Plant Physiology Photosynthesis, the light reaction
Photosynthesis • Photosynthesis is the process that converts solar energy into chemical energy • Nourishes almost the entire living world • Plants, algae • Autotrophs sustain themselves without eating anything derived from other organisms • Producers of the biosphere, make organic molecules from CO2and other inorganic molecules • Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from H2O and CO2 • Heterotrophs obtain their organic material from other organisms • Consumers of biosphere • Almost all Heterotrophs depend on photoautotrophs for food and O2
Photosynthesis converts light energy into chemical energy of food • Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria • The structural organization of these cells allows for the chemical reactions of photosynthesis
Chloroplasts: The Sites of Photosynthesis in Plants • Leaves are the major locations of Photosynthesis • Their green color is from Chlorophyll, the green pigment within chloroplasts • Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast • CO2 enters and O2 exits the leaf through microscopic pores called Stomata
Chlorophylls are Leaf Pigments • Chlorophylls collect light energy (absorbs it) in a resonant porphyrin group that hangs out like a kite on the surface of the thylakoid • Chlorophyll ainitiates the light-dependent reactions, primary pigment in plants and cyanobacteria • -absorbs violet-blue and red light • Chlorophyll bsecondary pigment absorbing light wavelengths that chlorophyll a does not absorb • Carotenoids are yellow and orange pigments that capture light energy and pass electrons to chlorophyll
The Structure of Chlorophyll • The positive charges on the Mg2+ ion attract electrons • The electrons bounce around the porphyrin ring
Pigments (molecules that absorb visible light) • Each pigment has a characteristic absorption spectrum, the range and efficiency of photons it is capable of absorbing. photon: a particle of light -acts as a discrete bundle of energy -energy content of a photon is inversely proportional to the wavelength of the light photoelectric effect: removal of an electron from a molecule by light -occurs when photons transfer energy to electrons
Absorption of Light Energy • Light energy is absorbed by electrons • The energy causes electrons to jump shells; the more energy absorbed, the further away electrons move from the nucleus • The energy may be shed as fluorescence • Or transferred in the form of an electron to another molecule
How Electrons Capture Energy • Electrons can absorb radiant energy. • Radiant energy comes in parcels called photons • When electrons absorb energy, they hop to a higher shell. • When electrons release energy, drop back to the lower shell. • The energy released is a kind of light energy called fluorescence.
The Visible Spectrum of Light • Photosynthesis uses only small visible portion of the electromagneticspectrum • Wavelengths of visible light most important forphotosynthesis. • The symbol for wavelength is λ
Energy for all life on Earth ultimately comes from photosynthesis. 6CO2 + 12H2O C6H12O6 + 6H2O + 6O2 Oxygenic photosynthesis is carried out by: cyanobacteria, 7 groups of algae, all land plants sunlight
Internal Structure of a Leaf Main site of Photosynthesis
The site of light harvesting or energy capture • The site of the start of carbohydrate synthesis (Inside of thylakoid) The Chloroplast
Photosynthesis Overview • Photosynthesis takes place in chloroplasts. • thylakoid membrane – internal membrane arranged in flattened sacs • containchlorophyll and other pigments • Organized into photosystems • Capture light and transfer energy (to pigment molecules) • grana – stacks of thylakoid membranes • stroma – semiliquid substance surrounding thylakoid membranes (houses the enzymes to make organic molecules)
Photosynthesis in the Chloroplast • The light-dependent reactions (the harvesting of light) occur on thylakoid membranes • The carbon fixation reactions (formation of carbohydrate) occur in the stroma
Photosynthesis Overview • Photosynthesis takes place in 3 stages: • Capturing energy from sunlight • Using the energy to make ATP and reduce NADP+ to NADPH • (nicotinamide adenine dinucleotide phosphate) • Using the ATP and NADPH to synthesize organic molecules (glucose) from CO2
Photosynthesis Overview Photosynthesis is divided into: light-dependent reactions -capture energy from sunlight -make ATP and reduce NADP+ to NADPH carbon fixation reactions (light-independent reactions) -use ATP and NADPH to synthesize organic molecules from CO2
Photosynthesis Overview • Photosynthesis takes place in the green portions of plants • Leaf of flowering plant contains mesophyll tissue • Cells containing chloroplasts • Specialized to carry on photosynthesis • CO2 enters leaf through stomata • Diffuses into chloroplasts in mesophyll cells • In stroma, CO2 fixed to C6H12O6 (sugar) • Energy supplied by light
Photosystem Organization A photosystem consists of 1. an antenna complex (light harvesting complex) of hundreds of accessory pigment molecules that gather photons and feeds energy to reaction center 2. a reaction center of one or more chlorophyll a molecules pass electrons out of photosystem (photochemical reactions) In summary, energy of electrons is transferred through the antenna complex to the reaction center.
Photosystem Organization At the reaction center (transmembrane protein complex), the energy from the antenna complex is transferred to chlorophyll a. This energy causes an electron from chlorophyll to become excited. The excited electron is transferred from chlorophyll a to an electron acceptor. Water donates an electron to chlorophyll a to replace the excited electron.
pheophytin Converting light to chemical energy
Light Reactions • Two electron pathways operate in the thylakoid membrane: the noncyclic pathway and the cyclic pathway. • Both pathways produce ATP; only the noncyclic pathway also produces NADPH. • ATP production during photosynthesis is called photophosphorylation; therefore these pathways are also known as cyclic and noncyclic photophosphorylation.
Light Reactions:The Noncyclic Electron Pathway • Takes place in thylakoid membrane • Uses two photosystems, PS-I and PS-II (consists of pigment complexes) • PS II captures light energy • Causes an electron to be ejected from the reaction center (chlorophyll a) • Electron travels down electron transport chain to PS I • Replaced with an electron from water • causes H+ to concentrate in thylakoid chambers • causes ATP production • PS I captures light energy (electrons and H) • Transferred permanently to a molecule of NADP+ • Causes NADPH production
Light Reactions:The Cyclic Electron Pathway • Uses only photosystem I (PS-I) • Begins when PS I complex absorbs solar energy • Electron ejected from reaction center • Travels down electron transport chain • Causes H+ to concentrate in thylakoid chambers • Which causes ATP production • Electron returns to PS-I (cyclic) • Pathway only results in ATP production
The Organization of the Thylakoid Membrane • PS II consists of a pigment complex and electron-acceptor molecules; it oxidizes H2O and produces O2. • The electron transport system consists of cytochrome complexes and transports electrons and pumps H+ ions into the thylakoid space. • PS I has a pigment complex and electron-acceptor molecules; it is associated with an enzyme (oxidoreductases) that reduces NADP+ to NADPH. • ATP synthase complex has an H+ channel and ATP synthase; it produces ATP.
ATP Production • Thylakoid space acts as a reservoir for hydrogen ions (H+) • Each time water is oxidized, two H+ remain in the thylakoid space • Electrons yield energy • Used to pump H+ across thylakoid membrane • Move H+ from stroma into the thylakoid space • Flow of H+ back across thylakoid membrane • Energizes ATP synthase • Enzymatically produces ATP from ADP + P • This method of producing ATP is called chemiosmosis