Download Photosynthesis

I. General Overview
Figure 1: Photosynthesis & Carbon Cycling
•
Photosynthesis:
II. Adaptations for Photosynthesis
Figure 2: Leaf (C3) Structure
Cuticle: waxy layer secreted by the epidermis that limits the amount of H2O loss from the leaf & entry of pathogens.
•
Epidermis (upper & lower): cuboidal cells that work in concert with the cuticle to limit water loss & prevent disease.
Provides protection for the leaf’s delicate inner structure.
•
1
Stomata: small openings in leaf’s underside. Allow the entry of CO2 into the leaf structure & the release of O2 & H2O
(v). Enable the transport of water from roots to leaves. Found on the upper leaf surface in floating species.
•
Guard Cells: pair of cells that surround each stomate to regulate the amount of transpirational water loss from the
leaf. When exposed to blue wavelengths in the morning, guard cells become permeable to K+ which serves to reduce
the water potential (Ψ)of their cytoplasm. As a result, the guard cells take in water & become turgid, causing them to
separate & open the stomates.
•
Figure 3: Guard Cells & Stomata (Daytime)
Figure 3.1: Guard Cells & Transpiration
Transpiration drives the upward movement of water molecules w/in the xylem tissue of the roots, stems, & leaves.
This occurs because water molecules are joined by H-bonds & exert a pull on one another as they evaporate. If soil H2O
availability is low, transpiration may lead to the desiccation of the plant body.
•
2
Figure 3.2: Guard Cells & Stomata (Night & Water Stress)
As plant cells lose water via transpiration during the night or during hot, dry conditions, they produce Abscissic Acid
(ABA) that opens K+ exit channels. The mass efflux of K+ out of the guard cells raises the ψ of their cytoplasm, causing
water to follow & the guard cells to become flaccid. This has the effect of closing the stomates to prevent continued
transpirational water loss.
•
Spongy Mesophyll: loosely packed cells that form a network of air spaces to increase surface area for CO2 uptake &
the loss of O2 & H2O (v).
•
Pallisade Mesophyll: columnar cells that act as the main site of photosynthesis, for they contain the highest
concentration of chloroplasts within the plant (20-100/cell).
•
Figure 4: Chloroplast (Pallisade Mesophyll)
Chloroplast are *Plastids that contain the photosynthetic pigments (mainly chlorophylls) for the conversion of sunlight
to chemical energy in the form of sugars.
a) Descended from relatives of modern Cyanobacteria (“blue-green algae”).
b) Consists of an inner & an outer membrane. The inner membrane encloses a fluid-filled space called the Stroma,
which contains enzymes for producing carbohydrates from CO 2 & H2O.
c) Suspended in the stroma are membranes that form a set of interconnected disk-like sacs called Thylakoids.
Thylakoid membranes contain chlorophyll & other photosynthetic pigments.
d) Thylakoid membranes, like the inner mitochondrial membrane, are involved in ATP synthesis. Thylakoids are
organized into stacks called Grana.
•
*Plastids function in pigment production (chromoplasts) as well as sugar synthesis (chlor0plasts) & starch storage (leucoplasts).
Vascular Bundle: contains conducting tissue that is continuous with the stem. Xylem serves to bring water & dissolved
solutes up to the leaf tissues & Phloem transports sugars to other parts of the plant.
•
3
III. Properties of Light
Figure 4: Electromagnetic Spectrum
EM Spectrum consists of all forms of radiation that travel in waves that are differentiated from one another by their
respective Wavelengths (λ).
•
The visible portion of the electromagnetic spectrum ranges from wavelengths of 380 nm (violet) to 760 nm (red).
Visible light is composed of small particles called Photons. The energy of a photon is inversely proportional to its λ.
•
Photosynthetic Pigments
• Substances that absorb specific λ’s of visible light are Pigments. The ability of a pigment to absorb various
wavelengths of visible light can be measured by placing a solution of the pigment in a Spectrophotometer.
Figure 5: Measuring Absorption: Spectrophotometer
4
Figure 5.1: Absorption Spectrum of Chlorophyll Pigments a & b
Blue
•
Red
Absorption Spectrum:
a) The absorption spectrum for the chlorophyll pigments suggests that they most effectively absorb blue & red visible
wavelengths.
Figure 5.2: Action Spectrum of Photosynthesis: T.W. Englemann
Blue
Red
Quantity to be Measured:
Method of Measure:
Results:
a) The distribution pattern of the aerobic bacteria around the spirogyra represented the first Action Spectrum for the
photosynthesis. This pattern illustrates how photosynthetic rate is dependent on specific λ’s of visible light.
b) When Englemann compared the action spectrum of photosynthesis with the absorption spectrum for the
chlorophylls, they very nearly matched, suggesting that chlorophyll played a vital role in driving photosynthesis.
5
•
Action Spectrum:
Figure 5.3: Chlorophyll Absorption Spectrum vs. Photosynthesis Action Spectrum
The action spectrum does not quite match the absorption spectrum for chlorophylls because secondary pigments
called Carotenoids absorb certain other visible wavelengths. Their presence increases the variety of visible
wavelengths that can drive photosynthesis.
•
Figure 6: Chlorophyll Structure & Excitation
•
Chlorophyll:
6
The types of pigments in the thylakoid membrane include:
a) Chlorophyll a: contains a methyl group (-CH3) on its porphyrin ring. Instrumental in the light reactions that convert
light energy into chemical energy. Receptive to wavelengths of 662nm (red) & 430nm (blue).
b) Chlorophyll b: contains a carbonyl group (-CHO) on its porphyrin ring. Does not participate in the light reactions
directly, but rather passes photons to chlorophyll a. Receptive to wavelengths of 642nm (red) & 453nm (blue).
c) Carotenoids: along with chlorophyll b, serve to channel photons to chlorophyll a. Where as chlorophyll a & b
absorb the same wavelengths, carotenes (hydrocarbons) & xanthophylls (oxygenated carotenoids) are able to
absorb wavelengths that chlorophyll a & b cannot, thus broadening the variety of wavelengths that can drive
photosynthesis.
•
IV. Chemistry of Oxygenic Photosynthesis
General Formula
6CO2 + 6H2O  C6H12O6 + 6O2 (∆G = +686 kcal/mol)
•
H2O (electron source) is completely oxidized to O2 in the presence of light. CO2 is reduced to the sugar PGAL.
Light Dependent Reactions
Figure 7: Photosystem (Thylakoid Membrane)
*Antenna pigments surround & channel photons to the reaction chlorophyll a molecule.
Two photosystems, named for the order of their discovery, function during the light reactions of photosynthesis:
a) Photosystem I: consist of an antenna complex surrounding a reaction center chlorophyll a complex that absorbs
maximally at wavelengths of 700nm (P700).
b) Photosystem II: consist of an antenna complex surrounding a reaction center chlorophyll a complex that absorbs
maximally at wavelengths of 680 nm (P680).
•
•
Light Dependent Reactions:
•
Photosystem:
7
Figure 7.1: Light Reactions: Noncyclic Electron Flow
Figure 7.2: Light Reactions: Noncyclic Electron Flow (Thylakoid View)
*Notice that both forms of chemical energy (ATP, NADPH) formed during noncyclic electron flow collect within the
stroma of the chloroplast, the site of the Calvin-Benson cycle.
8
•
Noncyclic Electron Flow:
a) Upon absorbing 2 photons, the antenna complex of photosystem II channels them to the reaction center chlorophyll
(a) molecule, P680. By absorbing these photons, 2e- in P680 jump to a higher energy state, whereby the molecule
becomes Photooxidized. These e- are captured by a Primary Electron Acceptor w/in the thylakoid membrane.
b) The photooxidation of the reaction center makes it a powerful oxidizing agent -during a process called Photolysis,
an enzyme “splits” water into ½O & 2H+, using its 2e- (from oxygen) to replace those lost by P680.
c) The donated e- travel along an Electron Transport Chain embedded in the thylakoid membrane. The e- cascade
down the chain & establish a proton gradient by transporting H+ from the stroma into the thylakoid space.
d) The proton gradient stores a great deal of potential energy, which is released as they rush back into the stroma via
ATP Synthases (chemiosmosis). As a result, ADP is Photophosphorylated to ATP. For every H2O molecule “split”, 1
ATP is produced via photophosphorylation, resulting in 12 total ATP’s.
e) Simultaneously, P700 receives 2e- from Photosystem II to replace those lost to its primary electron acceptor. The
donated e- are passed to the enzyme NADP+ Reductase, which stores them in the coenzyme NADPH. For every H2O
split, 1 NADP+ is reduced to NADPH, for a total of 12 NADPH’s.
f) End Products: 12 NADPH, 6O2, 12 ATP (NADPH & ATP are used during the Calvin Cycle to reduce CO2 to sugar).
Figure 7.3: Light Reactions: Cyclic Electron Flow
•
Cyclic Electron Flow:
a) May result when ATP is consumed at a high rate by the many reactions occurring in the stroma, leading to an ATP
deficit. Consequently, NADPH will begin to accumulate as the Calvin Cycle slows down (requires 18 ATP’s). This rise
in NADPH may stimulate a temporary shift from noncyclic to cyclic electron flow until ATP supply catches up with
demand.
9
Sugar Production
Figure 9: Calvin-Benson Cycle
•
Calvin-Benson Cycle:
a) During Carbon Fixation, 3CO2 are incorporated into the skeleton of the 5-carbon sugar RuBP via the enzyme
Rubisco, forming 3 molecules of a 6-carbon compound.
b) The 6-carbon compounds break down to form 6 molecules of the 3-carbon compound 3-phosphoglyceric acid. For
this, the Calvin Cycle is also known as the C3 Pathway & plants that initially fix carbon this way are C3 Plants.
c) 3-phosphoglyceric acid is phosphorylated via the hydrolysis of 6ATP to form 6 molecules of 1,3-diphosphoglyceric
acid, which is reduced by 6NADPH to form 6 molecules of the 3-carbon sugar PGAL (aka G-3-P).
d) In order for the cycle to continue, 5 of the 6 molecules of PGAL are recycled to make more RuBP for the carbon
fixation of additional CO2 –phosphorylated by 3 ATP.
e) Since 1 molecule of PGAL is equivalent to half a glucose molecule, another 3 CO 2 must enter the cycle to generate
another 6 PGAL’s. The 2 net PGAL’s generated by this represent the major products of the Calvin-Benson Cycle.
Fate of Photosynthetic Products
• Once formed, PGAL can be utilized by the plant in the following ways:
a) 50% of the PGAL will be converted into simple sugars that will be consumed during cell respiration to form ATP.
b) Some of the PGAL will be used to form the glucose required to form cellulose, the major structural polysaccharide
within plant cell walls.
c) Some of the PGAL will be modified to form other vital macromolecules (nucleic acids, lipids, amino acids, etc).
d) Excess PGAL will be converted to form starches to be stored mainly within the root system.
*All PGAL that is transported through the plant body via phloem tissue is first converted into the disaccharide sucrose.
10
V. Alternatives Mechanisms of C-Fixation
Figure 10: Conditions for Photorespiration
Under prolonged hot, arid conditions, guard cells lose turgidity & begin to close, preventing continued gas exchange.
As a result, CO2 levels within the leaf’s spongy mesophyll drop as it continues to be consumed for Calvin-Benson & the
levels of O2 by-product rises.
•
Under these conditions (high O2, low CO2), the enzyme RUBISCO fixes oxygen & introduces it into the Calvin-Benson
cycle. This leads to a metabolic pathway known as Photorespiration …
•
Figure 10.1: Photorespiration Pathway
Like cell respiration, this pathway consumes O2 & releases CO2 (hence the term “photorespiration”). However, this
pathway does not result in the formation of sugar (PGAL) & is thus seemingly wasteful. The benefit of photorespiration
under hot & arid atmospheric conditions is that RUBISCO acts as an antioxidant to reduce the amount of free oxygen
within the cell that can lead to the formation of reactive oxygen species (ROS) …
•
11
Figure 10.2: Antioxidant Function of RUBISCO
•
Photorespiration:
a) Although “wasteful” in the sense that it results in no PGAL produced, the need for photorespiration to prevent ROS
production outweighs the need for sugar production under prolonged hot & arid conditions.
b) Plants other than C3 plants enjoy the “best of both worlds” under these conditions, being able to continue sugar
production even though CO2 levels drop & O2 levels rise within the leaf tissue …
Figure 11: C4 (Hatch-Slack) Pathway
•
C4 Plants:
a) In contrast to C3 plants, C4 plants (sugar cane, grasses) possess leaves in which the palisade cells completely
surround those of the bundle sheath. This allows for the spatial separation of carbon fixation & the rest of the
Calvin-Benson cycle.
12
b) Carbon fixation initially occurs in the mesophyll cells in which CO2 is added to PEP (starting material) via the
enzyme PEP Carboxylase to form oxaloacetate & eventually malate (represents stored CO2). Compared to rubisco,
PEP carboxylase has a much higher affinity for CO2 relative to O2. As a result, it can fix CO2 instead of O2 even
during times when the stomata are closed & CO2 levels are too low for rubisco to be effective (avoids
photorespiration).
c) After CO2 fixation, the mesophyll cells export malate (stored CO2) to the bundle sheath cells through
plasmodesmata. Within the bundle sheath cells, the malate is degraded to release large amounts of CO2, which
establishes a high CO2 to O2 ratio favorable for RUBISCO to introduce CO 2 into Calvin Benson ( PGAL) & avoid
photorespiration.
Figure 11.1: C4 vs CAM Pathway
*CAM plants include succulent (fleshy) plants that store a large amount of water in their tissues (cacti, jades, etc). These plants live
in environments so hot & arid, that stomates can only be opened for gas exchange at NIGHT!
•
CAM Plants:
a) In contrast to C4 plants, CAM plants exhibit a temporal separation of carbon fixation & Calvin-Benson (c-fixation &
Calvin-Benson occur at different times).
b) During the night, with their stomata open, they take up CO2 & incorporate it into a malate (stored CO2), a mode of
carbon fixation known as Crassulacean Acid Metabolism (CAM).
c) The mesophyll cells of CAM plants store the malate in vacuoles until morning, when the stomata close. During the
day, when the light reactions can supply ATP & NADP for the Calvin Cycle, CO2 is released from the malate made the
night before to establish a high CO2 to O2 ratio so RUBISCO can fix CO2 which becomes reduced into sugar (PGAL).
*Although both pathways consume a greater amount of ATP in order to initially fix CO 2, this additional investment is
outweighed by the increased sugar output gained by avoiding photorespiration.
13