The following structures are found exclusively in plant cells and are absent in animal cells.
Cell wall
Though a cell wall is commonly found in prokaryotes and fungi as well as plants, their diversity is due to convergent evolution, not common ancestry, when it comes to these three groups of organisms. Plant cell walls are composed of cellulose are an excretion found outside the plasma membrane. They serve as a covering that provides structural support and gives shape to the cell.
Central Vacuole
The central vacuole is a large, membrane-bound structure that fills much of the plant cell. The membrane surrounding the central vacuole is called the tonoplast. The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That’s because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm (Figure 9.2.89.2.8). As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the plant’s cell walls results in the wilted appearance. The central vacuole also supports the cell’s expansion. When the central vacuole holds more water, the cell becomes larger without having to invest considerable energy in synthesizing new cytoplasm. Lastly, central vacuoles store nutrients, accumulate ions, or become a place to store wastes.
Plastids
Plastids are a group of storage organelle found in plants and algae. Chloroplasts are a type of plastid that store chlorophyll and other pigments for photosynthesis. Chromoplasts are plastids that store orange or yellow pigments, found in plants and fruit such as bell peppers. They are rich in carotenes and xanthophyls. Amyloplasts store starch and can be found in plants such as potato tubers, carrot roots, sweet potato roots, and grass seeds.
Chloroplasts store their pigments in interconnected sacs called thylakoids (Figure 9.2.99.2.9). These sacs are often found in stacks called grana (singular granum). The fluid portion of the double membraned chloroplast is called the stroma. Because the thylakoid stores chlorophyll a, b, and accessory pigments, it is the main region for the first reaction of photosynthesis, where sunlight is used to create molecular energy. In the stroma, the products of the first reaction are used to produce organic molecules such as glucose. The combination of these reactions allow these autotrophic organisms to produce their own organic food.
The chloroplast, like the mitochondria, contains its own DNA, ribosomes, and is double membraned.
