Transcript Photosynthesis

Photosynthesis
Photosynthesis
• Process by which plants use light energy
to make food molecules from CO2 and
H2O
– 6 CO2 + 6 H2O  C6H12O6 + 6 O2
enzymes and light
– Plants are autotrophs- self feeder- and are the
basis of all food chains and webs on earth
– Require sunlight to produce own food
• Photosynthesis occurs in chloroplasts
– Chlorophyll absorbs light energy
– Concentrated in cells of mesophyl-green
tissue in interior of leaf
• Each mesophyl cell has numerous chloroplasts
– Structure of chlorophyll
• Has outer membrane and inner membrane
• Inner membrane encloses thick material called
stroma
– Stroma is site of sugar synthesis from CO2
• Suspended in the stroma are elaborate, disc-like
membranous sacs called thylakoids
– Thylakoids are concentrated in sacs called grana
» Chlorophyll molecules embedded into thylakoid
membranes capture light energy
» Also in membrane is the machinery for converting
light energy to chemical energy
• Conversion reactions (fig 7.3c)
– Plants split O2 off of CO2 molecules
– Photosynthesis is a redox process and
happens in stages
• First stage- light reaction
– Absorption of light energy
– e- are excited and pass from reaction-center chlorophylls
to primary e- receptors
» In turn, each primary acceptor is oxidized as it
donates high energy e- to the first e- carrier of etransport chain
– Additional redox reactions shuttle e-’s from one carrier
molecule to the next, releasing ATP and NADPH
– NADPH, ATP and O2 gas are the products of the light
reactions
» ATP is released through a chemiosmosis process
called photophosphorylation
• Second stage: Calvin’s cycle (fig 7.10 a, b)
– Series of reactions that assemble sugar molecules using
CO2, ATP, and NADPH
– Steps
» Carbon fixation- rubisco (RuBP) enzyme combines 3
molecules of CO2 + 3 molecules of a 5C sugar
• End result- 6 3C acids (3-PGA)
» Energy consumption and redox reactions: 2 chemical
reactions consume energy from 6ATP and oxidize 6
NADPH. 6 molecules of 3-PGA are reduced
• End result- 6 molecules of G3P, and energy rich
molecule
» Release of 1 molecule of G3P leaves 5 G3P’s in
cycle. The single molecule of G3P leaving the cycle
is the net product of photosynthesis. Plant uses 2
G3P molecules to make 1 glucose. It takes 6
complete turns of the cycle to make 2 G3P to make 1
glucose molecule
» Regeneration of rubisco- a series of chemical
reactions uses ATP to rearrange 5 G3P to make 3
RuBP to start another turn of the cycle.
• Plants use sugars for fuels for cellular
respiration and as starting materials for
making other organic molecules
– Most plants make much more than what they
need so they store the excess in roots, tubers,
fruits, and chloroplasts
• Plants use CO2 directly from the air for the
Calvin cycle
– Dry weather can reduce rate of
photosynthesis
• Closing stomata reduces H2O loss, but also cuts
off CO2
– O2 levels will build up and RuBP incorporates O2 instead
of CO2
– Calvin cycle will produce a 2-C compound instead of a 3C
– Plant then breaks down 2C compound into CO2 and H2O
– Process is called photorespiration and yields no sugars
• C plants have special adaptations to save
H2O and prevent photorespiration
– When hot and dry, stomata stay shut most of
the time to conserve H2O
• Continues to make sugars through photosynthesis
using an enzyme that fixes C into a 4-C compound
– Plant can continue to fix C even when [CO2] is low
– 4C compound acts as a carbon shuttle by donating CO2
to calvin cycle in a nearby cell
• CAM plants conserve H2O by opening
stomata only at night
– As CO2 enters leaves, it is fixed into a 4C
compound and banked at night
– Leaf photosynthesizes during the day without
admitting any more CO2