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Text Sections
This Web Tutorial covers material throughout Chapter 8
Introduction
The energy that powers life comes from the Sun. How do plants capture the Sun's energy and put it to work? In this tutorial, you'll explore the details of photosyn- thesis—how a plant cell converts light energy into the chemical energy of sugars.
Learning Objectives
- Understand how plants use the sun’s energy to produce chemical energy during photosynthesis.
- Know the products of the light-dependent reactions.
- Discuss the function of the Calvin cycle.
W E B T U T O R I A L 8. 1
Photosynthesis
Narration
Overview of Photosynthesis and Cellular Respiration
The ultimate source of energy for most life on Earth is the Sun. Plants use the process of photosynthesis to capture the Sun's energy and put it to work. Cells derive the energy for life by respiration, using compounds that were originally cre- ated by photosynthesis. Cellular respiration and photosynthesis are complementa- ry processes.
In photosynthesis, plants use carbon dioxide, water, and light energy to produce sugar and other complex molecules. Oxygen is released as a by-product.
The complementary process of cellular respiration converts oxygen and sugar into carbon dioxide and water. In the process, some energy is released to make ATP, and the rest is lost as heat.
Reactions of Photosynthesis
Photosynthesis can be broken down into two sets of reactions: light-dependent and light-independent, both of which take place in the chloroplast.
The light-dependent reactions transform the energy in sunlight to chemical energy in the form of electrons with high potential energy. These reactions result in the production of oxygen from water. The light-independent reactions use ATP and NADPH to reduce carbon dioxide from the atmosphere to sugar, through a set of reactions called the Calvin cycle. The carbohydrates produced by the Calvin cycle are used in cellular respiration to make ATP for the cell.
Light-Dependent Reactions
The light-dependent reactions of photosynthesis occur within the thylakoid mem- branes inside the chloroplast.
The thylakoid membranes contain an array of proteins as well as chlorophyll and other pigments. This complex, called a photosystem, absorbs light and initiates the reactions of photosynthesis.
Light-dependent reactions have two photosystems. In photosystem I, excited elec- trons are used to produce NADPH. In photosystem II, they are used to produce ATP.
A photosystem absorbs photons of light, which bump electrons from chlorophyll molecules to electron acceptors within the photosystems. The chlorophyll mole- cules are left oxidized.
The electrons that feed photosystem II come from the splitting of water, which results in the production of protons (H+), as well as the liberation of oxygen.
molecule immediately breaks up into two three-carbon molecules called 3-phos- phoglycerate, or 3-PGA.
Next, the sugar must be energized. The next phase begins when 3-phosphoglycer- ate is phosphorylated by ATP and concludes when it has been reduced by electrons from NADPH. The product is the phosphorylated sugar glyceraldehyde-3-phos- phate, or G3P. The ADP and the NADP+^ will be recycled back to the light-depend- ent reactions.
One of the six G3P molecules is used to make many different substances, one of which is glucose. Many molecules of glucose can then come together to form large storage molecules known as starches.
The other five G3P molecules complete the Calvin Cycle. These molecules are used in the regeneration of RuBP, which is used to begin the cycle again. If the Calvin cycle didn't go through a regeneration phase, the chloroplast would run out of the RuBP molecules required to make glucose.
To make the equivalent of one new 6 carbon molecule such as glucose, and to regenerate the RuBP the cycle must run six times and fix 6 molecules of CO 2. If 6
carbon dioxide molecules react with 6 RuBP molecules, the chloroplast will form 12 molecules of 3-phosphoglycerate and use 12 molecules each of ATP and NADPH. Twelve G3P molecules will result. Two of these will be used to make glu- cose, leaving 10 to regenerate RuBP. The regeneration of RuBP costs the cell addi- tional ATP.
Note that the carbon atoms balance. Six 5-carbon molecules have a total of 30 car- bon atoms. Ten 3-carbon molecules also have 30 carbon atoms.
In addition to glucose, the G3P from the Calvin cycle can be used to make other organic molecules. The Calvin cycle allows plants to use carbon dioxide from the air to form the sugar molecules required by all organisms.
You should now be able to…
- List the reactants and products of photosynthesis.
- Explain how photosystems I and II transform energy during the light- dependent reactions.
- Describe the major phases of the Calvin cycle.
