Leguminous plants replenish the soil by the presence of Rhizobium. These plants contain legumes and nodules which reside in the bacteria called rhizobium. These bacteria convert the atmospheric nitrogen into a soluble form that can be absorbed by roots.

Reversible: Things or processes which can be reversed are called reversible. For example, a folded sheet of paper can be unfolded and hence folding a sheet of paper is reversible.

Irreversible: Things or processes which cannot be reversed are called irreversible. For example, when milk turns into curd; it cannot be changed back to milk and hence is an irreversible change.

Photosynthesis

• The process by which plants make their own food in the presence of sunlight, carbon-dioxide present in air, water, minerals and chlorophyll present in leaves is termed as photosynthesis.

The reaction that takes place in the process of photosynthesis can be written as:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The photosynthesis equation is as follows: 6CO2 + 6H20 + (energy) → C6H12O6 + 6O2 Carbon dioxide + water + energy from light produces glucose and oxygen. ... In most plants, water is supplied from the roots, with the leaves collecting carbon dioxide via the stomata and sunlight captured by the chloroplasts in the leaves. During this process, organisms such as plants go through the light-dependent and light-independent reactions to convert carbon dioxide and water into sugars and oxygen.

In order to obtain silk, mulberry trees are cultivated, silkworms are reared and their cocoons are collected to get silk fibres. The various steps involved are:

(i) Rearing of silkworms: A female silk moth lays hundreds of eggs at a time. The eggs are stored by the farmers at suitable temperature and humidity under hygienic conditions. When the eggs hatch, silkworms come out of the eggs. The silkworms are fed mulberry leaves. After 25-30 days, the silkworms stop eating and get ready to spin cocoons. The silkworms enclose themselves completely inside the silken cocoons in two or three days.

(ii) Processing of cocoons to obtain silk fibres: The pile of cocoons is placed in hot water. Hot water makes the silk fibres of cocoons to separate out. The long fibres are obtained by unwinding the threads from cocoons. This process is called reeling.

(iii) Converting silk fibres into silk cloth: Silk fibres obtained from cocoons are spun (twisted) to form silk threads called silk yarn. The silk yarn is then woven on looms into silk cloth by the weavers

A change which occurs without a change in the composition and chemical nature of the substance is called a physical change. It means that there is a change only in the physical properties of the substance. Properties like colour, hardness, rigidity, fluidity, density, melting point and boiling point are known as physical properties. Melting of ice or boiling of water is a physical change because ice, water and water vapours are chemically the same substance that is H₂O.

It is the mode of nutrition where two organisms live together for mutual benefit. For example, lichens. The association of algae and fungi is called lichen. Alga survives in water. Its need for water is fulfilled by the fungus which in turn consumes the food made by alga. The fungus in turn gives to the alga, water and minerals it obtains from the substratum on which it lives. This association of algae and fungi makes them look as if they are one single organism

Grass eating animals are called ruminant animals. Example – cow. They have special stomach to digest cellulose present in the grass. Their stomach is large and is divided into four compartments.

1. Rumen:
2. Omasum 
3. Abomasum
4. Reticulum

   Rumen is the first and the biggest compartment of a cow’s stomach. It contains cellulose digesting bacteria. When a cow eats grass as food, it does not chew it completely but swallow it and gets stored in the rumen. The bacteria present in the rumen start to digest cellulose and gets partially digested. The partially digested food/grass in the rumen of a cow is called cud. After some time, the cud is brought back into the mouth and is chewed thoroughly. That is why, a cow moves its jaws from side to side and chew continuously even when it is not eating grass.

   The process by which the cud is brought back from the stomach to the mouth of the animal and chewed again is called rumination. The animals which chew the cud are called ruminants. When the cud is thoroughly chewed in the mouth of the cow, it is swallowed and goes into the other compartments of the cow’s stomach and then into the small intestine for complete digestion and absorption.  

Silk obtained from silkworm and cotton obtained from cotton plants combine together to form the cotton silk fabric. Silk cotton fabric is a blend of both fibers, combining positive characteristics of each into a single textile. Cotton silk clothes are lightweight, silky and have a comfortable weave. Silk cotton fabric has a soft hand-feel and a sturdy, silken drape, along with a versatility that makes it suitable for an extended variety of garment uses.

All animals and non-photosynthetic plants are classified as heterotrophs since they are unable to prepare food. So these organisms resort to other various forms of nutrition. Hence, from an ecological perspective, heterotrophs are always secondary or tertiary consumers in a food chain.

Humans and other vertebrates rely on converting organic, solid or liquid food into energy. Other organisms such as fungi rely on converting dead organic matter into nutrients. In essence, heterotrophs break down complex food into its readily usable constituents.

In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with a long aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, from 4 to 28. Examples would be fats, oils, cholesterols, and steroids. Fatty acids are in fact carboxylic acids with long aliphatic chain, which can be saturated (containing only C-C single bonds) or unsaturated (containing multiple bonds between carbon atoms). Examples of saturated fatty acids are Palmitic acid, stearic acid etc.

• Villi are small finger-like outgrowths present that help in the process of absorption.
• These are located in the inner walls of the small intestine.
• Its function is to increase the surface area of the small intestinal wall to absorb the digested food.

• Photosynthesis occurs when plants use light energy to convert carbon dioxide and water into glucose and oxygen. Leaves contain microscopic cellular organelles known as chloroplasts.
• Each chloroplast contains a green-coloured pigment called chlorophyll. Light energy is absorbed by chlorophyll molecules whereas carbon dioxide and oxygen enter through the tiny pores of stomata located in the epidermis of leaves.

Without proper functioning of the bowels, waste can be trapped for days, weeks, months, or even years. Un-eliminated waste can cause toxins to back up in the colon, which sets an environment for auto-intoxication -- self-poisoning. In this case, the bowel walls become encrusted with un-eliminated fecal matter, hampering the absorption of vital nutrients and providing a breeding ground for unhealthy bacteria. Blood capillaries lining the bowel wall begin to absorb these toxins into the bloodstream, consequently polluting all of our organs and cells.

It is a collection of membranous sacs called thylakoids. The green coloured pigments called chlorophyll are found in the thylakoid membranes. It is the sight for the process of light-dependent reactions of the photosynthesis process.

Sugar is the one which forms the backbone of the DNA molecule. It is also called deoxyribose. The nitrogenous bases of the opposite strands form hydrogen bonds, forming a ladder-like structure.

Deoxyribonucleic acid; a nucleic acid that consists of two long chains of nucleotides twisted together into a double helix and joined by hydrogen bonds between complementary bases adenine and thymine or cytosine and guanine; it carries the cell's genetic information and hereditary characteristics via its nucleotides. Three major forms of DNA are double stranded and connected by interactions between complementary base pairs. These are terms A-form, B-form,and Z-form DNA.

• Veins –

1. Veins carry blood rich in carbon di oxide from all the cells of the body to the heart.
2. The pressure exerted by the walls of the veins is less and have thin walls.
3. Valves present in the veins ensure that the blood flows only towards the heart.

Oesophagus

• Oesophagus is also called the food pipe.
• It runs along the neck and the chest.
• The food from mouth after swallowing passes into oesophagus and is pushed down to stomach by a special movement called peristalsis.
• This peristaltic movement takes place through out the alimentary canal which pushes the food in downwards direction.
• At some times the stomach is not ready to take up food causing vomiting, where food is expelled out from the oesophagus to mouth by reverse peristaltic movement.

Ruminants are grass eating animals like the cow, ox, buffalo and sheep swallow the food without chewing. After feeding, they bring the food from the stomach back into the mouth and chew it leisurely. This process is called rumination and such animals are called as ruminants. Many different species of ruminant animals are found around the world. Ruminants include cattle, sheep, goats, buffalo, deer, elk, giraffes and camels. These animals all have a digestive system that is uniquely different from our own. Instead of one compartment to the stomach they have four.

The process by which green plants use sunlight to make food from carbon dioxide and water is called photosynthesis. The synthesis of food in plants occurs in leaves due to the presence of green pigment called chlorophyll. It is responsible for the absorption of light energy from the sun. The solar energy is captured by the leaves and stored in the plant in the form of food. During the process oxygen is released and a simple carbohydrate is produced which is called as glucose. The glucose is than converted into complex carbohydrate called as starch which is stored in various parts of plants. The plants also make oils as food, proteins as food and vitamins as food.

Country leading in silk production is China. In 2005 China accounted for 74% of the global silk production. India occupies second position after China. Rearing of silkworms for silk production is called sericulture. Silk Industry provides employment to 7.9 million people in India, and 20,000 weaving families in Thailand. China is the world's single biggest producer and chief supplier of silk to the world markets.