Transcript Photosynthesis - Bow High School

Photosynthesis
Each year at last 1017 kcal of free energy is
generated by the plant world at the expense of solar
energy. This is over 10 times greater than all the
fossil-fuel energy used per annum by mankind the
world over.
Ecosystem Trophic Levels
Plants provide the world’s organisms with food
Types of Organisms
1.
Autotrophs





2.
self-feeders
make their own food
also known as producers
need CO2 from the air, water from
the soil and sunlight
Plants
Heterotrophs




consumers
cannot make their own food
must obtain food from other
organisms, plants or animals
Animals
Primary Production


Plants produce carbohydrates by converting light energy
into chemical energy
Photosynthesis is the process by which plants produce
their own food
6CO2 + 6 H2O + light
C6H12O6 + 6O2
Divided into two steps
 Light Reaction – light dependant
 Calvin Cycle – light independent
Site of Photosynthesis

Takes place in chloroplast
Energy and Electrons
Light – Ultimate Source of Energy
Light contains photons
Short wave-length
High energy
Long wave-length
Low energy
•
•
•
•
Visible colors – those that are not absorbed
Pure energy
Amount of energy depends on wave-lengths
Excites electrons
WAVELENGTH
ENERGY
GAMMA RAYS X-RAYS
UV
V
INFRARED
RADIO WAVES
Visible Light
380 nm
430 nm
650 nm
750 nm
Action
Spectrum
Absorption
Spectrum
Spectrophotometer




Measures the ability for a
pigment to absorb or
transmit light
Chlorophyll absorbs red
and blue light 
transmits green
Carotenoids absorb
violet and blue light
~50 percent of light is
PAR
Photosynthetic Pigments

Chlorophylls



Chl a
Chl b


Carotenoids


carotenes
xanthophylls
Reaction Center Pigments

PSI [P700] and PSII [P680]
 Chl a
Accessory Pigments




Chl a
Chl b
carotenes
xanthophylls
Chlorophyll






Chl a in all higher plants
Chl b also in most higher plants
Chl c or d replaces chl b in some algae
Chl: porphyrin-like ring (loose e-’s) and phytol
tail to anchor it in thylakoid membrane
Center atom of Mg surrounded by 4 N
Fe necessary for the synthesis of chl
CHO
Chlorophyll a
polar
head
Reaction Center Pigment
PSI and PSII
phytol
tail
beta-carotene
chlorophyll
thylakoid membrane
carotenoids
Chloroplasts
-- a plastid found mostly in mesophyll (interior tissue) of


leaves
have three membrane systems:
- outer and inner membrane
- thylakoids - stacks of thylakoids are grana
stroma- fluid outside the thylakoids – contains DNA,
enzymes, ribosomes

thylakoid space
- interior space of the
thylakoid membrane
Photosynthesis Overview
Photosynthesis: two stages

Light reactions (“photo”) part – convert solar E
to chemical E (in form of ATP and NADPH)
-Oxygen is given off from the splitting of water
molecules
-NADP+ (different from NAD+ in cellular
resp.) reduced to NADPH as H+ and 2 e-‘s are
added
-ATP phosphorylation
-Occurs in THYLAKOIDS during day
Light Reactions:

NON CYCLIC Electron flow: Photons energize
electrons of Photosystem I (P700 for 700nm) and
Photosystem II(P680 for 680nm)


Both ATP and NADPH produced
CYCLIC Electron Flow

No NADPH produced, only ATP because only
Photosystem I used, and electrons cycle back from Fd
--More ATP needed in Calvin Cycle than
NADPH, overproduction of NADPH may
provide allosteric inhibition
ATP synthesis: similar to oxidative
phosphorylation in mitochondrion (cell.
Resp.):
--H+ pumped from stroma to thylakoid
space (across thylakoid membrane) as e-‘s
transferred along carriers
--H+ then diffuses from high to low
concentration through the ATP Synthase
proteins (located in thylakoid membranes)
thereby phophorylating ADP to ATP
--ATP formed in stroma where Calvin cycle
occurs, pH gradient that occurs is from pH of
5 in thylakoid space to pH of 8 in stroma
--Termed PHOTOPHOSPHORYLATION

Photosynthesis…
Light Reaction
Mechanical Analogy for the Light Reactions
Photosynthesis…
Dark Reactions (“synthesis”)




Carbon fixation – Carbon Dioxide incorporated
with other compounds
Electrons added to products of carbon fixation
to form carbohydrates
Calvin cycle energized by NADPH and ATP
occurs during the day (when NADPH and ATP
produced) in the STROMA
Dark Reactions:


Phase 1 – Carbon fixation, facilitated by RUBISCO
(enzyme – may be most abundant protein on earth)
 Results in two 3-carbon molecules called
phosphoglycerate (per CO2 molecule)
Phase 2: Reduction
 ATP used
 NADPH loses electrons – reduced? Or oxidized?
 one Glyceraldehyde-3-phosphate (G3P) molecule
produced for further anabolic processing (building
blocks of glucose and other carbohydrates)
 5 G3P molecules remain to be rearranged into 3
ribulose biphosphate (a 5-carbon molecule) to begin
new cycle
The Calvin Cycle
Review of Photosynthesis
If stoma are closed (because it’s hot and dry) then CO2 concentration
decreases in the leaf and RuBP starts fixing O2 -- wasting ATP,
NADPH and sugars.
“C4
photosynthesis”
summary
The light-dependent
reactions are the same
as in C3, but Carbon
fixing is done in two
steps in C4. This keeps
CO2 concentration high
in the bundle-sheath (bs)
cells)
C4 plants can photosynthesize even when
stomates are closed -- when C3 plants would
be losing productivity due to photorespiration.
CAM photosynthesis works like C4 except that in
CAM, the CO2 fixation and Calvin cycle are
separated in time (day/night) and space within cells
(cyto. vs. vacuole), whereas in C4, these functions are
separated among cells (mesophyll/bundle sheath)
C3, C4 & CAM
CO2
CO2
night
Mesophyll C4
cell
RuBP
Calvin
cycle
PGA
(C3)
Bundle CO2
sheath
cell
CO2
C4
day
CO2
Calvin
cycle
Calvin
cycle
PGAL
PGAL
CO2 fixation in a C4 plant
CO2 fixation in a CAM plant
PGAL
Mesophyll cell
CO2 fixation in a C3 plant
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A comparison of photosynthesis with and without
oxygen: evolutionary implications





Photosynthetic prokaryotes other than cyanobacteria have only one
photosystem and do not produce oxygen (Fig.19.13).
2H2D + CO2 light (CH2O) + H2O + 2D
Hydrogen donor:
water: green plants and cyanobacteria;
hydrogen sulfide (H2S): green and purple sulfur bacteria (anaerobes);
lactate, succinic acid or H2S2O3: other photosynthetic bacteria.
Hydrogen acceptor
 CO2
 NO2- or NO3- : NH3 is a product
Cyanobacteria were apparently the first organisms that developed the ability
to use water as the ultimate reducing agent in photosynthesis, chlorophyll a
rather than bacteriochlorophyll in this case.
Some herbicides inhibit photosynthesis


One of most important features is a higher rate of
detoxification of the herbicide in the desirable
plants compared to the weeds.
Amitrol: inhibit chlorophyll synthesis
Atrazine: inhibit oxidation of water
Diuron: inhibit electron transfer to plastoquinone
Bigyridylium: accept electrons by competing with
the electron acceptors in PSI
Paraquat: destroy the growth of marijuana
Indicator Solutions

DCPIP (1)
 Indicator
solution - acts in place of
plastocyanin Pc in electron transport chain
 Accepts electrons and gets reduced
 Changes color as it gets reduced

Phenol Red (2)
 Indicator
solution for Carbon dioxide
 Turns red in low CO2
 Turns yellow in high CO2