Features of Transport Systems (AQA A-Level Biology): Revision Notes
Features of Transport Systems
Why large organisms need transport systems
Diffusion works effectively for transporting substances over short distances, but larger organisms face significant challenges that make specialised transport systems essential.
In small organisms, materials can exchange directly across the body surface. However, as organisms increase in size, their surface area to volume ratio decreases dramatically. This creates a problem where the organism's surface area becomes insufficient to meet the metabolic demands of all internal cells.
As an organism grows larger, its volume increases much faster than its surface area. For example, if you double the length of each side of a cube, the surface area increases by 4 times, but the volume increases by 8 times. This mathematical relationship explains why larger organisms cannot rely on surface exchange alone.
Large organisms have developed specialist exchange surfaces to absorb nutrients and respiratory gases whilst removing waste products. These exchange surfaces are typically located in specific regions of the organism, such as the lungs for gas exchange or the intestines for nutrient absorption. A transport system becomes necessary to move materials between these exchange surfaces and the cells that require them throughout the body.
As organisms evolved to become larger and more complex, their tissues and organs became increasingly specialised and dependent on one another. This interdependence makes an efficient transport system even more critical for survival.
The need for a specialised transport system with a pump depends on two key factors:
- Surface area to volume ratio - the lower this ratio, the greater the need for transport
- Activity level - more active organisms have higher metabolic demands
Common features of transport systems
Large organisms face similar transport challenges, leading to the evolution of transport systems with several shared characteristics:
Transport medium
Most transport systems use a suitable medium to carry materials around the body. This is typically a liquid based on water, such as blood, because water dissolves substances readily and flows easily through vessels. In some cases, the medium can be a gas, such as air moving through the respiratory system.
Mass transport mechanism
Transport systems use mass transport, where the transport medium moves in bulk over large distances. This process is much more rapid than diffusion alone and allows efficient distribution of materials throughout large organisms.
Closed tubular system
A closed system of tubular vessels contains and directs the transport medium. These vessels form a branching network that distributes the transport medium to all parts of the organism, ensuring every cell receives the materials it needs.
Pumping mechanism
Transport systems require a mechanism to move the transport medium within the vessels. This creates a pressure difference between different parts of the system, driving flow from high to low pressure areas.
This pumping is achieved in two main ways:
- Animals use muscular contraction, either through body muscles or specialised pumping organs like the heart
- Plants rely on natural, passive processes such as water evaporation from leaves
Flow control mechanisms
Transport systems include mechanisms to:
- Maintain mass flow movement in one direction using structures like valves
- Control flow rate to suit the changing needs of different body parts
- Regulate the movement of water or gases, such as through intercostal muscles and the diaphragm during breathing in mammals
Key Points to Remember:
- Large organisms need transport systems because their surface area to volume ratio becomes too small to meet cellular demands through surface exchange alone
- All transport systems share common features: a suitable transport medium, mass transport mechanisms, closed tubular vessels, and pumping systems
- Transport systems move materials in bulk over long distances, with the final exchange to cells occurring via short diffusion pathways
- Mammals use double circulation to maintain high blood pressure and ensure rapid delivery of materials to active tissues
- The effectiveness of any transport system depends on having short diffusion distances at exchange surfaces combined with rapid bulk transport over longer distances