The need for transport systems in multicellular plants and animals in terms of size and surface area to volume ratios All cells need to take in substances from their environment, and get rid of unwanted substances. For example, a cell that is respiring aerobically has to take in oxygen and get rid of carbon dioxide. The exchange of substances between individual cells and their environments takes place by the physical process of diffusion (which includes osmosis) and the active transport and endocytosis or exocytosis. In a single cell organism such as amoeba, this can happen quickly enough by diffusion alone. This is because:

No point in the cell is very far from the surface, so it does not take longer for gases to diffuse from the cell surface membrane to the centre of the cell, or vice versa;

The surface area to volume ratio of the cell is relatively large-that is, it has a large amount of surface area compared to its total volume.

In a large and more complex organism, diffusion is no longer sufficient. This is because:

Cells may be too widely separated from each other and from their environments, so it would take too long for substances to diffuse all that way (distances too great for diffusion).

The surface area to volume ratio is much smaller-that is, it has a small amount of surface area compared to its total volume.

Cell requiring substances are at a distance from site production. Large organisms solve these difficulties in two ways:

They have transport systems that carry substances by mass flow (bulk transport of materials from one point to another as a result of a pressure difference between the two points) from one part of the body to another, rather than relying solely on diffusion.

They increase the surface area of parts of the body involved in exchange with the environment, for example, by having thin, flat leaves or by having a highly folded exchange surface.

Features of mass transport system:

Have a system of vessels that carry substances. Eg. Blood vessels in humans

and xylem vessels and phloem tubes in plants.

A way of making sure that substances are moved in the right directions, eg.

Nutrients in and waste out.

A means of moving materialsfast enough to supply the need of the organism

which involves mechanical methods like pumping of the heart, or ways of

maintaining a con.gradient like using active transport.

A suitable transport medium like blood.

Mass transport system in humans:

In humans it is called the cardiovascular system, which is made up of blood

vessels through which blood flows, a pump called heart to move blood through

the vessels, and the blood, the transport medium. The passage of blood through

the vessels is called the circulation.

Function of cardiovascular system:

Delivers materials needed by the cells of the body.

Carries away the waste products of their metabolism.

Carrying hormones from one part of the body to another.

Forming part of the defence system of the body

Distributing heat.

Components of blood: