Photosynthesis Overview and History

Photosynthesis Overview

• Photosynthesis converts inorganic materials into organic materials
• Performed by autotrophs (producers).
• A complex chemical reaction
• Involves water and carbon dioxide
• Light is required for the reaction
• Oxygen and high-energy carbohydrates are produced

History of Photosynthesis

• Jan van Helmont (1643): Plants gain most of their mass from water
• Joseph Priestley (1771): Plants release oxygen
• Jan Ingenhousz (1779): Plants need sunlight to produce oxygen
• Julius Robert Mayer (1845): Plants convert light energy to chemical energy
• Frederick Frost Blackman (1905): Photosynthesis comprises two reactions
• Cornelius B. Van Neil (1930): Oxygen given off during photosynthesis comes from water
• Melvin Calvin (1948): Traced the path of carbon (CO2) in forming glucose; the Calvin cycle; also known as the Dark Reaction
• Rudolph Marcus (1992): Won Nobel Prize for describing electron transfer in electron transport chain

Photosynthetic Organisms

• Examples include prayer plants, kelp, and nostoc (producers) – plants and algae

Photosynthesis Equation

• 6 H₂O + 6 CO₂ + (light) → 6 O₂ + C₆H₁₂O₆

Photosynthesis Redox Reaction

• Reduction: Addition of electrons
• Oxidation: Loss of electrons

Location of Photosynthesis

• Takes place in the mesophyll tissues of leaves and stems
• Chloroplasts are the organelles where photosynthesis occurs

Chloroplast Structure

• Chloroplasts have a double membrane
• Stroma is fluid in the chloroplast
• Thylakoid membranes are flattened sacs inside the stroma
• Chlorophyll is contained within the chloroplasts
• Grana are stacks of thylakoid membranes

Pigments

• Pigments are light-absorbing molecules
• Chlorophyll a and b are the dominant pigments, appearing green
• Chlorophyll absorbs violet, blue and red light best
• Chlorophyll mostly reflects green light
• Carotenoids (yellow and orange) absorb violet-blue-green light

Photosystems

• Consist of pigment complexes and electron acceptors
• "Antenna" for gathering light
• Photosystem I (PSI) discovered first
  • P700
  • Produce ATP and NADPH
• Photosystem II (PSII)
  • P680
  • Produce ATP

Quick Review Questions (Light Reactions)

• Where does photosynthesis occur within a plant?
• What is the specific name of the plant tissue where photosynthesis occurs?
• What is contained within the thylakoid membrane?
• What are grana?
• What is the location of the stroma?

Electromagnetic Spectrum

• Visible light is a part of the electromagnetic spectrum that we can see
• Plants absorb different wavelengths of light

Two Reactions of Photosynthesis

• Light Reaction (Light-Dependent Phase)
  • Converts solar energy into ATP and NADPH
• Calvin Cycle (Light-Independent Phase / Dark Reaction)
  • Produces glucose using CO₂ and energy.

Light Reaction Details

• Takes place in the thylakoid membranes
• Requires light
• Converts solar energy into chemical energy (ATP and NADPH is formed)
• Uses two photosystems (PSI & PSII) and two electron pathways (cyclic and noncyclic)

Noncyclic Electron Pathway

• Uses both photosystems
• Radiant/Solar energy is absorbed by PSI and PSII
• Water is split into O2, H+ and electrons
• Electrons move down the electron transport chain
• Electrons move to NADP+ and create NADPH
• H+ ions form a gradient which flow through ATP synthase and produce ATP
• ATP and NADPH are sent to the Calvin Cycle

Cyclic Electron Pathway

• Starts with PSI
• Absorbs solar energy
• Electrons move down the ETC
• Energy is captured and stored as a H+ gradient
• H+ flow down ATP synthase
• ATP is created
• Electrons return to PSI

Quick Review Questions (Electron Pathways)

• Which part of the electromagnetic spectrum can we see?
• Which wavelength of light is absorbed by the plant?
• Which wavelength of light is reflected by the plant?
• How is chlorophyll a different from chlorophyll b?
• What is the wavelength of light absorbed by photosystem I?

Calvin Cycle Details

• Takes place in the stroma
• Light is not required
• Uses ATP and NADPH from the light cycle to make glucose
• Three steps
  • CO₂ fixation
  • CO₂ reduction
  • Regeneration of RuBP (ribulose bisphosphate), which begins the cycle again.

Types of Photosynthesis

• C3 Plants: Most common, but less efficient in hot or dry conditions.
• C4 Plants: More efficient in hot or dry conditions due to special internal cellular structure, preventing photorespiration.
• CAM Plants: Open their stomata at night in order to conserve water for the hot conditions, and release CO2 during the day

Factors Affecting Photosynthesis

• Light Intensity
• Temperature (0°C – 35°C)
• Water availability

Quick Review Questions (Calvin Cycle)

• Where does the Calvin cycle occur in the chloroplast?
• What are the three major steps in the Calvin cycle?
• What is the end result of step 1?
• What is the end result of step 2?
• What are the end results of step 3?
• What factors affect photosynthesis?

G3P Uses

• G3P can be converted into other organic molecules (glucose, starch, sucrose, cellulose, fatty acids, amino acids)