Plants are multicellular eukaryotes. The kingdom (second largest taxonomic grouping) Plantae contains autotrophs capable of making their own food. With few exceptions, plants are photosynthetic. They use a group of light-capturing pigments called chlorophylls to harness the sun's energy and use it to convert carbon dioxide and water into organic compounds. This process makes oxygen as a by-product, which helps to oxygenate the atmosphere. In addition, plants are critical in the recycling of carbon in the ecosystem, as photosynthesis removes carbon dioxide from the atmosphere. Chlorophyll is also responsible for giving plants their green appearance. Chlorophyll is housed within organelles called chloroplasts, usually found inside leaf cells. Each plant cell is surrounded by a rigid polysaccharide-based wall, which offers protection but also limits the motility of the plant. Although plants can make their own organic compounds through photosynthesis, they must also perform cellular respiration to convert organic compounds into cellular energy.As land plants evolved, they developed characteristics to reduce their dependence on water. Hardy spores allowed plants to reproduce outside of the water, a waxy outer coating protected plants from dehydration, and vascular tissue in later plants provided the ability to deliver water and nutrients throughout a tall body. Another major plant adaptation is the alternation of generations life cycle, which includes both diploid and haploid reproductive stages. Land plants are most familiar in their diploid sporophyte stage. Most plants reproduce sexually, but many are also able to reproduce asexually. Reproductive adaptations include the seed, which protects the plant embryo and allows it to travel or wait out harsh conditions. Flowering plants, which make up the most diverse plant group, use insects, birds, and other animals to assist with pollination and seed distribution. Fragrant, brightly colored flowers attract pollinating insects. Fruits entice birds and other animals to ingest plant seeds and distribute them in their waste. Plants are primarily grouped based on the presence or absence of vascular tissue, seeds, and flowers.