chapter 6: photosynthesis · 2021. 1. 17. · photosystem • the light dependent reaction utilizes...
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CHAPTER 6: PHOTOSYNTHESIS
SUBTOPIC : 6.1 Overview of photosynthesis.
LEARNING OUTCOME : Outline the complete process of photosynthesis: Light dependent and light
independent reactions.
MAIN IDEAS /KEY
POINT EXPLANATION NOTES
OVERALL
REACTION OF
PHOTOSYNTHESIS
The conversion of light energy to chemical energy that is stored in
sugar or other organic compound using CO2 and water (occurs in
plants, algae & certain unicellular eukaryotes and some prokaryotes).
FIGURE 1: Overall reaction of photosynthesis
Chemical Equation for photosynthesis process:
6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O
• Reactants (Begins with): o Sunlight (Light Energy) o Water (from the ground) o Carbon dioxide (from the air)
• Products (Ends with): o Oxygen (Releases into atmosphere) o Glucose/ other organic compound (Stores chemical
energy)
o Water
sunlight
chlorophyll
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MAIN IDEAS /KEY
POINT EXPLANATION NOTES
• Involves REDOX reaction: o The reduction (gains electrons and hydrogen) of carbon
dioxide into sugars and the oxidation (loses electrons and
hydrogen ions) of water into molecular oxygen.
o Water is split and electrons transferred with H+ from water to CO2 reducing it to sugar.
o An endergonic redox process- energy is required to reduced carbon dioxide.
TIPS:
GER
Gain of
Electron-
Reduction
LEO-
Loss of
Electron-
Oxidation
THE STAGES OF
PHOTOSYNTHESIS
The anabolic pathway involves TWO STAGES:
FIGURE 2: Stages of photosynthesis
a) Light dependent reaction (Light reaction)
- the photo part of photosynthesis
- occurs in thylakoids
b) Light independent reaction (Calvin cycle)
- the synthesis part of photosynthesis
- occurs in stroma
SUBTOPIC : 6.2 Absorption spectrum of photosyntetic pigments.
LEARNING OUTCOME : State the photosynthetic pigments involved in photosynthesis
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MAIN IDEAS /KEY
POINT EXPLANATION
THE NATURE OF
SUNLIGHT
• Light is a form of energy known as electromagnetic energy, also called as electromagnetic radiation.
• Light travels in rhythmic waves. • The distance between the crests of electromagnetic waves is
wavelength ().
PHOTOSYNTHETIC
PIGMENTS
Photosynthetic pigments
Are "molecules that absorb specific wavelengths (energies) of light and
reflect all others."
Can be found in thylakoid membrane and intergranal lamella.
Are the substances that absorb visible light (narrow band between 380
to 750nm)
Each wavelength is different coloured light
TWO types of photosynthetic pigments:
Main pigment
▪ Chlorophyll a
• Most abundant pigment in plants.
• Absorbs light with wavelengths of 430nm (blue) and 662nm (red).
• Reflects green light strongly
Accessory pigments
▪ Chlorophyll b
• Has a structure similar to that of chlorophyll a
• Absorbs light (red and blue) of 453nm and 642 nm maximally.
• Appear green olive under visible light.
• Helps increase the range of light a plant can use for energy.
▪ Carotenoid
• Absorb light (violet and blue-green light) maximally between 460 nm and 550 nm
• Appear red, orange, or yellow
• Most important function: to be protecting the plant from free radicals formed from ultra violet or other radiation
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MAIN IDEAS /KEY
POINT EXPLANATION
ABSORPTION
SPECTRUM OF
PHOTOSYNTHETIC
PIGMENTS
Absorption spectrum is a visual representation of how well a particular
pigments absorbs different wavelength of visible light.
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SUBTOPIC : 6.3 Light dependent reaction.
LEARNING OUTCOME : Explain the cyclic and non-cyclic photophosphorylation
MAIN IDEAS /KEY
POINT EXPLANATION NOTES
LIGHT
DEPENDENT
REACTION
• Occurs in the thylakoid membrane of grana in presence of light. • Light energy converted into chemical energy (ATP and NADPH) to be
used during light independent reaction.
FIGURE 3: Light dependent reaction of photosynthesis
Photosystem
• The light dependent reaction utilizes two photosystems that are located in the thylakoid membrane.
• Chlorophyll molecules organized along with other small organic molecules and proteins into complexes called photosystem. A
photosystem acts like a light-gathering ‘antenna complex’
• 2 types of photosystems
• Photosystem I (PSI) Reaction center: P700
Absorption peak: 700 nm
• Photosystem II (PSII) Reaction center: P680
Absorption peak: 680 nm
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MAIN IDEAS /KEY
POINT EXPLANATION NOTES
FIGURE 4: Photosystem
Each photosystem is composed of :
• Reaction centre complex - organized association of protein
▪ holding special pair of chlorophyll a molecules - primary electron acceptor
▪ contain molecule capable for accepting electron and become reduced.
• several light harvesting complexes consists
- various pigment molecules bound to protein
- acts as an antenna for the reaction- center complex.
Excitation of Chlorophyll by Light
• When a chlorophyll molecule absorbs a photon of light - The molecule’s electron boosts to an orbital of higher energy
→ Pigment molecule in an excited state
→ unstable (does not remain long).
Thus, drop back to ground state and release excess energy as heat
FIGURE 5: Excitation of isolated chlorophyll molecule
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MAIN IDEAS /KEY
POINT EXPLANATION NOTES
Photosystem Harvest Light
• Photon strikes a pigment molecule in light harvesting complex (antenna molecule).
• Energy passed from one molecule to another until reaches reaction centre complex.
• An excited electron from reaction centre chlorophyll transferred to primary electron acceptor
FIGURE 6: How Photosystem Harvest Light
Photophosphorylation
• Process of generating ATP from ADP and phosphate by chemiosmosis
• Using a proton motive force generated across the thylakoid membrane of chloroplast
• Occurs during light dependent reaction • 2 types of photophosphorylation:
✓ Non cyclic photophosphorylation ✓ Cyclic photophosphorylation
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MAIN IDEAS /KEY
POINT EXPLANATION NOTES
Non-cyclic
Photophosphorylation
FIGURE 7: Non cyclic photophosphorylation
• Pigments in photosystem II (PS II) absorbs light, electron excited captured by the primary electron acceptor (pheophytin) thus
chlorophyll molecules in the reaction centre loses an electron &
become oxidized.
• Enzyme catalyze water splitting (water photolysis). Electrons from hydrogen atom are transferred to reaction center of PS II (P680)
Water Photolysis
FIGURE 8: Water Photolysis
The important of water photolysis
✓ To replace electron in photosystem II ✓ H2O → 2H+ +2e- + ½ O2
• The proton (H+) used to reduced NADP+ into NADPH
• Oxygen is given off or used in respiration
The electrons from primary electron acceptor of PS II are pass along an
electron transport chain, before ending up at an oxidized photosystem I
reaction center.
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MAIN IDEAS /KEY
POINT EXPLANATION NOTES
Electron transport chain (ETC)
FIGURE 9: Electron transport chain (ETC)
• A series of carriers that pass the electrons from one to another
• The energy lost when these electrons pass along the electron transport chain and will used to form ATP from ADP in during the process of
chemiosmosis
• At the bottom of this electron transport chain, the electrons fill an electron “hole” in an oxidized P700 center.
• At the same time,P700 absorbs photon and electrons are emitted and accepted by primary electron acceptor of PS I
• Then electron are passed to the electron transport chain, ferrodoxin (Fd) and then to an enzyme called NADP+ reductase (This chain
doesn’t create a proton gradient, does not produce ATP).
Cyclic
photophosphorylation
Photosystem I acts on its own, without photosystem II.
• The P700 molecule which is the reaction centre of photosystem I absorbs photon and energize the electrons.
• The electrons are emitted and taken up by primary electron acceptor • The electrons cycle back from ferredoxin (Fd) to the cytochrome
complex, then via a plastocyanin molecule (Pc) to a P700 chlorophyll in
the PS I reaction-center complex.
• No production of NADPH and no release of oxygen.
• Formation of ATP by chemiosmosis.
FIGURE 10:
Cyclic photophosphorylation
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MAIN IDEAS /KEY
POINT EXPLANATION NOTES
Summary • Light energy convert into chemical energy, ATP and NADPH.
• ATP and NADPH are used in light independent reaction.
Comparison of cyclic
and non-cyclic
photophosphorylation
Non-cyclic
Photophosphorylation
Cyclic
Photophosphorylation
Involve PS I and PS II Involve only PS I
e- flow is non-cyclic e-
flow is cyclic
Products are ATP, NADPH and O2 Product is ATP.
Last e-
acceptor is NADP+ Last e-
acceptor is PS 1 (P700)
1st e- donor is HO 1st e- donor is PS I (P700)
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SUBTOPIC : 6.4 Light independent reaction/ Calvin cycle
LEARNING OUTCOME : Explain Calvin cycle involving carbon fixation, reduction and regeneration of
RuBP
MAIN IDEAS
/KEY POINT EXPLANATION NOTES
LIGHT
INDEPENDENT
REACTION/
CALVIN CYCLE
• Use the ATP and NADPH (products from light dependent reaction) to reduce CO2 to sugar.
• Anabolic pathway - the cycle builds sugar from smaller molecules (CO2) - by using ATP as an energy source - and NADPH as reducing power of electron carrier.
FIGURE 11: Light Dependent Reaction And Light Independent Reaction
Carbohydrate produced directly from the Calvin cycle is three carbon sugar (C3),
Glyceraldehyde 3 - phosphate (G3P) / PGAL.
- 3 phases: i. Carbon fixation
ii. Reduction
iii. Regeneration of the CO2 acceptor ribulose bisphosphate (RuBP)
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MAIN IDEAS
/KEY POINT EXPLANATION NOTES
Phase 1: Carbon fixation
• CO2 (entering one at a time) attach to 3 molecules of RuBP (5C sugar). Catalyzed by RuBP carboxylase (rubisco).
• Produce unstable six-carbon compound that immediately split into half, forming two molecules of 3-phosphoglycerate (3C molecules) for each CO2
fixed.
Phase 2: Reduction
• Each molecule of 3-phosphoglycerate receives an additional phosphate group from ATP forming a molecules of 1,3-bisphosphoglycerate.
• NADPH reduces 1,3-bisphophoglycerate forming Glyceraldehyde 3-phosphate (G3P) / PGAL.
- 6 NADPH reduces 6 molecules of 1,3-bisphophoglycerate - Forming 6 molecules of Glyceraldehyde 3-phosphate (G3P) /
PGAL
• From 6 molecules of G3P (3C) formed only 1 molecule of G3P (3C) will be used to synthesis glucose (6C).
Phase 3: Regeneration of the CO2 acceptor ribulose bisphosphate (RuBP)
• Other 5 molecules (G3P) are rearranged to regenerate 3 molecules of RuBP (5C).
• Phase 3 spends 3 molecules of ATP.
The cycle repeated.
FIGURE 12: Calvin Cycle
• For the net synthesis of 1 G3P (3C), the Calvin cycle consumed: o 9 molecules of ATP o 6 molecules of NADPH
• To form 1 molecule of sugar (glucose), the cycle needs6 molecules of CO2.
• The complete cycle must occurs TWICE (2X)
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SUBTOPIC : 6.5 Alternative mechanisms of carbon fixation: C4 and Crassulacean Acid
Metabolism (CAM) pathways.
LEARNING OUTCOME : Explain photorespiration and state the alternative mechanism of carbon fixation
(C4 and Crassulacean Acid Metabolism (CAM) pathways)
MAIN IDEAS /KEY
POINT EXPLANATION NOTES
C3 PLANT • The most common and the most efficient at photosynthesis in cool and wet climate.
• The majority of plants are C3 plant, which have no special features to combat photorespiration.
PHOTORESPIRATION Photorespiration is a wasteful pathway that occurs when the Calvin
cycle enzyme rubisco acts on oxygen rather than carbon dioxide.
FIGURE 13: Photorespiration and Calvin Cycle
What is photorespiration?
• Stomata on leaf surface of C3 plant close during hot day to conserve water.
• This cause a drop in CO2 and an increase in O2 (by product of light dependent reaction) in the leaf.
• Thus, cause photorespiration..
“A metabolic pathway that consumes oxygen and ATP, releases
carbon dioxide and decreases photosynthetic output.
Generally occur on hot, dry, bright days when stomata close and the O2/
CO2 ratio in the leaves increase favouring the binding of O2 rather than
CO2 by rubisco.”
(Campbell, 10th edition)
Why photorespiration lowers photosynthetic output?
Photorespiration decreases photosynthetic output by fixing oxygen,
instead of carbon dioxide to Calvin cycle.
* Occur only in C3 plants.
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MAIN IDEAS /KEY
POINT EXPLANATION NOTES
FIGURE 14: Opening and Closing stomata
ALTERNATIVE
MECHANISM OF
CARBON FIXATION
In some plant species, alternate modes of carbon fixation have evolved
that minimize the photorespiration and optimize the Calvin cycle
even in hot, arid climates.
• There two photosynthetic adaptations: ✓ C4 pathway. ✓ Crassulacean Acid Metabolism (CAM). ✓
HATCH-SLACK
PATHWAYS (C4)
• C4 plant so named because they preface the Calvin cycle with an alternate mode of carbon fixation that forms four-carbon
compound as its first product.
Campbell10th Edition, page 246
Example: corn
FIGURE 15: C4 plant (corn)
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MAIN IDEAS /KEY
POINT EXPLANATION NOTES
LEAF ANATOMY OF
C4 PLANT
• In C4 plants, there are two types of photosynthetic cells: - Bundle sheath cells : arrange tightly packed sheath around the
veins of leaf.
- Mesophyll cells : loosely arranged between bundle sheath and leaf surface
• This arrangement are called Kranz anatomy.
• Involves enzyme called PEP carboxylase. - Present in the mesophyll cells.
• PEP carboxylase has a higher affinity towards CO2 compared to rubisco and will not fix O2.
• PEP carboxylase can fix carbon efficiently during hot and dry day when stomata are partially closed.
• Cause the CO2 concentration in leaf to decrease and O2 concentration increase.
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MAIN IDEAS /KEY
POINT EXPLANATION NOTES
FIGURE 16: C4 pathway
1. In mesophyll cells, the enzyme PEP carboxylase add carbon dioxide to phosphoenolpyruvate (PEP).
✓ Forming 4C product -oxaloacetate (OAA). 2. Oxaloacetate (OAA) reduce to malate. 3. Malate transported into the bundle-sheath cell via
plasmodesmata.
4. In bundle-sheath cell, malate release CO2 (decarboxylation) forming pyruvate
✓ CO2 enters Calvin cycle ✓ Pyruvate transported back to mesophyll cell &
phosphorylated to form PEP
✓ Allowing the following cycle to continue
CRASSULACEAN
ACID METABOLISM
(CAM) MECHANISM
FIGURE 17: CAM plant (pineapple)
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MAIN IDEAS /KEY
POINT EXPLANATION NOTES
• CAM plants : stomata open during the night and close during the day.
✓ To minimize photorespiration. • Closing stomata during the day helps desert plants to conserve
water.
• But, this prevents CO2 from entering the leaves.
FIGURE 18: CAM pathway
1. During the night, stomata are opened. ✓ CO2 enters the leaf tissue ✓ CO2 fixed with PEP catalysed by PEP carboxylase to form
oxaloacetate (OAA)
✓ OAA reduced into malate (NADH oxidized)
2. Reduced malate stored in the vacuole
3. During day, stomata closed, malate passed to the chloroplast ✓ Malate undergo decarboxylation release CO2 and form pyruvate
back (pyruvate converted to starch that helps the closure of
stomata)
✓ CO2 then enters Calvin cycle
Differences between C3,
C4 and CAM plant
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