Lecture 11: Photosynthesis: Light Reactions

Most of the Energy for Life Comes from Sunlight

• Total rate of energy delivery from sun to earth (at the top of the atmosphere) = 175,000 terrawatts
  • Terrawatt = 10^12 watts
• Rate of energy capture by plants from the sun = net primary productivity (NPP) = 100 terrawatts
  • Only ~ 0.06% of sunlight is captured
  • Captured in chemical bonds, especially in sugar
• Small amounts of energy are available to specialized microorganisms from chemical reactions not linked to sunlight
• It is estimated that 99% of the energy used by living cells comes from the sun
• Incorporation of sunlight into chemical bonds occurs through the process of photosynthesis
  • "Invented" by cyanobacteria about 2 billion years ago

Plants, Cyanobacteria and Algae are Autotrophs

• Autotrophs make their own macromolecules and do not require products from other living creatures for life
• Other types of organisms, including ourselves, are heterotrophs: require products from other species for energy and materials

An Overview of Photosynthesis

• In photosynthesis carbon dioxide and water are combined, using energy from sunlight, to form glucose
  • Oxygen is given off as a waste product
  • This is the source of the oxygen in the atmosphere

• Photosynthesis has 2 sets of reactions:
• Light reactions split water
  • Hydrogens are used to produce a reduced coenzyme, NADPH, and ATP
  • Oxygen is given off
• Once the NADPH and ATP are formed the rest of the reactions can take place in the dark
  • CO2 is reduced to glucose
  • A set of cyclic reactions: the Calvin cycle

Blue Light Has More Energy than Red Light

• Light energy has both a wave nature (like the waves seen on the surface of a pond) and a particulate nature (quanta)
  • Travels at an enormous velocity = 186,000 miles/sec = 300,000 kilometers/sec
  • Can travel through a vacuum
  • Compare with sound waves: velocity ~ 0.2 miles/sec, travels only through matter
• Light waves have a wavelength (the distance between the same part of 2 consecutive waves)
• Wavelength is associated with the color we see:
  • Violets have wavelengths near 400 nanometers (nm); reds are about 700 nm:

700 nM		400 nm
Low Energy		High Energy

• The light quanta of short wavelengths have more energy than the quanta of longer wavelength
• Red and orange light quanta have less energy than blue and violet quanta
• White light is a mixture of all the colors
• Black is the absence of light

When Light is Absorbed Electrons are Excited to Higher Energy Levels in Atoms and Molecules

• Light quanta are absorbed by matter when they have exactly the right energy to kick an electron in the matter to a higher energy orbit
  • If the energy of the quanta is too low or too high they will not be absorbed
• Different types of matter have electron orbits of different energy
  • This is why compounds absorb light of different energy and have different colors
  • The color of a material is determined by the light that is not absorbed.
• A chemical compound that has absorbed light is in an excited state
  • It is more chemically reactive than a compound with no electrons in higher energy orbits
  • Photosynthesis occurs because of excited electrons

Photosynthesis Starts with the Absorption of Light

• Leaves can absorb 90% of the light striking them
• Absorption is by a number of pigments including:
  
  

  Chlorophyll a		Main photosynthetic pigment
  Chlorophyll b		Accessory pigment: passes light to chlorophyll a
  Xanthophylls		More acessory pigments
  Carotene		Another accessory pigment

  
  
• Chlorophyll has a light-sensitive ring with a Mg atom in the center. It is anchored in chloroplast membranes by a hydrophobic tail
• Plants have chlorophylls a & b; algae also have 2 other types (c & d: see table, p. 533 of text)
• Chlorophyll absorbs mostly light in the blue and red regions of the spectrum; it appears green because it does not absorb green light
  • Light absorption is affected by the environment
  • Chlorophyll a has an absorption peak either at 680 or 700 nm depending upon what proteins it is associated with (photosystems I & II, see below)
• The use of accessory pigments allows photosythesis to use a large proportion of the visible light
• Once light is absorbed several things can happen. Suppose violet light is absorbed:
  • Violet light can be re-emmitted
  • The light energy can be converted to heat
  • Some of the energy can be given off as heat and the rest as light; the light will have a lower energy, which gives it a different color, such as red (fluorescence):

• If you have an organized photocenter the light energy can be passed from molecule to molecule and some of it can be used to make chemical bonds
  • Photocenter channels light into photosynthesis, reducing fluorescence and other forms of re-emmission
• Excitations by light do not last long, usually about 1 nanoseconds or less

Chlorophyll is Located in the Thylakoid Membranes of Chloroplasts

• Chloroplasts are greenish in color, about 5 microns in length (many variations in shape)
• Originated (probably) by endosymbiosis
• More complicated structure than mitochondria: 3 compartments (Compare mitochondria & chloroplasts.):
  • Have hollow stacked compartments
    • Compartments are called thylakoids
    • Stacks are called grana: about 10 compartments per granum
    • Stacks are interconnected
    • Thylakoids form by pinching off from chloroplast inner membrane
  • Thylakoid membranes have the photosynthetic apparatus
    • Chlorophyll and accessory pigments
    • Electron transport chain
    • Hydrogen pumps
    • ATP synthase

The Thylakoid Membranes of Chloroplasts Have 2 Photosystems

• In the chloroplast membranes the light-capturing pigments are organized into photosystems:
  • Accessory pigments (about 300-400 molecules) clustered around a reaction center containing chlorophyll a
  • Proteins hold system together and catalyze some steps
• There are two types of photosystem
  • Photosystem I:
    • Reaction center absorbs most efficiently at 700 nm (is called P700)
    • Passes electrons through acceptors that generate NADPH
  • Photosystem II:
    • Reaction center absorbs most efficiently at 680 nm (is called P680)
    • Passes electrons through acceptors that generate ATP
    • Splits water, making oxygen
• Photosystems work together: photosystem II feeds electrons to photosystem I
  • Emerson enhancement effect: photosynthesis requires light of 2 different wavelengths (around 680 and 700) at the same time
• Each granum has about 200 of each of the two photosystems

Electron Flow and Hydrogen Gradients Generate ATP and NADPH in the Chloroplast

• Photosystem II is excited first in the light reactions (for details see figures on p. 193)
  • The excited electron is passed to an electron transport system
  • To replace the electron lost from the reaction center an electron is pulled from water
    • This generates H+ ions and oxygen gas:
    • 2H20 -> 4 H+ + O2 + 4e-
  • As it is passed from one acceptor to another energy is released which is used to pump hydrogen ions from the stroma to the thylakoid compartment
  • The thylakoid compartment pH drops to around 5, compared to the stroma, which rises to about 8
  • The hydrogen gradient causes hydrogen ions to flow through the channels in ATP synthase and this generates ATP in the stroma

• While the electron is still in a partially excited state it is passed to photosystem I
  • A second photon excites the electron
  • It is passed to other acceptors, finally to NADP, where it produces NADPH in the stroma
• Final products of the light reactions are ATP and NADPH

The Light Reactions Split Water and Release Oxygen to the Atmosphere

• Oxygen is a waste product of photosynthesis
• All oxygen in atmosphere is believed to originate from photosynthesis
• Photosynthesis began with cyanobacteria, about 3 billion years ago
  • First oxygen released reacted with iron, producing dark red bands in the rocks (Fe rust)
  • Atmospheric oxygen accumulation began after all Fe had reacted, approximately 2 billion years ago
  • Oxygen atmosphere created a crisis for life, very toxic substance
  • Some forms adapted, took advantage of extra energy available from oxygen chemistry
• Oxygen atmosphere produced ozone layer
  • Protection against harmful UV light from sun

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