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Module 1: Ecological Interactions and Energetics

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Today, we move forward with our discussion on ecological energetics and look at some
Nutrient Cycles.
(Refer Slide Time: 00:22)

As always we begin with a definition, what is a nutrient? A substance used by an organism
to survive to grow and reproduce. Essentially when any organism is having food it is
getting a number of nutrients, those nutrients in our parlance we say that we are getting
proteins, carbohydrates, fats, minerals and so on. But, then if we look at these minerals
more fundamentally so, we are getting some elements out of these nutrients.
When are talking about say carbohydrates or fats we are getting carbon, hydrogen and
oxygen; when are talking about proteins we are getting nitrogen; when are talking about
other mineral salts we are getting say iron or we are getting copper or we are getting
selenium and all of these are different minerals or all of these are different nutrients when
we talk about in the parlance of ecology as well.

When we talk about say plant nutrition, a plant is not getting carbohydrates or fats or
proteins from somewhere else because a plant is a producer; it is producing it is own food.
But even when it is producing its own food it will require some nutrients, those nutrients
will be say carbon dioxide or water or some mineral salts that will use to make out of these
different food items. All of those substances will be called as nutrients.
A nutrient is a substance that is used by an organism to survive, grow and reproduce that
is to carry on its life functions, survival, growth and reproduction.
(Refer Slide Time: 01:59)

If we talk about nutrients there are certain nutrients that an organism requires in larger
concentrations or larger amounts and there are some nutrients that are required by the
organisms in the smaller amounts. For instance, if we talk about ourselves, we require
much more amount of carbohydrates, proteins and fats, then say we a requirement of say
sodium in the form of sodium chloride which is our common salt. So, for us, we will that
carbohydrates, proteins and fats are the nutrients that we require in larger quantities and
substances such as sodium chloride or may be some amount of selenium or some amount
of say magnesium is something that will require in very smaller quantities.
Similarly when we talk about the nutrition of plants and in this particular lecture, we will
be mostly concentrating ourselves with the nutrition of plants because once the plants have
made their food products, then those products get moved on with the different food chains
and food webs.

In the case of plants, when we talk about macro nutrients or nutrients that are required in
larger amounts we can talk about primary nutrients that is nitrogen, phosphorus and
potassium N, P, K. So, for instance whenever we are talking about fertilizers, we always
talk about the N, P, K fertilizer; the fertilizers that provide you nitrogen, phosphorus and
potassium. These are the primary nutrients. Or they can include the secondary nutrients
such as calcium, magnesium or sulphur. These are the nutrients that the plant require in
larger amounts.
Micro nutrients or trace elements are the nutrient that are needed in small or trace amounts
and in the case of plants it could include things like boron, copper, iron, chlorine,
manganese, zinc, molybdenum and so on and when we are talking about nutrients one
definition is about macro and micro nutrients. The second one is the about essential and
non essential nutrients.
(Refer Slide Time: 04:06)

Essential nutrients are those nutrients that are required by the plant and that cannot be
substituted by anything else. There are three criteria for data mining whether an element
is essential or not. The first one is that in the absence of the element, the plant should be
unable to complete their life cycle.
If you say that, suppose carbon is an essential element so, in the absence of carbon the
plant will be unable to complete its life cycle because all of its parts are made of carbon.
The second criteria is that the deficiency of an essential element cannot be met by

supplying some other element. Suppose in place of providing a plant with carbon dioxide,
we try to provide it with say sulphur dioxide or maybe you try to provide it with say
ammonia so, anything that does not have carbon.
Now, if you provide a plant with anything does not have carbon the deficiency of carbon
cannot be met by supplying some other element. So, it is a ‘sine qua non’ for the growth
of plants. It is an extremely essential thing and the plant cannot live without it. And, the
third one is that the element must be directly involved in the metabolism of the plant. These
are the three criteria to determine whether an element is an essential element or not.
Now, let us look at some essential elements and their rules.
(Refer Slide Time: 05:42)

Let us begin with nitrogen. Nitrogen is a essential element because if you do not give
nitrogen to a plant it would be able to complete its life cycle. Nitrogen cannot be replaced
by other element and the nitrogen is essential for the metabolism of the plant. Why?
Because nitrogen is a constituent of proteins so, different amino acids; so, when we say
the amino, the amino growth is the nitrogen growth. It is the constituent of different
proteins, it is there in the nucleic acids; so, when we talk about DNA or RNA that also
contains nitrogen. It is a constituent of several vitamins and hormones in the case of plants
and also in the case of animals as well.

Nitrogen is an essential element because it is a constituent of proteins, nucleic acids,
vitamins, hormones and so on. Another essential element is phosphorus. Phosphorus is a
constituent of nucleic acid again in the case of DNA or RNA we have phosphorus. It is a
constituent of ATP; ATP stands for adenosine triphospate which is the energy currency in
a cell. When a cell needs to move energy from one place to another or when a cell needs
to use energy for some particular purpose, it will be using the energy that is stored in the
ATP molecules. The ATP will get converted to ADP and it will release out energy for
some chemical reactions. And, phosphorus is essential part of ATP.
Phosphorus is also a constituent of the cell membrane of a cell. Cell membrane is the outer
layer that contains all the constituents of the cell inside the cell. It separates what is inside
the cell and what is outside the cell and phosphorus is a constituent of the cell membrane
and it is also a constituent of certain proteins. Here again phosphorus is an essential
element because the plant cannot live without it. You cannot replace it or substitute it with
anything else and it is involved in the metabolism in the plants because of it is role in
nucleic acids and ATP, cell membrane, proteins and so on.
Another essential element is potassium. When we talk about N, P, K this is N, this P, this
is K, the potassium is called kalium and we look at it is deep roots and kalium is presented
by K. Potassium is a part of cation-anion balance that is needed for maintaining cell
turgidity, opening and closing of stomata, activation of certain enzymes and so on. When
we say turgidity it is an expression of the pressure that is being contain inside a cell. So,
for instance if you have a cell and you take out the water that is inside so, the cell will
become placid. It will look like a balloon that is now not inflated. Turgidity is when you
take up balloon and you inflate it, then we say that balloon is turgid. In the case of plants,
in the case of a number of cells, this turgidity is not governed by the movement of air, but
by the movement of water.
And, for that case the amount of osmolarity in the cell is essential. If you more amount of
salt inside a cell. The salt will attract more amount of water from outside and will result in
the increase in the turgidity of the cell and potassium plays a key role there because it is
involved in the cation anion balance. And, the cell to turgidity in turn regulates the opening
and closing of the stomata and stomata are the pores in the leaves through which the
gaseous occurs in the case of plants and potassium is also involved in the activation of
certain enzymes.

Another essential nutrient is calcium; and calcium is used in the formation of calcium
pectate in the cell wall. And, calcium pectate especially plays a role in the cell division.
When you have a plant cell that is dividing into two cells. Calcium pectate will form a
layer that will separate the two cells or the two daughter cells. It is a part of the calcium
pectate in the cell wall, it is also involved in the activation of the certain enzymes and it
also plays a role in the calcium channels in the cell membranes.
We have magnesium; magnesium is a constituent of chlorophyll. So, just like our
haemoglobin contains iron inside it similarly chlorophyll contains a magnesium iron inside
it. Magnesium is important because it is involved in a formation of chlorophyll. It is a
constituent of chlorophyll plus it is required in the activation of certain respiration
enzymes. If you remove magnesium from the cell you would not have any chlorophyll and
the cell will not be able to respire.
Next is sulphur; sulphur is a constituent of amino acids cysteine and methionine and it is
also constitute of several vitamins and coenzymes. So, here are some of the essential
elements that the plant requires and this is not an exhaustive list, there are also a number
of other essential elements that are required by the plant.
So, we looked at essential and non-essential elements; we looked at macro nutrients and
micro nutrient. Macro nutrients is something that you require in large amounts, micro
nutrients is something that you require in smaller amounts. Let us now have a look at what
are things that a plant need.

(Refer Slide Time: 11:42)

We will divide this list into two parts one is the macro nutrient and the second one is the
micro nutrients. In the case of macro nutrients we can divide it into again three sub
categories; the first is the macro nutrients that are derive from air and water and this
includes carbon, hydrogen and oxygen.
(Refer Slide Time: 12:07)

Now, carbon is derived from the air in the form of carbon dioxide. Here you have carbon
and you have oxygen and hydrogen is derived from the water H2O. So, here you have
hydrogen and oxygen. Essentially the requirements of carbon, hydrogen and oxygen, these

are the three macro nutrients that the plant needs in very large amounts and these are met
from the air and water.
The second is primary macro nutrients and primary macro nutrients are nitrogen,
phosphorus and potassium N, P, K. We can remember it by N, P, K and these N, P, K, they
come from the soil. They come from mineral salts that are there in the soil or are added to
the soil in the form of fertilizers and the plant takes these macro nutrients along with the
water that gets absorbed by the roots. These are the primary macro nutrients.
Then, we have secondary and tertiary macro nutrients which includes sulphur calcium and
magnesium. Here again sulphur and magnesium are required in large quantities, but not as
large as N, P, K or as large as CHO. So, these are the macro nutrients that a plant needs.
(Refer Slide Time: 13:22)

Next we have a list of the micro nutrients. Now, remember again micro nutrients are
required in smaller quantities. In the case of plants it includes iron, molybdenum, boron,
copper, manganese, sodium, zinc, nickel, chlorine, cobalt, aluminium, silicon, vanadium,
selenium.
All of these micro nutrients, they play an important role in the activation of certain
enzymes, in the functioning of certain proteins as coenzymes in certain cases or in the case
of things like sodium and chlorine this also plays an important role in the water balance
inside the cell. In the case of water balance we are talking about the turgidity. If we have

more amount of sodium chloride or say potassium chloride inside the cell, so, it will absorb
more amount of water and so, it will become more and more turgid.
(Refer Slide Time: 14:20)

Then you have the situation that all the plants require all of these nutrients, but then these
nutrients are not present in an infinite quantity on earth. There is a certain repository of
these nutrients, but then the plants have been growing for ages. In that case how do they
get these nutrients if you have a certain nutrient stock? It is through the biogeochemical
cycles.
(Refer Slide Time: 14:50)

What we are saying here is that if you have a plant and this plant is growing in the soil and
the soil has certain amount of nutrients and then these plants get eaten up by animals, they
get eaten up by other carnivores animals and so on. Ultimately these nutrients that are
taken up by the roots of the plant, they should ultimately come back to the earth. So that,
if there is another plant that is growing, then it should have access to these nutrients.
The movement of the nutrients will occur in a cyclical manner. So, it moves from the soil
through the plants into the animals and then through the animals and the decomposers it
will move back to the soil. This forms a cycle and we call these cycles as biogeochemical
cycles. Because these involve biological process, they these involve geological process
and these also involve chemical processes. These are biogeochemical cycles.
(Refer Slide Time: 15:55)

They can be defined as a pathway by which a chemical substance moves through biotic
and abiotic compartments of the earth. It is a pathway, it is a route through which chemical
substances are moving through biosphere and abiotic components that is lithosphere,
which is the rocky portion of the earth, atmosphere that is the airy portion of the earth and
hydrosphere which is the watery portion of the earth; compartments of the earth. This is
essentially a biogeochemical cycle.

(Refer Slide Time: 16:30)

In general we can represent a biogeochemical cycle like this. You have a nutrient pool.
When we say a nutrient pool, it is a repository of the nutrients you can have this repository
either in the soil or maybe in the water or maybe in the air. One instance, if we are talking
about carbon, so, the pool of carbon is there in the air in the form of carbon dioxide and
then this pool is then utilized by the producers or the plants which also gained energy from
the sun to use these nutrients from the nutrient pool and then they make food or they make
certain biological molecules.
Now, from these plants it goes to the herbivores and then it goes to the carnivores and from
all three of these it moves through the decomposers back into the nutrient cycle. For
instance you have a dead leaf, so, this dead leaf will be eaten up by say earthworms and
after the earthworm has eaten it, they have increase the surface area. So, number of bacteria
and enzyme will act on it and ultimately they will all convert it back into carbon dioxide.
Whether it is a dead leaf or may be a dung of an animal or may be the carcass of an animal,
so all of these will be decomposed back into carbon dioxide or if we are talking about a
pool that is present in the soil, let us talk about say calcium that has been derived from the
soil. This calcium is taken up by the plants through their roots then it gets into some
biological molecules, from there it goes to the herbivores to the carnivores and then when
the decomposes are decomposing that dead and decayed parts of these animals or the dungs

and excreta of these animals. After that all that calcium will be then release back into the
nutrient pool which in this case will be the soil.
In this lecture we will have a look at certain biogeochemical cycles in more detail.
(Refer Slide Time: 18:36)

Let us begin with the nitrogen cycle. Now, in the case of nitrogen cycle, we do not one
pool, but we have two pools. The first pool is the atmospheric nitrogen. More than 70
percent the air that is surrounding us that is nitrogen and the second pool is that of the soil
nitrates which are present in the soil.
A plant is unable to use the atmospheric nitrogen directly, so it has to take up the nitrogen
in the form of nitrates which should come into the soil. How does this atmospheric nitrogen
reach the soil? It is through same biological fixation. We had talked about a mutualistic
relationships between these by bacteria by the name of rhizobium which like in the root
nodules of certain leguminous plants. These bacteria are able to fix this atmospheric
nitrogen. They convert the atmospheric nitrogen into a form that the plants are able to use.
Biological fixation is one way in which nitrogen moves from the atmospheric pool to the
soil pool.
The second one is lightning. When you have lightning, there is an intense amount of heat
and electrical discharge and in that case nitrogen and oxygen both the gases that are present

in the atmosphere, they can react together and they can form nitrates and nitrites which
then come down to the soil through rain. That is the second way.
And, the third process is industrial fixation. These days because we want to put more
amount of nitrates into the soil so that the plants can use them, especially for agricultural
production so, we are artificially converting the nitrogen that is there in the air into the
nitrates. That is the other way in which the atmospheric nitrogen can move, can get
converted into the soil nitrates. Here we are seeing that this pool is the nitrogen this pool
is moving into the, this pool.
But, then again because we are talking about cycles, so, if nitrogen can move from the first
pool to the second pool, it also be able to move from the second pool to the first pool. So,
if you have soil nitrates, so, there can be denitrification and volcanic activities that can
convert the nitrates that that are there in the soil or that are there in the rock back into
nitrogen and through both of these processes denitrification and volcanic activity, nitrogen
can move from the this pool of the soil into the atmospheric pool.
Now, that is the movement of the nitrogen between both of these pools, but then how does
it move through the biological system? These soil nitrates can be taken up by the plants
and then these nitrates can be used to form different components such as the proteins.
When the plant has formed certain proteins, the plant gets eaten up by the animals and then
plants and animals, when they are dead and decaying they are decomposed further by the
decomposers and then, so, nitrogen has moved from the soil nitrates to the plants, plants
to animals, and plants and animals to the decomposers. These decomposers can now
further break down these proteins and covert them either into ammonia or they can convert
into nitrates and in this process which is known a ammonification or nitrification, it can
then come back to the soil nitrates pool.
Here we are talking about the two things, one is the pool(s). So, these are the two pools
that we have and the second is fluxes; the fluxes are the rate at which the nutrient is moving
from one pool to another or from one pool into some other organic substance. If we talk
about the rate at which soil nitrates are taken up by all the plants on the earth, then we are
talking about the flux of nitrogen that is moving from the soil pool into the plants. That is
the generalised nitrogen cycle and we look at the sub processes in more detail now.

(Refer Slide Time: 22:57)

When we talk about nitrogen fixation, it is the conversion of atmospheric nitrogen into
ammonia and this occurs through biological fixation or lightning or industrial fixation. It
can be converted into either ammonia or it can be converted into nitrites and nitrates, but
mostly we say that the first stage is the conversion into ammonia. This is the conversion
of atmospheric nitrogen into ammonia which occurs through these three processes.
(Refer Slide Time: 23:28)

Now, the first one is called is biological nitrogen fixation. Biological nitrogen fixation is
the conversion of atmospheric nitrogen into ammonia and it occurs in the biological

organisms. So, in this case you have this nitrogen which is acted upon in the presence of
this enzyme called nitrogenase and it converts it into ammonia and it is done by rhizobium.
Rhizobium is a bacterium that lives in the root nodules in the leguminous plants, or it can
done by certain free living bacteria which is azotobacter or it can be done using some
cyanobacteria. In cyanobacteria, the prefix ‘cyano’ refers to blue. So, these include nostoc
and anabaena as well. So, these can also perform biological nitrogen fixation.
(Refer Slide Time: 24:21)

We can also get ammonia by the decomposition of organic nitrogen in the dead plants and
animals through the process of ammonification. In the process of ammonification, you
have the organic molecules that are rich in nitrogen and in this process of ammonification
they are converted into ammonia.
Ammonification generally happens because of the action of decomposers. If you have say,
a piece of egg that is lying around, you will have some bacterial growth and then it will
convert proteins, especially the albumin that is there in the egg and that will convert it into
ammonia and it will release it back. This is the process of ammonification.

(Refer Slide Time: 25:08)

Next, we have the process of nitrification. In the process of nitrification, the ammonia that
has been produced; now, ammonia is typically a toxic substance for most of the organisms.
So, it needs to be converted to something else so that it is gotten rid off. Nitrification is a
process in which there is a biological oxidation of ammonia into nitrites and nitrates.
In this case we have ammonia that is reacting with oxygen and it can be done through
organisms like nitrosomonas and nitrococcus and they will convert this ammonia into
nitrites. And then, these nitrites can be further oxidized using nitrobacters and converted
into nitrates. So, you have nitrites that are converted into nitrates and these nitrifying
bacteria, those that are converting ammonia into nitrites and nitrates, they are
chemoautotrophs. They are autotrophs, they are making their own food using chemical
reactions; so they are chemoautotrophs.

(Refer Slide Time: 26:17)

Now, in the industrial process, we have the Haber process. In the Haber process, we have
nitrogen and hydrogen that are reacted together. These two gases are reacted at high
temperature and pressure in the presence of catalyst to form ammonia and once we have
ammonia, it can then be further convert it into the nitrates using the Oswald process.
(Refer Slide Time: 26:41)

In the case of Oswald process, we have ammonia that reacts with the oxygen in the
presence of catalyst to form NO. Then NO is further oxidized in the presence of catalyst
to form NO2 and then NO2 is in further oxidized in the presence of catalyst to form NO3

and it is also reacted with water, so, it gives you HNO3. So, that is the nitric acid. Once
you have the nitric acid you can combine it with any base to get nitrate salt. So, if you add
HNO3 with NaOH you get NaNO3 which is sodium nitrate. So, that is all about the nitrogen
cycle.
(Refer Slide Time: 27:20)

Let us have a look at the carbon cycle. In the case of carbon cycle the main pool is the
carbon in the atmosphere, but then carbon is also stored in other pools especially like pools
like oceans or pools like soil carbon or pools like the biological carbon in the form of
forest. But, in this case when we are talking about the carbon cycle will mostly focus on
the atmospheric pool because that is the largest pool.
Here you have carbon in the atmosphere and then this carbon can be utilized in the process
of weathering and it can form carbon in the rocks.

(Refer Slide Time: 28:11)

What we are saying here is that, if you say have a rock that has calcium hydroxide, it can
react with carbon dioxide or carbon dioxide plus water because carbon dioxide normally
comes down with rain in the form of H2CO3 and in this case it will form CaCO3 + 2H2O.
Now, in this process what is happening is that the calcium hydroxide that is present in the
rocks, it is been acted upon by carbon dioxide and water to form calcium carbonate and
with the release of water. So, it was coming from H2O plus CO2. So, here we have CO2 in
the atmosphere that is been converted into CO2 in the rocks and in this process which is
known as weathering, so in the process of weathering when the reaction is happening, this
rock will also break down. Once it breaks down then further internal minerals will be more
and more available for weathering. In the process of weathering you can have carbon in
the atmosphere that becomes logged into rocks in the form of calcium carbonate.
And, then in a number of tectonic process, which is the process in which the plates of the
earth collide against each other, move past each other, move into the mantle or may be
give rise to volcanic activities, you can have the release of these carbon. So, if you say
have the CaCO3 and then it is heated up. So, it will form calcium oxide plus CO2. So, this
CO2 gets released into the atmosphere and this calcium oxide, in the presence of water, it
can act with water then it can form Ca(OH)2, once again.
So, what we are saying here is that carbon can very easily move from this one pool, which
is carbon in the atmosphere, to carbon in the lithosphere. So, that is the second pool.

Another pool is carbon that is present in the ocean water. In this case, we have that carbon
in the atmosphere can get dissolved in the water to form carbon in the ocean.
(Refer Slide Time: 30:30)

So, what we are saying here is that you have CO2 that is reacting with H2O to form H2CO3.
In this case, this carbon that was there in the atmosphere has now reached into the oceans.
This is the process in which, through dissolution the carbon can reach into the third pool
of carbon which is the ocean water and then from the ocean water it can come back to the
carbon pool through the release of carbon. In this process it is nothing, but you have the
reverse process. You have H2CO3 that can give rise to CO2 and H2O.
Now, once you have carbon in this pool, carbon in the atmosphere, it can be taken up by
the producers through the process of photosynthesis and converted into biomass. Once you
have this carbon in the form of biomass, in the form of say cellulose or different
carbohydrates or say protein molecules or fat molecules, once you have it in the biomass;
so, this biomass can get consumed by the other organisms, the herbivores and then from
herbivores into the carnivores and predators and so on and so, this biomass is converted
into different kinds of organic matter like food webs or it can reach into the soil and in all
of these processes, this biomass can also be used by the plants themselves. During
respiration it can be released back into the atmosphere or once it is reached in to the
animals from there it goes into the decomposers, it can then again release back into the
atmosphere or when the animals are respiring there also they are releasing carbon dioxide

back into the atmosphere. This is the process in which the carbon will move through the
different food chain and food webs.
But, then we can also convert this organic matter or rather they can also see the conversion
of this organic matter into one another pool which is the fossil fuels. In this case, this
process is known as lithification in which you have these plants and animals that get buried
inside the earth and slowly and steadily they get converted into things like petroleum or
coal.
This is the process in which carbon will reach this pool, which is the pool of the fossil
fuels and then these fossil fuels when they are burnt they release carbon back into the
atmosphere. In that case, this fossil fuel will release carbon back into the atmosphere
through the process of combustion. So, that is the carbon cycle.
(Refer Slide Time: 33:14)

Another cycle is the water cycle, because water is also an essential nutrient that is
providing two essential elements, hydrogen and oxygen. Now, water cycle is something
that we all know. There is water in different water bodies, it gets evaporated and forms
water vapour because of the heat that is given out by the sun and when then this water
vapour condenses it forms clouds; when these clouds are then further cooled down, so, all
these water forms a droplets, it falls down in the form of rain which you called as
precipitation.

Now, this rain can fall either into these big water bodies or it can fall into the land. Now,
on the land this water will be absorbed into the soil, it should percolate down or it will
move through run off. In the process of runoff, it is getting into the streams and rivers and
then ultimately reaching into the big water bodies like oceans and seas so, that is runoff,
or it can get accumulated in some terrestrial water bodies such as ponds or lakes.
And, here again you can have the process of evaporation that is happening or you can have
these plants that are absorbing the ground water and then in the process of transpiration
they are releasing it back into the atmosphere or you can have the situation in which the
percolated water, it moves below the ground and then it also reaches into the oceans.
Here again we see a cyclical process. You can start from any point and then, it will move
a complete cycle. So, this is the water cycle.
(Refer Slide Time: 34:55)

Another cycle is the phosphorus cycle. Now, in the case of the phosphorous cycle, the
main pool is the rock phosphates. The rock phosphates that is the phosphorus that is present
in the rocks. When these rocks undergo some amount of weathering, so, these phosphates
will be converted into the soil phosphates. These are more or less soluble forms of
phosphates that are there in the soil.