Chloroplasts

• photosynthesis: CO2 + H2O -> glucose + O2
• Chloroplasts:

  • surrounded by an outer and an inner membrane called envelope
  • Envelope contains stroma (aqueous)
  • Inner most compartment is the thylakoids space, enclosed by thylakoid membrane
• thylakoid membrane:

  • thylakoid membrane is formed into dense stacks called grana
  • chlorophyll is present
  • Responsible steps of photosynthesis which convert light energy into chemical energy
• Stages of photosynthesis:

  • light reactions
  • calvin cycle
• Light reaction:

  use solar energy to make ATP and NADPH which supplies chemical energy and reducing power to the the caliv cycle
• Structure of chloroplasts:

  A) inner membrane

  B) outer membrane

  C) grana

  D) chloroplast

  E) thylakoid

  F) stroma

  
• Calvin cycle incorporates CO2 into organic molecules, which are used to make glucose
• Light reaction contribute to H+ gradient by increasing H+ concentration in thylakoid space.

  1. photosystem II split water (on side of the membrane that faces the thylakoid space)
  2. Plastoquinone transfers electrons to the cytochrome complex, 4 H+ are transported into the thylakoid space, establishing a H+ gradient
  3. H+ is remove from stroma by NAPH+
  4. diffusion of H+ from thylakoid space to stroma powers ATP synthase. (store chemical energy in NADPH and ATP, and send energy to the carbohydrate-producing calvin cycle)
• Thylkoid stack

  A) stroma

  B) photosystem II

  C) light

  D) thylakoid space

  E) 4 H+

  F) cytochrome complex

  G) photosystem I

  H) NADP reductase

  I) NADP+ and H+

  J) NADPH

  K) pq

  L) pc

  M) fd

  N) ATP synthase

  O) ADP + Pi

  
• shape of chloroplast:

  lens-shaped
• Light reaction:

  • light reaction convert solar energy to chemical energy
  • Light energy absorbed by chlorophyll in the thylakoid membranes is used transfer electrons and H+ to the electron carrier NADP+ -> NADPH
  • Water is spilt during this process and oxygen given off as a by product
  • ATP is also generate from ADP and inorganic phosphate
  • The thylakoid membranes of the chloroplast are the site of the light reactiom
• Light absorption:

  Pigments absorb light energy as photons elevated to an orbital of higher energy.

  Only chlorophyll a can participate directly in the light reaction

  Pigments absorbs lights in the thylakoid membrane has a primary electron acceptor traps a high-energy electron that has absorbed the photon before it can drop back down.Only one localised pair of chlorophyll a molecule can actually donate their electrons to the primary electron acceptor. Known as the reaction center.
• Accessory pigments:

  Function as light-gathering antennae which collect energy from the photons and pass it inwards to the reaction centre.

  Chlorophyl b and carotenoids
• What is a photosystem?

  The reaction centre and the primary electron acceptor togethers
• chlorophyll absorbs violet blue and red light and transmits or reflects green light
• Way to measure wavelength absorption

  1. White light is separated into colors (wavelengths) by a prism.
  2. One by one, the different colors of light are passed through the sample.
  3. The transmitted light strikes a photoelectric tube, which converts the light energy to electricity.
  4. The electric current is measured by a galvanometer. The meter indicates the fraction of light transmitted through the sample, from which we can determine the amount of light absorbed.
• Evidence that chloroplasts pigments participate in photosynthesis:

  • algae filament exposed to different wavelengths of light
  • Placed aerobic bacteria with the algae
  • Higher concentration of oxygen allowed the aerobic bacteria to proliferate more
  • Accumulated near the parts of algae illuminated with red and blue light = the wavelenths that works most efficiently for photosynthesis
• What does an action spectrum?

  plots rate of photosynthesis against wavelength
• Why does the action spectrum not match chlorophyl a?

  accessory pigments, but matches closely as is the primary pigment
• Parts of a chlorophyll molecule

  porphyrin ring

  hydrocarbon tail
• Excitation of isolated chlorophyll:

  • photon hits chlorophyll molecule
  • energy derived from light elevates to higher energy state
  • electrons fall back to ground state if no other pigment state if no other pigments nearby
  • energy released as heat or fluorescence
• Photosystems have antennae pigment molecules which photons

  Energy transferred between pigments to the central pair chlorophyll

  electrons elevated to higher energy state and take by primary electron acceptor
• Photosystem energy transfer:

  photon -> light-harvesting complex -> reaction centre (chlorophyll a)
• Photosystem I

  absorbs best at 700nm

  reaction centre is called P700
• Photosystem II

  absorbs light best at 680nm

  reaction centre is called P680
• Non-cyclic electron flow

  Photosystem I and Photosystem II co-operate together to produce non-cyclic electron flow to generate NADPH and ATP to split water
• Process of non-cyclic electron flow

  1. Light excites electrons from P700

  2. Electrons pass to the electron acceptor

  3. Electrons passed to NADP+ to produce NADPH

  4. P700 becomes very oxidised
• Photosystem II P680 excitation

  1. P680 becomes excited

  2. Donates an electron to its primary acceptor

  3. Electron passes down an electron transport chain

  4. Electron accepted by oxidised P700

  5. Energy used to pump H+ out of thylakoid membrane

  6. H+ used by ATP synthase to produce ATP
• P680 is now in an oxidised state
• P680 gets electrons from water


  1. Water is split

  2. Oxygen is generated
• Photosystem II splits water, electrons replace those lost by P680 and generates O2 and H+.

  The electron given primary acceptor passes down the electron transport chain, this generates ATP.

  The electron reaches photosystem I no splitting water, electrons originally from water replace those lost at P700, linear movement of electrons
• Non-cyclic electron flow

  1. PSII splits water generating O2, H+ and 2 electrons

  2. The 2 electrons arrive at P680

  3. An electron is excited to the primary acceptor from P680

  4. The electron is passed from the primary electron acceptor to plastoquinone to cytochrome complex to plastocyanin

  5. The electron arrives at the reaction at P700 and is excited to the primary electron centre

  6. From the primary electron acceptor to ferrodoxin to NADP+ reductase
• P680


  Reaction centre in photosystem II
• P700


  Reaction centre
• This movement down the electron transport chain generates ATP
• NADP+ reductase


  Converts NADP+ to NADPH
• cyclic electron flow

  only uses photosystem I to produce ATP

  Electrons are passed from the primary electron acceptor through a chain which includes ferredoxin, the cytochrome, and a copper-containing protein (plastocyanin) electron is passed back to P700.
• cyclic electron flowonly uses photosystem I to produce ATPElectrons are passed from the primary electron acceptor through a chain which includes ferredoxin, the cytochrome, and a copper-containing protein (plastocyanin) electron is passed back to P700.

  Each stage the electron loses energy and this energy is used to pump hydrogen ion across the thylakoid membrane

  This H+ gradient across the membrane to drive ATP synthase

  DOESN'T produce NADPH or oxygen
• cyclic electron flow

  only uses photosystem I to produce ATP

  Electrons are passed from the primary electron acceptor through a chain which includes ferredoxin, the cytochrome, and a copper-containing protein (plastocyanin) electron is passed back to P700.

  Each stage the electron loses energy and this energy is used to pump hydrogen ion across the thylakoid membrane

  This H+ gradient across the membrane to drive ATP synthase

  DOESN'T produce NADPH or oxygen as PSII can't split water
• Calvin cycle

  • occurs in the stroma
  • Carbon enters the cycle in the form of CO2 and leaves as a sugar
  • Cycle requires ATP and consumes NADPH as a source of high energy electron
  • functions in the dark