Photosynthesis Earliest life forms survived by metabolizing high

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 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 (70 s)

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: § 6 CO 2 + 6 H 2 O C 6 H 12 O 6 + 6 O 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 (P 700) – Photosystem II absorbs at a peak of 680 nanometers (P 680)

§ 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

§ 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 (Ru. BP) – reaction is catalyzed by the enzyme rubisco § six carbon dioxide molecules are “fixed” to make one glucose § at the end of each cycle, Ru. BP 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 Ru. BP 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 Ru. BP • the 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 proceed • disadvantage 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