Concept 8.1 – Photosynthesis Feeds the Biosphere

Overview of Matter and Energy Flow

Photosynthesis is a fundamental biological process that sustains life on Earth by converting solar energy into chemical energy. It forms the basis of energy flow and matter cycling between plants, animals, and other organisms.

• Autotrophs: Organisms (such as plants and some bacteria) that produce their own food from inorganic substances using light or chemical energy. They are also called producers.

• Heterotrophs: Organisms (such as animals and fungi) that obtain energy by consuming other organisms. They are also called consumers.

• Energy Flow: Energy enters the biosphere primarily through photosynthesis, where light energy is converted into chemical energy stored in glucose and other organic molecules.

• Carbon Cycling: Photosynthesis removes CO2 from the atmosphere and incorporates it into organic molecules, which are then used by heterotrophs for energy and growth.

• Example: Plants use sunlight to synthesize glucose, which is then consumed by animals for energy.

Concept 8.2 – Photosynthesis Converts Light to Chemical Energy

Chloroplast Structure and Function

Photosynthesis occurs in the chloroplasts of plant cells, which contain specialized structures and pigments to capture light energy.

• Chloroplast Anatomy:

  • Thylakoids: Flattened sacs where light-dependent reactions occur.

  • Grana: Stacks of thylakoids.

  • Stroma: Fluid-filled space surrounding the grana, where the Calvin cycle takes place.

• Inputs and Outputs:

  • Inputs: Light energy, water (H2O), and carbon dioxide (CO2).

  • Outputs: Glucose (C6H12O6), oxygen (O2), and water.

• General Equation for Photosynthesis:

• Energy Transformation: Light energy is converted into chemical energy stored in glucose.

• Example: In a leaf, chloroplasts capture sunlight and use it to synthesize sugars from CO2 and H2O.

Concept 8.3 – Light Reactions Produce ATP and NADPH

Light Absorption and Pigments

The light reactions of photosynthesis capture solar energy and convert it into chemical energy in the form of ATP and NADPH.

• Light Energy and Wavelength: Light is a form of electromagnetic radiation. The energy of light is inversely related to its wavelength; shorter wavelengths have higher energy.

• Pigments: Molecules such as chlorophyll a, chlorophyll b, and carotenoids absorb specific wavelengths of light. Chlorophyll appears green because it reflects green light and absorbs red and blue wavelengths.

• Absorption Spectrum: A graph showing the wavelengths of light absorbed by a pigment. Action spectra show the effectiveness of different wavelengths in driving photosynthesis.

• Excitation of Electrons: When pigments absorb light, electrons are elevated to higher energy levels (excited state).

• Photosystems: Complexes of proteins and pigments (Photosystem II and Photosystem I) embedded in the thylakoid membrane. Each photosystem has a reaction center chlorophyll and a primary electron acceptor.

• Electron Transport Chain: Excited electrons move through a series of proteins, leading to the production of ATP (via chemiosmosis) and NADPH.

• ATP and NADPH Production:

  • ATP is produced by photophosphorylation as protons flow through ATP synthase.

  • NADP+ is reduced to NADPH, which carries high-energy electrons for use in the Calvin cycle.

• Example: In the light reactions, sunlight excites electrons in chlorophyll, which are transferred through the electron transport chain to generate ATP and NADPH.

Concept 8.4 – The Calvin Cycle Reduces CO2 to Sugar

Phases and Reactants of the Calvin Cycle

The Calvin cycle is a series of enzyme-catalyzed reactions that occur in the stroma of the chloroplast, using ATP and NADPH to convert CO2 into glucose.

• Major Reactants: CO2, ATP, NADPH.

• Major Products: G3P (glyceraldehyde-3-phosphate, a 3-carbon sugar), ADP, NADP+, and inorganic phosphate.

• Three Phases of the Calvin Cycle:

  1. Carbon Fixation: CO2 is attached to a five-carbon sugar (RuBP) by the enzyme rubisco.

  2. Reduction: ATP and NADPH are used to convert 3-phosphoglycerate into G3P.

  3. Regeneration of RuBP: Some G3P molecules are used to regenerate RuBP, enabling the cycle to continue.

• Example: For every three molecules of CO2 that enter the cycle, one G3P is produced, which can be used to form glucose and other organic molecules.

Concept 8.5 – Life Depends on Photosynthesis

Photosynthesis and the Biosphere

Photosynthesis is essential for life on Earth, providing the organic molecules and oxygen required by most living organisms. It forms the foundation of food webs and supports the energy needs of ecosystems.

• Oxygen Production: Photosynthesis releases O2 as a byproduct, which is vital for aerobic respiration in animals and other organisms.

• Food Source: All food chains ultimately depend on the organic molecules produced by photosynthetic organisms.

• Example: Forests and phytoplankton are major contributors to global oxygen and food production.