PHOTOSYNTHESIS (BIO 10)

cornelius van niel research

• Studied photosynthesis in purple sulfur bacteria

• Organisms were found to be autotrophs

• Bacteria did NOT produce oxygen as a by product, instead they released elemental sulfur (S)

• O2 is not released from CO2 in this bacteria

Thus, oxygen atoms released during plant photosynthesis must come from H2O!

Two main sets of reactions

1. Light-Capturing Reactions

• H2O ---> O2

• Water is split to form O2

• Its electrons are excited by light energy

• High energy electrons are transferred to the electron carrier NADP+, forming NADPH

1. Calvin Cycle Reactions

   • CO2 ---> sugar

   • Electrons and ATP are used to reduce CO2

Parts of Chloroplast

• Thylakoids- interior filled with flattened, vesicle-like structures

  • Pigments= molecules that absorb certain wavelengths of light and reflect or transmit others

  • Appear colored because people see only the wavelengths that aren't absorbed

  • Most common pigment in thylakoids is chlorophyll, which reflect green light

• Grana- interconnected stacks of thylakoids

• Lumen- space inside thylakoid (any inner membrane lining)

• Stroma- fluid filled space between thylakoids and the inner membrane (ex. Cytoplasm!)

how do pigments capture light energy?

• Electromagnetic radiation- form of energy

  • Electromagnetic spectrum displays range of wavelengths of electromagnetic radiation

• Light is a type of electromagnetic radiation!

  • As a wave, light can be characterized by its wavelength

  • As a particle, light exists in discrete packets called photons, which are absorbed, transmitted, or reflected when they strike an object

• Visible light= portion that humans can see

• Each photon and wavelength has a specific amount of energy

  • Shorter wavelengths have more energy than longer wavelengths (more complete wave cycles pass a fixed point every second to maintain constant speed)

Two Classes of Pigments

1. Chlorophylls

   • Absorb red and blue light

   • Reflect and transmit green light

2. Carotenoids

   • Absorb blue and green light

   • Reflect and transmit yellow, orange, and red light

Action Spectrum vs absorption spectrum

wavelengths that drive light-capturing reactions vs. how wavelength and photons influences amount of light absorbed by pigment

components of Chlorophyll a and b

• Long isoprenoid tail- interacts with proteins embedded in thylakoid membrane

• Head- consists of large ring structure with magnesium in the middle, light is absorbed

Carotenoids Xanthophylls

• Absorb light and pass energy on to chlorophyll

• When trees begin to die, their chlorophyll degrades and carotenoids absorb wavelengths of light, making the leaves appear yellow, orange, or red

• Extend range of wavelengths that can drive photosynthesis

• Protect chlorophylls from damage by stabilizing free radicals

What happens when chlorophyll absorbs a photon?

• Photon's energy is transferred to bonds in chlorophyll's head region

• Electron becomes "excited" (higher energy state)

• Red photons bump an electron up one energy level while blue photons bump an electron two energy levels

• Green photons are intermediate and are not absorbed

When electrons fall back to ground state…

• The absorbed energy is released as heat or as a combination of heat and light

• Fluorescence occurs when electron emits light (about 2% of absorbed red and blue photons produce fluorescence)

Photosystems

thylakoid membrane composed of 200-300 chlorophyll and accessory pigment molecules

• Photosystem II generates ATP and Photosystem I generates NADPH

• Pigment molecules have light-gathering antenna pigments, which guide energy towards reaction center

  • Resonance Energy Transfer= energy, but not electron, is passed to nearby chlorophyll molecule

  • Proteins organize and tune absorption potential of antenna pigments

  • After transferring energy, electron falls back to its ground state and energy is transferred inside photosystem

in which ways can energy be released from electrons?

1. Be emitted in the form of light via fluorescence

2. Be given off as heat

3. Excite electron in nearby pigment and induce resonance

4. Be transferred to electron acceptor in redox reaction

Light-Harvesting Complex

• Mobile accessory structure

• Transmits resonance energy to antenna pigment

• Resonance is relayed to special pair pigments (chlorophylls in the reaction center)

• Pheophytin= pigment molecule reduced with a high energy electron

• Electron is then passed down to ETC in thylakoid membrane

  • Similar to mitochondria ETC

  • Both contain quinones and cytochromes

  • Redox reactions result in protons transported from one side of the internal membrane to the other

  • Proton-motive force drives ATP production via ATP synthase

  • PQ's shuttle electrons from photosystem II across thylakoid membrane to cytochrome complex

Photophosphorylation

energy from light used to initiate synthesis of ATP in chloroplasts

oxygenic vs anoxygenic photosynthesis

photosystem II oxidizes, or "splits" water and produces oxygen (van Niel research!) vs. organisms have one single photosystem and use different electron donors (ex. Bacteria)

how does photosystem I work?

• Antennae pigments absorb photons and pass energy to reaction center

• Electrons in reaction center are excited

• Reaction center pigments are oxidized (excited electrons are passed to ferredoxin and then NADP+ reductase)

• NADP+ is reduced to form NADPH

 

At the end of the ETC, electron is passed to protein called PC

• Carries electron back across thylakoid membrane

• Forms link between ETC following both photosystems

• Electrons are donated to terminal electron acceptors (NADPH)

cyclic and noncyclic phosphorylation

• in algae and land plants

• ATP is produced via cyclic as well as noncyclic phosphorylation

• Photosystem I transfers excited electrons to ETC (moves backward instead of using electrons to reduce NADP+ !)

• Cyclic electron flow coexists with noncyclic and produces additional ATP

How do Cells Capture Carbon Dioxide?

• Plants are covered with waxy coating called cuticle

• Lipid layer prevents water from evaporating

• Prevents transport of CO2 and O2 across plasma membrane

• However, CO2 gets into photosynthesized tissues through pores

  • Two guard cells on surface of cells provide openings/pores

  • Entire structure is called a stoma

  • Open stomata allow CO2 to diffuse into the leaf and O2 to diffuse out

  • CO2 diffuses along concentration gradient into chloroplasts, maintained by the Calvin  Cycle

Carbon Fixation in the calvin cycle

Carbon Fixation= addition of carbon atoms from inorganic CO2 to organic compound, converts CO2 has to a biologically useful form

• Five carbon RuBP is initial reactant

• CO2 is fixed by enzyme Rubisco (most abundant enzyme in leaf tissue!)

  • Found in all photosynthetic organisms that use the Calvin cycle to fix carbon

  • Eight active sites

  • Very slow enzyme and inefficient due to O2 and CO2 competition to active sites

  • Photorespiration is produced when O2 and RuBP react in rubisco's active sites, undoes photosynthesis

three steps of Calvin Cycle

1. Fixation Phase- CO2 reacts with RuBP

   • Fixes carbon

   • produces two molecules of 3PGA

2. Reduction Phase- 3PGA is phosphorylated to ATP

   • Accepts electrons from NADPH (reduced)

   • Produces G3P

3. Regeneration Phase- G3P keeps cycle going

   • ATP is used to regenerate RuBP

Mechanisms for Increasing CO2 Concentration

In hot and dry conditions, stomata must close

• Stops transport of CO2 and O2

• Photosynthesis slows and photorespiration increases

• C4 pathway used by photosynthetic cells to increase CO2 concentrations (makes photosynthesis more efficient)

  • C4 plants fix CO2 using both pathways

  • Four carbon compound by enzyme PEP (instead of Rubisco)

  • PEP carboxylase fixes CO2 to phosphoenolpyruvates in mesophyll cells

  • Does not replace Calvin Cycle, instead serves as an additional fixation step

CAM plants pathway

• Photosynthetic pathway at night

• Resembles C4 pathway in some ways

• CO2 concentrator, preparatory step to Calvin Cycle

• Generates four-carbon organic acid in first CO2 fixation step

• Occurs in cacti and other species that keep their stomata closed on hot, dry days

What happens to the Sugar that is Produced by Photosynthesis?

• G3P molecules produced by Calvin cycle are used to make glucose and fructose by process of gluconeogenesis

• When sucrose is abundant, glucose is polymerized to form starch (in the chloroplast)

• Sucrose synthesis takes place in the cytosol

How is Photosynthesis Regulated?

• Presence of light triggers production of photosynthesis proteins

• High sugar inhibits production of photosynthetic proteins and stimulates production of proteins that process and store sugars (starch!)

• Rubisco is activated by regulatory molecules

  • Produces when light is available

  • Inhibited when CO2 is low