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.

Concept 8.1 Photosynthesis uses light energy to make food

Vocab

chloroplast: organelle in plant cells and certain unicellular organisms where photosynthesis takes place

chlorophyll: pigment that gives chloroplast its color and uses light energy to split water molecules during photosynthesis

stroma: thick fluid contained within the inner membrane of chloroplast

thylakoid: disk-shaped sac in the stroma where light reactions of photosynthesis take place

light reactions: the reactions which the sun's energy is converted into chemical energy, takes place in the thykaloid membrane

Calvin's cycle: cycle in plants that makes sugar from carbon dioxide, H+ions, and high energy electrons carried by NADH

Summary

• In most lants, the leave contain the most chloroplasts are the major sites for photosynthesis
  
• Chloroplasts are concentrated in cells of mesophyll, the inner layer of tissuechloroplasts are concentrated in cells of mesophyll, the inner layer of tissue
  
• Tiny pores on the surface of the leaf allow carbon dioxide to enter and oxygen to leave


• Veins carry water and other nutrients from the roots to the leaves, as well as deliver organic molecules produced in the leaves to other parts of the plant
  
• The chloroplasts structure is key to its function

Structuer of chloroplast:

-inner and outer membrane

-inner membrane encloses a thick fluid called stroma

-suspended in the stroma are disk-shaped sacs called thylakoids; each thylakoid is enclosed by a membrane

-thylakoids arranged in stacks called grana (singular, granum)

• Some steps of photosynthesis occur within the thylakoid membranes, while others occur in the stroma


• In contrast to the falling of electrons during cellular respiration, electrons from water are "boosted uphill" by the energy of sunlight


• Chloroplasts use these "excited" electrons along with carbon dioxide and water to produce sugar molecules
  
• Photosynthesis occurs in two main stages: the light reactions, and the Calvin cycle
  
• The light reactions convert energy in sunlight into chemical energy


• These reactions rely on the molecules built into the thylakoid membrane

Steps of light reaction:

1. chlorophyll molecules in the membrane capture energy from the light


2. chlorophyll then remove electrons from water using the captured light (this splits the water into oxygen and hydrogen ions)
   
3. oxygen is a waste product of photosynthesis
   
4. chloroplast uses electrons and hydrogen ions to make an energy-rich molecule called NADPH (electron carrier), similar to NADH in cellular respiration


5. chloroplast also use light energy to produce ATP


6. Overall result of light reactions: the conversion of light energy into chemical energy stored in two compounds: NADPH and ATP.

• The Calvin cycle creates sugar from the atoms in carbon dioxide plus the hyrdogen ions and the high energy electrons carried by NADPH


• Enzymes of the Calvin cycle are outside the thykaloid and dissolve in the stroma
  
• The ATP produced by the light reactions provides energy for making sugar in the Calvin cycle


• The Calvin cycle does not directly need light, but it relies on the light reactions for two inputs: ATP and NADPH

Concept Check

1) Draw and label a simple diagram of a chloroplast that includes the following structures: outer and inner membranes, stroma, thylakoids.

2) What are the reactants for photosynthesis? What are the products?

The reactants for photosynthesis are water and carbon dioxide and the products are sugar and oxygen.

3) Name the two main stages of photosynthesis. How are the two stages related?

The two main stages of photosynthesis are the light reactions and the Calvin cycle. The NADPH and ATP that are produced in the light reactions are used in the Calvin cycle for creating sugar. The Calvin cycle then returns certain substances and some electrons back to the light reactions.