Chapter 10 Photosynthesis – Part 1
Photosynthesis in nature
• Autotrophs:
biotic producers:
photoautotrophs;
chemoautotrophs;
obtain organic food without
eating other organisms
• Heterotrophs:
biotic consumers:
obtain organic food by
eating other organisms or
their by-products
(includes decomposers)
Principles of Energy Harvest (again!)
Photosynthesis
vs.
Cellular respiration
Principles of Energy Harvest Understand!
Photosynthesis
Cellular respiration
Endergonic
Exergonic
Products: O2,
C6H12O6
Products: CO2,
H2O, ENERGY
Reactants: CO2,
H2O, ENERGY
Reactants: O2,
C6H12O6
Chloroplasts
Mitochondria
Principles of Energy Harvest
Cell respiration is catabolic
• Breaks down glucose
Photosynthesis is anabolic
• Synthesizes glucose
ESSAY!
ESSAY!
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Powered by light, the green parts of plants
produce organic compounds and O2 from CO2
and H2O.
• Using glucose as our target product, the equation
describing the net process of photosynthesis is:
– 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2
• In reality, photosynthesis adds one CO2 at a time:
– CO2 + H2O + light energy -> CH2O + O2
– CH2O represents the general formula for a
sugar, (CH2O)n.
Deciphering the equation
• One of the first clues to the mechanism of
photosynthesis came from the discovery that the O2
given off by plants comes from H2O, not CO2:
CO2 + 2 H2O -> CH2O + H2O + O2
Did you understand?
• In the photosynthesis reaction,
CO2 + 2 H2O -> CH2O + H2O + O2
Where do each of these components of the products
come from?
–
–
–
–
Oxygen in glucose?
Oxygen in O2 gas?
Hydrogen in water?
Carbon in glucose?
Deciphering the equation – typical
AP Biology Exam question
• One of the first clues to the mechanism of
photosynthesis came from the discovery that the O2
given off by plants comes from H2O, not CO2:
CO2 + 2 H2O -> CH2O + H2O + O2
Deciphering the equation –
understand this!
Photosynthesis: an overview
• Redox process
• H2O is split, and
H (e- and H+) is
transferred to
CO2, reducing it
to sugar
• Detect redox
reaction with
DPIP in Lab 4A
Photosynthesis is a redox reaction
• Photosynthesis reverses the direction of electron
flow in respiration.
• Water is split and electrons transferred with H+ from
water to CO2, reducing it to sugar.
• Chemically: polar covalent bonds (unequal sharing)
are converted to nonpolar covalent bonds (equal
sharing).
– The reaction is strongly endergonic; light
boosts the potential energy of electrons as they
move from water so they can be used to make
sugar.
The chloroplast
•
•
•
•
•
•
Eukaryotic organelle
Site of photosynthesis
Pigment: chlorophylls
Plant cell: mesophyll
Gas exchange: stomata
Double membrane (or
is it triple?)
• Thylakoids in grana,
stroma
Chloroplasts are the size of medium to large bacteria
• A typical mesophyll cell has
30-40 chloroplasts, each about
2-4 microns by 4-7 microns
long.
• Each chloroplast has two
membranes around a central
aqueous space, the stroma.
• In the stroma are
membranous sacs,
the thylakoids.
The chloroplast: 3 membranes?
Chloroplasts - the sites of photosynthesis in plants
• Any green part of a plant has chloroplasts.
• However, the leaves are the major site of
photosynthesis for most plants.
– There are about half a million chloroplasts per
square millimeter of leaf surface.
• The color of a leaf comes from chlorophyll,
the green pigment in the chloroplasts.
– Chlorophyll plays an important role in the
absorption of light energy during photosynthesis.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Chlorophyll molecule contains Mg2+
Fig. 10.9
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Chloroplasts are found mainly in mesophyll cells
forming the tissues in the interior of the leaf.
• O2 exits and CO2 enters the leaf through
microscopic pores, stomata, in the leaf.
• Veins deliver water
from the roots and
carry off sugar from
mesophyll cells to
other plant areas.
Fig. 10.2
ESSAY!
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Two types of veins
• Xylem carries water and minerals from roots to
leaves (and all other parts)
• Phloem carries sugar water from leaves (to all
other parts)
ESSAY!
Fig. 10.2
Photosynthesis: 2 major steps
1. Light reactions
(“photo”)
• NADP+ (electron
acceptor nicotinamide
adenine dinucleotide
phosphate) to NADPH
• Photophosphorylation:
ADP ---> ATP
2. Calvin cycle
(“synthesis”)
• Carbon fixation:
carbon into organics
1. Photosystems: first Ps II, then Ps I
• Light harvesting units of
the thylakoid membrane
• Composed mainly of
protein and pigment
“antenna complexes”
1. “Antenna pigment”
molecules are struck by photons (light energy)
2. Energy is passed to reaction centers (redox location)
3. Excited e- from chlorophyll is trapped by a primary
e- acceptor
Understand the photosystem scheme
Noncyclic electron flow
Photosystem II (P680):
• photons excite chlorophyll e- to
an acceptor
• e- are replaced by splitting
of H2O (release of O2)
• e-‟s travel to Photosystem I
down an electron transport
chain
• as e- fall, ADP ---> ATP
(noncyclic
photophosphorylation)
Noncyclic electron flow
Photosystem I (P700):
• „fallen‟ e- replace excited
e- to primary e- acceptor
• 2nd ETC (Fd~NADP+
reductase) transfers e- to
NADP+ ---> NADPH
(...to Calvin cycle…)
• These photosystems
produce equal amounts
of ATP and NADPH
Light reactions animations
• http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::53
5::/sites/dl/free/0072437316/120072/bio13.swf::P
hotosynthetic Electron Transport and ATP
Synthesis
• http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::53
5::/sites/dl/free/0072437316/120072/bio12.swf::C
yclic and Noncyclic Photophosphorylation
The Calvin cycle – the “Dark” Reactions
•
1.
2.
3.
3 molecules of CO2 are „fixed‟
into glyceraldehyde 3phosphate (G3P):
Carbon fixation: each CO2 is
attached to RuBP (by “rubisco
enzyme”)
Reduction: electrons from
NADPH reduce to G3P; ATP
used up
Regeneration: G3P
rearranged to RuBP; ATP
used; cycle continues
Calvin cycle animations
• http://www.sinauer.com/cooper/4e/animations030
5.html
• http://www.cells.de/cellseng/1medienarchiv/Zellfu
nktionen/Memb_Vorg/Photosynthese/Dunkel_u_S
taerke/Calvin-Benson-Zyklus/calvin_lin.htm
Calvin Cycle, net synthesis
• For each G3P (and for 3 CO2)…….
Consumption of 9 ATP’s & 6 NADPH
(light reactions regenerate these
molecules)
• G3P can then be used by the plant to make
glucose and other organic compounds
Cyclic electron flow
• Alternative cycle when
ATP is deficient
• Photosystem I used
but not II; produces
ATP but no NADPH
• Why? The Calvin
cycle consumes more
ATP than
NADPH…….
• Cyclic
photophosphorylation
Alternative carbon fixation methods
Avoiding Excessive Photorespiration
• Hot/dry days; stomata close;
CO2 decreases, O2 increases in
leaves; O2 added to rubisco; no
ATP or food generated
Two Solutions…..
1. C4 plants: 2 photosynthetic
cells, bundle-sheath &
mesophyll; PEP carboxylase
(instead of rubisco) fixes CO2
in mesophyll; new 4C molecule
releases CO2 (grasses)
ESSAY!
Alternative carbon fixation methods
Avoiding Excessive Photorespiration
2. CAM plants: open
stomata during night,
close during day
(crassulacean acid
metabolism); cacti,
pineapples, etc.
Alternative carbon fixation methods –
Another common AP Biology exam question
A review of photosynthesis
Light and Photosynthesis - 1
• Light, like other form of electromagnetic
energy, travels in rhythmic waves.
• The distance between crests of
electromagnetic waves is called the
wavelength.
– Wavelengths of electromagnetic radiation
range from less than a nanometer (gamma
rays) to over a kilometer (radio waves).
• The electromagnetic spectrum.
• The most important segment for life is a narrow
band between 380 to 750 nm, visible light.
Light and Photosynthesis -2
• Other light properties are those of a discrete
particle, the photon.
• The amount of energy packaged in a photon is
inversely related to its wavelength.
– Photons with shorter wavelengths pack more energy.
• While the sun radiates a full electromagnetic
spectrum, the atmosphere selectively screens out
most wavelengths, permitting only visible light to
pass in significant quantities.
• When light meets matter, it may be reflected,
transmitted, or absorbed.
– Different pigments absorb photons of
different wavelengths.
– A leaf looks green
because chlorophyll,
the dominant pigment,
absorbs red and blue
light, while transmitting
and reflecting green
light.
• A spectrophotometer measures the ability of a
pigment to absorb various wavelengths of light.
http://www.youtube.com/
watch?v=V1vXCmhWw4
0&feature=related
– It beams narrow wavelengths of light through a
solution containing
a pigment and
measures the
fraction of light
transmitted at
each wavelength.
– An absorption
spectrum plots a
pigment’s light
absorption versus
wavelength.
• Photosynthesis pigments
– Chlorophyll a, the dominant pigment,
absorbs best in the red and blue
wavelengths, and least in the green.
– Other pigments
with different
structures have
different
absorption
spectra.
• Collectively, these photosynthetic pigments
determine an overall action spectrum for
photosynthesis.
– An action spectrum measures changes in some
measure of photosynthetic activity (for example, O2
release) as the wavelength is varied.
• The action spectrum of photosynthesis was first
demonstrated in 1883 through an elegant experiment
by Thomas Engelmann.
– In this experiment, different segments of a filamentous
alga were exposed to different wavelengths of light.
– Areas receiving wavelengths favorable to photosynthesis
should produce excess O2.
– Engelmann used the
abundance of aerobic
bacteria clustered
along the alga as a
measure of O2
production.
• The action spectrum of photosynthesis does not match
exactly the absorption spectrum of any one photosynthetic
pigment
• Only chlorophyll a participates directly in the light reactions
but accessory photosynthetic pigments absorb light and
transfer energy to chlorophyll a.
– Chlorophyll b, with a slightly different structure than
chlorophyll a, has a slightly different absorption
spectrum and funnels the energy from these wavelengths
to chlorophyll a.
– Carotenoids can funnel the energy from other
wavelengths to chlorophyll a and also participate in
photoprotection against excessive light.