Chapter 10: Photosynthesis
Mechanisms for Increasing CO2 Concentration
CAM and C4 Plants
Closing the stomata causes
CO2 delivery, and thus photosynthesis, to stop.
C4 photosynthesis and CAM function as
CO2 pumps. They minimize photorespiration when stomata are closed and CO2 cannot diffuse in directly from the atmosphere.
CAM Plants
During the night, CAM plants take in CO2 and temporarily fix it into organic acids. During the day, CO2 is released from the stored organic acids and used by the Calvin cycle, thus minimizing the effects of photorespiration.
The Calvin cycle has three phases:
Fixation Reduction Regeneration
The Fate of Sugar Produced by Photosynthesis
G3P molecules produced by the Calvin cycle are often used to make glucose and fructose, which can be combined to form sucrose. In rapidly photosynthesizing cells where sucrose is abundant, glucose is temporarily stored in the chloroplast as starch. Because starch is not water soluble, it is broken down at night and used to make more sucrose for transport throughout the plant.
C4 photosynthesis, carbon fixation and the Calvin cycle occur in separate types of cells. This occurs in a three-step process:
PEP carboxylase fixes CO2 in mesophyll cells. The 4-carbon organic acids produced travel to bundle-sheath cells. The four-carbon organic acids release a CO2 molecule, which rubisco uses to form 3-phosphoglycerate, thus initiating the Calvin cycle.
Photosynthesis contrasts with cellular respiration because:
Photosynthesis is endergonic (consumes energy). Reduces CO2 to sugar Cellular respiration is exergonic (releases energy). Oxidizes sugar to CO2
The Regulation of Photosynthesis
The rate of photosynthesis is finely tuned, to reflect changes in environmental conditions and use resources efficiently. For example, light triggers synthesis of photosynthetic proteins, and high sugar levels inhibit synthesis of photosynthetic proteins and stimulate production of proteins required for sugar processing and storage.
Plants Must Balance
Water Preservation and CO2 Delivery
In crassulacean acid metabolism (CAM) plants,
carbon fixation and the Calvin cycle are separated in time. These plants, which also live in hot, dry habitats, keep their stomata closed all day and open them only at night.
Stomata are normally open during the
day and closed at night.
The C4 pathway
occurs mostly in plants from hot, dry habitats, limits the damaging effects of photorespiration by spatially separating carbon fixation and the Calvin cycle. During carbon fixation, C4 plants incorporate CO2 into 4-carbon (C4) organic acids instead of 3-phosphoglycerate (performed by C3 plants).
One turn of the Calvin cycle fixes
one molecule of CO2.
In C4 plants, the reactions catalyzed by PEP carboxylase and rubisco are separated in
space
In CAM plants the reactions catalyzed by PEP carboxylase and rubisco are separated in
time
Chlorophylls have a long
"tail" made of isoprene subunits, and a "head" consisting of a large ring structure with a magnesium atom in the middle. Light is absorbed in the head
The Importance of Rubisco in the Reduction phase
3-Phosphoglycerate is phosphorylated by ATP and then reduced by electrons from NADPH. The product is the phosphorylated sugar glyceralaldehyde-3-phosphate (G3P). Some of the G3P that is synthesized is drawn off to manufacture glucose and fructose.
The overall reaction when glucose is the carbohydrate can be written as:
6 CO2 + 12 H2O + light energy ---> C6H12O6 + 6 O2 + 6 H2O
Chemiosmosis
A process for synthesizing ATP using the energy of an electrochemical gradient and the ATP synthase enzyme.
plastocyanin
A small protein that shuttles electrons at the end of photosystem II's ETC to photosystem I during photosynthesis. Carries the electron back across the thylakoid membrane and donates it to photosystem I, thus physically linking the two photosystems.
chloroplasts
A structure in the cells of plants and some other organisms that captures energy from sunlight and uses it to produce food.
OIL RIG
Oxidation is Loss (of electron), Reduction is Gain (of electron)
The Importance of Rubisco in the Fixation phase
The Calvin cycle begins when CO2 reacts with RuBP. This phase fixes carbon and produces two molecules of 3-phosphoglycerate.
The protons transported by plastoquinone result in a large
concentration of protons in the thylakoid lumen.
Photosynthesis is the
conversion of light energy to chemical energy stored in the bonds of carbohydrates. It consists of two linked sets of reactions.
Pigments that absorb blue and red photons are the most effective at
driving photosynthesis. Chlorophylls absorb these wavelengths - they are the main photosynthetic pigments.
Excited electrons in chloroplasts may
drop back down to a low energy state, causing fluorescence. excite an electron in a nearby pigment, inducing resonance. be transferred to an electron acceptor in a redox reaction.
protons diffuse down their
electrochemical gradient.
Photosynthetic reactions are linked by
electrons, which are released in the light-dependent reactions then uses these electrons and the potential energy in ATP to reduce CO2 to produce carbohydrates in the Calvin Cycle
How Does Photosystem II Work?
energy reaches the reaction center of the photosystem, the reaction center chlorophyll is oxidized a high-energy electron is donated to the electron acceptor pheophytin The electron is passed to an electron transport chain (ETC) in the thylakoid membrane, producing a proton gradient and driving ATP production via ATP synthase. This triggers chemiosmosis and ATP synthesis in the chloroplast.
Each photon and wavelength has a specific amount of
energy. The energy of a photon of light is inversely proportional to its wavelength. Shorter wavelengths such as ultraviolet light have more energy than longer wavelengths such as infrared light.
Photosystems I and II work together to produce an
enhancement effect, in which photosynthesis increases dramatically when cells are exposed to both red and far-red light.
Photosystem I and ATP synthase are much more common in the
exterior, unstacked membranes
the redox reactions that occur in the ETC result in protons being pumped
from one side of an internal membrane to another. The proton gradient that builds up drives ATP production via ATP synthase.
Thylakoids form stacks called
grana
Photosystem II is much more abundant in the
interior, stacked membranes of grana.
Rubisco is 'inefficient' because although it does catalyze the addition of CO2 to RuBP,
it also catalyzes the addition of O2 to RuBP. Oxygen and carbon dioxide compete at the enzyme's active sites, which slows the rate of CO2 reduction.
Stomata are
leaf structures where gas exchange occurs. They consist of two guard cells that change shape to open or close. When a leaf's CO2 concentration is low during photosynthesis, stomata open to allow atmospheric CO2 to diffuse into the leaf and its cells' chloroplasts.
Photosynthesis consists of two linked sets of reactions:
light-dependent reactions produce O2 from H2O, and Calvin cycle reactions produce sugar from CO2.
Pigments
molecules that absorb only certain wavelengths of light.
Fluorescence
occurs when a pigment absorbs a photon and the electron gets excited, but then falls back to its ground state. Some of the absorbed energy is released as heat and the rest is released as electromagnetic radiation (light).
three turns of the Calvin cycle are required to produce
one molecule of glyceraldehyde 3-phosphate (G3P)
How Does Photosystem II Obtain Electrons?
oxidizes water to replace electrons used during the light reactions. When excited electrons leave and enter the ETC, the photosystem becomes so electronegative that enzymes can remove electrons from water, leaving protons and oxygen.
Photosystem II "splits" water to replace its lost electrons and in the process produces
oxygen (oxygenic photosynthesis)
As a particle, light exists in discrete packets called
photons
The capture of light energy by photosystem II to produce ATP is called
photophosphorylation
When O2 and RuBP react in rubisco's active site, one of the products undergoes a process called
photorespiration. Photorespiration "undoes" photosynthesis because it consumes energy and releases fixed CO2.
When photorespiration occurs, the rate of _____ declines drastically
photosynthesis
The energy transformation of the light-dependent reactions and the carbon dioxide reduction of the Calvin cycle are two separate but linked processes in
photosynthesis.
Chlorophyll molecules work together in groups (+ carotenoids, proteins), forming a complex called a
photosystem
There are two types of reaction centers:
photosystem I and photosystem II
Thylakoid membranes contain large quantities of
pigments. The most common pigment is chlorophyll.
The ETC includes
plastoquinone (PQ), which shuttles electrons from pheophytin across the thylakoid membrane to a cytochrome complex.
An action spectrum shows the
rate of photosynthesis vs. wavelength.
Electrons in the electron transport chain participate in
redox reactions and are gradually stepped down in potential energy.
The CO2-fixing enzyme is called
ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco).
plastoquinone (PQ)
shuttles electrons from pheophytin across the thylakoid membrane to a cytochrome complex.
In plants, CO2 enters photosynthetic tissue through
stomata. The CAM and C4 pathways increase CO2 concentrations inside the leaves of some species and make photosynthesis more efficien
Photons may be absorbed, transmitted, or reflected when they
strike an object.
The _____ is the site of ATP production because the proton gradient established by photosystem II drives protons into the _____.
stroma
The fluid-filled space between the thylakoids and the inner membrane is the
stroma
Photosynthesis requires
sunlight, carbon dioxide, and water, and produces oxygen as a by-product.
Rubisco is found in all photosynthetic organisms that use
the Calvin cycle to fix carbon, and is thought to be the most abundant enzyme on Earth.
Photosynthesis occurs in
the chloroplasts of green plants, algae, and other photosynthetic organisms.
In the light-capturing reactions, excited electrons are used to produce
the electron carrier NADPH or are donated to an electron transport chain, which results in the production of ATP via chemiosmosis.
Chemiosmosis results when
the flow of protons through ATP synthase causes a change in its shape, driving the phosphorylation of ADP.
The tail of chlorophylls keeps
the molecule embedded in the thylakoid membrane.
The electromagnetic spectrum
the range of wavelengths of electromagnetic radiation.
The Calvin Cycle occurs in
the stroma.
Photosystem II and the cytochrome complex are located in the
thylakoid membraine
The photosystem itself and NADP+ reductase are anchored in the
thylakoid membrane.
Chloroplasts are surrounded by two membranes:
thylakoids: The internal membranes of chloroplasts that form flattened, vesicle-like structures grana: stroma: The fluid-filled space between the thylakoids and the inner membrane
At the reaction center, excited electrons are
transferred to a specialized chlorophyll molecule that acts as an electron acceptor. When this electron acceptor becomes reduced, the electromagnetic energy is transformed to chemical energy.
NADP+ reductase
transfers a proton and two electrons from ferredoxin to NADP+, forming NADPH.
Photosynthesis is the process of
using sunlight to produce carbohydrates.
Electromagnetic radiation that humans can see is called
visible light.
As a wave, light can be characterized by its
wavelength - the distance between two successive wave crests.
Carotenoids absorb
wavelengths of light not absorbed by chlorophyll, thus extending the range of wavelengths that can drive photosynthesis. They also stabilize free radicals, protecting chlorophylls from damage.
Carbon fixation is favored over photorespiration when a cell's
CO2 concentration is high and O2 concentration is low.
The discovery of the Calvin cycle clarified how the ATP and NADPH produced by light-capturing reactions allow cells to reduce
CO2 to carbohydrate
Fixation
Calvin Cycle's first Phase: CO2 reacts with ribulose bisphosphate (RuBP), producing two 3-phosphoglycerate molecules. The attachment of CO2 to an organic compound is called carbon fixation.
Reduction
Calvin Cycle's second phase: The 3-phosphoglycerate molecules are phosphorylated by ATP and reduced by NADPH to produce glyceraldehyde 3-phosphate (G3P).
A strong concentration gradient favoring entry of CO2 is maintained by the
Calvin cycle, which constantly uses up the CO2 in chloroplasts
NADPH Is an Electron
Carrier that can donate electrons to other compounds and thus reduce them. Photosystem I produces NADPH, which is similar in function to the NADH and FADH2 produced by the citric acid cycle.
thylakoids:
The internal membranes of chloroplasts that form flattened, vesicle-like structures grana:
Regeneration
The remaining G3P is used in reactions that regenerate RuBP.
The Importance of Rubisco in the Regeneration phase
The rest of the G3P keeps the cycle going by serving as the substrate for the third phase in the cycle: reactions that result in the regeneration of RuBP.
oxygenic photosynthesis
When Photosystem II "splits" water to replace its lost electrons and in the process produces oxygen Photosystem II is the only known protein complex able to oxidize water in this way.
Electromagnetic radiation is
a form of energy.
The Z scheme
a model of how photosystems I and II interact.
Light is
a type of energy electromagnetic radiation that acts both particle-like and wave-like.
The carotenoids
absorb blue and green light and reflect and transmit yellow, orange, and red light.
The chlorophylls (chlorophyll a and chlorophyll b)
absorb red and blue light and reflect and transmit green light.
Biologists use a graph called an ________ to study pigments.
absorption spectrum This spectrum plots the wavelength of light absorbed by pigment molecules.
The photosystem's antenna complex is
composed of accessory pigment molecules. resonance-When a red or blue photon strikes a pigment molecule in the antenna complex, the energy is absorbed and an electron excited. This energy is passed to another chlorophyll molecule, exciting another electron.
The reactions that produce sugar from carbon dioxide in the Calvin cycle are
Light-independent These reactions require the ATP and NADPH produced by the light-dependent reactions.
pheophytin
In photosystem II, a molecule that accepts excited electrons from a reaction center chlorophyll and passes them to an electron transport chain.
The Z scheme explains the enhancement effect
Photosynthesis is more efficient when both 680-nm and 700-nm wavelengths are available (hence the names of the pairs of reaction-center chlorophyll molecules), allowing both photosystems to run at maximum rates.
cyclic photophosphorylation
Photosystem I occasionally transfers electrons to photosystem II's electron transport chain to increase ATP production, instead of using them to reduce NADP+.
Summary of Photosystems I and II
Photosystem II produces a proton gradient that drives the synthesis of ATP. Photosystem I yields reducing power in the form of NADPH. Although several groups of bacteria have just one of the two photosystems, the cyanobacteria, algae, and plants have both.
How Does Photosystem I Work?
Pigments in the antenna complex absorb photons and pass the energy to the reaction center. Excited electrons from the reaction center of photosystem I are passed down an ETC of iron- and sulfur-containing proteins to ferredoxin. The enzyme NADP+ reductase transfers a proton and two electrons from ferredoxin to NADP+, forming NADPH. The photosystem itself and NADP+ reductase are anchored in the thylakoid membrane.
Carbon fixation
The attachment of CO2 to an organic compound
Electrons from Pheophytin Enter
an ETC in the thylakoid membrane. This ETC is similar in structure and function to the ETC in mitochondria.
A photosystem consists of two major elements:
an antenna complex and a reaction center, as well as proteins that capture and process excited electrons.
When a photon strikes chlorophyll, its energy can be transferred to
an electron in the chlorophyll head. The electron becomes excited, raised to a higher energy state.
ferredoxin
an iron sulfur protein that acts as another mobile electron carrier of the pathway; then transfers to the NADP+ reductase
Purple non-sulfur and purple sulfur bacteria, with their single photosystem, cannot oxidize water and thus perform
anoxygenic photosynthesis.
Energy is transferred inside the
antenna complex, from one molecule to the next, until it reaches the reaction center
redox reactions result in protons
being pumped from one side of the membrane to the other. Proton concentration inside the thylakoid increases1000-fold.
Carotenoids are classified into two groups:
carotenes and xanthophylls.
In the Calvin cycle, the enzyme rubisco
catalyzes the addition of CO2 to a five-carbon compound. Subsequent reactions use the ATP and NADPH synthesized in the light reactions, yielding a molecule required for carbohydrate production.
There are two major classes of pigments in plant leaves:
chlorophylls and carotenoids.
