Photosynthesis: Chapter 7

• Chapter 7 is about photosynthesis

Photosynthetic Organisms: Autotrophs

• All life on earth depends on solar energy
• Solar energy is transformed into chemical energy (carbohydrates) through photosynthesis
• Examples of photosynthetic organisms: Plants, Algae, and Cyanobacteria
• Photosynthetic organisms sustain themselves and other living things on earth (Heterotrophs)
• Photosynthetic organisms produce large amounts of carbohydrates and other organic molecules for growth and cellular functions
• Oxygen is produced in large amounts and forms the ozone layer
• Coal and oil provide 90% of the energy needed to power vehicles and factories

Photosynthesis

• Is a process that captures solar energy
• Transforms solar energy into chemical energy
• Energy ends up stored in a carbohydrate
• Both autotrophs and heterotrophs use organic molecules produced by photosynthesis as a source of chemical energy for cellular work

Photosynthesis Releases Oxygen

• Oxygen released comes from water
• Van Niel used the O18 isotope to prove this

Process of Photosynthesis

• Photosynthesis occurs in the green parts of the plant, usually the leaf (mesophyll tissues)
• Raw materials consist of water and CO2
• Chlorophyll and other pigments in the membrane of thylakoids absorb solar energy and energize electrons
• CO2 is then reduced to C6H12O6 in the stroma
• Reduction of CO2 to carbohydrate is associated with the stroma of the chloroplast
• CO2 in the air is taken in by the plant in the leaf through the stomata
• In the stroma, CO2 is fixed to an organic molecule and then reduced to a Sugar

Plants as Solar Energy Converters -Pigments

• Chemicals that absorb certain wavelengths of light
• Wavelengths not absorbed by pigments are reflected or transmitted
• The absorption spectrum is a graph showing the relative absorption of the colors of the rainbow
• Pigments found in chlorophyll absorb portions of visible light
• Chlorophyll is green because it absorbs much of the reds and blues and reflects green light
• Carotenoids are accessory pigments that absorb light in the violet-blue-green range and reflect yellow and orange light

Photosynthetic Reactions

• Photosynthesis is a redox reaction, CO2 is reduced, water is oxidized
• There are two sets of reactions in photosynthesis
  • Light reactions occur in the presence of light
  • Calvin cycle reactions occur in the stroma

Light Reactions

• Occur only in the presence of light and are energy-capturing reactions
• Chlorophyll absorbs solar energy, which energizes electrons
• Electrons move down an electron transport chain
• The electron transport chain:
  • Pumps H+ into thylakoids
  • Makes ATP out of ADP, and NADPH out of NADP+
• Calvin cycle reactions take place in the stroma

Overview of Photosynthesis

• Light reactions: Solar energy is converted to chemical energy(ATP and NADPH)
• Calvin cycle: ATP and NADPH reduces carbon into a carbohydrate

Light Reactions as Solar Energy Converters

• Light reactions consist of two alternate electron pathways:
  • Noncyclic pathway
  • Cyclic pathway
• Light reactions capture light energy using photosystems
• A photosystem is a pigment complex that helps collect solar energy
• Photosystems are located in the thylakoid membranes
• Both cyclic and noncyclic pathways produce ATP
• The noncyclic pathway also produces NADPH

Noncyclic Pathway

• Uses two photosystems: PS I and PS II
• Noncyclic pathway begins with photosystem II
• PS II captures light energy
• This causes an electron to be ejected from the reaction center (chlorophyll a)
• Electron travels down electron transport chain (ETC) to PS I
• It is replaced with an electron from water, which is split to form O2 and H+
• This causes H+ to accumulate in thylakoid chambers (inside)
• The H+ gradient is used to produce ATP.
• PS I captures light energy and ejects an electron
• The electron is transferred permanently to a molecule of NADP+
• Causes NADPH production

Photosystem II

• Consists of a pigment complex and electron acceptors
• Receives electrons from the splitting of water
• Oxygen is released as a gas

Electron transport chain

• Consists of cytochrome complexes and plastoquinone
• Carries electrons between PS II and PS I
• Also pumps H+ from the stroma into the thylakoid space

Photosystem I

• Has a pigment complex and electron acceptors
• Adjacent to the enzyme that reduces NADP+ to NADPH

ATP synthase complex

• Has a channel for H+ flow
• H+ flow through the channel drives ATP synthase to join ADP and P to each other

Light Reactions

• Takes place in the thylakoids
• Chlorophyll absorbs solar energy and energizes electrons
• Energized electrons move down an electron transport system
• Energy is captured and used for ATP production
• NADP+ (electron carrier) accepts electrons and becomes NADPH
• During the light reactions solar energy is converted to chemical energy (ATP, NADPH)

ATP Production

• Thylakoid space acts as a reservoir for H+ ions
• Each time water is oxidized, two H+ remain in the thylakoid space
• Transfer of e in the ETC yields energy
• This energy is used to Pump H+ across the thylakoid space
• The flow of H+ back across the thylakoid Membrane energizes ATP synthesis known as Chemiosmosis

Light Reactions: Cyclic Electron Pathway

• PSI absorbs solar energy, energized electrons escape from reactions center to electron acceptors, electron transport system, energy captured and stored as H+ gradient, and ATP production occurs
• Here electrons of PSI do not go to NADP+, they return to PSII (Cyclic)
• Advantage: allows more ATP to be produced since the Calvin cycle needs more ATP than NADPH

Plants as Carbon Dioxide Fixers - Calvin Cycle.

• Takes place in the stroma
• A cyclical series of reactions, known as C3 photosynthesis
• Atmospheric CO2 is reduced into a carbohydrate
• ATP and NADPH formed during the light reaction carry out this reduction
• During the Calvin Cycle Chemical energy ATP and NADPH, are used to create chemical energy in the form of a carbohydrate
• Involves three stages
  • Carbon dioxide fixation
  • Carbon dioxide reduction
  • RuBP regeneration

Steps of the Calvin Cycle

• Step 1: Fixation of Carbon Dioxide, CO₂ is attached to 5-carbon RuBP by the enzyme RuBP carboxylase
  • Results in a 6-carbon molecule
  • This splits into two 3-carbon molecules(3PG)
  • Reaction is accelerated by RuBP carboxylase (Rubisco)
  • CO₂ is "fixed" because it is part of a carbohydrate
• Step 2: Reduction of Carbon Dioxide
  • 3PG is reduced to BPG
  • BPG is then reduced to G3P
  • Electrons and energy are required for this stage
  • This stage utilizes NADPH and some ATP produced in the light reactions
  • G3P is reduced and chemically able to store more energy and form larger organic molecules such as glucose.
• Step 3: Regeneration of RuBP
  • RuBP used in CO2 fixation must be replaced
  • Every three turns of Calvin cycle:Five G3P (a 3-carbon molecule) are used to remake three RuBP (a 5-carbon molecule) - 5×3=3 × 5.
• Importance of the Calvin cycle:G3P (glyceraldehyde-3-phosphate) can be converted to many other molecules
• The hydrocarbon skeleton of G3P can form:Fatty acids and glycerol to make plant oils, Glucose phosphate (simple sugar), Fructose (which with glucose = sucrose), Starch and cellulose, and Amino acids

Fate of G3P

• G3P is converted to:
  • Glucose phosphate
  • Fatty acid
  • Amino acid synthesis
  • Sucrose
  • Starch
  • Cellulose

C3 Photosynthesis (wheat, rice, oat)

• C3 plants use RuBP carboxylase (Rubisco) to fix CO2 in mesophyll cells
• 3 RuBP +3 CO2→ 6 3PG
• If weather is hot and dry stomata close to conserve water - CO2 in mesophyll decreases → 02 increases
• Rubisco allows O2 to bind RuBP, and produces 1 PG instead of 6 PG & release of CO2
• This is photorespiration, where in presence of light, O2 is taken up and CO2 is released

C4 Photosynthesis (corn, sugar cane)

• In mesophyll cells, PEP carboxylase, has high affinity to CO2.
• Photorespiration does not occur because PEPcase doesn't combine with O2
• СО2 + PЕР(C3) → Oxaloacetate (C4) → Malate
• Oxaloacetate is reduced to malate
• Malate is pumped to bundle sheath cells where the Calvin cycle takes place
• This helps C4 plants to avoid photorespiration in hot climates

CAM Photosynthesis (cactus, pineapple)

• Crassulacean-acid metabolism
• Prevalent in succulent plants that grow in desert environment
• Night PEPCase + CO2 → C4 molecules (malate) stored in large vacuoles in mesophyll cells
• Day -C4 molecules release CO2 that enter the Calvin cycle
• This helps conserve water because the stomata will be closed but CO2 will be available