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Nutrition and Respiration


Life in different organisms consists of a series of similar processes such as respiration, excretion, etc. All these processes are performed by living organisms to sustain their life. All living organisms show some similarity in their activities. They all have to eat and digest their food, derive energy and remove waste materials from their bodies. It is through such processes that living organisms maintain their lives. The study of how they carry out these processes is called physiology.

The branch of biology that is concerned with studying the vital functions of plants and animals, such as nutrition, respiration, reproduction and excretion is called physiology.


The word nutrition is derived from a Latin word nutrire, which means to nourish. It is the process of consuming food.

The process by which organisms obtain energy (in the form of food) for their growth, maintenance, and repair is called nutrition.

All living organisms perform some or the other activity all the time. To carry on these activities, energy is required. This energy comes from the food that they eat.


Any material containing nutrients such as carbohydrates, proteins and fats, which are required by living organisms in order to obtain energy for growth and maintenance is called food. Food contains substances that are usable for the organism. Such substances are called nutrients. On the basis of functions, nutrients can be divided into:

  • Energy-yielding nutrients: carbohydrates and fats.

  • Body-building nutrients: proteins and some minerals.

  • Regulating nutrients: vitamins and minerals.

Organisms mainly exhibit two modes of nutrition:

  1. Autotrophic

  2. Heterotrophic

Autotrophic nutrition

The term autotroph is derived from two Greek words, auto meaning ’self’ and troph meaning ’nutrition’ In autotrophic mode of nutrition, an organism manufactures its own food from simple inorganic raw materials in the presence of sunlight. All the green plants and some bacteria have autotrophic nutrition and are called autotrophs. All autotrophs are, thus, producers of food.

Those green plants are autotrophic, which use raw materials such as carbon dioxide, water, mineral salts and sunlight to synthesize organic compounds in the form of sugars. This mode of nutrition in green plants is mainly achieved by the process of photosynthesis. During the process of photosynthesis, oxygen is released, which purifies air.

Chemical reaction of photosynthesis:














A few bacteria are chemosynthetic autotrophs. They do not require light to prepare their own food. They make use of carbon dioxide and other inorganic substance such as hydrogen suplhide to prepare food.

Heterotrophic nutrition

The term heterotroph is derived from two greek words: hetero meaning ’different’, and troph meaning ’nutrition’ All animals and few plants (fungi and bacteria) are heterotrophs. Heterotrophs, directly or indirectly, depend upon autotrophs for their nutrition. Therefore, heterotrophs are consumers. Most of the heterotrophs take in complex materials as food and break them down or decompose them into simpler forms.

Heterotrophic nutrition is different in lower and higher organisms.

Lower heterotrophic organisms derive their food in two modes:

  1. Saprophytic nutrition

  2. Parasitic nutrition

Saprophytic nutrition:

The word ’Saprophytic’ is derived from the Greek words Sapro meaning ’rotten’ and phyto refers to ’plants’ In this mode, organisms obtain their food from dead and decaying bodies such as rotten leaves, plants and decaying organic matter.


 Saprophytic nutrition 

Fungi (mould, mushroom, yeast) and many bacteria are saprophytic organisms. They break up complex organic molecules into simpler ones. The simpler molecules are then used by plants.

Parasitic nutrition:

Para refers to ’feeding beside’ and sites refers to grains. In this mode of nutrition, organisms obtain their food from living organisms. A parasite is literally one which lives on or inside the body of another living organism (host) and absorbs nutrients from the body of the host. It may damage or even kill the host in the process.

For example: All viruses, several bacteria, some fungi such as rust (Puccinia), smut (Ustilago) and plants such as Cuscuta and Visum.


Parasites, which often produce diseases in the host organisms are called pathogens. Examples: Viruses and bacteria.

Holozoic nutrition

Holozoic nutrition means feeding on soild food.


Feeding of complex organic matter by ingestion which is subsequently digested and absorbed is called holozoic nutrition.

Most of the animals have holozoic nutrition. Examples: Amoeba, dog, frog, man.




The chemical process by which green plants synthesize organic compounds (glucose) from carbon dioxide (CO2) and water (H2O) in the presence of sunlight is called photosynthesis.

It takes place in chloroplasts in green leaves. It is the primary mode for the production of food in green plants. The process of photosynthesis can be represented by the following equation:

6CO2     +    12H2  -------------->   C6H12O   +    6O2    + 6H2O

Here, the energy is trapped by plants and is converted into chemical energy.


The process of photosynthesis in plant leaf 


It is the green pigment present in green plants. Chlorophyll is localized in small bodies known as chloroplasts. Chloroplasts are photosynthetic organelles of the cell.

The chlorophyll consists of:

  1. Chlorophyll-a
  2. Chlorophyll-b
  3. Xanthophyll
  4. Carotene

Chlorophyll-a (blue-green pigment) and chlorophyll-b (yellow-green pigment) are common in most of the plants. These are the pigments, which help in photosynthesis. Chemically, chlorophylls are porphyrins. Other porphyrins are haemoglobin and other respiratory pigments. Chlorophyll contains magnesium. This is similar to that of haemoglobin, which contains iron. Chlorophyll is present in chloroplasts – the photosynthetic organelles of palnts. Xanthophyll is a yellow-coloured pigment whereas carotene is an orange-coloured pigment.


Sun is the natural source of light energy. Photosynthesis can also occur under artificial light. The rate of photosynthesis is highest in red light.

Conversion of solar energy into chemical energy

Solar energy is trapped by the photosynthetic pigments (chlorophyll) in plants and is converted into chemical energy (carbohydrates). Plants fix solar energy into chemical energy and this chemical energy is consumed as food by other living beings. So, all animals indirectly derive their nutrition from plants. The entire process of photosynthesis involves entrapping, converting and storing solar energy.


Chloroplasts are green-coloured plastids (in Greek, chloro means ’green’) containing the pigments chlorophyll-a and chlorophyll-b, DNA and RNA. They are the most common plastids in plants and are very important, since they perform function of photosynthesis.

Chloroplasts are homogeneously distributed in plant cells. These are mobile and show active and passive movements. Generally, chloroplasts are biconvex, but may be found in varying shapes such as filamentous, saucer-shaped, spheroid, ovoid, discoid, club-shaped, etc. Their size and number depends upon the species.


Site of photosynthesis in plants 

Essential raw materials for photosynthesis

The essential raw materials for photosynthesis are:

  • Carbon dioxide (CO2)
  • Water

Carbon dioxide (CO2):

Carbon dioxide forms only about 0.32 per cent of the total atmosphere. The source of carbon dioxide in atmosphere is respiration, microbial decomposition and combustion (of wood, coal, petroleum and natural gas).

Green plants utilize carbon dioxide from the atmosphere during photosynthesis, whereas aquatic plants make use of the carbon dioxide dissolved in water. In day time, when light is available, plants fix carbon dioxide during photosynthesis. But at night, due to the absence of light, no photosynthesis takes place. Plants utilize starch (in catabolism) and release carbon dioxide. The rate of photosynthesis remains low in the shade and during early morning and late evening hours. During this time, carbon dioxide released in respiration may be sufficient for photosynthesis.

This stage, when no net carbon dioxide uptake is done by plants, is termed as compensation point.


During photosynthesis, plants absorb water through roots with the help of xylem. Along with water, plants also absorb minerals and salts, which also help in photosynthesis. Out of the total water absorbed by plants, only one per cent is used in photosynthesis.





Mechanism of photosynthesis

Photosynthesis takes place in the green leaves of a plant.

Leaves are flat, thin and broad in structure, which are modified for two functions: photosynthesis and transpiration.

The upper surface of the leaf is called the upper epidermis and the lower surface is called the lower epidermis. The epidermis has a wax-like covering called the cuticle layer. There are a number of openings on the lower epidermis of the leaf called the stomata. Each stoma is guarded by two guard cells. The stomata open during daytime and close during the night. Mesophyll tissues in leaves are present between upper and lower epidermis.

Mesophyll tissues are composed of palisade cells. Photosynthesis takes place in the palisade cells. The palisade cells may have 300 or more chloroplasts. Water passes into the palisade cells by osmosis from the vein and mid-rib of leaves. Carbon dioxide diffuses in from the atmosphere through the stomata. Sunlight is absorbed by the chlorophyll. By using sunlight, carbon dioxide and water are combined in the chloroplast with the help of a number of enzymes to produce sugar. Sugar is then converted into starch for storage purposes. During the process of metabolism, starch is distributed to various parts of the plant through phloem. The oxygen gas produced during photosynthesis diffuses out to the atmosphere through the stomata of the leaf.


The photosynthetic reaction can be summarized as:

 CO2  +   H2O  -------------->  Organic matter (starch)   +   O2
       green plants       

It was observed that photosynthesis not only requires light, but other materials to proceed. Earlier, it was thought that carbon dioxide was split into carbon and oxygen. Carbon combines with water to form glucose.

 6CO2  +  6H2O  -------------->  C6H12O6  +  6O2

The process of photosynthesis occurs in two steps – light reaction and dark reaction.

Light reaction or photochemical phase


The photosynthetic pigments absorb light energy in the form of photons. Chlorophyll when excited emits electron which move to nearby electron acceptor molecules.

  1. Chlorophylls and accessory pigments absorb light of specific wavelengths.

  1. Photolysis of water takes place. The oxidized chlorophyll molecule takes an electron from water which splits to release oxygen.

  1. H2O ----> 2H+ + O2 + e

  1. NADP+ gets reduced to NADPH in the presence of the enzyme, ferredoxin - NADP - reductase. Oxygen released during photosynthesis comes from water.

Dark reaction or thermochemical phase


NADPH and ATP production takes place in light reaction. This is utilized by stroma of chloroplast to synthesize carbohydrate from carbon dioxide. This is dark reaction.

It occurs in stroma of chloroplasts. Here, carbon from carbon dioxide is utilized to form carbohydrates where products of light reactions are used.

The path of carbon was demonstrated by Melvin Calvin (1954).

Calvin cycle regenerates ADP and NADP required for the light reaction. The dark reaction is slow. It does not regenerate adequate ADP and NADP and hence, is a ’rate limiting’ step.

 Calvin cycle 

Factors influencing photosynthesis

Factors influencing photosynthesis are:

Light intensity and quality: Plants respire, i.e. they take in oxygen and release carbon dioxide in the absence of light. Sufficient light is essential for photosynthesis.

The rate of photosynthesis increases with the increasing intensity of light. This continues till carbon dioxide becomes a limiting factor. At a certain point, the plant becomes light-saturated. Thus, light is no longer a limiting factor in determining the rate of photosynthesis. Now concentration of carbon dioxide matters.

The photosynthetic rate is also influenced by light. Blue and red regions of the visible light are photosynthetically most effective.

Availability of carbon dioxide: Carbon dioxide enters through the stomata of leaves in terrestrial plants. As stomata close, the availability of carbon dioxide for photosynthesis decreases and the photosynthetic rate is reduced to zero. In aquatic plants (submerged), carbon dioxide directly enters in the form of bicarbonates or carbonates through the epidermis and reaches the photosynthetic cells.

Carbon dioxide acts as a limiting factor under field conditions during clear summer days, when plants are provided with adequate water.

Availability of water: Water becomes a limiting factor in field conditions during prolonged drought periods and during hot weather. Reduction of water results in closing of stomata which affects photosynthesis. Decrease in water results in dehydration due to which enzyme activity is hampered.

Temperature: During cool days temperature becomes a limiting factor. Also increase in temperature above 30o C results in decreased photosynthesis. Changes in temperature influence enzyme-controlled reactions. C4 plants show higher temperature optimum for photosynthesis than the C3 plants.

Internal factors: Age of leaf, its anatomy and chlorophyll content also affect photosynthesis. The photosynthetic activity is maximum when leaf is fully expanded. But it decreases with the age of leaf. The number of stomata, their opening and closing, venation of leaf, volume of intercellular spaces influence the rate of photosynthesis.

Generally, chlorophyll content is not a limiting factor for photosynthesis. This is evident from the less chlorophyll content in sunplants to have high rate of photosynthesis.

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