It is important to note that photosynthesis is the only natural process, which liberates oxygen for the use, all other living organisms. An amount of oxygen equivalent to that present in the Earth's entire atmosphere, is produced by photosynthesis every two years. In this lesson, you will learn how plants carry out photosynthesis.

• After completing this lesson, you will be able to:
• define photosynthesis
• name the different pigments found in chloroplasts and describe the ultrastructure of chloroplasts with a diagram.
• explain the main aspects of the process of photosynthesis.
• enumerate the steps involved in the light and dark reactions of photosynthesis.
• define the terms absorption spectrum, electron acceptor, photophosphorylation and action spectrum.
• distinguish between, absorption spectrum and action spectrum; light and dark reactions; cyclic and non-cyclic phosphorylation, C and C photosynthesis.
• list the environmental variables and internal factors affecting photosynthesis.
• describe the principle of limiting factor giving suitable graphs.

These photo synthetic pigments belong to three categories:

(i) Chlorophylls (ii) Carotenoids (iii) Phycobilins

The photo synthetic pigments of higher plants fall into three classes, the chlorophylls, carotenoids and phycobillms. The role of the pigments is to absrob light energy, thereby converting it to chemical energy. These pigments are located on the chloroplast membranes and the chloroplasts are usually arranged within the cells so that the membranes are at right angles to the light source for maximum absorption.

Carotenoids and Phycobilins absorb light in the wavelength range not absorbed by chlorophyll and they transfer this energy to chlorophyll. Therefore, carotenoids and phycobilins are known as accessory pigments.

When investigating a process such as photosynthesis that is activated by light, it is important to establish the action spectrum for the process and to use this to identify the pigments involved. An action spectrum is a graph showing the effectiveness of different wavelengths (VIBGYOR) of light in stimulating the process of photosynthesis, where the response could be measured in terms of oxygen produced at different wavelengths of light. An absorption spectrum is a graph of the relative absorbance of different wavelengths of light by a pigment. An action spectrum for photosynthesis is shown in Fig. 13.2 together with an absorption spectrum for the combined photo synthetic pigments. Note the close similarity, which indicates that the pigments, chlorophyll in particular, are those responsible for absoiptiqn of light in photosynthesis.

All wavelengths of light are not equally effective in photosynthesis i.e. the rate of photosynthesis is more in some and less in others.

Wavelength Action

B. There are two pigment systems, which work in the trapping of solar energy in photosynthesis. These are called photosystem I and photosystem II. The differences between them are given in the following table 13.1

chlorophyll a ^{light enersy} Chlorophyll a⁺ + e-

(Reduces Chlorophyll) (Chlorophyll oxidised) + (Excited electron)

The fate of the electrons is summarised in Fig. 13.3. The pathway shown is sometimes known as the 'Z-Scheme' from its shape. Remember that losing an electron is oxidation, gaming an electron is reduction.

First, an electron from photosystem I or n is boosted to a higher energy level, i.e. it acquires excitation energy. It is captured by an electron acceptor. This represents the important conversion of light energy to chemical energy. The electron acceptor is thus reduced and a positively charged (oxidised) pigment is left in photosystem. The electron then travels downhill, in energy terms, from one electron acceptor to another in a series of oxidation - reduction (redox) reactions. This electron flow is "coupled" to the formation of ATP in both cyclic and non-cyclic pathways; in addition, NADP is reduced in the non-cyclic pathway.

Non-cyclic photophosphorylation is initiated by light shining on photosystems 1 Andean. Exited electrons from P 680 (PSII) and P 700 (PSI) reduce electron acceptors. P 680 and P 700 are now positively charged (oxidised). P 680 is neutralised by electrons from water; electrons flow downhill from the latter to P 680 via electron carrier and oxygen is produced as a waste product of photosynthesis.

P 700 is neutralised by electrons moving downhill via chain of electron carriers, the energy from this flow being coupled to ATP production. Upto two ATP molecules may be made per pair of electrons, but this number is probably variable. Finally, electrons pass downhill from Ferredoxin to NADP and combine with hydrogen ions to form NADPH,. Note that the excess hydrogen ions are available from the 'splitting' of water. (Fig. 13.3)

The overall equation for non-cyclic photophosphorylation is

light energy HO + NADP + 2 ADP + 2Pi,———————— '/2 0, + NADPH, + 2 ATP

Cholorophyll

(maximum of 2 ATP may be less than 2)

Extra ATP can be made via cyclic photophosphorylation. The efficiency of energy conversion in the light reactions is high and estimated at about 39%.

(i) Light Reaction (Also called Hill Reaction after the name of the discoverer)

(ii) Dark Reaction

(i) Light Reaction It is an early step in photosynthesis and occurs in the grana of the chloroplast. These are closely tied with absorption of light, and they stop immediately as the light is shut off. These are in two phasies.

Phase -1 (Photolysis):- The light energy absorbed by the chlorophyll breaks the water molecule as follows:

4H₂O —^ 4H⁺ + 4 OH-40H- -> 4(OH) + 4e- (electron) 4(OH) —^ 2H.O+0, All together, 4ILO -> 2H^O + 4H⁺ + 4e-

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The essential point in this phase of light reactions are:

(i) Water is broken down into high reactive ions (the process is known as Photolysis ("Photo" = light, "lysis" breaking)

(ii) Oxygen is produced which is given out of the leaf. (iii) Highly reactive If and electrons are available.

Photosystem II (described earlier) picks up the electrons produced during photolysis. For this it has to absorb light energy. The excited electrons come out of the chlorophyll molecules and pass on to a system of electron carriers. The electrons lose most of then- energy which gets used in forming an energy-rich compound called ATP (this process is known as photophosphorylation which means "addition of inorganic phosphorus through light-energy"). In other words light energy has been converted into chemical energy.

energy ADP + Pi———-^ATP

The electrons ultimately reach photosystem I. Here they are given another doze of light energy. These energy rich electrons activates a special compound NADP (nicotinamide adenine dinucleotide phosphate). The activated NADP combine with hydrogen (produced during photolysis) and gives rise to NADPH

NADP + e⁺ + H⁺ ——————————^ NADPH

A bit of Chemistry to remember:

Addition of "H" to a compound is called reduction and the compound thus produced is called reductant.

A compound that can readily take up "H" is called hydrogen acceptor.

Removal of "H" from a compound is called oxidation. So what can you say about the reactive nature of NADP?

Thus, these were the light reactions of photosynthesis. And where do they occur? They occur in grana of chloroplast.

(ii) Dark Reactions - These reactions do not require light energy (but it does not mean that they occur at night).

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The main features of the dark reactions can be summarised as follows.

It occurs in the stroma of the chloroplasts.

The hydrogen of NADPH is used to combine with Cowboy utilizing energy from ATP to form organic compounds.

There are two major pathways by which this combination (fixation) of CO occurs depending upon whether the product is a 3-carbon substance or a 4-carbon substance. Accordingly, they are called C, system and C^ system

Fig. 13.5 C, cycle of photosynthesis. Initial product is a 3-carbon compound

The whole process of photosynthesis can be summarized as shown in Table 13.5

2. The leaf cells possess two types of chloroplasts

3. Within the cells of bundle sheath the chloroplasts are larger and lack grana but contain numerous starch grains.

4. The mesoohyll cells of the leaf contain smaller chloroplasts which have well developed grana but do not accumulate starch.

The leaves of C₄ plants are thus compartmentalised (block or groups) and exhibit a division of labour with respect to the fixation of CO₂ into C₄ acids (mesophyll cell chloroplasts) and the subsequent formation of phosphorylated sugars (bundle sheath cell chloroplasts). Due to specialized leaf anatomy C₄ plants photosynthesize more efficiently than do C₄ plants.

The characteristics of C₄ plant leaves are as follows:

5. Examples - maize, sugarcane, grasses, etc.

C₃, and C₄ plants differ from each other in relation to their leaf anatomy, CO₂, fixing enzymes, efficiency etc. as shown in Table 13.5

PGA then reacts with NADPH which was formed in the light reaction to produce a 3-Carbon sugar called triose with attached phosphate. This reaction is mediated by ATP that was produced in light reaction. It once again revert to ADPs. Two molecules of triose phosphate can be converted into a single molecule of glucose or some other hexose sugar (containing 6 carbon atoms).In order to keep the cycle going on, the 5-Carbon sugar compound RuBP is regenerated through a series of reactions in C- system.

m plants which cany out C cycle, the CO fixation occurs to produce the first stable intermediate compound in the form of a 4-carbon compound instead of 3-compound (fig. 13.6). This 4-carbon product then breaks down to release CO., which is then picked up by an enzyme to follow the same cycle as in C₄ plants.

(a) Light: The rate of photosynthesis increases with increase in the intensity of light. However, extremely high intensitites of light do not increase the rate of photosynthesis. Optimum light intensity for photosynthesis varies with the species of the plant.

(b) Temperature: In general increase in temperature results in increase in the rate of photosynthesis when other factors are not limiting. Photosynthesis is restricted to a temperature range in which the enzymes remain active i.e. between 0° C and 60° C. Only biochemical part of photosynthesis (dark phase) which is controlled by enzymes is temperature-dependent.

(c) Water: Even less than 1% of the total water absorbed by plants is utilised as raw material in photosynthesis. Therefore water rarely becomes a limiting factor.

(a) Leaf age: Generally, as leaves grow the rates of photosynthesis increases up to a stage just before maturity and then it decreases slowly. Old, senscent leaves eventually become yellow and are unable to photosynthesis because of chlorophyll-breakdown and loss of functional chloroplasts.

(b) Chlorophyll content: No direct correlation has so far been observed between rate of photosynthesis and chlorophyll content.

(c) Leaf Anatomy: Rate of photosynthesis in a leaf is influenced by the variation in: (i) number, structure and distribution of stomata, (ii) size and distribution of intercellular spaces (iii) relative proportion and distribution of palisade and spongy tissues, (iv) thickness of cuticle and (v) size and distribution of vascular system. As mentioned earlier, the leaves of C^ plants are more productive due to specialised anatomy.

When chemical .process is affected by more than one factor its rate is limited by that factor which is nearest its minimum value; it is that factor which directly affects a process if its quantity is changed.(fig. 13.7)

The principle was first established by Blackman in 1905. Since then it has been shown that different factors, such as carbon dioxide concentration and light intensity, interact and can be limiting at the same time, although one is often the major factor.

Fig. 13.7 Blackman's Law of Limiting Factor Rate of photosynthesis as a function of light intensity, CO^ concentration, and temperature. At low light intensities, light is the limiting factor. At higher light intensities, temperature and CO concentration are the limiting factors.

• Green plants are capable of synthesizing carbohydrates from CO₂ and H₂O in the presence of light, by the process of photosynthesis.
• During photosynthesis 'light energy", which is captured by the photosynthetic pigments (chlorophyll, carotenoids and xanthophylls) present in the chloroplasts, is converted into chemical energy.
• Photosynthesis in general is expressed by the following equation:

6CO₂ +12H₂.O Chlorophyll C₆H₁₂O₆ +6H₂O+6O₂

• Photosynthesis comprises two sets of reactions:
• Light reactions: which take place in grana only in the presence of light.
• Dark reactions: Which occur in the stroma of chloroplast and is independent of light.
• Light energy is used for splitting of water, and actual reduction of CO takes place in the dark reaction.
• Light reaction is further divisible into photosystem-I and photosystem-II.
• During dark reactions CO, is accepted by Ribulose biphosphate (RuBP) and 3-PGA (3 phosphoglyceric acid) is formed which by further cyclic reactions (Calvin Cycle) leads to the formation of carbohydrates as well as in regeneration of RuBP.
• In C₄ plants like maize. Sorghum, the primary acceptor of CO₂ is in mesophyll cells and the first detectable product of dark reaction in oxaloacetic acid (OAA), whereas in the bundle sheath cells CO₂ fixation occurs viz. Calvin cycle.
• Leaf anatomy of C₄ plants is known as "Kranz anatomy" and is characterized by the presence of a sheath of parenchyma cells around a vascular bundle (bundle sheath). Cells of this sheath have larger chloroplasts which lack grana and are filled with starch grains. In contrast mesophyll cells contain chloroplasts which are smaller but have well developed grana.
• Rate of photosynthesis is influenced by (i) environmental factors such as light, temperature, carbon dioxide concentration and water, and (ii) internal factors which include age of leaf, chlorophyll content and leaf anatomy.

1. Describe briefly the process of photosynthesis.
2. Write short notes on: (i) Ultra structure of chloroplast and (ii) Pigments involved in photosynthesis.
3. Mention the path of electrons in the light reaction of photosynthesis.
4. Describe the reactions occurring during dark reaction of photosynthesis (Calvin Cycle).
5. Distinguish between C^ plants and C plants.
6. Describe the factors which influence the rate of photosynthesis.
7. With the help of a suitable graph explain the principle of limiting factors.
8. Distinguish between cyclic and non-cyclic photophosphorylation.
9. Give one example each of an electron donor and an electron acceptor
10. Give four important differences between photosynthesis and Respiration

1. What is Kranz Anatomy?
2. List three main characteristics of C^ plants.

1. What is Kranz Antomy?
2. What is the role of NADP in photosynthesis.