Editing Water (section)

==Effects on life==
[[File:Auto-and heterotrophs.svg|thumb|upright|Overview of [[photosynthesis]] <span style="color:green;">(green)</span> and [[cellular respiration|respiration]] <span style="color:red;">(red)</span>]]

From a [[biology|biological]] standpoint, water has many distinct properties that are critical for the proliferation of life. It carries out this role by allowing [[organic compound]]s to react in ways that ultimately allow [[Self-replication|replication]]. All known forms of life depend on water. Water is vital both as a [[solvent]] in which many of the body's solutes dissolve and as an essential part of many [[metabolism|metabolic]] processes within the body. Metabolism is the sum total of [[anabolism]] and [[catabolism]]. In anabolism, water is removed from molecules (through energy requiring enzymatic chemical reactions) in order to grow larger molecules (e.g., starches, triglycerides, and proteins for storage of fuels and information). In catabolism, water is used to break bonds in order to generate smaller molecules (e.g., glucose, fatty acids, and amino acids to be used for fuels for energy use or other purposes). Without water, these particular metabolic processes could not exist.

Water is fundamental to both photosynthesis and respiration. Photosynthetic cells use the sun's energy to split off water's hydrogen from oxygen.<ref>{{Cite web|title=Catalyst helps split water: Plants|url=https://asknature.org/strategy/catalyst-helps-split-water/|access-date=10 September 2020|website=AskNature|language=en-US|archive-date=28 October 2020|archive-url=https://web.archive.org/web/20201028194047/https://asknature.org/strategy/catalyst-helps-split-water/|url-status=live}}</ref> In the presence of sunlight, hydrogen is combined with {{chem|C|O|2}} (absorbed from air or water) to form glucose and release oxygen.<ref>{{cite book | last=Hall | first=D.O. | date=2001 | title=Photosynthesis, Sixth edition | url=https://books.google.com/books?id=6F7yuf1Sj30C&dq=process+of+photosynthesis&pg=PR7 | publisher=University of Cambridge | isbn=0-521-64497-6 | access-date=26 August 2023 | archive-date=5 October 2023 | archive-url=https://web.archive.org/web/20231005012445/https://books.google.com/books?id=6F7yuf1Sj30C&dq=process+of+photosynthesis&pg=PR7 | url-status=live }}</ref> All living cells use such fuels and oxidize the hydrogen and carbon to capture the sun's energy and reform water and {{chem|C|O|2}} in the process (cellular respiration).

Water is also central to acid-base neutrality and enzyme function. An acid, a hydrogen ion ({{chem|H|+}}, that is, a proton) donor, can be neutralized by a base, a proton acceptor such as a hydroxide ion ({{chem|O|H|−}}) to form water. Water is considered to be neutral, with a [[pH]] (the negative log of the hydrogen ion concentration) of 7 in an ideal state. [[Acids]] have pH values less than 7 while [[Base (chemistry)|bases]] have values greater than 7.

===Aquatic life forms===
{{Further|Hydrobiology|Marine life|Aquatic plant}}
Earth's surface waters are filled with life. The earliest life forms appeared in water; nearly all fish live exclusively in water, and there are many types of marine mammals, such as dolphins and whales. Some kinds of animals, such as [[amphibian]]s, spend portions of their lives in water and portions on land. Plants such as [[kelp]] and [[algae]] grow in the water and are the basis for some underwater ecosystems. [[Plankton]] is generally the foundation of the ocean [[food chain]].

Aquatic vertebrates must obtain oxygen to survive, and they do so in various ways. Fish have [[gills]] instead of [[lungs]], although some species of fish, such as the [[lungfish]], have both. [[Marine mammal]]s, such as dolphins, whales, [[otter]]s, and [[pinniped|seals]] need to surface periodically to breathe air. Some amphibians are able to absorb oxygen through their skin. Invertebrates exhibit a wide range of modifications to survive in poorly oxygenated waters including breathing tubes (see [[Siphon (insect)|insect]] and [[Siphon (mollusc)|mollusc siphons]]) and [[gills]] (''[[Carcinus]]''). However, as invertebrate life evolved in an aquatic habitat most have little or no specialization for respiration in water.

{{gallery
|align=center
|File:Blue Linckia Starfish.JPG|Some of the [[biodiversity]] of a [[coral reef]]
|File:Diatoms through the microscope.jpg|Some marine [[diatom]]s – a key [[phytoplankton]] group
|File:VonDamm Crustaceans.jpg|[[Squat lobster]] and [[Alvinocarididae]] shrimp at the Von Damm [[Hydrothermal vent|hydrothermal field]] survive by altered water chemistry.}}