ch10.pdf - Photosynthesis Light-Independent Reactions Light \u25a0 \u25a0 Energy from the sun travels to Earth in the form of light Sunlight is a mixture of

ch10.pdf - Photosynthesis Light-Independent Reactions Light...

This preview shows page 1 out of 46 pages.

You've reached the end of your free preview.

Want to read all 46 pages?

Unformatted text preview: Photosynthesis Light-Independent Reactions Light ■ ■ Energy from the sun travels to Earth in the form of light. Sunlight is a mixture of different wavelengths, many of which are visible to our eyes and make up the visible spectrum. Light ■ Our eyes see the different wavelengths of the visible spectrum as different colors: red, orange, yellow, green, blue, indigo, and violet. Pigments ■ Plants gather the sun’s energy with light-absorbing molecules called pigments. ■ The plants’ principal pigment is chlorophyll. Pigments ■ ■ ■ There are two types of chlorophyll found in plants: chlorophyll a and chlorophyll b. Both pigments absorb light very well in the blue-violet and red regions of the visible spectrum, but not in the green region, as shown in the graph. Leaves do not absorb but rather reflect green light, which is why plants look green. Pigments ■ ■ Plants also contain red and orange pigments known as accessory pigments. Accessory pigments such as carotene absorb light in other regions of the spectrum. Pigments ■ ■ Most of the time, the green color of the chlorophyll overwhelms the other pigments. As temperatures drop and chlorophyll molecules break down, however, the red and orange pigments may be seen. Photosystems ■ Each cluster of pigment molecules (chlorophylls and accessory pigments) is referred to as a Photosystem. ■ There are 2 types of Photosystems: ■ Photosystem I ■ Photosystem II Chloroplasts ■ ■ Photosynthesis takes place inside organelles called chloroplasts. Chloroplasts contain saclike photosynthetic membranes called thylakoids, which are interconnected and arranged in stacks known as grana. The thylakoids contain the Photosystems. Chloroplasts ■ ■ Pigments are located in the thylakoid membranes. The fluid portion outside of the thylakoids is known as the stroma. Photosynthesis Oxygen Overview of Photosynthesis ■ Photosynthesis is the process that provides energy for almost all life. ■ During Photosynthesis, autotrophs combine the energy of the sun with water and carbon dioxide to create carbohydrates. Oxygen is released as a byproduct of this reaction. ■ PHOTOSYNTHESIS CAN BE SUMMARIZED BY THE EQUATION: 6CO2 + 6H2O + LIGHT → C6H12O6 + 6O2 In this equation the six-carbon sugar GLUCOSE and OXYGEN are the Products. ■ Photosynthesis occurs in stages Stage 1 (Light-dependent reactions) Energy is captured from the sun. Water is split into hydrogen ions, electrons, and oxygen gas. The oxygen gas diffuses out of the chloroplasts. (Temporary storage) The light energy is converted to chemical energy which is temporarily stored in ATP and NADPH. (energy carriers) Stage 2 (Light-independent reactions or Calvin Cycle) The chemical energy stored in ATP and NADPH powers the formation of organic compounds using carbon dioxide. Light-dependent Reactions ■ ■ ■ ■ All of the reactions of photosynthesis that are directly dependent upon light are known as the light reactions or light-dependent reactions. The light reactions occur in the grana (stacks of thylakoids). Many chlorophyll molecules are found embedded in the membranes of the thylakoids. The purpose of the light reactions is to convert light energy into chemical energy in the form of ATP & NADPH. ATP & NADPH are energy carriers. They carry energy to the 2nd part of photosynthesis, known as the light-independent reactions (Calvin Cycle reactions). The Light-dependent Reactions The light reactions can be split into the following specific steps: ■ Light absorption & splitting of water ■ Production of ATP ■ Movement of electrons through electron acceptors to power a hydrogen pump ■ Re-energizing electrons so they can produce NADPH Light Absorption and Splitting of Water ■ ■ ■ ■ Light strikes a photosystem containing of a reaction-center complex with a special pair of chlorophyll a molecules. Excited electrons from chlorophyll pass into an electron transport chain (discussed next). In addition, the light energy powers enzymes (Photosystem II) that break up water molecules splitting them into electrons, H+ ions, and oxygen. Oxygen is released to the air when water is split. The chlorophyll accepts the energized electrons from water to replace those passed on to the electron transport chain. The primary electron donor of photosystem II is a special form of chlorophyll a known as P680 (P = pigment, 680 = wavelength). Production of ATP ■ ■ ■ Protons (H+ ions) accumulate in the thylakoid as water is split. The H+ ions diffuse through a protein channel that contains the enzyme ATP synthase. In the process, ATP, an energy carrier, is generated. The H+ ions now found on the outside of the thylakoid will power the next part of the light-dependent process. Movement of Electrons Through Electron Acceptors The cytochrome protein complex catalyzes the electron transfer between plastoquinone and plastocyanin, but also aids the transport of two protons into the lumen (interior) of the thylakoid membranes. Re-energizing Electrons So They Can Produce NADPH P700, or photosystem I primary donor, refers to the reaction-center chlorophyll a molecules in photosystem I. Ferredoxin (FD) is a small iron-sulfur protein. When NADP+ and a suitable enzyme (NADP+ Reductase) are present, two ferredoxin molecules, carrying one electron each, transfer two electrons to NADP+, which picks up a proton (i.e., a hydrogen ion) and becomes NADPH. Light-Dependent Reactions ■ View the Video From the Text The Light-Independent Reactions: Producing Sugars ■ ■ During the light-independent reactions, commonly referred to as the Calvin cycle, plants use the energy contained in ATP and NADPH to build stable high-energy carbohydrate compounds that can be stored for a long time. This process takes place in the stroma. The Calvin Cycle ■ The Calvin Cycle involves the process of carbon fixation. • This is the process of assimilating carbon from a non-organic compound (ex. CO2) and incorporating it into an organic compound (ex. carbohydrates). CARBON FIXATION 32 Calvin Cycle ■ ■ ■ Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P) For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2 The Calvin cycle has three phases: ■ ■ ■ Carbon fixation (catalyzed by the enzyme Rubisco) Reduction Regeneration of the CO2 acceptor (RuBP) Carbon Fixation ■ Carbon dioxide molecules enter the Calvin cycle from the atmosphere. ■ An enzyme (Rubisco) in the stroma of the chloroplast combines carbon dioxide molecules with 5-carbon compounds (RuBP) that are already present in the organelle, producing two 3-carbon compounds that continue into the cycle. Carbon Dioxide Enters the Cycle ■ For every 6 carbon dioxide molecules that enter the cycle, a total of twelve 3-carbon compounds (PGA) are produced. Reduction ■ Other enzymes in the chloroplast then convert the 3-carbon compounds into higher-energy forms (G3P or PGAL) in the rest of the cycle, using energy from ATP and high-energy electrons from NADPH. Sugar Production ■ At midcycle, two of the twelve 3-carbon molecules are removed from the cycle. ■ These molecules become the building blocks that the plant cell uses to produce sugars, lipids, amino acids, and other compounds. Regeneration ■ The remaining ten 3-carbon molecules are converted back into six 5-carbon molecules (RuBP) that combine with six new carbon dioxide molecules to begin the next cycle. The Calvin Cycle ■ ■ View the Video Nature's Smallest Factory The End Results ■ ■ ■ The two sets of photosynthetic reactions work together—the light-dependent reactions trap the energy of sunlight in chemical form, and the light-independent reactions use that chemical energy to produce stable, high-energy sugars from carbon dioxide and water. View the Video In the process, animals, including humans, get food and an atmosphere filled with oxygen. Photosynthesis in Review Factors Affecting Photosynthesis ■ Temperature Photosynthesis depends on enzymes that function best between 00 and 350 C. Temperatures above or below this range may damage the enzymes. Factors Affecting Photosynthesis ■ Light intensity Increasing light intensity increases photosynthesis until the plant reaches a maximum rate of photosynthesis. Factors Affecting Photosynthesis ■ Water Availability ■ ■ Water is a necessary ingredient of the light-dependent reactions C3, C4, and CAM Plants The Photosynthetic Process in C3, C4 and CAM Plants ■ View the Video ...
View Full Document

  • Winter '19

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

Stuck? We have tutors online 24/7 who can help you get unstuck.
A+ icon
Ask Expert Tutors You can ask You can ask You can ask (will expire )
Answers in as fast as 15 minutes