Photosynthesis - 1
The vast majority of living organisms obtain their fuel molecules, directly or
indirectly, from the process of photosynthesis. The process of photosynthesis
transforms light energy into chemical energy, and uses that chemical energy to
produce organic molecules, typically glucose, from water and carbon dioxide
molecules.
These organic molecules are the basis for the macromolecules used for structure
and metabolism of living organisms as well as the fuel molecules we use in cell
respiration. The process of photosynthesis also produces oxygen gas as a "byproduct", the very same molecule that is used in aerobic cell respiration, without
which most organisms on earth would not survive.
To review: Organisms that obtain energy and carbon from their physical
surroundings are autotrophs. The vast majority of autotrophs are
photoautotrophs that manufacture their organic molecules by the process of
photosynthesis. (Organisms that obtain their organic fuel molecules pre-formed
from the environment are heterotrophs. Animals, fungi, many protists and many
bacteria are heterotrophs.) Most photosynthetic organisms are plants or protists
that contain chlorophyll. Many prokaryotes are also photosynthetic. The
Cyanobacteria have chlorophyll pigments; some Bacteria, such as the purple sulfur
bacteria, have different light-capturing pigments, and slightly different
photosynthetic mechanisms. They are studied in microbiology.
Not all autotrophs are photosynthetic; a tiny proportion of our organic fuel is
manufactured by chemosynthetic organisms (or chemoautotrophs).
Chemosynthetic autotrophs use energy from chemical reactions involving inorganic
atoms and molecules, such as S, Fe, H and N, to make organic compounds.
Chemosynthesis is restricted to a very few groups of bacteria, mostly the
Archaea. However, chemosynthesis sustains some deep sea-bed ecosystems that
surround sulfur vents. The chemosynthetic bacteria are also discussed in
microbiology.
Photosynthesis - 2
The Process of Photosynthesis
As stated, photosynthesis involves the transformation of light energy to chemical
energy. The chemical energy is then used to manufacture carbohydrate molecules,
primarily glucose. In eukaryotic organisms, and in the Cyanobacteria, the process
of photosynthesis also produces oxygen. The photosynthetic bacteria produce
organic carbon molecules, but do not produce oxygen. We will discuss the primary
photosynthetic processes of plants only.
Photosynthesis occurs in all parts of plants that contain the green pigment,
chlorophyll, which is located in the chloroplasts. In most plants, however,
photosynthesis occurs mostly in leaves, where chloroplasts are concentrated.
the laboratory, we shall look closely at the leaf structure as it relates to its
function on photosynthesis. At this time we shall turn to the pathways of
photosynthesis.
In
Photosynthesis involves two stages, occurring in separate locations within
chloroplasts. In the light-dependent reactions of photosynthesis, light energy
is transformed into chemical energy in the form of energy transfer molecules in a
series of re-dox reactions that transfer electrons and hydrogen from water to the
energy transfer molecule NADP+ . (Recall that a number of metabolic processes,
including cell respiration, involve oxidation-reductions with electrons being "carried"
along a gradient of electron transfer molecules.) The light-dependent reactions
are known as photophosphorylations, because they involve producing ATP.
They take place on the thylakoid membranes of the grana.
In the Calvin cycle (sometimes called the light-independent reactions), the energy
from the light reactions is used to manufacture carbohydrate molecules, which
form glucose. These reactions occur in the stroma of the chloroplast.
The two "stages" of photosynthesis are linked by the products of the lightdependent reactions. These products are ATP and NADPH.
The overall chemical equation for photosynthesis* is:
Chlorophyll
6CO2 + 12H2O + energy Æ C6H12O 6 + 6H2O + 6O2
Chlorophyll
(Carbon dioxide + water + light energy Æ glucose + water + oxygen)
*
The process of photosynthesis directly produces 12 molecules of the 3-carbon
compound, glyceraldehyde-3-phosphate (G3P). Two of these molecules are
metabolized to glucose during a "complete" photosynthesis. The remaining 10
molecules of G3P are recycled in the photosynthetic process.
Photosynthesis - 3
Photosynthetic Requirements
In order to do photosynthesis, water, CO2 , chlorophyll and light energy must be
available. We shall briefly discuss each of these requirements.
1.
Water
Water is the hydrogen and electron donor for the process of
photosynthesis. Light energy is used to split water molecules, forming 2H + ,
2e - , and Oxygen during the process of photosynthesis.
Water is obtained from the environment, absorbed by roots and conducted
throughout the plant in the xylem of the vascular system. Water needed
for photosynthesis is but one of the demands for water in plants. (How
water moves within plants is discussed in Biology 203.)
2.
Carbon Dioxide
Carbon dioxide provides the carbon source for manufacturing the
carbohydrates in photosynthesis.
Carbon dioxide diffuses from the atmosphere, through pores in leaf
surfaces, called stomata, which are formed by a pair of guard cells. The
carbon dioxide then diffuses to the photosynthetic cells of the leaf
mesophyll. The rate of diffusion of carbon dioxide and availability of carbon
dioxide often limit the rate and amount of photosynthesis that occurs in a
plant. (The mechanisms of stomatal operation are discussed in Biology 203,
although we will observe stomata in the leaf surfaces in the laboratory.)
3.
Chloroplasts
The pigments needed for photosynthesis are located in the chloroplasts.
Recall that the chloroplast has a double membrane with a series of internal
stacked membranes. Light energy is captured by the pigments located on
the special membranes in the chloroplast called thylakoids, which are
folded into disk-shaped stacks called grana. The interior compartments of
thylakoids serve as reservoirs for hydrogen ions (H + ) that are needed for
producing ATP.
The reactions of photosynthesis that are involved in the transformation of
light energy to chemical energy, the light-dependent reactions, occur on
the thylakoid membranes of the chloroplast.
The reactions needed to produce carbohydrates occur in the stroma region
of the chloroplast. Enzymes are located here. These reactions are known as
the Calvin cycle.
Photosynthesis - 4
4.
Light Waves
Light is a form of electromagnetic radiation. Visible light is a combination of
many wavelengths that we can see as different colors (of the rainbow) in the
range of 380 - 750 nm. Each wavelength is associated with a specific
photon, or particle of energy. In general, shorter wavelengths have more
energy.
The light absorbing photosynthetic pigments do not absorb all wavelengths of
light equally. Some light energy cannot be absorbed (and is reflected
instead) and some is transmitted, or passed through the chloroplasts.
The absorption of different wavelengths, or absorption spectrum, of light
by the photosynthetic pigments can be demonstrated in the laboratory using
spectrophotometers. The light waves most absorbed and most useful to
photosynthesis are reds and blues. Not surprisingly, green light is absorbed
poorly. One can also measure rates of photosynthesis in different
wavelengths to generate an action spectrum. This is done by growing
plants in light boxes that have just one wavelength of light.
Absorption Spectrum
Action Spectrum
Englemann's Action Spectrum of Spirogyra and Bacteria attracted to the Oxygen evolved.
Photosynthesis - 5
5.
The Light Absorbing Pigments of Photosynthesis
Chlorophyll is the primary pigment that absorbs light energy in
photosynthesis. In plants, there are two forms of chlorophyll (a, which has a
methyl group, and b, which has an aldehyde group) as well as important
accessory pigments, the carotenes. Each pigment absorbs certain
wavelengths, and collects and concentrates light energy for the
photosynthetic process. In addition the carotenes may function to protect
chlorophyll molecules from being damaged by too intense light. The red and
blue phycocyanin pigments can also absorb light and serve as accessory
pigments in photosynthesis.
When pigment molecules absorb energy from light, electrons in the molecules
are excited and moved to a higher energy orbital in an excited state. The
energy level of a photon absorbed and the rise in energy level to the higher
energy orbital must match, which is why only certain wavelengths can be
absorbed by certain pigments. This rise in energy is temporary. Electrons
fall back to their ground state if the energy is not transferred elsewhere.
When the electrons fall back to their ground state energy is given off as
heat, or sometimes as light (fluorescence). Chlorophyll a does both,
fluorescing as red light.
Excitation of Isolated Chlorophyll Molecules
Excitation of Chlorophyll in Chloroplasts
Photosynthesis - 6
The photosynthetic pigment molecules do not work alone. They are arranged
on the thylakoids of the chloroplast in clusters of about 300 pigment
molecules in a protein matrix to form a light-harvesting or antenna
complex that gathers and transfers energy to a photochemical reaction
center, embedded in a transmembrane protein complex. The reaction
center has a special chlorophyll a molecule along with the primary
electron acceptor (which accepts the electrons released from chlorophyll
a). There are two such complexes found in the chloroplasts, called
Photosystem I and Photosystem II. Each has a unique electron acceptor.
The reaction centers of Photosystems I and II are activated by slightly
different wavelengths of light. The reaction center in Photosystem I
absorbs light of 700nm best. The reaction center of Photosystem II absorbs
wavelengths of 680nm. Each thylakoid has thousands of the two different
photosystems. Both are needed for photosynthesis.
6.
Electron Transport System Molecules (Energy Transfer Molecules)
As discussed previously, many chemical reactions of metabolism are coupled
oxidation-reductions that utilize a chain of electron transport molecules.
These electron carriers specialize in oxidizing and reducing at specific energy
levels to minimize energy loss in energy transfers. Both photosynthesis and
cellular respiration rely on such molecules. In the process of photosynthesis,
the electron transport carriers are embedded in the thylakoid membranes.
Some of the molecules which specialize in these energy transfers in the
thylakoids are: NADP+, plastoquinone, two cytochromes, plastocyanin and
ferredoxin
Photosynthesis - 7
The Photosynthesis Pathways
Stage I
Light (Dependent) Reactions - Cyclic and Non-Cyclic
Photophosphorylation (Electron Flow)
• The light-dependent reactions transform light energy into chemical energy that
is trapped and carried by ATP and NADPH to the Calvin Cycle.
• The light-dependent reactions require chlorophyll and occur in the thylakoid
membranes of the grana of the chloroplast.
• During the light-dependent reactions water is split in an enzyme-mediated
reaction located in photosystem II into:
+
H 2 O -----> 2H + 2e- + 1 /2 O 2
Splitting water produces oxygen and provides electrons (transferred to
photosystem II) and Hydrogen for the reduction of NADP+ to NADPH (NADP gains
H + and electrons; the water is oxidized because it loses the H+ and e-).
There are two events in the light reactions: noncyclic and cyclic electron flow (or
noncyclic and cyclic photophosphorylation)
1.
Non-Cyclic Photophosphorylation (Non-cyclic Electron Flow)
Uses
Photosystem I
Photosystem II
Electron Transport System
Inputs
Water
Light energy
Energy Transfer Molecules
ADP and Pi
NADP+
Outputs
ATP
NADPH (reduced form) (from NADP+, the oxidized form)
O2
Organization of the Light Reaction Components in the Thylakoid
Photosynthesis - 8
We often diagram the processes of the light reactions to show the relative energy
levels of the electrons as they are moved, even though, as shown, the molecules of
photosystems I and II and the electron transfer molecules are embedded within the
thylakoid membranes, organized in a way that best facilitates the process. It is
useful to be familiar with both perspectives.
Light-Dependent Reactions of Photosynthesis or Non-Cyclic Electron Flow
How non-cyclic electron flow works:
• Light energy hitting photosystem II excites an electron in the p680 reaction
center causing the chlorophyll molecule to lose an electron to the primary
electron acceptor. A total of two electrons are actually lost from chlorophyll.
The electrons are transferred to the electron transport molecules and passed
slowly “down” the chain releasing energy. Some of this energy is used to
produce a H+ gradient within the thylakoid. This gradient ultimately drives the
mechanism that produces ATP by chemiosmosis.
•
The oxidized chlorophyll a molecule needs to replace its electrons. The
electrons lost by photosystem II are replaced by the splitting of water.
Water’s electrons are passed to photosystem II; its H+ is used in the H+ gradient
(and can be passed to NADP+) and its oxygen is released as oxygen gas
molecules.
•
Light energy also hits photosystem I, exciting its reaction center chlorophyll a
electrons. They are picked up by the primary electron acceptor, transferred to
ferredoxin and then to NADP+, which will also pick up a H+ forming NADPH.
•
The electrons released from photosystem II, once they have passed through the
electron transport system, are used to replace the electrons lost by chlorophyll
in photosystem I.
Photosynthesis - 9
To summarize, the low energy electrons from water are elevated in
energy by passing through both photosystem II and photosystem I and
trapped by NADP + where their potential energy will be used for the
high-energy reduction of carbon in the Calvin cycle. Along the way,
ATP, needed for the endergonic Calvin cycle, is also produced.
Non-cyclic electron flow is sometimes referred to as the "Z" scheme, because the
electrons do an energy "zig-zag" as they move from water to photosystem II to
photosystem I to NADP+ . The process of using both photosystems has an
enhancement effect so that the overall rate and efficiency of light capture is
higher than if there were just one photosystem of if the two photosystems worked
independently of each other.
2.
Cyclic Photophosphorylation (Cyclic Electron Flow)
Uses
Photosystem I
Electron Transport System
Inputs
Light energy
Outputs
ATP (from ADP and Pi)
Cyclic Photophosphorylation only uses Photosystem I and produces only ATP.
Cyclic electron flow dates to the earliest photosynthetic bacteria functioned
to generate energy, not to facilitate the synthesis of organic fuel molecules.
The electrons captured could not be used in the reduction of carbon. In
plants, cyclic photophosphorylation still functions to generate ATP as a
complement to the non-cyclic flow.
In cyclic photophosphorylation, electrons released from the chlorophyll p700
are returned back to Photosystem I after passing through the chain of
electron carriers. ATP is produced by chemiosmosis.
Cyclic Photophosphorylation
Photosynthesis - 10
Chemiosmotic synthesis of ATP in the chloroplast
In photosynthesis, the molecules of electron transport system are located in the
thylakoid membrane. The energy released from electron transport systems is
used to move Hydrogen ions (H+ ) from the stroma into the inner thylakoid
compartments by active transport. This concentration of H+ in the inner
compartment of the thylakoid establishes a concentration and an electrical
gradient (the proton-motive force) in the thylakoid compartment that has an
inherent (potential) energy value. This is not insignificant. The pH gradient goes
from pH 5 within the thylakoid membrane to pH 8 in the stroma (a 1000 X
difference).
The accumulated H+ ions diffuse through the ATP synthase protein complex
channels in the thylakoid membranes that are coupled to ATP synthesis. As the H+
ions flow down the gradient in the protein channels, their energy is used to make
ATP from ADP and P on the other side of the thylakoid membrane in the stroma.
Note:
Peter Mitchell won a Nobel prize in 1978 for determining the chemiosmotic
theory of ATP synthesis. ATP is synthesized in the mitochondria during
cell respiration by a similar mechanism.
Chemiosmotic synthesis of ATP in the Thylakoids
Photosynthesis - 11
Stage II
The Calvin Cycle and Carbon Fixation
The second set of reactions for photosynthesis is known as the Calvin cycle. The
reactions of the Calvin cycle do not use light energy for the energy source. They
use the ATP produced in the light reactions for their energy source, and the
energy transfer molecule, NADPH to provide Hydrogen and electrons for the
reduction of carbon.
Carbohydrate molecules are produced in Calvin Cycle in the stroma of the
chloroplast.
In the Calvin cycle carbon dioxide combines with a 5-carbon sugar (Ribulose
bisphosphate) and undergoes a reduction to form 3-carbon molecules. These 3carbon intermediates can be used to regenerate the 5-carbon sugar or be
metabolized to form the carbohydrate, glucose. They can also be used for the
synthesis of the organic molecules needed for cell structure and function.
The requirements for the Calvin cycle are:
• Carbon dioxide (CO2)
• NADPH from the light-dependent reactions
• ATP from the light-dependent reactions
• Ribulose bisphosphate, regenerated in the cycle
• Appropriate enzymes for each step in the cycle. Of these, Ribulose
bisphosphate carboxylase/oxidase (Rubisco) is especially important.
The Metabolic Intermediate in Process is:
• G3P (Glyceraldehyde-3-Phosphate)
The Calvin cycle produces:
• Glucose (carbohydrate)
• Ribulose bisphosphate, regenerated in the cycle
• Water
The parts of the Calvin cycle are
• CO2 Fixation
• Reduction
• Regeneration
• Output or Surplus
The process we are discussing was determined using radioisotopes of 14C. The
researchers (Calvin, Benson and others) won a Nobel prize for this. This pathway
is called 3-carbon photosynthesis, (because of the 3-C G3P intermediate) to
distinguish it from an alternative pathway known as 4-carbon (C-4)
photosynthesis, to be discussed in a bit. To some, the Calvin cycle looks like a
carbon cut-and-paste dance. It is. It's easy to follow the maze if one counts
carbons. It also helps to remember that without this happening, you'd starve!
Photosynthesis - 12
Overview of the Calvin Cycle
The "Parts"
Carbon Fixation and the Reduction Phase of the Calvin Cycle
This must repeat 6 times to form 1 glucose.
Æ
Æ
C O 2 Fixation
6CO 2 + 6RuBP (Ribulose 1,5-Bisphosphate) Æ 6Hexose Phosphate (not shown) Æ
12 PGA (3-Phosphoglycerate)
Reduction of PGA to G3P
12 PGA (3-Phosphoglycerate) +12ATP Æ 12 1,3-Biphosphoglycerate +12NADPH Æ
12 G3P (Glyceraldehyde-3-Phosphate) + (12ADP +12Pi +12NADP+)
Summary (CO2 Fixation and Reduction of PGA to G3P)
6CO 2 + 6 RuBP + 12 ATP + 12 NADPH Æ 12 G3P (or PGAl).
Photosynthesis - 13
Fate of G3P
The 12 molecules of G3P produced will be used for two purposes: Regeneration of
ribulose bisphosphate (RuBP) and synthesis of glucose (the output from the cycle).
The Output (Surplus Phase) - Producing Glucose
The remaining 2 molecules of G3P are converted into one Glucose molecule
Æ
2
Æ
Æ
Æ
2 G3P --------------------------------------------------------------------------> 1 Glucose
Although glucose is the typical end product of photosynthesis, plants do not store
or transport glucose. Sucrose (synthesized from fructose-phosphate and glucosephosphate) is the typical solute translocated throughout the plant, and, as learned,
plants typically store starch. In addition, plants are capable of synthesizing all of
their organic molecules (amino acids, lipids, etc.) from photosynthetic
intermediates, notably G3P and glucose phosphate. Plants are very versatile in
their synthetic abilities compared to animals. There are also whole groups of plant
products, called secondary metabolites, synthesized directly or perhaps as byproducts of plant activity. Some secondary metabolites are protective in nature,
such as toxins; some, such as lignin, are used in plant structure.
Regeneration of ribulose bisphosphate (RuBP)
This will use 10 of the 12 G3P molecules from the reduction phase. Only the
carbon backbone is illustrated. Water, ADP and P are given off in the process.
10
+ 6 ATP Æ
6
Summary
10 G3P + 6 ATP ----> 6 RuBP + 6 H2O (+ 6 ADP)
+ 6ADP + 6 H2 O
Photosynthesis - 14
How the regeneration of RuBP works:
(Take G3P in groups of 5 to produce 3 Rubs. Repeat to get 6RuBPs regenerated)
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
remove "head"
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
+ ATP ----> RuBP
remove "head" C
C
C
C
C
C
C
+ ATP ----> RuBP
C
C
C
C
C
+ ATP ----> RuBP
C
C
C
C
C
+ ATP ----> RuBP
C
C
C
C
C
+ ATP ---->
RuBP
C
C
C
C
C
+ ATP ---->
RuBP
remove "head"
C
C
C
C
C
C
C
remove "head"
Photosynthesis - 15
Calvin Cycle - the Biochemistry Version:
Photosynthesis - 16
Photosynthesis Productivity
It takes 18 ATP and 12 NADPH to make one molecule of glucose. Much of the light
energy that hits the surface of plants is not absorbed and not available. And much
of the light that hits earth does not hit photosynthetic surfaces of plants.
The common limit to photosynthesis is the availability of carbon dioxide, which
diffuses from the atmosphere into the leaves through pores in the leaf surface,
called stomata. Unfortunately, these pores also permit diffusion of water from
the interior of the leaf, and can be a significant source of water loss to the plant.
(As much as 90% of the water taken in to a plant is lost this way. This
evaporation of water through the stomata (called transpiration) is also used by
the plant to generate a tension that serves to pull water up through the xylem
from the roots to stems and leaves, so this water loss is not a completely
negative thing for the plant. Transpiration will be studied in Biology 203.)
Under hot and dry conditions, many plants must close their stomata to minimize
water loss. During these times the ratio of oxygen to carbon dioxide in the leaf
increases, which favors a process that competes with photosynthetic output
called photorespiration.
Photorespiration
Rubisco, the enzyme that brings CO2 and RuBP together, works only when the
concentration of CO2 is high relative to the level of O2, and when CO2 levels drop,
the enzyme, Rubisco, combines RuBP with O2 which is then exported from the
chloroplast and broken down into other compounds (in mitochondria and
peroxisomes) that actually release CO2 rather than reduce it to PGA -> G3P. The
Calvin cycle is subverted. Photorespiration decreases the photosynthetic output of
the plant. This reduction in photosynthesis can be significant!
Comparison of CO2 fixation and photorespiration with Rubisco
Photosynthesis - 17
C-4 Photosynthesis
Some plants have evolved mechanisms to minimize the effects of
photorespiration. Such plants have an internal anatomy (the Kranz anatomy) that
separates the oxygen producing light-dependent reactions of photosynthesis from
the Calvin cycle. These two processes occur in modified chloroplasts that are
located in different cells within the leaves. In addition, these plants have
alternative methods of trapping and accumulating carbon dioxide so that
photosynthesis can proceed when stomata are closed. The photosynthetic
mechanism used in these plants if called C-4 photosynthesis. The typical
photosynthesis is called C-3 Photosynthesis.
Modified (Kranz) anatomy of the C-4 plant leaf
Light reactions occur in leaf mesophyll cells of C-4 plants, just as in C-3 plants.
The chloroplasts of the C-4 plant mesophyll cells have lots of grana. Recall that
oxygen is produced only in the light reactions. (C-3 and C-4 leaf structure will be
reviewed in lab.)
The Calvin Cycle, however, occurs in the chloroplasts of special bundle sheath
cells, cells that surround the veins of the leaf. These chloroplasts have almost no
grana, so very little, if any, oxygen is produced in these cells. The bundle sheath is
the outer layer of the leaf veins, which normally add strength to the leaf.
C-3 Leaf Structure
C-4 Leaf Structure
Modified carbon dioxide fixation system in the C-4 plant.
CO 2 combines with RuBP in the first step of the Calvin cycle. In C-4 plants, CO2 is
fixed before the Calvin cycle starts. Any CO2 that enters the leaf, at any time,
can be combined with a common metabolic intermediate, PEP (phosphoenol
pyruvate) in mesophyll cells, forming a 4-carbon acid, Oxaloacetic acid
(oxaloacetate) that is converted to malic acid (malate).
Photosynthesis - 18
Malate is transferred to the bundle sheath cells where CO2 can be released for refixation in the Calvin cycle forming the 3-carbon pyruvate by-product. Pyruvate is
returned to the mesophyll cells where it is converted back to PEP.
This is an efficient trap for CO2 since the 4-carbon acid can accumulate during
non-light periods, concentrating CO2 when photosynthesis cannot occur, and can be
used during periods of low moisture when stomata are closed to prevent water
loss. The separation of Calvin cycle from O2 production raises the efficiency when
O 2 levels are high (bright sunlight and higher temperatures). However,
regenerating PEP requires ATP, so C-4 photosynthesis may not always be more
productive than the C-3 pathway, as shown when temperatures are moderate and
moisture levels are adequate. The pathway is genetic; plants can't choose.
C-3 and C-4 Photosynthesis rate comparisons for CO 2 and O 2 levels and for Temperature
Note C-3 plants perform better at lower temperatures.
Photosynthesis - 19
CAM – A Water Conservation Mechanism
Some C-3 plants close stomata during the daylight hours to conserve water. They
open their stomata during the night and use the CO2 acid trap of C-4 plants. They
store the acids accumulated during the dark in the vacuole. In the daytime, this
CO 2 is released for "normal" C-3 photosynthesis, while the stomata can remain
closed to prevent excessive water loss. The name, CAM, is derived from the types
of plants in which it was first discovered, Crassuleans, and for the fact the CO2
trapped forms acids. A significant difference between C-4 plants and CAM plants
is that CAM plants do not have Kranz anatomy. They perform the Calvin cycle in
the same cells as the light reactions. It is strictly a water conservation benefit
for photosynthesis.
Comparison of C-4 and CAM Plants