Photosynthesis : a Brief Introduction

Photosynthesis 光合作用 is from the Greek word φώτο [photo] "light" and σύνθεσις [synthesis] “putting together”.  It is a chemical process that converts carbon dioxide into organic compounds, using the energy from sunlight.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Chloroplast

In plants and algae, photosynthesis occurs in chloroplasts.  A typical plant cell contains about 10-100 chloroplasts.

The chloroplasts is enclosed by a phospholipid membrane.

Within the membrane is an aqueous fluid ‘stroma’.

The stroma contains flattened discs ‘thylakoids’. 

Liquid bounded within thyakoid membrane is ‘lumen’.

Stacks of thylakoids are ‘granum’ (singular), ‘grana’ (plural).

Grana are connected by ‘lamellae’.

Thylakoid membranes contains light-harvesting-proteins : ‘photosystem I – P700’ & ‘photosystem II – P680’

Thylakoid membranes also contains : electron transport chain ‘cytochrome b6f complex’ and ATP synthase ‘CF1FO-ATP’.

Thylakoids are the site of light-dependent reactions.

Stroma is the site of light-independent reactions.

Photosynthetic pigments

Pigments are chemical compounds which reflect only certain wavelength of visible light.

Chlorophylls are greenish pigments which contain a porphyrin ring. 

Chlorophyll-a, found in all plants, algae and cyanobacteria.

Chlorophyll-b, found in plants and algae.

Chlorophyll-c, found in chromista and dinoflagellates.

Carotenoids

Carotene, an orange pigment.

Xanthophyll, a yellow pigment

Phaeophytin-a, a gray-brown pigment.

Phaeophytin-b, a yellow-brown pigment.

Bacteriochlorophylls are greenish pigments, found in Protobacteria, purple bacteria, green sulfur bacteria, Chloroflexi, and Heliobacteria.

Phycobiliproteins

Phycocyanin, a light blue accessory pigment, found in Cynobacteria.

Phycocyanin, a light blue accessory pigment, found in Cyanobacteria.

Allophycocyanin, a blue accessory pigment, found in Cyanobacteria and Rhodophyta.

Phycoerythrocyanin, a magenta accessory pigment, found in Cyanobacteria and Rhodophyta.

Fucoxanthin, a brown pigment, found in brown algae, diatoms and kelps.

Phycoerythrin, red pigment, found in Cyanobacteria, Rhodophyta and Cryptomonads.

Phycobilins, are red/blue pigments, found in Cyanobacteria, Rhodophyta, Cryptomonads and Glaucophtes.

Order & Kinetics

The overall process of photosynthesis takes place in 4 stages:

1. absorption of light

2. transport of electrons

3. synthesis of ATP

4. fixation of carbon

Reactions in stage 1, 2 & 3 are dependent of light energy, thus are called light dependent reactions. 

Reactions in stage 4 are indirectly dependent on light energy, thus called light-independent reactions or dark reactions.  However, in contrary to its name, dark reactions are not confined to the dark nor are they totally independent from light.  Still, CAM plants’ dark reactions do happen at night.

1. Absorption of light  ( Hill Reaction & Photolysis )

Site : Thylakoid membrane

photons + P680 → P680* → P680⁺  + e

2 H₂O + 4 P680⁺ → 4 H⁺ + O₂ ( to atmosphere )+ 4 P680

- O₂ is released to the atmosphere

- 4 H⁺ (proton) enter photophosphorylation to form ATP

- 4 e^- exit photosystem II, enter photosystem I via a electron transfer chain

2. Transport of electrons ( z-sheme )

Site : Thylakoid membrane 

Electron transport chain PS2 :( Pheo – PQ – b6f – Pc – PS1 )

Electron transport chain PS1 : ( Rd – FNR )

Site : Chloroplast stroma : NADP⁺ + 2 H⁺ + 2 e^-  → NADPH + H⁺

Overall reaction of stage 1 & 2 : 2 H₂O + NADP⁺ → 2 H⁺ + O₂ + 2 NADPH

3. ATP synthesis ( photophosphorilation )

Site : Thylakoid membrane

- 4 e^-  transport via b6f synthesis ATP to pump  H⁺ from stroma into lumen.

- Extra H⁺ inside lumen creates a gradient, resulting in H⁺ flow back via F₀F₁ complex out to stroma, creating another ATP.

4. Fixation of carbon ( Calvin Cycle )

Site : Stroma

- carbon is  fixed into organic matter with the help of ATP and NAPDH from the light reactions.

6 CO₂ + 18 ATP^4- + 12 NAPDH + 12 H₂O  → C₆H₁₂O₆ + 18 ADP^3- + 18P_i^2- + 12 NADP⁺ + 6 H⁺