Life processes

Sexual Reproduction : Why and how!

What is the significance of sexual reproduction?

In the previous article on breeding we learned about the various methods of asexual reproduction. So, what are the limitations of asexual reproduction?

During the process of reproduction, the genetic code present in the DNA is copied. This process is quite efficient with a tiny margin of error. However, errors do happen, and these mistakes are the source of genetic variation in nature. Genetic variation is necessary for propagation and survival of the species. In asexual reproduction, as each progeny is an exact copy of the parent, the scope of genetic variation is quite limited. Therefore, to ensure genetic variation, organisms have developed the sexual form of reproduction.

Apart from the most primitive form of life like eukaryotes and prokaryotes, most plants and animals reproduce sexually. So, what does sexual reproduction mean?

Can a hen produce a chick by herself? No, the chicken needs the rooster to produce a chick. Similarly, a cow cannot produce a calf by herself. She needs a bull to produce progeny. Thus, in sexual reproduction, both males and females are required to produce an offspring.

The male and female of a species produce gametes. Gametes are specialised cells that combine to produce progeny. The male gamete is usually mobile while the female gamete is stationary and has stores of energy. The male gamete is also called sperm, and the female gamete is called ova or ovum.

In sexual reproduction, the male and female gamete combine to produce a zygote. However, this creates a unique problem. Each time the gametes fuse, the amount of DNA would double. If this process continues, then the amount of DNA will grow exponentially and over a period there will only be DNA on earth.

So, how does nature work around this problem?

Simple, nature keeps the amount of DNA constant across generations by halving the amount of DNA present in gametes. For example, all human cells contain 46 chromosomes. However, the gametes i.e. Sperm and Ova contain only 23 chromosomes.

When the sperm combines with the Ovum, the number of chromosomes is restored back to 46. Thus, the amount of DNA is preserved across generations.

Thus, the non-reproductive cells have twice the amount of DNA as compared with gametes. The non-reproductive cells are diploid, and the gametes are haploid.

As sexual reproduction requires the presence of male and female gametes, organisms have well developed male and female reproductive organs. The male and female reproductive organs are the cause of the differences in the bodies of the male and female organisms.

We shall learn more about sexual reproduction in plants and humans in another article.

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Life processes: Human excretory system

In our previous article, we had discussed the process of excretion in general. In this article, we shall discuss the human excretory system in detail. We have previously learnt that carbon dioxide, urea and other solutes are the primary waste products of metabolism. Lungs are the primary organs responsible for the excretion of carbon dioxide while the kidneys are the main organs that excrete urea and other nitrogenous waste products from the body.

The human excretory system comprises of two kidneys, the ureters, the urinary bladder and the urethra. The kidneys are two bean-shaped organs that occupy a place over the loins. The kidneys receive nearly 25% of the cardiac output. The kidneys are made up of small filtering units called nephrons. Each kidney contains nearly a million nephrons. Each nephron has an artery that brings blood into it and another which takes blood out of it. The ?former is called as the afferent arteriole, and the latter is called as the efferent arteriole. The afferent arteriole, inside the nephron, leads to a network of fine tubes with a single layer of cells lining the inside of the tubes. This tuft of tubes is called the glomerulus. Blood enters the glomerulus under pressure, and the plasma is filtered out into the space between the glomerulus and the wall of the nephron. This plasma then flows through a series of tubes, where glucose, amino acids and other essential substances are reabsorbed. Within these tubules, the urine is concentrated and finally delivered to a structure called as the renal pelvis. From the renal pelvis, the urine flows through two long tubes called ureters (one for each kidney).

The ureters join the urinary bladder, a storage organ of the human excretory system. The urinary bladder is a hollow muscular organ that holds urine till it is voided outside through the urethra. The internal and external sphincters are two circular muscles that control the voiding of urine.

As the bladder fills up with urine, it sends signals through nerves to neurons within the spinal cord. A micturition reflex is set up; that is characterised by contraction of the urinary bladder and relaxation of the internal sphincter. This process is involuntary, yet we can hold urine as the external sphincter is under voluntary control.

The human excretory system is similar in both males and females up to the bladder. In men, the urethra merges with the genital tract to form the urogenital tract, while, in women, the urethra and the genital tract remain separate, although nearby.

The human excretory can function adequately, even when a part or one whole kidney is damaged. It is for this reason that diseases of the kidney are typically found late in the course?after substantial damage has occurred to the kidneys. It is therefore, important that we take adequate precautions to safeguard our kidneys. A minimum urine output of 1200 ml/day is required for the proper health of the kidneys. One of the easiest ways to improve the functioning of the kidneys is to drink plenty of water. We should drink at least eight glasses of water every day.

To conclude, the human excretory system, cComprising two kidneys, ureters, urinary bladder and urethra?is the primary mode of excretion of nitrogenous waste substance.The human excretory system produces urine, the final waste product. The urine consists urea, salts, solutes and other nitrogenous waste products dissolved in water. This urine is then voided out through the urethra.

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Life Processes: Excretion

Excretion is the process that removes waste products of metabolism from the body. In our previous articles on life processes, we learnt about nutrition, respiration and transportation. Each of these processes produces waste that needs elimination from the body. The process of respiration produces carbon dioxide. ?Similarly, many solutes and nitrogenous substances are produced as waste products of metabolism. These also require to be removed from the body.

The mode of excretion of waste products is different from one organism to another. A unicellular organism like amoeba or bacteria can excrete the waste products by simple diffusion. However, simple diffusion by itself is unsuitable for a multicellular organisms like humans.

Plants to excrete waste products of metabolism. They have a well-developed system of excretion, comprising the stomata in the leaves. These stomata serve to expel oxygen that is a waste product of plant metabolism. The stomata also remove excess water from the leaves. Apart from the stomata, plants also excrete waste products through secretions like resin and gum.

Before we study human excretion, we shall discuss the types of wastes produced during metabolism.

Metabolism is a process of growth, repair and development. During this process, energy-rich compounds like glucose and fats are broken down and the synthesis of proteins and other complex molecules occurs. During this process, the primary waste products produced are carbon dioxide, water, solutes and nitrogenous compounds.

Carbon dioxide and water are the waste products of respiration. Metabolism of glucose produced six molecules of carbon dioxide. The plasma dissolves most of the carbon dioxide. Here it exists in its ionised form- bicarbonate. The following chemical equation will illustrate this reaction clearly:-

CO2 + H2O?? = H+ + HCO3

The above reaction is a reversible reaction. In the lungs, the hydrogen and bicarbonate combine to form water and carbon dioxide. The lungs then expel the carbon dioxide into the air. An enzyme called carbonic anhydrase is the catalyst this reaction. Plasma, lungs and red cells are the primary sources of this enzyme.

Proteins are nitrogenous substances that are the building blocks of life. Their synthesis leads to the formation of nitrogenous waste products. In humans, the nitrogenous waste products are excreted as urea. Urea, other solutes and water was all excreted by the kidneys. We shall discuss the human excretory system in detail elsewhere.

To conclude, excretion is a vital life process that helps removal of waste products of metabolism from the body.

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The human circulatory system

The human circulatory systems consist of the heart, arteries, and veins. The lungs also form an important component of the circulatory system. It has been discussed previously in our article on human respiration.

The heart, a muscular organ is the centre of the human circulatory system. The heart is located in the chest, towards the left side. It contains four chambers- the right atrium, the right ventricle, the left atrium and left ventricle. The left atrium and ventricle are connected in series and form the left side of the heart. The left heart receives blood (oxygenated) from the lungs and pumps it to various parts of the body. The pulmonary veins, four in number, ?bring oxygenated blood from the lungs to the left atrium. The outflow from the left heart goes into a large blood vessel (in fact, the biggest blood vessel in the body) – the aorta. The aorta has many branches at different levels that supply blood to the various organs.

The right side of the human circulatory system consists of the right atrium and ventricle that are connected in series. The right heart receives deoxygenated blood from the various parts of the body and pumps it through the pulmonary artery into the lungs. Within the lungs, by the process of simple diffusion, oxygen is taken in by the haemoglobin present in red cells, and the carbon dioxide is excreted out into the air. This entire process is called respiration.

The heart pumps blood through the body continuously. In never stops during a person?s lifetime. The pumping action of the heart is finely co-ordinated. The atria relax to receive the blood. Once it is filled up, the atria contract, pushing the blood into the ventricles. During this process, the ventricles are in a relaxed state. Once the ventricles fill up with blood, they contract, pumping the blood into the blood vessels. The pumping action of the heart is noticeable in many ways. We can feel the heart pumping if we put our hands over the left side of the chest. The heart produces sounds while pumping blood. The heart sounds are of two types- lub and dub. These sounds are heard by placing a stethoscope on our chest. A doctor is trying to listen to the heart sounds when (s) he is listening to your chest.

The function of the heart can also be felt by feeling the pulse. When the heart pumps blood, it does so in waves. These waves are palpable be putting our finger on any artery. The commonest way to palpate a pulse is by placing a finger on the wrist where the radial artery is palpable.

Thus, to summarise, the human circulatory system is a system consisting of a central pump (the heart) that is connected to a network of tubes (the blood vessels). It functions to transport nutrients, oxygen and waste products through the body.

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Life processes: Human circulation

The human circulation is a specialised system that performs the task of transport of nutrients, oxygen and waste products of life various life processes in the human body. The human circulation is similar in many ways to transportation in plants. There are many distinguishing features as well.

The human circulation also has a well-developed system of tubes to transport various substance across the body. These tubes are called arteries and veins. A series of small tubes called capillaries, connect the arteries and veins. Blood, a liquid connective tissue, flows within this network of tubes. Plasma is the aqueous component of the blood. Plasma is mostly water in which salts, proteins and cells are dissolved or suspended.

The cells that are suspended are mainly red blood cells, white blood cells and platelets.

The red blood cells (RBC?s), as their name denotes, are red in colour. The red cells are the predominant cells in the blood. The RBC?s contain a pigment-haemoglobin. Haemoglobin is the oxygen-carrying molecule in the blood. As the human body is a complex structure, its energy requirements are high. Simple diffusion is insufficient for this purpose. Therefore, the human circulation contains haemoglobin, which can bind to 4 molecules of oxygen, thus greatly enhancing the efficiency of oxygen transport.

We all have seen how plumbing works in our houses. There is a tank that is used to store water. A network of tubes then conveys this water to our houses. A pump fills the tank with water. ?Will water flow in the taps if there is no pump? No. Similarly, the human circulation also requires a pump to circulate blood through the body. The heart pumps blood into the circulatory system. The heart is a muscular organ about the size of one?s fist and is made up of 4 chambers.

This system of chambers separates the human circulation into two closed, parallel systems- the arterial system or arteries for transporting oxygenated blood and the venous system or veins for carrying de-oxygenated blood.

Also to carrying oxygen, human circulation also plays other roles. The blood circulates to the intestines from where it absorbs the products of digestion. It takes waste products of life processes from cells to the kidneys and lungs for excretion. It also serves to transport some signals that modulate growth, development and metabolism, i.e. hormones.

Now that we have learnt about the human circulation, what will happen if one part of the system dysfunctions? The pump can get damaged, the tubes can get damaged, there could be a block in the plumbing, and there could be an abnormal leak. All these can cause problems with the functioning of human body and can lead to diseases like heart attack, thrombosis, bleeding, etc.

Thus, to summarise, the human circulation is the process that transports nutrients, oxygen and waste matter through the human body.

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Life Processes: Transportation in Plants

Transportation in plants is an important life process. In our previous articles on life processes, we learnt that plants utilise water and carbon dioxide to produce oxygen and energy in the form of starch. The water and other nutrients like nitrogen and phosphorus are present in the soil and plants absorb these from the soil. Plants have specialised tissue called roots that perform this function. The roots of the plants not only anchor the plant to earth, they also have fine fibrils that increase the surface area available for absorption of water and nutrients manifold.

Once water and other nutrients are absorbed, they need to be transported to the leaves, where the actual process of photosynthesis happens. In small plants, transportation of water and nutrients can happen by diffusion. However, diffusion is unsuitable for moving water and nutrients across large distances, especially in very tall trees. Therefore, trees have a well-developed system of transportation- namely the xylem and phloem. The xylem is a specialised system of transportation in plants that helps move water and nutrients absorbed from the roots to the leaves. There are two processes going on here, firstly the salt ions absorbed by the roots create a concentration gradient. This concentration gradient helps in the movement of water across the cell membranes. Secondly, the stomata in the leaves lose water by a process called as transpiration. Transpiration creates a negative suction pressure that helps move the water and other nutrients through the xylem. This system is quite similar to a wick. Have you ever noted that if you dip one of a wick in water or oil, in a matter of few minutes, the water or oil will slowly move across the whole length of the wick? This process is called the capillary action. Thus, transportation in plants is dependent on diffusion, capillary action of the transportation systems and also transpiration. Together, these processes can transport many litres of water from the soil to the leaves in any day. During the night, this process does not stop but continues at a slower rate as transpiration is shut down.

Similarly, substances like glucose, amino acids and fatty acids move from the leaves to storage organs like roots, fruits and seeds. The phloem is a specialised tissue that transports these substances from the leaves to other parts of the plants. Unlike the xylem, transportation through the phloem is driven by energy. Therefore, this is an active process and can proceed even against a concentration gradient. Plants use the energy stored ATP bonds to accomplish this task.

Thus, we have learnt that transportation in plants is a specialised process by which plants transport water and other nutrients to the leaves and then transport the finished products to their organs of storage. The Xylem and Phloem are specialised systems that perform this task. Students of class 10 science should be familiar with the process of transportation in plants as this is an important topic.

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Human Respiration: The vital force of life!

Human respiration is the process by which human beings take in oxygen and excrete carbon dioxide. In our previous articles, we learnt about respiration in general. In this article, we will look at human respiration.

Respiration is an important life process. Respiration releases the energy stored in food for use by cells. As humans are multicellular organisms, we have developed a specialised set of organs to help with respiration.

The organ system that performs to work of human respiration is called the respiratory system. The respiratory system consists of the lungs, which are the primary organs of respiration, along with the airways and the circulatory system.

A robust circulatory system is necessary for the function of human respiration. Therefore, these systems are often referred to as the cardio-respiratory system.

The respiratory system starts from the nose and oral cavity. Both the nose and the oral cavity can take in air, but the nose is designed to be an air passage. The nose draws the air in and it flows through the larynx, trachea, and bronchi to reach the lungs. Airways is the collective name given to these structures. The airways, apart from being conduits of the inspired air, also modify the air by humidifying it. The nose also contains fine hair, which filter particulate matter from the air. Also, the airways also bring the temperature of the inspired air to body temperature. The airways consist of a scaffolding of cartilage. These cartilages prevent the collapse of airways during breathing.

The lungs are large spongy organs that occupy most of the thoracic cavity in humans. The lungs are composed of thousands of units called as alveoli. These alveoli are the basic gas exchange units of the lung. The walls of the alveoli are comprised of a single layer of endothelial cells. Beneath these cells, lie the blood vessels of the lung. The partial pressure of oxygen in the inspired air is greater than the partial pressure of oxygen in the blood. Therefore, oxygen diffuses into the blood from the air. Similarly, the partial pressure of carbon dioxide is greater in the blood. Therefore, carbon dioxide diffuses from blood into the air.

Humans and other advanced living beings have developed a sophisticated system to improve the efficiency of oxygen transport. Simple diffusion is good enough of single-celled organisms, but we humans need to extract large quantities of oxygen from the air. In human respiration, this function is performed by the haemoglobin. The Haemoglobin is a pigment that binds to oxygen. Each molecule of this pigment can bind four molecules of oxygen, thus enhancing the efficiency of human respiration. This oxygen reacts with glucose within the cells to produce energy that is stored by cells as ATP.

One question naturally arises. What is the difference between breathing and respiration?

Breathing refers to the mechanical aspects of respiration. On the other hand, respiration refers to the entire process, from taking in air through the nose, right up to the aerobic respiration that occurs at the level of the individual cell.

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Respiration: Aerobic Vs. Anaerobic

In the previous articles, we learnt that respiration is an important life process. Respiration is the process by which cells convert the food we eat into energy. This process can occur in the presence of oxygen or without oxygen. The former is called aerobic respiration, and the latter is called anaerobic respiration.

Carbohydrates are broken down into glucose during the process of nutrition or digestion. This glucose is utilised by the cells for their energy requirements. Glucose is a six-carbon molecule that produces a ?three-carbon molecule in the cytoplasm of the cell. This three carbon molecule is called pyruvate, and this process does not require oxygen.

The mitochondria then take up the pyruvate, where it is broken down to carbon dioxide and water in the presence of oxygen. Everyone molecule of pyruvate generates three molecules of carbon dioxide. The energy released in the process of respiration is used by the cells to produce a molecule called Adenosine Triphosphate (ATP).

ATP is the energy currency of the living world. Wherever there is a need for energy, the phosphate bonds in ATP are broken down and the energy stored in these bonds for the needs of the cells.

The pyruvate can also be converted into lactic acid by metabolism in the absence of oxygen. You may have suffered from cramps after strenuous muscular activity. These cramps are due to the accumulation of lactic acid in the muscles.

The aerobic respiration process is dependent on the presence of oxygen. So, what is the source of oxygen for all living organisms?

The air we breathe is 21% oxygen. Single-celled organisms like bacteria and fungi can take in oxygen directly from the air by a process of diffusion. Similarly, plants and trees have stomata on the underside of the leaves. It is through this stomata that the exchange of oxygen and carbon dioxide takes place.

During daytime, plants use carbon dioxide and water in the process of photosynthesis. This process produces oxygen, and it diffuses out of the leaves through the stomata. During the night, photosynthesis cannot occur, therefore, oxygen is taken up by the plants and carbon dioxide is released into the air.

Single-celled organisms can take in oxygen by the process of diffusion. However, direct diffusion from the atmosphere is not suitable for multi-cellular organisms like humans. Therefore, multicellular organisms have developed a specialised system to absorb oxygen from the atmosphere. In humans, the respiratory system consisting of the nose, trachea, bronchi and lungs comprise the respiratory system.

We will learn more about the human respiratory system in the next article.

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Class 10 Science: Human Nutrition

In the previous articles on nutrition, we discussed the basics of nutrition including autotrophic and heterotrophic nutrition. In this article, we will discuss human nutrition.

Human beings are heterotrophs. We depend on other living organisms of food. The food consumed by us varies in complexity from simple sugars like glucose to complex carbohydrates, fats and proteins. The individual cells of the human body can only utilise elemental molecules like glucose, amino acids and fatty acids. Therefore, the complex molecules in our diet need to be broken down before absorption. Humans have developed a specialised organ system to accomplish this task- the digestive system.

The digestive system begins in the mouth or oral cavity and terminates in the anus. From the mouth to anus, the digestive system is one long tube. However each part of this tube is distinct, in both anatomy and functional characteristics.

The oral cavity is the first part of the digestive tract. There are some major functions performed by the mouth and most involve preparing the food for the digestive process.

Have you ever wondered why our mouth fills up with fluid when we see or smell food? This fluid is called saliva, and its function is twofold. The primary function is to moisten the food to enable easy swallowing. Secondly, the saliva contains many enzymes, which are biological catalysts, which begin the process of digestion. ?Many glands line the oral cavity; these are called salivary glands. There are the parotid glands on both sides of the jaw that produce the bulk of saliva. Then there is the submandibular gland located beneath the floor of the mouth and then there are many small salivary glands that line the oral cavity.

The oral cavity also contains teeth whose primary function is to masticate the food into a soft, semisolid consistency. Semisolid food is easy to swallow, and it also ensures mixing of the food with enzymes.

Once the food is moistened and chewed, it is taken and transported to the stomach through the oesophagus. The oesophagus is a long muscular tube whose sole purpose transportation of food from the mouth to the stomach. Transportation of food to the stomach is not dependent upon gravity. One can eat even in a lying position, so how does the oesophagus transport food to stomach. It does so by a series of co-ordinated movements called peristalsis. In fact, all segments of the digestive system express this form of action. Now consider a food bolus that is being swallowed. The muscles behind the bolus push the food forward while the muscles ahead of the bolus relax, thus ensuring seamless transport of food from one end to another. The digestive system has developed an intricate web of nerves which co-ordinate this process. This web is called the enteric nervous system.

The next stop for food is the stomach. The stomach is a hollow muscular tube that expands as food enters the stomach. The stomach secretes acid and some enzymes like pepsin. The acidic environment in the stomach serves two purposes. One, it helps to kill ingested bacteria, secondly, it facilitates the action of enzymes like pepsin (an enzyme that aids digestion of proteins).

Acidity is a common problem in humans. You might have seen many of your family or even yourself suffering from burning sensation in the stomach, especially when one is hungry. This is due to the secretion of acid by the stomach. So, how does the stomach protect itself from the action of acid? The cells lining the stomach produce a slimy fluid called mucus. The mucous coats the surface of the stomach and prevents damage to the lining of the stomach by acid. When this mucous becomes depleted, then symptoms of acidity occur (of course there are many other reasons for acidity).

From the stomach, the food then travels to the intestines. The intestine is a long hollow tube that lies within the abdomen. There are three parts of the gut- duodenum, jejunum and ileum.

Two principal digestive organs secrete their enzymes into the first part of the intestine- liver and pancreas. Liver secretes bile that helps with the digestion of fat. Fat is a very complex molecule and is not soluble in water while the enzymes can only act in a water-based medium. Bile acts on the fat like soap acts on grease. It emulsifies the fat to make it water soluble. It also increases the surface area for enzymes to act.

Also, the pancreas also secretes its enzymes into the duodenum. Pancreas also secretes copious amounts of dilute bicarbonate into the duodenum. This bicarbonate neutralises the acid (remember stomach secretes acid) and enables enzymes like trypsin and chymotrypsin to act on the proteins.

The food then travels along the length of the intestine where carbohydrates, proteins and fats are broken down by the above enzymes into their elemental forms, namely glucose, amino acids and fatty acids. They then travel to the ileum, where they enter the bloodstream by absorption through the cells lining the ileum. ?The cells lining the intestine have fine hair like projections called villi. These increase the surface area of the intestine manifold and help with the absorption of nutrients.

From the ileum, the food bolus, which now only contains indigestible fibre, moves to the colon. Here the food residue is compacted by absorption of water and then expelled out of the body through the anus.

To summarise, the humans are dependent on other living beings for their nutrition. The human digestive system does the task of breaking the food into its basic unit?and its absorption. The alimentary is a highly evolved and well-organised system with different components. Each organ has a particular role to play in human nutrition as discussed above. Students of class 10 science are advised to familiarise themselves with the basics of human nutrition before their board exams.

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Heterotrophic Nutrition

In our earlier articles on class 10 science, we learnt that organisms require energy to maintain the various life processes. Organisms obtain energy from the environment, either by self-synthesis or by depending on other living organisms. The former are called autotrophs and they are discussed in detail in the previous article. In this article, we shall examine living organisms that depend on other living organisms for food. This is called as heterotrophic nutrition.

Have you heard of a food pyramid or food chain? This is a graphic representation of the sources of food for living organisms. At the bottom of the pyramid lie the autotrophs, for eg. Grass, plants, algae, plankton etc. These form the food for organisms above them in the food chain. So a deer depends on grass and plants for its food. Similarly, a tiger or lion eats other animals like deer. So a tiger or lion is above the deer on a food chain. How organisms obtain nutrition is dependent on the environment and each organism is uniquely adapted to its environment.

Organisms like fungi and some bacteria break down food into its elemental form outside the body and absorb the elemental form for energy. Other organisms like birds, animals and humans consume complex molecules, which are then broken down into its elemental forms and absorbed from the digestive tract. In a process involving millions of years, each organism has developed certain specialised structures and organs to help with digestion and absorption of food. These structures will vary depending on the type of food each organism eats.

Herbivores like cows and deer have a long small intestine to help with digestion of cellulose while a carnivore like a tiger only has a short small intestine as meat is easily digested. Some organisms have further specialised organs which aid the process of nutrition.? Have you seen a cow? It always seems to be chewing something. This is known as chewing cud. A cow needs to eat a lot of grass to fulfil its energy requirements, therefore it chews the grass just enough to moisten the food and swallows it. The cow?s stomach is specialised in the sense that it has four partitions. In the first partition or the rumen, the swallowed food is mixed with digestive enzymes and softened. This is called cud. This cud is regurgitated back into the mouth where it is chewed and swallowed back. It seems a funny way of eating, but there is an evolutionary reason for this way of eating. Cows and other herbivores are ?flight animals?. It means that they run from danger. It is believed that the reason for this type of eating is that it allows the animal to eat as much it needs in quick time before needing to flee from that area. They can then regurgitate the cud and re-chew it at a safer area.

To summarise, heterotrophic nutrition (organisms) is nutrition obtained from other living beings. Each organism is adapted to its environment and the type of nutrition is dependent upon the type and availability of the food in the environment.