Photosynthesis: Using Light to Make Food

Introduction

Photosynthesis is a fundamental biological process by which plants, algae, and some bacteria convert light energy into chemical energy, producing food and oxygen essential for life on Earth. This chapter explores the basic concepts, reactions, and significance of photosynthesis, including the light reactions and the Calvin Cycle.

Main Topics

7.1 Photosynthesis Powers Most Life on Earth

• Definition: Photosynthesis is the process by which autotrophic organisms use sunlight to synthesize organic compounds from carbon dioxide and water.

• Importance: Provides the primary energy source for nearly all living organisms and produces oxygen as a byproduct.

• Example: Green plants, algae, and cyanobacteria perform photosynthesis.

7.2 Photosynthesis Occurs in Chloroplasts in Plant Cells

• Chloroplasts: Specialized organelles in plant cells where photosynthesis takes place.

• Structure: Contains inner and outer membranes, thylakoids (stacked as grana), and stroma (fluid-filled space).

• Example: Leaf mesophyll cells are rich in chloroplasts.

7.5 Photosynthesis Occurs in Two Stages: Light Reactions and Calvin Cycle

• Light Reactions: Occur in the thylakoid membranes; convert light energy into chemical energy (ATP and NADPH).

• Calvin Cycle: Occurs in the stroma; uses ATP and NADPH to fix carbon dioxide into glucose.

• Equation:

7.6 Visible Radiation Absorbed by Pigments Drives the Light Reactions

• Pigments: Molecules such as chlorophyll a, chlorophyll b, and accessory pigments absorb specific wavelengths of light.

• Spectrum: The spectrum of electromagnetic radiation includes visible light, which is used in photosynthesis.

• Example: Chlorophyll absorbs red and blue light most efficiently.

7.7 Photosystems Capture Solar Energy

• Photosystems: Complexes of proteins and pigments in the thylakoid membrane that capture light energy.

• Types: Photosystem I and Photosystem II.

• Function: Initiate electron transport chains that generate ATP and NADPH.

7.9 Light Reactions Take Place Within the Thylakoid Membranes

• Location: Thylakoid membranes within chloroplasts.

• Process: Light energy excites electrons, leading to the production of ATP and NADPH.

• Equation:

7.10 ATP and NADPH Power Sugar Synthesis in the Calvin Cycle

• Calvin Cycle: Uses ATP and NADPH to convert CO2 into glucose.

• Key Intermediate: Glyceraldehyde 3-phosphate (G3P) is produced and used to form glucose and other carbohydrates.

• Equation:

Key Vocabulary

• Photosynthesis

• Light Reaction

• Calvin Cycle

• NADP+/NADPH

• Spectrum of Electromagnetic Radiation

• Chlorophyll a/b and accessory pigments

• Proton (H+) pump / proton gradient

• ATP synthesis

• CAM and C4 carbon fixation

• Chloroplast structure: inner/outer membranes, thylakoids, stroma, grana

• Stroma

• Spongy mesophyll

• Palisade mesophyll

• Epidermis & cuticle

• Stoma/stomata

• Vascular tissue

• Glyceraldehyde 3-phosphate (G3P)

Photosynthesis: Two Main Stages

Additional Info

• CAM and C4 Pathways: Adaptations in some plants to minimize water loss and optimize carbon fixation under different environmental conditions.

• Stomata: Pores on leaf surfaces that regulate gas exchange and water loss.

• Vascular Tissue: Xylem and phloem transport water, nutrients, and sugars throughout the plant.