Important words and concepts from Chapter 10,
Campbell & Reece, 2002 (1/29/2005):
by Stephen T. Abedon (abedon.1@osu.edu)
for Biology 113 at the Ohio State University
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Course-external links are
in brackets Click [index] to access site index Click here to
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(1) Chapter title: Photosynthesis
(a)
Photosynthesis is how most ecosystems make the
food that serves as their energy (and reducing-electrons and
fixed-carbon) foundation
(b)
[photosynthesis (Google Search)] [photosynthesis] [index]
(a)
Organisms that obtain their carbon and energy
(especially the former) without eating other organisms are called autotrophs
(b)
Some autotrophs (though not all) obtain their energy from photons
(i.e., light)
(c)
In most ecosystems,
in fact, the organisms at the base of all food chains are photoautotrophs (i.e., autotrophs that obtain
energy from photons)
(d)
The most common of the terrestrial autotrophs are the green plants
(e)
See Figure 10.1:
Photoautotrophs
(f)
[autotroph or autotrophs
(Google Search)] [index]
(a)
Heterotrophs are organisms that obtain their carbon (in particular) and their energy by
eating other organisms
(b)
[heterotroph or heterotrophs
(Google Search)] [index]
(4) Chloroplast
structure, review
(a)
See Figure 10.2, Focusing in
on the location of photosynthesis in a plant
(b)
Recall that a chloroplast may be
differentiated into the following structures (going from outside to in):
(i)
Outer membrane
(ii)
Intermembrane space
(iii)
Inner membrane
(iv)
Stroma
(v)
Thylakoid
(vi)
Thylakoid space (or compartment or lumen)
(c)
Recall additionally that the thylakoids are derived from the inner
membrane (think of them as sealed off cristae-equivalents)
(d)
[chloroplast (Google Search)] [chloroplast links (MicroDude)] [index]
(5) Photosynthesis,
overall reaction
(a)
The overall reaction of photosynthesis may be described in shorthand
as:
(i)
CO2 + H2O + energy à CH2O + O2
(b)
See Figure 10.3, Tracking
atoms through photosynthesis (and, no, I don’t expect you to know this figure,
though do note that the actual reaction is perhaps more complex than expected
with water appearing on both sides of the equation)
(c)
Note that here energy is “light energy,” i.e., photons
(d)
Note also that CH2O represents a one carbon unit of carbohydrate
(recalling that carbohydrate molecules have more than one carbon)
(e)
In terms of glucose, the above equation may be written
as:
(i)
6CO2 + 6H2O + energy
à C6H12O6
+ 6O2
(f)
Note how this equation is essentially the reverse of the equation for
the oxidation of glucose:
(i)
C6H12O6 + 6O2 à 6CO2 + 6H2O
+ energy
(g)
[photosynthesis (Google Search)] [photosynthesis links
(MicroDude)] [index]
(a)
The CO2 given off by cellular respiration is ultimately taken up as
a substrate of photosynthesis
(b)
The O2 given off by photosynthesis is ultimately taken up as
a substrate (final electron acceptor) of cellular respiration
(c)
All of the atmospheric molecular oxygen, in fact, came/comes from
photosynthesis
(d)
See Figure 9.1: Energy flow
and chemical recycling in ecosystems
(e)
[carbon cycle, oxygen cycle (Google Search)] [carbon cycle links
(MicroDude)]
[index]
(7) Two reactions of photosynthesis
(a)
Photosynthesis is not a single reaction pathway but two, one
dependent on the other
(b)
See Figure 10.4, An overview
of photosynthesis: cooperation of the light reactions and the Calvin cycle
(c)
The light reactions represents the “photo”
part of photosynthesis
(i)
This is where the energy of
photons is captured
(ii)
The light reaction supplies the energy and reducing electrons to drive forward the Calvin cycle
(d)
The Calvin
Cycle
represents the “synthesis” part of photosynthesis
(i)
This is where the “fixing” of CO2
(leading to the making of carbohydrate)
occurs
(ii)
The Calvin cycle serves to store the energy and reduced electrons generated by the light reaction, i.e., store
energy and reduced electrons as carbohydrate
(e)
Both reactions are anabolic, involving the phosphorlyation of ATP
(not, in this case, as a product of catabolism), reduction of NADP+ (both products of the light
reactions) or carbohydrate formation (the product of the dark reactions)
(f)
[photosynthesis two reactions
(Google Search)] [stages of photosynthesis
(Online Biology Book)]
[photosynthesis links (MicroDude)]
[index]
(8)
Light
reaction of photosynthesis
(a)
The light reactions of photosynthesis involve the following
(i)
Capture of a photon of light by a chlorophyll photosystem
(ii)
Conversion of photon energy to energy stored by electrons (chemical energy)
(iii)
Capture of this chemical energy by the reaction center
chlorophyll of a photosystem
(iv)
Oxidation of the reaction center chlorophyll by the
primary electron acceptor
(vi)
Oxidation of water
(vii)
Cyclic electron flow
(b)
Overall these reactions produce molecular oxygen
(O2; a waste product given off by one of the light reactions of
photosynthesis), ATP, and NADPH
(c)
[photosynthesis light reactions,
photosynthesis light reaction
(Google Search)] [light reactions links
(MicroDude)]
[index]
(a)
In photosynthesis, reducing electrons are carried
by a phosphorylated derivative of NAD+ called NADP+
(b)
Analogous to the reduction of NAD+,
the reduction of NADP+ occurs thusly:
(i)
NADP+ + 2e- + 2H+ à NADPH + H+
(c)
NAD+ and NADP+ otherwise act equivalently, the
former in cellular respiration, the latter in
photosynthesis
(d)
[NADP reduction (Google Search)] [conversion of NADP+
to NADPH (Davidson College)] [NADPH oxidase page
(David Lambeth)] [NADPH oxidase (KUMC Sophomore Pathology)]
[index]
(a)
The wavelength of light is, among other things, a measure of the energy
associated with a photon
(b)
Wavelength is also equivalent to the perceived color of a photon
(c)
See Figure 10.5, The electromagnetic
spectrum
(d)
The chloroplasts of green plants do not absorb green light
(photons); this is why plants are green
(e)
See Figure 10.6, Why leaves
are green; interactions of light with matter in a chloroplast
(f)
[wavelength of light,
wavelength of light
photosynthesis (Google Search)] [the nature of light (Online Biology Book)]
[the physics of photosynthesis
(MIT Biology Hypertextbook)]
[index]
(a)
The molecule that does the actual absorption of photons in chloroplasts is chlorophyll
(b)
Chlorophyll is a complex, non-protein, organic-molecule that is
embedded, more or less, in the thylakoid membrane
(c)
See Figure 10.9, Location
and structure of chlorophyll molecules in plants
(d)
The major function of a chlorophyll molecule is to absorb a photon,
using the energy of that photon to boost a specific
chlorophyll electron to a higher energy
shell
(e)
Chlorophylls are arrayed into photosystems
(f)
[chlorophyl (Google Search)] [chlorophyll and accessory
pigments (Online Biology Book)]
[index]
(a)
Photosystems are two-dimensional arrays of chlorophyll molecules
(b)
These arrays are found embedded together in the membranes of the thylakoids of chloroplasts
(c)
These arrays serve as photon-gathering antennae
(d)
Green plants employ two slightly different photosystems termed
photosystem I and photosystem II, the former of which (photosystem I) is
involved in both non-cyclic and cyclic electron flow
(e)
See Figure 10.11, How a
photosystem harvests light
(f)
[photosynthesis photosystem or
photosystems (Google Search)] [index]
(a)
When a photon is captured by one chlorophyll of a photosystem, the energy of that photon is passed from chlorophyll to
chlorophyll until the energy reaches the photosystem reaction center
(b)
See Figure 10.11, How a
photosystem harvests light
(c)
Note that it is energy, not electrons (or photons) that are passed from
chlorophyll to chlorophyll molecule with a photosystem
(d)
[reaction center photosynthesis
(Google Search)] [index]
(14) Primary
electron acceptor
(a)
The reaction center of a photosystem is a chlorophyll
molecule that is associated with the primary electron acceptor
(b)
The primary electron acceptor accepts the now high-energy electron from the reaction center
chlorophyll and passes that electron on to an electron transport chain
(c)
See Figure 10.11, How a
photosystem harvests light
(d)
[primary electron acceptor,
primary electron acceptor
photosynthesis (Google Search)] [index]
(a)
Chlorophyll minus its electron
(i.e., having passed it on to the primary electron acceptor) is a very strong oxidizer
(b)
To proceed with another round of photon absorption, the reaction
center chlorophyll must obtain a replacement electron
(c)
This replacement electron, in green plants, is obtained, in certain
circumstances, from water:
(i)
Chlorophyll2+
+ H2O
à chlorophyll0 + 2H+
+ ½O2
(d)
Note that the O2 generated by photosynthesis
comes from water, not from CO2!
(recall that similarly the O2 used in cellular respiration serves to generate water
rather than CO2)
(e)
The H+ are generated in the thylakoid space, thus contributing to a
chemiosmotic proton gradient
(f)
[oxidation of water,
oxidation of water
photosynthesis (Google Search)] [index]
(a)
See Figure 10.12, How
noncyclic flow during the light reactions generates ATP and NADPH
(b)
Non-cyclic electron flow through the various photosystems (I and
II) is employed to produce ATP as well as reduce NADP+
(c)
Note that the production of both of these molecules is driven by light
energy that has been converted to chemical energy rather than derived from the
catabolism of food molecules (such as glucose)
(d)
Note if the figure (more or less in order):
(i)
Photon capture by photosystem II
(ii)
Oxidation of photosystem II
(iii)
Primary electron acceptor
(iv)
Oxidation of water
(v)
Electron transport
(vi)
Photophosphorylation
(vii)
Reduction of photosystem I
(viii)
Photon capture by photosystem I
(ix)
Oxidation of photosystem I
(xi)
NADP+ reduction
(e)
Note that photosystem I and photosystem II absorb photons of light at
slightly different wavelengths (700 nm and 680 nm,
respectively)
(f)
See Figure 10.13, A
mechanical analogy for the light reactions
(g)
[non-cyclic electron flow,
non-cyclic electron flow
photosynthesis (Google Search)] [index]
(a)
See Figure 10.12, How
noncyclic flow during the light reactions generates ATP and NADPH
(b)
Cyclic electron flow is the same as non-cyclic flow except:
(i)
Involves only photosystem I
(ii)
Electron donated to ETS, not NADP+
(iv)
Electron reduces photosystem I
(c)
See Figure 10.14, Cyclic
electron flow
(d)
The idea is that, because the photosystem I electron serves as the source of electrons for photosystem I,
the system is considered cyclic
(e)
Note that cyclic electron flow generates ATP, but no NADPH +
H+
(f)
This essentially gives a plant an opportunity to produce more ATP than
NADPH when it is to the plants advantage to do so (particularly, the Calvin cycle requires more
ATP than NADPH)
(g)
[cyclic electron flow,
cyclic electron flow
photosynthesis (Google Search)] [index]
(a)
See Figure 10.15, Comparison
of chemiosmosis in mitochondria and chloroplasts
(b)
Photophosphorylation is equivalent to oxidative phosphorylation except that:
(i)
It is the energy of
photons that generates the proton-motive force
rather than the high energy electrons found in chemical bonds (though in both cases it is electron transfer that is used to pump hydrogen ions)
(ii)
The high proton concentration is generated in the thylakoid space
rather than in the intermembrane space
(c)
Note that since the thylakoids are
generated from the inner membrane of the chloroplast, the
thylakoid space of chloroplasts and intermembrane space of mitochondria are equivalent (though, of course, not
identical; think pinching inward of the inner membrane to generate the
thylakoids)
(d)
Various other reactions contribute to the proton motive force that is found across the
thylakoid membrane including:
(i)
The oxidation of water generates protons in the
thylakoid space
(ii)
The reduction of NADP+ sequesters
protons otherwise found in the stroma (i.e., outside of the thylakoid space)
(e)
See Figure 10.16, The light
reactions and chemiosmosis: the organization of the thylakoid membrane
(f)
FAQ: Photophosphorylation,
explain please. Photophosphorylation is similar to oxidative
phosphorylation in that both involve a chemiosmotic mechanism of ADP
phosphorylation. The difference is that while hydrogen ion pumping is driven in
oxidative phosphorylation by electrons donated by NADH or FADH2, in
photophosphorylation the electrons that drive pumping are donated by
chlorophyl. Similarly, the energy of oxidative phosphorylation is ultimately
derived from chemical bonds while the energy of photophosphorylation is ultimately
derived from photons. Alternatively, you might consider photophosphoryaltion as
the chemiosmotic phosphorylation that makes ATP during photosynthesis. See in
particular Figure 10.15 of your text, page 195 [NOTE, PAGE NUMBER/FIGURE NUMBER MAY BE INCORRECT].
(g)
[photophosphorylation
(Google Search)] [index]
(19)
Calvin cycle (carbon
fixing, dark reaction of
photosynthesis)
(a)
The Calvin cycle employs CO2
captured from the atmosphere and ATP and NADPH generated by the light reaction
(b)
The “job” of the Calvin cycle is to convert atmospheric CO2
into carbohydrates
(c)
The Calvin cycle technically doesn’t require light to progress (and hence
is also know as the “dark reaction” of photosynthesis),
though, of course, the Calvin cycle does require the products (ATP and NADPH)
of the light reaction (that is, these have to come from somewhere and in a
functioning plant these substrates of the Calvin cycle come from
the light reactions of photosynthesis)
(d)
See Figure 10.16, The light
reactions and chemiosmosis: the organization of the thylakoid membrane
(e)
Note that the ATP and NADPH are generated in the stroma
(f)
Note that the Calvin cycle takes place in the stroma
(g)
See Figure 10.17, The Calvin
cycle
(h)
Don’t worry about knowing figure 10.16
(i)
[Calvin cycle, carbon fixing, carbon fixation, photosynthesis dark reaction
or reactions (Google Search)] [index]
(a)
Don’t forget that plants also have mitochondria and are able to do cellular respiration—this is how plants derive energy from
that stored in the sugars synthesized by the Calvin cycle
(b)
[mitochondria, mitochondria and
photosynthesis (Google Search)] [index]
(a)
Autotroph
(b)
Calvin cycle
(d)
Carbon fixing
(e)
Chlorophyll
(f)
Chloroplast
structure, review
(h)
Dark reaction of photosynthesis
(i)
Heterotrophs
(j)
Light reaction of photosynthesis
(k)
Mitochondria
(l)
NADP+
reduction
(p)
Photosynthesis,
overall reaction
(q)
Photosystems
(s)
Reaction center
(t)
Two reactions of
photosynthesis