CONCEPT 8.2) The Light Reactions Convert Light Energy to Chemical Energy

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KEY TERMS:

wavelength: distance between adjacent waves.
electromagnetic spectrum: range of types of electromagnetic energy from gamma waves to radio waves.
pigment: chemical compound that determines a substance's color paper.
chromatography: laboratory technique used to observe the different pigments in a material
photosystem: cluster of chlorophyll and other molecules in a thylakoid.

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SUMMARY

• Sunlight is a form of electromagnetic energy that travels in waves.
• Visible light: 400 nm (nanometers) violet ~ 700 nm, red.
• Shorter wavelengths energy > Longer wave lengths energy
• Energy wavelengths shorter than the visible light can cause damage on organic molecules (proteins, nucleic acids). //eg. Ultraviolent radiation
• A substance's color is due to chemical compounds: pigments.
• When light shines on a material containing pigments: ----> 1. absorbed 2. transmitted 3. reflected
• Chloroplasts absorb well: blue-violet, red-orange light
• Chloroplasts do NOT absorb well: green light (transmit, reflect) -> why plants are GREEN!!!

- Paper chromatography is used to observe the different pigments in a green leaf:

• Pigments travel at different rates depending on how easily they dissolve, how strongly they are attracted to the paper.
• This laboratory technique can be used to separate and analyze the pigments in a leaf.

Harvesting Light Energy

• Within the thylakoid membranes is the chlorophyll, other molecules all together: photosystems.
• Each photosystem contains a few hundred pigment molecules (+ chlorophyll a, b & carotenoids).
• A pigment molecule absorbs light energy --> One of the pigment's electrons gains energy ("ground state" to "excited state"-unstable) --> excited electron falls back to ground state --> exits an electron in the next pigment molecule --> so on...."jumping" --> arrives at the reaction center --> primary electron acceptor (traps the excited electron from the chlorophyll a molecule) //
• -----> Now energy is able to make ATP and NADPH in the rest of the thylakoid membrane.

Chemical Products of the Light Reactions


-light strikes photosystem and transfer excited electrons to the primary electron acceptor.
-electrons split water and releases oxygen
-excited electron goes through electron transport chain and pumps H+ions across the membrane to thylakoid.
-light excited electrons get ransferred to NADP.


"Water-splitting photosystem"- releases oxygen (waste product), hydrogen ions:

• First photosystem traps light energy --(light-excited electrons)--> electron transport system//
• --> release of energy --> chloroplast use to make ATP

"NADPH-producing photosystem":

• Photosystem transfer excited electrons & hydrogen ions --> NADP+ --> light reactions (light energy --> chemical energy of ATP & NADPH) --> produces NADPH
• Sugar is to be produced in the Calvin Cycle process.

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CONCEPT CHECK 8.2

1. Explain why a leaf appears green.
A leaf appears green because chloroplasts doesn't absorb green light well. Unlike other colors such as blue-violet and red-orange lights that are well absorbed, green light is rather transmitted or reflected because of the particular wave-length that is not able to get absorbed in the chloroplast pigments.


2. Describe what happens when a molecule of chlorophyll a absorbs light.
When chlorophyll a absorbes light, it makes the molecule get excited and makes it transfer the energy to another near molecule.

3. Besides oxygen, what two molecules are produced by the light reactions?
Hydrogen ions and NADPH is produced by the light reactions, besides oxygen.

4. Where in the chloroplast do the light reactions take place?
It takes place in the chloroplast's thylakiod membrane.

CONCEPT 8.1) Photosynthesis Uses Light Energy to make food.

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KEY TERMS:

Chloroplast: organelle found in some plant cells and certain unicellular organisms where photosynthesis takes place. These contain chemical compounds called chlorophylls.
Chlorophyll: pigment that gives a chloroplast its green color; uses light energy to split water molecules during photosynthesis.
Stroma: thick fluid contained in the inner membrane of a chloroplast
Thylakoid: disk-shaped sac in the stroma of a chloroplast; site of the light reactions of photosynthesis.
Light reactions: chemical reactions that convert the sun's energy to chemical energy; take place in the membranes of thylakoids in the chloroplast.
Calvin cycle: cycle in plants that makes sugar from carbon dioxide, H+ ions, and high-energy electrons carried by NADPH.

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SUMMARY

Looking at the structure of a plant leaf.....
- Leafs in a plant contain the most chloroplasts and are the major sites for photosynthesis to take place.
- Chloroplasts are concentrated in the cells of the mesophyll (inner layer of tissure).
- On the surface of the leaf, there are pores called stomata, where CO2 enters and O2 exits through.
-Veins carry water and nutrients, and also other organic molecules produced in the leaves, through stomatas and to other parts of the plant.

Looking inside the chloroplasts' structure...
Chloroplast
~ inner& outer membrane
~ inner: thick fluid - stroma
~ stroma: Many disk-shaped sacs: thylakoids
~ Thylakoids:
+ each has a membrane surrounding the interior space.
+ Thylakoids arranged in stacks are called grana.

- Photosynthesis is a process of plants and other producers, or autotrophs, converting the energy of sunlight into useful energy that is stored in organic molecules. - Photosynthesis is the opposite of cellular respiration.

- The chloroplast uses "excited" electrons, along with CO2 and hydrogen ions, to produce sugar molecules.

-Photosynthesis occurs in 2 main stages: theLight Reactions & the Calvin Cycle.

The Light Reaction

1. Sun light energy>> chemical energy

2. Chlorophyll molecules in the membrane captures energy.

3. Chloroplasts use these energy to remove electrons from water. ---> Splits H2O --> Oxygen ("waste product" go out through stomata) & Hydrogen ions

4. Electron & Hydrogen ions in chloroplast ---> NADPH (energy carrier)

5. Chloroplast use the stored energy to make ATP too.

+++ Overall: Light energy stored ---> chemical energy ---> NADPH & ATP

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The Calvin Cycle

- NADPH & ATP made by light reactions provides energy to make sugar from the atoms in:

CO2 + Hydrogen ions + high-energy electrons carried by NADPH

- Enzymes for the process located outside the thylakoids, dissolved in stroma.

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CONCEPT CHECK 8.1

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?
Reactants: CO2, H2O
Products: Glucose, Oxygen
6 O2 + 6 H2O ->->-> C6H12O6 + 6 O2

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

Two main stages: the light reactions, the Calvin Cycle

They are related in that the light reactions recieve is the process of storing the sun light energy and converting them into chemical energy, making NADPH and ATP rich in energy. Then the Calvin Cycle uses these NADPH and ATP to make sugar for the chloroplast.