Photosynthesis Overview

PHOTOSYNTHESIS: USING LIGHT TO MAKE FOOD (CH. 5)

Lecture Objectives:

• Review autotrophs vs heterotrophs

• Identify the reactants and products of the photosynthesis equation

• Know the 2 steps of photosynthesis:

  • Light reactions

  • Calvin Cycle

Autotrophs vs Heterotrophs

• Autotrophs

  • Definition: Organisms that can produce their own food.

  • Example: Plants (specifically photoautotrophs) that use sunlight and carbon to generate chemical energy (glucose).

• Heterotrophs

  • Definition: Organisms that cannot produce their own energy and must consume other organisms.

  • Example: Humans and other animals.

  • Importance: All life depends on photosynthesis, as it is the source of energy for heterotrophs.

Sunlight as Energy Source

• Sunlight is the ultimate source of energy in most ecosystems.

• Photosynthetic organisms capture sunlight to manufacture glucose from CO2 and H2O, releasing O2 into the atmosphere.

• Cellular respiration is performed by both photosynthetic and non-photosynthetic organisms to break down sugars and generate energy, releasing CO2 and H2O.

Inputs and Outputs of Photosynthesis

• Reactants (inputs):

  • Sunlight

  • Water (H₂O)

  • Carbon Dioxide (CO₂)

• Products (outputs):

  • Glucose (sugar)

  • Oxygen (O₂)

Location of Photosynthesis

• Occurs in organelles called chloroplasts.

  • Gas exchange of CO2 and O2 occurs in openings called stomata.

  • Chloroplasts contain thylakoids that house chlorophyll molecules, the pigment responsible for absorbing light.

The Photosynthesis Equation

• Reflects the reactants and products:

  \text{6CO}2 + \text{6H}2\text{O} \xrightarrow{sunlight} \text{C}6\text{H}{12}\text{O}6 + \text{6O}2

Two Stages of Photosynthesis

1. Light Reactions

   • Energy carriers are generated using:

     • Sunlight

     • Water (H₂O)

   • Outputs:

     • Oxygen (O₂)

     • ATP and NADPH (energy carriers)

2. Calvin Cycle (Light-Independent Reactions)

   • Uses energy from ATP and NADPH.

     • Carbon dioxide (CO₂)

   • Output:

     • Glucose (sugar)

   • Significance: Carbon fixation converts inorganic carbon (CO₂) to organic carbon (sugars) for living organisms.

   • Involves the most abundant enzyme on the
     planet: rubisLight Energy Characteristics

• Light energy is compose of energy packets called photons.

• Light travels in waves, and different wavelengths (visible light range: 380-750 nm) have varying energies.

• Chlorophyll absorbs mainly blue-violet and red light, reflecting green light, which is why plants appear green.

Light Dependent Reactions

• Process involves:

  • Capture of light energy by pigments (photosystems) in thylakoids.

  • Water-splitting releases O₂.

  • Generation of ATP and NADPH through electron transport chain.

Key Components in Light Reactions

• Photosystems: Protein complexes that absorb light.

• Reaction center chlorophyll: Transfers energy and electrons.

• Water-splitting reaction (Photosystem II) occurs here, producing O₂ and providing electrons for the transport chain.

Calvin Cycle Details

• Occurs in the stroma of chloroplasts.

• Utilizes:

  • CO₂

  • ATP

  • NADPH

• Generates glucose and other organic compounds (cellulose, starch).

Importance of Photosynthesis

• Photosynthesis is foundational to all energy-generating processes in ecosystems.

• It drives the energy flow and is essential for cellular respiration in heterotrophs.