Pearson Biology Chapter 8 Class Notes Photosynthesis Chemical Energy and ATP Why is ATP useful to cells? Energy is the Ability to do Work. Cells use Adenosine Triphosphate (ATP) to Store and Release Energy for short periods of time. ATP consists of : Energy is Stored between its Phosphate Groups. Energy is Released by Breaking the Bonds between the 2 nd and 3 rd Phosphate Groups (This releases a pair of electrons) ATP Storing and Releasing Energy Adenosine Diphosphate (ADP) Has two phosphate groups Contains some energy, but not as much as ATP. Like a Rechargeable Battery ATP can Release the Energy by breaking the bonds between the 2nd and 3rd Phosphate Groups. Using Biochemical Energy ATP is not good for storing energy over the long term. Glucose carries 90 times the energy it takes to put a single Phosphate Group on to ATP Cells stay more efficient by keeping only a small supply of ATP on hand. Cells can regenerate ATP from ADP as needed by using the energy in foods like glucose. Cells Use the Biochemical Energy in ATP: Cells Use the Biochemical Energy in ATP: Keep Homeostasis Grow and develop Movement of Cilia and Flagella Move materials around (Active Transport) Responses to chemical signals and environmental changes Build new molecules.. Synthesis of proteins 1
Heterotrophs and Autotrophs What happens during the process of Photosynthesis? Autotrophs Convert the Energy of Sunlight into Chemical Energy... and Store it in the Bonds of Carbohydrates Think About It How would you design a system to capture the energy of sunlight and convert it into a useful form? Photosynthetic organisms capture energy from sunlight with Pigments. Photosynthesis Reaction Formula: 6 CO 2 + 6 H 2 O + C 6 H 12 O 6 + 6 O 2 CO 2 = Carbon Dioxide H 2 O = Water C 6 H 12 O 6 = Glucose O 2 = Oxygen Gas SUNLIGHT Sun s Spectrum Energy from the sun travels to Earth by Wavelengths Some are Visible Some are Not Objects Reflect, Transmit, or Absorb these Wavelengths Chlorophyll CO 2 + H 2 O + C 6 H 12 O 6 + O 2 White Light When we pass white light through a PRISM it separates into its component colors This array of colors that are visible to our eyes is called the Visible Spectrum...It Ranges from VIOLET to RED (nanometers = nm) 400 nm 700 nm Pigments = Compounds that absorb light waves The light wave that a pigment does not absorb is reflected. This is the color that we see. Major Plant Pigments = Chlorophyll a Chlorophyll b Light is absorbed in blue-violet and red regions of the visible iibl spectrum, but not tin the green region This is why leaves are Green 400 nm 700 nm VIOLET RED Absorbed Reflected Absorbed 2
Pigments (Continued) Plants also contain Red and Orange Pigments (like Carotene) that absorb light in other regions of the spectrum. FYI - The Green of the Chlorophyll overwhelms the other pigments as temperatures drop chlorophyll molecules break down. the red and orange pigments may be seen Chloroplasts Chloroplasts contain Interconnected Saclike Photosynthetic Membranes called Thylakoids, arranged in stacks known as Grana, surrounded by Stroma Pigments are located in the Thylakoid Membrane Chloroplast Anatomy = Organelles found in plant and algae cells Surrounded by a pair of Outer and Inner membranes Outer Membrane Inner Membrane Thylakoids = System of membranes inside the inner membrane Appear as flattened stacks Grana (singular = Granum) = Connected stacks of Thylakoids 3
Energy Collection Compounds that absorbs light absorb energy..chlorophyll easily absorbs visible light.this light energy is transferred to electrons These high-energy electrons make photosynthesis work. Electron Carrier Compound accepts a Pair of High- Energy Electrons from Chlorophyll and Transfers them (along with most of their energy) to another Molecule Stroma = Solution surrounding the Grana High-Energy Electrons FYI - High-energy Electron are like hot potatoes If you wanted to move a potato from one place to another, you would use an oven mitt a carrier to transport it. Plants use Electron Carriers to Transport High-energy Electrons from Chlorophyll to other molecules. NADP + (Nicotinamide Adenine Dinucleotide Phosphate) = a carrier molecule that accepts and holds two highenergy electrons, along with a hydrogen ion (H + ). it is converted into NADPH. Photosynthesis Reaction Formula: 6 CO 2 + 6 H 2 O + C 6 H 12 O 6 + 6 O 2 CO 2 = Carbon Dioxide H 2 O = Water C 6 H 12 O = Glucose 6 = Oxygen Gas O 2 Photosynthesis involves two sets of reactions: Stage 1 = Light-Dependent { directly involve sunlight } because they require the direct involvement of light and light-absorbing pigments. Use energy from Sunlight to Convert ADP and NADP + into the energy carriers ATP and NADPH Water is required as a source of electrons and hydrogen ions. Oxygen is released as a byproduct. Reactions occur within the Thylakoid Membranes Stage 1 of Photosynthesis Stage 1 of Photosynthesis = Light-Dependent Reactions Occur within the Thylakoid Membrane 4
Light-Dependent Reactions: Generating ATP and NADPH Photosystems { II and I } Clusters of chlorophyll and proteins within the thylakoid that absorb sunlight and generate highenergy electrons that are then passed to a series of electron carriers embedded in the thylakoid membrane. Photosystem II comes before Photosystem I Photosystem II Light energy is absorbed by electrons in pigments in photosystem II These High-energy electrons are passed to the Electron Transport Chain ETC = a series of electron carriers that move the high-energy electrons during ATP-generating reactions. Photosystem II (Continued) New electrons come from the breakdown of Water to replace the ones that left chlorophyll and travelled into the ETC Enzymes of the inner surface of the Thylakoid break up each water into 2 electrons, 2 H + ions, and 1 oxygen atom 2H 2 O 4e - + 4H + + O 2 Photosystem II (Continued The 2 electrons replaced the high-energy electrons that were lost to the electron transport chain. Oxygen gas is released into the air This reaction is the source of most of the oxygen in Earth s atmosphere. The H + ions are released into the space inside the Thylakoid (Continued) Electron Transport Chain Energy from electrons travelling through the ETC is used by the proteins in the electron transport chain to pump additional H + ions from the Stroma into the Thylakoid space. Electron Transport Chain (Continued) At the end of the electron transport chain, the electrons pass to photosystem I. 5
Photosystem I Because energy was used to pump H + ions across the thylakoid membrane electrons do not contain as much energy when they reach the end of fthe ETC at tthe beginning of Photosystem I. Pigments in the Photosystem I use energy from light to re-energize these electrons. Photosystem I (Continued) At the end of a short second ETC the NADP + (in the stroma) picks up the high-energy electrons and H + ions. And becomes NADPH. Hydrogen Ion Movement and ATP Formation H + ions accumulate in the thylakoid space from: the splitting of water. and. from being pumped in from the stroma. Hydrogen Ion Movement and ATP Formation (continued) The buildup of H + ions makes the stroma negatively charged relative to the space within the thylakoids. This gradient.. the difference in both charge and H + ion concentration across the membrane. provides the energy to make ATP. Hydrogen Ion Movement and ATP Formation (continued) Hydrogen Ion Movement and ATP Formation (continued) The thylakoid membrane contains a protein enzyme called ATP Synthase that allows H + ions to pass through it Powered by the gradient. H + ions pass through the ATP Synthase and force it to rotate. As it rotates. ATP Synthase binds ADP and a phosphate group to produce ATP ADP + P group ATP 6
Hydrogen Ion Movement and ATP Formation (continued) = Chemiosmosis Enables light-dependent electron transport to produce not only NADPH (at the end of the electron transport chain), but ATP as well. Summary of Light-Dependent Reactions The light-dependent reactions produce oxygen gas and convert ADP and NADP + into the energy carriers ATP and NADPH. During Stage 2 = Light-Independent ATP and NADPH provide the energy needed to build high-energy sugars from low-energy carbon dioxide. The Light-Independent Reactions: Producing Sugars Stage 2 = Light-Independent { No light is required } Occcur in the Stroma of the Chloroplast Commonly referred to as the Calvin cycle ATP and NADPH molecules produced in the lightdependent reactions are used to produce high-energy sugars from carbon dioxide These carbohydrate compounds can be stored for a long time. Carbon Dioxide ( CO 2 ) Enters the Calvin Cycle An enzyme in the stroma of the chloroplast combines CO 2 molecules with 5-carbon compounds that are already present in the organelle producing 3-carbon compounds that continue in the cycle. For every 6 CO 2 molecules that enter the cycle, a total of twelve 3-carbon compounds are produced. 6CO 2 +6(5C) (5-C) 12 (3-C) Carbon Dioxide ( CO 2 ) Enters the Calvin Cycle Other enzymes in the chloroplast then convert the 3-carbon compounds into higher-energy forms 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. 7
Sugar Production (continued) Summary of the Calvin Cycle The remaining ten 3- carbon molecules are converted back into six 5-carbon molecules that combine with six new carbon dioxide molecules to begin the next cycle. The Calvin cycle uses 6 molecules of carbon dioxide to produce a single 6-carbon sugar molecule. Summary of the Calvin Cycle (continued) The energy for the reactions is supplied by compounds produced in the light-dependent reactions. Summary of the Calvin Cycle (continued) The plant uses the sugars produced by the Calvin cycle to meet its energy needs and to build macromolecules needed for growth and development. When other organisms eat plants, they can use the energy and raw materials stored in these compounds. 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 lightindependent reactions use that chemical energy to produce stable, high-energy sugars from carbon dioxide and water. In the process, animals, including humans, get food and an atmosphere filled with oxygen. Factors Affecting Photosynthesis: Temperature Light Intensity Availability of Water 8
Temperature The reactions of photosynthesis are made possible by enzymes that function best between 0 C and 35 C. Temperatures above or below this range may affect those enzymes, slowing down the rate of photosynthesis or stopping it entirely. Light High light intensity increases the rate of photosynthesis. After the light intensity reaches a certain level, however, the plant reaches its maximum rate of photosynthesis, as is seen in the graph. Water Because water is one of the raw materials in photosynthesis, a shortage of water can slow or even stop photosynthesis. Water loss can also damage plant tissues. Plants that live in dry conditions often have waxy coatings on their leaves to reduce water loss. They may also have biochemical adaptations that make photosynthesis more efficient under dry conditions. Photosynthesis Under Extreme Conditions In order to conserve water, most plants under bright, hot conditions close the small openings in their leaves that normally admit carbon dioxide. This causes carbon dioxide within the leaves to fall to very low levels, slowing down or even stopping photosynthesis. C4 and CAM plants have biochemical adaptations that minimize water loss while still allowing photosynthesis to take place in intense sunlight. C4 Photosynthesis C4 plants have a specialized chemical pathway that allows them to capture even very low levels of carbon dioxide and pass it to the Calvin cycle. The name C4 plant comes from the fact that the first compound formed in this pathway contains 4 carbon atoms. CAM Plants Members of the Crassulacae family, such as cacti and succulents, incorporate carbon dioxide into organic acids during photosynthesis in a process called Crassulacean Acid Metabolism (CAM). The C4 pathway requires extra energy in the form of ATP to function. C4 organisms include crop plants like corn, sugar cane, and sorghum. 9
CAM Plants CAM plants admit air into their leaves only at night, where carbon dioxide is combined with existing molecules to produce organic acids, trapping the carbon within the leaves. During the daytime, when leaves are tightly sealed to prevent water loss, these compounds release carbon dioxide, enabling carbohydrate production. CAM plants include pineapple trees, many desert cacti, and ice plants. 10