Plants

Plants are photosynthetic eukaryotes. Photosynthesis is the act of coverting light energy to chemical energy. Phtosynthesis evolved in bacteria, speficially cyanobacteria, and plants and algae are responsible for most of the transfer of energy in the biosphere. Photosynthetic eukaryotes first developed when an archae-like cell engulfed an aerobic bacterium. This is known as endosymbiosis and created a proto-eukaryote. This eukaryote then englufed cyanobacteria, a photosynthetic prokaryote, which then turned into a chloroplast to produce a photosynthetic eukaryote. The first cyanobacteria were like completely identical to modern-day cyanobacteria because they first used electrons from hydrogen sulfide, which later produced our oxygen-rich atmosphere. 

What is and what is not a plant is very unclear and debated. The main criteria of plants are cellulose in the cell wall, they store food as starch in plastids, they have phragmoplasts and plasmodesmata, and they photosynthesize. This creates a general idea of what is a plant, but there are some algae we consider plants that do not fit all this criteria. Nowadays, what people consider plants are mostly our modern-day land plants and green algae.