Photosynthesis Overview and Mechanisms

Photosynthesis Overview

• Photosynthesis is the process plants, algae, and some bacteria use to convert light energy into chemical energy, primarily glucose. A byproduct is oxygen.

• Equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

• Reactants: Carbon dioxide (CO₂) from air, water (H₂O) from soil, and light energy.

• Products: Glucose (C₆H₁₂O₆) and oxygen (O₂).

Location and Structures

• Chloroplasts: These organelles in plant cells contain chlorophyll, which absorbs light energy for photosynthesis. Chlorophyll is found in the thylakoid membranes within the chloroplasts.

• Leaves: The primary site of photosynthesis in plants.

• Stomata: Pores on leaves enabling CO₂ entry and O₂ exit.

• Xylem: Transports water from roots to leaves.

• Phloem: Transports glucose produced in leaves to other plant parts.

Stages of Photosynthesis

Light-dependent Reactions (Light Reactions)

• Location: Thylakoid membranes within chloroplasts.

• Input: Water (H₂O) and light energy.

• Process: Chlorophyll absorbs light energy, splitting water into oxygen (O₂), protons (H⁺), and electrons (e⁻). Electron energy produces ATP (energy carrier) and NADPH (high-energy electron carrier).

• Output: Oxygen (O₂), ATP, and NADPH.

Calvin Cycle (Light-independent Reactions)

• Location: Stroma of the chloroplasts.

• Input: Carbon dioxide (CO₂) and ATP and NADPH from light reactions.

• Process: ATP and NADPH convert CO₂ into glucose (C₆H₁₂O₆) in enzyme-driven steps. Crucial enzyme in this step is RuBisCO.

• Output: Glucose (C₆H₁₂O₆), used for plant energy and growth.

Factors Affecting Photosynthesis

• Light Intensity: Higher light, faster photosynthesis (until a maximum point).

• CO₂ Concentration: Higher CO₂ concentration generally increases photosynthesis rate.

• Temperature: Optimal temperature range is around 0°C to 35°C; extremes hinder the process.

• Water Availability: Water is a key reactant, so lack of it decreases photosynthesis.

Importance of Photosynthesis

• Oxygen Production: Photosynthesis creates Earth's atmosphere's oxygen, vital for most life.

• Energy Source: Provides fundamental energy for nearly all life. Plants (through photosynthesis) are at the bottom of the food chain.

• Carbon Dioxide Reduction: Helps lower atmospheric CO₂ levels, important for climate regulation.

Photosynthesis vs. Cellular Respiration

• Photosynthesis: Converts light energy into chemical energy, producing glucose and oxygen. Occurs in plants, algae, and some bacteria.

• Cellular Respiration: Breaks down glucose and oxygen to release energy (ATP). Occurs in the mitochondria of all cells.

Photosynthesis in Different Organisms

• Plants (most common organisms)

• Algae (aquatic photosynthetic organisms)

• Cyanobacteria (photosynthetic bacteria)

Experimental Evidence

• Early experiments (like Priestley and Ingenhousz) demonstrated plant uptake of CO₂ and release of O₂.

• Variegated leaves (green and non-green patches): Only green parts (containing chlorophyll) photosynthesize.

Key Concepts

• Chlorophyll: Absorbs sunlight.

• ATP & NADPH: Energy carriers in the Calvin Cycle.

• Carbon Fixation: Converting CO₂ into organic molecules (glucose).