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To clarify, I think the answer should be able to explain:

  1. What are animals?
  2. What are plants?
  3. What's the difference between animals and plants (How do biologists differentiate them, if they differentiate them by moving ability of course it's simple, but we know they don't)?
  4. How do those differences affect their evolutionary paths so that most animals can move around and most plants cannot? I know there are exceptions, but that's just the exceptions that show the rule.
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up vote 10 down vote accepted

Animals and plants are both classified as Eukaryotes, and as such can form large, complex, multi-cellular organisms. There are several major differences at the cellular level that distinguish the 2 Kingdoms (Animalia and Plantae). Without getting technical, the most crucial difference in relation to your question is that plants contain chlorophyll, and as such can generate their own food using sunlight.

This is very important when you consider that plants (by definition, those Eukaryotes that contain chlorophyll) have therefore evolved to be very efficient at converting sunlight to 'energy', whereas animals have evolved without this option and have to eat what food they need. So they have instead evolved mechanisms that allow them to forage, or even to hunt, for their food.

At some point, a unicellular organism incorporated chlorophyll, leading to multicellular plants in the course of evolution. The ancestor of animals evolved independently from the last common ancestor of plants and animals. Therefore, plants and animals are quite diverged, e.g. in mating 'rituals' (in plants this involves flowers and seeds - whereas in animals there are mating and gestation periods), and systemic changes, such as the immune system and the circulatory system (these are generally much more complex in animals than plants). All of which has stemmed from the difference in evolutionary pressures applied by the choice of food-source.

As you have pointed out, there are exceptions to all rules, so not all plants have chlorophyll, but as a rule it is the case! (Wikipedia: "Some plants are parasitic and may not produce normal amounts of chlorophyll or photosynthesize.").

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Also, plants get their nutrients via their roots, so it is quite handy for them to stay in the ground! :) – nico May 21 '12 at 14:29
Awesome answer. Not all plants have chlorophyll? I guess I'll look at wikipedia then. – Jim Thio May 23 '12 at 15:20
So what's exactly biologist use to differentiate plants and animals? Chlorophyll? – Jim Thio May 27 '12 at 15:33
@Jim Thio: think of red algae for instance – nico May 27 '12 at 19:49
@nico the rhodophyta (red algae) all have chlorophyll, they are red because they also have red accessory pigments. An example of a plant lacking chlorophyll is the parasitic Hydnora africana – Richard Smith-Unna Jun 8 '12 at 22:49

Plants are multicellular organisms that utilize diffuse inorganic molecules (CO2, NO3, etc...) for energy and growth and thus they have evolved anatomical features that maximize the efficient concentration of these molecules (i. e., the root - shoot).

Animals are multicellular organisms that utilize concentrations of organic molecules for energy and growth (i.e., food) and thus they have evolved anatomical features that maximize the efficient harvest of these molecules. Note that not all animals move. There are many sessile filter feeders.

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