplest overall form, oxygenic photosynthesis may be represented by
$$
6\mathrm{CO_2}+6\mathrm{H_2O}+\text{light energy}
\longrightarrow \mathrm{C_6H_{12}O_6}+6\mathrm{O_2}.
$$
Aerobic utilization of the resulting <glucose> has the complementary overall balance
$$
\mathrm{C_6H_{12}O_6}+6\mathrm{O_2}
\longrightarrow6\mathrm{CO_2}+6\mathrm{H_2O}+\text{released energy}.
$$
These balanced equations show complementary material transfers without making the underlying pathways literal reversals of one another. \b[Plants also perform <cellular respiration>, by day as well as by night.] They can therefore release <carbon dioxide> while consuming <oxygen>; a sufficiently illuminated leaf may simultaneously show net uptake of <carbon dioxide> because <photosynthesis> exceeds respiration. Stating that plants only purify air and animals only spoil it would erase this essential distinction between gross processes and net exchange.
Other relations close material cycles. Animals return organic matter through excretion and death; microbial decomposition and mineral transformations return nutrients in forms available for plant uptake. Most atmospheric <nitrogen> is not directly available to ordinary plants merely because animals have breathed or excreted near them: <nitrogen fixation>, <ammonification> and often <nitrification> involve particular microorganisms. Plants and animals also interact through <pollination>, <seed dispersal>, grazing, parasitism and shelter. Such relations may benefit both partners, one partner at the other's expense, or different partners under different conditions; interdependence is not universal mutual benevolence.
A closed illuminated aquarium containing plants and animals gives an instructive experiment, provided gas exchange with air, illumination, nutrient supply and organismal amounts are specified. Dissolved <oxygen> and <carbon dioxide> should be followed over light and dark periods. Survival alone is an ambiguous readout: microorganisms, stored food and air exchange may support it temporarily. A more controlled comparison includes illuminated and dark plant vessels and animal-only vessels. The expected alternation of net gas exchange makes the physiological distinction visible while exposing why no simple rule guarantees indefinite balance in any arbitrarily stocked closed jar.
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