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All animals including human beings depend on plants for their food. Have

you ever wondered from where plants get their food? Green plants, in fact,

have to make or rather synthesise the food they need and all other organisms

depend on them for their needs. The green plants make or rather synthesise

the food they need through photosynthesis and are therefore called autotrophs.

You have already learnt that the autotrophic nutrition is found only in plants

and all other organisms that depend on the green plants for food are

heterotrophs. Green plants carry out ‘photosynthesis’, a physico-chemical

process by which they use light energy to drive the synthesis of organic

compounds. Ultimately, all living forms on earth depend on sunlight for

energy. The use of energy from sunlight by plants doing photosynthesis is

the basis of life on earth. Photosynthesis is important due to two reasons: it

is the primary source of all food on earth. It is also responsible for the release

of oxygen into the atmosphere by green plants. Have you ever thought what

would happen if there were no oxygen to breath? This chapter focusses on

the structure of the photosynthetic machinery and the various reactions

that transform light energy into chemical energy.

13.1 WHAT DO WE KNOW?

Let us try to find out what we already know about photosynthesis. Some

simple experiments you may have done in the earlier classes have shown

that chlorophyll (green pigment of the leaf), light and CO

are required for

photosynthesis to occur.

You may have carried out the experiment to look for starch formation

in two leaves – a variegated leaf or a leaf that was partially covered with

black paper, and exposed to light. On testing these leaves for the presence

of starch it was clear that photosynthesis occurred only in the green parts

of the leaves in the presence of light.

HOTOSYNTHESIS IN

IGHER

LANTS

HAPTER

13.1 What do we

Know?

13.2 Early

Experiments

13.3 Where does

Photosynthesis

take place?

13.4 How many

Pigments are

involved in

Photosynthesis?

13.5 What is Light

Reaction?

13.6 The Electron

Transport

13.7 Where are the

ATP and NADPH

Used?

13.8 The C

Pathway

13.9 Photorespiration

13.10 Factors

affecting

Photosynthesis

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Another experiment you may have carried out

where a part of a leaf is enclosed in a test tube

containing some KOH soaked cotton (which

absorbs CO

), while the other half is exposed to air.

The setup is then placed in light for some time. On

testing for the presence of starch later in the two

parts of the leaf, you must have found that the

exposed part of the leaf tested positive for starch

while the portion that was in the tube, tested

negative. This showed that CO

was required for

photosynthesis. Can you explain how this

conclusion could be drawn?

13.2 EARLY EXPERIMENTS

It is interesting to learn about those simple

experiments that led to a gradual development in

our understanding of photosynthesis.

Joseph Priestley (1733-1804) in 1770

performed a series of experiments that revealed the

essential role of air in the growth of green plants.

Priestley, you may recall, discovered oxygen in

1774. Priestley observed that a candle burning in

a closed space – a bell jar, soon gets extinguished

(Figure 13.1 a, b, c, d). Similarly, a mouse would

soon suffocate in a closed space. He concluded that

a burning candle or an animal that breathe the air,

both somehow, damage the air. But when he placed a mint plant in the

same bell jar, he found that the mouse stayed alive and the candle

continued to burn. Priestley hypothesised as follows: Plants restore to

the air whatever breathing animals and burning candles remove.

Can you imagine how Priestley would have conducted the experiment

using a candle and a plant? Remember, he would need to rekindle the

candle to test whether it burns after a few days.

How many different

ways can you think of to light the candle without disturbing the set-up?

Using a similar setup as the one used by Priestley, but by placing it

once in the dark and once in the sunlight, Jan Ingenhousz (1730-1799)

showed that sunlight is essential to the plant process that somehow

purifies the air fouled by burning candles or breathing animals.

Ingenhousz in an elegant experiment with an aquatic plant showed that

in bright sunlight, small bubbles were formed around the green parts

while in the dark they did not. Later he identified these bubbles to be of

oxygen. Hence he showed that it is only the green part of the plants that

could release oxygen.

(a)

(c)

(b)

(d)

Figure 13.1 Priestley’s experiment

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It was not until about 1854 that Julius von Sachs provided evidence

for production of glucose when plants grow. Glucose is usually stored as

starch. His later studies showed that the green substance in plants

(chlorophyll as we know it now) is located in special bodies (later called

chloroplasts) within plant cells. He found that the green parts in plants is

where glucose is made, and that the glucose is usually stored as starch.

Now consider the interesting experiments done by T.W Engelmann

(1843 – 1909). Using a prism he split light into its spectral components

and then illuminated a green alga, Cladophora, placed in a suspension

of aerobic bacteria. The bacteria were used to detect the sites of O

evolution. He observed that the bacteria accumulated mainly in the region

of blue and red light of the split spectrum. A first action spectrum of

photosynthesis was thus described. It resembles roughly the absorption

spectra of chlorophyll a and b (discussed in section 13.4).

By the middle of the nineteenth century the key features of plant

photosynthesis were known, namely, that plants could use light energy

to make carbohydrates fr

om CO

and water. The empirical equation

representing the total process of photosynthesis for oxygen evolving

organisms was then understood as:

CO H O CH O O

Light

2 2 2 2

+  → +[ ]

where [CH

O] represented a carbohydrate (e.g., glucose, a six-carbon

sugar).

A milestone contribution to the understanding of photosynthesis was

that made by a microbiologist, Cornelius van Niel (1897-1985), who,

based on his studies of purple and green bacteria, demonstrated that

photosynthesis is essentially a light-dependent reaction in which

hydrogen from a suitable oxidisable compound reduces carbon dioxide

to carbohydrates. This can be expressed by:

2 2

2 2 2 2

H A CO A CH O H O

Light

+  → + +

In green plants H

O is the hydrogen donor and is oxidised to O

. Some

organisms do not release O

during photosynthesis. When H

S, instead

is the hydrogen donor for purple and green sulphur bacteria, the

‘oxidation’ product is sulphur or sulphate depending on the organism

and not O

. Hence, he inferred that the O

evolved by the green plant

comes from H

O, not from carbon dioxide. This was later proved by using

radioisotopic techniques. The correct equation, that would represent the

overall process of photosynthesis is therefore:

6 12 6 6

2 2 6 12 6 2 2

CO H O C H O H O O

Light

+  → + +

where C

represents glucose. The O

released is from water; this

was proved using radio isotope techniques. Note that this is not a single

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reaction but description of a multistep process called photosynthesis.

Can you explain why twelve molecules of water as substrate are used

in the equation given above?

13.3 WHERE DOES PHOTOSYNTHESIS TAKE PLACE?

You would of course answer: in ‘the green leaf’ or ‘in the chloroplasts’,

based on what you earlier r

ead in Chapter 8. You are definitely right.

Photosynthesis does take place in the green leaves of plants but it does so

also in other green parts of the plants. Can you name some other parts

where you think photosynthesis may occur?

You would recollect from previous unit that the mesophyll cells in the

leaves, have a large number of chloroplasts. Usually the chloroplasts align

themselves along the walls of the mesophyll cells, such that they get the

optimum quantity of the incident light. When do you think the

chloroplasts will be aligned with their flat surfaces parallel to the walls?

When would they be perpendicular to the incident light?

You have studied the structure of chloroplast in Chapter 8. Within

the chloroplast there is membranous system consisting of grana, the

stroma lamellae, and the matrix stroma (Figure 13.2). There is a clear

division of labour within the chloroplast. The membrane system is

responsible for trapping the light energy and also for the synthesis of ATP

and NADPH. In stroma, enzymatic reactions synthesise sugar, which in

turn forms starch. The former set of reactions, since they are directly light

driven are called light reactions (photochemical reactions). The latter

are not directly light driven but are dependent on the products of light

reactions (ATP and NADPH). Hence, to distinguish the latter they are called,

by convention, as dark reactions (carbon reactions). However, this should

not be construed to mean that they occur in darkness or that they are not

light-dependent.

Figure 13.2 Diagrammatic representation of an electron micrograph of a section of

chloroplast

Outer membrane

Inner membrane

Stromal lamella

Grana

Stroma

Ribosomes

Starch granule

Lipid droplet

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13.4 HOW MANY TYPES OF PIGMENTS ARE

INVOLVED IN PHOTOSYNTHESIS?

Looking at plants have you ever wondered why

and how there are so many shades of green in

their leaves – even in the same plant? We can

look for an answer to this question by trying to

separate the leaf pigments of any green plant

through paper chromatography. A

chromatographic separation of the leaf pigments

shows that the colour that we see in leaves is

not due to a single pigment but due to four

pigments: Chlorophyll a (bright or blue green

in the chromatogram),

chlorophyll b (yellow

green), xanthophylls (yellow) and carotenoids

(yellow to yellow-orange). Let us now see what

roles various pigments play in photosynthesis.

Pigments are substances that have an ability

to absorb light, at specific wavelengths. Can you

guess which is the most abundant plant

pigment in the world? Let us study the graph

showing the ability of chlorophyll a pigment to

absorb lights of different wavelengths (Figure

13.3 a). Of course, you are familiar with the

wavelength of the visible spectrum of light as

well as the VIBGYOR.

From Figure 13.3a can you determine the

wavelength (colour of light) at which chlorophyll

a shows the maximum absorption? Does it

show another absorption peak at any other

wavelengths too? If yes, which one?

Now look at Figure 13.3b showing the

wavelengths at which maximum photosynthesis

occurs in a plant. Can you see that the

wavelengths at which there is maximum

absorption by chlorophyll a, i.e., in the blue and

the red regions, also shows higher rate of

photosynthesis. Hence, we can conclude that

chlorophyll a is the chief pigment associated

with photosynthesis. But by looking at Figure

13.3c can you say that there is a complete

one-to-one overlap between the absorption

spectrum of chlorophyll a and the action

spectrum of photosynthesis?

Figure 13.3a Graph showing the absorption

spectrum of chlorophyll a, b and

the carotenoids

Figure 13.3b Graph showing action

spectrum of photosynthesis

Figure 13.3c Graph showing action

spectrum of photosynthesis

superimposed on absorption

spectrum of chlorophyll a

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These graphs, together, show that most of the photosynthesis takes

place in the blue and red regions of the spectrum; some photosynthesis

does take place at the other wavelengths of the visible spectrum. Let us

see how this happens. Though chlorophyll is the major pigment

responsible for trapping light, other thylakoid pigments like chlorophyll

b, xanthophylls and carotenoids, which are called accessory pigments,

also absorb light and transfer the energy to chlorophyll a. Indeed, they

not only enable a wider range of wavelength of incoming light to be utilised

for photosyntesis but also protect chlorophyll a from photo-oxidation.

13.5 WHAT IS LIGHT REACTION?

Light reactions or the ‘Photochemical’ phase

include light absorption, water splitting, oxygen

release, and the formation of high-energy

chemical intermediates, ATP and NADPH.

Several protein complexes are involved in the

process. The pigments are organised into two

discrete photochemical light harvesting

complexes (LHC) within the Photosystem I (PS

I) and Photosystem II (PS II). These are named

in the sequence of their discovery, and not in

the sequence in which they function during the

light reaction. The LHC are made up of

hundreds of pigment molecules bound to

proteins. Each photosystem has all the pigments

(except one molecule of chlorophyll

a) forming

a light harvesting system also called antennae

(Figure 13.4). These pigments help to make

photosynthesis more efficient by absorbing

different wavelengths of light. The single chlorophyll a molecule forms

the reaction centre. The reaction centre is different in both the

photosystems. In PS I the reaction centre chlorophyll a has an absorption

peak at 700 nm, hence is called P700, while in PS II it has absorption

maxima at 680 nm, and is called P680.

13.6 THE ELECTRON TRANSPORT

In photosystem II the reaction centre chlorophyll a absorbs 680 nm

wavelength of red light causing electrons to become excited and jump

into an orbit farther from the atomic nucleus. These electrons are picked

up by an electron acceptor which passes them to an electrons transport

Photon

Reaction

centre

Pigment

molecules

Primary acceptor

Figure 13.4 The light harvesting complex

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212 BIOLOGY

system consisting of cytochromes (Figure

13.5). This movement of electrons is downhill,

in terms of an oxidation-reduction or redox

potential scale. The electrons are not used up

as they pass through the electron transport

chain, but are passed on to the pigments of

photosystem PS I. Simultaneously, electrons

in the reaction centre of PS I are also excited

when they receive red light of wavelength 700

nm and are transferred to another accepter

molecule that has a greater redox potential.

These electrons then are moved downhill

again, this time to a molecule of energy-rich

NADP

. The addition of these electrons reduces

NADP

to NADPH + H

. This whole scheme of

transfer of electrons, starting from the PS II,

uphill to the acceptor, down the electron

transport chain to PS I, excitation of electrons,

transfer to another acceptor, and finally down hill to NADP

reducing it to

NADPH + H

is called the Z scheme, due to its characterstic shape (Figure

13.5). This shape is formed when all the carriers are placed in a sequence

on a redox potential scale.

13.6.1 Splitting of Water

You would then ask, How does PS II supply electrons continuously? The

electrons that were moved from photosystem II must be replaced. This is

achieved by electrons available due to splitting of water. The splitting of

water is associated with the PS II; water is split into 2

, [O] and electrons.

This creates oxygen, one of the net products of photosynthesis. The

electrons needed to replace those removed from photosystem I are provided

by photosystem II.

2 4 4

2 2

H O H O e → + +

+ −

We need to emphasise here that the water splitting complex is associated

with the PS II, which itself is physically located on the inner side of the

membrane of the thylakoid. Then, where are the protons and O

formed

likely to be released – in the lumen? or on the outer side of the membrane?

13.6.2 Cyclic and Non-cyclic Photo-phosphorylation

Living organisms have the capability of extracting energy from oxidisable

substances and store this in the form of bond energy. Special substances like

ATP, carry this energy in their chemical bonds. The process through which

Electron

transport

system

acceptor

Light

Photosystem II

Photosystem I

NADPH

NADP

LHC

O 2e + 2H + [O]2

ADP+iP

ATP

Figure 13.5 Z scheme of light reaction

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ATP is synthesised by cells (in mitochondria and

chloroplasts) is named phosphorylation. Photo-

phosphorylation is the synthesis of ATP from

ADP and inorganic phosphate in the presence of

light. When the two photosystems work in a

series, first PS II and then the PS I, a process called

non-cyclic photo-phosphorylation occurs. The

two photosystems are connected through an

electron transport chain, as seen earlier – in the

Z scheme. Both ATP and NADPH + H

are

synthesised by this kind of electron flow (Figure

13.5).

When only PS I is functional, the electron is

circulated within the photosystem and the

phosphorylation occurs due to cyclic flow of

electrons (Figure 13.6). A possible location

where this could be happening is in the stroma

lamellae. While the membrane or lamellae of the grana have both PS I

and PS II the stroma lamellae membranes lack PS II as well as NADP

reductase enzyme. The excited electron does not pass on to NADP

but is

cycled back to the PS I complex through the electron transport chain

(Figure 13.6). The cyclic flow hence, results only in the synthesis of ATP,

but not of NADPH + H

. Cyclic photophosphorylation also occurs when

only light of wavelengths beyond 680 nm are available for excitation.

13.6.3 Chemiosmotic Hypothesis

Let us now try and understand how actually ATP is synthesised in the

chloroplast. The chemiosmotic hypothesis has been put forward to explain

the mechanism. Like in respiration, in photosynthesis too, ATP synthesis is

linked to development of a proton gradient across a membrane. This time

these are the membranes of thylakoid. There is one difference though, here

the proton accumulation is towards the inside of the membrane, i.e., in the

lumen. In respiration, protons accumulate in the intermembrane space of

the mitochondria when electrons move through the ETS (Chapter 14).

Let us understand what causes the proton gradient across the

membrane. We need to consider again the processes that take place during

the activation of electrons and their transport to determine the steps that

cause a proton gradient to develop (Figure 13.7).

(a) Since splitting of the water molecule takes place on the inner side of

the membrane, the protons or hydrogen ions that are produced by

the splitting of water accumulate within the lumen of the thylakoids.

Figure 13.6 Cyclic photophosphorylation

Photosystem I

Light

acceptor

Electron

transport

system

Chlorophyll

P 700

ADP+iP

ATP

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(b) As electrons move through the photosystems, protons are transported

across the membrane. This happens because the primary accepter of

electron which is located towards the outer side of the membrane

transfers its electron not to an electron carrier but to an H carrier.

Hence, this molecule removes a proton from the stroma while

transporting an electron. When this molecule passes on its electron

to the electron carrier on the inner side of the membrane, the proton

is released into the inner side or the lumen side of the membrane.

membrane. Along with electrons that come from the acceptor of

electrons of PS I, protons are necessary for the reduction of NADP

NADPH+ H

. These protons are also removed from the stroma.

Hence, within the chloroplast, protons in the stroma decrease in

number, while in the lumen there is accumulation of protons. This creates

a proton gradient across the thylakoid membrane as well as a measurable

decrease in pH in the lumen.

Why are we so interested in the proton gradient? This gradient is

important because it is the breakdown of this gradient that leads to the

synthesis of ATP. The gradient is broken down due to the movement of

protons across the membrane to the stroma through the transmembrane

Figure 13.7 ATP synthesis through chemiosmosis

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PHOTOSYNTHESIS IN HIGHER PLANTS

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channel of the CF

of the ATP synthase. The ATP synthase enzyme consists

of two parts: one called the CF

is embedded in the thylakoid membrane

and forms a transmembrane channel that carries out facilitated diffusion

of protons across the membrane. The other portion is called CF

and

protrudes on the outer surface of the thylakoid membrane on the side

that faces the stroma. The break down of the gradient provides enough

energy to cause a conformational change in the CF

particle of the ATP

synthase, which makes the enzyme synthesise several molecules of energy-

packed ATP.

Chemiosmosis requires a membrane, a proton pump, a proton

gradient and ATP synthase. Energy is used to pump protons across a

membrane, to create a gradient or a high concentration of protons within

the thylakoid lumen. ATP synthase has a channel that allows diffusion of

protons back across the membrane; this releases enough energy to activate

ATP synthase enzyme that catalyses the formation of ATP.

Along with the NADPH produced by the movement of electrons, the

ATP will be used immediately in the biosynthetic reaction taking place in

the stroma, responsible for fixing CO

, and synthesis of sugars.

13.7 WHERE ARE THE ATP AND NADPH USED?

We learnt that the products of light reaction are ATP, NADPH and O

. Of

these O

diffuses out of the chloroplast while ATP and NADPH are used to

drive the processes leading to the synthesis of food, more accurately, sugars.

This is the biosynthetic phase of photosynthesis. This process does not

directly depend on the presence of light but is dependent on the products

of the light reaction, i.e., ATP and NADPH, besides CO

and H

O. You may

wonder how this could be verified; it is simple: immediately after light

becomes unavailable, the biosynthetic process continues for some time,

and then stops. If then, light is made available, the synthesis starts again.

Can we, hence, say that calling the biosynthetic phase as the dark

reaction is a misnomer? Discuss this amongst yourselves.

Let us now see how the ATP and NADPH are used in the biosynthetic

phase. We saw earlier that CO

is combined with H

O to produce (CH

or sugars. It was of interest to scientists to find out how this reaction

proceeded, or rather what was the first product formed when CO

is taken

into a reaction or fixed. Just after world war II, among the several efforts

to put radioisotopes to beneficial use, the work of Melvin Calvin is

exemplary. The use of radioactive

C by him in algal photosynthesis

studies led to the discovery that the first CO

fixation product was a

3-carbon organic acid. He also contributed to working out the complete

biosynthetic pathway; hence it was called

Calvin cycle after him. The

first product identified was 3-phosphoglyceric acid or in short PGA.

How many carbon atoms does it have?

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Scientists also tried to know whether all plants have PGA as the first

product of CO

fixation, or whether any other product was formed in

other plants. Experiments conducted over a wide range of plants led to

the discovery of another group of plants, where the first stable product of

fixation was again an organic acid, but one which had 4 carbon

atoms in it. This acid was identified to be oxaloacetic acid or OAA. Since

then CO

assimilation during photosynthesis was said to be of two main

types: those plants in which the first product of CO

fixation is a C

acid

(PGA), i.e., the C

pathway, and those in which the first product was a C

acid (OAA), i.e., the C

pathway. These two groups of plants showed

other associated characteristics that we will discuss later.

13.7.1 The Primary Acceptor of CO

Let us now ask ourselves a question that was asked by the scientists who

were struggling to understand the ‘dark reaction’. How many carbon atoms

would a molecule have which after accepting (fixing) CO

, would have 3

carbons (of PGA)?

The studies very unexpectedly showed that the acceptor molecule

was a 5-carbon ketose sugar – ribulose bisphosphate (RuBP). Did any

of you think of this possibility? Do not worry; the scientists also took

a long time and conducted many experiments to reach this conclusion.

They also believed that since the first product was a C

acid, the primary

acceptor would be a 2-carbon compound; they spent many years trying

to identify a 2-carbon compound before they discovered the 5-carbon

RuBP.

13.7.2 The Calvin Cycle

Calvin and his co-workers then worked out the whole pathway and showed

that the pathway operated in a cyclic manner; the RuBP was regenerated.

Let us now see how the Calvin pathway operates and where the sugar is

synthesised. Let us at the outset understand very clearly that the Calvin

pathway occurs in all photosynthetic plants; it does not matter whether

they have C

or C

(or any other) pathways (Figure 13.8).

For ease of understanding, the Calvin cycle can be described under

three stages: carboxylation, reduction and regeneration.

1. Carboxylation – Carboxylation is the fixation of CO

into a stable organic

intermediate. Carboxylation is the most crucial step of the Calvin cycle

where CO

is utilised for the carboxylation of RuBP. This reaction is

catalysed by the enzyme RuBP carboxylase which results in the formation

of two molecules of 3-PGA. Since this enzyme also has an oxygenation

activity it would be more correct to call it RuBP carboxylase-

oxygenase

or RuBisCO.

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2. Reduction – These are a series of reactions that lead to the formation

of glucose. The steps involve utilisation of 2 molecules of ATP for

phosphorylation and two of NADPH for reduction per CO

molecule

fixed. The fixation of six molecules of CO

and 6 turns of the cycle are

required for the formation of one molecule of glucose from the pathway.

3. Regeneration – Regeneration of the CO

acceptor molecule RuBP is

crucial if the cycle is to continue uninterrupted. The regeneration

steps require one ATP for phosphorylation to form RuBP.

Ribulose-1,5-

bisphosphate

Atmosphere

+ H

Carboxylation

ADP

Regeneration

3-phosphoglycerate

Triose

phosphate

Reduction

ATP

NADPH

ADP

+NADP

Sucrose, starch

ATP

Figure 13.8 The Calvin cycle proceeds in three stages : (1) carboxylation, during which

combines with ribulose-1,5-bisphosphate; (2) reduction, during which

carbohydrate is formed at the expense of the photochemically made ATP

and NADPH; and (3) regeneration during which the CO

acceptor ribulose-

1,5-bisphosphate is formed again so that the cycle continues

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Hence for every CO

molecule entering the Calvin cycle, 3 molecules

of ATP and 2 of NADPH are required. It is probably to meet this difference

in number of ATP and NADPH used in the dark reaction that the cyclic

phosphorylation takes place.

To make one molecule of glucose 6 turns of the cycle are required.

Work out how many ATP and NADPH molecules will be required to make

one molecule of glucose through the Calvin pathway.

It might help you to understand all of this if we look at what goes in

and what comes out of the Calvin cycle.

In Out

Six CO

One glucose

18 ATP 18 ADP

12 NADPH 12 NADP

13.8 THE C

PATHWAY

Plants that are adapted to dry tropical regions have the C

pathway

mentioned earlier. Though these plants have the C

oxaloacetic acid as

the first CO

fixation product they use the C

pathway or the Calvin cycle

as the main biosynthetic pathway. Then, in what way are they different

from C

plants? This is a question that you may reasonably ask.

plants are special: They have a special type of leaf anatomy, they

tolerate higher temperatures, they show a response to high light intensities,

they lack a process called photorespiration and have greater productivity

of biomass. Let us understand these one by one.

Study vertical sections of leaves, one of a C

plant and the other of a C

plant. Do you notice the differences? Do both have the same types of

mesophylls? Do they have similar cells around the vascular bundle sheath?

The particularly large cells around the vascular bundles of the C

plants are called bundle sheath cells, and the leaves which have such

anatomy are said to have ‘Kranz’ anatomy

. ‘Kranz’ means ‘wreath’ and

is a reflection of the arrangement of cells. The bundle sheath cells may

form several layers around the vascular bundles; they are characterised

by having a large number of chloroplasts, thick walls impervious to

gaseous exchange and no intercellular spaces. You may like to cut a

section of the leaves of C

plants – maize or sorghum – to observe the

Kranz anatomy and the distribution of mesophyll cells.

It would be interesting for you to collect leaves of diverse species of

plants around you and cut vertical sections of the leaves. Observe under

the microscope – look for the bundle sheath around the vascular

bundles. The presence of the bundle sheath would help you identify

the C

plants.

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Now study the pathway shown in Figure 13.9. This pathway that has

been named the Hatch and Slack Pathway, is again a cyclic process. Let

us study the pathway by listing the steps.

The primary CO

acceptor is a 3-carbon molecule phosphoenol

pyruvate (PEP) and is present in the mesophyll cells. The enzyme

responsible for this fixation is PEP carboxylase or PEPcase. It is important

to register that the mesophyll cells lack RuBisCO enzyme. The C

acid

OAA is formed in the mesophyll cells.

It then forms other 4-carbon compounds like malic acid or aspartic

acid in the mesophyll cells itself, which are transported to the bundle

sheath cells. In the bundle sheath cells these C

acids are broken down

to release CO

and a 3-carbon molecule.

The 3-carbon molecule is transported back to the mesophyll where it

is converted to PEP again, thus, completing the cycle.

The CO

released in the bundle sheath cells enters the C

or the Calvin

pathway, a pathway common to all plants. The bundle sheath cells are

Figure 13.9 Diagrammatic representation of the Hatch and Slack Pathway

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rich in an enzyme Ribulose bisphosphate carboxylase-oxygenase

(RuBisCO), but lack PEPcase. Thus, the basic pathway that results in

the formation of the sugars, the Calvin pathway, is common to the C

and

plants.

Did you note that the Calvin pathway occurs in all the mesophyll

cells of the C

plants? In the C

plants it does not take place in the

mesophyll cells but does so only in the bundle sheath cells.

13.9 PHOTORESPIRATION

Let us try and understand one more process that creates an important

difference between C

and C

plants – Photorespiration. To understand

photorespiration we have to know a little bit more about the first step of

the Calvin pathway – the first CO

fixation step. This is the reaction

where RuBP combines with CO

to form 2 molecules of 3PGA, that is

catalysed by RuBisCO.

RuBP CO PGA

RuBisCo

+  → ×

2 3

RuBisCO that is the most abundant enzyme in the world (Do you

wonder why?) is characterised by the fact that its active site can bind to

both CO

and O

– hence the name. Can you think how this could be

possible? RuBisCO has a much greater affinity for CO

when the CO

: O

is nearly equal. Imagine what would happen if this were not so! This

binding is competitive. It is the relative concentration of O

and CO

that

determines which of the two will bind to the enzyme.

In C

plants some O

does bind to RuBisCO, and hence CO

fixation is

decreased. Here the RuBP instead of being converted to 2 molecules of

PGA binds with O

to form one molecule of phosphoglycerate and

phosphoglycolate (2 Carbon) in a pathway called photorespiration. In

the photorespiratory pathway, there is neither synthesis of sugars, nor of

ATP. Rather it r

esults in the release of CO

with the utilisation of ATP. In

the photorespiratory pathway there is no synthesis of ATP or NADPH.

The biological function of photorespiration is not known yet.

In C

plants photorespiration does not occur. This is because they

have a mechanism that increases the concentration of CO

at the enzyme

site. This takes place when the C

acid from the mesophyll is broken

down in the bundle sheath cells to release CO

– this results in increasing

the intracellular concentration of CO

. In turn, this ensures that the

RuBisCO functions as a carboxylase minimising the oxygenase activity.

Now that you know that the C

plants lack photorespiration, you

probably can understand why productivity and yields are better in these

plants. In addition these plants show tolerance to higher temperatures.

Based on the above discussion can you compare plants showing

the C

and the C

pathway? Use the table format given and fill in the

information.

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PHOTOSYNTHESIS IN HIGHER PLANTS

221

TABLE 13.1 Fill in the Columns 2 and 3 in this table to highlight the differences

between C

and C

Plants

Characteristics C

Plants C

Plants Choose from

Cell type in which the Calvin Mesophyll/Bundle sheath/both

cycle takes place

Cell type in which the initial Mesophyll/Bundle sheath /both

carboxylation reaction occurs

How many cell types does the Two: Bundle sheath and

leaf have that fix CO

. mesophyll

One: Mesophyll

Three: Bundle sheath, palisade,

spongy mesophyll

Which is the primary CO

acceptor RuBP/PEP/PGA

Number of carbons in the 5 / 4 / 3

primary CO

acceptor

Which is the primary CO

PGA/OAA/RuBP/PEP

fixation product

No. of carbons in the primary 3 / 4 / 5

fixation product

Does the plant have RuBisCO? Yes/No/Not always

Does the plant have PEP Case? Yes/No/Not always

Which cells in the plant have Mesophyll/Bundle sheath/none

Rubisco?

fixation rate under high Low/ high/ medium

light conditions

Whether photorespiration is High/negligible/sometimes

present at low light intensities

Whether photorespiration is High/negligible/sometimes

present at high light intensities

Whether photorespiration would be High/negligible/sometimes

present at low CO

concentrations

Whether photorespiration would be High/negligible/sometimes

present at high CO

concentrations

Temperature optimum 30-40 C/20-25C/above 40 C

Examples Cut vertical sections of leaves of

different plants and observe under

the microscope for Kranz anatomy

and list them in the appropriate

columns.

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222 BIOLOGY

13.10 FACTORS AFFECTING PHOTOSYNTHESIS

An understanding of the factors that affect photosynthesis is necessary.

The rate of photosynthesis is very important in determining the yield of

plants including crop plants. Photosynthesis is under the influence of

several factors, both internal (plant) and external. The plant factors include

the number, size, age and orientation of leaves, mesophyll cells and

chloroplasts, internal CO

concentration and the amount of chlorophyll.

The plant or internal factors are dependent on the genetic predisposition

and the growth of the plant.

The external factors would include the availability of sunlight,

temperature, CO

concentration and water. As a plant photosynthesises,

all these factors will simultaneously affect its rate. Hence, though several

factors interact and simultaneously affect photosynthesis or CO

fixation,

usually one factor is the major cause or is the one that limits the rate.

Hence, at any point the rate will be determined by the factor available at

sub-optimal levels.

When several factors affect any [bio] chemical process, Blackman’s

(1905) Law of Limiting Factors comes into effect. This states the following:

If a chemical process is affected by more than one factor, then its

rate will be determined by the factor which is nearest to its minimal

value: it is the factor which directly affects the process if its quantity is

changed.

For example, despite the presence of a green

leaf and optimal light and CO

conditions, the

plant may not photosynthesise if the temperature

is very low. This leaf, if given the optimal

temperature, will start photosynthesising.

13.10.1 Light

We need to distinguish between light quality, light

intensity and the duration of exposure to light,

while discussing light as a factor that affects

photosynthesis. There is a linear relationship

between incident light and CO

fixation rates at

low light intensities. At higher light intensities,

gradually the rate does not show further increase

as other factors become limiting (Figure 13.10).

What is interesting to note is that light saturation

occurs at 10 per cent of the full sunlight. Hence,

except for plants in shade or in dense forests, light

is rarely a limiting factor in nature. Increase in

Figure 13.10 Graph of light intensity on the

rate of photosynthesis

Rate of photosynthesis

Light intensity

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PHOTOSYNTHESIS IN HIGHER PLANTS

223

incident light beyond a point causes the breakdown of chlorophyll and a

decrease in photosynthesis.

13.10.2 Carbon dioxide Concentration

Carbon dioxide is the major limiting factor for photosynthesis. The

concentration of CO

is very low in the atmosphere (between 0.03 and

0.04 per cent). Increase in concentration upto 0.05 per cent can cause an

increase in CO

fixation rates; beyond this the levels can become damaging

over longer periods.

The C

and C

plants respond differently to CO

concentrations. At

low light conditions neither group responds to high CO

conditions. At

high light intensities, both C

and C

plants show increase in the rates of

photosynthesis. What is important to note is that the C

plants show

saturation at about 360 µlL

-1

while C

responds to increased CO

concentration and saturation is seen only beyond 450 µlL

-1

. Thus, current

availability of CO

levels is limiting to the C

plants.

The fact that C

plants respond to higher CO

concentration by

showing increased rates of photosynthesis leading to higher productivity

has been used for some greenhouse crops such as tomatoes and bell

pepper. They are allowed to grow in carbon dioxide enriched atmosphere

that leads to higher yields.

13.10.3 Temperature

The dark reactions being enzymatic are temperature controlled. Though

the light reactions are also temperature sensitive they are affected to a

much lesser extent. The C

plants respond to higher temperatures and

show higher rate of photosynthesis while C

plants have a much lower

temperature optimum.

The temperature optimum for photosynthesis of different plants also

depends on the habitat that they are adapted to. Tropical plants have a

higher temperature optimum than the plants adapted to temperate

climates.

13.10.4 Water

Even though water is one of the reactants in the light reaction, the effect of

water as a factor is more through its effect on the plant, rather than directly

on photosynthesis. Water stress causes the stomata to close hence reducing

the CO

availability. Besides, water stress also makes leaves wilt, thus,

reducing the surface area of the leaves and their metabolic activity as well.

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224 BIOLOGY

SUMMARY

Green plants make their own food by photosynthesis. During this process carbon

dioxide from the atmosphere is taken in by leaves through stomata and used for

making carbohydrates, principally glucose and starch. Photosynthesis takes place

only in the green parts of the plants, mainly the leaves. Within the leaves, the

mesophyll cells have a large number of chloroplasts that are responsible for CO

fixation. Within the chloroplasts, the membranes are sites for the light reaction,

while the chemosynthetic pathway occurs in the stroma. Photosynthesis has two

stages: the light reaction and the carbon fixing reactions. In the light reaction the

light energy is absorbed by the pigments present in the antenna, and funnelled to

special chlorophyll a molecules called reaction centre chlorophylls. There are two

photosystems, PS I and PS II. PS I has a 700 nm absorbing chlorophyll a P700

molecule at its reaction centre, while PS II has a P680 reaction centre that absorbs

red light at 680 nm. After absorbing light, electrons are excited and transferred

through PS II and PS I and finally to NAD forming NADH. During this process a

proton gradient is created across the membrane of the thylakoid. The breakdown

of the protons gradient due to movement through the F

part of the ATPase enzyme

releases enough energy for synthesis of ATP. Splitting of water molecules is

associated with PS II resulting in the release of O

, protons and transfer of electrons

to PS II.

In the carbon fixation cycle, CO

is added by the enzyme, RuBisCO

, to a 5-

carbon compound RuBP that is converted to 2 molecules of 3-carbon PGA. This

is then converted to sugar by the Calvin cycle, and the RuBP is regenerated. During

this process ATP and NADPH synthesised in the light reaction are utilised. RuBisCO

also catalyses a wasteful oxygenation reaction in C

plants: photorespiration.

Some tropical plants show a special type of photosynthesis called C

pathway.

In these plants the first product of CO

fixation that takes place in the mesophyll,

is a 4-carbon compound. In the bundle sheath cells the Calvin pathway is carried

out for the synthesis of carbohydrates.

EXERCISES

1. By looking at a plant externally can you tell whether a plant is C

or C

? Why and

how?

2. By looking at which internal structure of a plant can you tell whether a plant is

or C

? Explain.

3. Even though a very few cells in a C

plant carry out the biosynthetic – Calvin

pathway, yet they are highly productive. Can you discuss why?

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PHOTOSYNTHESIS IN HIGHER PLANTS

225

4. RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do

you think RuBisCO

carries out more carboxylation in C

plants?

5. Suppose there were plants that had a high concentration of Chlorophyll b, but

lacked chlorophyll a, would it carry out photosynthesis? Then why do plants

have chlorophyll b and other accessory pigments?

6. Why is the colour of a leaf kept in the dark frequently yellow, or pale green?

Which pigment do you think is more stable?

7. Look at leaves of the same plant on the shady side and compare it with the

leaves on the sunny side. Or, compare the potted plants kept in the sunlight with

those in the shade. Which of them has leaves that are darker green ? Why?

8. Figure 13.10 shows the effect of light on the rate of photosynthesis. Based on the

graph, answer the following questions:

(a) At which point/s (A, B or C) in the curve is light a limiting factor?

(b) What could be the limiting factor/s in region A?

9. Give comparison between the following:

(a) C

and C

pathways

(b) Cyclic and non-cyclic photophosphorylation

and C

plants

2020-21