Jan. 18, 2011
B4730/5730
Plant Physiological Ecology
Subcellular Processes II
Chloroplast and Mitochondrion
Biology
• Chloroplasts and mitochondria arose from
endosymbiotic evolution
– Both affected by bacterial antibiotics
– Both reproduce by binary fission, neither can
reproduce by themselves
– Both have own genome and chimeric proteins
– Both have ATPase to generate ATP from
chemiosmosis
• Differences between chloroplasts and
mitochondria
– Membrane structure
– Direction of pH gradient during chemiosmosis
Respiration
• Four steps respiration
–
–
–
–
1) Glycolysis
2) Pyruvate Transport
3) Krebs cycle
4) Electron transport chain and oxidative phosphorylation
• Glycolysis in cytoplasm
– Results in two pyruvate molecules
• Pyruvate Transport removes CO2
• Krebs cycle occurs in the mitochondrial matrix
– Remaining 2 C molecule is broken into carbon dioxide
• Glycolysis and the Krebs cycle both pass electrons to
NAD+ and form NADH
• Electron transport chain receives the electrons from
NADH and passes them to O2
– Powers pH gradient and ATPase
Respiration Accounting
• Two paths of energy production during respiration
– Most energy flows from glucose -> NADH -> electron transport
chain -> proton-motive force -> ATP
– Some energy comes directly from substrate-level
phosphorylation
• Each NADH can yield ~3 ATP and FADH2 ~2 ATP
– Some NADH shuttles or transport proteins use 1 ATP resulting in
a net gain of 2
• ~38 ATP generated under optimum conditions
– 34 ATP from oxidative phosphorylation
– 4 ATP from substrate-level phosphorylation
– Some energy from proton-motive force may be used for other
work
• Respiration pathways used for other purposes
Fig. 7.10
Brooker Biology 2007
Fermentation
• Aerobic conditions occur when oxygen is
present and anaerobic conditions occur when
oxygen is absent
– Glycolysis occurs under aerobic or anaerobic
conditions because NAD+ is the oxidizing agent
– Glycolysis produces two ATP in aerobic or anaerobic
conditions
• Fermentation occurs under anaerobic conditions
and is limited by the supply of NAD+
• Fermentation requires two phases
– 1) glycolysis
– 2) regeneration of NAD+ by transferring electrons to
pyruvate from NADH
Light Properties and
Photosynthesis
• Light behaves both as a wave and a particle
– Photons have no mass but contain distinct amount of
energy
– Amount of energy in photon inversely proportional to
its wavelength
– Sun radiates entire electromagnetic spectrum but
visible light passes through the earth’s atmosphere
easiest
• When light meets matter three possible
outcomes
– 1) Reflectance or light bounces off the matter
– 2) Transmittance or light passes through the matter
– 3) Absorption or light is absorbed by the matter
Photosynthesis I
• Photosynthesis is composed of two stages
– light reactions in the thylakoids
– Calvin cycle in the stroma
• The light reactions convert solar energy to
chemical energy
– Light absorbed by chlorophyll drives a transfer of
electrons from water to NADP+ to form NADPH
– Water is split and oxygen is given off as a waste
product
– ATP is formed through photophosphorylation
– No sugar is produced
Fig. 8.2
Brooker Biol. 2007
Photosynthesis II
• Calvin cycle or dark reactions incorporate
CO2 from air into organic compounds
– Carbon fixation incorporation of CO2 into
organic compounds
– Fixed carbon is reduced to carbohydrates by
NADPH
– Additional energy provided by ATP
– ATP and NADPH provided by the light
reactions
– Dark reactions do not need light but usually
occur during the daytime
3 RuBP (15C)
Carbon Fixation
5 G3P (15C)
6 PGA (18C)
Reduction
3 ATP
Rubisco
3 CO2 (3C)
RuBP Regeneration
Calvin
Cycle
6 ATP
6 NADPH
6 G3P (18C)
Output
1 G3P (3C)
Pinus radiata (closed)
and Populus deltoides (open);
Ow et al. GCB 09
Photosynthesis, O2 and H2O
• Plants face two major problems
– 1) whenever stomata open to allow CO2 to diffuse to
the locations of carbon fixation, H2O invariably leaves
– 2) Rubisco fixes both CO2 and O2
• Transpiration loss of H2O from plants
– Stomatal physiology tries to maximize photosynthesis
while minimizing transpiration
– Stomatal closure decreases CO2 concentrations and
increases O2 concentrations promoting O2 fixation
• Photorespiration fixation of O2 by Rubisco
– Photorespiration requires light
– Photorespiration produces no ATP
– Photorespiration uses organic material from the
Calvin cycle
Alternative Pathways of
Photosynthesis
• Three major photosynthetic pathways based on
which molecule first incorporates CO2
– 1) C3 plants fix CO2 into 3-PGA (3 carbon)
– 2) C4 plants initially fix CO2 into a 4 carbon molecule
before passing it to the Calvin cycle
– 3) CAM plants initially fix CO2 into organic acids
• C4 and CAM photosynthetic pathways minimize
transpiration and photorespiration at the cost of
additional energy for carbon fixation
– Temporal or spatial separation
– Light reactions same for all pathways