Plants absorb nutrients through photosynthesis, which takes solar energy to help convert nutrients in the soil and air into energy. A plant's leaves absorb carbon, hydrogen and oxygen from the air and atmospheric carbon dioxide and oxygen. Other nutrients, such as potassium, nitrogen trioxide, magnesium or iron, are absorbed through the root systems. The roots absorb these nutrients through a scientific process known as diffusion, which forces particles in areas of lower higher density (the soil) to move to areas of lower density (the roots).
On a cellular level, these nutrients enter the roots and are absorbed through cell walls by a process known as osmosis. The cell membrane is the site at which nutrients are transported within the cell to various proteins and enzymes that make them into usable energy. After the nutrients form energy in the form of sugars, transporter proteins and ionic channels transport the nutrients throughout the plant, supplying the various leaves, stems, branches or buds with sustenance.
Animals intake nutrients by consuming certain foods in their natural environments. Animals may be herbivores, carnivores or omnivores. Herbivores obtain nutrients by eating plants, and they need diets with a lot of carbon, hydrogen, oxygen, potassium, magnesium and zinc. Carnivores eat other animals, and they need diets higher in protein and fat. Omnivores eat both plants and animals, and they need a mix of protein, fat and other nutrients.
On a cellular level, animal cells absorb nutrients in a similar fashion to plants. Although they don't have cell walls, the membrane of animals cells absorbs the nutrients through osmosis. The nutrients, after passing through the membrane, are transported by proteins that take the nutrients to the mitochondria, which is where enzymes turn the nutrients into glucose and energy. Once this process is finished, proteins carry the energy to various places in the cell and other parts of the body to be used.