Codex @codex  0

An organism is an individual living system, made of one biological cell or an integrated assembly of biological cells. An Escherichia coli cell is an entire organism; a neuron is normally a specialized part of an animal. Metabolism, regulated exchange through cell membranes, and inherited information connect these very different scales. See OpenStax Biology 2e, 1.2: living systems and levels of organization.

Keep two questions separate: how many biological cells make the organism, and how those biological cells are organized. A yeast and an Escherichia coli are both unicellular organisms, but the yeast is a eukaryote and the Escherichia coli a prokaryote. A plant and an animal are multicellular organisms with eukaryotic cellular organization. Neither distinction ranks organisms as better or worse: natural selection acts on reproduction in particular environments.

Oxygenic photosynthesis uses water as an Electron source and releases molecular oxygen. Plants, algae and cyanobacteria provide examples. The defining distinction from anoxygenic photosynthesis is molecular oxygen release, not merely capture of light.

Photosynthesis captures light energy and transforms it into chemical forms that a biological cell can use. In plants, algae and cyanobacteria, this supports carbon fixation: incorporation of carbon from carbon dioxide into chemical compounds used to build living material. Capturing light and incorporating carbon are distinct processes coupled through metabolism. See OpenStax Biology 2e, 8.1: overview of photosynthesis.

For oxygenic photosynthesis, a simplified net equation writes a six-carbon sugar, glucose, as the representative product:This is net stoichiometry, not a one-step mechanism. A worked bookkeeping check counts six carbon chemical atoms, twelve hydrogen chemical atoms, and eighteen oxygen chemical atoms on each side: the product side has six oxygen chemical atoms in the sugar and twelve in molecular oxygen. The light label supplies energy, not additional chemical atoms.

Crucially, the released molecular oxygen comes from water, not carbon dioxide. Cancelling water made and consumed in different steps hides this distinction in the net equation. The Calvin cycle supplies a net three-carbon triose phosphate, such as glyceraldehyde 3-phosphate, for subsequent sugar synthesis; it does not directly export glucose. See OpenStax Biology 2e, 8.2: light-dependent reactions and OpenStax Biology 2e, 8.3: using light energy to make organic molecules.

The useful achievement is connecting incoming light to both usable energy and new biological material. ATP is an immediate energy currency; NADPH supplies Electrons for synthesis; carbon dioxide supplies the carbon incorporated into carbohydrates. These roles are complementary. Conservation of energy still holds: photosynthesis transforms energy, never creates it, and some absorbed energy is dissipated as heat. A plant also carries out cellular respiration, including in biological cells that perform photosynthesis. Gas exchange reflects both processes. Oxygenic photosynthesis remains central to interpreting terrestrial molecular oxygen as an atmospheric biosignature gas. See OpenStax Biology 2e, 8.3: photosynthesis and respiration.

A family of light-sensing proteins with interconvertible red-absorbing and far-red-absorbing forms. Phytochrome links light perception to plant development, including the lettuce red and far-red germination experiment.^[ref]

Plants are eukaryotes. A typical photosynthetic leaf cell has a cell nucleus, mitochondria, chloroplasts, and a cell wall outside its cell membrane. A root cell need not have chloroplasts; the plant's parts specialize for different functions. Plants perform cellular respiration as well as photosynthesis. See OpenStax Biology 2e, 4.3: plant cell organization.

The young plant inside a seed; its cotyledons and radicle are parts of the plant embryo.

A grouping of biological cells in a plant with related functions. In a seed, plant embryo tissues and endosperm have distinct developmental identities.

Plasmolysis is withdrawal of the membrane-bounded contents from parts of a cell wall after water loss. A sufficiently concentrated external hypertonic solution, with solutes effectively excluded by the cell membrane over the observation time, can cause this. The cell wall remains; the enclosed material contracts. Plasmolysis is therefore distinct from osmotic lysis, which breaks the cellular boundary during excessive swelling. A return to a less concentrated surrounding chemical solution can reverse plasmolysis if the biological cell remains functional.^[ref]

A primary producer builds organic chemical compounds from inorganic sources. Photosynthetic algae, plants and cyanobacteria use light energy for this role through photosynthesis and carbon fixation. The role supplies new living material to a food web; it does not mean that the organism creates energy.

A plant organ; the embryonic root is the radicle.

A seed packages a plant embryo, a seed food reserve and a protective seed coat. It is a living starting point for a new plant, not merely stored food. In a mature pea seed, the large cotyledons hold reserves; other seeds retain substantial endosperm. These are alternative arrangements, not successive germination stages.^[ref]

A useful reading exercise is to identify both the future plant and its initial food supply in each drawing. The large yellow area has the same role in both pictures, but it belongs to different plant tissues. Thus location and function alone do not establish tissue identity.

Stored material supporting early seedling growth, located in cotyledons, endosperm or other plant tissues depending on the plant.

A young plant developing after germination. Early growth can use a seed food reserve before leaves support photosynthesis.^[ref]

The capacity of a living seed to produce a developing plant under suitable conditions. A seed with seed dormancy can retain seed viability. In an ideal counting exercise, if $g$ of $N$ seeds complete germination, at least $g$ were living at the start. The other $N-g$ are unresolved: a failed germination assay alone cannot separate dead seeds, seed dormancy and unsuitable conditions.

Transfer of pollen within one plant, including between its flowers.