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A cell wall is a supporting structure outside a cell membrane. It helps preserve shape and bears the load when osmosis raises internal pressure. The cell membrane supplies the selective permeability described in its own article; the cell wall supplies mechanical reinforcement. These functions cooperate: a porous supporting layer can restrain expansion without being the boundary that retains dissolved solutes. Turgor pressure makes this cooperation visible in a firm leaf; plasmolysis and a lysozyme wall-removal experiment separate the two functions experimentally.^[ref]

The material is not universal. Most bacteria use peptidoglycan, whose linked sugar chains and amino acid cross-links are explained below. A plant cell wall uses cellulose; a fungal cell wall contains chitin and glucans. Animals lack a cell wall, some bacteria are naturally wall-free, and archaea have different wall chemistry. Thus a cell wall is neither a universal cellular component nor one chemical recipe.^{[ref][ref][ref]}

The bacterial outer membrane in typical Gram-negative bacteria is an additional membrane outside the peptidoglycan, not another name for that supporting mesh. The cell envelope includes the boundary layers together. An extra barrier can change access to the peptidoglycan, so response to lysozyme cannot be inferred from the mere presence of a cell wall.^[ref]

A mitochondrion is an organelle with an outer mitochondrial membrane and an inner mitochondrial membrane. The latter encloses the mitochondrial matrix and folds into cristae; the intermembrane space lies on its opposite side. This architecture separates a chemical workspace from a membrane that couples electron transport to ATP formation.^[ref]

For an original location exercise, place soluble label A in the mitochondrial matrix and soluble label B in the interior of a crista. In the depicted geometry, B can reach the peripheral intermembrane space through the fold's opening without crossing a lipid bilayer. A must cross the inner mitochondrial membrane to reach either location. A fold projecting into the mitochondrial matrix therefore does not make its enclosed fluid part of the mitochondrial matrix. Following a continuous path through the colored regions proves the distinction; merely asking whether a label is “inside the mitochondrion” loses it.

The functional connection is equally spatial. The mitochondrial respiratory chain pumps protons from the mitochondrial matrix side across the inner mitochondrial membrane. Their return through ATP synthase can drive ATP formation. Oxidative phosphorylation develops the mechanism and chemiosmosis gives its free energy bookkeeping. Compartmentalization is useful because it lets a shared electrochemical gradient couple different proteins without direct transfer of a chemical intermediate between each pair.

Under aerobic conditions, mitochondria supply much of the ATP in many eukaryotic biological cells; the contribution varies with biological tissue, demand and oxygen supply. Glycolysis also makes ATP in the cytosol. Mature mammalian red blood cells lack mitochondria, and bacteria have no mitochondria at all. Neither all ATP production nor all biological cells fit the familiar mitochondrial picture.^[ref]

Mitochondria descend from a bacterial endosymbiosis and retain a genome and ribosomes, although many required proteins are encoded by nuclear genes. Endosymbiotic theory develops the ancestry argument; organelle protein import explains delivery of nuclear-encoded proteins. A functioning isolated mitochondrion demonstrates biochemical autonomy under supplied conditions, rather than independently establishing that ancestry.

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.^[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.^[ref]

A second relation is gas exchange. In its simplest overall form, oxygenic photosynthesis may be represented byAerobic utilization of the resulting glucose has the complementary overall balanceThese balanced equations show complementary material transfers without making the underlying pathways literal reversals of one another. 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.