Modern Biology Chapter 6: Photosynthesis
Plant cell
6-1: Capturing the Energy in Light
Energy for life processes
• photosynthesis: process by which green
plants convert solar energy into chemical
energy
– produces carbohydrates
– produces oxygen
Energy for life processes
• chloroplast structure
– double membrane surrounds entire organelle
– thylakoids: flattened sacs inside double
membrane
– grana: stacks of thylakoids
– stroma: fluid surrounding thylakoids inside
double membrane
Energy for life processes
• sunlight
– provides heat and energy to earth
• white light from sun contains mixture of
colors of light
– wavelength of light determines its color
– only a small portion of sun-light is visible to
humans
• The sun emits all visible
wavelengths of light
• Green plants absorb
red, orange, blue
and violet
•They reflect yellow and green
Overview of photosynthesis
From • CO
2
Air
From • H O
2
Soil
From • Light
Sun
energy
To
air
• O2
To • C H O
6 12 6
plant
Energy for life processes
• pigment:
colored
substance
that reflects
or absorbs
light
Energy for life processes
• chlorophyll
– type of pigment in thylakoid membranes
• two types of chlorophyll
• chlorophyll a absorbs light in red end of spectrum
• chlorophyll b absorbs light in blue end of the
spectrum (accessory pigment)
• green light is not absorbed, but reflected giving the
leaves the appearance of being green
– by absorbing light pigments also absorb energy
Energy for life processes
• Cartenoids: other accessory pigments
– absorb different colors depending on chemical
structure
– become apparent when chlorophylls fade (fall
colors)
THE LIGHT REACTIONS
The light reactions consist of three
basic components
• Photosystem 2
• Photosystem 1
• ATP synthase (chemiosmosis)
Photosystem 2
• water-plastoquinone oxidoreductase
• Uses the energy from sunlight to split the
water molecule into three parts
2H2O  4 H+ + 4 e- + O2
Photosystem 1
• plastocyanin: ferredoxin oxidoreductase
• Uses the energy from sunlight to move the
electrons onto NADP+ for transport to the
next phase of the process
ATP synthase
• Synthesizes ATP using a concentration
gradient created by photosystem II
Light reactions
• Light and the energy associated with it are
absorbed into photosystem I and
photosystem II
Light-dependent reactions a.k.a.
light reactions
• Electron transport occurs within membranes
Light-dependent reactions a.k.a.
light reactions
• photosystem II (PSII)
– accessory pigments absorb light and acquire
energy (E) (step 1)
– energy is passed along membrane pigments
until it reaches a specific pair of chlorophyll a
molecules
Light-dependent reactions a.k.a.
light reactions
• photosystem II (PSII)
– electron transport
• E forces e- to increase E level (e- are said to be
“excited”)
• excited e- leave chlorophyll a
• chlorophyll a is oxidized
• PEA donates e• e- reduces primary e- acceptor (PEA) (step 2)
• e- transported down ETC (step 3)
• each transfer, the e- loses some E
• E is used to move p+ into thylakoid
Light-dependent reactions a.k.a.
light reactions
• photosystem I (PSI)
–
–
–
–
light absorbed by PSI (step 1b)
e- move from chlorophyll a to PEA (step 4)
e- lost are replaced by e- from PSII
PEA of PSI donates e- to NADP+ (step 5)
• brings e- to edge of thylakoid membrane by stroma
• e- combine with p+ and NADP+
• NADP+ reduced to NADPH
Light-dependent reactions a.k.a.
light reactions
• replacing e- (step 6)
– recall e- from chlorophyll in PSII replace ethat leave chlorophyll on PSI
– e- from PSII need to be replaced or both ETCs
cease
Light-dependent reactions a.k.a.
light reactions
• replacing e- (step 6)
– replacement e- come from water
•
•
•
•
•
enzyme in thylakoid splits water molecule
2H2O  4 H+ + 4 e- + O2
p+ (H+) remain in thylakoid
O2 diffuses out and leaves the plant
replace e- lost by chlorophyll in PSII
Summary of Light Reactions
• Summary: what is produced during the light
reactions
– p+ concentration gradient
– NADPH
Summary of Light Reactions
• Summary: what is produced during the light
reactions
– p+ concentration gradient
– NADPH
Chemiosmosis
• potential E from gradient is harnessed by
ATP Synthase in thylakoid membrane
– ATP Synthase serves two functions
– Catalyzes ADP + (P)  ATP
– Acts as carrier protein for p+
• as H+ ions pass through ATP Synthase, ATP
is produced
Section
6.2:
The
Calvin
cycle
Stomata
Open
Closed
Section 6.2: The Calvin cycle
• Light-independent reactions
• Many names
– Calvin (or Calvin-Benson) cycle (men who
first described cycle)
– Dark reactions (does not directly require light)
– Carbon fixation (incorporation of C into
organic substances)
Section 6.2: The Calvin cycle
• sugars are long term energy storage (much
more stable than ATP of NADP+)
• requires carbon dioxide (CO2 ) and water
(H2O)
– CO2 enters plants through stomata (little tiny
pores controlled by the plant)
– H2O enters plant through osmosis, capillarity or
stomata
Step 1
• after diffusing into the stroma from the
cytosol, CO2 joins with a 5-C sugar (RuBP)
to produce 2 3-C molecules of PGA
(process is known as carbon-fixation)
Step 2
• PGA is converted into PGAL
– 2PGA + 2ATP + 2NADPH2PGAL + 2ADP
+ 2NADP+ +2 phosphate
Step 3 and 3B
• Most PGAL converted back into RuBP
– 2PGAL + ATP  RuBP + ADP + phosphate +
some fixed C
• Some PGAL leave Calvin cycle as fixed C
(3B)
Balance Sheet
• Each turn of Calvin cycle results in fixation
of 1 CO2
• Three times around Calvin cycle results in 1
PGAL
– each turn requires 3 ATP and 2 NADPH
• 2 ATP from step 2
• 1 ATP from step 3
– 3 turns requires 9 ATP and 6 NADPH
…
• PGAL and other organic molecules like
carbohydrates are formed and then used all
over the cell for a variety of functions.
• 6CO2+ 6H2O + energy  C6H12O6 + 6O2
http://bcs.whfreeman.com/thelifewire/conte
nt/chp08/0802003.html
Alternative Pathways
• C4 pathway
– when CO2 is low, enables plants to continue
fixing carbon
– grasses, corn
– uses less water, but moves much more slowly
Alternative Pathways
• CAM pathway
– when very hot and dry, enables plant to
continue to fix carbon
– cacti, pineapples
– stomata open at night instead of during the day
– uses less water, but moves much more slowly