Metabolic Pathways pt2; Photosynthesis

Photosynthesis

Builds reduced organic molecules from CO2 and H2O

• Endergonic reactions (require energy from sunlight)

• Anabolic (builds complex molecules)

• Done by photoautotrophs (light, self, feeder)

  • Producers of the biosphere

  • Eaten by heterotrophs (consumers)

• Occurs in 2 main stages; Light Reactions, Calvin Cycle

Light Reactions

• Captures light energy to energize electrons

• Makes NADPH and ATP

• Consists of: photosystem I and II, ETC, and ATP synthase

• Electrons are removed from the process to form O2

Calvin Cycle

• Uses energy from ATP and NADH to reduce CO2 to make 3 carbon sugars → glucose

• ADP and NADP+ are oxidized and recycled

• Occurs in light but also proceeds in dark until NADPH and ATP run out

Chloroplast

Organelle in which photosynthesis takes place

• Surrounded by a double membrane

• CO2 enters the plant via stomata and water is absorbed by the roots

• A typical mesophyll cell has about 30-40 chloroplasts

• Fluid inside is called the stroma

  • Calvin cycle occurs in the stroma

Thylakoids

A third membrane system inside the chloroplasts, which is stacked to form grana

• The thylakoid intermembrane space is called the lumen which is where light reactions occur

• Contains the chlorophyll pigment which absorbs and transforms the photon energy

When photon hits matter (3)

I. Reflected

II. Transmitted straight through

III. Absorbed

What happens depends on the energy of the photon and the molecule it interacts with

Pigments

Absorb wavelengthds of light

• If it is not absorbed it is reflected

• Different pigments will absorb different wavelengths

Absorption spectrum

The pigments light absorption versus the wavelength

Action spectrum

Shows the photosynthetic activity at different wavelengths

Excited state

When a photon is absorbed by a molecules, an electron is converted to a higher-energy state (excited state)

• The electron is boosted to a higher orbital where it has more potential energy

Ground state

Normal state of the electron

Excited → Ground

1. Energy may be dissipated as heat

2. Energy may be re-emitted in the form of less energetic and longer wavelength photon (bit of heat also released)

3. Energy may be transferred to another molecule

Porphyrin ring

Light-absorbing head of molecule, Mg at the centre

Hydrocarbon tail

interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts

Photosystem

are composed of reaction-centre complex, surrounded by many light-harvesting complexes

Reaction centre

Embedded in the thylakoid membrane where energy is funnelled into.

• Consists of specially arranged chlorophyll molecules called P680 and P700

• P = pigment, ### = wavelength absorbed best

• Pass on the electrons to the primary electron acceptor which gets reduced

Photosystem II

1. Light is captured in the light harvesting complex and transferred to the reaction centre to the pair of special chlorophylls (P680 absorbs light energy and ejects electrons)

2. The primary electron acceptor gets reduced and captures the electron and shuttles it to photosystem I

3. This oxides P680 to P680+ — the strongest oxidant in biological systems → he want that cookie baddd

• • The oxidized P680+ rips an electron from water reducing it back to P680

  • This result in the use of 2 H2O molecules to make one O2 removing 4 e in the process (oxygen

  • The H+ ions released stay inside the thylakoid lumen to help make a proton motive force

ETC

4. After electron gets ejected from PSII gets captured by the primary electron acceptor;

   1. Some of the energy is dissipated and used to drive the proton H+ gradient into the thylakoid lumen

   2. The electron carriers are plastoquinone (Pq) a cytochrome complex, and plastocyanin (Pc)

   3. The cytochrome complex is the one that pumps H+ into the thylakoid lumen

5. ATP is not made directly

   1. Protons build up in the lumen by splitting water, and the cytochrome complex pumping proton in to the lumen

   2. This proton motive force is used to make ATP through ATP synthase embedded in the thylakoid membrane via Chemiosmosis

6. The electrons are passed to the photosystem I (PS1) where they are re-energized

7. Photoexcited electrons are passed to a second ETC to the protein ferrodoxin (Fd)

8. Once ferredoxin has two electrons, it passes them onto NADP+ reductase to form NADPH → Calvin cycle

Cyclic electron flow

Sometimes, photoexcited electrons can by-pass photosystem II into photosystem I

• Like a short circuit, electrons cycle back form ferredoxin to cytochrome complex back to P700

Advantage: supplements ATP synthesis

Disadvantage: no NADPH production (→ no Calvin cycle → no sugar production) and no O2

The Calvin Cycle

• Endergonic, therefore anabolic pathway of 11 enzymes that builds the 3-carbon sugar glyceraldehyde 3-phosphate (G3P).

• The initial sugar (5 carbon) ribulose biphosphate is used and regenerated

• Each turn through the cycle fixes 1 molecule of CO2

C3 plant

All steps of photosynthesis occurs in mesophyll cells in the middle of the leaf

C4 plant

Have extra cell called bundle sheathes associated with the veins

• Calvin cycle only occurs in the chloroplasts of the bundle sheaths

• CO2 is fixed into a 4-carbon molecule in the mesophyll

• Then transferred to the bundle sheaths which unloads the CO2 where it is concentrated around Rubisco reducing the binding of Rubisco to oxygen

PEP carboxylase

Add carbon from CO2 to PEP (phophoenolpyruvate)

• Mesophyll export to bundle sheaths

• CO2 enters Calvin Cycle

CAM plants

Store organic intermediates during the night and release them for Calvin Cycle during the day

Fossil Fuels

Products of ancient photosynthesis