Classification of Organisms by Food Acquisition

Heterotrophs and Autotrophs

Organisms can be classified based on how they obtain food molecules. This distinction is fundamental in biology and relates to energy flow in ecosystems.

• Heterotrophs: Must consume organic food molecules built by other organisms.

• Autotrophs: Can produce their own organic food molecules by internal biochemical processes. There are two main types:

  • Chemotrophs: Synthesize food using energy from chemical reactions involving inorganic molecules, without using light energy.

  • Phototrophs: Use light energy to convert inorganic molecules into organic food molecules. The process is called photosynthesis.

Examples of Phototrophic Organisms:

• Plants: Multicellular, land-adapted organisms that perform photosynthesis in leaves.

• Algae: Unicellular and multicellular aquatic organisms using chloroplasts for photosynthesis.

• Cyanobacteria: Prokaryotic organisms capable of photosynthesis, but lacking chloroplasts.

Photosynthesis: Overview and Chemical Reaction

General Reaction and Energetics

Photosynthesis is the process by which light energy is converted into chemical energy in the form of glucose. Most photosynthesis occurs in the leaves of plants.

• The overall chemical reaction is:

• Carbon dioxide is reduced: Gains electrons and hydrogen atoms to become glucose.

• Water is oxidized: Loses electrons and hydrogen to become oxygen.

• Endergonic reaction: Requires input of light energy.

• Anabolic process: Builds larger glucose molecules from smaller carbon dioxide molecules.

Leaf Structure and Gas Exchange

Leaf Anatomy and Stomata

Leaves are specialized organs for photosynthesis, containing cells called mesophyll that are rich in chloroplasts. Gas exchange occurs through stomata, pores surrounded by guard cells.

• Stomata: Allow carbon dioxide to enter and oxygen to exit the leaf.

• Guard cells: Regulate the opening and closing of stomata.

• Transpiration: Water vapor exits through stomata, aiding in nutrient transport and cooling.

Light and Pigments in Photosynthesis

Visible Light Spectrum and Pigments

Photosynthesis requires light in the visible spectrum (wavelengths 400–700 nm). Pigments in chloroplasts absorb specific wavelengths to capture energy.

• Chlorophyll a: Main pigment, absorbs red, blue, and violet light.

• Accessory pigments: Expand the range of absorbed light.

  • Chlorophyll b: Absorbs blue and red light.

  • Carotenoids: Orange/yellow pigments, e.g., beta-carotene.

  • Xanthophylls: Yellow pigments, absorb blue and violet light.

Chloroplast Structure:

• Double membrane organelle.

• Contains thylakoids (membranous sacs) stacked into grana.

• Stroma: Fluid surrounding the thylakoids, site of the Calvin cycle.

Photosystems and Electron Transport

Photosystem II and Photosystem I

Photosystems are complexes of pigments and proteins that absorb light and initiate electron transport.

• Photosystem II (PS II): Absorbs light, excites electrons, splits water (photolysis) to replace lost electrons.

• Photosystem I (PS I): Receives electrons from electron transport chain, further excites them, and transfers them to NADP+ to form NADPH.

Electron Transport and ATP Formation:

• Electrons move through an electron transport chain, pumping protons into the thylakoid space.

• Proton gradient drives ATP synthesis via photophosphorylation.

• NADPH is produced by reduction of NADP+ (enzyme: NADP+ reductase).

Phases of Photosynthesis

Light-Dependent Reactions

These reactions occur in the thylakoid membranes and require light.

• Light energy is absorbed by pigments in PS II.

• Electrons are excited and transferred to electron acceptors.

• Water is split (photolysis) to replace electrons, releasing O2.

• Electrons move through the electron transport chain, generating ATP and NADPH.

Calvin Cycle (Light-Independent Reactions)

Occurs in the stroma and uses ATP and NADPH to fix carbon dioxide into glucose.

• CO2 is attached to ribulose bisphosphate (RuBP) by the enzyme Rubisco.

• Forms an unstable 6-carbon intermediate, which splits into two 3-phosphoglycerate (3-PGA) molecules.

• ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P).

• Some G3P is used to regenerate RuBP; the rest is used to synthesize glucose and other organic molecules.

Alternative Photosynthetic Pathways

C4 Photosynthesis

Adaptation in plants from hot, dry environments to minimize photorespiration.

• CO2 is initially fixed into a 4-carbon compound (oxaloacetate) in mesophyll cells.

• Oxaloacetate is converted to malate, which releases CO2 in bundle sheath cells for the Calvin cycle.

Crassulacean Acid Metabolism (CAM)

Adaptation in succulents and other plants in arid environments.

• Stomata open at night to fix CO2 into organic acids (e.g., malate).

• During the day, CO2 is released from acids for the Calvin cycle while stomata are closed to conserve water.

Key Table: Comparison of Photosynthetic Pathways

Key Terms and Definitions

• Autotroph: Organism that produces its own food from inorganic sources.

• Heterotroph: Organism that consumes organic molecules produced by other organisms.

• Photosynthesis: Process by which light energy is converted to chemical energy in glucose.

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

• Stomata: Pores in leaves for gas exchange.

• Rubisco: Enzyme that catalyzes the first step of the Calvin cycle.

• Photophosphorylation: Production of ATP using light energy.

• NADPH: Reduced coenzyme, electron carrier in photosynthesis.

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table entries and some explanations have been logically inferred and supplemented for a self-contained study guide.