Notes-Part-1-Class-10-CBSE-NCERT-Science-Chapter-5-Life Processes

Topics to be learn : Part-1 What are life processes? Nutrition Respiration Topics to be learn : Part-2 Transportation Excretion

Introduction :

• Biology is the study of living organisms and their life processes.
• All live or living organisms respire. Also some movements either visible like locomotion from one place to another and growth related movements or invisible movement such as movement of molecules are the criteria to decide whether something is alive or not.
• Virus body is made up of Nucleic Acids (DNA or RNA) and protein cover called capsid. It is crystalline in nature. When this nucleoprotein structure enters the host cell, it undergoes reproduction. So, there is controversy whether they are alive or not.

What are life processes?

Life processes : Life processes are the processes which work together to perform maintenance work in living organisms.

• The molecular movements are essential to carry out different life processes like nutrition, respiration, excretion, growth, repair, transportation, etc.

Living organisms : Living organisms are Well-organized structures. They have the ability to perform the basic life processes in order to maintain and sustain life. This feature differentiates them from nonliving organisms. They may have tissues, which are made up of cells and further cells have smaller components in them.

• Living organisms on earth depend on carbon based molecules. So, organisms obtain carbon molecules from the environment either in the form of CO₂ as in green plants or in the form of food.
• Living organisms may be unicellular or multicellular.
• Unicellular organisms like Amoeba, Euglena are single celled organisms. They do not need specific organs for performing different life processes. They are directly in contact with the environment and simple diffusion is sufficient for nutrition, excretion and gaseous exchange.
• In complex multicellular organisms like man, higher plants, etc. need specific organ systems to carry out life processes arise. In multicellular organisms all cells are not in direct contact with surrounding environment hence diffusion is insufficient to meet the oxygen requirement of multicellular organisms.

Processes essential for maintaining life :

• Nutrition: It provides energy and materials (nutrients) required to sustain life.
• Respiration: It breaks down food (especially glucose) in the cells of organisms to release energy in the presence or absence of oxygen.
• Transportation: It is the process for carrying food and oxygen from one place to another in the body. Also it transports Wastes from body cells to excretory tissues.
• Excretion: It is the process to remove the by-products produced during other life processes.

Outside raw materials used for by an organism :

Ans. Raw materials are the substances which cannot be directly utilized by the organisms. These are to be processed inside the body like photosynthesis in plants and digestion in man. Raw materials taken by plants from outside are carbon dioxide, Water and sunlight.

Animals take various materials from outside which are:

• Food: To supply energy, materials like carbohydrates, fats, proteins, minerals,
• vitamins etc., for working and maintenance of cells/tissues.
• Water: To provide medium in the cells of body and to carry out all the metabolic reactions necessary for life.
• Oxygen: To break down food so as to release energy.

 Nutrition :

Nutrition is a process of intake as well as utilization of nutrients by an organism.

Nutrient :

• Nutrient is a substance which an organism obtains from its surroundings.
• Nutrients are components of food which are needed in the body to get energy and raw materials required for growth and other life processes.

Two major types of nutrition : Autotrophic and Heterotrophic.

Autotrophs :

• Autotrophs make their own food from raw materials like CO₂ and water in presence of sunlight and chlorophyll.
• They take in simple inorganic substances and change them into complex organic food.
• Examples, all green plants.

Heterotrophs :

• Heterotrophic plants and animals adopt various modes of nutrition.
• Heterotrophs cannot make their own food. They depend on autotrophs for their food supply directly or indirectly.
• Animals do not have chloroplasts in the cells of their body. So, they cannot convert solar energy into chemical energy.
• The food taken in by the animals is in the complex form. It must be broken down into simpler form hence there is an elaborate digestive system in animals that helps in the digestion of food which is aided by various enzymes.
• Examples, all animals, fungi and non-green plants like Cuscuta.

Autotrophic Nutrition :

Photosynthesis : Photosynthesis  is the manufacture of simple carbohydrates like glucose from carbon dioxide and water in the presence of sunlight and chlorophyll.

• Chloroplast is the site of photosynthesis in a leaf‘.
• Chlorophyll is the photosynthetic pigment present in a chloroplast
• Green plants are able to synthesise all the organic molecules associated with life by photosynthesis.
• Aquatic plants utilize the carbon dioxide dissolved in water around them to manufacture food.
• Food is usually stored in the form of complex carbohydrates like starch and cellulose.

Events occur during photosynthesis :

• Absorption of light energy by chlorophyll, which is converted into chemical energy.
• Photolysis of water which is the splitting of water molecules into hydrogen and oxygen.
• Carbon fixation which is the reduction of carbon dioxide to carbohydrates.

These steps need not take place one after the other immediately.

Chemical equation of photosynthesis :

6CO₂ + 12H₂O \(\frac{sunlight}{chlorophyll}>\)  C₆H₁₂O₆(s) + 6H₂O(g) + 6O₂(g)

• Photosynthesis process supplies continuous oxygen in the atmosphere

Know This : In most of the plants, all the events of photosynthesis take place one after the other immediately. But desert plants take carbon dioxide during night and then convert it into an intermediate compound. During the day time this intermediate compound is further converted into final product of photosynthesis. Therefore, in desert plants all the events of photosynthesis do not occur one after the other immediately. Plant cells can change their shape with the help of cytoskeleton present in their cytoplasm during critical conditions.

Role of water, chlorophyll and Stomata in photosynthesis:

(i) Water: Water (H₂O) undergoes photolysis during light reaction. As a result of this electrons (e⁻) and protons (H⁺) are produced which are utilised for the synthesis of ATP and NADPH.

H₂O → 2H⁺ + O + 2e⁻

It also releases oxygen

O + O → O₂ ↑

Oxygen is obtained by the breakdown of water.

(ii) Chlorophyll: It traps solar (light) energy to initiate the process of photosynthesis.

• Chlorophyll containing palisade parenchyma cells and spongy parenchyma cells of mesophyll. In this cells of the leaves photosynthesis take place.

(iii) Stomata: They help in exchange of gases. Carbon dioxide needed in photosynthesis is taken in through stomata. Oxygen produced during photosynthesis is given out through stomata.

Cross section of Leaf :

If we observe a cross-section of a leaf under the microscope (Fig.), there are some cells contain green dots. These green dots are cell organelles called chloroplasts which contain chlorophyll, which is essential for photosynthesis.

Stomata :

• Each stoma is a minute pore surrounded by two guard cells. The guard cells are kidney-shaped and contain numerous chloroplasts.
• The inner walls of guard cells are comparatively thicker than the outer walls.

Functions of guard cells:

• The guard cells regulate the opening and closing of the stomatal pore.
• The guard cells swell when water flows into them causing the stomatal pore to open.
• Because inflow of water in guard cells causes the stretching and bulging of outer thin walls in convex shape. This drags thick walls apart leading to opening of the pore.
• When there is outflow of water from guard cells, the outer thin walls come to their original position resulting in closure of stomatal pore.

Major functions of stomata :

• Stomata are essential for exchange of gases (oxygen and carbon dioxide) between the plant and the atmosphere for carrying on respiration and photosynthesis.
• Normally, plants eliminate excess water in the form of vapours through stomatal openings. This process is called transpiration.

Functions of stomata in desert plants :

• Stomata remain closed during the day in desert plants.
• In deserts during day time, temperature is very high so stomata are closed to
• reduce the loss of water due to transpiration.
• Therefore, desert plants are adapted to take up carbon dioxide at night when stomata are open.
• This CO₂, taken up during night, changes into an intermediate compound.
• This intermediate compound is acted upon by the energy absorbed by the chlorophyll during the day to prepare food.

Sources of raw material:

• Other than sunlight from sun, autotrophs also need other raw materials for building their body.
• Water used in photosynthesis is taken up from the soil by the roots in terrestrial plants.
• Other materials like nitrogen, phosphorus, iron and magnesium are taken up from the soil.
• Nitrogen is an essential element used in the synthesis of proteins and other compounds. This is taken up in the form of inorganic nitrates or nitrites. Or it is taken up as organic compounds which have been prepared by bacteria from atmospheric nitrogen.

Heterotrophic Nutrition :

Other than autotrophic—saprophytic, parasitic and holozoic are the three modes of nutrition. All these are different modes of heterotrophic nutrition.

• Saprophytic Nutrition : In this mode of nutrition, organism releases some enzymes on dead and decomposing materials. The enzymes break down them into simple nutrients which are absorbed by the organism as food.
• Holozoic Nutrition: In this type of nutrition, organism takes in whole material and breaks it down inside the body. For example, animals eating grass, fruits, insects, fish etc.
• Parasitic Nutrition: In this type of nutrition, organisms obtain their food from other living organisms without killing them. For example, Cuscuta (amarbel), leeches, tape-worms, etc.

Parasitic Nutrition :

• Some organisms derive their nutrition from other living plants or animals (hosts). This is known as parasitic mode of nutrition and such organisms are called parasites.
• Some parasites are ectoparasites. They live outside the body of the host and derive their nutrition from the host such as Cuscuta, leech, blood sucking mosquitoes.
• Some parasites live inside the body of the host and are called endoparasites. For example, Tapeworm, some fungi, etc.

How do Organisms obtain their Nutrition?

Process of nutrition in Amoeba :

• Engulfing of Food: Amoeba obtains food by phagocytosis (a type of holozoic
• nutrition). Amoeba engulfs its food by using pseudopodia which are temporary extensions of the cell surface. These pseudopodia fuse over the food particle forming a food vacuole.

• Digestion of Food: Inside the food vacuole, complex substances of the food are broken down into simpler ones. The digested simple substances diffuse from food vacuole into cytoplasm. The remaining undigested material is moved near the surface of the unicellular body of Amoeba and thrown out.

Know This : Pseudopodia also help Amoeba in locomotion and excretion. In Paramoecium, which is also a unicellular organism, the cell has a definite shape and food is taken in at a specific spot. Food is moved to this spot by the movement of cilia which cover the entire surface of the cell.

Intracellular digestion : When digestion of food takes place within the cell it is called intracellular digestion, e.g., Amoeba.

Extracellular digestion : When digestion of food takes place outside the cells, but in the alimentary canal, it is known as extracellular digestion, e.g., man, frog, etc.

 Nutrition in Human Beings :

Ingestion :

• The process by which organism takes in the food is called ingestion.
• Normally ingested food is complex in nature.
• In man, food is ingested through the mouth

Digestion :

• The Process of mechanical, enzymatic and chemical break down of ingested complex food material into simpler and soluble molecules is called digestion. In man, it starts from mouth and completes in the small intestine.
• The digestion of protein in man starts in stomach. The gastric juice of stomach contains pepsin enzyme, hydrochloric acid and mucus.
• Pepsin enzyme converts complex protein molecules into smaller molecules—peptones in the presence of acidic medium which is due to hydrochloric acid.
• The mucus protects the inner lining of the stomach from the action of the acid under normal conditions.
• In duodenum, pancreatic juice enzyme trypsin also acts upon protein molecules to convert it into peptides and peptones.
• In small intestine intestinal juice enzymes convert proteins, peptones and peptides into amino acids. Proteins are absorbed as amino acids by the intestinal villi.
• The process of absorption of useful part of the digested food through the epithelial surface of villi of small intestine is called absorption.

Egestion :

• The process of removal of undigested food from the body is known as egestion.
• In man, undigested food is removed through the anus.

Function of Liver :

• Liver does not secrete any digestive enzyme yet it is known as the largest digestive gland. It does not secrete any digestive enzyme. It secretes bile juice which is very important for the digestion of food.
• Bile juice does two functions: (i) Bile juice emulsifies fats and facilitates digestion of fats. (ii) It also helps to make the chyme (semi-solid food coming from stomach into intestine) alkaline. In this alkaline medium, pancreatic and intestinal enzymes digest the food.
• Liver weighs between 1-2 kg and is the largest organ in the body. Liver has two lobes, right and left. The right lobe is much larger than the left lob. It is soft and reddish-brown in colour.

Emulsification : Emulsification means a fine dispersion of one liquid in another. In reference to digestion of food, breakdown of large fat globules in the small intestine by the bile juice from liver into fine smaller fat globules so as to increase the efficiency of enzyme action, is called emulsification.

Salivary glands :

• There are three pairs of salivary glands located in the mouth.
• Salivary glands secrete their secretion called saliva in the mouth cavity.
• Saliva contains salivary amylase enzyme or ptyalin which is an amylolytic enzyme. It breaks polysaccharide starch into disaccharide maltose.

Role or Functions of saliva:

• Saliva moistens the food thus helps in making food soft so that it can be chewed and swallowed easily.
• Human saliva contains an enzyme called salivary amylase or ptyalin that converts starch into maltose (a sugar).

Small intestine :

• The small intestine is the site where digestion of food components is almost completed. It is also the place where digested food is absorbed into the blood to transport it to each and every cell of the body.
• The small intestine is designed to provide maximum area for absorption of digested food and its transfer into the blood for its circulation into the body. For this, the inner lining of the small intestine has numerous finger like projections called villi.

Villi :

• The finger-like structures present on the inner surface of the small intestines are called villi. About five millions of villi are present in the intestines, thus they increase the absorptive surface of the intestine considerably and facilitate quick absorption of the digested food.
• The villi are richly supplied with blood vessels which take the absorbed food to each and every cell of the body, where it is utilised for obtaining energy, building up new tissues and the repair of old tissues.

Pancreas:

The leaf-shaped organ presents above the intestine that regulates the metabolism of sugar in the blood is called the pancreas.

Functions of pancreas:

• Pancreas secretes pancreatic juice. The pancreatic juice contains digestive enzymes. The trypsin enzyme for digesting proteins, pancreatic amylase for breakdown of starch and pancreatic lipase to digest fats.
• Pancreas also secretes, two hormones, known as insulin and glucagon. Insulin helps regulate blood sugar levels by signaling cells to absorb glucose from the bloodstream and convert it into energy or store it for later use.
• Pancreatic juice is poured into the duodenum (uppermost part of small intestine) with the help of pancreatic duct.

Significance of peristaltic movement that occur all along the gut during digestion :

• The lining of the digestive canal has muscles that contract rhythmically in order to push the food forward. This movement is called peristaltic movement which occurs all along the gut from upward to downward, i.e., from anterior to posterior.
• This movement is necessary to move the food in a regulated manner along the digestive tract so that it can be processed properly in each part of alimentary canal.

Functions of stomach :

• It stores food for variable duration. For example, carbohydrates remain in stomach for about 1-2 hours, proteins for 3 hours and fats for 3-6 hours,
• Walls of stomach undergo periodic muscular contractions. This helps in
• churning and mixing of food with gastric juices.
• Wall of stomach has numerous gastric glands which secrete gastric juices. Gastric juices contain enzymes and hydrochloric acid which help in digestion.
• Stomach regulates movement of partially digested food into the small intestine.

Gastric glands : It present in the wall of the stomach secrete gastric juice which contains hydrochloric acid, pepsin enzyme and mucus.

• Hydrochloric acid kills bacteria and also makes the medium acidic for the action of pepsin enzyme. It makes food soft. HCI also stops the action of saliva.
• Pepsin enzyme helps in breaking proteins into peptones.
• Mucus protects the inner lining of the stomach from the action of the HCl under normal conditions.

Bile juice :

• Bile juice is secreted by liver and is temporarily stored in gall bladder.
• It does not contain any enzyme. It is an alkaline juice, yellowish-green in colour. Its composition is water, sodium bicarbonates, bile pigments and bile salts.
• Bicarbonates of bile juice makes it alkaline. It helps in (a) making the chime alkaline so that pancreatic enzymes can act on it (b) breaking down the bigger fat globules into very globules thereby increasing the efficiency of enzyme action.

Large intestine :

• The unabsorbed food is sent into the large intestine where its wall absorb more water from this material.
• The rest of the material is removed from the body via the anus.
• The exit of this waste material is regulated by the anal sphincter.

Know This : Dental caries : Dental caries or tooth decay causes gradual softening of enamel and dentine. It begins when bacteria acting on sugars produce acids that softens or demineralises the enamel. Masses of bacterial cells together with food particles stick to the teeth to form dental plaque. Saliva cannot reach the tooth surface to neutralise the acid as plaque covers the teeth. Brushing the teeth after eating removes the plaque before the bacteria produce acids. If untreated, microorganisms may invade the pulp, causing inflammation and infection.

Respiration :

• Respiration is a biochemical phenomenon by which food is oxidised to release
• It occurs in all living cells.
• A process which releases energy is called exothermic reaction. So respiration is an exothermic reaction.
• Gases involved in respiration are called respiratory gases. Two respiratory gases are carbon dioxide and oxygen.
• Respiration is the slow oxidation of organic substances (glucose) which results in step-wise release of energy.
• Less than 50% energy is liberated as heat. Light is liberated in rare cases.
• The heat evolved during respiration is step-wise, hence it does not harm the cells and organelles.
• Many intermediates are produced which are used for the synthesis of many organic compounds.
• Most of the energy is trapped in ATP molecules.
• Respiration reactions are catalysed by enzymes.
• Catabolic process : It is a process in which bigger molecules are broken into smaller molecules. Respiration is a catabolic process.
• Anabolic process : It is a process in which smaller molecules combine to form bigger molecules. Photosynthesis is an anabolic process.

Anaerobic respiration and Aerobic respiration :

The food material taken in during the process of nutrition is used in cells to

provide energy for various life processes. Diverse organisms do this in different ways – some use oxygen to break-down glucose completely into carbon dioxide and water, some use other pathways that do not involve oxygen. In all cases, the first step is the break-down of glucose, a six-carbon molecule, into a three-carbon molecule called pyruvate.

Different ways in which glucose is oxidised to provide energy in various organisms:

• First step of breakdown of glucose (6 carbon molecule) takes place in the cytoplasm of cells of all organisms. This process yields a three carbon molecule compound called Pyruvate or pyruvic acid.
• Further breakdown of pyruvate takes place in different ways in different organisms.

(i) Anaerobic respiration:

• This process takes place in the absence of oxygen. In this case pyruvate is converted into ethanol and carbon dioxide.

C₆H₁₂O₆ —> 2C₂H₅OH + 2CO₂ + Energy

• This type of respiration is observed in yeast and bacteria.
• Less amount of energy (58 kcal) is released during anaerobic respiration.
• Anaerobic respiration is also called fermentation.
• Muscle cells of our body also carry out anaerobic respiration during excessive exercise or during short supply of oxygen.
• When there is lack of oxygen, especially during vigorous activity in our muscles, pyruvate is converted into lactic acid (3 carbon molecule).
• Formation of lactic acid in muscles causes fatigue and cramps.

(ii) Aerobic respiration:

• It occurs in cytoplasm and mitochondria cells.
• Breakdown of food in the presence of oxygen is called aerobic respiration.

It has following two steps:

• First step: The first step which occurs in cytoplasm is called glycolysis. It does not require oxygen. In this step glucose is partially broken down into an intermediate compound (pyruvic acid) and only a small amount of energy is released.

• Second step: This step requires oxygen. In this step, the intermediate compound formed above in first step undergoes complete oxidation. As a result CO₂ and H₂O are produced and a large amount of energy (686 kcal) is released. The complete oxidation of glucose takes place in the mitochondria.

Pyruvate + O2 \(\underrightarrow{mitochondria}\) CO2 + H₂O + Energy

ATP : The energy released during cellular respiration is immediately used to synthesise a molecule called ATP which is used to fuel all other activities in the cell.

• ATP stands for Adenosine triphosphate.

Formation of ATP: The energy released during respiration is used to make ATP

molecules from ADP (adenosine diphosphate) and inorganic phosphate (P).

ADP+ (P) \(\underrightarrow{energy\,from\,respiration}\) ADP~(P) = ATP

When the terminal phosphate linkage in ATP is broken using water, the energy equivalent to 30.5 kJ/mol is released.

ATP + water —> ADP + (P) + Energy

Uses of ATP: ATP is the energy currency for most cellular processes. ATP is used to

supply energy in the cells for the various functions such as contraction of muscles, protein synthesis, conduction of nervous impulse, etc.

Breathing:

The mechanism by which organisms obtain oxygen from the environment and

release carbon dioxide is called breathing.

• It is a mechanical process.
• During breathing, atmospheric air rich in O₂ is taken into the body and the CO₂ rich air is given out of the body.
• The air that we breathe in contains 21% oxygen and 0.04% carbon dioxide and the air that we breathe out contains 16.4% oxygen and 4.45% carbon dioxide.
• This process does not release energy.
• It is known as external respiration.

Respiration in aquatic animals :

• Most of the aquatic animals like prawns, fish, tadpoles use gills as the respiratory organs. Respiration through gills is known as branchial respiration.
• The blood flowing in the capillaries of gills absorb oxygen dissolved in water and gives carbon dioxide to the water passing over them by diffusion through thin epithelium.
• Since the amount of dissolved oxygen in water is quite low compared to the amount of oxygen in the air, the rate of breathing in aquatic organisms is much faster than that seen in terrestrial organisms.

Common features of respiratory organs in aquatic and terrestrial :

Common features of respiratory organs (gills, lungs and skin) as given below :

• A large surface area to absorb sufficient oxygen.
• Thin walls for easy exchange of respiratory gases by diffusion.
• Such respiratory organs have rich blood supply for transport of gases.
• Respiratory surface is very fine and delicate and the surface is always moist.

Mechanism of breathing in human being :

External Respiration (breathing) comprises of inspiration and expiration of air.

(i) Inspiration: During inspiration the volume of the thoracic cavity is increased by two movements, for intake of air.

• The muscles of the diaphragm contract and flatten the diaphragm from its domed position.
• The ribs are raised upward and outward by the contraction of the intercostal muscles.

As a result, the size of chest cavity (thoracic cavity) increases and air rushes into the lungs.

(ii) Expiration: The muscles attached to ribs and muscles of diaphragm relax. As a result, chest cavity becomes smaller in size and air is forced outward through the trachea and nose.

Breathing rate :

• Under normal conditions, the rate of breathing of a normal person is 15 to 18
• times per minute. During vigorous exercise, the rate of breathing increases by about 20 to 25 times per minute.
• Reason: During vigorous exercise, the demand of oxygen increases to release more energy for extra work. Hence, respiratory centre of brain allows increase in breathing rate.

Sequence of parts through which atmospheric air reaches the last part of our lungs :

External nostrils —> Nasal cavity —> Nasopharynx —> Larynx —> Trachea —> Bronchi —> Bronchioles —> Alveolar duct —> Alveolus or alveolar sac.

Nasal Cavity:

• It provides passage to air from external nostrils to pharynx through internal
• In nasal cavity, air is warmed to the body temperature, moistened to avoid friction, and dust particles are trapped by mucus secreted by mucus glands.
• Nasal cavity has fine hair which filters dust particles, pollen etc., from the air passing through them.

Nasopharynx : It is a junction between the nasal cavity and the larynx in man. It is guarded by eplglottis which closes the passage of air while swallowing food.

Trachea : Trachea is supported by 16-120 incomplete C-shaped rings of cartilage. These cartilagenous rings ensure that the  walls of trachea do not collapse even when there is a little air in it.

Lungs :

• In the lungs, the air passage (wind pipe) divides into smaller tubes, called bronchi which in turn form bronchioles.
• The bronchioles terminate in balloon-like structures, called alveoli.
• The alveoli present in the lungs provide maximum surface for exchange of gases. The alveoli have very thin walls and contain an extensive network of blood vessels to facilitate exchange of gases.
• The alveoli increase the surface for exchange of respiratory gases.
• Alveoli walls are thin which facilitate easy exchange of respiratory gases (O₂ and CO₂) by diffusion.
• Alveoli walls are richly supplied with blood capillaries.

Breathing cycle and exchange of gases :

• The blood brings CO₂ from the body parts to be released into the alveoli, and the oxygen in the alveolar cavity is taken up by the alveolar blood capillaries to be transported to all the cells in the body. This forms the breathing cycle (taking in air and giving out air) which is rhythmic.
• But during breathing cycle, when air is taken in and released out, the lungs always contain a residual volume of air. So that there is sufficient time for oxygen to be absorbed and for the carbon dioxide to be released. Thus, exchange of gases is a continuous process.

Diaphragm : Part which separates chest cavity from abdominal cavity

Respiratory pigment in human beings :

Respiratory pigment in human beings is haemoglobin.

Role of haemoglobin:

• Haemoglobin is present in Red blood corpuscles (RBCs).
• Haemoglobin has high affinity for oxygen.
• In lungs, haemoglobin takes up oxygen from the air.
• Blood carries oxy-haemoglobin complex to oxygen deficient body tissues where haemoglobin releases oxygen.

Transportation of oxygen and carbon dioxide in human beings :

• Transport of oxygen: In human beings, the respiratory pigment, called haemoglobin present in the red blood corpuscles carry oxygen to different tissues of the body. The respiratory pigment present in the blood takes up the oxygen from the air in the lungs and carry it to tissues which are deficient in oxygen.
• Transport of carbon dioxide: Carbon dioxide is more soluble in water. Therefore, it is mostly transported in dissolved form through blood plasma. Blood plasma transports carbon dioxide from the tissues to the lungs.
• In tissues, oxygen is used up in respiration and carbon dioxide is released. So, there is less concentration of O₂ and more concentration of carbon dioxide. On the contrary, blood coming from lungs has more concentration of oxygen and less concentration of carbon dioxide. Due to the difference in concentration, oxygen diffuses from blood into tissues and carbon dioxide from tissues into blood.

Respiration in unicellular organisms :

• Unicellular organisms like Amoeba, Paramoecium, Euglena, etc. respire through body surface by the process of diffusion.
• Concentration of oxygen is more in the water than in the body of Amoeba, so oxygen from water diffuses into these unicellular organisms (like Amoeba) through plasma membrane.
• After metabolism, when concentration of carbon dioxide increases inside the Amoeba than in the water, carbon dioxide diffuses out through plasma membrane.

Branchial respiration: Aquatic animals like fish, prawns and tadpoles respire by gills: Respiration through gills is known as branchial respiration.