The Basics of Photosynthesis – 2

Part – 1

Whenever high light falls on plants as a first line of defence Xanthophyll cycle runs. Due to further light intensity Triplet chlorophyll and other Reactive Oxygen Species (ROS) like O2-, O-, H2O2, OH-, O3 are produced, which are countered by second line of defence as antioxidants like beta carotene, vitamin C, vitamin E, SOD, reduced glutathione etc. If ROS is too high then antioxidants also fails, and they cause oxidation i.e. Photooxidation of D1 protein of PS-II which results in photoinhibition of photosynthesis (temporary process). Photo oxidised D1 is excised out and new D1 is incorporated.

1. Zeaxanthin formation
2. Degradation of D1 of PS-II  
3. Thermal dissipation

All three are part of photoprotection

• FNR stands for Ferredoxin NADP Reductase
• Fd stands for Ferredoxin
• PS-II present in stalked region/ appressed membrane
• PS-I & ATP synthase are present in unstalked/Non-appressed membrane and stomal lamellae protruding into stroma because PS-I form NADPH & ATP- Synthase form ATP.
• Cytochrome b6f complex is uniformly distributed.
• Carotenoids are accessory pigments which give protection from light and are integral membrane compounds.

Quantasome:

Also known as Photosynthetic Unit or Antenna complex. It consists of core/ central chlorophyll-a, above which chlorophyll-b and outermost beta- carotene. These three together make up 280 molecules which makes up one quantasome.

• Light should fall on this reaction centre

• Chlorophyll-a is a universal pigment because reaction starts from here only
• Quantasomes are present on the inner face of thylakoid
• Quantasome discovered by Park and Biggin.

Chlorophyll Biosynthesis:

• Starts from Glutamic acid, which is converted into Glutamate 1 semialdehyde which gets converted into delta Aminolaevulinic Acid, which is converted to porphobilinogen and then into Protoporphyrin IX. If Magnesium added then protoporphyrin IX gets converted in to chlorophyll as seen in plants and if iron added then converted into heme as seen in animals.

Lumen of Thylakoid:

• It is filled with water and salt and photooxidation/photolysis of water takes place here.
• Photooxidation/Photolysis of water

1)It occurs in lumen and associated with PS-II

2)It needs an enzyme which is known as water oxidising enzyme (WOE) complex or oxygen evolving complex (O.E.C.) or Mn++ protein complex or Mn₄CaO₅

3) Mn++ is an essential cofactor in WOE complex which undergoes S0, S1, S2, S3 &S4 oxidation states.

4) To explain the photolysis of water KOK gave the concept of “S” state mechanism i.e. oxidation of water molecule is 5 step, S-state mechanism.

5) If single water molecule is broken, o is released which is damaging therefore in plants two molecules of water are broken.

Try Z amino acid domain is found in D1 which gives two electrons to PS-II. Also only two electrons are required by PS-II, so first two electrons will go then other two will go.

• All these electrons released at step 1,2,3 &4 goes to TyrZ protein of D1. From TryZ they go to PS-II
• TryZ takes electrons from water and gives it to PS-II known as Ferry boat.

Out of two water molecules, four hydrogen ions makes proton gradient, four electrons catched by TyrZ and oxygen given to atmosphere.

These 8 photons requirement is called quantum requirement. These 8 photons will produce one O2, So 1 photon will produce 1/8 O₂i.e. 12.5% O2. 12.5% is the quantum yield.

Photosynthesis:

Photosynthesis is divided into 2 types of reactions i.e. Light reaction and dark reaction