Lecture Prepared by: Dr. Laxmi Kant Pandey PHOTOSYNTHESIS Lecture Prepared by: Dr. Laxmi Kant Pandey

LIGHT HARVESTING COMPLEX

Definition The light-harvesting complex (or antenna complex) is an array of protein and chlorophyll molecules embedded in the thylakoid membrane of plants, which transfer light energy to one chlorophyll a molecule at the reaction center of a photosystem.

Role of Photosystem and Photosynthesis

Bacterial antenna complex The antenna complexes are generally composed of two types of polypeptides (alpha and beta chains). This is arranged in a ring-like fashion creating a cylinder that spans the membrane; the proteins bind two or three types of bacteriochlorophyll (BChl) molecules and different types of carotenoids  

Light-harvesting complexes of green plants The antenna pigments are predominantly  chlorophyll b., xanthophylls, and carotenoids. Chlorophyll a is known as the core pigment. Each antenna complex has between 250 and 400 pigment molecules

PHOTOSYNTHESIS OVERVIEW Photosynthetic process in plants into four stages Absorption of light, Electron transport through photosystem, leading to the reduction of NADP+ to NADPH, Generation of ATP, and Conversion of CO2 into carbohydrates (carbon fixation).

All four stages of photosynthesis are tightly coupled and controlled so as to produce the amount of carbohydrate required by the plant. All the reactions in stages 1 – 3 are catalyzed by proteins in the thylakoid membrane. The enzymes that incorporate CO2 into chemical intermediates (stage 4 or dark reaction)and then convert it to starch are soluble constituents of the chloroplast stroma

OVERVIEW OF PHOTO PHOSPHORYLATION It takes place during photosynthesis (an anabolic process) in thylakoids (of grana) in chloroplast. Ultimate source of energy for photophosphoryla­tion is light. It is associated with photosynthetic pigment systems and elec­tron transport in chloroplasts (generation of NADPH) and is of two types- cyclic and non-cyclic photophosphorylation. Energy rich ATP molecules and NADPH produced by this pro­cess are mainly utilised in dark reaction (Calvin Cycle) of photo­synthesis for the synthesis of carbohydrates.

PHOTOSYNTHETIC PIGMENTS CHLOROPHYLLS a, b, c, d, f ACCESSORY PIGMENTS: carotenoids phycobillins

Chlorophylls Chlorophyll is a term used for several closely related green pigments found in cyanobacteria and the chloroplasts of plants and algae. Its name is derived from the Greek words ‘chloros’ ("green") and ‘phyllon’ ("leaf"). Chlorophyll is vital for photosynthesis, which allows plants to absorb energy from light. Chlorophyll absorbs light most strongly in the blue portion of the electromagnetic spectrum.

Chlorophyll a Chlorophyll b Chlorophyll c1 Chlorophyll c2 Chlorophyll d Chlorophyll f Occurence Universal Mostly plants Various algae Cyanobacteria Formula C55H72O5N4Mg C55H70O6N4Mg C35H30O5N4Mg C35H28O5N4Mg C54H70O6N4Mg

Structure of Chlorophyll a and b Pyrrole ring I II IV III V Cyclopentanone ring Ester bond

Accessory Pigments Carotenoids Phycobillins

Carotenoids Carotenoids, also called tetraterpenoids. These are organic pigments that are found in the chloroplasts of plants and some other photosynthetic organisms, including some bacteria. There are over 600 known carotenoids; they are split into two classes, xanthophylls (which contain oxygen) and carotenes (which are purely hydrocarbons, and contain no oxygen). Carotenoids serve two key roles in plants and algae: they absorb light energy for use in photosynthesis, and they protect chlorophyll from photodamage. 

Phycobillins Phycobilins (from Greek: (phykos) meaning "alga", and from Latin: bilis meaning"bile") are light- capturing bilanes found in cyanobacteria and in the chloroplasts of red algae. They are unique among the photosynthetic pigments in that they are bonded to certain water-soluble proteins, known as phycobiliproteins. Phycobiliproteins then pass the light energy to chlorophylls for photosynthesis. The phycobilins are especially efficient at absorbing red, orange, yellow, and green light, wavelengths that are not well absorbed by chlorophyll a.

Phycobilines……… There are four types of phycobilins Phycoerythrobilin, which is red Phycourobilin, which is orange Phycoviolobilin (also known as phycobiliviolin) Phycocyanobilin (also known as phycobiliverdin), which is blue. phycobilins consist of an open chain of four pyrrole rings (tetrapyrrole) and are structurally similar to the bile pigment bilirubin,

Xanthophyll

Light light, a form of electromagnetic radiation, has properties of both waves and particles. When light interacts with matter, it behaves as discrete packets of energy (quanta) called photons. energy of a photon, ϵ, is proportional to the frequency of the light wave: ϵ = hγ, h is Planck’s constant (1.58 × 10−34 cal·s, or 6.63 × 10−34 J·s), γ is the frequency of the light wave. γ = c ÷ λ, where c is the velocity of light (3 × 1010 cm/s in a vacuum).

Absorption of Light The energy of the absorbed light is used to remove electrons from an donor (water, in green plants), forming oxygen. and then to transfer the electrons to a primary electron acceptor, a quinone designated Q, which is similar to CoQ.

Many photosynthetic bacteria use molecules such as hydrogen gas (H2) or hydrogen sulfide (H2S) as the ultimate source of electron.

One of the strongest pieces of evidence for the involvement of chlorophylls and β- carotene in photosynthesis is that the absorption spectrum of these pigments is similar to the action spectrum of photosynthesis The action spectrum of photosynthesis in plants; that is, the ability of light of different wavelengths to support photosynthesis.

680 nm 650 nm

Light Absorption by Reaction-Center Chlorophylls Causes a Charge Separation across the Thylakoid Membrane

42 kcal/mol.

According to this model, the ground state of the reaction-center chlorophyll(P) is not a strong enough reductant to reduce Q; that is, an electron will not move spontaneously from P to Q. However, the excited state of the reaction center chlorophyll, P* (after absorption of light), is an excellent reductant and rapidly (in about 10−10 seconds) donates an electron to Q, generating P+ and Q−. The acceptor, Q−, is a powerful reducing agent capable of transferring the electron to plasto quinone and ultimately to NADP+. The powerful oxidant P+ can remove electrons from water molecules to regenerate the original P.