Monday, February 29, 2016


You have learnt that a variety of end products are formed during metabolism and of these nitrogenous wastes are important.
Plants do not have any specific organ to excrete these wastes. As in animals, carbon dioxide, water, ammonia and other nitrogenous wastes are also formed in plants.
In the plants, carbon dioxide released during respiration is sent out through stomata of leaves. Due to this, the heat in the plant body is reduced.

 Even when leaves are absent, as in the germinating seeds. Carbon dioxide formed during respiration is released into the atmosphere. Plants have the capacity to utilize the by-products of one metabolic activity as raw materials for another metabolic activity.

oxygen released during photosynthesis is utilized for respiration. Carbon dioxide released during respiration is utilized for photosynthesis. Plants can convert nitrogen and ammonia into nitrates. However, several compounds are synthesized by the plants for their own use, especially for defence.

 As plants are sessile(do not have the capacity to move), they cannot defend from the predators by running away.
 Majority of plants do not have organs for protection against the herbivores. Hence, several plants prepare chemicals and store them in roots, leaves, seeds and even in stem.

Most of these chemicals are unpleasant to taste and hence herbivores usually do not prefer to eat such plants. Some of the chemicals are toxic and may even kill the animal that eats them.

 Some of the plants secrete chemicals when injured. These chemicals  seal the wound and help the plant to recover from the injury.

Though plants produce these chemicals for their use, man has found other uses for these chemicals and benefited immensely from these chemicals.

 Some of the plants release attractants for other Organisms which will help the plant in pollination, seed dispersal or even in their nutrition.
For example, plants having root nodules, secrete chemicals to attract rhizobia in to the surroundings of the roots and form a symbiotic relationship with the rhizobium. These compounds are called secondary metabolites.

The biochemical substances produced in plants are of two types-primary metabolites and secondary metabolites, Food materials like carbohydrates, fats and proteins are the primary metabolites.

Alkaloids, Tannins, Resins, Gums, Latex etc., are some of the secondary metabolites in plants.


Excretion is the elimination of wastes that are formed during the metabolism of various substances in the body.

The main substances excreted are nitrogenous substances like Ammonia, Urea and Uric acid. Nitrogenous wastes are formed in the metabolism of amino acids and nucleic acids.

Along with these excess water, certain salts and organic substances are also excreted. The main purpose of excretion is to maintain ionic balance in the body.

If the metabolic wastes are not discharged from the body, they become toxic and harm the body.

 All the animals do not excrete the type of nitrogenous wastes.
Some animals, like crabs excrete ammonia, while frogs and man excrete urea, insects, reptiles and birds excrete uric acid.

The type of the product excreted may change during different stages of development and availability of water.

Aquatic animals prefer to excrete ammonia while animals like reptiles and birds, to whom water is very scarce, excrete uric acid.




Connective tissue helps in binding the other tissues and organs together and provides a frame work and support to various organs in the body.

This tissue also plays a major role in the transport of material from one tissue to another. 
It also helps in the body defense, body repair and in the storage of fat.

There are different types of connective tissues each performing a different function.

Areolartissue is one type of connective tissue which joins different tissues. It helps in packing these tissues and thus help to keep the organs in pace.
These Cells called fibroblasts are the major components in this type of connective tissue. These cells secrete fibrous material which holds the other tissues in position. These cells also help in repair of the tissues when they are injured.

Cartilage is a type of connective tissue found in the joints of bones, tip of ribs, of the nose, external ear and in trachea. Embryos of several vertebrates do not have bone but have cartilage. The entire skeleton of fishes like sharks is made of cartilage. Cartilage is hard but not as hard as bone.
 Boneis another type of connective tissue. It is a major component of the skeletal system of  several vertebrates. Bone is made calcium, phosphate and calcium carbonate.
These salts are secreted by cells called osteocytes. These are present in the central hollow portion of the bone called marrow.
Ligament is yet another types of connective tissue that connects bones at the joints and holds them in position. It is made us. These fibres are made of a protein called collagen.
 Tendon is a type of connective tissue which is also made of fibres. The tendon joins the muscles to the bone. It is also made of collagen.

Adipose or fat tissue is a type of connective tissue which store fat. This tissue is present beneath the skin, around the kidneys and in bone marrow. This tissue is made of large numbers of cells called adipocytes or fat cells.

Cytoplasm of these cells is filled with fat is used for the production of energy required by the body during starvation. The layer of fat, especially, below the skin, prevent heat loss from the body.
Blood is called fluid connective tissue. It differs from other from types of connective tissues. There are several different types of cells in blood each one has a different function.
All the cells in the blood float freely in the plasma. Extracellular space is filled with a fluid called plasma. There are no fibres in blood. A normal adult humans being has about 5 liters of blood. Chief component of blood is water.

Besides water, it also has several nutrients such as glucose, amino acids, proteins, vitamins, and hormones etc. required for the body and excretory products such as lactic acid, urea, salts etc.

plasma also contain factors responsible for blood clotting. Cells present in blood are called corpuscles and they are of three types- red blood cells, white blood cells and blood platelets.
 Red blood cells, also known as erythrocytes, are red in color . In mammals, red blood cells do not have mitochondria, nucleus, endoplasmic reticulum, lysosomes and ribosomes.
 In fishes, amphibians and reptiles, they have a nucleus. These cells are concave on both the sides.
 They have red colored protein called hemoglobin, which helps in the transport of oxygen and carbon dioxide .One ml of human blood has about 500 crores of red blood cells. In adults humans, red blood cells are formed in bone marrow and live for 120 days I blood.

The second type of cell present in blood are white blood cells. These cells do not have hemoglobin, hence they are colorless. These cells are less in number.

There are two kinds of white blood cells, they are granulocytes and a granulocytes.

There are three types of granulocytes, neutrophils, basophils and eosinophils. These three kinds of cells attack and destroy the micro organisms that enter the blood . all these types of cells have granules in their cytoplasm. Hence they are called granulocytes.

There are two types of a granulocytes- they are lymphocytes and monocytes. Lymphocytes secrete antibodies towards foreign material that enter into blood . monocytes move like amoeba and along with granulocytes they attack the foreign materials and engulf them. The foreign material are destroyed inside these cells.

Blood platelets are a separates group of cells which do not have a nucleus. They are disk  like bodies. Whenever a blood vessel is injured, platelets accumulate at the site of injury and help in the formation of a blood clot.
The clot seals the wound in the blood vessel and prevent blood loss. Several chemicals present in the blood required for clot formation.




On the basis of their function, animal tissues are of four major types. They are epithelial tissue, connective tissue, muscle tissue and nervous tissue.

Let us examine the structure the structure and functions of these four kinds of tissues in animals ( including man).

Epithelial tissue: epithelial tissue forms a covering over the surface of the body as well as on the outside and inside of several internal organs example skin is an epithelial tissue on the outer side of the body. Lining of the digestive tract is a covering of epithelial cells on the internal organ.
Epithelial cells perform several functions- they offer protection, produce structure like hair, nails, feather and horns, in the form of gland cells, they secrete a variety of chemical and may involve in receiving external stimuli.
Cells in the epithelial are very close to each other and are joined together by a cementing substance. Epithelial cells occur in various shapes, flat, cube like, long and cylindrical.
Epithelialcells lining the digestive and respiratory system of several animals have hair like projections called cilia. They help in moving the food particles or mucus.
Epithelial cells may be present in a single layer or may occur in multiple layer stratified epithelium.