It is important to note that snakes are venomous, and not poisonous as they are so often described.
So what is the difference between the two?
Venom needs to be injected either by a sting (insects), or in the case of snakes, a bite. This means that venomous animals have specific highly evolved delivery apparatus with which to inject venom.
In addition to specialised delivery methods, venomous animals also need to have specialised organs specifically designed for that purpose.
Poison needs to be ingested, or touched. Poison is generally distributed over a greater area, and generally secreted.
For example: It is possible to touch, even eat, a venomous snake and suffer no ill effects. The opposite is true for a poison dart frog (Dendrobates).
Conversely, you may allow a poison frog to bite you, not so a venomous snake.
So what is the difference? Well although both venomous and poisonous organisms may contain identical toxins, it is the delivery method of those toxins, and the way it is transported, or abosrobed in the body that differentiates between the two.
What is venom?
It is a cocktail, consisting of hundreds of different proteins and enzymes. Its proteinaceous nature was first established back in 1843 by Charles Lucien Bonaparte, the nephew of emperor Napoleon.
A vast majority of these proteins and enzymes are completely harmless to man. However, a tiny percentage of those enzymes present are considered toxins.
There are roughly 20 types of toxic enzymes present in venomous snakes. Each of these enzymes serve a specific function. The majority of venomous snakes only employ 6-12 of these toxins. No single venomous species employs all 20 toxins.
An enzyme can be described as a biological catalyst. In snakes, the enzymes speed up the chemical reaction within the afflicted organism to such an extent that the organism either dies or is immobilised.
Here are a couple of examples of the enzymes. This information was sourced from www.chm.bris.ac.uk A more detailed explanation can be found at this site.
Cholinesterase : Present in Mambas (Dendroaspis) this attacks the nervous system.
Adenosine triphosphatase : Present in most snakes and believed to be the central agents resulting in shock.
Polypeptide toxins : Disrupt the nerve-impulse transmission causing heart or respiratory failure.
There are three distinct types of venom:
Nuerotoxic : A fast acting toxin responsible for attacking the nervous system. Results in paralysis, seizures, incoherence, respiratory failure and eventually death. eg Mambas, and non-spitting cobras.
Cytotoxic : This causes immense tissue damage, and necrosis. eg Puff-adder
Haemotoxic : Responsible for attacking the cardio-vascular system causing eventual organ failure. This process is slower than aforementioned toxins. eg Pit vipers, Boomslang, and vine snake.