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Photosynthesis Chapter 08
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Chapter 8, Section 1: Energy and Life
The ability to do work Nearly every activity in life uses energy Building new molecules Contracting muscles Carrying out active transport Comes in many forms Light, heat, electricity Can be stored in chemical compounds Old bonds break New bonds form Chemical fuel needed by organisms Adenosine triphosphate (ATP)
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Chapter 8, Section 1: Energy and Life
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Chapter 8, Section 1: Energy and Life
Adenosine Triphosphate made up of: Adenine – nitrogen base Ribose – 5-carbon sugar 3 phosphate groups Key to ATP’s ability to store and release energy Releasing energy ATP loses one phosphate group to become ADP (adenosine diphosphate) Broken chemical bond between phosphate groups releases energy Storing energy ADP gains one phosphate group to become ATP Adding phosphate groups stores energy
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Chapter 8, Section 1: Energy and Life
ATP: Fully charged battery ADP: Partially charged battery Using Biochemical Energy Carry out active transport Example: sodium-potassium pump Powering movement of cell and within cell Example: muscle contraction, cilia, flagella Synthesis of macromolecules Example: proteins, carbohydrates, etc. Produce light Example: bioluminescence Most cells have only enough ATP for a few seconds Regenerate ATP from ADP as needed by cells
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Chapter 8, Section 1: Energy and Life
Heterotrophs and Autotrophs All organisms need a source of energy: food Heterotrophs Obtain food by consuming other living things Autotrophs Use light energy or chemical energy to produce food Nearly all life on earth depends on the ability of autotrophs to capture the energy of sunlight and store the energy in the form of food Process known as photosynthesis Photo- meaning “light” -synthesis meaning “putting together” In the process of photosynthesis, plants convert the energy of sunlight into chemical energy stored in the bonds of carbohydrates.
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Chapter 8, Section 2: Photosynthesis: An Overview
Chlorophyll and Chloroplasts Energy from sunlight seen as “white” light Mixture of different wavelengths of light Red, orange, yellow, green, blue, indigo, and violet (ROYGBIV) Pigments Light absorbing molecules Chlorophyll – principal pigment: green Chlorophyll a, b, c, and d Absorbs red and blue light Reflects green light Carotenoids – red, orange, and yellow Absorbs blue and green light Reflects red, orange, and yellow
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Chapter 8, Section 2: Photosynthesis: An Overview
Chlorophyll and Chloroplasts Chloroplasts Found in eukaryotic cells Contain sac-like membranes called thylakoids Contain pigments Chlorophyll Carotenoids Interconnected Arranged in stacks One stack: granum Multiple stacks: grana Fluid portion of chloroplast: stroma Found outside thylakoids
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Chapter 8, Section 2: Photosynthesis: An Overview
Chlorophyll and Chloroplasts Energy Collection Light is a form of energy Anything that absorbs light absorbs energy Chlorophyll collects visible light especially well Large portion of light energy collected is transferred directly to electrons in the chlorophyll molecule By raising the energy levels of the electrons, light energy can produce a steady supply of high-energy electrons Makes photosynthesis work
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Chapter 8, Section 2: Photosynthesis: An Overview
Electron-carrier Molecules Think of high-energy electrons as being similar to a hot potato If you wanted to move the potato, you’d need an oven mitt (a carrier) to move it Plants use a carrier to move high-energy electrons from chlorophyll to other molecules NADP+ (nicotinamide adenine dinucleotide phosphate) is a carrier molecule Accepts and holds 2 high-energy electrons A hydrogen ion When these are attached, NADP+ is called NADPH NADPH can carry the high-energy electrons and hydrogen ion to chemical reactions elsewhere in the cell
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Chapter 8, Section 2: Photosynthesis: An Overview
Reactants and Products of Photosynthesis Plants use the sugars made in photosynthesis to Produce complex carbohydrates such as starches Provide energy for synthesis of other compounds Proteins and lipids produce
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Chapter 8, Section 2: Photosynthesis: An Overview
Light-dependent reactions Also called light reactions First set of reactions in photosynthesis Occur in thylakoid membranes Require sunlight Require light-absorbing pigments Water is required as a source of electrons Oxygen is released as a by- product (waste)
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Chapter 8, Section 2: Photosynthesis: An Overview
Light-independent reactions Second set of reactions in photosynthesis Occurs in stroma outside the thylakoids Also called Calvin Cycle Requires no sunlight Requires no light-absorbing pigments Carbon dioxide from the atmosphere is required as a source of carbon Sugars are produced
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Chapter 8, Section 2: Photosynthesis: An Overview
Light-dependent and light- independent reactions are interdependent NADPH and ATP produced by light-dependent reactions to supply energy for light- independent reactions NADP+ and ADP produced by light-independent reactions to supply light-dependent reactions with needed compounds Two sets of reactions work together to Capture energy of sunlight Transform energy into energy-rich compounds such as carbohydrates
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Chapter 8, Section 3: Process of Photosynthesis
Light-dependent Reactions Reactions directly involve the absorption of energy from sunlight Use energy from sunlight to Produce oxygen Convert ADP to ATP Convert NADP+ to NADPH Thylakoids contain clusters of chlorophyll and proteins known as photosystems Absorb sunlight Generate high-energy electrons that are passed to electron transport chain
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Chapter 8, Section 3: Process of Photosynthesis
Photosystem II First photosystem in light-dependent reactions Named photosystem II because found after photosystem I First event: Light absorption by pigments in thylakoid membranes Light energy passed to electrons to make them higher-energy High-energy electrons passed to electron transport chain (ETC) Water splits to replace electrons lost to ETC From each water molecule: 2 electrons, 2 hydrogen ions, one oxygen atom Oxygen released to atmosphere Source of nearly all oxygen on Earth
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Chapter 8, Section 3: Process of Photosynthesis
Photosystem II Hydrogen ions left behind when water is split are released inside the thylakoid Energy from electrons being passed down the ETC is used to pump hydrogen ions from the stroma into the thylakoid space The electrons are passed to a second photosystem called photosystem I
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Chapter 8, Section 3: Process of Photosynthesis
Photosystem I Because energy was used to pump hydrogen ions across the thylakoid membrane into the thylakoid space Electrons are not as high-energy when they reach photosystem I Pigments absorb more light energy to re- energize the electrons High-energy electrons pass to short ETC At end of ETC NADP+ molecules in the stroma pick up high-energy electrons and hydrogen ions to become NADPH Used in light-independent reactions
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Chapter 8, Section 3: Process of Photosynthesis
Photosystem I Remember in photosystem II hydrogen ions accumulated in thylakoid space Other hydrogen ions were “pumped” in from the stroma Buildup of hydrogen ions makes the stroma negatively charged when compared to thylakoid space Gradient – difference in charge and hydrogen ion concentration across the thylakoid membrane Gradient provides energy to make ATP
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Chapter 8, Section 3: Process of Photosynthesis
Photosystem I Hydrogen ions cannot pass directly across the thylakoid membrane Enzyme used to move the hydrogen ions across thylakoid membrane Called ATP Synthase As hydrogen ions pass across the thylakoid membrane, it causes ATP synthase to rotate As ATP synthase rotates, it binds ADP and a phosphate group together to make ATP Process called chemiosmosis Enables light-dependent electron transport to produce NADPH and ATP
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Chapter 8, Section 3: Process of Photosynthesis
Summary of Light- dependent Reactions Reactants: sunlight, water, ADP, NADP+ Products: oxygen, ATP, NADPH Provides energy needed to build high-energy sugars from low-energy carbon dioxide
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Chapter 8, Section 3: Process of Photosynthesis
Light-independent Reactions Also called the Calvin Cycle Also called the dark reactions Occurs in the stroma Does not require sunlight for energy Can occur in sunlight Energy provided by ATP and NADPH produced in light-dependent reactions Use carbon dioxide from atmosphere to Produce high-energy sugars Convert ATP to ADP Convert NADPH to NADP+ Both ADP and NADP+ sent back to light-dependent reactions
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Chapter 8, Section 3: Process of Photosynthesis
Light-independent Reactions Carbon dioxide molecules enter Calvin Cycle from the atmosphere Carbon dioxide is combined with a 5-carbon compound already present in the cycle The 6 6-carbon compounds are converted to 12 3-carbon compounds ATP and NADPH from the light-dependent reactions provide energy to make the compounds higher energy compounds The ADP and NADP+ used here will be sent back to the light-dependent reactions
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Chapter 8, Section 3: Process of Photosynthesis
Light-independent Reactions At mid-cycle, two of the 3-carbon molecules leave the cycle Used to produce sugars Also lipids and amino acids can be produced using the Calvin Cycle The remaining 10 3-carbon molecules are converted into 6 5-carbon molecules These molecules will combine with carbon dioxide molecules from the atmosphere to begin the next cycle.
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Chapter 8, Section 3: Process of Photosynthesis
Summary of Light-independent Reactions Reactants: carbon dioxide, ATP, NADPH Products: sugars, ADP, NADP+ Provides ADP and NADP+ for use in the light- dependent reactions Provides sugars needed for energy in both producers and consumers
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Chapter 8, Section 3: Process of Photosynthesis
Summary of Light-independent Reactions Calvin Cycle uses 6 molecules of carbon dioxide to produce a single 6-carbon sugar molecule Energy required for Calvin cycle supplied by products of light-dependent reactions ATP and NADPH Plant uses sugars to Meet energy needs Build macromolecules for growth and development When other organisms eat plants, sugars are used to provide energy and raw materials stored in its compounds
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Chapter 8, Section 3: Process of Photosynthesis
Summary of Light-independent Reactions Calvin Cycle uses 6 molecules of carbon dioxide to produce a single 6-carbon sugar molecule Energy required for Calvin cycle supplied by products of light-dependent reactions ATP and NADPH Plant uses sugars to Meet energy needs Build macromolecules for growth and development When other organisms eat plants, sugars are used to provide energy and raw materials stored in its compounds
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Chapter 8, Section 3: Process of Photosynthesis
End Results Light-dependent reactions use water, sunlight, ADP and NADP+ To produce oxygen, ATP, and NADPH Light-independent reactions use carbon dioxide, ATP, and NADPH To produce sugars, ADP, and NADP+
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Chapter 8, Section 3: Process of Photosynthesis
End Results Two sets of photosynthetic reactions work together Light-dependent reactions trap the energy of sunlight in chemical form Light-independent reactions use chemical energy to produce stable, high- energy sugars from carbon dioxide and water Heterotrophs get food and an oxygen-filled atmosphere
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Chapter 8, Section 3: Process of Photosynthesis
Factors Affecting Photosynthesis Temperature Photosynthesis is controlled by enzymes that function best between 0-35°C Temperatures greater or less than these temperatures may slow or even stop photosynthesis Light Intensity High light intensity increases the rate of photosynthesis After light intensity reaches a certain level, the rate of photosynthesis levels off Water Availability Shortage of water can slow or stop photosynthesis Plants may adapt photosynthesis to adjust for water availability
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Chapter 8, Section 3: Process of Photosynthesis
Photosynthesis Under Extreme Conditions Most plants under bright, hot conditions close the small openings (stomata) in their leaves that normally admit CO2 Keeps plants from losing too much water Can cause CO2 levels to drop very low Slows or stops photosynthesis Two ways plants have adapted to extremely hot, dry conditions: C4 photosynthesis CAM plants
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Chapter 8, Section 3: Process of Photosynthesis
Photosynthesis Under Extreme Conditions C4 photosynthesis Specialized chemical pathway Allows capture of very low levels of carbon dioxide First compound formed in Calvin Cycle contains 4 carbon atoms First compound formed in typical photosynthesis contains 6 carbon atoms Permits photosynthesis to keep working under intense light and heat Examples: corn, sugar cane, and sorghum
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Chapter 8, Section 3: Process of Photosynthesis
Photosynthesis Under Extreme Conditions CAM plants Stands for: Crassulacean Acid Metabolism (CAM) Includes members of the family Crassulaceae Cells admit air into their leaves only at night CO2 is combined with existing molecules to produce organic acids “Trap” carbon within the leaves During daytime leaves are tightly sealed Prevents water loss CO2 is released to begin carbohydrate production Examples: desert cacti, pineapple trees, ice plants
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