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.

Photosynthesis light-capturing reactions involve the following steps:

  • Sunlight is absorbed by pigments like chlorophyll, exciting electrons to a higher energy state.

  • These excited electrons are then transferred to an electron transport chain (ETC).

  • Water molecules are split (2 H2O → 4 H+ + 4 e- + O2), producing oxygen as a waste product.

  • The resulting hydrogen ions (protons) accumulate in the thylakoid lumen, creating a proton gradient.

  • This proton gradient drives hydrogen ions through ATP synthase, leading

The Calvin Cycle involves these steps:

  • Carbon Fixation: CO_2 is converted into an organic form.

  • Key Enzyme: Rubisco helps with CO_2 fixation.

Three-Step Process:

  1. Fixation Phase: CO_2 combines with RuBP, producing 3PGA.

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

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

Outcome:

  • 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.