photosynthesis
C1.3 Molecules
C1.3 Photosynthesis
1. Converting Light Energy to Chemical Energy in Photosynthesis
Photosynthesis is the process through which light energy is converted into chemical energy. It involves several forms of energy, primarily chemical energy, heat (thermal energy), and light (radiant energy). During photosynthesis, light is absorbed by pigments, particularly chlorophyll, which then transforms this light energy into chemical energy. The resultant carbon compounds produced during photosynthesis store the chemical energy that was initially present in the light.
2. Carbon Dioxide is Converted to Glucose in Photosynthesis
The molecular composition of glucose is represented by the ratio of atoms: 1 carbon, 2 hydrogens, and 1 oxygen (C₁H₂O₁). In contrast, carbon dioxide (CO₂) consists of 1 carbon and 2 oxygens. To convert carbon dioxide into glucose, it is necessary to remove one oxygen atom and add hydrogen. This hydrogen is obtained by splitting water (H₂O) into hydrogen and oxygen – a process that only occurs in the presence of light since energy is required for this reaction. Specifically, twelve water molecules are split to yield:
Half of the hydrogen atoms produced are incorporated into glucose, while the remaining half is used for the reduction of carbon dioxide into water:
3. Equation for Photosynthesis
The overall equation summarizing the photosynthesis process is as follows:
Oxygen produced during this process is a by-product derived from the splitting of water molecules to release hydrogen. Various organisms, including plants, algae, and cyanobacteria, perform photosynthesis. These organisms utilize a portion of the oxygen generated during the process for aerobic respiration (e.g., the roots of plants) while most of it diffuses into the environment.
In aquatic environments, oxygen produced can be visualized as bubbles that rise to the surface. In terrestrial environments, the diffused oxygen escapes into the atmosphere without being visible.
4. Chromatography
Chromatography can be used to separate and identify leaf pigments as follows
Tear up a leaf into small pieces.
Grind the leaf pieces with sharp sand and propanone using a mortar and pestle to extract the pigments.
Transfer a sample of the extract to a watch glass.
Evaporate to dryness using hot air from a hair-dryer.
Dissolve the dried pigments by adding a few drops of propanone.
Build up a concentrated pigment spot approximately 10 mm from the end of a chromatography strip.
Suspend the strip in a tube with the base dipping into a running solvent.
Remove the strip when the solvent has nearly reached the top, then draw a pencil line to indicate how far the solvent moved.
The pigment in each spot can be quantified based on color and Rf value, defined as the distance moved by the pigment spot over the distance moved by the solvent. Typical Rf values include:
- Carotene: 0.9
- Chlorophyll a: 0.65
- Chlorophyll b: 0.6
- Xanthophylls: 0.3-0.5
5. Wavelengths of Light Absorbed in Photosynthesis
The Sun emits a broad spectrum of electromagnetic radiation, with significant amounts in the range of 400 nm to 700 nm effectively reaching the Earth's surface. This wavelength range is utilized in photosynthesis as well as by the human eye. Violet light, the shortest wavelength, carries more energy per photon than red light, which has the longest wavelength.
Pigments like chlorophyll enable an electron within the molecule to move from a lower energy state to an excited state upon absorbing a photon of light. This excited state marks the transformation of solar energy into chemical energy during photosynthesis. Photosynthetic pigments can relay these excited electrons to other molecules, with much of the energy ultimately incorporated into glucose and other carbon compounds.
Only specific wavelengths of light carry the requisite energy to elevate an electron to a higher state, and it is these wavelengths that photosynthetic pigments absorb while others are reflected; the result is an absorption spectrum representing the light absorbed.
6. Comparing Absorption and Action Spectra
The absorption spectrum illustrates the relative amounts of photosynthetic activity at various light wavelengths. Experimentally, this involves varying the wavelength while measuring the rate of photosynthesis through indicators like oxygen production or carbon dioxide consumption.
In ponds, for instance, bubbles of oxygen emerging from pondweed can provide direct countable data.
The absorption spectrum for the two prevalent chlorophyll types indicates that chlorophyll efficiently absorbs red and blue light but very little green light, which is predominantly reflected, giving chlorophyll its characteristic green appearance. Accessory pigments like xanthophyll and carotene absorb other wavelengths, enabling plants to optimize light energy intake even in the presence of green light.
The action spectrum of photosynthesis, documenting the light wavelengths effectively utilized in photosynthesis, showcases peaks in both blue and red light while indicating the presence of some green utilization due to accessory pigments.
7. Limiting Factors on the Rate of Photosynthesis
Photosynthesis is influenced by several factors, but typically only one factor is the limiting factor at a given time. This entity is deemed the limiting factor as it is closest to its minimum threshold. The main limiting factors affecting photosynthesis include:
Temperature
Light intensity
Carbon dioxide concentration
When designing experiments to evaluate the effects of these limiting factors, key considerations include:
Investigating one limiting factor at a time, which serves as the independent variable.
Selecting an appropriate range for the independent variable, starting from the minimum level to the point where it no longer limits the process.
Implementing a reliable measurement method for the rate of photosynthesis, generally quantified through oxygen production over a specific timeframe.
Maintaining all control variables constant to ascertain that the independent variable is the sole influencer on photosynthesis rates.
The proposed methods for varying the three limiting factors include:
Temperature: Place pondweed in water at varying temperatures using a thermostatically controlled water bath.
Light Intensity: Adjust the distance from a light source, utilizing a lux meter to measure light intensity (calculated as ).
Carbon Dioxide Concentration: Start with boiled, cooled water (absent CO₂) before adding quantified amounts of sodium hydrogen carbonate (NaHCO₃) to enhance CO₂ levels.
8. Carbon Dioxide Enrichment Experiments
Before the late 18th century, atmospheric carbon dioxide levels stood at approximately 270 ppm. However, human activities have since escalated this figure by over 50%, with predictions that CO₂ concentration might surpass double its pre-industrial level in the 21st century. Such a drastic rise could significantly affect various ecosystems. Research indicates increased rates of photosynthesis induced by heightened CO₂ levels, suggesting that CO₂ is frequently a limiting factor.
Experiments involving carbon dioxide have been conducted in controlled environments such as greenhouses and through Free Air Carbon Dioxide Enrichment (FACE) projects, which release CO₂ in outdoor settings while monitoring variables to assess impact on plant growth and ecosystems.
9. Photosystems
Photosystems - molecular complexes made up of chlorophyll and other pigment molecules designed to catalyze photosynthetic reactions.
- located within thylakoid membranes of chloroplasts, which are structured sacs that organize chlorophyll. There are two principal types of photosystems:
PS I (Photosystem I), located in the stroma, and
PS II (Photosystem II), which interacts directly with the thylakoid membrane.
Cyanobacteria feature their own photosystems, although not enclosed within chloroplasts and organized differently.
10. Pigment Molecules in a Photosystem
Each photosystem comprises two functional units:
1) a reaction centre that emits excited electrons and
2) antenna complexes that capture light energy and direct it towards the reaction centre.
Antenna complexes contain a diverse array of pigment molecules arranged precisely, enabling several advantages:
Higher light interception rates due to numerous pigment molecules improve energy transfer to the reaction centre.
By encompassing multiple types of pigments, a broader spectrum of wavelengths can be absorbed resulting in greater sunlight energy utilization.
Energy is transferred from one pigment to another via excitation energy transfer, thereby funneling energy efficiently to the reaction centre.
11. Photolysis of Water in PS II
In photosystem II (PS II), the absorption of light energizes chlorophyll (P680), causing it to expel an electron, which results in the reduction of P680 until it replaces the missing electron sourced from water. This photolysis of water occurs at the Oxygen-Evolving Complex (OEC) within PS II and involves binding and splitting two water molecules, yielding four electrons, four protons, and one molecular oxygen compared to the general equation:
The protons contribute to a proton gradient across the thylakoid membrane, while the oxygen is released as a by-product, facilitating the accumulation of oxygen in the atmosphere, which is vital for aerobic respiration.
12. ATP Production by Chemiosmosis in Thylakoids
The formation of ATP occurs during the light-dependent reactions within thylakoids, primarily through chemiosmosis, a process adapting mechanisms seen in mitochondria. ATP synthase, embedded within the thylakoid membranes, utilizes a proton gradient established by chains of electron carriers. As excited electrons move through these carriers, they release energy, which is leveraged to transport protons into the thylakoid space. Two mechanisms provide electrons:
Cyclic Photophosphorylation: Emission of excited electrons from PSI that re-enter the process after traversing the electron carrier chain.
Non-Cyclic Photophosphorylation: Emission of excited electrons from PS II that flow through carriers to PSI.
13. NADP is Reduced by PS I
The reduction of NADP occurs when excited electrons are emitted from PS I and transferred to an electron carrier (ferredoxin) on the thylakoid membrane's surface, which in turn passes electrons to an enzyme (NADP reductase), necessitating two electrons for the reduction process, depicted as follows:
Electrons for PS I are replenished via PS II's contributions, combining to generate reduced NADP (NADPH).
14. Thylakoids
Thylakoids consist of interdependent components crucial for the photosynthetic process. Noteworthy features include:
Their membranes are impermeable to protons aiding in the establishment of a proton gradient.
The enclosed volume allows for rapid gradient development.
They are composed of phospholipids, necessary for the retention of hydrophobic pigment molecules.
The relationships among thylakoid components ensure effective photosynthesis, as their functions include supplying electrons to the electron transport chain, utilizing energy to propel protons, facilitating ATP synthesis, and sustaining the cyclic electron flow needed for photosystems.
15. Rubisco
Rubisco is an enzyme that catalyzes the initial step of carbon fixation, combining carbon dioxide with ribulose bisphosphate (RuBP), a five-carbon sugar. The unstable intermediate formed from this reaction executes a rapid splitting into two molecules of glycerate 3-phosphate, depicted as:
16. Synthesis of Triose Phosphate
Carbon fixation does not directly utilize light energy hence represents a light-independent phase within photosynthesis. The necessity for high concentrations of Rubisco in the chloroplast stroma arises from its relative slowness and ineffectiveness at low CO₂ levels. Due to its prevalence among photosynthetic organisms, Rubisco remains Earth's most abundant enzyme. The conversion of glycerate-3-phosphate to triose phosphate involves a reduction reaction where reduced NADP provides hydrogen and ATP offers energy, represented as:
This reaction constitutes part of the Calvin cycle which is light-independent and can occur in darkness provided ATP and reduced NADP are present.
17. The Calvin Cycle
To sustain the Calvin cycle, each RuBP consumed must be regenerated. Five triose phosphates produced during the light-independent reactions are converted back into three RuBP molecules, requiring ATP as energy. Thus, among every six triose phosphates created, five contribute to RuBP regeneration while one exit the cycle for carbohydrate production.
18. Using the Products of the Calvin Cycle
All carbon utilized by photosynthesizing organisms originates from the Calvin cycle. Linking two triose phosphates results in glucose formation, which can subsequently convert into carbohydrates such as sucrose for transport or starch for storage. Pathways leading to fatty acids and amino acids commence with intermediates produced in the Calvin cycle, interconnected with aerobic respiration products as well. Nutrient minerals provide all elements excluding carbon, hydrogen, and oxygen in the compounds synthesized by photosynthetic organisms, with nitrogen supplied as nitrate or ammonium ions, sulfur as sulfate, and phosphorus as phosphates.
19. Interdependence of Photosynthesis Reactions
The reactions dependent on light must rely on ATP and reduced NADP from the light-independent reactions. Conversely, the light-dependent reactions require ADP and NADP produced from light-independent reactions to maintain a continuous cycle of photosynthesis.