Plant Biology - Photosynthesis Overview

Plant Biology - Winter 2025

• Instructor: Dr. Junyan Zhang
• Course: BIOL2010 Plant Biology

Photosynthesis (2)

• Noncyclic photophosphorylation vs. Cyclic photophosphorylation
• Light-dependent Reactions vs. Carbon-Fixation Reactions
• C₃, C₄ and CAM Photosynthesis

Noncyclic electron flow and photophosphorylation

• PS I and II are spatially separated
• PSII feeds into PSI
• Photophosphorylation is the process of utilizing light energy from photosynthesis to convert ADP to ATP
• Two types:
  • Cyclic Photophosphorylation
  • Non-cyclic Photophosphorylation

Where did ATP come from?

• Non-cyclic photophosphorylation
  • This process is known as non-cyclic photophosphorylation because the lost electrons of P680 of Photosystem II are occupied by P700 of Photosystem I and are not reverted to P680
  • The complete movement of electrons is unidirectional or in a non-cyclic manner

Cyclic Photophosphorylation

• PSI can work independently of PS II in cyclic electron flow, this is called cyclic photophosphorylation
• This process uses chlorophyll P700 and generates only ATP

Non-cyclic vs. Cyclic Photophosphorylation

• Diagram illustrating the path of electrons in non-cyclic and cyclic photophosphorylation
• Light is captured by chlorophyll
• Electrons are excited and move through electron carriers
• ATP and NADPH are generated in non-cyclic photophosphorylation
• Only ATP is generated in cyclic photophosphorylation

Light-dependent Reactions

• Two paces of photosynthesis: Light Reaction and Dark Reaction
• Light reactions occur in the grana of the chloroplast
• Light reactions convert light energy and the splitting of water to chemical energy as ATP and NADPH, and release H and oxygen
• Notice that one of the main products of photosynthesis is said to be sugars, but the light reactions don't form any sugars

Carbon-Fixation Reactions

• Carbon-fixation reactions (also known as light-independent reactions or dark reactions) occur in the stroma of the chloroplast
• Carbon-fixation reactions use the energy from the light-dependent reactions to convert CO₂ and H to sugar

Carbon-Fixation Reactions

• ATP and NADPH generated by the light reaction are used to fix and reduce carbon and to synthesize simple sugars
• Carbon comes in the form of CO₂ in air to plants
• In water-living photosynthetic organisms (algae and bacteria), CO₂ is dissolved into their surrounding water

Calvin Cycle is C₃ photosynthesis

• Calvin Cycle (Light-Independent Reaction)
• The term "dark reaction" is confusing because these reactions can occur in the light or the dark
• A better term is "light-independent" reactions
• The energy-source molecules, ATP and NADPH, which were generated from electron transfer chains, and electrons originally derived from photolysis, are used to make sugars
• The Calvin cycle starts and ends with ribulose bisphosphate (RuBP)
• RuBP carboxylase/oxygenase (Rubisco) which catalyzes the first reaction is the world's most abundant enzyme
• Rubisco is both a carboxylase and an oxygenase

Light Reactions and Dark Reactions

• Light reactions:
  • Harvests photons of light and produces ATP and NADPH
  • Water is required as a source of electrons
  • Oxygen is produced as a by-product
  • This reaction requires light
• Dark reactions:
  • ATP and NADPH used to fix CO₂ as energy-rich sugars in the Calvin cycle
  • The first product is a molecule with 3 C atoms
  • Produces sugars and starch as a product
  • Doesn't require light, but happens during daytime

Practice question-1

• The Calvin cycle occurs in the stroma.
  • True

Practice question-2

• The Calvin cycle consumes ATP and NADPH.
  • True

C₃ Photosynthesis

• C₃ plants do all the photosynthesis at once in the mesophyll cells
• In hot, dry weather, the stomata close to conserve water In moderate temperature, C₃ plants do well with water supply

C₄ Photosynthesis

• An extra set of reactions are inserted between light-dependent and light-independent reactions
• These reactions act as a CO₂ pump concentrating CO₂ at the site of CO₂ fixation in the Calvin cycle
• This pathway requires spatial separation of the initial and final fixation of CO₂
• This occurs in mesophyll and bundle sheath
• In hot and dry environments, C₄ photosynthesis is more efficient than C₃

Examples of C₄ Plants

• Many grass species use this pathway, including corn, sugarcane and sorghum

CAM Photosynthesis

• Crassulacean Acid Metabolism (CAM) type photosynthesis evolved to reduce water loss
• Stomata open at night, minimal water loss
• All reactions for photosynthesis occur in mesophyll cells
• Carbon reactions occur only at night in the cooler, moister air to avoid photorespiration
• Used by plants like this cactus that live in desert conditions

Examples of CAM Plants

• Crassula tabularis
• Golden star cactus
• Pineapple

C₃ vs. C₄ vs. CAM Photosynthesis

• C₃ species have spatial separation
• C₄ species have temporal separation

CO₂-H₂O Trade-off

• To acquire CO₂ for photosynthesis, plants have to open their stomata
• Opening stomata leads to increased loss of water
• Under dry conditions, plants close their stomata which is a problem for photosynthesis

Plants use different carbon fixation pathways

• C₃ photosynthesis is used by most plant species (approximately 95%)
• C3, C4, and CAM plants each do photosynthesis using different pathways for fixing carbon
• Each pathway has its own advantages and disadvantages depending on the environment the plant is in

This quiz covers key concepts of photosynthesis including noncyclic and cyclic photophosphorylation, as well as the differences between light-dependent reactions and carbon fixation. Test your understanding of C₃, C₄, and CAM pathways and the mechanisms of ATP production in plants.