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LifeEarth happens to be the only known planet having a life. There are beings wh0 live, die and become part of nature again. The living organism can be differentiated from the inanimate entities on various parameters of life processes.Life ProcessMaintenance of living organism is essential even if they are moving, resting or even sleeping.The processes which together perform the function of maintenance of ‘life’ are called as life processes.Nutrition, respiration, circulation, excretion are examples of essential life processes.In unicellular organisms, all these processes are carried out by that single cell.In multicellular organisms, well-developed systems are present to carry out the processes.NutritionNutritionThe process of acquiring food that is needed for nourishment and sustenance of the organism is called nutrition.There are two main modes of nutrition, autotrophic and heterotrophic.Heterotrophic nutrition has subtypes as holozoic, saprophytic and parasitic nutrition.Autotrophic NutritionIf an organism can nourish\xa0itself by\xa0making its own food\xa0using sunlight or chemicals such mode of nutrition is called as autotrophic nutrition.Plants photosynthesize (use light energy) and are called photoautotrophs.Few bacteria use chemicals to derive energy and are called chemoautotrophs.PhotosynthesisPhotosynthesis is an important process by which food is formed.The plants make food using sunlight and water, which provides nourishment to other organism and themselves.Chlorophyll present in the green parts absorbs light energy.This light energy is used to split water into hydrogen and oxygen.Hydrogen is then used to reduce carbon dioxide into carbohydrates, typically glucose.Chlorophyll is essential for photosynthesis and stomata to facilitate intake of carbon dioxide.StomataStomata are pores on the leaves that help in exchange of gases.They are mostly found on the underside of the leaf.Each stoma is guarded by guard cells, which control the opening and closing of the pore.The water content of the guard cells is responsible for their function.Saprophytic NutritionSome organism feed on\xa0dead and decaying organic matter. This mode of nutrition is called saprophytic nutrition.The food is partially\xa0digested outside the body and then it is absorbed.E.g. Fungi are saprophytes.Parasitic NutritionSome organisms feed on the expense of another organism and in turn causing it harm. This is called parasitic mode of nutrition.These organisms live on the body or in the body of a host organism and derive the nutrients directly from the body of the host.E.g. Leech is an ectoparasite while Ascaris\xa0is an endoparasite. Cuscuta is a parasitic plant.Nutrition in AmoebaAmoeba feeds by holozoic mode of nutrition.It engulfs the food particle using pseudopodia, the process is called phagocytosis.The engulfed food gets enclosed in a food vacuole.As the food vacuole passes through the cytoplasm, digestion, absorption and assimilation take place.When the food vacuole opens to outside, egestion of undigested food takes place.Nutrition in ParamoeciumParamoecium also exhibits holozoic nutrition.However, they have cilia that help them to engulf the food through the oral groove.A food vacuole is created enclosing the food.It moves through the cytoplasm, the process is called cyclosis.Food digested in the food vacuole is absorbed by the cytoplasm.Undigested food is given out to a tiny pore\xa0called **** pore or cytopyge.Nutrition in HumansHumans are omnivores, they can eat plant-based food as well as animal-based\xa0food.Being more complex, humans have a very complicated nutrition system.The digestive system has an alimentary canal and associated digestive glands, which together function to nourish the body.There are five stages in human nutrition; Ingestion, Digestion, Absorption, Assimilation and Egestion.Four stages\xa0i.e. ingestion, digestion, absorption and egestion take place in the alimentary canal while assimilation of food takes place in the whole body.Alimentary CanalAlimentary canal in humans is a long tube of varying diameter.It starts with the mouth and ends with the ****.Oesophagus, stomach, small intestine and large intestine are the parts of the alimentary canal.\xa0MouthIt is the opening of the alimentary canal and helps in ingestion of food.The buccal cavity which is present behind the mouth is also commonly referred to as the mouth.The buccal cavity has teeth and tongue.The set of teeth helps in the mastication of food.The tongue has taste buds on it and thus helps in tasting the food.The salivary glands open also in the buccal cavity and pour saliva which initiates the process of digestion.TeethTeeth are the hard structures present in the buccal cavity.They help us to cut, shear and masticate the food we eat.Vertical section of a tooth shows four layers as enamel, dentine, cement and dental pulp.Enamel is outermost, shiny, highly mineralized and hardest part of the human body.Dentine makes the bulk of the tooth and contains 70% inorganic salts.Cement is present at the lining of a tooth and bony socket.The dental pulp is the central soft part of a tooth and contains nerve endings, blood and lymph vessels along with connective tissue.There are four types of teeth in humans, Incisors, canines, molars and premolars, each with a specific function.Incisors cut the food, canines tear the food while molars and premolars crush it.The dental formula in adult humans is 2:1:2:3.Oesophagus & StomachOesophagusThe swallowed food passes into the oesophagus.It is a muscular tube, about 25 cm long, with a sphincter (valve/opening) at each end.Its function is to transport food and fluid, after being swallowed, from the mouth to the stomach.Food is pushed down by peristaltic movements.StomachThe stomach is a thick-walled bag-like structure.Its receives food from the oesophagus at one end and opens into the small intestine at the other end.The inner lining of the stomach secretes mucous, hydrochloric acid and digestive juices.Food is churned into a semi-solid mass in the stomach and is called chyme.Enzymes present in the gastric juice break down the\xa0food.Hydrochloric acid\xa0helps in partial digestion of proteins and also kills harmful bacteria.Mucus secreted by the wall of the stomach\xa0resists the action of HCl on itself.Small IntestineThe small intestine is the longest part of the alimentary canal, about 20 feet long in humans.It has regions, duodenum, the region which follows stomach, jejunum\xa0is the middle part and ileum is the later region which continues further into the large intestine.The internal surface of the small intestine is folded into finger-like\xa0projections called villi.A common pancreatic duct from pancreas\xa0and liver opens into the duodenum.Most of the chemical digestion and absorption takes place in the small intestine.Large IntestineThe large intestine in humans is about 5 feet long.It has two regions, colon ( about 1.5 m) and rectum (10 cm in length in the adult).The region of large intestine after ileum is called colon while the last part is called the rectum.Colon has three regions as, ascending colon, transverse colon and descending colon.At the base of the ascending colon, a small finger-like out-growth is seen and is called\xa0an appendix.It houses many useful bacteria required for digestion of food.Rectum opens to outside by ****.The **** has internal and external **** sphincters.PeristalsisA constant wave-like movement of the alimentary canal right from the oesophagus to the small intestine is called as peristalsis.Muscles present in the wall of the alimentary canal are responsible for peristalsis.This movement helps to push the food through the alimentary canal.Digestive GlandsSeveral glands produce digestive juices that help in digestion of the food.Salivary glands, Gastric glands, Liver, Gallbladder, Pancreas are few to name.Salivary glands secrete saliva which initiates digestion in the mouth itself.Gastric glands\xa0present in the wall of the stomach secrete hydrochloric acid and enzyme pepsin.The liver secretes bile which is stored in the gallbladder. Bile helps in digestion of fats.The pancreas secretes many digestive enzymes and its secretion is called as pancreatic juice.Enzymes like trypsin, chymotrypsin, lipase, amylase are present in the pancreatic juice.PancreasThe pancreas is a long, flat gland present behind the stomach in humans.It is one of the major digestive glands and is of mixed nature i.e. endocrine as well as exocrine.As an endocrine organ, it secretes two hormones called insulin and glucagon which maintain the blood sugar level.As an exocrine gland, it secretes pancreatic juice which is nothing but a mixture of many digestive enzymes.The digestive enzymes secreted by the pancreas include trypsin and chymotrypsin and proteases which digest proteins.It also includes amylase which digests the starch content of the food.Pancreatic lipases are the pancreatic enzymes that help in digestion of fats.\xa0Holozoic NutritionThe mode of nutrition in which animals take their food as a whole is called as holozoic nutrition.In holozoic nutrition, food passes through five steps as ingestion, digestion, absorption, assimilation and egestion.Physiology of DigestionMechanical digestion of food takes place in the buccal cavity where teeth masticate the food, saliva gets mixed and it turns into a bolus.Digestion of starch starts in the buccal cavity itself, with the action of salivary amylase present in the saliva.Salivary amylase converts starch into maltose.In the stomach, the churning of food takes place due to the muscular contraction and relaxation of its wall. It breaks down the food into simpler substances.Digestion of proteins starts in the stomach with the action of pepsin. Proteins are broken down into smaller fragments called peptide by the action of pepsin.The bolus after mixing with gastric juice, turn into a fine soluble form known as the chyme.Chyme enters into the small intestine where complete digestion takes place due to the action of various enzymes present in the pancreatic juice, bile and intestinal juice.The digested food is completely absorbed by the villi and microvilli of the small intestine.Undigested food\xa0then enters into the large intestine.The colon is responsible for absorption of water and salts whereas rectum stores the undigested food temporarily before defaecation.Digestive System in Other AnimalsDigestive systems in different animals vary in structure and function.The structure of the\xa0digestive\xa0system depends on the food habits of the animal.Alimentary canal in herbivores is long as the cellulose content of their plant-based diet takes a long time to digest.On the other hand, the alimentary canal of carnivorous animals is comparatively shorter because meat gets digested faster.Anatomy of Digestive TractThe alimentary canal in humans is approximately 30 feet (9m) long.It starts with the mouth and ends in the ****.Between these two openings, the alimentary canal is the tube of varying diameter.Oesophagus, stomach, small intestine (divided into three regions as duodenum, jejunum and ileum) and large intestine(having two regions as colon and rectum) are the parts of the alimentary canal.Salivary glands, pancreas and liver act as major digestive glands.Glands present in the wall of the stomach and small intestine also contribute towards digestion of food.Role of HClHydrochloric acid in the stomach is secreted by the gastric glands present in its wall.pH of the gastric acid\xa0is usually between 1.5\xa0to\xa03.5This acid serves the following functions:Converts inactive pepsinogen and pro-rennin into active pepsin and rennin respectively.Provides acidic medium for protein digestion.Kills bacteria entered through food and prevents infection.Prevents putrefaction of food in the stomach.A thick layer of mucus secreted by the mucous glands of the stomach prevent itself from the action of the gastric acid.Excess acid\xa0damages gastric mucosa and causes gastric and duodenal ulcers.Salivary GlandsSalivary glands are the exocrine glands that secrete saliva and through a system of ducts, it is poured into the mouth.In humans, three major pairs of salivary glands are present, parotid, submandibular and sublingual.In healthy individuals between 0.5 to 1.5 litres of saliva is produced per day.Saliva serves the following functions in the oral cavity.It lubricates and protects the soft and hard tissues of the oral cavityIt also gives protection from dental cariesSaliva prevents microbial growth in the oral cavity.Saliva can encourage soft tissue repair by decreasing clotting time and increasing wound contractionSaliva contains the enzyme amylase that hydrolyses starch into maltose and dextrin. Hence saliva allows digestion to occur before the food reaches the stomachSaliva acts as a solvent in which solid particles can dissolve in and enter the taste buds located on the tongue.\xa0Heterotrophic NutritionWhen an organism depends on others for food, such a mode of nutrition is called as a heterotrophic mode of nutrition.These organisms depend on autotrophs for their nutritional requirements.E.g. Animals which eat plants as their food are called herbivores.Animals which eat other animals as their food are called carnivores.Holozoic, saprophytic and parasitic nutrition are all types of heterotrophic nutrition.Glandular EpitheliumMany small glands present in the inner layer of the stomach and intestine take part in the digestion of food.These glands are present in the epithelial lining of the stomach and intestine.The glands present in different regions of the stomach are called gastric glands.They are responsible for the secretion of mucus, hydrochloric acid and enzymes like pepsinogen.The glands present in the epithelial lining of the small\xa0intestine and large intestine are called intestinal glands.Glands of the small intestine are responsible for the secretion of intestinal juice also called succus entericus.Intestinal juice contains\xa0hormones,\xa0digestive enzymes, alkaline mucus, substances to neutralize\xa0hydrochloric acid\xa0coming from the\xa0stomach.Intestinal juice completes the digestion started by the pancreatic juice.Glands of the large intestine are associated with absorption of water and electrolytes.Villi and Micro VilliComplete digestion and absorption of food take place in the small intestine.Pancreatic juice coming from the pancreas, bile from the liver and intestinal juice secreted by the intestinal glands complete the digestion of food material.All the digested nutrients are absorbed by the long finger-like projections present in the ileum of the small intestine.These small finger-like\xa0projections of the inner wall of intestine are called as villi (singular: villus).Each villus has its cell membrane of the lumen side again folded into microscopic processes, called microvilli.Villi increase the internal surface area of the intestinal walls making available a greater surface area for absorption.Digested nutrients pass into the semipermeable villi through diffusion.Villi also help in chemical digestion of food by secreting digestive enzymes.\xa0LiverThe liver is the largest and major digestive gland of humansLiver, in humans, is located in the upper right-hand portion of the abdomen.This\xa0organ is dark reddish-brown in colour due to an extensive blood supply.Some of the important functions of the liver are as follows:It secretes bile that helps in digestion.It filters the blood coming from the digestive tract before passing it to the rest of the body.It detoxifies various metabolites and drugsThe liver makes proteins important for blood clotting and other functions.It stores and releases glucose as needed.It processes\xa0haemoglobin, from the dead and worn out RBCs, for the iron content (the liver stores iron).Conversion of harmful ammonia to urea takes place in the liver.Digestive JuicesPancreatic juice, bile and intestinal juice (succus entericus) are collectively called as digestive juices.A common duct from digestive glands pours the secretions into the duodenum.Chyme enters into the small intestine where complete digestion takes place due to the action of various enzymes.In the duodenum, the acidity of chyme is turned to alkalinity\xa0by the action of bile coming from the liver. This is necessary for pancreatic enzyme action.Bile also emulsifies the fats into smaller globules.Pancreatic and intestinal amylases break down the carbohydrates into glucose.Trypsin and chymotrypsin are the proteases responsible for the breakdown of proteins finally into amino acids.Lipase is the enzyme which acts on the emulsified fats and breaks them down into glycerol and fatty acids.Water Absorption in Large IntestineThe large intestine is not involved in the digestion of food or absorption of nutrients.The major function\xa0of the large\xa0intestine\xa0is to absorb water from the remaining indigestible food matter and make the stool solid.The large intestine also helps in absorption of vitamins made by bacteria that normally live in the large intestine.The innermost layer of the large intestine also acts as a barrier and protects from microbial infections and invasions.Rectum stores the undigested food temporarily until defaecation.RespirationIntroduction to RespirationRespiration broadly means the exchange of gases.Animals and plants have different means of exchange of gases.At a cellular level, respiration means the burning of the food at the for generating the energy needed for other life processes.Cellular respiration may take place in the presence or absence of oxygen.\xa0Respiration in HumansThe human respiratory system is more complex and involves breathing, exchange of gases and cellular respiration.A well defined respiratory system helps breathing and exchange of gases.Breathing involves the inhalation of oxygen and exhalation of carbon dioxide.The gaseous exchange takes place in the lungs and oxygen is supplied to all cells of the body.Cellular respiration takes place in each and every cell.Respiratory SystemThe human respiratory system involves the nose, nasal cavities, pharynx, larynx, trachea/windpipe, bronchi, bronchioles and alveoli.Bronchioles and alveoli are enclosed in a pair of lungs.The rib cage, muscles associated with the rib cage and diaphragm, all help in inhalation and exhalation of gases.Exchange of gases takes place between an alveolar surface and surrounding blood vessels.Alveoli provide a large surface area for exchange of gases.Physiology of RespirationBreathing in humans is facilitated by the action of internal intercostal and external intercostal muscles attached to the ribs and the diaphragm.When the dome-shaped diaphragm contracts and becomes flattened and the rib cage is expanded due to the action of intercostal muscles, the volume of the lungs increases, pressure there drops down and the air from outside gushes in. This is inhalation.To exhale, the diaphragm relaxes, becomes dome-shaped again, chest cavity contracts due to the action of intercostal muscles, the volume inside the lungs decreases, pressure increases and the air is forced out of the lungs.Inhaled air increases the concentration of oxygen in the alveoli, so oxygen simply diffuses into the surrounding blood vessels.Blood coming from cells has more concentration of carbon dioxide than outside air and thus carbon dioxide simply\xa0diffuses out of the blood vessels into the alveoli.Thus, breathing takes place due to the combined action of intercostal muscles and diaphragm while the exchange of gases takes place due to simple diffusion.Inhalation and ExhalationThe process of taking in air rich in oxygen is called\xa0inhalation.Similarly, the process of giving out air rich in carbon dioxide is called\xa0exhalation.One breath comprises one inhalation and one exhalation.A person breathes several times in a day.The number of times a person breathes in one minute is termed as his/her\xa0breathing rate.DiffusionDiffusion is the\xa0movement of molecules from high concentration area to the low concentration area without spending any energy.\xa0Cellular RespirationCellular respiration is set of metabolic reactions occurring inside the cells to convert biochemical energy obtained from the food into a chemical compound called adenosine triphosphate (ATP).Metabolism\xa0refers to a set of chemical reactions carried out for maintaining the living state of the cells in an organism. These can be divided into two categories:Catabolism\xa0– the process of breaking molecules to obtain energy.Anabolism\xa0– the process of synthesizing all compounds required by the cells.Therefore, respiration is a catabolic process, which breaks large molecules into smaller ones, releasing energy to fuel cellular activities.Glycolysis, Krebs cycle and electron transport chain are the important processes of the cellular respiration.\xa0Aerobic RespirationAerobic respiration is a process in which the food i.e. glucose is converted into energy in the presence of oxygen.The general equation of aerobic respiration as a whole is as given below-Glucose + oxygen\xa0⇒Carbon-dioxide + Water + EnergyThis type of respiration takes place in animals, plants and other living organisms.Respiration in Lower AnimalsLower animals lack a sophisticated respiratory system like lungs, alveoli etc.Respiration\xa0in them\xa0takes place by simple exchange mechanisms.Animals like earthworms take in gases through their skin.Fishes have gills for gaseous exchange.Insects have a tracheal system, which is a network of tubes, through which air circulates and gaseous exchange takes place.Frogs breathe through their skin when in water and through their lungs when on land.Respiration in MusclesRespiration in muscles can be anaerobic when there is not enough oxygen.Glucose gets broken down to\xa0carbon dioxide and lactic acid.This results in the accumulation of lactic acid that makes the muscles sore.This type of anaerobic respiration is also known as lactic acid fermentation.ATPIt is the energy currency of the cell.ATP stands for Adenosine Tri-Phosphate.This molecule is created as a result reaction like photosynthesis, respiration etc.The three phosphate bonds present in the molecule are high energy bonds and when they are broken, a large amount of energy is released.Such released energy is then used for other metabolic reactions.\xa0Respiration in PlantsUnlike animals and humans, plants do not have any specialized structures for gaseous exchangeThey have stomata (present in leaves) and lenticels (present in stems) which are involved in the exchange of gases.Compared to animals, plant roots, stems, and leaves respire at a very lower rate.TranspirationTranspiration is a biological process in which water is lost in the form\xa0of water vapour from the aerial parts of the plants.This process occurs mainly through the stomata where the exchange of gases (oxygen and carbon dioxide) occurs.Transpiration helps in the transportation of water from roots to upper parts of plants and this is explained by ‘transpirational pull theory’.Loss of water, especially from leaves, acts as a straw effect and pulls water upwards from roots.Transpiration also acts as an excretory mechanism in plants as it helps to get rid of excess water.Why Do We Need LungsIn unicellular organisms like an amoeba exchange of gases takes place through a general body surface by osmosis.In lower animals like an earthworm, the gaseous exchange takes place through their moist skin.The requirement of oxygen is sufficiently met by these ways.But as the animal starts becoming more and more complex, for example, human, the requirement of oxygen cannot be met alone by diffusion.Moreover, diffusion will not be able to supply oxygen to the deep-seated\xa0cells.This difficulty has led to the evolution of a\xa0more complex mechanism of gaseous exchange and that is the development of lungs.The alveoli present in the lungs provide a large surface area required for the necessary gas exchange.Transportation in Human BeingsTransportationAll living organisms\xa0need a few necessary components like air, water, and\xa0food for their survival.On our regular basis, animals ensure these elements by breathing, drinking and eating.The required\xa0elements are transported to their body cells and tissues by a transportation system.In plants, the vascular tissue is responsible for transporting the substances.Transportation in HumansTransportation in humans is done by the circulatory system.The circulatory system in humans mainly consists of blood, blood vessels and heart.It is responsible for the supply of oxygen, nutrients, removal of carbon dioxide and other excretory products.It also helps to fight the infections.HeartThe muscular organ which is located near the chest slightly towards the left in the\xa0thoracic region.The heart is the main pumping organ of the body.The human heart is divided into four chambers\xa0which are involved in the transportation of oxygenated and deoxygenated blood.The upper two chambers are called atria whereas the lower two chambers are called as\xa0ventricles.\xa0The flow of blood through the\xa0heart is as follows:Blood VesselsBlood vessels carry blood throughout the body.There three types of blood vessels; arteries, veins and blood capillaries.Arteries carry oxygenated blood and veins carry deoxygenated blood.Gaseous exchange takes place between blood and cells at capillaries.Difference between Arteries and Veins\xa0Blood PressureThe pressure exerted by the blood when it flows through the blood vessels is called blood pressure.There are two different variants of blood pressure; the systolic and the diastolic blood pressure.The pressure exerted on the walls of arteries when the\xa0heart is filling\xa0with blood is called\xa0diastolic\xa0pressure. It constitutes the\xa0minimum\xa0pressure on arteries.The normal range of diastolic blood pressure should be 60 – 80 mm Hg.The pressure\xa0exerted on the walls of arteries when the\xa0heart is pumping\xa0the blood is called\xa0systolic\xa0pressure. It constitutes the\xa0maximum\xa0pressure applied on the arteries.The normal range of systolic blood pressure should be 90 – 120 mm Hg.BleedingBleeding occurs when the blood vessels rupture.Bleeding is stopped by the platelets that help in the clotting of blood at the site of the injury.Blood Clotting\xa0is the process of forming a\xa0clot in order to prevent excess loss of blood from the body.It is a gel-like mass which is formed by the platelets and a fibre-like protein in the blood.Double CirculationIn the human body, blood circulates through the heart twice.Once it goes through the heart during pulmonary circulation and second time during systemic circulation.Hence, circulation in human beings is called\xa0double circulation.\xa0Transportation in PlantsTransportation in PlantsTransportation is a vital process in plants.The process involves the transportation of water and necessary\xa0nutrients\xa0to all parts of the plant for its survival.Food and water transportation takes place separately in plants.Xylem transports water and phloem transports food.\xa0PhloemThe phloem is responsible for translocation of\xa0nutrients\xa0and sugar like carbohydrates, produced by the leaves to areas of the plant that are metabolically active.Sieve tubes, companion cells, phloem fibres, and phloem parenchyma cells are the components of this tissueThe flow of material through phloem is bidirectional.\xa0TranslocationTransport of food in the plant through phloem via a process such as mass flow is called as\xa0translocation.Photosynthates i.e. sugars and organic molecules such as amino acids, organic acids, proteins and inorganic solutes like potassium, magnesium, nitrate, calcium, sulfur and iron from source tissues (mature leaves) to the sink cells (areas of growth and storage) are transported through the phloem.Material like sucrose is loaded from leaves to phloem using the energy of ATP.Such a transfer increases the osmotic pressure causing movement of water from nearby cells into\xa0in phloem tissue and the material gets transported through the phloem.The same pressure is also responsible for the transfer of substances from phloem to tissues where food is required.Thus the bulk flow of material through phloem takes place in response to an osmotically generated pressure difference.\xa0XylemXylem tissue transports water in plants from root to all other parts of the plant.Xylem tissue is made up tracheids, vessels, xylem fibres and xylem parenchyma.The flow of water and minerals through xylem is always unidirectional.Root PressureConduction of water through the xylem, from\xa0roots to upper parts of plants, is due to many forces acting together.One of the forces responsible for this is root pressure.Root pressure is osmotic pressure within the cells of a root system that causes sap to rise through a plant stem to the leaves.Root pressure helps in the initial transport of water up the roots.Transport of WaterWater is absorbed by the roots and is transported by xylem to the upper parts of the plant.Imbibition, osmosis, root pressure and transpiration are the forces that contribute towards the upward movement of water, even in the tallest plants.Imbibition is a process in which water is absorbed by the solids. E.g. seeds take up water when soaked.Osmosis is a process where water moves from the area of its lower concentration to the area of its higher concentration.At the roots, the cells take up ions by an active process and this results in the difference of concentration of these ions.It leads to movement of water, in the root cells, by osmosis.This creates a continuous column of water that gets pushed upwards. This is root pressure.Transpiration contributes to the upward movement of water by creating a staw effect.It pulls the water column upwards as there is a continuous loss of water from leaves.All these forces act together for water transport through the xylemExcretion in HumansExcretionExcretion is the process of removal of metabolic waste material and other non-useful substances.Organisms like animals have an advanced and specialized system for excretion.But plants lack a well-developed excretory system like that in animals.They do not have special organs for excretion and thus\xa0excretion in plants is not so complex.Excretion in Unicellular OrganismIn unicellular organisms such as amoeba and bacteria, the waste product is removed by simple diffusion through the general body surface.Unicellular organisms like the amoeba, paramecium excrete excess through tiny organelles called contractile vacuoles.Undigested food in unicellular animals is excreted\xa0when the food vacuole merges with the general body surface and opens to outside.Excretory System of HumansThe excretory system in humans includesa pair of kidneys,a pair of ureters,a urinary bladder andurethra.It produces urine as a waste product.KidneysPaired kidneys are the main excretory organs of the body.They are basically the filtration units of the human body.Each kidney is made up many tiny filtration units called\xa0nephrons.Kidneys\xa0perform crucial functions like:Filtering waste materials, medications, and toxic substances from the blood.Regulation of osmolarity i.e. fluid balance of the body.Regulation of ion concentration in the body.Regulation of pH.Regulation of extracellular fluid volume.Secreting hormones that help produce red blood cells, promote bone health, and regulate blood pressureNephronNephrons are the structural and functional unit of kidney.Each kidney has millions of nephrons and it forms the basic structural and functional unit of the kidney.Each nephron has two parts: Malpighian body and renal tubule.Malpighian body is made up of cup-like structure called Bowman’s capsule which encloses a bunch of capillaries called glomerulus.They together filter waste materials along with many useful substances.Renal tubule has regions called proximal convoluted tubule, Loop of Henle and distal convoluted tubule.These regions\xa0absorb\xa0back\xa0useful\xa0substances into the blood and also filter remaining waste substances.The output from nephrons is called urine.\xa0HaemodialysisWhen the kidneys fail, it results in a lot of complications and to\xa0compensate this situation a technology called dialysis has been developed.It uses a machine filter called a dialyzer or artificial kidney.This is to remove excess water and salt, to balance other electrolytes in the body and remove waste products of\xa0metabolism.Blood from the body is removed and flowed through a series of tubes made up of a semipermeable membrane.A dialysate flows on the other side of the membrane, which draws impurities through the membrane.Excretion in PlantsThe cellular respiration, photosynthesis, and other metabolic reactions produce a lot of excretory products in plants.Carbon dioxide, excess water produced during respiration and nitrogenous compounds produced during protein metabolism are the major excretory products in plants.Plants produce two gaseous waste products i.e. oxygen during photosynthesis and carbon dioxide during respiration.Excretion of gaseous waste in plants takes place through stomatal pores on leaves.Oxygen released during photosynthesis is used for respiration while carbon dioxide released during respiration is used for\xa0photosynthesis.Excess water is excreted by transpiration.Organic by-products generated by the plant\xa0are stored in different forms in different parts.The gums, oils, latex, resins, etc. are some waste products stored in plant parts like barks, stems, leaves, etc.Eventually, plants shed off these parts.Few examples of the excretory products of plants are\xa0oil produced from orange, eucalyptus, jasmine, latex from the rubber tree, papaya tree, and gums from acacia.Sometimes plants even excrete into the soil.