TRANSPORT SYSTEM

The transport system carries food and oxygen to all tissues and removes waste materials of metabolism, convenes hormones from ductless glands to their target organs and the body temperatures are equalized.

 

Needs for transportation

In Amoeba and many simple multicellular organisms food products and oxygen are taken into cells and distributed and the waste materials are eliminated by diffusion. In many other complicated plants and animals, many of their cells are too far away to be in direct contact with food and oxygen and some means had to be provided to carry these substances to them and to remove their metabolic wastes.

The primary purpose of the transport system is to transport materials throughout the plant and animal bodies.

Other functions of transport system include:

1. Regulation of body temperature in the vertebra by transporting and distributing heat.
2. Regulation of water balance in the animals.
3. Auxins in plants and animal hormones (secreted by ductless glands) are carried from the site of production to the target tissues by the transport system.
4. Regions of active cell division need substances to enable them build new protoplasm for the purpose of forming new cells. The transport system serves these needs.
5. Excess useful substances that can be stored are transported to tissues specialized to store them.
6. Substances required by reproductive cells for development are moved to them by the transport system.

 

Surface area/volume ratio

Numerous micro-organisms, unicellular organisms and simple multicellular organisms are found in aquatic habitat of various types. They are bathed by water all the time. Essential materials move from water directly into their bodies with minimum difficulty. On the reverse route metabolic wastes leave their bodies into the surrounding water by simple diffusion following diffusion gradient. In the larger and more complex organisms simple diffusion can no longer satisfy these needs. We now talk of short distance to cover in the less complex organisms and long distance to cover in the more complex organisms. In the simple organisms surface area is comparatively larger than the volume of the cell. This means that the surface area to volume ratio is high and therefore the distance from the environment into the cell and from the cell to the environment is short. Simple diffusion can transport materials from the environment into the cells and eliminate waste from the cells to the exterior. Consequently there is no need for a transport system.

In the large and more complex organisms, the surface area is comparatively far smaller than the volume. This means that the surface area to volume ratio is low and therefore the distance from the environment into the body is very great or long. The organisms therefore, have to develop an efficient transport system to cope with these problems.

 

Substances have to move greater distances in complex organisms

As the organisms become larger the body becomes more complex in structure, specialization of parts for specific function leads to a division of labour among different parts. All the living cells in the body of complex organisms need materials to release energy needed for carrying out their assigned function to the well being of the organisms. The cells are numerous and far from the source of these long distances has to be met by special transport system. The distances to cover in complex organisms are greater than those to cover in the smaller, simple or less complex organisms whose bodies are bathed by water. You can now appreciate the great distances that have to be covered in transporting digested food absorbed into the villi in the small intestine of vertebrates to the cells of the brain, the skin etc or soil nutrients in the root-hairs through the tall stem to the branches, leaves, flowers and fruits in higher plants.

 

Materials for transportation

In the living things, there is a life long process of continuous exchange of materials within the cells in the body and between the entire organism and the environment. As a host of essential materials enter the organisms body for their vital functioning so also waste products and unwanted substances leave the body to the environment. Materials for transportation include: water, gases, digested food and other nutrients and various secretions such as enzymes, hormones, excretory products and excess materials for elimination and or storage.

 

Media of transportation

The media of transportation in the living things are:

• Cytoplasm.
• Water.
• Body fluid.
• Blood and lymph.
• Cell sap.
• Latex.

 

Cytoplasm in cell: The unicellular plants and animals live in aquatic habitat of all types (freshwater, brackish water and marine or salt water). Their bodies are surrounded and bathed by water. Nutrients and air dissolved in the surrounding water diffuse into the bodies of these organisms for the purpose of building their bodies and releasing energy.

Similarly metabolic wastes such as ammonia and carbon (IV) oxide leave their bodies into the surrounding water by simple diffusion. The nutrients and oxygen on entering the cells are distributed or circulated to all parts of the cell by streaming of the cytoplasm in the cell. The Golgi bodies ribosomes and mitochondria concerned with carbohydrate, protein synthesis and final internal respiration are found scattered through out the cytoplasm makes it possible for the Golgi bodies, ribosome and mitochondria to receive their share of food and oxygen. The streaming of cytoplasm in cell serves as the medium of transportation in the unicellular forms such as Amoeba, Paramecium and Euglena, simple multicellular forms like Spirogyra and fungi, the non-vascular plants (Pteridophyta) and the roundworms special media of transportation of food and oxygen to the more bulky and complex body had to be developed to assist cytoplasmic streaming. Similar media for the removal of metabolic waste were also developed. Whereas the same cytoplasmic streaming brings metabolic waste such as ammonium compound to the periphery of the cell for diffusion to the exterior.

 

Body fluid in the invertebrates

In the invertebrate phyla from the Platyhelminthes through the Nematoda, Annelida to the Arthropoda large volume of body fluid flowing and bathing numerous cells serve as the transport medium for animals. The food and oxygen needed for the body well being is distributed in the body fluid to reach all living cells. The waste products of cellular metabolism are carried in the body fluid to the excretory organs for excretion. For this plan excretory organs lie free in the space called coelom where it is bathed by body fluid called coelomic fluid. Under this arrangement these excretory organs of the following animals are not supplied by blood vessels as seen in the vertebrates.

Although the earthworm has a blood vascular system the body fluid in the coelom still serves as transport medium for distribution of food and the carrying of wastes to the nephridia which lie free in it. Many other invertebrates have no vascular system and so the body fluid serves the needs of transportation for the animals.

 

Blood and lymph in the vertebrates

The fish, toad and frog, lizards, birds and mammals have large bodies with complex internal structures which cannot be served or reached by body fluid alone. As a result special vascular systems with special fluid tissues have been developed to meet the needs of carrying food, oxygen, various secretions and water to all living cells of the body and to remove from them waste products of cellular metabolism to excretory organs (kidneys) for excretion. The special fluid tissues are:

1. Blood in the blood circulatory system.
2. Lymph in the lymphatic circulatory system.

 

Advantages of the blood and lymph over the cytoplasm and body fluid as transport media in the lower animals

1. The development of special respiratory pigment haemoglobin for transporting oxygen and plasma for carbon IV oxide during the process of gaseous exchange is faster and more efficient for active animals.
2. The plasma of blood transports digested food, mineral salts, other organic compounds such as hormones, drugs and water to all cells and urea, ammonium compound and other surplus substances such as water plus dissolved salts to excretory organs for excretion.
3. The lymph carries fatty acids and later hands them over to the blood for onward transportation to the liver for final distribution.
4. The lymph fills the gap created by the blood circulation by reaching all the cells that cannot be reached by the blood.
5. The lymph returns body fluid to the blood and when this is disturbed by certain monotonous activity of the body disease called dropsy is developed. This is characterized by the swelling of the legs with the accumulated unreturned body fluid.

The blood being pumped by the heart and the lymph propelled by muscular contraction are in constant circulation round the body supplying food, oxygen, water, hormones, drugs and heat to all the cells and removing from them urea ammonium compounds, carbon (IV) oxide, surplus heat, water mineral salts to excretory organs (kidneys skin) for excretion. This meets the physiological requirements to sustain the life of large complex and active animals in the vertebrate group.

 

Cell sap and latex in most plants

The flowering plants form the major vegetation of the whole world, cell sap like body fluid in animals is widely spread in them. But latex which is an excretory product stored away in the dead cells of the cortical tissue occurs in some families of plants like Euphorbiaceae such as Euphorbia hirta, Poinsettia, Pedilanthus, Jatropha, Rubber, Allamanda, Thivetia, Ficus and Rawolfia.

Both cell sap and latex are fluid tissues as water is their basic component. The flowering plants are large and have complex tissues with many of them attaining great heights. The movement of absorbed soil water and manufactured food is greatly polarized. That is from the roots to the shoot tips and from the shoot to roots below. The sideways distribution of water, food, oxygen and carbon (IV) oxide is by the cell sap and latex.