All living creatures require energy and nutrients to survive. Animals can be divided into autotrophs and heterotrophs according to how they acquire this energy and nutrients. Autotrophs make their own food from inorganic nutrients and obtain energy from non-living sources. Heterotrophs must consume other living creatures to gain the energy and nutrients they need to live. Plants, as autotrophs, must make their own food and do so from sunlight, carbon dioxide and water through a process called photosynthesis.
Photosynthesis consists of two distinct series of reactions called the light reactions and dark reactions. In the light reactions through a process known as photophosphorylation, sunlight energizes an electron in chlorophyll which is then transported along a membrane to create a proton gradient. This proton gradient is then used to generate energy needed in the dark reactions. In the dark reactions, the energy harnessed in the light reactions drives a series of reactions that use carbon dioxide to make sugar.
The photosynthetic reactions of plants occur in specialized structures within the cell called chloroplasts. Chloroplasts contain stacks of membranes called thylakoid membranes where the photosynthetic process takes place. The membranes form enclosed spaces that allow a proton gradient to be created and then utilized to generate high energy bonds in ATP, an energy molecule used by cells.
Organisms from bacteria to whales must convert their food into energy and organic compounds for other life processes. Many organisms, including plants and animals, use oxygen to perform aerobic respiration. However, in the case of plants, this food energy is made inside the cells rather than obtained from other creatures.
Both plants and animals have mitochondria within their cells to facilitate the conversion of food to energy. Mitochondria are specialized cellular organelles with folded membranes inside. These membranes facilitate the electron transport chain, where high energy electrons are used to create a proton gradient that drives ATP production. The electron transport in the mitochondria and photophosphorlyation in chloroplasts conceptually are very similar, though the details vary greatly.
Plants as Producers
While photosynthesis is important to plants for their food, in many ecosystems the plants also make the food for other organisms. Animals such as the zebra eat plants to obtain their needed energy and nutrients. In turn, predators such as the lion eat the zebra for their required food. This fundamental role of plants in ecosystems leads them to be called primary producers, since through photosynthesis, they are able to harness the energy and nutrients within their physical environment and render it usable to other living creatures.