PHOTOSYNTHESIS
Differences in plant and animal nutrition
Plants characteristically synthesize complex organic substances from simple organic raw materials. In green plants the energy for this process is sunlight. Plants can use this energy because they possess the green pigment chlorophyll. Photosynthesis, or "light-synthesis" is a "self-feeding" or autotrophic process.
Animals, on the other hand, must obtain complex organic substances by eating plants or other animals. The reason for this is that they lack chlorophyll. Among these "other-feeders", or heterotrophs, we distinguish between "solid-feeders" or phagotrophs, and "liquid-feeders" or osmotrophs. Whereas phagotrophic organisms take in solid and often living food, osmotrophic ones absorb or suck up liquid food, usually from dead or rotting animals.
Plants and animals, then have characteristically different feeding methods. However, we cannot define plants as autotrophs and animals as heterotrophs. The reason for this is that many plants lack chlorophyll and feed heterotrophically, and some animals do have chlorophyll and feed autotrophically.