The Reactions of Photosynthesis Unit C: Section 6.2 The Reactions of Photosynthesis

Photosynthesis Reactions There are two sets of reactions that make up the process of photosynthesis: The Light-Dependent Reactions solar energy is trapped and used to generate two high-energy compounds ATP (adenosine triphosphate) NADPH (reduced nicotinamide adenine dinucleotide phosphate)

The Light-Independent Reactions the energy of ATP and the reducing power of NADPH are used to reduce carbon dioxide to make glucose.

Photosynthetic Pigments During the light-dependent reactions, the pigments within the thylakoid membranes absorb light energy from the sun. A pigment - is a compound that absorbs certain wavelengths of visible light, while reflecting others that give the pigment it’s specific colour.

The three main pigments plants use during photosynthesis are chlorophyll a, chlorophyll b, and beta carotene. Chlorophyll a and b absorb red and blue wavelengths of light, and reflect green light.

Beta carotene is a member of a very large class of pigments called carotenoids that absorb blue and green light, and reflect yellow, orange and red light. Having a variety of pigments enables plants to use a greater percentage of the sun’s light because they are able to absorb different wavelengths of light.

Chlorophyll a and b absorb blue and red. Beta carotene absorbs blue and green.

The Light-Dependent Reactions Chlorophyll and other pigments are arranged in the thylakoid membranes in clusters called photosystems. The chloroplasts of plants have two photosystems, called photosystem I (PSI) and photosystem II (PSII). Each photosystem is made up of pigment molecules that include a dozen or more chlorophyll molecules, as well as a few carotenoids for absorbing light energy.

Each photosystem also contains a specialized chlorophyll a molecule, called the reaction center that accepts electrons from all the other pigments. When a reaction center has received the energy passed on to it (in the form of an electron), an electron already in the reaction center becomes “excited” or moves to the next energy level.

The “excited” electron then gets passed on to an electron accepting molecule, which becomes reduced after receiving the electron. During photosynthesis the light energy from the pigments is first sent to photosystem II.

When the electron leaves the reaction center in photosystem II, the reaction center is missing an electron. This electron must be replaced before photosystem II can absorb more light energy. The new electron comes from a process called photolysis that splits a water molecule into two H+ ions and one O2- ion. When 2 H2O molecules are split the oxygen ions come together to make oxygen gas, releasing their 4 e- as they neutralize.

From the electron acceptor, the energized electron is transferred along a series of electron carrying molecules known as the electron transport system or electron transport chain. With each transfer along the electron transport system the electron loses a small amount of energy that is eventually used to make ATP.

When the events of the previous steps are taking place, light energy is absorb by photosystem I. This energy is transferred to a reaction center, where an electron becomes “excited”. Once again the electron is passed to an electron accepter that gets reduced.

** Reduction = gain of elections** In photosystem I the lost electron is replaced by the electron coming from photosystem II along the electron transport system. The electron received by the electron acceptor from photosystem I is given to a NADP+ molecule, reducing it to NADPH. ** Reduction = gain of elections** This is the end of the Light-Dependent Reactions.

PS I PS II

Making ATP: Chemiosmosis The lost energy from the electron transport system is used to pump hydrogen ions up their concentration gradient (low to high, using energy). The hydrogen ions are pumped from the stroma into the inner space of the thylakoid, called the thylakoid lumen.

Once a high concentration of hydrogen ions is created in the thylakoid lumen, the ions naturally want to move from a high concentration to low concentration, back into the stroma. The only means for these molecules to move back across the membrane, down the concentration gradient, is through a special molecule called ATP synthase which is embedded in the thylakoid membrane.

The movement of molecules down their concentration gradient, releases small amounts of energy. This energy is used by the ATP synthase to add a free phosphate molecule to an ADP molecule to produce ATP. This process is known as chemiosmosis.

Light-Independent Reactions When there is a sufficient amount of NADPH and ATP in the stroma of the chloroplasts, the energy of these molecules can be used to synthesize glucose in the presence or absence of light. The series of reactions by which carbohydrates are produced is called the Calvin-Benson Cycle.

The Calvin-Benson cycle can be summarized in three steps: Fixing of carbon Reduction Replacing RuBP

The Calvin-Benson Cycle The Fixing of Carbon The carbon atom of a carbon dioxide molecule becomes chemically bonded to a pre-existing molecule in the stroma. This molecule is a five carbon compound called ribulose bisphosphate (RuBP). The resulting compound is an unstable six-carbon compound that immediately breaks into two identical three carbon compounds called PGA.

The Calvin-Benson Cycle 2) Reduction The newly formed PGA molecules are in a low energy state, and in order to convert them into a high energy state so they can be used, they are first activated by ATP and then reduced by NADPH. The result of the reduction is two molecules of PGAL (glyceraldehyde-3-phosphate). At this point some of the reduced PGAL molecules leave the cycle and may be used to make glucose. The remaining PGAL molecules move to the third stage of the cycle.

The Calvin-Benson Cycle 3) Replacing RuBP The remaining PGAL molecules that were not used to make glucose are used to replenish the RuBP in the stroma. The energy required to break and reform the chemicals bonds necessary to make the five carbon compound RuBP is supplied by ATP.

***IMPORTANT NOTE*** The Calvin-Benson must be completed 6 times in order to form 1 glucose molecule. Of the 12 PGAL molecules that are produced in six cycles, 10 are used to regenerate RuBP, and 2 are used to make glucose. 2 PGAL’s = 1 Glucose (3C’s) (6C’s)