Chapter 7: Photosynthesis – Using Light to Make Food

Introduction to Photosynthesis

Overview

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, producing sugars that fuel biological activity. This process is essential for life on Earth, as it forms the foundation of most food chains and regulates atmospheric gases.

• Photosynthesis removes CO2 from the atmosphere and stores it in plant matter.

• The burning of sugar in cellular respiration releases CO2 back to the environment.

• Example: Poison ivy produces energy for growth by photosynthesis, converting light energy to the chemical energy of sugar.

7.1 Photosynthesis Fuels the Biosphere

Autotrophs and Heterotrophs

• Autotrophs are organisms that sustain themselves without consuming organic molecules from other organisms. They make their own food through photosynthesis.

• Photoautotrophs use light energy to produce organic molecules (e.g., plants, algae).

• Chemoautotrophs are prokaryotes that use inorganic chemicals as their energy source.

• Heterotrophs are consumers that feed on plants, animals, or decompose organic material.

Photoautotrophs are crucial for ecosystems, providing food, materials (cotton, wood), and energy for various human uses.

7.2 Photosynthesis Occurs in Chloroplasts in Plant Cells

Chloroplast Structure and Function

• Chlorophyll is a light-absorbing pigment responsible for the green color of plants and plays a central role in converting solar energy to chemical energy.

• Chloroplasts are concentrated in the mesophyll cells of leaves.

• Stomata are tiny pores that allow CO2 to enter and O2 to exit the leaf.

• Veins deliver water absorbed by roots to the leaf tissues.

Chloroplasts have an inner and outer membrane, with internal structures called thylakoids (stacked into grana) and a fluid-filled stroma.

7.3 Scientists Traced the Process of Photosynthesis Using Isotopes

Source of Oxygen in Photosynthesis

• Early scientists debated whether the O2 produced in photosynthesis came from CO2 or H2O.

• Experiments using the heavy isotope 18O showed that the oxygen released comes from water, not carbon dioxide.

7.4 Photosynthesis is a Redox Process, as is Cellular Respiration

Redox Reactions in Photosynthesis and Respiration

• Photosynthesis and cellular respiration are both redox (oxidation-reduction) processes.

• In photosynthesis: CO2 is reduced to sugar, and H2O is oxidized to O2.

• Equation: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

• Photosynthesis requires energy input (light), while respiration releases energy.

• Light energy is captured by chlorophyll, converted to chemical energy, and stored in sugars.

7.6 Visible Radiation Absorbed by Pigments Drives the Light Reactions

Light and Pigments

• Sunlight is electromagnetic energy; visible light is a small part of the electromagnetic spectrum.

• Light travels in waves; wavelength is the distance between wave crests.

• Light consists of photons, discrete packets of energy. Shorter wavelengths have higher energy.

• Plant pigments (e.g., chlorophyll) absorb some wavelengths and reflect or transmit others (green is reflected/transmitted).

• Chloroplasts contain several pigments:

  • Chlorophyll a: absorbs blue-violet and red, reflects green.

  • Chlorophyll b: absorbs blue and orange, appears olive-green.

  • Carotenoids: broaden absorption spectrum, provide photoprotection by dissipating excess energy.

7.7 Photosystems Capture Solar Energy

Photosystems and Energy Transfer

• Pigments in chloroplasts absorb photons, raising electrons to a higher energy state.

• In the thylakoid membrane, pigment molecules are organized into photosystems (light-harvesting complexes and reaction-center complex).

• Energy is transferred from pigment to pigment until it reaches the reaction center, where a primary electron acceptor captures the excited electron.

• This is the first step in converting light energy to chemical energy.

Summary Table: Key Components of Photosynthesis

Component	Location	Function
Chlorophyll	Thylakoid membrane	Absorbs light energy
Stomata	Leaf surface	Gas exchange (CO2 in, O2 out)
Thylakoid	Chloroplast	Site of light reactions
Stroma	Chloroplast	Site of Calvin cycle (sugar synthesis)

Example: Photosynthesis Equation

The overall chemical equation for photosynthesis is:

$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

This equation summarizes the conversion of carbon dioxide and water into glucose and oxygen, powered by light energy.

Additional info: Later sections (not shown here) would cover the details of the light reactions, Calvin cycle, and global significance of photosynthesis.