Photosynthesis  Earliest life forms survived by metabolizing high-energy inorganic molecules  About 3 billion years ago, some primitive organisms evolved the ability to photosynthesize  they combined carbon dioxide and water to make glucose  further evolution resulted in more efficient photosynthesis that produced molecular oxygen as a by-product  as oxygen began to accumulate in the atmosphere, cells evolved the ability to use the oxygen for cellular respiration

All living things are either producers or depend on producers  most producers are photoautotrophs – absorb and convert light energy into stored chemical energy in organic molecules through the process of photosynthesis (plants, algae and cyanobacteria)  Heterotrophs – consumers and decomposers that obtain organic compounds from producers or other consumers  photosynthesis sustains almost all living things in the biosphere

The Visible Light Spectrum – light travels in waves  Includes all the colors of the rainbow – violet has the shortest wavelength and red has the longest

 Light is composed of particles or packets of energy called photons  When a pigment molecule absorbs a photon of light, one of its electrons is energized and is then accepted by an electron acceptor molecule in photosynthesis  Absorption spectrum – spectrophotometers are used to measure the relative abilities of different pigments to absorb different wavelengths of light – absorption spectrum is a plot of the absorption of light of different wavelengths

Action spectrum – shows how effective various wavelengths of light are in causing photosynthesis

Chloroplasts – organelles containing the green pigment, chlorophyll  Bounded by an outer and inner membrane  inner membrane encloses a fluid-filled stroma which contains the enzymes for the dark reactions  thylakoids are suspended in the stroma – fluid filled sacs (stack of thylakoids is a granum) – thylakoid membranes contain chlorophyll and the enzymes that catalyze the light reactions  Chloroplasts have their own DNA (circular chromosome) and ribosomes (70s)

Pigments  chlorophyll a is the main photosynthetic pigment  absorbs light mostly in the red and blue regions of the spectrum  chlorophyll a – pigment that initiates the light reactions  chlorophyll b – acts as an accessory pigment  carotenoids – other accessory pigments which are yellow and orange – absorb different wavelengths of light to broaden the spectrum of light available for photosynthesis – they pass the energy to chlorophyll a

Photosynthetic equation:  6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2  involves many steps – divided into two major sets of reactions: light-dependent reactions, and the light-independent reactions (dark reactions or carbon fixation reactions)  Light-dependent reactions take place in the thylakoid membrane  Light-independent reactions take place in the stroma

The Light-Dependent Reactions  Energy from the sun is temporarily stored in the molecules of ATP and NADPH – energy is then used to run the light-independent reactions

Chlorophyll molecules and enzymes that run the light-dependent reactions are embedded in the membranes of the thylakoids

 pigments are organized in the thylakoid membrane into units called antenna complexes  each antenna complex traps light and transfers the energy to a reaction center

Antenna complex and associated enzymes are organized into Photosystems Reaction center in Photosystem I absorbs at a peak of 700 nanometers (P700) – Photosystem II absorbs at a peak of 680 nanometers (P680)

 Noncyclic Photophosphorylation – both photosystems are used and electrons are ultimately passed to NADPH – electron “hole” left behind is filled by splitting water and removing electrons – O 2 gas is formed and released  continuous, one-way flow of electrons

 ATP is generated by chemiosmosis  called photophosphorylation because light energy is used to phosphorylate ADP called photophosphorylation because light energy is used to phosphorylate ADP called photophosphorylation because light energy is used to phosphorylate ADP

 Cyclic Photophosphorylation – simplest light-dependent reaction  energy from the sun is used to cycle electrons through Photosystem I to produce ATP (used by ancient bacteria)

Light-independent reactions (Carbon Fixation Reactions or Dark reactions)  energy from ATP and NADPH is used to form glucose from CO 2  Most plants go through the Calvin (C 3 ) Cycle

 CO 2 enters the cycle by reacting with ribulose biphosphate (RuBP) – reaction is catalyzed by the enzyme rubisco  six carbon dioxide molecules are “fixed” to make one glucose  at the end of each cycle, RuBP is reformed

Photorespiration  Plants need to have their stomata (pores in the leaves) open in order for gas exchange to occur (CO 2 in and O 2 out)  When it gets too hot, the stomata close and CO 2 concentrations drop and O 2 concentrations increase  RuBP also tends to combine with O 2 in a process called photorespiration  O 2 is used up and CO 2 is generated (as in cellular respiration) however photorespiration does not produce any energy and stops the C 3 pathway  if this continues for a long enough time (dry, hot weather) then the plant dies from lack of energy (glucose)

C 4 plants reduce photorespiration  C 3 plants have almost all their chloroplasts in the mesophyll cells  plants that are adapted to dry, hot weather often use the C 4 pathway - have chloroplasts in both the mesophyll AND bundle-sheath cells (surround the vascular bundles)

the mesophyll cells contain a 3-carbon molecule called PEP instead of RuBPthe mesophyll cells contain a 3-carbon molecule called PEP instead of RuBP the CO 2 combines with PEP to form a 4- carbon molecule of oxaloacetatethe CO 2 combines with PEP to form a 4- carbon molecule of oxaloacetate oxaloacetate acts as a shuttle to transport carbon dioxide to the bundle-sheath cells where it releases CO 2 and increases CO 2 concentrations to allow the regular C 3 cycle to proceedoxaloacetate acts as a shuttle to transport carbon dioxide to the bundle-sheath cells where it releases CO 2 and increases CO 2 concentrations to allow the regular C 3 cycle to proceed disadvantage is that C 4 pathway uses more energy (ATP) than the C 3 pathwaydisadvantage is that C 4 pathway uses more energy (ATP) than the C 3 pathway

Comparison of C 3 and C 4 Plants  Most plants are C 3 plants  Called C 3 because first product of carbon fixation is a 3 carbon compound (PGA)  C 4 plants – called C 4 because first compound formed in carbon fixation is 4 carbons (oxaloacetate)  C 4 plants thrive in deserts and mid-summer when sun is plentiful but water scarce  C 3 plants do well in cool, wet, cloudy climates because C 3 pathway is more energy-efficient