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Photosynthesis (play/

foʊtoʊˈsɪnθəsɪs/; from

the Greek φώτο- [photo-],

"light," and σύνθεσις

[synthesis], "putting

together", "composition")

is a process used by

plants and other

organisms to convert the

light energy captured

from the sun into

chemical energy that can

be used to fuel the

organism's activities.

Photosynthesis occurs in

plants, algae, and many

species of bacteria, but

not in archaea.

Photosynthetic organisms

are called

photoautotrophs, since

they can create their own

food. In plants, algae, and

cyanobacteria,

photosynthesis uses

carbon dioxide andwater,

releasing oxygen as a

waste product.

Photosynthesis is vital for

all aerobiclife on Earth. In

addition to maintaining

normal levels of oxygen in

theatmosphere,

photosynthesis is the

source of energy for

nearly all life on earth,

either directly, through

primary production, or

indirectly, as the ultimate

source of the energy in

their food,[1] the

exceptions being

chemoautotrophs that live

in rocks or around deep

seahydrothermal vents.

The average rate of

energy capture by

photosynthesis globally is

immense, approximately

130 terawatts,[2][3][4]

which is about six times

larger than thepower

consumption of human

civilization

.[5] As well as energy,

photosynthesis is also the

source of the carbon in all

the organic compounds

within organisms' bodies.

In all, photosynthetic

organisms convert

around 100–115 thousand

million metric tons (i.e.,

100–115petagrams) of

carbon into biomass per

year.[6][7]

Although photosynthesis

can happen in different

ways in different species,

some features are always

the same. For example,

the process always begins

when energy from light is

absorbed byproteins

called photosynthetic

reaction centers

that contain chlorophylls.

In plants, these proteins

are held insideorganelles

called chloroplasts, while

in bacteria they are

embedded in theplasma

membrane. Some of the

light energy gathered by

chlorophylls is stored in

the form ofadenosine

triphosphate (ATP). The

rest of the energy is used

to removeelectrons from

a substance such as

water. These electrons are

then used in the reactions

that turn carbon dioxide

into organic compounds.

In plants, algae and

cyanobacteria, this is done

by a sequence of

reactions called theCalvin

cycle, but different sets of

reactions are found in

some bacteria, such as

thereverse Krebs cycle in

Chlorobium. Many

photosynthetic organisms

haveadaptations that

concentrate or store

carbon dioxide. This helps

reduce a wasteful process

calledphotorespiration

that can consume part of

the sugar produced

during photosynthesis.

Schematic of

photosynthesis in plants

The first photosynthetic

organisms probably

evolved about 3,500

million years ago, early in

theevolutionary history of

life, when all forms of life

on Earth were

microorganisms and the

atmosphere had much

more carbon dioxide.

They most likely used

hydrogen or hydrogen

sulfide as sources of

electrons, rather than

water.[8] Cyanobacteria

appeared later, around

3,000 million years ago,

and drastically changed

the Earth when they

began tooxygenate the

atmosphere, beginning

about 2,400 million years

ago.[9] This new

atmosphere allowed the

evolution of complex life

such as protists.

Eventually, no later than a

billion years ago, one of

these protists formed a

symbiotic relationship

with a cyanobacterium,

producing the ancestor of

many plants and algae.

[10] The chloroplasts in

modern plants are the

descendants of these

ancient symbiotic

cyanobacteria.[11]

Overview

Photosynthesis changes

sunlight into chemical

energy, splits water to

liberate O2, and fixes CO2

into sugar.

Photosynthetic organisms

arephotoautotrophs,

which means that they

are able tosynthesize

food directly from carbon

dioxide and water using

energy from light.

However, not all

organisms that use light

as a source of energy

carry out photosynthesis,

sincephotoheterotrophs

use organic compounds,

rather than carbon

dioxide, as a source of

carbon.[1] In plants, algae

and cyanobacteria,

photosynthesis releases

oxygen. This is called

oxygenic photosynthesis.

Although there are some

differences between

oxygenic photosynthesis

inplants, algae, and

cyanobacteria, the overall

process is quite similar in

these organisms.

However, there are some

types of bacteria that

carry outanoxygenic

photosynthesis

, which consumes carbon

dioxide but does not

release oxygen.