Imagine a life without photosynthesis? If you can’t, it’s because life would simply not exist without it. This process, which is the defining metabolism of all plants, evolved billions of years ago when early heterotrophic organisms were so abundant that the organic molecules they depended on for life were becoming scarce. Under the pressures of competition for nutrients, cells evolved that were able to make their own energy-rich molecules from inorganic materials. Without the evolution of these autotrophic organisms, life on Earth would have ceased to exist.
These photosynthetic organisms grew in population and changed the face of the entire planet. Because the process of photosynthesis releases oxygen, oxygen began to accumulate in the atmosphere. This was not an overnight change. In fact, it took millions of years, and by 700 million years ago, atmospheric levels of oxygen began to approach modern levels.
The increase in oxygen led to the formation of the ozone layer, which absorbs ultraviolet rays from the sunlight. By 450 million years ago, the Earth was protected enough from these ultraviolet rays that living organisms could survive on the surface layers of the water, on the shores, and life came to land for the first time. The increase in oxygen, which made the Earth aerobic, paved the way for more efficient use of carbon-containing molecules formed by photosynthesis, thus allowing respiration. This process, yields more energy than any anaerobic, or oxygenless, process. The increase in free oxygen also led to the first eukaryotic cells.
As photosynthetic organisms became successful in the open waters, the mineral sources they depended on began to deplete. Because of this, life began to develop toward the shores, where the water was rich in minerals and nutrients carried down from the mountains. In this new environment, these organisms faced the challenges of a more complicated environment. These organisms were up for the challenge, and began to evolve and adapt to the new environment. They developed strong cell walls for support, as well as structures that would anchor their bodies to the rocky surfaces. This was the first step to plants colonizing the land.
As plants evolved from aquatic to terrestrial environments, several obstacles stood in the way. One challenge was the distribution of water and nutrients to each cell. In water, transport was able to occur directly from the surrounding environment. Plants also needed to find a way to withstand the forces of gravity, and provide support so it could grow tall. To solve these problems, roots and stems developed. Roots anchor the plant to the ground and collect water and nutrients to distribute throughout the plant body. Stems provide support, as well as houses the vascular system, which conducts substances between the photosynthetic and non photosynthetic parts of the plant body. The vascular system includes a vascular bundle has two major components: the