9_Photosynthesis

Cell & Molecular Biology: Photosynthesis

Learning Objectives

• Understand the properties of light.

• Analyze pigment structure and organization in energy acquisition from light.

• Explain the function of photosystems in light energy conversion to chemical energy.

• Recognize the significance of water in photosynthesis.

• Identify reactants, products, and energy transformations in the light reactions and Calvin cycle.

• Relate chloroplast structure to function.

• Compare and contrast cellular respiration and photosynthesis.

Connecting Photosynthesis and Cellular Respiration

• Photosynthesis Equation: 6CO2 + 6H2O + energy → C6H12O6 + 6O2

• Differences Between Processes:

  • Occur in different organelles.

  • Involve different enzymes.

  • Utilize different reactions.

Introduction to Photosynthesis

• Definition: Photosynthesis is the process by which organisms use sunlight to manufacture carbohydrates.

• Autotrophs: Organisms that produce their own food (e.g., plants).

• Heterotrophs: Organisms that must obtain sugars from other organisms (e.g., animals).

Photosynthesis Overview

• Converts electromagnetic energy into chemical energy.

• Requirements:

  • Sunlight, carbon dioxide, and water.

• By-products: Oxygen is produced as a by-product.

Types of Photosynthetic Reactions

1. Light-Capturing Reactions

   • Produce O2 from H2O.

   • Use ATP and electrons to reduce CO2.

2. Calvin Cycle Reactions

   • Produce sugar from CO2.

   • Water is split to form O2.

   • Excited electrons create ATP and convert NADP+ to NADPH.

Photosynthesis Location

• Chloroplasts:

  • Surround by double membranes.

  • Interior contains thylakoid membranes arranged in grana.

  • Stroma is the fluid-filled space between thylakoids.

Solar Energy and Light

• Electromagnetic Radiation: Energy form with light being a key type.

• Characterization of Light:

  • Wavelength corresponds to distance between wave crests.

  • Photons are discrete packets of light energy.

The Electromagnetic Spectrum

• Includes gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves, and radio waves.

• Visible Light Wavelength: Ranges approximately from 400 nm (blue) to 700 nm (red).

• Energy levels are highest at shorter wavelengths and lowest at longer wavelengths.

Pigments in Chloroplasts

• Thylakoid membranes contain pigments absorbing specific light wavelengths.

• Common Pigment: Chlorophyll, which reflects green light and imparts green color to plants.

Absorption of Light by Pigments

• Chlorophyll a and b:

  • Similar structures with a long isoprenoid tail and a magnesium-containing head.

  • Effective in absorbing specific wavelengths of light.

Accessory Pigments: Carotenoids

• Function: Extend the range of wavelengths absorbed and help transfer energy to chlorophyll.

• Color Change in Trees: Carotenoids visible in autumn after chlorophyll degradation signify an adaptation in perception of light.

Photosynthesis: How Light Energy Affects Electrons

• Absorption of a photon by chlorophyll excites electrons, moving them to a higher energy state.

• If an electron returns to the ground state, energy is released as heat or light (fluorescence).

Photosystems

• Complex of chlorophyll and accessory pigments serving as light-gathering antennas.

• Function to direct energy to the reaction center.

Photosystem II & ATP Production

• Converts electromagnetic energy into chemical energy (ATP).

Light Energy Conversion to Chemical Energy

• Electron Transport Chain (ETC): Similarity between thylakoid and mitochondrial ETCs in structure and function.

• Protons are transported, creating a proton-motive force driving ATP production via ATP synthase.

Water’s Role in Photosynthesis

• Water Splitting Reaction: 2 H2O → 4 H+ + 4 e- + O2.

• Oxygen as a waste product from this reaction while electrons replenish those lost in Photosystem II.

Photosystem I and NADPH Production

• Produces NADPH, an essential electron carrier in photosynthesis.

• Processes high-energy electrons derived from photosynthetic reactions.

The Z-Scheme Model

• Electron Flow: Water and light energy drive the production of O2, ATP, and NADPH.

• NADP+ acts as the final electron acceptor from Photosystem I.

Mechanism of Carbon Dioxide Capture

• Cuticle: Waxy layer that prevents water loss and CO2 gas exchange.

• Stomata: Pores for gas exchange, facilitated by guard cells.

  • Allows CO2 entry and O2 exit, maintained by a concentration gradient through the Calvin cycle.

Calvin Cycle Overview

• Carbon Fixation: Conversion of CO2 into a usable organic form.

• Key Enzyme: Rubisco, critical in facilitating CO2 fixation.

Three-Step Process of the Calvin Cycle

1. Fixation Phase: CO2 combines with RuBP, producing 3PGA.

2. Reduction Phase: 3PGA gets phosphorylated and reduced to G3P.

3. Regeneration Phase: G3P is used to regenerate RuBP using ATP.

Outcome of the Calvin Cycle

• Converts energy from ATP and NADPH into high-energy sugars like G3P, leading to glucose and starch synthesis.

• Significance: Forms the basis of energy and structural components in many organisms.

Overall Summary of Photosynthesis

• Location: Chloroplasts

• Main Reactants: Light, H2O, CO2

• Products: Sugars (e.g., glucose), O2, ATP, NADPH

• Process: Divided into light reactions and the Calvin cycle.

Comparing Cellular Respiration and Photosynthesis

Key Questions to Consider:

• Location of the electron transport chain (ETC).

• Source of high-energy electrons in each process.

• Mechanisms of ATP production.

• Final electron acceptors in cellular respiration versus photosynthesis.

• Characteristics of high-energy electron carriers.