15_Photosynthesis

Photosynthesis Notes

Definition of Photosynthesis

Photosynthesis: A complex biochemical process utilized by plants, certain bacteria, and protists to convert sunlight into chemical energy in the form of glucose from carbon dioxide (CO2) and water (H2O). This vital process forms the foundation for life on Earth as it provides energy for nearly all ecosystems.

• By-product: Oxygen (O2), released as a crucial component for aerobic respiration in living organisms.

• Word equation: Carbon dioxide + Water → Glucose + Oxygen

Key Components Involved

• Chlorophyll: The green pigment found in the chloroplasts of photosynthetic organisms responsible for absorbing light energy needed for the photosynthetic process.

  • Types of chlorophyll: All photosynthetic organisms possess chlorophyll a; some also include accessory pigments such as chlorophyll b, xanthophylls, and carotenoids (e.g., beta-carotene), which assist in light absorption.

Absorption Notes

• Chlorophyll a primarily absorbs wavelengths from the violet-blue and red-orange regions of the light spectrum, while it reflects green-yellow-orange wavelengths, giving plants their characteristic green color.

Leaf Structure and Function

• Leaves as Solar Collectors: Leaves are specialized structures equipped with photosynthetic cells, maximizing light absorption and gas exchange.

  • Raw Materials: Water absorbed from the roots via xylem and carbon dioxide entering through stomata on leaf surfaces are vital inputs for photosynthesis.

  • Gas Exchange: Oxygen and sugars are released through stomata, allowing for energy distribution within the plant.

  • Transpiration Loss: Water loss through evaporation in stomata is significant; for instance, a mature cottonwood tree can lose up to 100 gallons of water per hour in hot, dry conditions.

Chloroplast Structure

• Thylakoids: Membrane-bound structures within chloroplasts that house the chlorophyll and are arranged in stacks known as grana (singular: granum), surrounded by a fluid called stroma.

• Chloroplasts: These organelles possess three membrane compartments: the outer membrane, the inner membrane, and the thylakoid membrane, distinguishing eukaryotic organisms from prokaryotes, which do not contain chloroplasts.

Stages of Photosynthesis

Photosynthesis occurs in two main stages:

1. Light-Dependent Reactions (occur in thylakoid membranes):

   • Require sunlight to produce ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).

   • Key processes include:

     • Photoactivation of chlorophyll: Exciting electrons upon light absorption.

     • Water Splitting (Photolysis): Light energy splits water molecules to provide electrons and H+ ions, releasing oxygen as a by-product.

     • Electron Transport: Electrons move through a series of proteins in the thylakoid membrane, generating ATP and NADPH through chemiosmosis.

2. Light-Independent Reactions (Calvin Cycle; occur in stroma):

   • Utilize ATP and NADPH to convert CO2 into carbohydrates (e.g., glucose).

   • Initial product: Glyceraldehyde 3-phosphate (GALP), which is pivotal for glucose formation.

   • The cycle regenerates Ribulose bisphosphate (RuBP) to maintain the ongoing process.

Light-Dependent Reactions Details

• Photoexcitation: The absorption of light by chlorophyll, leading to the excitement of electrons.

• Photoionization: The phenomenon occurring when light energy splits water molecules, producing free oxygen and energized electrons.

• Z Scheme: Describes the flow of electrons through the two photosystems: Photosystem II (PS II) absorbing at 680 nm and Photosystem I (PS I) at 700 nm, with PS II operating before PS I despite its designation.

ATP Synthesis

• Non-Cyclic Phosphorylation: A pathway that produces both ATP and NADPH through the movement of electrons.

• Movement of Electrons: This process establishes a proton (H+) gradient across the thylakoid membrane.

• Chemiosmosis: H+ ions flow back into the stroma through ATP synthase, driving ATP synthesis through phosphorylation.

Cyclic Phosphorylation

• A mechanism that generates additional ATP without producing NADPH, playing a crucial role in supplying energy for the Calvin Cycle.

The Calvin Cycle

Process:

• Carbon dioxide combines with RuBP to form an unstable six-carbon sugar, which rapidly splits into two three-carbon molecules known as glycerate 3-phosphate (GP).

• GP is phosphorylated and reduced to GALP using ATP and NADPH produced during light-dependent reactions.

Outputs:

• One GALP molecule is used for glucose synthesis, while others contribute to the regeneration of RuBP.

Factors Affecting Photosynthesis Rate

• Light Intensity: Directly influences the rate of light-dependent reactions; however, limitations may arise due to other factors.

• Carbon Dioxide Levels: Increased CO2 concentration boosts the efficacy of light-independent reactions until a limiting factor is reached.

• Temperature Influence: Enzyme activity is affected by temperature; the rate of photosynthesis presumably increases until a peak temperature is reached, after which it declines due to enzyme denaturation.

• Wavelengths of Light: The absorption peaks for PSI and PSII are at specific wavelengths of light (700 nm for PSI and 680 nm for PSII), highlighting the significance of light quality in photosynthesis efficiency.

Summary of the Photosynthesis Process

• Photosynthesis is a vital process that efficiently converts light energy into chemical energy, generates oxygen, and sustains ecosystems by forming the base of food webs through the production of glucose.