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PHOTOSYNTHESIS
Overview
Photosynthesis—process that converts solar energy into chemical energy (of food)
o Occurs in plants, algae, certain other unicellular eukaryotes, and some prokaryotes
Organisms feed not only themselves but also most of living world
o Leaves are major location
Autotrophs—sustain themselves without eating anything derived from other organisms
o Producers of biosphere, producing organic molecules from CO2 and other inorganic molecules
o Includes almost all plants
Use energy of sunlight to make organic molecules
Heterotrophs—obtain organic material from other organisms
o Almost all depend on photoautotrophs for food and O2
Earth’s supply of fossil fuels was formed from remains of organisms that died hundreds of millions
years ago
o Represent stores of solar energy from distant past
Chloroplasts
Structurally similar to and likely evolved from photosynthetic bacteria
Structural organization of organelles allows for chemical reactions of photosynthesis
Found mainly in cells of mesophyll
o Interior tissue of leaf
o Each contains 30-40 chloroplasts
Has envelope of two membranes surrounding stroma
o Stroma—dense fluid
Thylakoids—connected sacs in chloroplast which compose a third membrane system
o May be stacked in columns called grana
o Chlorophyll resides in thylakoid membranes
Stomata—microscopic pores through which CO2 enters and O2 exits the leaf
Tracking atoms through photosynthesis
Photosynthesis is a complex series of reactions
o 6CO2 + 12H2O + light energy→ C6H12O6 + 6O2 + 6H2O
Overall chemical change during photosynthesis is reverse of one that occurs during cellular respiration
Splitting of water
Chloroplasts split H2O into hydrogen and oxygen, incorporation electrons of hydrogen into suga
molecules
o Releases oxygen as a byproduct
Photosynthesis as a redox process
Photosynthesis reverses direction of electron flow compared to respiration
Redox process in which H2O is oxidized and CO2 is reduced
Endergonic because energy boost is provided by light
Nature of light energy
Electromagnetic radiation—form of energy
, 2
Light—type of energy like electromagnetic radiation that acts both particle-like and wave-like
o As a particle—exists in discrete packets called photons
o As a wave—can be characterized by wavelength
Wavelength—distance between two successive wave crests
Electromagnetic spectrum
Range of wavelengths of electromagnetic radiation
Visible light—electromagnetic radiation that humans can see
Each photon and wavelength has a specific amount of energy
o Energy of a photon of light is inversely proportional to its wavelength
Shorter wavelengths such as ultraviolet light have more energy than longer wavelengths
such as infrared light
Photosynthetic pigments
Pigments—molecules that absorb only certain wavelengths of light
Two major classes in plant leaves
o Chlorophylls—absorb red and blue light and reflect and transmit green light
o Carotenoids—absorb blue and green light and reflect and transmit yellow, orange, and red light
Function in photoprotection
Absorb excessive light that would damage chlorophyll
Each pigment has a specific absorption spectrum
o Absorption spectrum—graph used to study pigments
Plots wavelength of light absorbed by pigment molecules
Action spectrum—shows rate of photosynthesis vs wavelength
o Pigments that absorb blue and red photons are the most effective at driving photosynthesis
Chlorophylls absorb these wavelengths → most likely main photosynthetic pigments
Chlorophyll A—main photosynthetic pigment
o Accessory pigments—broaden spectrum used for photosynthesis
Includes chlorophyll B
o Difference in absorption spectrum between chlorophylls A and B is due to a slight structura
difference between pigment molecules
Light absorption
Photons may be absorbed, transmitted, or reflected when they strike an object
o Energy can be transferred to an electron in chlorophyll head
Electron becomes excited and is raised to a higher energy state
In chlorophyll, red and blue photons can be absorbed and excite electrons to different states
o Red photons raise electrons to state 1
o Higher-energy blue photons raise electrons to state 2
o Green photons (intermediate energy level) are not easily absorbed by chlorophyll
Photosystems
Consists of a reaction-center complex surrounded by light-harvesting complexes
o Reaction-center complex—type of protein complex
o Light-harvesting complex—pigment molecules bound to proteins
Transfer energy of photons to reaction center
Primary electron acceptor—accepts excited electrons and is reduced as a result
PHOTOSYNTHESIS
Overview
Photosynthesis—process that converts solar energy into chemical energy (of food)
o Occurs in plants, algae, certain other unicellular eukaryotes, and some prokaryotes
Organisms feed not only themselves but also most of living world
o Leaves are major location
Autotrophs—sustain themselves without eating anything derived from other organisms
o Producers of biosphere, producing organic molecules from CO2 and other inorganic molecules
o Includes almost all plants
Use energy of sunlight to make organic molecules
Heterotrophs—obtain organic material from other organisms
o Almost all depend on photoautotrophs for food and O2
Earth’s supply of fossil fuels was formed from remains of organisms that died hundreds of millions
years ago
o Represent stores of solar energy from distant past
Chloroplasts
Structurally similar to and likely evolved from photosynthetic bacteria
Structural organization of organelles allows for chemical reactions of photosynthesis
Found mainly in cells of mesophyll
o Interior tissue of leaf
o Each contains 30-40 chloroplasts
Has envelope of two membranes surrounding stroma
o Stroma—dense fluid
Thylakoids—connected sacs in chloroplast which compose a third membrane system
o May be stacked in columns called grana
o Chlorophyll resides in thylakoid membranes
Stomata—microscopic pores through which CO2 enters and O2 exits the leaf
Tracking atoms through photosynthesis
Photosynthesis is a complex series of reactions
o 6CO2 + 12H2O + light energy→ C6H12O6 + 6O2 + 6H2O
Overall chemical change during photosynthesis is reverse of one that occurs during cellular respiration
Splitting of water
Chloroplasts split H2O into hydrogen and oxygen, incorporation electrons of hydrogen into suga
molecules
o Releases oxygen as a byproduct
Photosynthesis as a redox process
Photosynthesis reverses direction of electron flow compared to respiration
Redox process in which H2O is oxidized and CO2 is reduced
Endergonic because energy boost is provided by light
Nature of light energy
Electromagnetic radiation—form of energy
, 2
Light—type of energy like electromagnetic radiation that acts both particle-like and wave-like
o As a particle—exists in discrete packets called photons
o As a wave—can be characterized by wavelength
Wavelength—distance between two successive wave crests
Electromagnetic spectrum
Range of wavelengths of electromagnetic radiation
Visible light—electromagnetic radiation that humans can see
Each photon and wavelength has a specific amount of energy
o Energy of a photon of light is inversely proportional to its wavelength
Shorter wavelengths such as ultraviolet light have more energy than longer wavelengths
such as infrared light
Photosynthetic pigments
Pigments—molecules that absorb only certain wavelengths of light
Two major classes in plant leaves
o Chlorophylls—absorb red and blue light and reflect and transmit green light
o Carotenoids—absorb blue and green light and reflect and transmit yellow, orange, and red light
Function in photoprotection
Absorb excessive light that would damage chlorophyll
Each pigment has a specific absorption spectrum
o Absorption spectrum—graph used to study pigments
Plots wavelength of light absorbed by pigment molecules
Action spectrum—shows rate of photosynthesis vs wavelength
o Pigments that absorb blue and red photons are the most effective at driving photosynthesis
Chlorophylls absorb these wavelengths → most likely main photosynthetic pigments
Chlorophyll A—main photosynthetic pigment
o Accessory pigments—broaden spectrum used for photosynthesis
Includes chlorophyll B
o Difference in absorption spectrum between chlorophylls A and B is due to a slight structura
difference between pigment molecules
Light absorption
Photons may be absorbed, transmitted, or reflected when they strike an object
o Energy can be transferred to an electron in chlorophyll head
Electron becomes excited and is raised to a higher energy state
In chlorophyll, red and blue photons can be absorbed and excite electrons to different states
o Red photons raise electrons to state 1
o Higher-energy blue photons raise electrons to state 2
o Green photons (intermediate energy level) are not easily absorbed by chlorophyll
Photosystems
Consists of a reaction-center complex surrounded by light-harvesting complexes
o Reaction-center complex—type of protein complex
o Light-harvesting complex—pigment molecules bound to proteins
Transfer energy of photons to reaction center
Primary electron acceptor—accepts excited electrons and is reduced as a result
