Unit 3 (8)

Photosynthesis Notes

Overview of Photosynthesis

  • Photosynthesis is the process that converts solar energy into chemical energy.

  • It nourishes almost the entire living world directly or indirectly.

Autotrophs and Heterotrophs

  • Autotrophs:

    • Organisms that sustain themselves without eating anything derived from other organisms.

    • They are the producers of the biosphere.

    • Produce organic molecules from CO2 and other inorganic molecules.

    • Examples: Almost all plants are photoautotrophs.

  • Heterotrophs:

    • Obtain organic material from other organisms.

    • Act as the consumers of the biosphere.

    • Almost all heterotrophs, including humans, depend on photoautotrophs for food and oxygen.

Organisms Involved in Photosynthesis

  • Photosynthesis occurs in:

    • Plants

    • Algae

    • Certain protists

    • Some prokaryotes

  • These organisms not only feed themselves but also most of the living world.

Structural Organization of Photosynthetic Cells

  • Photosynthetic cells have enzymes and molecules grouped together in membranes, allowing for efficient chemical reactions.

  • Chloroplasts:

    • Sites of photosynthesis in plants.

    • Structurally similar to and likely evolved from photosynthetic bacteria.

Chloroplast Structure and Function

  • Major locations of photosynthesis:

    • Leaves, which act as nature’s solar panels.

    • Green color is due to chlorophyll, the green pigment within chloroplasts.

    • Chloroplasts found mainly in mesophyll cells of leaves, with each mesophyll cell containing 30-40 chloroplasts.

  • Gas Exchange:

    • CO2 enters and O2 exits the leaf through stomata.

  • Thylakoids:

    • Membrane-bound structures within chloroplasts where chlorophyll is located.

    • Can be stacked into columns known as grana.

  • Stroma:

    • The dense fluid interior of chloroplasts.

Photosynthesis Reactions

  • Photosynthesis can be summarized by the equation:
    6CO<em>2+12H</em>2O+extlightenergy<br>ightarrowC<em>6H</em>12O<em>6+6O</em>2+6H2O6 CO<em>2 + 12 H</em>2O + ext{light energy} <br>ightarrow C<em>6H</em>{12}O<em>6 + 6 O</em>2 + 6 H_2O

  • Chloroplasts split H2O into hydrogen (incorporating into sugar) and oxygen (released as by-product).

  • Photosynthesis as a Redox Process:

    • Water is oxidized and CO2 is reduced.

    • It is an endergonic process (requires energy) boosted by light.

Stages of Photosynthesis

Light Reactions

  • Occur in thylakoids and are responsible for the following:

    • Splitting H2O

    • Releasing O2

    • Reducing NADP+ to NADPH

    • Generating ATP from ADP through photophosphorylation.

Calvin Cycle

  • Occurs in stroma and involves:

    • Forming sugar from CO2 using ATP and NADPH.

    • Initiated by carbon fixation (incorporating CO2 into organic molecules).

Light Reactions

  • Chloroplasts function as solar-powered chemical factories transforming light energy into ATP and NADPH.

  • Nature of Sunlight:

    • Light is a form of electromagnetic energy.

    • Travels in rhythmic waves.

  • Wavelength:

    • Distance between crests of the waves. Determines the type of electromagnetic energy.

  • Electromagnetic Spectrum:

    • Range of electromagnetic energy; visible light has wavelengths that produce colors we see.

  • Light behaves as particles called photons.

Photosynthetic Pigments

  • Pigments: Substances that absorb visible light.

    • Different pigments absorb different wavelengths.

    • Chlorophyll reflects and transmits green light, making leaves appear green.

  • Spectrophotometer: Measures a pigment's ability to absorb various wavelengths.

  • Absorption Spectrum: Graph plotting a pigment's absorption versus wavelength.

    • Chlorophyll a absorbs violet-blue and red light best.

  • Action Spectrum: Profiles effectiveness of different wavelengths of radiation in driving photosynthesis. First demonstrated by Theodor W. Engelmann in 1883.

Functioning of Photosystems

  • A Photosystem is a reaction-center complex associated with light-harvesting complexes (pigment molecules bound to proteins).

    • Primary electron acceptor in reaction center accepts excited electrons.

    • Solar-powered transfer of an electron from chlorophyll a to primary acceptor is the first step of light reactions.

Types of Photosystems

  • Photosystem II (PS II):

    • Functions first, absorbs light best at 680 nm.

    • Reaction-center chlorophyll a called P680.

  • Photosystem I (PS I):

    • IAbsorbs light best at 700 nm; reaction-center chlorophyll a called P700.

Linear Electron Flow

  • Flow of electrons through photosystems using light energy to produce ATP and NADPH.

  • Steps include:

    • Photon strikes pigment, exciting P680.

    • Excited electron is transferred to primary electron acceptor.

    • Enzymes split H2O; electrons replenish P680.

    • Electrons transported down electron transport chain, driving ATP synthesis via proton gradient.

Chemiosmosis

  • Process utilized by both chloroplasts and mitochondria for ATP production, but with different energy sources.

    • Mitochondria create ATP from food energy; chloroplasts from light energy.

  • Differences in spatial organization:

    • Mitochondria pump protons into the intermembrane space; chloroplasts into thylakoid spaces.

Calvin Cycle

Overview

  • Uses ATP and NADPH to reduce CO2 to sugar, regenerating its starting material.

  • Three phases:

    1. Carbon fixation: Incorporating CO2 into ribulose bisphosphate (RuBP) using rubisco enzyme. Produces 3-phosphoglycerate.

    2. Reduction: Converts 3-phosphoglycerate to G3P using ATP and NADPH.

    3. Regeneration: Rearranging of G3P to regenerate RuBP, requiring additional ATP.

Mechanisms of Carbon Fixation

  • Plants adapt to dehydration by closing stomata, decreasing CO2 access and promoting photorespiration.

  • C4 Plants: Incorporate CO2 into a four-carbon compound, minimizing photorespiration.

  • CAM Plants: Open stomata at night, storing CO2 as organic acids for use in the daytime.

Importance of Photosynthesis

  • Converts solar energy to chemical energy stored in organic compounds.

  • Produces O2 and supports life processes.

  • Plants store excess sugar as starch, providing energy and carbon skeletons for cellular functions.

Connections to Other Processes

  • Photosynthesis and cellular respiration are interconnected processes vital for energy transformation within cells.

  • Provides the foundational cycle of oxygen and carbohydrates necessary for life.




MORE DETAILS

Overview of Photosynthesis

  • Photosynthesis is the process that converts solar energy into chemical energy, primarily in the form of sugars.

  • It nourishes almost the entire living world directly or indirectly, forming the base of most food webs.

Autotrophs and Heterotrophs

  • Autotrophs:

    • Organisms that sustain themselves without eating anything derived from other organisms.

    • They are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules.

    • Examples: Almost all plants, algae, and certain prokaryotes are photoautotrophs.

  • Heterotrophs:

    • Obtain organic material from other organisms.

    • Act as the consumers of the biosphere, consuming autotrophs or other heterotrophs.

    • Almost all heterotrophs, including humans, depend on photoautotrophs directly or indirectly for food and oxygen.

Organisms Involved in Photosynthesis

  • Photosynthesis occurs in:

    • Plants

    • Algae

    • Certain protists

    • Some prokaryotes (e.g., cyanobacteria)

  • These organisms not only feed themselves but also most of the living world by producing organic compounds and oxygen.

Structural Organization of Photosynthetic Cells

  • Photosynthetic cells have enzymes and molecules grouped together in membranes, allowing for efficient chemical reactions.

  • Chloroplasts:

    • Sites of photosynthesis in plants.

    • Structurally similar to and likely evolved from photosynthetic bacteria, retaining their own DNA and ribosomes.

Chloroplast Structure and Function

  • Leaves are the major locations of photosynthesis, often referred to as nature’s solar panels.

  • The green color is due to chlorophyll, the main green pigment within chloroplasts.

  • Chloroplasts are found mainly in mesophyll cells of leaves, with each mesophyll cell typically containing 30-40 chloroplasts.

  • The chloroplast is enclosed by a double membrane (inner and outer membranes).

  • Gas Exchange:

    • CO2 enters and O2 exits the leaf through microscopic pores called stomata.

  • Thylakoids:

    • Membrane-bound sac-like structures within chloroplasts where chlorophyll is located and light reactions occur.

    • Can be stacked into columns known as grana (singular: granum).

  • Stroma:

    • The dense fluid-filled interior of chloroplasts, surrounding the thylakoids, where the Calvin cycle takes place.

Photosynthesis Reactions

  • Photosynthesis can be summarized by the equation:
    6CO<em>2+12H</em>2O+light energy→C<em>6H</em>12O<em>6+6O</em>2+6H2O6 CO<em>2 + 12 H</em>2O + \text{light energy} \rightarrow C<em>6H</em>{12}O<em>6 + 6 O</em>2 + 6 H_2O

  • Chloroplasts split H<em>2OH<em>2O into hydrogen (incorporating into sugar) and oxygen (released as a by-product). The atoms in the glucose molecule come from CO2 and H</em>2OH</em>2O.

  • Photosynthesis as a Redox Process:

    • Water is oxidized (loses electrons) and CO2 is reduced (gains electrons).

    • It is an endergonic process (requires energy input) boosted by light energy, which increases the potential energy of electrons.

Stages of Photosynthesis
Light Reactions

  • Occur in the thylakoids and are responsible for the following transformations of light energy into chemical energy:

    • Splitting H2OH_2O

    • Releasing O2 as a by-product

    • Reducing NADP+ (nicotinamide adenine dinucleotide phosphate) to NADPH by adding electrons and H+H^+ ions.

    • Generating ATP from ADP (adenosine diphosphate) through photophosphorylation (the addition of a phosphate group using light energy).

    • ATP and NADPH are energy-carrying molecules that power the Calvin cycle.

Calvin Cycle

  • Occurs in the stroma and involves:

    • Forming sugar (specifically G3P, glyceraldehyde-3-phosphate) from CO2.

    • Utilizes the ATP and NADPH produced during the light reactions.

    • Initiated by carbon fixation (incorporating CO2 into organic molecules).

Light Reactions

  • Chloroplasts function as solar-powered chemical factories transforming light energy into the chemical energy of ATP and NADPH.

  • Nature of Sunlight:

    • Light is a form of electromagnetic energy, also known as electromagnetic radiation.

    • Travels in rhythmic waves.

  • Wavelength:

    • The distance between crests of the waves. Determines the type of electromagnetic energy.

  • Electromagnetic Spectrum:

    • The entire range of electromagnetic energy or radiation; visible light has wavelengths ranging from approximately 380380 nm (violet) to 750750 nm (red), which produce the colors we see.

  • Light also behaves as particles called photons, each with a fixed amount of energy.

Photosynthetic Pigments

  • Pigments: Substances that absorb visible light.

  • Different pigments absorb different wavelengths of light and reflect or transmit others.

  • Chlorophyll reflects and transmits green light, which is why leaves appear green.

  • Spectrophotometer: An instrument that measures a pigment's ability to absorb various wavelengths by shining light through a sample and measuring the fraction of light transmitted at each wavelength.

  • Absorption Spectrum: A graph plotting a pigment's absorption versus wavelength, showing the wavelengths best absorbed.

  • Chlorophyll a, the primary photosynthetic pigment, absorbs violet-blue and red light best.

  • Accessory pigments like chlorophyll b and carotenoids broaden the spectrum of light that can be used for photosynthesis and also provide photoprotection (scavenging harmful free radicals).

  • Action Spectrum: Profiles the effectiveness of different wavelengths of radiation in driving photosynthesis. This was first demonstrated by Theodor W. Engelmann in 1883 using Spirogyra and aerobic bacteria.

Functioning of Photosystems

  • A Photosystem is a reaction-center complex (a complex of proteins associated with two chlorophyll a molecules) associated with light-harvesting complexes (various pigment molecules bound to proteins).

  • Light-harvesting complexes funnel the energy of photons to the reaction center.

  • A primary electron acceptor in the reaction center accepts excited electrons from the reaction-center chlorophyll.

  • This solar-powered transfer of an electron from chlorophyll a to the primary acceptor is the first step of the light reactions.

Types of Photosystems

  • Photosystem II (PS II):

    • Functions first (but was discovered second), absorbs light best at a wavelength of 680680 nm. Its reaction-center chlorophyll a is called P680.

  • Photosystem I (PS I):

    • Absorbs light best at 700700 nm; its reaction-center chlorophyll a is called P700.

Linear Electron Flow

  • The predominant pathway for light energy conversion, involving the flow of electrons through photosystems using light energy to produce ATP and NADPH.

  • Steps include:

    • A photon strikes a pigment molecule in a light-harvesting complex, and the energy is passed to other pigment molecules until it excites P680 in PS II.

    • The excited electron from P680 is transferred to the primary electron acceptor, forming P680+P680^+ (a strong oxidizing agent).

    • Enzymes split H2OH_2O, providing electrons to replenish P680+P680^+. O2 is released.

    • Electrons are transported down an electron transport chain from PS II to PS I, which includes a cytochrome complex and plastocyanin. This exergonic fall of electrons drives the pumping of protons (H+H^+) into the thylakoid space.

    • The proton gradient is then used by ATP synthase to drive ATP synthesis (ADP+Pi→ATPADP + P_i \rightarrow ATP).

    • Light energy excites P700 in PS I, and its excited electron is transferred to PS I's primary electron acceptor.

    • Electrons from the electron transport chain (from PS II) replenish P700+P700^+.

    • Electrons from PS I's primary acceptor pass down a second electron transport chain to ferredoxin (Fd).

    • The enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+, reducing it to NADPH. NADPH will provide the reducing power for the Calvin cycle.

Chemiosmosis

  • Process utilized by both chloroplasts and mitochondria for ATP production, but with different energy sources and spatial organization.

  • Mitochondria create ATP from chemical energy stored in food; chloroplasts from light energy.

  • Differences in spatial organization:

    • Mitochondria pump protons from the mitochondrial matrix into the intermembrane space through an inner membrane.

    • Chloroplasts pump protons from the stroma into the thylakoid spaces through the thylakoid membrane.

Calvin Cycle
Overview

  • Uses ATP and NADPH to reduce CO2 to sugar (G3P), regenerating its starting material (RuBP).

  • It is a metabolic pathway that occurs in the stroma and is cyclical.

  • Three phases:

    1. Carbon fixation: Incorporating CO2 into ribulose bisphosphate (RuBP), a five-carbon sugar, catalyzed by the enzyme rubisco (RuBP carboxylase-oxygenaseRuBP \text{ carboxylase-oxygenase}). This produces an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate.

    2. Reduction: 3-phosphoglycerate is converted to glyceraldehyde-3-phosphate (G3P) using ATP for phosphorylation and NADPH for reduction. For every three CO2 molecules fixed, six G3P molecules are produced, but only one G3P exits the cycle as a net product (the other five are used for regeneration).

    3. Regeneration: The remaining five G3P molecules are rearranged to regenerate three molecules of RuBP, requiring additional ATP for this process.

  • For the net synthesis of one G3P molecule, the cycle consumes 99 molecules of ATP and 66 molecules of NADPH.

Mechanisms of Carbon Fixation

  • Plants adapt to dehydration by closing stomata, which conserves water but also decreases CO2 access and promotes photorespiration (a process that consumes O2 and organic fuel and releases CO2 without producing ATP or sugar).

  • C4 Plants:

    • Incorporate CO2 into a four-carbon compound in mesophyll cells (using PEP carboxylase, which has higher affinity for CO2 than rubisco), minimizing photorespiration by concentrating CO2 in bundle-sheath cells where rubisco then operates.

    • Examples: sugarcane, corn.

  • CAM Plants (Crassulacean Acid Metabolism):

    • Open stomata at night, storing CO2 as organic acids (e.g., malate) in vacuoles.

    • During the daytime, stomata close, and the CO2 is released from the organic acids for use in the Calvin cycle, thus conserving water.

    • Examples: pineapples, cacti, succulents.

Importance of Photosynthesis

Converts solar energy to chemical energy stored in organic compounds, which can be utilized by virtually all life forms.

  • Produces O2, which is essential for aerobic respiration in most organisms.

  • Plants store excess sugar as starch for energy reserves and provide carbon skeletons for other organic molecules needed for cellular functions and growth.

Connections to Other Processes

  • Photosynthesis and cellular respiration are interconnected processes vital for energy transformation and nutrient cycling within cells