Transport Systems: Plants vs Animals

All living organisms need to be able to move (transport) particles such as molecules or ions, into themselves (i.e. into the body of the organism) and to be able to move such particles around within the organism. They also need to be able to remove unwanted substances from their bodies. This is necessary in order for organisms to breathe, feed, and maintain the internal environment of the organism within acceptable limits (homeostasis), i.e. to continue to live.

Different types of organisms, such as - but not only - plants and animals, have different types of internal transport systems via which fluids containing substances necessary for the life of their cells are moved around the organism.

Due to the huge variety of different types of animals and plants it is useful to specify some categories of organisms in order to compare transport systems in plants and animals. The following table compares the basic transport mechanisms in mammals with those in flowering plants.

Transport in *Mammals
*with some comments re. other types of animals

Transport in flowering plants

Transport systems
(around entire organism, not just within a cell or specific organ)

The main circulatory system in vertebrates is the Blood System which consists of:

In addition, lymphatic drainage has a (more limited) role in transporting fluids, which contain dissolved particles 'solutes' from tissues.

*Many invertebrates e.g. molluscs and arthropods, have open circulatory systems in which the circulatory fluid is hemolymph rather than blood.

Plants have 2 types of transport tissue:

  • Xylem moves water and solutes from the roots to the leaves
  • Phloem moves food substances from leaves to the rest of the plant (both up and down the plant)

Tubes through which fluid flows around the transport system(s)

Blood vessels have different structures according to their role and position in the system, e.g. to convey higher pressure blood from heart vs lower-pressure (oxygen-poor) blood returning to the heart.

Types of blood vessels, all of which are composed of living tissues, include:

The specialized tubes for fluid transport within plants include:

  • Xylem vessels
  • Tracheids
  • Phloem sieve tubes

Some of these consist of living cells (e.g. phloem), while others are composed of dead cells (e.g. xylem vessels).

What causes fluid to move?

Blood is pumped by the heart (mammals have a double-pump, four chamber, heart), which is controlled by the nervous system.

  • Xylem
    • transpiration pull (transpiration is movement of water up the plant and out of the leaves)
  • Phloem
    • active pumping of sugars into the phloem
    • hydrostatic pressure

Fluid flow rate

Fluid flow rate varies with the part of the body and other factors, e.g. physical activity level. Overall mammals have a moderate flow rate, i.e. greater than that of plants.

Fluid flow rate in xylem and phloem is relatively slow.

Controlled by pumping action of the heart and by vasoconstriction and vasodilation of the fluid (blood, in mammals) vessels - which can be affected by e.g. hormones and environment conditions such as extremes of temperature.

Rate of flow in the xylem is affected by external factors such as temperature, wind and humidity.

Transport of water
(Water is very important for life.
It acts as a solvent in which many different substances can be transported around organisms.)

Blood plasma is normally at least 90% water.

In xylem sap.

Substances transported

In fluid (blood, in mammals) systems, examples incl.

In fluid system(s),
examples incl.

  • *Sucrose, which is a disaccharide
  • Amino acids
  • Fatty acids
  • Glycerol
  • Vitamins
  • Minerals
  • Hormones

Note that respiratory gases move via the inter-connecting air spaces between cells - as opposed to via fluid systems.

Transport of *carbohydrate
i.e. food, so in animals incl. humans - as released by the digestive system

Glucose in solution in blood plasma

Sucrose in solution in phloem sap

Transport of oxygen
for breathing, i.e. cell respiration

Oxygen combines with haemoglobin (in the red blood cells), forming oxyhaemoglobin, which is an efficient way for oxygen to be transported around the body via the blood system.

This applies to closed circulatory systems, such as those of mammals.

In contrast, hemolymph (the circulatory fluid in open circulatory systems) is not necessarily used for oxygen transport. Many animals that have open circulatory systems, e.g. most insects, respire directly from their body surfaces. For more info see gas exchange in insects.

Oxygen (in the form of O2 gas) can reach cells by diffusing through the air spaces between the cells.

Transport of carbon dioxide
CO2 is a waste product of metabolism.

Carbon dioxide diffuses into red blood cells where it is converted to hydrogencarbonate ions (HCO3-) and hydrogen ions (H+). Haemoglobin binds to the H+ ions, acting as a buffer to prevent a decrease in blood pH. In addition, some carbon dioxide dissolves in the blood plasma (which is usually at least 90% water).

Carbon dioxide (in the form of CO2 gas) can reach cells by diffusing through the air spaces between the cells.

Gas Exchange surface
by which oxygen (O2) can enter the organism and carbon dioxide (CO2) can be released from it

Alveoli (plural; singular="alveolus") in the lungs - see diagram below & on page re. internal respiration for more about gas exchange in humans

All cell surfaces that are in contact with air.

Examples include the palisade and spongy mesophyll cells in leaves.

See also what is transport in biology?, types of circulation in animals, types of circulation in plants, and types of movement across biological membranes e.g. cell membranes.

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