Photosynthesis: Structure, Function, and Mechanisms

Photosynthesis

Introduction to Photosynthesis

Photosynthesis is the process by which solar energy is converted into chemical energy in the form of organic molecules, primarily glucose. This process is fundamental to life on Earth, as it provides the energy and organic matter required by most living organisms.

• Autotrophs are organisms that produce their own food from inorganic substances. They are the producers in ecosystems.

• There are two main types of autotrophs:

  • Chemoautotrophs: Use energy from inorganic compounds (e.g., hydrogen sulfide, iron) to build organic molecules from CO2. Most are bacteria and archaea in extreme environments.

  • Photoautotrophs: Use light energy to convert CO2 into organic molecules. Includes plants, algae, and cyanobacteria.

• Heterotrophs cannot make their own food and must consume other organisms. They are the consumers in ecosystems, including animals, fungi, and many prokaryotes.

Overview of Photosynthesis

• Photosynthesis Equation:

$6\ \mathrm{CO}_2 + 6\ \mathrm{H}_2\mathrm{O} + \text{Light Energy} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{O}_2$

• Photosynthesis is a redox reaction:

  • Water is oxidized (loses electrons) to form O2.

  • CO2 is reduced (gains electrons) to form glucose.

• The process is the reverse of cellular respiration.

Sites of Photosynthesis

• Leaves are the primary sites of photosynthesis in plants.

• Chlorophyll is the green pigment in chloroplasts that absorbs light energy.

• Stomata are microscopic pores in leaves that allow gas exchange (CO2 in, O2 out).

• Mesophyll is the interior tissue of the leaf where most chloroplasts are found (30-40 per cell).

• Thylakoids are membrane sacs within chloroplasts, often stacked into grana.

• Stroma is the dense fluid outside the thylakoids.

The Structure of the Chloroplast

• Outer membrane and inner membrane surround the organelle.

• Thylakoid lumen is the internal space of the thylakoid.

• Granum is a stack of thylakoids.

• Stroma is the aqueous fluid surrounding the thylakoids.

An Overview of Photosynthesis

• Photosynthesis has two main stages:

  1. Light Reactions (in thylakoids): Split water, release O2, produce ATP and NADPH.

  2. Calvin Cycle (in stroma): Forms sugar from CO2 using ATP and NADPH.

The Nature of Sunlight

Properties of Light

• Light is a form of electromagnetic energy (radiation).

• Light consists of discrete energy particles called photons.

• Wavelength determines the type of electromagnetic radiation.

Pigments

• Pigments absorb certain wavelengths of visible light; others are reflected or transmitted.

• Chlorophyll is the main pigment in chloroplasts, reflecting green light and absorbing other wavelengths.

• Accessory pigments (chlorophyll b, carotenoids) broaden the spectrum of light used for photosynthesis.

Absorption Spectrum: A graph showing which wavelengths are absorbed by different pigments. Chlorophyll a absorbs best at blue-violet and red wavelengths, chlorophyll b at blue and orange, and carotenoids at blue-green.

Phytoplankton and Global Oxygen

• Phytoplankton are microscopic, photosynthetic organisms in aquatic ecosystems, including cyanobacteria and algae.

• They produce 50-85% of the world's oxygen via photosynthesis.

The Light Reactions

Photosystems

• A photosystem is a protein complex in the thylakoid membrane that converts light energy into chemical energy.

• Photosystems contain pigment molecules that transfer energy to special chlorophyll a molecules.

• There are two types: Photosystem II (P680) and Photosystem I (P700).

Light Reactions Overview

• Capture light energy to synthesize ATP and NADPH.

• Electrons move from water (donor) to NADP+ (acceptor), forming NADPH.

• Key components:

  • Photosystem II

  • Electron transport chain

  • Photosystem I

  • NADP+ reductase

  • ATP synthase

Photosystem II

• Light excites electrons in P680 chlorophyll a molecules.

• High-energy electrons are captured and passed to the electron transport chain.

• Light energy also splits water into O2, H+, and electrons (photolysis).

• Electrons from water replace those lost by P680.

Electron Transport Chain

• Electrons lose energy as they move through the chain.

• Energy is used to pump H+ ions into the thylakoid space, creating a proton gradient.

Photosystem I

• Light excites electrons in P700 chlorophyll a molecules.

• Electrons are transferred to NADP+ reductase.

• Electrons from the electron transport chain replace those lost by P700.

NADP+ Reductase

• Transfers electrons to NADP+, reducing it to NADPH.

• NADPH carries high-energy electrons to the Calvin cycle.

Chemiosmosis

• The proton gradient drives ATP synthesis via ATP synthase.

• H+ diffuses from the thylakoid space to the stroma.

• ATP produced is used in the Calvin cycle.

Comparison: Chemiosmosis in chloroplasts is similar to that in mitochondria, but the direction of proton flow and the source of energy differ.

The Calvin Cycle

Calvin Cycle Overview

The Calvin cycle uses ATP and NADPH from the light reactions to convert CO2 into sugar. It occurs in the stroma of the chloroplast and consists of three main phases:

1. Carbon Fixation: CO2 is attached to ribulose bisphosphate (RuBP) by the enzyme rubisco, forming 3-phosphoglycerate (3-PGA).

2. Reduction: ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a three-carbon sugar.

3. Regeneration of RuBP: Some G3P is used to regenerate RuBP, enabling the cycle to continue. This step also requires ATP.

• For every three turns of the cycle, one G3P is produced and can be used to form glucose and other organic molecules.

• Two G3P molecules combine to form one glucose molecule (cycle must turn six times for one glucose).

Fate of G3P

• G3P can be used to synthesize glucose, fatty acids, amino acids, and other organic compounds.

Summary Table: Key Components of Photosynthesis

Component	Location	Function
Chlorophyll	Thylakoid membrane	Absorbs light energy
Photosystem II	Thylakoid membrane	Splits water, initiates electron transport
Electron Transport Chain	Thylakoid membrane	Creates proton gradient, transfers electrons
Photosystem I	Thylakoid membrane	Transfers electrons to NADP+
NADP+ Reductase	Thylakoid membrane	Reduces NADP+ to NADPH
ATP Synthase	Thylakoid membrane	Produces ATP via chemiosmosis
Calvin Cycle	Stroma	Fixes CO2, produces G3P (sugar)

Key Equations

• Overall Photosynthesis:

$6\ \mathrm{CO}_2 + 6\ \mathrm{H}_2\mathrm{O} + \text{Light Energy} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{O}_2$

• Redox Summary:

$\text{CO}_2$ is reduced to $\text{C}_6\text{H}_{12}\text{O}_6$ $\text{H}_2\text{O}$ is oxidized to $\text{O}_2$

Additional info:

• Phytoplankton are crucial for global oxygen production and aquatic food webs.

• Accessory pigments help plants utilize a broader range of the light spectrum.

• ATP and NADPH produced in the light reactions are essential for the Calvin cycle.