Classification of Organisms by Food Acquisition

Heterotrophs and Autotrophs

Organisms can be classified based on how they obtain their food molecules. The two main categories are heterotrophs and autotrophs.

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

• Autotrophs: Are able to produce their own organic food molecules by internal biochemical processes. There are two main types of autotrophs:

  • Chemoautotrophs: Synthesize food from chemical energy, usually by oxidizing inorganic molecules.

  • Photoautotrophs: Use light energy to convert inorganic molecules into organic molecules. The process that converts light energy to chemical energy is photosynthesis.

Examples of photoautotrophs:

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

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

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

Photosynthesis Overview

General Reaction and Characteristics

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

• Overall reaction of photosynthesis:

• Endergonic: Requires absorption of light energy for the reactions to occur.

• Anabolic: Carbon dioxide is built into larger glucose molecules.

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

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

Leaf Structure and Gas Exchange

Photosynthesis mainly takes place in the leaves, which are adapted to maximize light absorption and gas exchange.

• Palisade cells: Contain many chloroplasts and are located near the leaf surface.

• Stomata: Pores on the leaf surface surrounded by guard cells, allowing for gas exchange.

• Oxygen: Released as a byproduct of photosynthesis.

• Water: Lost through transpiration via the stomata.

Light and Pigments in Photosynthesis

Visible Light and Pigments

Photosynthesis is driven by light in the visible spectrum (wavelengths 400–700 nm). Pigments in plants absorb specific wavelengths of light.

• Main pigments:

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

  • Chlorophyll b: Accessory pigment, absorbs blue and red light.

  • Carotenoids: Orange accessory pigments, e.g., beta carotene.

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

• Stroma: Fluid inside the chloroplast surrounding the grana.

• Thylakoid membrane: Contains photosystems and pigment complexes.

Photosynthesis: The Two Main Phases

Light-Dependent Reactions

These reactions occur in the thylakoid membranes and require light to produce ATP and NADPH.

• Light is absorbed by pigments in photosystem II (PS II), exciting electrons.

• Electrons move from P680 to the primary electron acceptor.

• Photolysis: Water is oxidized, releasing electrons to replace those lost by P680.

• Electrons pass through the electron transport chain, pumping hydrogen ions into the thylakoid space.

• Light contacts pigments of photosystem I (PS I), exciting electrons again.

• Electrons are transferred to NADP+ to form NADPH.

• Hydrogen ions move through ATP synthase to produce ATP from ADP and phosphate.

Key enzymes:

• NADP+ reductase: Reduces NADP+ to NADPH.

• ATP synthase: Produces ATP via photophosphorylation.

Calvin Cycle (Light-Independent Reactions)

The Calvin Cycle uses ATP and NADPH to convert carbon dioxide into glucose. It occurs in the stroma of the chloroplast.

• Carbon dioxide is fixed to ribulose bisphosphate (RuBP) by the enzyme Rubisco.

• The unstable 6-carbon product splits into two 3-carbon molecules: 3-phosphoglycerate (3-PGA).

• 3-PGA is converted to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.

• Excess G3P is converted to glucose and other organic molecules.

• Some G3P is recycled to regenerate RuBP.

Variations in Photosynthetic Pathways

C4 Photosynthesis

C4 photosynthesis is an adaptation found in plants in hot, dry environments. It separates carbon fixation and the Calvin cycle into different cell types to minimize water loss and photorespiration.

• CO2 reacts with phosphoenolpyruvate (PEP) to form oxaloacetate, a 4-carbon molecule.

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

Crassulacean Acid Metabolism (CAM)

CAM photosynthesis is an adaptation in succulents and other plants in arid environments. Stomata open at night to absorb CO2, which is stored as malate and released during the day for photosynthesis.

• Stomata open at night to absorb CO2 and form oxaloacetate and malate.

• During the day, malate breaks down to release CO2 for the Calvin cycle.

Key Terms and Concepts

• Photorespiration: A process where oxygen is used instead of carbon dioxide, reducing photosynthetic efficiency.

• Photophosphorylation: The production of ATP using light energy in the thylakoid membrane.

• Rubisco: The enzyme responsible for carbon fixation in the Calvin cycle.

• ATP and NADPH: Energy carriers produced in the light-dependent reactions and used in the Calvin cycle.

Summary Table: Types of Photosynthetic Pathways