Chapter Six (Photosynthesis)

Transcription

1 Chapter Six (Photosynthesis) 1 SECTION ONE: THE LIGHT REACTIONS OBTAINING ENERGY Organisms can be classified according to the way they obtain energy. Autotrophs are organisms that use energy from sunlight or inorganic substances to make organic compounds. They mostly use the process of photosynthesis to convert light energy from the sun into chemical energy. Animals and other organisms that have to get energy from food instead of directly from sunlight or other inorganic compounds are called heterotrophs ophs. Auto means self and hetero means other. So remember, autotrophs make food by themselves and heterotrophs need others for food. Photosynthesis involves a complex series of chemical reactions in which the product of one reaction is consumed by the net reaction. When such reactions are linked in this way, it is referred to as a biochemical pathway. Light energy Plants convert light energy to chemical energy. Caterpillars get energy by eating plants. Birds get energy by eating caterpillars. OVERVIEW OF PHOTOSYNTHESIS Carbon dioxide and water PHOTOSYNTHESIS by autotrophs CELLULAR RESPIRATION by autotrophs and heterotrophs Organic compounds and oxygen The diagram to the left shows how autotrophs use photosynthesis to produce organic compounds from carbon dioxide (CO 2 ) and water. The oxygen (O 2 ) and some of the organic compounds produced are then used by cells in cellular respiration, where CO 2 and water are produced. Therefore, the products of photosynthesis are reactant of cellular respiration. This reaction goes both ways.

2 Photosynthesis can be divided into two stages: 1. Light Reactions in which light energy from the sun is converted into chemical energy, which is temporarily stored in ATP and NADPH, which is the energy carrier molecule. 2. Calvin Cycle where organic compounds are formed using CO 2 and the chemical energy stored in ATP and NADPH. 2 Photosynthesis can be summarized using the equation: CAPTURING LIGHT ENERGY The first stage of photosynthesis includes the light reactions, which require light to happen. This begins with light being absorbed in the chloroplasts of plant and algae cells. If you recall from Chapter 4, chloroplasts are surrounded by a double membrane. Inside the inner membrane is a system of membranes called thylakoids, arranged as flattened pockets. They are connected and stacked on top of each other called grana. Surrounding the grana is a solution called the stroma. The stroma of a thylakoid can be compared to the cytoplasm of a cell. Lights and Pigments Although light from the sun appears white, it is actually comprised of several colors. White light can be separated into its component colors by passing the light through a prism. The resulting colors range from red at one end to violet at the other, and is called the visible spectrum. Each color in the visible spectrum has a different wavelength, measured in nanometers.

3 When white light strikes an object, its component colors can be reflected, transmitted, or absorbed by the object. Many objects contain pigments, compounds that absorb light. Most pigments absorb certain colors stronger than the other. The absorbed colors are removed from the visible spectrum, so the light that is reflected or transmitted by the pigment no longer appears white. For example, the pigments in a green shirt absorb all the colors except for green. This leaves green to be reflected and transmitted, so the shirt looks green. Chloroplast Pigments Located in the membrane of the thylakoids are several pigments, of which chlorophylls are the most important. While there are several different types of chlorophyll, the most important types are chlorophyll a and chlorophyll b. Chlorophyll a absorbs less blue light but more red light than chlorophyll b does. Neither of them absorbs much green light, and they allow it to be reflected or transmitted. This is why leaves and other parts of plants that have a lot of chlorophyll look green. Only chlorophyll a is directly involved in the light reactions of photosynthesis. Chlorophyll b assists chlorophyll a in doing so; therefore it is an accessory pigment. Other compounds in the thylakoid membrane include carotenoids, which also function as accessory pigments. Accessory pigments absorb the colors that chlorophyll a cannot absorb, and so they enable plants to capture more of the energy in light. In the leaves of plants, the chlorophylls are generally present in larger numbers, and so mask the colors of other pigments. But in parts of a plant not involved in photosynthesis, such as fruit and flowers, the colors of other pigments are visible. Additionally, when plants lose their chlorophylls in the fall, their leaves take on the colors of the carotenoids. CONVERTING LIGHT ENERGY TO CHEMICAL ENERGY Once the pigments in the chloroplast have captured light energy, the next step is to convert the light energy into chemical energy. This chemical energy is temporarily stored in ATP and NADPH. Oxygen is given off during these reactions. Chlorophylls and carotenoids are clustered in groups of a few hundred pigment molecules in the thylakoid membrane. Each group of pigment molecules and the proteins they are embedded in are called a photosystem. Two types of photosystems are photosystem I and photosystem II. They have similar kinds of pigments, but have different roles in the light reactions. 3

4 The light reactions begin when accessory pigment molecules in both photosystems absorb light. When doing so, they acquire some of the energy carried by the light. In each photosystem, this energy is passed on to other pigment molecules into it reaches a specific pair of chlorophyll a molecules, which can also absorb light. Step (1) Light energy forces the electrons to enter a higher energy level in the chlorophyll a molecules of photosystem II. They are now excited and have enough energy to leave the chlorophyll a molecules. Step (2) The acceptor of those lost electrons is a molecule in the thylakoid membrane called the primary electron acceptor. Step (3) ( The primary electron acceptor donates the electrons to a series of molecules which transfer electrons from one molecule to the next. It is naturally called an electron transport chain. As the electrons move from molecule to molecule in the chain, they lost most of the original energy they had when they were excited. This lost energy is used to move protons (H + ) into the thylakoid. Step (4) Light is absorbed by photosystem I at the same time that light was absorbed by photosystem II. The electrons from the chlorophyll a molecules in photosystem I move to another primary electron acceptor. The electrons lost by these chlorophyll a molecules are replaced by the electrons that passed through the electron transport chain from photosystem II. Step (5) The electrons that photosystem I now has are donated to a different electron transport chain, which brings them to the outer side of the thylakoid membrane. There the electrons combine with a proton and NADP +, an organic molecule that accepts electrons during redox reactions. This causes NADP + to be reduced to NADPH. 4

5 Replacing Electrons in Light Reactions The electrons from chlorophyll molecules in photosystem II replace the electrons that leave chlorophyll molecules in photosystem I. The replacement electrons are provided by water molecules. An enzyme inside the thylakoid splits water molecules into protons, electrons, and oxygen. 5 2H 2 O 4H + + 4e For every two molecules of water split, four electrons become available to replace those lost by the chlorophyll molecules in photosystem II. The protons produced are left inside the thylakoid, and the oxygen diffuses out of the chloroplast to leave the plant. Making ATP in Light Reactions An important part of the light reactions is chemiosmosis, a process in which ATP in synthesized. Chemiosmosis relies on a concentrated gradient of protons across the thylakoid membrane. Some protons are produced by the splitting of water molecules, and other protons are pumped from the stroma to the interior of the thylakoid. The energy required to pump these protons is supplied by the excited electrons passing through the electron transport chain of photosystem II. The concentration of protons is higher inside the thylakoid than in the stroma. The concentration gradient represents potential energy, which is harvested by an enzyme called ATP synthase, located in the thylakoid membrane. It makes ATP by adding a phosphate group to adenosine diphosphate (ADP). ATP synthase converts the potential energy of the proton concentration gradient into chemical energy stored in ATP. Some of the protons in the stroma area also used to make NADPH. Together, ATP and NADPH provide energy for the second set of reactions in photosynthesis.

6 6 SECTION 1 REVIEW 1. Explain why both autotrophs and heterotrophs depend on photosynthesis to obtain the energy they need for life processes. 2. Describe the role of chlorophylls in the biochemical pathways of photosynthesis. 3. List the three substances that are produced when water molecules are broken down during the light reactions. 4. Explain why the splitting of water is important to the continuation of the light reactions. 5. Name the product of the process known as chemiosmosis. CRITICAL THINKING 6. Thinking about the main roles of pigments in photosynthesis, explain how the pigments in colored objects such as clothes differ from plant pigments. 7. The molecule that precedes the electron transport chains of both photosystem I and photosystem II is an electron acceptor. What is the original molecule that is the electron donor for both of these systems? 8. Explain how the light reactions would be affected if there were no concentration gradient of protons across the thylakoid membrane. SECTION TWO: THE CALVIN CYCLE CARBON FIXATION The Calvin cycle is a series of enzyme-assisted chemical reactions that produces a three-carbon sugar. In this cycle, carbon atoms from carbon dioxide in the atmosphere are bonded, or fixed, into organic compounds. This incorporation is called carbon fixation. A total of three CO 2 molecules must enter the Calvin cycle to produce each three-carbon sugar. The Calvin cycle occurs within the stroma of the chloroplast. Step (1) CO 2 diffuses into the stroma from the surrounding cytosol. An enzyme combines each CO 2 molecule with a five-carbon molecule called ribulose bisphosphate (RuBP). The six-carbon molecules that result are very unstable, and immediately split into two three-carbon molecules, which are called 3-phosphoglycerate (3-PGA). Step (2) Each molecule of 3-PGA is converted into another three-carbon molecule, glyceraldehyde 3-phosphate in a two-step process. Each 3-PGA molecule received a phosphate group from a molecule of ATP. The compound then receives a proton from NADPH and releases a phosphate group. The ADP, NADP +, and phosphate that are produced can be used again in light reactions. Step (3) One of the G3P molecules leaves the Calvin cycle and is used to make organic compounds in which energy is stored for later use.

7 Step (4) The remaining G3P molecules are converted back into RuBP by adding phosphate groups from ATP molecules. They then re-enter the Calvin cycle. The most common pathway for carbon fixation, the Calvin cycle is named for Melvin Calvin. He was the American biochemist who worked out the chemical reactions in the cycle. Plant species that fix carbon exclusively through this cycle are classified as C 3 plants because of the three-carbon compound initially formed in the process. 7 ALTERNATIVE PATHWAYS Plant species living in hot, dry climates fix carbon through alternative pathways. Under such conditions, plants can rapidly lose water to the air through small pores called stomata, usually located on the undersurface of the leaves. By partially closing their stomata when the air is hot and dry, plants can reduce water loss. They are the major passageways through which CO 2 enters and O 2 leaves a plant. When a plant s stomata are partly closed, level of CO 2 falls as it is consumed in the Calvin cycle. At the same time, the level of O 2 in the plant rises as light reactions generate O 2. Both a low CO 2 level and a high O 2 level restrains carbon fixation by the Calvin cycle. Alternative pathways help plants deal with such a problem. The C 4 Pathway One alternative pathway enables certain plants to fix CO 2 into four-carbon compounds, and is called the C 4 pathway. Plants that use it are known as C 4 plants. During the hottest part of the day, these plants have their stomata partially closed. Certain cells in C 4 plants have an enzyme that can fix carbon into four-carbon

8 compounds even when the CO 2 level is low and the O 2 level is high. Such plants lose only about half as much water as C 3 plants when producing the same amount of carbohydrates. 8 The CAM Pathway Certain other plants have different adaptations to hot, dry climates, and fix carbon through a pathway called the CAM pathway (crassulacean acid metabolism). They open their stomata at night and close them during the day, the opposite of what other plants do. At night, they take in CO 2 and fix it into organic compounds, while during the day; CO 2 is released from these compounds and enters the Calvin cycle. Since their stomata open at night, when the temperature is lower, CAM plants grow fairly slowly, but lose less water than either C 3 or C 4 plants. A SUMMARY OF PHOTOSYNTHESIS Photosynthesis occurs in two stages, both of which occur inside the chloroplasts of plant cells and algae. 1. The light reactions Energy is absorbed from sunlight and converted into chemical energy, which is temporarily stored in ATP and NADPH. 2. The Calvin cycle Carbon dioxide and the chemical energy stored in ATP and NADPH is used to form organic compounds. Photosynthesis is an ongoing cycle: the products of the light reactions are used in the Calvin cycle and vice versa. Additionally, the other products of the Calvin cycle are used to produce a variety of organic compounds. Many plants produce surplus carbohydrates, which can be stored as starch in the chloroplasts and in structures such as roots and fruits. The stored carbohydrates provide the chemical energy that autotrophs and heterotrophs depend on. The simplest overall equation for photosynthesis can be written as follows: CO 2 + H 2 O (CH 2 O) In the above equation, (CH 2 O) represents the general formula for a carbohydrate. It is often replaced by the carbohydrate glucose, giving the following equation: 6CO 2 +H 2 O C 6 H 12 O

9 However, glucose is not a direct product of photosynthesis, and is included to emphasize the relationship between photosynthesis and cellular respiration, where glucose plays a key role. Photosynthesis and cellular respiration together create an ongoing cycle. The light reactions are sometimes referred to as light-dependent reactions, because energy from light is required for the reactions. The Calvin cycle is then referred to as light-independent reactions or the dark reactions, because the Calvin cycle does not require light directly. FACTORS THAT AFFECT PHOTOSYNTHESIS Light Intensity The rate of photosynthesis increases as light intensity increases, since higher light intensity excites more electrons. Thus, the light reactions will proceed more rapidly. However, at a certain point all of the available electrons are excited, and the maximum rate is reached. This means the rate will stay level regardless of further increases. Carbon Dioxide Levels Increases levels of C0 2 also stimulate photosynthesis until the rate of photosynthesis levels off. Temperature Increasing temperature speeds up the chemical reactions involved in photosynthesis. The rate of photosynthesis increases as the temperature does over a certain range, but then it peaks at a certain temperature. This is when many of the enzymes that catalyze the reactions become ineffective and the stomata being to close. These conditions cause the rate to decrease when the temperature is further increased. SECTION 2 REVIEW 1. Name the part of the chloroplast where the Calvin cycle takes place. 2. Describe what can happen to the three-carbon molecules made in the Calvin cycle. 3. Distinguish between C 3, C 4, and CAM plants. 4. Explain why the light reactions and the Calvin cycle are dependent on each other. 5. Explain why increased light intensity might not result in an increased rate of photosynthesis. 9 CRITICAL THINKING 6. What would happen to photosynthesis if all of the three-carbon sugars produced in the Calvin cycle were used to make organic compounds? 7. Explain how a global temperature increase could affect plants 8. Explain how the world would be different if C 4 plants and CAM plants had not evolved.

10 CHAPTER HIGHLIGHTS 10 SECTION 1: The Light Reactions Photosynthesis converts light energy into chemical energy through series of reactions known as biochemical pathways. Almost all life depends on photosynthesis. Autotrophs use photosynthesis to make organic compounds from carbon dioxide and water. Heterotrophs cannot make their own organic compounds from inorganic compounds and therefore depend on autotrophs. White light from the sun is composed of an array of colors called the visible spectrum. Pigments can absorb certain colors of light and reflect or transmit the other colors. The light reactions of photosynthesis begin with the absorption of light by chlorophyll a and accessory pigments in the thylakoids. Excited electrons that leave chlorophyll a travel along two electron transport chains, resulting the production of NADPH. The electrons are replaced when water is split into electrons, protons, and oxygen in the thylakoid. Oxygen is released as a byproduct of photosynthesis. SECTION 2: The Calvin C Cycle The ATP and NADPH produced in the light reactions drive the second stage of photosynthesis, the Calvin cycle. In the Calvin cycle, C0 2 is incorporated into organic compounds, a process called carbon fixation. The Calvin cycle produces a compound called G3P. Most G3P molecules are converted into RuBP to keep the Calvin cycle operating. However, some G3P molecules are used to make other organic compounds, including amino acids, lipids, and carbohydrates. Plants that fix carbon using only the Calvin cycle are known as C 4 plants. Some plants that evolved in hot, dry climates fix carbon through alternative pathways the C 4 and CAM pathways. These plants carry out carbon fixation and the Calvin cycle either in different cells or at different times. Photosynthesis occurs in two stages. In the light reactions, energy is absorbed from sunlight and converted into chemical energy; in the Calvin cycle, carbon dioxide and chemical energy are used to form organic compounds. The rate of photosynthesis increases and then reaches a plateau as light intensity of CO 2 concentration increases. Below a certain temperature, the rate of photosynthesis increases as temperature increases. Above that temperature, the rate of photosynthesis decreases as temperature increases.