Download Photosynthesis includes light reactions and dark reactions

Photosynthesis includes light reactions
and dark reactions
1. light reactions: The light reactions require light. Energy
of light is converted to chemical energy and conserved
as
 “high energy” compound ATP
 reducing power of NADPH
2.dark reactions : The light-independent reactions occur
either in the light or in the dark.
– NADPH and ATP produced by the light reactions are used in
the reductive synthesis of carbohydrate from CO2 and water
Synthesis of Glucose
The “goal” of photosynthesis is to synthesize the
carbohydrate “glucose”.
– Carbon dioxide is reduced to glucose (see equation
below).
– The electrons needed for this reduction come from
water.
– The energy needed for this reduction comes from
light (ATP, NADPH).
– The equation is:
Energy + 6CO2 + 6H2O  C6H12O6 + 6O2
Sun
light
photosystem
• Two kinds of photosystems are involved in
photosynthesis in plants.
• Photosystem I (PSI) is defined as
containing reaction center chlorophylls with
maximal light absorption at 700 nm.
• Photosystem II (PSII) absorbs light about
680 nm.
• both
of
the
two
photosystems
are
pigment/protein complexes that are located in
thylakoids
The roles of photosystem I (PS I) and
photosystem II (PS II)
• PS I provides reducing power NADPH
• PS II uses light energy to drive two
chemical reactions - the split of water
producing O2 and releasing electrons
into an electron transport chain
(Photosynthetic Electron Transport).
Types of photosynthesis
• C3
– The majority of plants
• C4
– CO2 temporarily stored as 4-C organic acids resulting in
more efficient C exchange rate
– Advantage in high light, high temperature, low CO2
– Many grasses and crops (e.g., corn, sorghum, millet, sugar
cane)
• CAM
– Stomata open during night
– Advantage in arid climates
– Many succulents (e.g., cacti, euphorbs, bromeliades,
agaves)
Cellular Respiration
• Breakdown of glucose begins in the
cytoplasm: the liquid matrix inside the cell
• At this point life diverges into two forms and
two pathways
– Anaerobic cellular respiration (aka fermentation)
– Aerobic cellular respiration
Cellular Respiration Reactions
• Glycolysis
– Series of reactions which break the 6-carbon
glucose molecule down into two 3-carbon
molecules called pyruvate
– Process is an ancient one-all organisms from
simple bacteria to humans perform it the same
way
– Yields 2 ATP molecules for every one glucose
molecule broken down
– Yields 2 NADH per glucose molecule
Anaerobic Cellular Respiration
• Some organisms thrive in environments with little or no
oxygen
– Marshes, bogs, gut of animals, sewage treatment ponds
• No oxygen used= ‘an’aerobic
• Results in no more ATP, final steps in these pathways serve
ONLY to regenerate NAD+ so it can return to pick up more
electrons and hydrogens in glycolysis.
• End products such as ethanol and CO2 (single cell fungi (yeast)
in beer/bread) or lactic acid (muscle cells)
Aerobic Cellular Respiration
• Oxygen required=aerobic
• 2 more sets of reactions which occur in a
specialized structure within the cell called the
mitochondria
– 1. Kreb’s Cycle
– 2. Electron Transport Chain
Kreb’s Cycle
• Completes the breakdown of glucose
– Takes the pyruvate (3-carbons) and breaks it
down, the carbon and oxygen atoms end up in
CO2 and H2O
– Hydrogens and electrons are stripped and loaded
onto NAD+ and FAD to produce NADH and FADH2
• Production of only 2 more ATP but loads up
the coenzymes with H+ and electrons which
move to the 3rd stage
CO2 effects on photosynthesis
• C4 > C3 at low CO2
• But, C3 > C4 at high CO2
*At high CO2, C3 more efficient than C4 at all temps.
(photosynthesis only, not other processes)
Photosynthetic N-use efficiency
• C4 plants need (have) less leaf N than C3
• Photosynthesis higher per unit N in C4
• Humans are increasing global N, which
benefits C3 more than C4
• Increasing CO2 decreases leaf N content,
more in C3 than C4
Photosynthetic water-use efficiency
• C4 plants use less water than C3
• (cause stomates open less)
• Water availability may increase or decrease
in the future.