Photosynthesis: Using Light to Make Food

Introduction to Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert solar energy into chemical energy, producing food and oxygen essential for most life on Earth. This process underpins the energy flow in ecosystems and is central to the global carbon and oxygen cycles.

• Biofuels are energy sources derived from living material, ultimately storing solar energy captured by photosynthesis.

• Photosynthesis converts solar energy into the chemical energy of sugars, which can be used by living cells or transformed into biofuels.

7.1 Photosynthesis Powers Most Life on Earth

• Photoautotrophs are organisms (such as plants, algae, and some protists and bacteria) that use light energy to synthesize organic molecules from inorganic substances.

• Heterotrophs are organisms that obtain energy by consuming other organisms or organic matter.

• Photoautotrophs are the producers in ecosystems, forming the base of food chains.

• Checkpoint: Photoautotrophs require sunlight, carbon dioxide (CO2), and water (H2O) from the environment to make their own food.

7.2 Photosynthesis Occurs in Chloroplasts in Plant Cells

• Chloroplasts are organelles surrounded by a double membrane, containing stacks of thylakoids and a fluid called stroma.

• Chlorophyll is a pigment in chloroplasts that absorbs light energy, initiating the process of photosynthesis.

• Checkpoint: CO2 enters leaves through stomata, and water is absorbed by roots and transported via xylem to the chloroplasts.

7.3 Scientists Traced the Process of Photosynthesis Using Isotopes

Isotope experiments clarified the source of oxygen released during photosynthesis.

• Using heavy oxygen isotope (O-18), scientists showed that the O2 produced comes from water, not CO2.

• Photosynthesis equation:

• Checkpoint: Most of the mass of organic matter produced by photosynthesis comes from CO2 in the air.

7.4 Photosynthesis Is a Redox Process

Photosynthesis involves the transfer of electrons (redox reactions), similar to cellular respiration but in reverse direction.

• In photosynthesis, H2O is oxidized (loses electrons) and CO2 is reduced (gains electrons).

• Cellular respiration is an exergonic process (releases energy), while photosynthesis is endergonic (requires energy input).

• Checkpoint: Photosynthesis is endergonic; cellular respiration is exergonic.

7.5 Photosynthesis Occurs in Two Stages, Linked by ATP and NADPH

• The light reactions occur in the thylakoid membranes, producing ATP and NADPH.

• The Calvin cycle occurs in the stroma, using ATP and NADPH to fix carbon and synthesize sugars.

• Carbon fixation is the incorporation of CO2 into organic molecules.

• Checkpoint: To produce sugar from CO2 in the dark, chloroplasts need ATP and NADPH.

The Light Reactions: Converting Solar Energy to Chemical Energy

7.6 Visible Radiation Absorbed by Pigments Drives the Light Reactions

• Sunlight is electromagnetic energy; only certain wavelengths (visible light) are absorbed by photosynthetic pigments.

• Chlorophyll absorbs blue and red light, reflecting green (least effective for photosynthesis).

• Carotenoids absorb excess light, protecting chlorophyll from damage (photoprotection).

• Checkpoint: Green light is least effective at driving photosynthesis because it is reflected, not absorbed.

7.7 Photosystems Capture Solar Energy

• Photosystems are complexes in thylakoid membranes composed of light-harvesting complexes and a reaction-center complex.

• Photoexcited electrons from chlorophyll a are transferred to a primary electron acceptor.

• Checkpoint: In intact chloroplasts, energy is used for chemical work, not released as heat and light.

7.8 Two Photosystems Connected by an Electron Transport Chain

• Electrons move from photosystem II to photosystem I via an electron transport chain, generating ATP and reducing NADP+ to NADPH.

• Photosystem II regains electrons by splitting water, releasing O2.

• Checkpoint: Two photons are required to move electrons from water to NADPH—one for each photosystem.

7.9 The Light Reactions Take Place Within the Thylakoid Membranes

• During photophosphorylation, the electron transport chain pumps H+ into the thylakoid space, creating a gradient.

• H+ flows back through ATP synthase, driving ATP synthesis.

The Calvin Cycle: Reducing CO2 to Sugar

7.10 ATP and NADPH Power Sugar Synthesis in the Calvin Cycle

• The Calvin cycle consists of four main steps:

  1. Carbon fixation

  2. Reduction

  3. Release of G3P (glyceraldehyde-3-phosphate)

  4. Regeneration of RuBP (ribulose bisphosphate)

• Inputs: CO2, ATP, NADPH; Output: G3P (used to build glucose and other organic molecules).

The Global Significance of Photosynthesis

7.12 Photosynthesis Provides Food and O2 For Almost All Living Organisms

• About 50% of carbohydrates made by photosynthesis are used in cellular respiration by plant mitochondria.

• Sugars are also used to synthesize proteins, lipids, and cellulose (main component of cell walls).

• Checkpoint: Photosynthesis is essential for life, providing food and oxygen for nearly all organisms.

7.13 Scientific Thinking: Rising Atmospheric Levels of Carbon Dioxide May Affect Plants

• Scientists use laboratory and field studies to assess the effects of elevated CO2 on plant growth and ecosystems.

• Long-term studies help understand the impact of climate change on plant communities.

7.14 Connection: Reducing Both Fossil Fuel Use and Deforestation May Moderate Climate Change

• CO2 and other greenhouse gases trap heat in the atmosphere, causing the greenhouse effect.

• Global warming is a long-term increase in Earth's average temperature, driven by increased greenhouse gases.

• International agreements, such as the Paris climate conference (2015), aim to reduce emissions and limit warming.

• Checkpoint: The greenhouse effect is the warming of Earth due to atmospheric gases trapping heat radiated from the planet's surface.

Summary Table: The Two Stages of Photosynthesis

Key Terms and Definitions

• Autotroph: An organism that makes its own food from inorganic substances.

• Heterotroph: An organism that obtains food by consuming other organisms.

• Photoautotroph: An autotroph that uses light energy to synthesize organic compounds.

• Chloroplast: Organelle where photosynthesis occurs in plants and algae.

• Thylakoid: Membranous sac within chloroplasts; site of the light reactions.

• Stroma: Fluid-filled space in chloroplasts surrounding the thylakoids; site of the Calvin cycle.

• Chlorophyll: Green pigment that absorbs light energy for photosynthesis.

• Photosystem: Cluster of pigments and proteins that capture light energy.

• ATP (Adenosine Triphosphate): Main energy carrier in cells.

• NADPH: Electron carrier used in the Calvin cycle.

• Calvin Cycle: Series of reactions that synthesize sugar from CO2 using ATP and NADPH.

• Carbon fixation: Incorporation of CO2 into organic molecules.

• Photophosphorylation: Production of ATP using the energy of sunlight.

• Greenhouse effect: Warming of Earth due to atmospheric gases trapping heat.

Summary of Key Concepts

• Photosynthesis is essential for life, providing food and oxygen for most organisms.

• It occurs in two stages: the light reactions (in thylakoids) and the Calvin cycle (in stroma).

• Light energy is captured by pigments and used to produce ATP and NADPH.

• The Calvin cycle uses ATP and NADPH to fix carbon and synthesize sugars.

• Photosynthesis helps regulate atmospheric CO2 and influences global climate.

Additional info: For a deeper understanding, students should review the mechanisms of C3, C4, and CAM photosynthesis, and the impact of environmental factors on photosynthetic efficiency.