This table shows the surface area and volume of cubes increasing in size to prove that the SA/V ratio decreases as the cube or any object becomes larger.
During the course of evolution organisms have changed from single celled to multicellular organisms. Single celled, microscopic organisms have a large surface area: volume ratio, therefore they are able to efficiently exchange and remove materials. However, as these organisms grow larger they eventually become multicellular, which means they need to somehow increase their surface area for the needed amount of materials to be exchanged. Multicellular organisms such as sea anemones and tapeworms are elongated and flat to have a larger surface area causing the SA/V ratio to also increase. This means that the diffusion of materials between the organism and its environment requires only a short distance and time.
As for larger organisms, they have specialised organs to increase its surface area, for example lungs, intestines and leaves. Each specific organ exchanges specific materials. This allows materials to be exchanged more efficiently and quicker. To connect these organs together in the body a transport system is needed, which is a network of tubes or vessels. In plants the transport system is the vascular system, which involves the xylem and phloem and in animals have the circulatory system.
The gases, which are exchanged between organisms and its environment, are mainly oxygen and carbon dioxide. Organisms have specific gas exchange organs to enable this to happen. In animals, the lungs are responsible for the exchange of gases whereas the leaves of a plant help it to respire. Gas exchange happens by diffusion. The gas exchange organs are adapted to give maximum diffusion by having a large surface area, an efficient transport system and thin membranes so that diffusion can occur quickly.
In the lungs of animals, the millions of alveoli give it a large surface area for the exchange of carbon dioxide and oxygen. When oxygen is drawn into the lungs it diffuses out of the alveoli through the wall, which consists of only one cell, and the capillary wall, also only of one cell. The oxygen is then able to reach the haemoglobin in the blood. Carbon dioxide is removed by diffusing into the alveoli and then breathed out through the nose or mouth into the air. As alveoli have a large surface area: volume ratio, the rate of diffusion is speeded up which enables gases to be exchanged at the required time.
Gas exchange in plants happen through the leaf. As leaves of a plant are required for photosynthesis, its structure is vital for the exchanging of carbon dioxide and oxygen. The leaf is adapted for this as it is made up of freely packed mesophyll cells and air spaces, which gives the leaf a large surface area for gas exchange. It also has stomata which allows diffusion of gases to take place between the air spaces. This gives the leaf a great advantage as therefore diffusion happens at a high rate which also maximises photosynthesis.