Photosynthesis Overview

• Definition: Photosynthesis is the process by which plants, some bacteria, and protistans convert sunlight energy into glucose from carbon dioxide and water.

• Word Equation:

  • Carbon dioxide + Water → Glucose + Oxygen.

• Chlorophyll: The green pigment involved in this process.

  • Types of Chlorophyll: Chlorophyll a is present in all photosynthetic organisms; accessory pigments such as chlorophyll b, xanthophylls, and carotenoids absorb additional wavelengths of light.

  • Chlorophyll Structure:

    • Lipid-soluble hydrocarbon tail (C20H39-).

    • Flat hydrophilic head with a magnesium ion; differing side-groups determine type.

• • The arrangement of these components allows chlorophyll to efficiently capture light energy and convert it into chemical energy during photosynthesis.

• The arrangement of these components allows chlorophyll to efficiently capture light energy during the process of photosynthesis.

This process is essential for the production of glucose, which serves as an energy source for plants and other organisms.

• Additionally, photosynthesis releases oxygen as a byproduct, which is crucial for the survival of aerobic organisms.

This energy is then used to drive the synthesis of glucose from carbon dioxide and water, highlighting the crucial role of chlorophyll in sustaining life on Earth.

Furthermore, the overall equation for photosynthesis can be summarized as: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂, illustrating how carbon dioxide and water are transformed into glucose and oxygen through the energy harnessed from sunlight.

• • • Chlorophyll a: Primary pigment involved in photosynthesis, absorbing light mainly in the blue-violet and red wavelengths.

    • Chlorophyll b: Assists chlorophyll a by capturing light energy and transferring it for photosynthetic processes.

      • Carotenoids: Accessory pigments that absorb light in the blue and green wavelengths, providing additional energy for photosynthesis and protecting chlorophyll from photo-damage.

        • Phycobilins: Pigments found in red algae and cyanobacteria that absorb light in the orange and red wavelengths, further enhancing the light-harvesting capability of photosynthetic organisms.

          • Xanthophylls: A type of carotenoid that absorbs light in the blue-green wavelengths and plays a role in photoprotection by dissipating excess light energy.

            • Anthocyanins: Water-soluble pigments that can appear red, purple, or blue depending on pH, contributing to light absorption and providing protection against UV radiation.

              • Chlorophyll a: The primary pigment in photosynthetic organisms, crucial for capturing light energy and converting it into chemical energy during photosynthesis.

Leaf Structure and Function

• Leaves as Solar Collectors: Main sites for photosynthesis; contain numerous photosynthetic cells.

• Transport Processes:

  • Water enters roots and is transported through xylem to leaves.

  • Carbon dioxide enters through stomata; oxygen exits in a similar fashion.

• Stomata Functionality:

  • Guard cells regulate the opening and closing of stomata to balance gas exchange and water loss.

  • Example: Cottonwood trees can lose about 100 gallons of water per hour.

• Chloroplast Structure:

  • Composed of thylakoids (site of photosynthesis); contains chlorophyll and is organized into stacks called grana in the stroma.

Stages of Photosynthesis

Light-Dependent Reactions

• Process Initiation: Light energy excites electrons in chlorophyll, causing oxidation and the splitting of water molecules (photolysis).

  • Key Reactions Involved:

    • Photophosphorylation: Light energy is captured to make ATP.

    • Photolysis: Splits water into oxygen, hydrogen ions, and electrons.

    • NADP+ Reduction: Electrons reduce NADP+ to NADPH.

Light-Independent Reactions (Calvin Cycle)

• Function: Uses ATP and NADPH from light reactions to convert CO2 into carbohydrates, initially forming glyceraldehyde 3-phosphate (GALP).

  • Carbon Fixation: CO2 joins with ribulose 1,5-biphosphate (RuBP) to start the cycle.

Light-Dependent Processes

Photoexcitation and Photoionization

• Electron Excitation: Light energy increases electron energy, leading to photoionization (loss of electron).

• Photosystems: Comprised of PSII (P680) and PSI (P700) that facilitate electron transfer in a Z scheme, which enables ATP production.

ATP Synthesis Processes

Non-Cyclic Phosphorylation

• Production of ATP and NADPH:

  • Electrons from PSII are transferred to an electron transport chain while NADP+ is reduced to NADPH.

Cyclic Phosphorylation

• Functionality: Provides additional ATP for the Calvin cycle using only PSI without producing NADPH. The electrons return to PSI to sustain the cycle.

Calvin Cycle Overview

Steps in Carbon Fixation

• Initial Steps: CO2 is captured and combined with RuBP, producing unstable six-carbon sugar, quickly decomposing to produce glycerate 3-phosphate (GP).

  • Forms of GP: Phosphorylated by ATP, reduced by NADPH to glyceraldehyde 3-phosphate (GALP).

• Product Formation:

  • One GALP is utilized to synthesize glucose and other organic compounds, while the other returns to regenerate RuBP.

Factors Affecting Photosynthesis Rate

• Key Limiting Factors:

  • Light intensity: Increases photosynthesis until another factor becomes limiting.

  • Carbon dioxide concentration: Higher levels increase incorporation into carbohydrates until limited by another factor.

  • Temperature: Enzymatic reactions have an optimal temperature range; deviations can decrease the rate of photosynthesis.

• Light Wavelengths: PS II absorbs best at 680 nm; PSI at 700 nm; effective light wavelengths enhance photosynthesis efficiency.

more details on

the role of chlorophyll pigments: Chlorophyll a absorbs light primarily in the blue (around 430 nm) and red (around 680 nm) wavelengths, while chlorophyll b absorbs light in the blue (around 450 nm) and orange-red (around 640 nm) ranges, allowing plants to capture a broader spectrum of light for photosynthesis.

• Accessory Pigments: Other pigments, such as carotenoids, assist in capturing light energy and protecting chlorophyll from photodamage by absorbing excess light.

  These accessory pigments not only expand the absorption spectrum but also play a crucial role in photoprotection, ensuring that the plant can efficiently manage light energy under varying light conditions.

  • Accessory pigments like xanthophylls and anthocyanins also contribute to light absorption and can help in dissipating excess energy, further enhancing the plant's ability to adapt to different environmental light levels.

• The presence of these

  pigments is essential for optimizing photosynthetic efficiency, as they allow plants to utilize a broader range of wavelengths for energy production.

s

s

s

s

s