1. Pick up a copy of the notes and the diagram.

The Light Reactions

Light Reactions The light reactions consist of those reactions that require the presence of light in photosynthesis. Light energy from the sun and water are used to produce ATP and another energy carrier known as NADPH. These two products are important because they are necessary for the actual production of organic molecules (such as glucose) that occurs during the dark reactions.

The Photosynthesis Equation 6CO 2 + 6H 2 O C 6 H O 2 light

 What’s necessary for the light reactions? NADP, ADP, water, and sunlight.

What’s produced in the light reactions? Products of the light reactions (light dependent reactions) go on to the Calvin Cycle NADPH, ATP, and oxygen as a byproduct.

Light Reactions Step by Step Recall that photosynthesis occurs within the chloroplast of a cell. The light reactions occur in the thylakoids of the chloroplasts. The thylakoid membrane is composed of a phospholipid bilayer that is very similar to the membrane found in a cell.

Within the membrane of the thylakoid there are two clusters of pigments that we refer to as photosystem I and photosystem II. Remember that pigments absorb light. The pigments present in these photosystems are chlorophyll a, chlorophyll b, and caretonoids. The most important event associated with both photosystems is the capturing of light.

When a photon of light strikes photosystem II, it excites an electron. At the same time an enzyme binds to two water molecules and splits the water into hydrogen ions (H+ or protons) and releases an oxygen atom (O2). Note: This is why water is necessary for photosynthesis to occur and this is where oxygen is released as a byproduct of photosynthesis.

Two electrons are released in the process of photolysis described above. These electrons can be traced all the way through photosystems I and II. The proteins and enzymes that pass these electrons through out the thylakoid are known as the Electron Transport Chain.

A primary electron acceptor accepts the light-energized electrons that have been energized in photosystem II and passes them to a proton pump (“pp” in the diagram).

This proton pump moves the protons (H+) across the thylakoid membrane against their concentration gradient where they build up in the thylakoid interior. This will have importance later. This is a form of active transport. The protons then exit the thylakoid interior through a special protein channel that is ATP synthase. Recall that ATP synthase is the enzyme that is associated with the synthesis of ATP molecules.

As protons pass through the ATP synthase down the concentration gradient, they fuel ATP synthesis. In this process a phosphate is added to a molecule of ADP (adenosine diphosphate) and ATP is made and released into the stroma of the chloroplast. This process is known as photophosphorylation.

Now back to the electrons that left photosystem II and energized the proton pump. After fueling the proton pump (pp) the electrons are now de-energized but they are still not finished. Another electron carrier carries the electrons to photosystem I (EC).

These electrons are again reenergized in Photosystem I as light is absorbed. These reenergized electrons are passed to another primary electron acceptor (“PEA2” in the diagram) and carried to an enzyme called NADP Reductase (“R” in the diagram). Electrons that leave Photosystem I are replaced by the electrons from Photosystem II.

The NADP Reductase combines NADP with protons (H+) found in the stroma to make NADPH. The NADPH combined with the ATP produced in the light reactions now move on to the Calvin Cycle.

The Dark Reactions The dark reactions are also known as the light independent reactions or the Calvin Cycle. Named after Melvin Calvin. The light independent reactions do not require the presence of light to occur. The dark reactions are the “synthesis” part of “photosynthesis.” Carbon fixation occurs.

What’s Necessary for the Dark Reactions? The dark reactions cannot take place without the ATP and NADPH that was produced during the light reactions. Recall that ATP and NADPH are energy carriers. Carbon dioxide from the environment is also necessary for the dark reactions.

What’s produced by the dark reactions? Glucose (a 6 carbon sugar) and other energy storing organic compounds are made. NADP+ and ADP are produced to be reused by the light reactions.

Glucose Glucose is a monosaccharideGlucose is a monosaccharide C 6 H 12 O 6C 6 H 12 O 6 One Molecule of glucose Stores 90 Times More Chemical Energy Than One Molecule of ATPOne Molecule of glucose Stores 90 Times More Chemical Energy Than One Molecule of ATP

Carbon dioxide molecules enter the cycle and are added to five- carbon molecules. 6- Carbon molecules are formed. Energy is added. The six-carbon molecules split to form three- carbon molecules. More energy is added and the molecules are rearranged into higher-energy molecules. A high-energy three-carbon molecule exits for every 3 CO2 molecules that enter. After 2 three-carbon molecules have exited, they bond to form 1 six-carbons sugar. Three-carbon molecules are changed back to five-carbon molecules by energy from ATP. Refer to 111 in the book.

Alternative Pathways Many plants in extreme conditions (particularly hot and dry environments) have alternative photosynthesis pathways to make organic compounds. C 4 Plants CAM Plants

C 4 Plants These plants fix carbon dioxide into four- carbon molecules instead of three-carbon molecules during the Calvin Cycle. These plants partially close stomata (pores found in leaves) during the day to avoid water loss. Examples: Corn and sugar Cane

CAM Plants Occurs in plants that live in extreme heat. Allow carbon dioxide to enter only at night. Examples: cacti, orchids, and pineapples

Factors Affecting the Rate of Photosynthesis Amount of available waterAmount of available water TemperatureTemperature Amount of available light energyAmount of available light energy