8.2 The light reactions convert light energy into chemical energy

Vocab

wavelength: distance between adjacent waves

electromagnetic spectrum: range of electromagnetic types of energy from gamma waves to radio waves

pigment: chemical compound that determines a compounds color

paper chromatography: lab technique used to observe the different pigments in a material

photosystem: cluster of chlorophyll and other molecules in a thylakoid

Summary

• Sunlight is a form of electrogmagnetic energy
  
• Different forms of electromagnetic energy have characteristic wavelengths
  
• Shorter wavlengths have more energy than longer wavelengths


• Visible light, which can be seen in different colors by our eyes, only make up a small fraction of the electromagnetic spectrum
  
• Visible light ranges from wavelengths of about 400 nanometers, violet, to about 700 nanometers, red


• Wavelengths shorter than those of visible have enough energy to damage organic molecules such as proteins and nucleic acids
  
• When light hits a substance with pigments, three things can occur with the different wavelengths: they can be absorbed, transmitted, or reflected


• The pigments in leaf chloroplasts absorb blue-violet and red-orange light very well


• The cholorplast convert some of the light into chemical energy


• However, green light is either transmitted (pass through) or reflected on the chloroplast; thus, giving leaves their green color


• Paper chromatography is a lab technique that can be used to observe the different pigments in a green leaf

Steps of Paper Chromatography

1. the leaf is pressed on a piece of filter paper to deposit a "stain"


2. the paper is then sealed in a cylinder containing solvents, and is worked on under a vented hood
   
3. As the solvents move up the paper, the different pigments are dissolved and move up the paper


4. Different pigments dissolve at different rates, depending on how easily they dissolve and how strongly they are attracted to the paper
   
5. Several pigments spread out on the paper

• Chlorophyll A mainly absorbs blue-violet and red light, and reflects green light; plays a major role in light reactions
  
• Chloroplasts also contain "helper" pigments, including: chlorophyll B that mainly absorbs blue and orange light and reflects yellow-green light; and several types of carotenoids, which mainly absorb blue-green light and reflect yellow-orange light
  
• Within a thylakoid membrane, clusters of chlorophyll and other molecules make up photosystems; each containing a few hundred pigment molecules including chlorophyll A, B, and carotenoids.




• Each time that a pigment molecule obtains light energy, one of its electrons turns from "ground state" to "excited state"; which is very unstable


• Almost immediately, the pigment molecule returns to its ground state, and passes the energy to the next pigment molecule


• This cycle continues until the energy is transferred to what is called the reaction center of the photosystem


• Within the reaction center is a chlorophyll A molecule located adjacent to a molecule called a primary electron acceptor


• The primary electron acceptor traps the excited electron from the chlorophyll a molecule


• Other groups of molecules inside the thylakoid membrane use the trapped energy to make ATP and NADPH


• In light reactions, two photosystems are involved

Steps of Light Reactions:

1. first photosystem traps light energy and transfers the light-excited electrons to an electron transport chain; this photosystem can be considered as a "water-splitting photosystem" since the electrons are replaced by the splitting of a molecule of water. This process releases oxygen as a waste product, as well as hydrogen ions


2. excited electrons travel along an electron transport chain, and pump H+ ions across the membrane into the thylakoid


3. light-excited electrons in the second photosystem are transferred to NADP +, and are replaced by the electrons coming from the electron transport chain


4. the "backflow" of hydrogen ions out from the thylakoid provide power for ATP production

• The electron transport chain linking the two photosystems releases energy which the chloroplasts use to make ATP; this mechanism of ATP production is similar to that of cellular respiration


• In both systems, H+ ions are pumped across a membrane through an electron transport chain; the inner mitochondrial membrane in respiration and the thylakoid membrain in photosynthesis


• Second photosystem also can be thought of as "NADPH-producing photosystem"








• This photosystem produce NADPH through transferring excited electrons and hydrogen ions into NADP+

Concept Check

1. A leaf appears green since certain pigments in a leaf reflect or transmit green light from the sun.

2. When a molecule of chlorophyll a absorbs light, it transfers the excited electron to the electron transfer molecule; it absorbs blue-violet and red light, and reflects mainly green light.

3. Besides oxygen, two other molecules produced by light reaction are NADPH and ATP.

4. The light reactions take place in the thylakoid membranes in the chloroplast.