8 Photosynthesis

• Objectives

• Describe the structure of a chloroplast.

• Identify the overall reactants and products of photosynthesis.

• Key Terms

• chloroplast

• chlorophyll

• light reactions

• Calvin cycle

• photosynthesis is the process by which plants and other producers convert the energy of sunlight into the energy stored in organic molecules

• The Structure of ChloroplastsThe cellular organelle where photosynthesis takes place is called a chloroplast. Chloroplasts contain chemical compounds called chlorophylls that give these organelles a green color. In most plants, the leaves contain the most chloroplasts and are the major sites of photosynthesis.

• Within a leaf, the chloroplasts are concentrated in the cells of the mesophyll, the inner layer of tissue

• Tiny pores called stomata (singular, stoma) are found on the surface of the leaf. Carbon dioxide enters the leaf and oxygen exits the leaf through the stomata.

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• The chloroplast's structure is key to its function. Like a mitochondrion, a chloroplast has an inner and an outer membrane. The inner membrane encloses a thick fluid called stroma. Suspended in the stroma are many disk-shaped sacs called thylakoids.

• Overview of PhotosynthesisYou have read that cellular respiration involves the process of electron transfer. The "fall" of electrons from glucose to oxygen releases energy, which is then used to make ATP. The opposite occurs in photosynthesis. Electrons from water are boosted "uphill" by the energy from sunlight. The chloroplast uses these "excited" electrons, along with carbon dioxide and hydrogen ions, to produce sugar molecules

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• Photosynthesis occurs in two main stages, each with many steps: the light reactions and the Calvin cycle

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• The Light Reactions The light reactions convert the energy in sunlight to chemical energy. These reactions depend on molecules built into the membranes of the thylakoids.

• First, chlorophyll molecules in the membranes capture light energy.

• .Then the chloroplasts use the captured energy to remove electrons from water. This splits the water into oxygen and hydrogen ions. The oxygen is a "waste product" of photosynthesis. It escapes to the atmosphere through the stomata of leaves.

• What becomes of the water's electrons and hydrogen ions? Chloroplasts use them to make an energy-rich molecule called NADPH. (an electron carrier.) The chloroplasts also use the captured light energy to generate ATP. The overall result of the light reactions is the conversion of light energy to chemical energy stored in two compounds: NADPH and ATP.

• The Calvin Cycle The Calvin cycle makes sugar from the atoms in carbon dioxide plus the hydrogen ions and the high-energy electrons carried by NADPH. The enzymes for the Calvin cycle are located outside the thylakoids and dissolved in the stroma. The ATP made by the light reactions provides the energy to make sugar.

• The Calvin cycle is sometimes referred to as the "light-independent reactions" because, unlike the light reactions, it does not directly require light to begin. However, this doesn't mean that the Calvin cycle can continue running in a plant kept in the dark. The Calvin cycle requires two inputs supplied by the light reactions, ATP and NADPH.

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• Recall that the overall equation for photosynthesis is:

• 6 CO2 + 6 H2O C6H12O6 + 6 O2

• The light reactions, which take place in the thylakoid membranes, convert light energy to the chemical energy of ATP and NADPH. The light reactions use the reactant water from the equation and release the product oxygen. The Calvin cycle, which takes place in the stroma, uses ATP and NADPH to convert carbon dioxide to sugar

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• By converting light energy to chemical energy, photosynthesis is the first step in the flow of energy through an ecosystem. Some of that chemical energy then passes from producers to consumers. Even when people eat meat, you can trace its stored energy back to photosynthesis.Photosynthesis is the ultimate source of all the food you eat and all the oxygen you breathe.