PHOTOSYNTHESIS
Chapter 10
BASIC VOCABULARY
Autotrophs – producers; make
their own “food”
Heterotrophs – consumers;
cannot make own food
LEAF STRUCTURE
Stomata (stoma) – microscopic pores that allow
water, carbon dioxide and oxygen to move into/out
of leaf
Chloroplasts – organelle that performs
photosynthesis
Found mainly in mesophyll – the tissue of the
interior leaf
Contain chlorophyll (green pigment)
Stroma – dense fluid in chloroplast
Thylakoid membrane – inner membrane of
chloroplast
Grana (granum) – stacks of thylakoid membrane
Figure 10.2 Focusing in on the location of photosynthesis in a plant
PHOTOSYNTHESIS SUMMARY
6CO2 + 6H20 + light energy
C6H12O6 + 6O2
Oxygen comes from water, not CO2
Two parts:
Light Reactions
The Calvin Cycle (Dark
Reactions or Light
Independent)
Figure 10.3 Tracking atoms through photosynthesis
Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the
Calvin cycle
LIGHT
Photons – discrete packets of light
energy
Chlorophyll a – (blue-green)only
pigment that is directly used in light
reactions
Chlorophyll b – (yellow-green)
accessory pigment
Carotenoids - (yellow-orange)
Figure 10.6 Why leaves are green: interaction of light with chloroplasts
Figure 10.8 Evidence that chloroplast pigments participate in photosynthesis:
absorption and action spectra for photosynthesis in an alga
PHOTOEXCITAION
When photons hit chlorophyll
and other pigments, electrons
are excited to an orbital of
higher energy
In solution when the excited
electrons fall, they give off
energy (a photon) and
fluoresce
Figure 10.9 Location and structure of chlorophyll molecules in plants
LIGHT REACTIONS
Photosystems:
Made of proteins and other
molecules surrounding
chlorophyll a
Contain a primary electron
acceptor
Photosystem I – P700
Photosytem II – P680
Figure 10.11 How a photosystem harvests light
Require light to occur
Two pathways:
Noncyclic (predominant
route)
Cyclic
Noncyclic animation
Another animation
NONCYCLIC ELECTRON FLOW
Photosystem II absorbs light
Two electrons excited and captured
by primary electron acceptor
“Hole” in photosystem II is filled by
2 electrons that come from the
splitting of water
H2O
2H+ + ½ O2 + 2e-
Figure 10.12 How noncyclic electron flow during the light reactions generates ATP
and NADPH (Layer 1)
Oxygen is released
Excited electrons pass from
primary electron acceptor down
an electron transport chain to
photosystem I (filling its “hole”)
ATP is made by
photophosphorylation as
electrons fall down ETC
Figure 10.12 How noncyclic electron flow during the light reactions generates ATP
and NADPH (Layer 3)
Photons excite 2 electrons from
Photosystem I and are captured by
its primary electron acceptor
Electrons then move down another
ETC to ferredoxin (Fd)
Fd gives electrons to NADP+
(nicotinamide dinucleotide
phosphate) making NADPH
The enzyme that helps this transfer
of e- is called NADP+ reductase
Figure 10.12 How noncyclic electron flow during the light reactions generates ATP
and NADPH (Layer 4)
Figure 10.12 How noncyclic electron flow during the light reactions generates ATP
and NADPH (Layer 5)
Figure 10.13 A mechanical analogy for the light reactions
Figure 10.14 Cyclic electron flow
CYCLIC ELECTRON FLOW
Only Photosystem I is used
Fd passes electrons back to
Photosystem I via ETC
Some ATP made
No NADPH made
No oxygen released
Used when cell needs more
ATP than NADPH
ETC
MITOCHONDRIA CHLOROPLAST
Food (chemical
energy) to ATP
(chemical energy)
ATP synthase
Pumps H+ into
intermembrane
space
Light energy to
ATP (chemical
energy)
ATP synthase
Pumps H+ into
thylakoid space
Figure 10.15 Comparison of chemiosmosis in mitochondria and chloroplasts
Figure 10.17 The Calvin cycle (Layer 1)
Figure 10.17 The Calvin cycle (Layer 2)
Figure 10.17 The Calvin cycle (Layer 3)
CALVIN CYCLE
Also called Dark Reactions because
light is not needed; however products
from light reactions are needed.
Carbon Fixation – initial incorporation
of carbon into organic molecules
CO2 attaches to a 5-carbon sugar called
ribulose bisphosphate (RuBP)
The enzyme that catalyzes this is called
rubisco
Calvin cycle animation
Immediately splits into two 3-carbon molecules
called 3-phosphoglycerate
3-phosphoglycerate is phosphorylated by ATP
(from light reactions) making
1,3-bisphosphoglycerate
1,3-bisphosphoglycerate is reduced by taking
electrons from NADPH making glyceraldehyde
3-phosphate (G3P)
One G3P molecule leaves cycle to be used by
plant
The remaining G3P’s are converted into RUBP in
several steps and by getting phosphorylated by
ATP
Recall, G3P is the sugar formed
by splitting glucose in glycolysis
G3P can be made into glucose,
sucrose, cellulose etc. by plant
C3 PLANTS – have a problem
Examples : rice, wheat, and soy beans
Problem - produce less food when
stomata are closed during hot days
because low CO2 starves Calvin Cycle and
rubisco can accept O2 instead of CO2
High oxygen levels = O2 passed to RUBP
(not CO2) and Calvin cycle stops
When this oxygen made product splits, it
makes a molecule that is broken down by
releasing CO2
This process is called photorespiration.
Occurs during daylight (photo)
Uses O2 and makes CO2 (respiration)
NO ATP made (unlike respiration) and
NO food made
Early earth had low O2 so this would not
have mattered as much
Photorespiration drains away as much as
50% of carbon fixed by Calvin Cycle in
many plants.
C4 PLANTS – have a solution
Examples: sugarcane, corn and
grasses
Leaves contain bundle-sheath cells
and mesophyll cells
Bundle sheath surrounds veins of
leaf (location of Calvin cycle)
Mesophyll – between bundle and
surface
In mesophyll cells: CO2 fixed to
phosphoenolpyruvate (PEP)
PEP carboxylase is the enzyme that does this
PEP carboxylase has higher affinity for CO2
than rubisco so less danger of O2 interfering
The fixed CO2 is then taken to Calvin cycle (in
bundle-sheath) as part of a 4-carbon molecule
(malate)
Malate gives CO2 to Calvin cycle
Figure 10.18 C4 leaf anatomy and the C4 pathway
CAM PLANTS – have another solution
(crassulacean acid metabolism)
Examples: succulent plants
(pineapples and cacti etc.)
Open stomata at night and close
during day
At night CO2 is fixed into organic
acids in mesophyll and then taken
to Calvin cycle (also in mesophyll)
during day.
Figure 10.19 C4 and CAM photosynthesis compared
PHOTOSYNTHESIS FACTS
50% of organic material made is
used by plant in respiration
Organic molecules often leave
leaves as sucrose
Large amounts of cellulose are
made (for cell walls)
“And no process is more important
than photosynthesis to the welfare
of life on Earth.”
(Campbell and Reece, 2005)
Figure 10.20 A review of photosynthesis