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By the end of this section, you will be able to:
  • Explain how plants absorb energy from sunlight
  • Describe short and long wavelengths of light
  • Describe how and where photosynthesis takes place within a plant

How can light be used to make food? When a person turns on a lamp, electrical energy becomes light energy. Like all other forms of kinetic energy, light can travel, change form, and be harnessed to do work. In the case of photosynthesis, light energy is converted into chemical energy, which photoautotrophs use to build carbohydrate molecules ( [link] ). However, autotrophs only use a few specific components of sunlight.

A photo shows the silhouette of a grassy plant against the sun at sunset.
Photoautotrophs can capture light energy from the sun, converting it into the chemical energy used to build food molecules. (credit: Gerry Atwell)

What is light energy?

The sun emits an enormous amount of electromagnetic radiation (solar energy). Humans can see only a fraction of this energy, which portion is therefore referred to as “visible light.” The manner in which solar energy travels is described as waves. Scientists can determine the amount of energy of a wave by measuring its wavelength    , the distance between consecutive points of a wave. A single wave is measured from two consecutive points, such as from crest to crest or from trough to trough ( [link] ).

The illustration shows two waves. The distance between the crests (or troughs) is the wavelength.
The wavelength of a single wave is the distance between two consecutive points of similar position (two crests or two troughs) along the wave.

Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun and other stars. Scientists differentiate the various types of radiant energy from the sun within the electromagnetic spectrum. The electromagnetic spectrum    is the range of all possible frequencies of radiation ( [link] ). The difference between wavelengths relates to the amount of energy carried by them.

The illustration lists the types of electromagnetic radiation in order of increasing wavelength. These include gamma rays, X-rays, ultraviolet, visible, infrared, and radio. Gamma rays have a very short wavelength, on the order of one thousandth of a nanometer. Radio waves have a very long wavelength, on the order of one kilometer. Visible light ranges from 380 nanometers at the violet end of the spectrum, to 750 nanometers at the red end of the spectrum.
The sun emits energy in the form of electromagnetic radiation. This radiation exists at different wavelengths, each of which has its own characteristic energy. All electromagnetic radiation, including visible light, is characterized by its wavelength.

Each type of electromagnetic radiation travels at a particular wavelength. The longer the wavelength (or the more stretched out it appears in the diagram), the less energy is carried. Short, tight waves carry the most energy. This may seem illogical, but think of it in terms of a piece of moving a heavy rope. It takes little effort by a person to move a rope in long, wide waves. To make a rope move in short, tight waves, a person would need to apply significantly more energy.

The electromagnetic spectrum ( [link] ) shows several types of electromagnetic radiation originating from the sun, including X-rays and ultraviolet (UV) rays. The higher-energy waves can penetrate tissues and damage cells and DNA, explaining why both X-rays and UV rays can be harmful to living organisms.

Absorption of light

Light energy initiates the process of photosynthesis when pigments absorb the light. Organic pigments, whether in the human retina or the chloroplast thylakoid, have a narrow range of energy levels that they can absorb. Energy levels lower than those represented by red light are insufficient to raise an orbital electron to a populatable, excited (quantum) state. Energy levels higher than those in blue light will physically tear the molecules apart, called bleaching. So retinal pigments can only “see” (absorb) 700 nm to 400 nm light, which is therefore called visible light. For the same reasons, plants pigment molecules absorb only light in the wavelength range of 700 nm to 400 nm; plant physiologists refer to this range for plants as photosynthetically active radiation.

Questions & Answers

what conversation
Sally Reply
what is biology?
KINEI Reply
the scientific study of the life and structure of plants and animals.
Boame
when is the right time to play sex in order to prevent pregnancy?
Guor
After finishing school
Hellen
study of living things n non living things
Benjamin
study of living organsm
Hellen Reply
which element is the largest
Haja Reply
francium
Izzati
francium
Paul
how many groups do we have in the periodic table
Theola Reply
seven groups
Moses
the one that makes eight is the noble gases
Moses
okay thank you
Theola
how many times can a man release before the lady get pregnant
Smart Reply
nearly a hundred million
Izzati
one
Moses
what do you by one?
Izzati
two times
Theola
just only one time
Okello
what is a function of cell sap
Martin
it repair worn out tissue
Moses
when is the right time for playing sex to prevent pregnancy?
Guor
what is biology
Darkoa Reply
it's study of all living organisms and their characteristics
Ice
by what proces do ameaba reproduce?
Okello Reply
hi dear
Janet
hi my sist gudmorning
Okello
Hi
Muah
hello
Bob
yes
Mathew
hi
promise
what is biology
promise
amoeba reproduce asexually
Chidi
gudevening member
Okello
good chidi
Okello
hello
Felix
hello
Felix
please guys help me with my assignment 😃
Mary Reply
please guys help me with my assignment 😃
Mary
please guys help me with my assignment 😃
Mary
same here
Rachel
lol I am just learning idk how to do assignment
Izzati
lol idk how I am just learning lol
Izzati
lol I think I understand why we repeat this
Izzati
what is active transport
Amona Reply
is the movement's of water & mineral salts from the reason of highly concentrated to the reason of low concentration across semi permeable membrane & it take place in plant
Okello
in what situation would the use of a scanning microscope be ideal and why?
Anthony Reply
what is euglina
Ruth Reply
no idea
Enock
Euglena is a unicellular organism that has both animal- like and plant- like features.
Eugenia
define hybridization?
Guor
Which of the following statements regarding membrane transport is false? 1. Glucose is transported only by facilitated diffusion 2.Each protein carrier will only bind and b transport one type of solute.
#2. Each protein carrier will only bind and b transport one type of soluble
Only
#2 is false for the regarding membrane transport.
Only
Tanks a lot
Thanks a lot
what is gene
Okello Reply
what is somatic cell
garaadmaxamed Reply
what is Biology?
garaadmaxamed
is the study of living things
garaadmaxamed
what is organ
Chan

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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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