Question

Q1. State briefly two laws of thermodynamics. (1.5+1.5 3 pts According to the first law of thermodynamics, the energy According to the second law of thermodynamics, every energy Q2. During photosynthesis Calvin cycle uses the chemical energy of ATP and NADPH to do EXACTLY what? Name the KEY regulatory enzyme in Calvin cycle (1+1=2pts) Q3. Label (image below) correctly the spectrum of wavelength where photosynthesis can happen and where it cannot? Explain briefly [0.5+0.5+2 = 3pts] Chloro- phyll a Chlorophyllb Absorption of light by chloroplast pigments Carotenoids 700 500 600 Wavelength of light (nm) Rate of photosynthesis (measured by Ó, release) 500 600 7001

Answer 1

Q1..

First law of thermodynamics is the law of conservation of energy which is stated as

• Energy can neither be created nor destroyed although it may be converted from one form to another.
• The total energy of the universe that is, system and the surrounding remains constant though it may change from one form to another.

According to second law of thermodynamics,

• Every energy exchanges, and if no energy leaves or enters the system , potential energy of the state will be less than that of the initial state.This is known as entropy which is the measure of molecular disorder or randomness of system.

Q2..

Dark reaction / C3 cycle /Calvin - Benson cycle / Biosynthetic phase /Photosynthetic carbon reduction cycle :

• Biosynthetic phase of photosynthesis utilizes assimilatory power such as ATP and NADPH produced during light reaction are used to to fix and reduce carbon dioxide into carbohydrates.
• This reaction does not require light and hence it is called as dark reaction.
• The primary acceptor molecule is five carbon ketose sugar called Ribulose bisphosphate (RuBP).

Regulatory enzyme in Calvin cycle :

• The enzyme for carbon dioxide fixation is Ribulose biphosphate carboxylase oxygenase (RuBisCO). This enzyme's active site has ability to bind carbon dioxide and oxygen. But, it has higher affinity to Carbon dioxide than for Oxygen and binding is competitive. The relative concentration of both will determines which of the two will bind to the enzyme.
• Calvin cycle consists of three stages -carboxylation , reduction and regeneration.

In Calvin cycle,

IN (utilised)	OUT (produced)
6 CO​​​​​2	1 glucose
18 ATP	18 ATP
12 NADPH	12 NADP

Q3..

Photosynthetic pigments are substances that have ability to absorb light at specific wavelength .The chromatographic separation of leaf pigments shows the colour of leaves is due to the four pigments such as chlorophyll a ,chlorophyll b , xanthophylls and carotene. Of these, chlorophyll a is the primary photosynthetic pigment. All pigments other than chlorophyll a are called accessory pigments.

In chromatographic separation,

• Chlorophyll a is bright or blue-green
• chlorophyll b is yellow-green
• xanthophylls is yellow
• Carotene is yellow to yellow -orange.

The accessory pigments absorb light of different wavelength and transfer energy to chlorophyll molecules there also called antenna molecules.carotenoids protect the plant from excess heat and prevent photooxidation hence they are called as shield pigments.

First graphic curve- Absorption spectrum : shows the amount of energy of different wavelength of light absorbed by pigments.

Second graphic curve - action spectrum : showing relative rate of of photosynthesis at different wavelengths of light.

These graph shows that most of photosynthesis takes place in blue and red regions of spectrum and some photosynthesis takes place at other wavelength of visible spectrum. These graphs depicts that maximum photosynthesis occurs at wavelength at which there is maximum absorption of chlorophyll a, that is, in the blue and red region. Chlorophyll a 680 (P680) and chlorophyll a 700 (P700) function as trap centre for PS- 11 and PS-1 respectively. Photosynthetically active radiation (PAR) is between 400 to 700 nm.

Chlorophyll a and b absorb water from quanta from blue and red region. Maximum absorption peak for different forms of chlorophyll a is 670 - to 673nm , 680 to 683nm and 695 to 705nm and green light is least effective and it is reflected.Thus, red light induces highest rate of photosynthesis and green light induces least rate of photosynthesis.