5.2 Photosynthesis

Photosynthesis: Light-Dependent Reactions

Learning Objectives

• Plants absorb energy from sunlight.

• Wavelength of light affects its energy and color.

• Photosynthesis location and process within a plant.

Light Energy

• Light is a form of energy that can be transformed and harnessed.

• Autotrophs convert light energy into chemical energy to build carbohydrates.

• Autotrophs use a specific component of sunlight.

Electromagnetic Radiation

• The sun emits electromagnetic radiation (solar energy).

• Humans can only see a fraction of this energy, called visible light.

• Energy travels in waves, and the amount of energy is determined by its wavelength.

• Electromagnetic spectrum: range of all possible wavelengths of radiation.

• Longer wavelength = less energy; shorter wavelength = more energy.

• The sun emits various types of radiation, including X-rays and UV rays.

Absorption of Light

• Light energy enters photosynthesis when pigments absorb the light.

• Pigments absorb only visible light for photosynthesis in plants.

• Visible light exists as a rainbow of colors, with violet and blue having shorter wavelengths (higher energy) and red having longer wavelengths (lower energy).

Pigments

• Different pigments absorb certain wavelengths (colors) of visible light and reflect the color of the wavelengths they cannot absorb.

• Chlorophyll a: absorbs blue and red light, reflects green (hence plants appear green).

• Other pigments: chlorophyll b, carotenoids.

• Absorption spectrum: the specific pattern of wavelengths a pigment absorbs.

• Photosynthetic organisms use a mixture of pigments to absorb energy from a wider range of visible-light wavelengths.

Light-Dependent Reactions

• Purpose: convert light energy into chemical energy (ATP and NADPH).

• Occur in photosystems within thylakoid membranes.

• A pigment molecule absorbs a photon, exciting an electron in chlorophyll.

• Chlorophyll donates the electron.

• Water is split to replace the electron, forming oxygen (O2) and hydrogen ions (H+).

• Photosystem II transfers the electron to the electron transport chain.

• Energy from the electron fuels membrane pumps that move hydrogen ions into the thylakoid space, creating an electrochemical gradient.

• The electron is then accepted by a pigment molecule in photosystem I.

Energy Carriers: ATP and NADPH

• Energy from sunlight is stored in ATP and NADPH.

• ATP stores energy with a phosphate group, while NADPH stores energy with a hydrogen atom.

• These molecules release energy in the Calvin cycle, becoming ADP and NADP+.

• The electrochemical gradient of hydrogen ions in the thylakoid space is used to generate ATP through chemiosmosis.

• Hydrogen ions flow through ATP synthase, which attaches a third phosphate to ADP, forming ATP (photophosphorylation).

• The electron from the electron transport chain is re-energized in photosystem I and used to form NADPH from NADP+ and a hydrogen ion.

• Solar energy is stored in ATP and NADPH, which are used to make sugar molecules.