Life Processes Class 10 Notes Science Chapter 5

Different types of life Processes - Life Processes, Class 10, Science

LIFE PROCESS - NUTRITION

What is life process ?

All the living organisms including human beings perform a number of activities such as nutrition, respiration, excretion, growth and reproduction. These activities are characteristics of living organisms, and through such activities they maintain their lives. These maintenance function of living organisms are known as life processes.

Chemical reactions which take place within cells or organisms during various vital activities are called biochemical reactions.

Metabolism is a word used to describe the sum total of all the chemical and physical changes that are constantly taking place in living matter and are necessary for life. The word metabolite refers to a substance which undergoes various changes during metabolism. For example, carbon dioxide and water are metabolites used in the process of photosynthesis.

The metabolic pathways are of two types :

(i) Anabolic pathways or biosynthetic pathways in which biosynthesis of organic compounds occurs,
or in other words, complex substances are synthesized from simpler ones ; for example photosynthesis.

(ii) Catabolic pathways in which the breakdown of complex organic substances into simpler ones occurs (as in respiration)

In anabolic pathways or processes of anabolism energy is used (endothermic reactions), while in catabolic pathways or catabolism, energy is released (exothermic reactions).

Criteria to define something is alive :- 

1. Nutrition :- The processes by which the organisms obtain and utilise the nutrients (food).

2. Respiration :- The process that involves breakdown of respiratory substrates through oxidation and release of usable energy.

3. Transport :- The process in which the substances absorbed or synthesized in one part of the body are carried to other parts of the body.

4. Excretion :- The process involved in removal of the excess or toxic wastes from the body.

5. Control and coordination :- The process which helps the living organisms to receive information from the surroundings and behave accordingly in order to survive in the changing environment around them.

6. Growth and development :- Permanent increase in the size of the organisms is called growth. The whole series of changes which an organism goes through during its life cycle, is called development.

7. Movement and Locomotion

Definition :- The basic functions performed by living beings to sustain themselves are called life processes.

Nutrition

Nutrients are inorganic as well as organic substances which the organisms obtain from their surroundings in order to synthesize their body constituents and use them as a source of energy.

The process of intake of nutrients and its utilization by an organism in various biological activities.

OR

A process to transfer a source of energy from outside the body of the organism (food), to the inside is called nutrition.

There are various types of nutrients on the basis of function they perform :-

e.g. Energy foods :_ Carbohydrates and fats. 

Body building foods :_ Proteins and mineral salts.

Regulating foods :_ Vitamins and minerals. 

Modes of nutrition :_ Method of obtaining food by the organism is called mode of nutrition.

(A) Autotrophic (Holophytic) nutrition :_ The mode of nutrition in which the organisms prepare (or synthesize) their own organic food by using inorganic raw material (CO₂ & H₂O). They are also called autotrophs.

e.g. Plants, Photosynthetic and chemosynthetic bacteria and cyanobacteria etc.

(B) Heterotrophic nutrition :_ The mode of nutrition in which the organisms derive their nutrition from other organisms. They take ready made organic food from other dead or living plants or animals. The living organisms showing heterotrophic nutrition, are called heterotrophs.

e.g. All animals, fungi, many bacteria and some non-green plants (insectivorous plants) and man.

Types of heterotrophic nutrition :_ Depending upon the mode of obtaining food, the heterotrophic nutrition is of following types :

(i) Holozoic nutrition (Holo-Complete + Zoon-animal)

The mode of nutrition in which all animals take in complex solid food material is called holozoic nutrition.

It contains following steps :-

(i) Ingestion :- Taking in complex organic food through mouth opening.

(ii) Digestion :- Change of complex food into simple diffusible form by the action of enzymes.

(iii) Absorption :- Passing of simple, soluble nutrients into blood or lymph.

(iv) Assimilation :- Utilization of absorbed food for various metabolic processes.

(v) Egestion :- Expelling out the undigested food.

e.g. All animals including vertebrates and Invertebrates.

Depending upon the type of the food habit, animals are divided into three categories :-

(a) Herbivores :_ Animals that depend up on green plants are known as herbivores. e.g. Goat, Cow, Deer, Rabbit.

(b) Carnivores :_ Animals which eat flesh of other animals as food are called as carnivores. e.g. Lion, Tiger.

(c) Omnivores :_ Animals which eat both plants and animals as food are known as omnivores. e.g. Rat, Pigs, Crows, Cockroaches and Humans.

(ii) Saprotrophic (Sapro - Rotten ; Trophos - Feeder) Nutrition :_  In this type of nutrition the organisms obtain their food from decaying organic substances. Organisms are also called saprotrophs. e.g. Bacteria, Fungi.

 (iii) Parasitic nutrition (para-other) :_ 

The mode of nutrition in which one organism (called parasite) derive its food from other living organism (Host) is called parasitic nutrition. e.g. Tape worm, Ascaris, Plasmodium, Liver flukes, Cuscuta etc.

Differences between Autotrophic & Heterotrophic nutrition :-

Characters Autotrophic Nutrition Heterotrophic nutrition

(1) Source of Energy Sunlight or chemical energy Readymade food

(2) Mode of Nutrition Photosynthesis or Chemosynthesis Feeding upon dead or living plants or animals

(3) Occurrrence Found in green plants, Found in Animals, fungi, Blue-green algae, certain Most of the bacteria Bacteria

Differences between holozoic and saprotrophic nutrition :-

Feature Holozoic nutrition Saprotrophic nutrition

1. Nature of food Solid food (Whole plant or animal Liquid food (Dead and decaying organic or their parts) is ingested matter) is ingested

2. Site of digestion Inside the body form Outside the body as enzymes are released on the food material that convert solid food into simple soluble

Examples Most of the Animals Fungi _ Yeast, Slime moulds, Bacteria.

 Animals which depend upon the blood of other animals known as sanguivores. e.g. Bedbug, Mosquito, Leech etc.

Some organisms take in predigested food through their body wall by the process of diffusion. This process of nutrition is known as osmotrophic nutrition. e.g. Tapeworm, Trypanosoma.

Mutualistic nutrition :_ The mutualistic nutrition can be defined as the interdependent nutrition in which each organism is dependent mutually on the other. e.g. The lichens share mutualistic nutrition between a fungus and a Algae.

Nutrition in unicellular organisms (ExampleAmoeba) :- 

Food - Amoeba is a holozoic and omnivorous animal. It feeds upon microscopic organisms like bacteria, Paramecium, Diatoms, Algae and dead organic matter.

Mechanisms. Nutrition in Amoeba involves the following steps:

(i) Ingestion:- Amoeba has no mouth, so ingestion may occur at any point of body surface but generally it occurs at the advancing end of the body. Ingestion occurs with the help of pseudopodia. The opening of food cup gradually becomes narrower and narrower, and finally closes. So the food is finally enveloped and taken inside a food-vacuole (called phagosome) along with a drop of water. 

(ii) Digestion:-   Amoeba shows intracellular and vacuolar digestion. In the cytoplasm, food vacuole fuses with lysosomes containing digestive enzymes. In this , the complex and non-diffusible nutrients are changed into simple and diffusible nutrients. Medium inside the food vacuole is first acidic but later becomes alkaline, (as in the alimentary canal of man).

(iii) Absorption and assimilation :_ In absorption, the diffusible nutrients pass through vacuolar membrane into cytoplasm by diffusion and are then distributed to all the body parts by streaming movements of cytoplasm called cyclosis. Due to this, the size of food vacuole gradually decreases.

In the cytoplasm, a part of the absorbed food is oxidised to produce energy, most of simple nutrients are combined to synthesize complex compounds.

(iv) Egestion :_ Amoeba has no anus, so egestion may occur at any point on the body surface.

competition Window 

1. Food :- The substance which is palatable, delicious enough and energy provider is called food.

Chemically food consists of six essential components :-

(i) Carbohydrates (ii) Fats (iii) Proteins (iv) Minerals (v) Vitamins (vi) Water

2. Digestion :- Digestion is a catabolic process, in which the complex, non-diffusable and larger components of the food, are broken down into their respective simpler, diffusable and smaller form with the help of various hydrolytic enzymes in the alimentary canal of living organisms.

3. Intracellular and Extra-cellular Digestion :-

Intracellular Digestion : This type of digestion occur inside the cell cytoplasm. The food inside the cell occurs as food vacuole. The digestive enzyme in this case are secreted inside the cell. They digest the contents of the food vacuole. So the entire process of digestion occurs inside the cell. e.g. Protozoans [Amoeba], Sponges

Extracellular Digestion :- It takes place outside the cell [i.e. in the intercellular space or a cavity formed by many cells or tissue]. In all animals this cavity is found as a large canal, called Alimentary canal. 

4. Hydrolysis :- It is a kind of catabolic reaction in which a compound is broken [lysis break] down into smaller compounds, with the help [addition] of water [hydro = water].

5. Carbohydrates :- These are the hydrates of carbon in which the ratio among carbon, hydrogen & oxygen is 1 : 2 : 1. Carbohydrates are the quickest source of energy.

On the basis of their composition, carbohydrates are of following types :

(a) Monosaccharides :- The simplest sugars are called monosaccharides. These sugars cannot be further degraded to produce more sugars. e.g. Glucose, Fructose, Galactose, Ribose and Deoxyribose.

(b) Oligosaccharides :- These are complex sugars, formed by the polymerisation of a few [1 to 10] units of monosaccharides.

Sucrose Glucose + Fructose

Maltose Glucose + Glucose

Lactose Glucose + Galactose

(c) Polysaccharides :- These are most complex carbohydrates, which are the polymers of thousand of units of monosaccharides. e.g. Starch Stored food in plants, Glycogen Stored food material in Animals.
Cellulose Constituent of cell wall.

6. Fats :- These are energy rich compounds. These are the esters of higher fatty acids. [Esters are formed by the addition of alcohol with acids] Alcohol generally Glycerol.

7. Proteins :- Proteins are the polymers of amino acids. Amino acids are held together by means of peptide bond to form polypeptide chains.

8. On the basis of gross size of food, the mechanism in different animals may be of two main types :-

(a) Microphagy  Feeding on microscopic organisms. e.g. Amoeba, Paramecium.

(b) Macrophagy  Feeding on larger forms of organisms. e.g. Majority of non-chordates and some chordates.

9. In Paramecium, ingestion is aided by beating of cillia. It has definite food passage, mouth (cytostome) and anus (cytopyge).

10. Food vacuole is commonly called temporary stomach or gastriole as it is the site of storage of food.

11. Most common mode of ingestion in Amoeba is circumvallation. In this, pseudopodia extend and form a cup-like structure, called food cup, around the prey.

 Digestive system of human :-

Human digestive system consists of the alimentary canal and digestive glands and it involves mastication, swallowing, digestion of food and elimination of undigested matter.

It consists two parts :-

[A] Alimentary canal:_ The alimentary canal is basically a long tube extending from the mouth to the anus.

It is differentiated into following parts.

(1) Mouth :_ It is a transverse slit bounded by movable lips. The lips serve to close and open mouth, holding the food in between and also help in speaking.

(2) Buccopharyngeal cavity / Mouth cavity : Mouth leads into the mouth cavity or oral or buccal cavity. The roof of mouth cavity is formed by palates i.e., hard and soft palate, the floor by tongue and the sides by the cheeks. The other conspicuous structures are the teeth and salivary glands.

(a) Tongue :_ The floor of the mouth cavity is occupied by muscular, large, mobile tongue. It remains attached on its under surface to the floor by fold of mucous membrane called the lingual frenum. The tongue is covered with mucous membrane and its upper surface is raised into lingual papillae which contain microscopic taste buds.

(i) Lingual papillae :- Our tongue has a rough uppper surface due to three types of lingual papillae filiform, fungiform and circumvallate.

(ii) Taste buds :- Tongue is an organ of taste, richly supplied with sensory nerves which end in taste buds in the papillae of the upper surface. The taste buds for sweet taste are located on the anterior end of the tongue, for bitter taste at the posterior end, for sour taste on its sides and for salty taste on a small part just behind the anterior end of the tongue.

Functions of tongue :-

(i) It acts like a spoon during ingestion of food.

(ii) It brings food under teeth for mastication

(iii) It moves food in buccal cavity for mixing of saliva.

(iv) It helps in swallowing food.

(v) It cleans teeth by removing small food particles from their surface.

(vi) It helps in speaking.

(vii) It is the main organ of taste.

(viii) It keeps the mouth moist by the secretion of both mucus and serous or water like fluid.

(ix) In dogs during panting it helps in thermoregulation by quick evaporation of water of saliva.

(x) In some mammals tongue is used to clean skin by licking.

(b) Teeth :_ Thecodont (Teeth present in bony socket), Heterodont (Teeth are of four types) and diphyodont (Teeth that come two time in life).

Teeth are of following types :

(i) Incisors  Biting the food.

(ii) Canines  Wearing and tearing of food.

(iii) Premolars Crushing and grinding the food.

(ii) Molars  Crushing and grinding the food.

How can the dental caries be prevented ?

(i) Avoid sugar rich eatable.

(ii) Regular brushing of teeth after meals.

(iii) Vigorous chewing of fibrous food.

(iv) Consumption of water containing 1 ppm of fluoride.

(c) Palate :- The septum like structure which separates nasal path from oral path. The hard palate is the anterior part of the roof of mouth cavity. It is supported by palatine and maxillary bones that is why it is known as hard palate. There are no premaxillary bones present in our hard palate. The lower surface of hard palate covered with mucous membrane of mouth cavity is raised into transverse ridges called palatine rugae which serve to grip the food firmly. Soft palate is the posterior muscular part having no skeletal support. It carries a fleshy cone shaped projection called uvula which hangs down from it in the middle line. Uvula serve to block the internal nares during swallowing by rising up, preventing entry of food in the nasal cavities.

Various types of teeth

(iii) Pharynx :

The buccal cavity opens into a short narrow chamber called pharynx or throat. Pharynx is incompletely divided into three parts by soft palate the nasopharynx dorsal to the soft palate, oropharynx below the soft palate and laryngopharynx into which both these parts communicate round the freely hanging uvula. The oropharynx and laryngopharynx are associated with swallowing. The laryngopharynx communicates with oesophagus through gullet and with larynx through glottis. The glottis is guarded by an elastic and muscular flap called epiglottis which closes glottis during swallowing to prevent food from entering into wind pipe. Thus pharynx serves two ways (i) as a passage between nose and wind pipe (trachea) (ii) as food passage between mouth cavity and oesophagus. Resonance of voice also occurs due to pharynx.

(iv) Oesophagus (food pipe) :_ It conducts the food by peristalsis.

The oesophagus is a collapsible muscular tube 25-30 cm long and about 25-30 mm thick, leading from the pharynx to stomach. It runs down the neck, behind trachea and through thorax, finally piercing the diaphragm to open into stomach. The upper 1/3rd part is composed of voluntary muscles and the lower 2/3rd of involuntary muscles. The muscular coat has a peristaltic action for driving the food towards the stomach. There are no digestive glands but only mucous glands in oesophagus.

(v) Stomach :_ It is a thick, muscular and J-shaped sac present on the left side of upper part of abdomen. Beneath diaphragm lying to the left side of abdomen is sickle-shaped stomach. It is the widest part of alimentary canal, size and shape of which varies according to the contents and sex. The stomach of a woman in general is more slender and smaller than that of a man. It can be distinguished into three regions Fundic part, Body part and Pyloric parts. Pyloric part is the lower end of stomach which narrows down like a funnel and curves to the right like 'J' leading into duodenum. The exit is guarded by a pyloric sphincter or pylorus. The partly digested paste like food is forced into intestine through pyloric sphincter, due to peristaltic waves of stomach.

Functions of Stomach :-

(1) Temporary storage of food.

(2) Partial digestion of food by gastric juice.

(3) Churning of food.

(4) The stomach regulates the flow of partially digested food into the small intestine.

Intestine :- It distinguished into two parts :-

(a) Small Intestine (b) Large Intestine

(a) Small intestine :_  It is a long (about 6 meter) narrow (average diameter 4 cm), tubular and coiled part. It is differentiated into anterior duodenum, middle jejunum and posterior ileum. It is mainly concerned with completion of digestion and absorption of food.

(i) Duodenum : This proximal part starts after pyloric end of stomach. It is about 25 cm long lying against the posterior abdominal wall. It is curved like 'C' or a horseshoe and ends behind the stomach. A common bile duct and a pancreatic duct opens in middle of 'C' of duodenum by a common aperture over a raised area called ampulla of vater.

(ii) Jejunum : The Jejunum (a latin word meaning empty) is so called because it is always found empty after death.

(iii) Ileum : It is the last part of small intestine.

Small intestine designed to absorb digested food as :

(i) It is lengthy about 6 meters.

(ii) Inner lining has two types of folds called villi and microvilli.

(iii) Each villus has blood capillaries and a lymph capillary.

(b) Large intestine :- It is shorter (about 1.5 meter) and wider (Average diameter 6 cm) than small intestine. It is differentiated into caecum, colon and rectum. It is the site of aborption of water from digested food. It helps in formation and temporary storage of faeces.

Caecum is vestigeal in human beings.

Anus :- It lies at the base of trunk and is for egestion.

[B] Digestive glands:- They secrete digestive juices which contain digestive enzymes. These are of following types.

(i) Salivary glands :- In man, there are three pairs of salivary glands. These secrete saliva which contains a digestive enzyme called ptyalin or Salivary amylase.

Saliva is an antiseptic as it kills germs and bacteria due to presence of an enzyme called Lysozyme. Saliva makes the food soft, slippery and helps in digestion of starch due to presence of salivary amylase enzyme.

Our mouth starts watering on eating food of our interest. This water is basically the saliva secreted by the salivary gland which get activated on eating or seeing or thinking of a food.

Involuntary contraction & relaxation movement is called peristalsis.

(ii) Gastric glands :- Gastric glands are present in the wall of stomach and secrete gastric juice.

(iii) Liver :- It is lobed and reddish-brown coloured largest gland of body present in the right side of upper part of the abdomen. It synthesize and secretes bile juice. Gall bladder is present below the right lobe of liver. It stores and secretes bile.

(iv) Pancreas :- It is yellow- coloured gland present just behind the stomach. It secretes pancreatic juice. Pancreatic duct carries pancreatic juice to small intestine.

(v) Intestinal glands :- These lie in the wall of small intestine and secrete intestinal juice.

[C] Physiology of nutrition :-

(i) Ingestion :- Man is omnivorous in feeding and is holozoic.

Ingestion involves carrying the food to the mouth with the help of hands and cutting of food with incisors or canines depending upon the nature of food.

(ii) Digestion :- In man, digestion is started in buccal cavity and completed in intestine.

(a) In buccal cavity :- Here, food is chewed with the help of premolars and molars which increases the rate of action of salivary amylase. Food is mixed with saliva of salivary gland.

Starch Maltose

(b) In stomach :-

Food is mixed with gastric juice which contains mucus, hydrochloric acid, pepsin, rennin and a weak lipase enzyme. Mucus, lubricates the food and protects the inner lining of the stomach from the action of acids.

Functions of Hydrochloric acid

(i) Stops the action of salivary amylase in stomach.

(ii) Kills the bacteria present in the food.

(iii) Activates pepsin.

(iv) Provides acidic medium.

Pepsin hydrolyses proteins into proteoses and peptones.

Lipase enzymes hydrolyses small amounts of fats into fatty acids and glycerol.

Curdling of milk is done by the enzyme rennin. (Rennin is not found in human beings, it found only in cattles)

(c) In small intestine :-

The small intestine is the site of the complete digestion of carbohydrates, proteins and fats.

Food is mixed with three digestive juices, bile juice, pancreatic juice and intestinal juice.

Bile juice provide alkaline medium and emulsifies fats [conversion of larger fat globules into smaller fat droplets] but is a non enzymatic digestive juice so has no chemical action on food.

Pancreatic juice contains trypsin, pancreatic amylase and pancreatic lipase enzymes which digest the peptones, starch and fats into peptides, maltose and fatty acids.

Intestinal juice contains aminopeptidase, intestinal amylase, maltase, sucrase and lipase enzymes:_
 

Peptides  Amino acid

Maltose  Two glucose

Fats  Fatty acid + Glycerol

Lactose Glucose + Galactose

Sucrose Glucose + Fructose

(iii)Absorption :-

Absorption of the digested food occurs through the epithelial surface of the villi & microvilli of small intestine.

Inner surface of small intestine is raised into 4 millions of finger-like folds called villi.

Each cell of villus is with electron microscopic processes called microvilli.

Each villus is with blood capillaries and a lymph capillary.

(iv) Assimilation:- It is a process by which absorbed nutrients are utilized to resynthesize complex molecules like carbohydrates, proteins and fats inside the cells.

(v) Egestion :- Removal of waste products from the body is known as egestion.

1. Emulsification :- Emulsification is the phenomenon of physically breaking of large sized fat globules into large number of fat droplets by the bile-salts of the bile juice. This increases the surface area for digestion of fats by the lipase enzyme.

2. Enzymes are classified into 3 groups :-

(a) Amylolytic enzymes _ Carbohydrate digestion

(b) Proteolytic enzymes _ Protein digestion

(c) Lipolytic enzymes _ Lipid digestion 

3. Alimentary Canal :- The digestive canal where the entire process of digestion is accomplished, called alimentary canal.

It is the site for ingestion, digestion, absorption and egestion of food material.

In man it is about 78 meter long.

The alimentary canal of herbivores is longer than the alimentary canal of carnivores, because herbivores have to digest the cellulose, which is difficult to digest.

4. Mucus in stomach :- Mucus is a viscous secretion. The mucus also forms a thick coating over the mucosal cells and prevents them from the harmful effects of HCl and pepsin.

5. Pepsin is the chief proteolytic enzyme.

6. Castle's Intrinsic factor :- This intrisic factor is secreted by fundic glands of the stomach mucosa. It combines with vitamin B₁₂ [which is known as extrinsic factor].

The vitamin B₁₂ from this combined complex can easily be extracted and absorbed by the intestinal mucosa. Intrinsic factor, thus, helps in the absorption of vitamin B₁₂.

7. Colitis :- An inflammation of the colon and rectum is called colitis. Inflammation of the mucosa reduces absorption of water and salts, producing watery, bloody faces and in severe cases, dehydration and salt depletion.

8. Digestion of proteins in man starts from stomach. In buccal cavity there is no digestion of proteins because saliva contains no proteolytic (protein digesting) enzyme.

Nutrition in plants 

Green plants are autotrophic. They synthesize their own food by the process of photosynthesis. Autotrophic plants are able to produce food so they are known as producers.

Photosynthesis :- Photosynthesis is a process by which green plants synthesis organic food (carbohydrate) from carbon dioxide and water using solar energy by chlorophyll pigments. The sugar produced is stored in the form of starch in plants.

Importantce of photosynthesis :-

Photosynthesis is an anabolic process in nature for providing food supply to the living organisms. It purifies the atmospheric air, by consuming CO₂ and evolving oxygen.

The over all equation of photosynthesis is :_

6CO₂ + 12 H₂O C₆H₁₂O₆ + 6H₂O + 6O₂

Requirements for photosynthesis :-

(1) Sunlight (2) Photosynthetic pigment (3) Carbondioxide (4) water

CO₂ and water work as raw materials which are obtained from the atmosphere and the soil respectively.

1. Sunlight :- Sun is a natural source of light for photosynthesis. Sunlight is an electromagnetic spectrum. Photosynthetic pigments absorb only visible/white light from electromagnetic spectrum. White light (380 nm to 760 nm) is composed of wavelength of seven different colours violet, indigo, blue, green, yellow, orange and red (VIBGYOR).

2. Photosynthetic pigment :- These are chlorophylls carotenoids (carotenes and xanthophylls) and phycobilins. These pigments absorb only visible light. Chl-a and b absorb only blue and red light and reflect green light.

3. Carbondioxide : All the plants need carbondioxide to form carbohydrates. The carbon dioxide is obtained by the plants from the atmospheric air. In the terrestrial plants, the CO₂ enters into the cells of leaves through tiny pores called stomata which always remain present on the surface of leaves.

Stomata :- These are tiny pores or microscopic aperture guarded by two kidney shaped or bean shaped guard cells.

Functions :-

(i) Massive amount of a gaseous exchange take place in the leaves through stomata. Exchange of gasses also occurs across the surface of stem, root and leaves.

(ii) Large amount of water is lost through stomata.

Guard cells :- These are kidney shaped cells which cover single stoma. They contain chloroplast also.

Function :- They regulate the opening and closing of the stoma and also perform photosynthesis.

Opening and Closing of Stomata :-

When the guard cells swell due to the entry of water, the stomata gets opened. But when the guard cells shrink due to the loss of water, the stomata gets closed.

Desert plants take up CO₂ at night and prepare an intermediate which is acted upon by the energy absorbed by the chlorophyll during the day and form glucose.

In aquatic plants, CO₂ is obtained from the water where it remains present in dissolved form. Such plants absorb carbondioxide in solution form, all over their surface from the surrounding water.

4 Water :

Water is always needed by the plants for its use during photosynthesis. Inside the chloroplasts of the leaves, the water molecules split into hydrogen and oxygen with the help of light energy of solar light. Some mineral salts like N, P, K, Fe, Mg required by the plants are also transported to different parts of the plant along with the water. Nitrogen is an essential element used in the synthesis of proteins and other compounds (chlorophyll, DNA and RNA). Nitrogen is taken up in the form of inorganic nitrates or nitrites which have been prepared by symbiotic bacteria from atmospheric N₂.

Site of Photosynthesis :- Green plastid (Chloroplast or Kitchen of the cell).

When we observe the cross section of a leaf under microscope, we can see the mesophyll cells full of green dots. These green dots are chloroplasts containing chlorophylls.

Chloroplasts :- They are green coloured plastids. Their green colour is due to the presence of green pigments the chlorophylls. Each developed chloroplast has two distinct areas - grana and stroma.

Grana (Singular-granum) :- The light reaction of photosynthesis takes place in this part of chloroplast. In a granum large number of lamellae remain arranged like a stack of coins. These lamellae are called as thylakoids, which contain chlorophyll pigments.

Stroma :-  It forms the matrix of the chloroplast. The dark reactions of photosynthesis take place in stroma.

Mechanism of photosynthesis

During photosynthesis following events occur :-

(i) Absorption of light energy by chlorophyll.

(ii) Conversion of light energy into chemical energy and spliting of water molecule into hydrogen & O₂

(iii) Reduction of CO₂ to carbohydrates.

All these events can be categorised into two main phases :-

(A) Light phase (B) Dark phase

(A) Light reaction :- This step occurs inside thylakoid membranes of chloroplasts. In this step firstly absorbtion of solar energy by chlorophyll molecules takes place.

During light reaction, light energy breaks up water molecules into hydrogen and oxygen and this process is called photolysis of water. Light energy converts into chemical energy in the form of ATP and NADPH₂.

Oxygen is released as a by product of light reaction occuring during photosynthesis.

(B) Dark reaction :- In this step synthesis of carbohydrates from carbondioxide takes place. It is not light dependent hence it is called as dark reaction. This reaction occurs inside stroma of chloroplasts where light energy is not captured.

During this reaction, the chemical energy formed during light reaction (ATP and NADPH₂) is utilised for the fixation and conversion of carbon dioxide into a simple carbohydrate, that is glucose. The glucose molecules thus formed are further converted by the cellular machinery into various chemicals required by the plants.

Difference between light and dark reactions. 

Factors affecting photosynthesis :_ 

(a) Light (b) Water (c) Temperature (d) CO₂ (e) Oxygen 

(a) Light

The source of light for planet earth is sun, although some marine algae also utilise the light of moon. Out of the total solar energy, only 2% solar energy is used in photosynthesis. The quality and intensity of light also affects photosynthesis.

Quality Red and blue lights are most effective in photosynthesis. But the rate of photosynthesis is maximum in red light. There is no photosynthesis in presence of green light because green parts of plants reflect whole of the green light.

Intensity  The increase in intensity of light increases photosynthesis. Intensity of sunlight µ Rate of photosynthesis At very high light intensity the photosynthesis is decreased due to photo-oxidation of the constituents (solarization).

(b) Water

Water is an essential raw material in photosynthesis. Only 1% of the absorbed water is utilised in photosynthesis. Less availability of water reduces the rate of photosynthesis. Under water deficient conditions the stomatal aperture remains closed to reduce the loss of water by transpiration. As a consequence the entry of CO₂ is also stopped into the leaves.

c) Temperature 

The rate of photosynthesis increases by increase in temperature upto 40°C. Above this temperature, there is a decrease in the photosynthesis. Similarly, low temperature also inhibits photosynthesis. The temperature affects photosynthesis by affecting the activity of enzymes. We know that the dark reaction of photosynthesis involves several enzymes. These enzymes function at a specific range of temperature. Low temperature lowers the activity of enzymes and high temperature causes inactivation of enzymes.

(d) Carbon-dioxide

Atmosphere is the main source of CO₂ for terrestrial plants. In atmosphere CO₂ is present at the tune of 0.03%. The rate of photosynthesis increases by increasing the concentration of CO₂. But after a certain limit, the excess concentration of CO₂ proves to be toxic to the cells.

(e) Oxygen

Over concentration of oxygen stops photosynthesis. It increases the rate of respiration manifold and disturbs the excited condition of the chloroplast. In these conditions photosynthesis is not possible.