# 8.2 The light-dependent reactions of photosynthesis

 Page 1 / 30
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.

in which of the following stages of the cell cycle are chromosomes pulled apart to opposite poles of the cell
Anaphase
Jonah
what is digestive system
Digestive system is a system in mammals associated with the breaking down or assimilation of food substances into simple soluble and diffusable substances by mechanical and chemical means
Emmanuel
what is symbiotic factors of an organisms
what are Nephron's
cells of the kidneys
Nanmwa
what is biology
Describe cellular event during meiosis
what is the difference between compound light microscope and electron microscope
Hy can u help me plz
Khurram
i need all meterial related to MCAT
Khurram
what is tracheids
what is the branches of biology
botany zoology microorganisms
Jyoti
what are prokaryotic cell
Mutala
What is abiotic
what is prokaryotit
Deoxyribonucleic Acid
Gd evening guys pls some should help me with the excretory function of the kidney
Fonki
separate waste (urine) from the body
Simon
what is the full meaning of DNA
dioxyribonucleic acid
Theophilus
deoxyribonucleic acid
Simon
What is life
what are oil immersion lenses
no idea
Favour