Photosynthesis

Overview of Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water. This process is fundamental to life on Earth, as it provides the primary energy source for most ecosystems.

• Definition: Photosynthesis is an endergonic (energy-requiring) process that occurs in the chloroplasts of plant cells.

• General Equation:

• Inputs: Carbon dioxide (CO2), water (H2O), and sunlight

• Outputs: Glucose (C6H12O6) and oxygen (O2)

• Location: Chloroplasts in mesophyll cells of leaves

Photosynthesis in the Ecosystem

Photosynthesis is the foundation of most food chains, providing energy and organic molecules for other organisms. It is closely linked to cellular respiration, which occurs in mitochondria and releases energy for cellular work.

• Photosynthesis: Converts light energy to chemical energy (glucose)

• Cellular Respiration: Breaks down glucose to release energy (ATP)

• Energy Flow: Sunlight → Photosynthesis → Glucose → Respiration → ATP → Cellular Work

Redox Reactions in Photosynthesis and Respiration

Both photosynthesis and respiration involve oxidation-reduction (redox) reactions, where electrons are transferred between molecules.

• Photosynthesis: CO2 is reduced to form glucose; H2O is oxidized to form O2.

• Respiration: Glucose is oxidized to form CO2; O2 is reduced to form H2O.

Equations:

Photosynthesis:

Respiration:

Autotrophic and Heterotrophic Organisms

Types of Organisms Based on Nutrition

Organisms are classified by how they obtain energy and organic molecules.

• Autotrophs (Producers): "Make their own food" by synthesizing complex organic compounds from simple molecules.

• Photoautotrophs: Use light energy (e.g., plants, algae)

• Chemoautotrophs: Use energy from inorganic chemical reactions (e.g., some bacteria)

• Heterotrophs: Cannot make their own food; obtain energy by consuming other organisms (e.g., humans, animals, fungi)

Photosynthetic Pigments and Light Absorption

Properties of Light and Pigments

Photosynthetic organisms use pigments to absorb light energy. Light behaves both as a wave and as discrete particles called photons. The energy of light is related to its wavelength; shorter wavelengths have higher energy.

• Visible Light: Wavelengths from about 400 nm (violet) to 700 nm (red)

• Pigments: Molecules that absorb specific wavelengths of light

• Main Pigments: Chlorophyll a, chlorophyll b, carotenoids, xanthophylls

• Chlorophyll: Absorbs blue and red light; reflects green light (plants appear green)

• Carotenoids: Absorb blue/green light; transmit yellow, orange, or red light

Action Spectrum of Photosynthesis

The action spectrum shows the effectiveness of different wavelengths of light in driving photosynthesis. Chlorophyll absorbs most strongly in the blue and red regions.

• Absorption Spectrum: Range of wavelengths absorbed by a pigment

• Action Spectrum: Rate of photosynthesis at different wavelengths

Photosystems and Light Reactions

Structure and Function of Photosystems

Photosystems are complexes of proteins and pigments in the thylakoid membranes of chloroplasts. They capture light energy and initiate the light-dependent reactions of photosynthesis.

• Photosystem II (PSII): Absorbs light, splits water, releases O2, and transfers electrons

• Photosystem I (PSI): Absorbs light and helps produce NADPH

• Light-Harvesting Complex: Contains many pigment molecules that transfer energy to the reaction center

• Excited Electrons: When a photon strikes chlorophyll, an electron is raised to a higher energy state

Calvin Cycle (Light-Independent Reactions)

Stages of the Calvin Cycle

The Calvin Cycle uses ATP and NADPH from the light reactions to fix carbon dioxide and produce carbohydrates. It occurs in the stroma of the chloroplast.

• Carbon Fixation: CO2 is attached to a five-carbon sugar (RuBP) by the enzyme RuBisCO

• Reduction: ATP and NADPH are used to convert 3-phosphoglycerate (3-PGA) into glyceraldehyde-3-phosphate (G3P)

• Regeneration: Some G3P is used to regenerate RuBP, allowing the cycle to continue

• Output: Glucose and other carbohydrates

Gas Exchange and Water Loss in Plants

Stomata and Gas Exchange

Stomata are pores on the leaf surface that regulate gas exchange and water loss. When stomata are closed (e.g., in hot, dry conditions), CO2 uptake decreases, and photorespiration may increase.

• Stomata: Open to allow CO2 in and O2 out; close to prevent water loss

• Photorespiration: Occurs when O2 is used instead of CO2 by RuBisCO, reducing photosynthetic efficiency

Adaptations to Hot, Dry Environments

C4 and CAM Photosynthesis

Some plants have evolved alternative pathways to minimize photorespiration and conserve water.

• C4 Plants: Spatial separation of initial CO2 fixation and the Calvin Cycle; common in grasses

• CAM Plants: Temporal separation; stomata open at night to fix CO2, reducing water loss

Learning Objectives

• Describe how the equations and processes of respiration and photosynthesis are complementary.

• Explain how photosynthesis happens in two stages: light-dependent reactions and the Calvin Cycle.

• Summarize the general properties of light and how pigments absorb different wavelengths.

• Identify the mechanisms and differences between C3, C4, and CAM photosynthesis.

• Explain how photosynthesis and respiration are linked, and describe the process of photorespiration.