# 8.2 The light-dependent reactions of photosynthesis

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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.

## 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] ).

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.

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.

what conversation
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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
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francium
Izzati
francium
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how many groups do we have in the periodic table
seven groups
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the one that makes eight is the noble gases
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okay thank you
Theola
how many times can a man release before the lady get pregnant
nearly a hundred million
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one
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what do you by one?
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two times
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just only one time
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it repair worn out tissue
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when is the right time for playing sex to prevent pregnancy?
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it's study of all living organisms and their characteristics
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hi dear
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hi my sist gudmorning
Okello
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yes
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promise
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promise
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gudevening member
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good chidi
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hello
Felix
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Felix
please guys help me with my assignment 😃
please guys help me with my assignment 😃
Mary
please guys help me with my assignment 😃
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same here
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lol I am just learning idk how to do assignment
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lol idk how I am just learning lol
Izzati
lol I think I understand why we repeat this
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no idea
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Euglena is a unicellular organism that has both animal- like and plant- like features.
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define hybridization?
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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
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is the study of living things
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