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Many photosynthetic organisms have a mixture of pigments; using them, the organism can absorb energy from a wider range of wavelengths. Not all photosynthetic organisms have full access to sunlight. Some organisms grow underwater where light intensity and quality decrease and change with depth. Other organisms grow in competition for light. Plants on the rainforest floor must be able to absorb any bit of light that comes through, because the taller trees absorb most of the sunlight and scatter the remaining solar radiation ( [link] ).

The photo shows undergrowth in a forest.
Plants that commonly grow in the shade have adapted to low levels of light by changing the relative concentrations of their chlorophyll pigments. (credit: Jason Hollinger)

When studying a photosynthetic organism, scientists can determine the types of pigments present by generating absorption spectra. An instrument called a spectrophotometer    can differentiate which wavelengths of light a substance can absorb. Spectrophotometers measure transmitted light and compute from it the absorption. By extracting pigments from leaves and placing these samples into a spectrophotometer, scientists can identify which wavelengths of light an organism can absorb. Additional methods for the identification of plant pigments include various types of chromatography that separate the pigments by their relative affinities to solid and mobile phases.

How light-dependent reactions work

The overall function of light-dependent reactions is to convert solar energy into chemical energy in the form of NADPH and ATP. This chemical energy supports the light-independent reactions and fuels the assembly of sugar molecules. The light-dependent reactions are depicted in [link] . Protein complexes and pigment molecules work together to produce NADPH and ATP.

 Illustration a shows the structure of PSII, which is embedded in the thylakoid membrane. At the core of PSII is the reaction center. The reaction center is surrounded by the light-harvesting complex, which contains antenna pigment molecules that shunt light energy toward a pair of chlorophyll a molecules in the reaction center. As a result, an electron is excited and transferred to the primary electron acceptor. A water molecule is split, releasing two electrons which are used to replace excited electrons. Illustration b shows the structure of PSI, which is similar in structure to PSII. However, PSII uses an electron from the chloroplast electron transport chain also embedded in the thylakoid membrane to replace the excited electron.
A photosystem consists of a light-harvesting complex and a reaction center. Pigments in the light-harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center. The light excites an electron from the chlorophyll a pair, which passes to the primary electron acceptor. The excited electron must then be replaced. In (a) photosystem II, the electron comes from the splitting of water, which releases oxygen as a waste product. In (b) photosystem I, the electron comes from the chloroplast electron transport chain discussed below.

The actual step that converts light energy into chemical energy takes place in a multiprotein complex called a photosystem    , two types of which are found embedded in the thylakoid membrane, photosystem II    (PSII) and photosystem I    (PSI) ( [link] ). The two complexes differ on the basis of what they oxidize (that is, the source of the low-energy electron supply) and what they reduce (the place to which they deliver their energized electrons).

Both photosystems have the same basic structure; a number of antenna proteins to which the chlorophyll molecules are bound surround the reaction center    where the photochemistry takes place. Each photosystem is serviced by the light-harvesting complex , which passes energy from sunlight to the reaction center; it consists of multiple antenna proteins that contain a mixture of 300–400 chlorophyll a and b molecules as well as other pigments like carotenoids. The absorption of a single photon    or distinct quantity or “packet” of light by any of the chlorophylls pushes that molecule into an excited state. In short, the light energy has now been captured by biological molecules but is not stored in any useful form yet. The energy is transferred from chlorophyll to chlorophyll until eventually (after about a millionth of a second), it is delivered to the reaction center. Up to this point, only energy has been transferred between molecules, not electrons.

Questions & Answers

how does meiosis produced
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wana Reply
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Hope Reply
effect of planning beans of using fertilizer
Elizerbeth Reply
what do we mean by transgenic organisms?
FADILAT Reply
what is or are the functions of the Islets of Langarhaans
FADILAT
They are the regions of the pancreas that contains the endocrine cell
Iyadi
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is the study of living organism and their interection with one another and their environment.
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what is soil
Mukisa Reply
the top layer of the earth in which plant's, tree's
Ahmad
type of soil
Asunta
function of cell wall
Nthati Reply
function of cell wall
Asunta
To protect the cell from bursting
Maurice
to protect the cell from bursting
Deborah
to protect all other internal components of the cell
Olaoye
What is escherichia coli
Tumise Reply
It's an example of gram negative Bacteria
Abdulrasheed
in what type of cells is meiosis taking place?
Rhyeann Reply
sex cells
Eric
hlo
Amit
reproductive system of earthworm plzz describes
Amit
procryotic cell and some eucaryotic cell
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Reproductive or sex cells
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applications of biology
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what is dormancy?
Aliyu Reply
hello guys what's the difference between prokaryotes and eukaryotes
Nwachukwu Reply
hlo what are the applications of biology?
Namawejje
eukaryotic cells have DNA in their nucleus while prokaryotic cells have their DNA present freely in their cytoplasm.
FADILAT
deviation from mendelian
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what is lethal allele
Ogali
a lethal allele is an allele that can cause poor development or even death of an organism
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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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