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⁺

Plant Growth : once a mystery

It is like a mustard seed, which is the smallest seed you plant in the ground. Yet when planted, it grows and becomes the largest of all garden plants . . .

Mark 4 31-32 ( NIV )

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From a tiny seed, plants are capable to grow to a majestic size.

World largest single stem tree, by volume, is a Giant Sequoia ( Sequoiadendron giganteum ), nicknamed General Sherman, stands 83.8 m tall, and the trunk alone is estimated to weigh over 1,814 metric tonne; is grown from a seed not more than 5mm in length !

How does plant gain its mass ? What caused them to grow ?

Man had since awed by the miraculous growth of plant…   Plant seems to be simply emerged from the earth. Of course some suspected the earth provided the substances for plant growth, as did Aristotle, but what exactly ?

Jean Baptiste van Helmont, in 1648 published in his book Ortus medicinae an experiment he performed on a willow tree planted in a pot for 5 years. He weighted the young tree, soil and the water he added. After 5 years, the plant gained about 164 pounds, the amount of soil remain relatively unchanged. Thus, he deducted that the tree’s weight gain had come from water.

Today, we already know that plant’s mass consists of about 80% water, 19% organic matter, and 1% minerals.

Through a process known as photosynthesis, green plants and certain other organisms transform light energy into chemical energy. Water, carbon dioxide, and minerals are converted into oxygen and energy-rich organic compounds.

At the same time, plants rely on soil for supplying water and mineral elements. Mineral elements are essential and beneficial for optimum plant growth.