Photosynthesis

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1 Student Expectations: Cellular Energy Understand that cellular energy is temporarily stored in the nucleotide ATP (adenosine triphosphate) Describe how energy is released by ATP When the outer phosphate bond in a molecule of ATP is broken, energy is released ATP ADP + P + energy Photosynthesis Know that autotrophs or producers use sunlight to make their own food through the process of photosynthesis Identify the reactants in photosynthesis as CO 2 and H 2 O + energy Identify the products of photosynthesis as C 6 H 12 O 6 and O 2 Understand that due to the Law of Conservation of Mass the balanced formula for photosynthesis is: 6CO 2 + 6H 2 O + energy C 6 H 12 O 6 + 6O 2 Terminology Explain the energy conversions in a cell during photosynthesis; light energy is converted to chemical energy stomata) Light Dependent Stroma Energy is absorbed from the Sun by the pigments, chlorophyll and carotenoids, in the thylakoids of chloroplasts When light strikes the thylakoid, energy is transferred to electrons, freeing them from the chlorophyll. These electrons are replaced by from the water molecules which are split by an enzyme in the thylakoid. Oxygen atoms released from the water molecules, combine to form oxygen gas. The electron energy is harvested as they pass through electron transport chains forming ATP and NADP. Light Independent Takes place within the stroma of chloroplasts Molecules of CO 2 are split apart, rearranged, and bonded with hydrogen to make a simple 3-carbon compound using the ATP produced from the light dependent reactions. After the 3-carbon compounds are produced, they are used to make glucose and other organic compounds. (Calvin Cyclemost common method of carbon dioxide fixation which is the transferring of carbon dioxide to organic compounds) ATP Photosynthesis Autotroph Heterotroph Pigment Chlorophyll Carotenoid Thylakoid NADPH Calvin cycle NADH Stoma (plural: Unit Summary 8-1 Energy & Life Plants and some other types of organisms are able to use light energy from the sun to produce food. Organisms that make their own food are called autotrophs. Other organisms cannot use the sun s energy directly. These organisms, called heterotrophs, obtain energy from the foods they consume. One of the principal chemical compounds that cells use to store and release energy is adenosine triphosphate, or ATP. ATP consists of adenine, a 5-carbon sugar called ribose, and three phosphate groups. Adenosine di-phosphate (ADP) is a similar compound that has only two phosphate groups instead of three. 1 Photosynthesis -

2 When a cell has energy available, it can store small amounts of energy by adding a third phosphate group to ADP, producing ATP. The energy stored in ATP can be released by breaking the bond between the second and third phosphate groups. Because a cell can subtract this third phosphate group, it can release energy as needed. The characteristics of ATP make it exceptionally useful as the basic energy source of all cells. Cells use energy from ATP to carry out many important activities, including active transport, synthesis of proteins and nucleic acids, and responses to chemical signals at the cell surface. 8 2 Photosynthesis: An Overview In the process of photosynthesis, plants use the energy of sunlight to convert water and carbon dioxide into high-energy carbohydrates and oxygen. High-energy carbohydrates include sugars and starches. Oxygen is a waste product of the process. Research into photosynthesis began centuries ago. In 1643, Jan van Helmont concluded that trees gain most of their mass from water. In 1771, Joseph Priestly determined that plants release a substance that can keep a candle burning. We now know that substance is oxygen. In 1779, Jan Ingenhousz concluded that plants need sunlight to produce oxygen. The experiments performed by van Helmont, Priestly, and Ingenhousz led to work by other scientists who finally discovered that in the presence of light, plants transform carbon dioxide and water into carbohydrates and plants also release oxygen. The overall equation for photosynthesis can be shown as follows: 6CO2 + 6 H2O light C6H12O6 + 6O2 Carbon dioxide + water light sugars + oxygen Photosynthesis uses the energy of sunlight to convert water and carbon dioxide into high-energy sugars and oxygen. Plants use the sugars to produce complex carbohydrates such as starches. Plants obtain the carbon dioxide they need for photosynthesis from the air or from the water in which they grow. In addition to water and carbon dioxide, photosynthesis requires light and chlorophyll. Plants gather the sun s energy with light-absorbing molecules called pigments. The plants principal pigment is chlorophyll. There are two main types of chlorophyll: chlorophyll a and chlorophyll b. The wavelengths of sunlight you can see make up the visible spectrum, which contains all the colors. Chlorophyll absorbs light in the blueviolet and red regions very well. But it does not absorb light in the green region well. Green light is reflected by leaves, which is why plants look green. Any compound that absorbs light absorbs the energy in light. When chlorophyll absorbs sunlight, much of the energy of the light is transferred directly to the electrons in the chlorophyll molecule, raising the energy level of the electrons. There are other pigments that aid in photosynthesis known as carotenoids. 8 3 The Reactions of Photosynthesis In plants and other photosynthetic prokaryotes, photosynthesis takes place inside the chloroplasts. Chloroplasts contain saclike photosynthetic membranes called thylakoids. Thylakoids are arranged 2 Photosynthesis -

3 in stacks called grana. Proteins in the thylakoid membrane organize chlorophyll and other pigments into clusters known as photosystems. These photosystems are the light-collecting units of chlorophyll. The reactions of photosynthesis occur in two parts: (1) the light-dependent reactions and (2) the light-independent reactions, which are also called the Calvin cycle. When sunlight excites electrons in chlorophyll, the electrons gain a great deal of energy. A carrier molecule is a compound that can accept a pair of high-energy electrons and transfer them along with most of their energy to another molecule. One of these carrier molecules is NADP+. In the process of photosynthesis, NADP+ accepts and holds 2 high-energy electrons along with a hydrogen ion (H+). This converts the NADP+ into NADPH. The light-dependent reactions take place within the thylakoid membranes. These reactions require light. The light-dependent reactions use energy from light to produce oxygen gas and convert ADP and NADP+ into the energy carriers ATP and NADPH. Photosynthesis begins when pigments in a photosystem called photosystem II absorb light. A series of reactions follows. The reactants are water, ADP, and NADP+. The products are oxygen gas, ATP, and NADPH. The oxygen gas produced by photosynthesis is the source of nearly all the oxygen in Earth s atmosphere. The Calvin cycle takes place in the stroma of chloroplasts and does not require light. During the Calvin cycle, plants use the energy of ATP and NADPH products of the light-dependent reactions to produce high-energy sugars. The Calvin cycle uses carbon dioxide in its series of reactions. As photosynthesis proceeds, the Calvin cycle works steadily removing carbon dioxide from the atmosphere and turning out energy-rich sugars. Six carbon dioxide molecules are needed to produce a single 6-carbon sugar. Many factors affect the rate of photosynthesis. Such factors include availability of water, temperature, and intensity of light. The Need for Energy All organisms require energy for: Movement Active transport Production of proteins Cell division Photosynthesis Cellular respiration All other cellular reactions adenine ribose 3 phosphate groups Cellular energy is stored in the ATP Molecule: 3 Photosynthesis -

4 ADP Molecule or Adenosine Di-Phosphate: Cells recycle the ADP to make new ATP to store more energy for future use. Many proteins have spots where ATP attaches to provide energy for the protein to do its job, and then the ADP is released for recycling. ADP ATP Molecule or Adenosine Tri-Phosphate: Cells have enough ATP to last for a few seconds. ATP must constantly be made. ATP transfers energy very well. ATP is NOT good at energy storage. ATP ADP to ATP Conversion Reaction: Photosynthesis Summary The What: Is the process that uses the sun s energy to make. Purpose is to trap sun s energy and store it in glucose (food for the plant). Carried out by green plants and some bacteria known as autotrophic can make their own food. Reaction: 6 CO2 + 6 H2O C6H12O6 + 6 O2 This process is known as a biochemical pathway: when products of one reaction are used as the reactants for the next. 4 Photosynthesis -

5 The Where: Mainly occurs in the leaves: Stoma - pores Mesophyll cells - Occurs in the Chloroplasts inside the Mesophyll Cell Inside the Chloroplast - Thylakoids are green because they contain chlorophyll. Chlorophyll a is the green pigment in plants that absorbs light energy. Absorbs light in the violet-blue and orange-red wavelengths Chlorophyll B is a yellow pigment that absorbs blue wavelengths Pigment is a light-absorbing compound. The How: A Look inside Photosynthesis There are two separate reactions that occur inside the process of Photosynthesis: Light (Dependent) Reaction Dark (Light Independent) Reaction / Calvin Cycle Photosynthesis The Specifics The LIGHT Reaction - First step of photosynthesis that traps sunlight and makes electrons and ATP to run the dark reaction. Occurs inside the thylakoid membranes. Uses light energy. 5 Photosynthesis -

6 Produce Oxygen from water. Converts ADP to ATP. Also converts NADP+ into the energy carrier NADPH. Overview: 6 Photosynthesis -

7 Stage 1: Photosystem II absorbs light energy. Electrons are energized and passed to the (ETC). Lost electrons are replaced from the splitting of water into 2H+, free electrons, and oxygen. 2H+ pumped across thylakoid ETC Electron transport chain membrane. Stage 2: High-energy electrons are moved to photosystem I through the Electron Transport Chain (ETC). Energy is used to transport H+ from the stoma to the inner thylakoid membrane. NADP+ is converted to 2 molecules of NADPH when it picks up 2 electrons & H+. Stage 3: Powers ATP synthesis. 7 Photosynthesis -

8 In the thylakoid membrane is an enzyme called ATP Synthase. As H+ ions pass through the thylakoid membrane, down their concentration gradient, the enzyme binds them to a molecule of ADP (hydrolysis). Forms 3 molecules of ATP that is used to fuel cellular activities. The DARK Reaction / Calvin Cycle: Sunlight is not necessary for this reaction to occur. Occurs in the stroma Uses ATP and NADPH from light reaction as energy. Atmospheric C02 is used to make sugars like glucose and fructose. A 3-carbon molecule called Ribulose Biphosphate (RuBP) is used to regenerate the Calvin cycle. 3-CO2 molecules enter the cycle to form several intermediate compounds (PGA). The 3 carbon molecule used to make glucose is PGAL (Glyceraldehyde-3-phosphate). Two turns of the Calvin Cycle are required to make one molecule of sugar and other compounds. 8 Photosynthesis -

9 Overview: Stage 1: CO2 from the atmosphere undergoes fixation and creates 6 copies of an intermediate molecule called: 3-Phosphoglycerate (3-PGA) Stage 2: ATP and NADHP lose a phosphate group to become ADP and NADH. 3-PGA converts to G3P Glyceraldehyde 3-phosphate and ultimately Glucose Stage 3: 5 of the G3P molecules use 3 ATP molecules to regenerate Ribulose biphosphate in order to go through the Calvin Cycle again. 9 Photosynthesis -

10 Photosynthesis Biochemical Summary Consumed Produced Light Reaction Dark Reaction 12 H2O 6 CO2 18 ATP 12 - NADPH 12 - NADPH 18 ATP 6 O2 1 Sugar 6 H2O Concept Review: 1. Give several examples of life processes that require energy. 2. What is the major light absorbing pigment in plants? 3. What two things are formed by plants when carbon dioxide & water are combined using sunlight as the energy? 4. Do all organisms require energy? 5. What is the pigment that absorbs violet-blue wavelengths? 6. Only, not heterotrophs, carry on photosynthesis. 7. When the products of one chemical reaction are used as the reactants for the next reaction, the series of reactions is known as a pathway. 8. Plants cells use light to make what two energy carrying molecules? 9. What sugar is the final product of photosynthesis? 10. Electrons are transported from one molecule to another by atoms. 11. Can the dark reactions of photosynthesis occur during the daytime or only in the dark? 12. What is the original source of energy for all living things on earth? 13. Stacks of thylakoids called grana are suspended in the fluid inside chloroplasts called. 14. Light travels to plants as tiny packets of radiant energy called. 15. What happens to the electrons of chlorophyll when they are stuck by sunlight? 16. What gas is put back into our atmosphere by photosynthesis? 17. Complex carbohydrates are made during what cycle during photosynthesis? 18. Electrons raised to a higher energy level when struck by light enter what chain? 19. What important energy carrier molecule in photosynthesis picks up hydrogen atoms? 10 Photosynthesis -

11 20. Oxygen made during photosynthesis comes from the splitting of what molecule? 21. How many turns of the Calvin cycle produce 1 molecule of sugar? 22. What are the net molecules consumed during the Calvin cycle? What and how many are produced? 23. Complete the Concept map below 11 Photosynthesis -