ch8 photosythesis

• Organisms can be classified based on how they obtain energy
• Types of organisms based on energy source

  • Autotrophs
  • Heterotrophs
• Autotrophs


  Able to produce their own organic molecules through photosynthesis

  
• Heterotrophs


  Live on organic compounds produced by other organisms
• All organisms use cellular respiration to extract energy from organic molecules
• Photosynthesis


  6CO2 + 12H2O + Light → C6H12O6 + 6H2O + 6O2

  
• Types of photosynthesis


  • Anoxygenic photosynthesis
  • Oxygenic photosynthesis

  
• Anoxygenic photosynthesis


  Carried out by four different bacterial groups: purple bacteria, green sulfur bacteria, green non-sulfur bacteria, heliobacteria

  
• Oxygenic photosynthesis


  Carried out by cyanobacteria, seven groups of algae, and all land plants (containing chloroplasts)

  
• Stages of photosynthesis


  • Light-dependent reactions
  • Carbon fixation reactions or light-independent reactions

  
• Light-dependent reactions


  Require light to capture energy from sunlight and make ATP and reduce NADP+ to NADPH

  
• Carbon fixation reactions or light-independent reactions


  Do not require light, use ATP and NADPH to synthesize organic molecules from CO2
• Chloroplast


  • Thylakoid membrane - internal membrane (PL bilayer) organized into flattened sacs called thylakoid
  • Grana - stacks of flattened sacs of thylakoid membrane
  • Stroma lamella - connect grana
  • Stroma - semiliquid surrounding thylakoid membranes analogous to mitochondrial matrix, contains enzymes to incorporate CO2 into organic compounds using energy from ATP and NADPH
• Components of a leaf

  • Cuticle
  • Epidermis
  • Stoma
  • Mesophyll
• Cuticle


  Waxy protective layer covering the epidermal cells of leaves, limits water loss
• Epidermis


  Single layer of plant cells that forms a boundary between all plant organs and the external environment
• Stoma


  Pore (tiny opening) in the epidermis of leaves, stems, and other organs that facilitates gas exchange (O2 and CO2)
• Mesophyll


  Two layers of cells inside of the plant's leaves, the palisade parenchyma cells and the spongy mesophyll, flooded with chloroplasts and most directly responsible for photosynthesis

  
• Within the chloroplast's thylakoid membrane, chlorophyll molecules are organized into photosystems

  
• Overview of photosynthesis


  1. Light-dependent reactions begin when a chlorophyll molecule absorbs a photon of light, generating ATP and NADPH

  2. Electrons lost from chlorophyll are replaced by oxidizing water, producing O2

  3. ATP and NADPH are used in the Calvin cycle in the stroma to reduce CO2 and produce organic molecules

  
• Jan Baptista van Helmont demonstrated that the substance of the plant was not produced only from the soil


  1580–1644
• Jan Ingenhousz proposed that plants carry out a process that uses sunlight to split carbon dioxide into carbon and oxygen (O2 gas)


  1730–1799

  
• F.F. Blackman came to the conclusion that photosynthesis is a multistage process, with light-dependent and light-independent reactions


  1866–1947

  
• Light is the limiting factor at low light intensities, but temperature and CO2 concentration are the limiting factors at higher light intensities

  
• C. B. van Niel found that purple sulfur bacteria do not release O2 but accumulate sulfur, and proposed a general formula for photosynthesis


  1897–1985

  
• Robin Hill demonstrated that light energy could be harvested and used in a reduction reaction


  1899–1991
• Photon


  Particle of light that acts as a discrete bundle of energy, with energy content inversely proportional to the wavelength

  
• Photoelectric effect


  Removal of an electron from a molecule by light, with chloroplasts acting as photoelectric devices absorbing sunlight and transferring excited electrons to a carrier

  
• Types of pigments in photosynthesis

  • Chlorophylls
  • Carotenoids

  
• Chlorophyll a


  Main pigment in plants and cyanobacteria, only pigment that can act directly to convert light energy to chemical energy, absorbs violet-blue and red light

  
• Chlorophyll b


  Accessory pigment that absorbs light wavelengths that chlorophyll a does not

  
• Structure of chlorophyll


  Porphyrin ring with magnesium ion at center, tail that anchors the pigment molecule to the thylakoid membrane

  
• The action spectrum corresponds to the absorption spectrum for chlorophylls
• Carotenoids


  Can absorb photons with a wide range of energies, also have a protective role as antioxidants
• Phycobiloproteins


  Important accessory pigments in low-light ocean areas
• Fall leaf colors are produced by carotenoids and other accessory pigments, as chlorophyll is no longer present to mask their colors

  
• Photosystem


  Consists of an antenna complex that gathers photons and feeds the energy to a reaction center with chlorophyll a molecules that pass excited electrons out of the photosystem

  
• Antenna complex


  Hundreds of accessory pigment molecules that gather photons and channel the captured light energy to the reaction center

  
• Reaction center


  1 or more chlorophyll a molecules that pass excited electrons out of the photosystem

  
• Light-dependent reactions


  1. Primary photo event - photon captured by pigment

  2. Charge separation - energy transferred to reaction center, electron excited

  3. Electron transport - electrons move through carriers to reduce NADP+

  4. Chemiosmosis - produces ATP