Chapter7

Biochemical Pathways: Photosynthesis

Introduction

• Photosynthesis is the process through which organisms utilize light energy to synthesize high-energy organic molecules.

• Key organisms: Plants, algae, and some bacteria known as photosynthetic autotrophs.

• Importance: 99.9% of all life on Earth relies on photosynthesis for energy.

• Energy flow: Heterotrophs consume autotrophs for energy.

Photosynthesis Overview

Key Structures

• Chloroplasts: Organelles where photosynthesis occurs, containing the pigment chlorophyll in membranous sacs called thylakoids.

• Granum (pl. grana): Stack of thylakoids, suspended in the stroma, a fluid-filled space.

Energy Processing

• Light energy is converted into ATP (adenosine triphosphate), which is then used to produce organic molecules like glucose.

Detailed Breakdown of Photosynthesis

Chemical Equation

• Overall reaction: Light energy + Carbon dioxide + Water → Glucose + Oxygen.

Three Main Events of Photosynthesis

1. Light-Capturing Events: Chlorophyll absorbs certain light wavelengths, exciting its electrons.

2. Light-Dependent Reactions: Use the energy from excited electrons to create ATP and NADPH.

3. Light-Independent Reactions: Utilize ATP and NADPH to convert CO2 into glucose.

Light-Capturing Events

Properties of Light

• Visible Light: Comprised of different wavelengths perceived as colors.

• Pigments: Molecules that absorb specific wavelengths; unabsorbed wavelengths are reflected (the visible color).

• Chlorophyll: Main photosynthetic pigment with two forms (a and b), absorbing blue and red light while reflecting green.

Light-Dependent Reactions

Mechanism

• Excited electrons from chlorophyll are transferred through an electron transport chain (ETC).

• Energy released pumps protons, creating a concentration gradient.

• ATP Synthase: Utilizes proton diffusion to produce ATP.

• Water is split to replenish electrons in chlorophyll, releasing oxygen.

• Location: Reactions occur in thylakoid membranes, ATP and NADPH then move to the stroma.

Light-Independent Reactions (Calvin Cycle)

Process

• ATP and NADPH provide energy and electrons to convert CO2 into organic sugars.

• RuBisCo: Enzyme that facilitates the first step, combining CO2 with ribulose.

• The initial product is a 6-carbon molecule, which quickly splits into two 3-carbon molecules.

• NADPH reduces these molecules to form Glyceraldehyde-3-phosphate (G3P), which is a building block for glucose and other organic compounds.

Products and Uses of G3P

• G3P can:

  • Be converted into glucose.

  • Recycle ribulose for the Calvin Cycle.

  • Synthesize sugars, fats, and proteins (amino acids).

  • Participate in glycolysis for energy production.

Plant Metabolism

• Photosynthetic products are used for:

  • Creating fats, proteins, and carbohydrates.

  • Producing protective toxins and medicinal compounds.

  • Synthesizing essential vitamins that humans cannot produce.

Interrelationships Between Autotrophs and Heterotrophs

• Autotrophs (like plants) convert light energy into food, while Heterotrophs (like animals) consume autotrophs for energy.

• Plants use produced sugars and oxygen, while animals contribute CO2 and nutrients back into the ecosystem.

• This cycle highlights the interconnected nature of life, ensuring mutual benefit and energy transfer.