C1.3 Photosynthesis SL notes
EVERYTHING YOU NEED TO KNOW FOR BIOLOGY SL AND HL
Overview of Photosynthesis
Photosynthesis is the process of producing glucose from water and carbon dioxide in the presence of chlorophyll and enzymes, using light energy and releasing oxygen as a by-product.
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It occurs in chloroplasts of plant cells.
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Light energy is trapped by chlorophyll and converted into chemical energy in molecules like glucose and ATP—the primary sources of chemical energy for life.
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Glucose is temporarily stored as starch and later used for metabolism.
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Plants use glucose to produce carbohydrates (e.g., cellulose for cell walls), lipids, proteins, and growth factors.
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Glucose is also used in respiration by plants and other organisms.
Conversion of Carbon Dioxide to Glucose
Hydrogen from water is used in photosynthesis to convert carbon dioxide into glucose.
This process releases oxygen as a by-product.
Oxygen as a By-Product
Water molecules are split during photosynthesis, releasing oxygen.
Other photosynthetic organisms like green algae, red and brown algae, and cyanobacteria also produce glucose and oxygen.
Absorption and Action Spectra
Photosynthetic pigments in plant membranes absorb light.
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The absorption spectrum shows how much light is absorbed at different wavelengths.
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The action spectrum shows the rate of photosynthesis at different wavelengths.
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The rate of photosynthesis is measured by:
Oxygen production
Carbon dioxide consumption
Different Pigments and Their Absorption Spectra
When a pigment absorbs light energy, it excites an electron to a higher energy level.
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This excitation requires a specific wavelength of light.
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Different pigments absorb different wavelengths, resulting in unique absorption spectra.
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Excited electrons use their energy to form chemical bonds, converting light energy into chemical energy.
Limiting Factors in Photosynthesis
The Law of Limiting Factors states:
“A process dependent on multiple factors is limited by the factor at its least favorable value.”
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Photosynthesis is affected by:
Light intensity
Water availability
Temperature
CO₂ concentration
Chlorophyll content
Chloroplast function
If any one of these factors is scarce, the rate of photosynthesis decreases.
Measuring the Rate of Photosynthesis
· The rate can be measured by:
o Oxygen production
o Carbon dioxide intake
· A photosynthometer is used to measure the rate under different conditions.
CO₂ Enrichment Experiments
Method 1: Greenhouse CO₂ Enrichment
· CO₂ levels are controlled in a greenhouse environment.
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· Higher CO₂ concentrations are achieved by burning fuels or using other sources.
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· A control greenhouse (with normal CO₂ levels) is used for comparison.
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· The effect of CO₂ enrichment is measured by:
o Total biomass produced
o Fruit/vegetable yield
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· Limitations:
o Artificial conditions do not account for natural factors like rainfall and sunlight.
Method 2: Free-Air CO₂ Enrichment (FACE)
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· CO₂ is released in an open-air setting around a circular area (1–30 m in diameter).
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· Pipes release CO₂ continuously, and sensors monitor its levels.
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· This method mimics natural conditions more accurately.
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· Limitation: It is very expensive to conduct.
C1.3 Photosynthesis
HL notes
Stages of Photosynthesis
1. Light-Dependent Reactions
Capture light energy and occur in the chloroplast membranes.
2. Light-Independent Reactions (Calvin Cycle)
Synthesize carbohydrates and occur in the stroma of the chloroplast.
Photosystems
Photosystems are arrays of pigment molecules that absorb light energy and emit excited electrons.
They are always located in membranes and found in cyanobacteria and chloroplasts of photosynthetic eukaryotes.
Structure of a Photosystem
Reaction Center:
The core of the photosystem where energy conversion occurs.
Light-Harvesting Complex (LHC):
Surrounds the reaction center and consists of chlorophyll and accessory pigments bound to protein complexes.
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Process of Photoactivation:
A pigment in the LHC absorbs a photon, passing energy between pigment molecules.
Energy reaches the reaction center complex, exciting chlorophyll a molecules.
Chlorophyll a emits 2 electrons, which are captured by the primary electron acceptor.
Types of Photosystems
Photosystem I (P700): Absorbs light best at 700 nm.
Photosystem II (P680): Absorbs light best at 680 nm.
· There are two types of photosystem present in green plants and Cyanobacteria identified by the wavelength of light that the chlorophyll a of the reaction centre absorbs:
· Photosystem I has a reaction centre that is activated by light or wavelength 700 nm. This reaction centre is referred to as P700.
· Photosystem II has a reaction centre that is activated by light of wavelength 680nm. This reaction centre is referred to as P680.
Light-Dependent Stage
Key Processes:
Photoactivation of chlorophyll
Photolysis of water → releases oxygen
Electron Transport Chain (ETC) → Cyclic or Non-cyclic electron flow
Chemiosmosis → ATP synthesis
Formation of reduced NADP (NADPH)
Photoactivation of Chlorophyll
A photon strikes a pigment in the LHC.
Energy is transferred between molecules until reaching the reaction center.
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Chlorophyll a ejects an excited electron, which is captured by the primary electron acceptor.
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Excited electrons follow either a cyclic or non-cyclic pathway in the electron transport chain.
Photolysis of Water
P680 (Photosystem II) is linked to a water-splitting enzyme.
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The enzyme splits a water molecule into:
2H⁺ (protons)
2e⁻ (electrons)
½ O₂ (oxygen gas)
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Electrons refill the “electron holes” in P680.
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Oxygen atoms combine to form molecular O₂, which is released.
Non-Cyclic Electron Flow
Involves both Photosystem I (PS I) and Photosystem II (PS II).
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Begins with the release of high-energy electrons from the reaction centers.
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The electrons do not return to their original reaction center (hence, non-cyclic).
Electrons are captured by the primary electron acceptor.
Electron Flow from P680 (Photosystem II)
Electrons pass from PS II to PS I via the Electron Transport Chain (ETC).
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The ETC consists of plastoquinone (PQ), a cytochrome complex, and plastocyanin (PC) (names not necessary to memorize).
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As electrons fall to a lower energy level (exergonic fall), ATP is synthesized through chemiosmosis.
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The electron from PS II fills the electron holes in P700 (PS I), which had also ejected an electron earlier.
Electron Flow from P700 (Photosystem I)
Photoexcited electrons from P700 pass down a second ETC via a primary electron acceptor.
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This ETC consists of ferredoxin and NADP⁺ reductase.
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Electrons are used to reduce NADP⁺ into NADPH, which is then used in the Calvin Cycle.
Photophosphorylation
Formation of ATP due to the flow of electrons during the light-dependent stage of photosynthesis.
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Two types:
Non-Cyclic Photophosphorylation → ATP is formed through non-cyclic electron flow (involves both PS I and PS II).
Cyclic Photophosphorylation → ATP is formed through cyclic electron flow (involves only PS I).
Cyclic Electron Flow
Electrons follow a cyclic route, involving only PS I.
Electrons cycle back from ferredoxin to the cytochrome complex, plastocyanin, and back to P700.
Cyclic Photophosphorylation
ATP formation via cyclic electron flow during the light-dependent stage of photosynthesis.
Advantages of the Structured Array of Pigments in a Photosystem
Wider Light Absorption:
Accessory pigments absorb different wavelengths of light, maximizing light capture and increasing the rate of photosynthesis.
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Higher Efficiency:
The structured arrangement ensures energy is efficiently transferred to chlorophyll a, ensuring sufficient energy to excite electrons.
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Optimized Energy Conversion:
Without this structured arrangement, light energy would be inefficiently absorbed, leading to low electron emission and poor chemical energy conversion.
Generation of Oxygen by Photolysis of Water in Photosystem II
ATP and Reduced NADP (NADPH) are the final products of the light-dependent stage.
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These molecules provide chemical energy for the Calvin Cycle.
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Protons (H⁺) and electrons (e⁻) generated by photolysis are used in photosynthesis, but oxygen is released as a waste product.
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This oxygen release transformed Earth’s atmosphere, leading to major geological and biological changes.
Chemiosmosis in Chloroplasts
Energy coupling mechanism that links the electron transport chain (ETC) to ATP synthesis.
Protons (H⁺) are actively pumped from the stroma into the thylakoid lumen by the ETC in the thylakoid membrane.
Protons diffuse back into the stroma through ATP synthase, driving ATP synthesis.
Similarities between chemiosmosis in the mitochondria and chloroplast
· Electron carriers are similar
· Both use ATP synthase to make ATP
· Both transform redox energy to a proton motive force
Chemiosmosis: Mitochondria Vs. chloroplast
Reduction of NADP by Photosystem I (PSI)
NADP⁺ is reduced by accepting two electrons from Photosystem I (PS I).
It also accepts one hydrogen ion (H⁺) from the stroma, forming NADPH.
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Terminology Consistency: Always pair "NADP and reduced NADP" or "NADP⁺ and NADPH" correctly.
Thylakoids as Systems for Light-Dependent Reactions
ATP and NADPH synthesis occur on the stroma side of the thylakoid membrane.
These products are then used in the Calvin Cycle in the stroma.
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Photolysis of water occurs inside the thylakoid space, providing electrons, protons, and oxygen gas.
Calvin Cycle (Light-Independent Stage)
The Calvin Cycle consists of three major steps:
Carbon Fixation – CO₂ is fixed by Rubisco (Ribulose-1,5-bisphosphate carboxylase-oxygenase).
Synthesis of Triose Phosphate (G3P) – Uses ATP and NADPH to form sugars.
Regeneration of RuBP – Uses ATP to regenerate Ribulose-1,5-bisphosphate (RuBP), ensuring the cycle continues.
1. Carbon Fixation (CO₂ Fixation)
CO₂ combines with RuBP (Ribulose Bisphosphate) in the presence of Rubisco (Ribulose Bisphosphate Carboxylase).
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This reaction occurs in the stroma of the chloroplast.
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It forms an unstable 6-carbon compound, which immediately splits into two molecules of 3-carbon compound: Glycerate 3-phosphate (GP).
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Glycerate 3-phosphate is the first product of carbon fixation.
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Rubisco is the most abundant enzyme on Earth but is slow and inefficient at low CO₂ concentrations, requiring high amounts in the stroma.
2. Synthesis of Triose Phosphate (TP) Using Reduced NADP and ATP
Glycerate-3-phosphate (GP) is reduced to Triose Phosphate (TP) using ATP and NADPH from the light-dependent stage.
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This is a reduction reaction, where NADPH donates electrons.
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Two molecules of TP leave the Calvin Cycle to form sugar phosphates, which can later be converted into:
Glucose (monosaccharide)
Sucrose (disaccharide)
Cellulose & Starch (polysaccharides)
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Most TP molecules are used to regenerate RuBP rather than form sugars.
3. Regeneration of RuBP Using ATP
10 molecules of TP are converted into 6 molecules of RuBP, ensuring the Calvin Cycle continues.
This process requires ATP.
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If glucose is the final product, five-sixths of all TP must be recycled into RuBP.
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If less RuBP is produced, the cycle slows down, and the light-dependent reactions may not proceed efficiently due to limited CO₂ fixation.
Synthesis of Carbohydrates, Amino Acids, and Other Carbon Compounds Using Calvin Cycle Products
The Calvin Cycle provides intermediates that serve as starting points for various metabolic pathways.
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Glucose can combine with fructose to form sucrose (a common transport sugar in plants).
Glucose phosphate is a precursor for cellulose (structural polysaccharide) and starch (storage polysaccharide).
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Fatty acids and glycerol are synthesized from glycerate 3-phosphate (GP), leading to lipid production.
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Amino acids are formed when nitrogen is added to the hydrocarbon backbone of GP, enabling protein synthesis.
All carbon compounds in plant tissues originate from carbon fixed in the Calvin Cycle, which then enters the food chain.
Interdependence of Light-Dependent and Light-Independent Reactions
Light-independent reactions (Calvin Cycle) rely on ATP and reduced NADP from the light-dependent stage.
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If the light-dependent reactions stop, the Calvin Cycle will halt once ATP and reduced NADP supplies are depleted.
Light-dependent reactions depend on the Calvin Cycle to regenerate NADP and ADP, which are required for ATP and NADPH production.
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If CO₂ is unavailable, the Calvin Cycle stops, leading to a shortage of NADP and ADP, slowing down the light-dependent reactions.
Photosystem II (PSII) is most affected, as it initiates non-cyclic photophosphorylation, which requires NADP as an electron acceptor.
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Without NADP, electron flow ceases, and PSII can no longer function, disrupting the entire photosynthetic process.
