Notes

• Photosynthesis and Light Energy Conversion

  • Photosynthesis is the process by which cells synthesize organic compounds from inorganic molecules in the presence of sunlight.
  • It requires a photosynthetic pigment called chlorophyll and is found in certain organisms like plants and some bacteria.
  • The process involves converting light energy into chemical energy.
  • Photosynthetic pigments capture light energy from the sun to create chemical energy in the form of ATP.

  
• Utilization of Chemical Energy in Photosynthesis


  • The chemical energy produced in photosynthesis is used for synthesizing organic compounds, such as glucose.
  • Anabolic reactions utilize the chemical energy to synthesize organic compounds from simple inorganic molecules.
  • The synthesized organic compounds can contribute to cellular structure or serve as an energy source through catabolic digestion in processes like cell respiration.
• Light Spectrum


  The electromagnetic spectrum comprises the full range of all types of radiation energy (travels through space in particles or waves)

  • The sun emits its peak power in the visible region of this spectrum (white light = 400nm – 700nm)
  • Colours represent different wavelengths of visible light and range from red (longest) to violet (shortest)
  • The colours of the visible spectrum (from longest to shortest) are red, orange, yellow, green, blue, indigo, violet

  
• Pigment Molecules


  In photosynthetic organisms, the absorption of light is mediated by specific pigment molecules (e.g. chlorophyll)

  • Each pigment molecule contains electrons at discrete and specific energy levels (according to the pigment’s atomic configuration)
  • These electrons can absorb light at specific frequencies (or wavelengths) and become energised and delocalised (ionised)
  • The energy from these excited electrons can be harnessed by the cell to make chemical energy (ATP – via photophosphorylation)

  
• Photosynthetic Pigments and Chromatography


  Photosynthetic organisms utilize multiple pigments, such as chlorophyll a, xanthophylls, and carotenes, to absorb light.

  • Chromatography is an experimental technique used to separate mixtures of compounds.
  • It allows for the separation and identification of photosynthetic pigments present in a mixture.
• Techniques for Separating Photosynthetic Pigments 


  Two common chromatography techniques for separating photosynthetic pigments are:

  • Paper Chromatography: This method uses paper (cellulose) as the stationary bed.
  • Thin Layer Chromatography: This technique involves a thin layer of adsorbent (e.g., silica gel) that provides faster migration and better separation.
• Solvent Choice


  Since photosynthetic pigments contain a non-polar tail for membrane anchorage, an organic solvent such as acetone is used to dissolve the pigments during chromatography.

  
• Absorption Spectrum


  • The absorption spectrum indicates the wavelengths of light that are absorbed by each pigment, such as chlorophyll.
  • It focuses on the pigment's ability to absorb light energy for photosynthesis.
  • The level of absorbance can be quantitatively measured using a spectrophotometer.
  • The spectrum displays peaks in the blue region (~450 nm) and red region (~670 nm) of the visible spectrum.
  • There is a trough in the green/yellow portion (~550 nm) of the visible spectrum.
• Action Spectrum


  • Action spectra indicate the overall rate of photosynthesis at each wavelength of light.
  • The photosynthetic rate can be measured by the rate of carbon dioxide consumption or the level of oxygen production.
  • Action spectra show the effectiveness of different wavelengths in driving photosynthesis.
• There is a strong correlation between the cumulative absorption spectra of all pigments and the action spectrum, indicating that the absorbed light energy contributes to the overall photosynthetic rate.

  Collectively, this demonstrates that photosynthetic pigments absorb red and blue light most effectively and reflects green light more than other colours.

  
• Photosynthesis is a two step process


  The light dependent reactions convert light energy from the Sun into chemical energy (ATP)

  The light independent reactions use the chemical energy to synthesise organic compounds (e.g. carbohydrates)
• Light Dependent Reactions


  In plants, the light dependent reactions occur within the membranous discs called thylakoids (which are arranged into stacks called grana)

  • Light is absorbed by photosynthetic pigments (such as chlorophyll), resulting in the production of ATP (chemical energy)
  • Light is also absorbed by water, which is split (photolysis) to produce oxygen and hydrogen (carried by NADPH)
  • The hydrogen and ATP are used in the light independent reactions, the oxygen is released from stomata as a waste product

  
• Light Independent Reactions


  In plants, the light independent reactions occur within the fluid-filled interior of the chloroplast called the stroma

  • ATP and hydrogen (carried by NADPH) are transferred to the site of the light independent reactions
  • The hydrogen is combined with carbon dioxide to form complex organic compounds (e.g. carbohydrates, amino acids, etc.)
  • The carbon is fixed by the enzyme Rubisco, with ATP providing the chemical energy required to join the molecules together
  • This process is also commonly known as the Calvin cycle

  
• Temperature and Photosynthesis


  Temperature impacts the rate of photosynthesis by influencing the frequency of successful enzyme-substrate collisions.

  • At low temperatures, photosynthetic rates are low due to insufficient kinetic energy for frequent collisions.
  • High temperatures cause a decrease in photosynthetic rates as enzymes begin to denature and lose functionality.
  • Optimum temperature reflects the conditions most suitable for photosynthetic enzymes, such as Rubisco.
  • The highest photosynthetic rates occur at temperatures that align with the optimum conditions for photosynthetic enzymes.

  
• pH and Photosynthesis 


  • pH affects the rate of photosynthesis by altering the charge and solubility of the enzymes involved.
  • Photosynthetic rates are highest at a pH that reflects optimum physiological conditions, typically around pH 7.
  • Any pH condition outside the optimal range can lead to enzyme denaturation, resulting in reduced photosynthetic rates.

  
• Factors Affecting Photosynthesis


  • There are several variables that can potentially affect the rate of photosynthesis within an organism.
  • Temperature and pH can alter the functionality of photosynthetic enzymes, thereby influencing reaction rates.
  • Availability of carbon dioxide, as a required input for photosynthesis, plays a crucial role in determining reaction rates.
  • Light intensity also influences the rates of photosynthesis by determining the activation of photosynthetic pigments.
• Law of limiting factors


  A chemical process's rate will be limited by the condition closest to its minimum value
• Experimental techniques for investigating limiting factors


  1. Vary the concentrations of carbon dioxide

  2. Vary light intensity

  3. Vary temperature
• Experimental techniques


  • Used to investigate the effects of limiting factors on the rate of photosynthesis.
  • Help in understanding how various factors independently or collectively influence the rate of photosynthesis. By manipulating these factors within controlled experiments, researchers can determine their impact on the photosynthetic process and identify the limiting factors
• Carbon Dioxide as a Photosynthetic Input


  Carbon dioxide is the primary source of carbon used in the light-independent reactions of photosynthesis to synthesize organic compounds.

  • Increasing concentrations of carbon dioxide generally lead to higher rates of photosynthesis until enzyme saturation occurs, resulting in a plateau.
  • The concentration of carbon dioxide can be experimentally regulated using sodium bicarbonate tablets, which release carbon dioxide when dissolved in water through the formation of carbonic acid.

  
• Light as a Photosynthetic Input


  Light is required for the photoactivation of pigments and the subsequent production of chemical energy (ATP) in photosynthesis.

  • Increasing light intensity typically leads to higher rates of photosynthesis until pigment photoactivation saturates, resulting in a plateau.
  • Light intensity can be experimentally regulated by controlling the distance of the light source, measured using a lux meter.
  • The specific wavelength or color of light also affects photosynthetic rates, showcasing the importance of light quality in the process.

  
• Measuring Photosynthesis


  Photosynthesis can be measured directly via the uptake of CO2 and production of O2 – or indirectly via a change in biomass

  • It is important to recognise that these levels may be influenced by the relative amount of cell respiration occurring in the tissue
• CO2 Uptake


  • Carbon dioxide uptake can be measured by placing leaf tissue in an enclosed space with water
  • Water free of dissolved carbon dioxide can initially be produced by boiling and cooling water
  • Carbon dioxide interacts with the water molecules, producing bicarbonate and hydrogen ions, which changes the pH (↑ acidity)
  • Increased uptake of CO2 by the plant will lower the concentration in solution and increase the alkalinity (measure with probe)
  • Alternatively, carbon dioxide levels may be monitored via a data logger
• O2 Production


  • Oxygen production can be measured by submerging a plant in an enclosed water-filled space attached to a sealed gas syringe
  • Any oxygen gas produced will bubble out of solution and can be measured by a change in meniscus level on the syringe
  • Alternatively, oxygen production could be measured by the time taken for submerged leaf discs to surface
  • Oxygen levels can also be measured with a data logger if the appropriate probe is available
• Biomass (Indirect)


  • Glucose production can be indirectly measured by a change in the plant’s biomass (weight)
  • This requires the plant tissue to be completely dehydrated prior to weighing to ensure the change in biomass represents organic matter and not water content
  • An alternative method for measuring glucose production is to determine the change in starch levels (glucose is stored as starch)
  • Starch can be identified via iodine staining (turns starch solution purple) and quantitated using a colorimeter
• O2 Enrichment and Predicting Plant Growth


  CO2 enrichment involves increasing carbon dioxide levels above the typical concentration found in fresh air.

  It is used in experiments to predict future rates of photosynthesis and plant growth, particularly in response to human activities.

  Combustion of fossil fuels and deforestation contribute to increased carbon dioxide concentration in the atmosphere.

  Higher carbon dioxide levels generally promote plant growth and maturation, but excessive concentrations can cause damage.
• Methods for CO2 Enrichment Experiments

  Enclosed Greenhouse Experiments: These experiments are conducted within controlled greenhouse environments where carbon dioxide levels can be regulated and manipulated. Free Air Carbon Dioxide Enrichment (FACE) Experiments: These experiments maintain open-air conditions while artificially increasing carbon dioxide levels around specific plant populations.

  • Both methods aim to simulate the effects of increased carbon dioxide levels on photosynthesis and plant growth under realistic conditions.
• Considerations and Limitations of FACE Experiments


  • FACE experiments allow for the measurement of CO2 enrichment effects on larger trees and consideration of competition among plant species.
  • However, due to the open nature of these experiments, certain conditions like sunlight cannot be directly controlled.
  • FACE experiments provide insights into the responses of ecosystems to elevated CO2 levels and help in understanding the broader implications of CO2 enrichment.
• Enclosed Greenhouse Experiments



  Carbon dioxide levels can be artificially increased in indoor greenhouses by adding CO2 from compressed gas tanks or by adding fermentation buckets that continuously produce CO2

  • Enclosed greenhouses act as a closed system, which allow for the control of a range of extraneous variables (such as temperature and light)
  • However the conditions do not reflect those that occur in the natural environment and only plants that grow in small spaces can be measured

  
• Free Air Carbon Dioxide Enrichment (FACE) Experiments


  Free Air Carbon Dioxide Enrichment (FACE) experiments involve placing pipes around an experimental area to emit carbon dioxide (CO2).

  • Sensors monitor the concentration of CO2 and adjust the flow from the pipes to maintain desired levels.
  • FACE experiments aim to create open systems that simulate natural conditions, including rainfall and temperature fluctuations.
  • These experiments provide a more realistic setting for studying the effects of CO2 enrichment on photosynthesis and plant growth.

  
• Xanthophylls


   They are a class of oxygen-containing carotenoid pigments found in plants and algae. They are responsible for the yellow to orange coloration in many leaves and vegetables and play a role in the process of photosynthesis by protecting chlorophyll from damage by excessive light.
• Carotenes 


  They are a class of hydrocarbon carotenoid pigments found in many fruits and vegetables, contributing to their red, orange, or yellow color. They are important for photosynthesis in plants and serve as precursors to vitamin A in animals and humans.
• Chromatography is an experimental technique by which mixtures can be separated


  • A mixture is dissolved in a fluid (called the mobile phase) and passed through a static material (called the stationary phase)
  • The different components of the mixture travel at different speeds, causing them to separate
  • A retardation factor can then be calculated (Rf value = distance component travels ÷ distance solvent travels)
• Why are plants green

  Plants are green because of the pigment chlorophyll, which reflects green light while absorbing blue and red light for photosynthesis
• Chlorophyll a


  Primary pigment in photosynthesis

  Absorbs light mainly in the blue-violet and red regions (430-450 nm & 640-680 nm)

  Reflects green light
• Chlorophyll b


  • Accessory pigment that complements chlorophyll a
  • Absorbs light primarily in the blue and red-orange regions (450-500 nm & 600-650 nm)
  • Reflects green light