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Plant cells have several structures not found in other eukaryotes. In particular, organelles called chloroplasts enable plants to trap solar energy into energy-rich molecules; cell walls allow plants to have rigid structures as diverse as tree trunks and flexible leaves; and vacuoles allow plant cells to change size. What is the origin of chloroplasts? Like mitochondria, chloroplasts likely arose from an ancient symbiosis, in this case cells with a nucleus surrounding a photosynthetic prokaryote. Chloroplasts actually resemble modern cyanobacteria, which are still similar to cyanobacteria from 3 million years ago. However, the evolution of photosynthesis goes even further, to the earliest cells that evolved the ability to capture light energy and use it to produce energy-rich molecules. As these organisms evolved into the ability to separate water molecules and use electrons from these molecules, photosynthetic cells began to produce oxygen, with dramatic consequences for the evolution of all life on Earth (Fig. 1). Today, chloroplasts retain small circular genomes similar to cyanobacteria, albeit much smaller. (The mitochondrial genomes are even smaller than the chloroplast genomes.) Most of the sequences encoding the chloroplast proteins have been lost, so these proteins are now encoded by the nuclear genome, synthesized in the cytoplasm, and transported from the cytoplasm to the chloroplast. What is the function of chloroplast membranes? Like mitochondria, chloroplasts are surrounded by two membranes. The outer membrane is permeable to small organic molecules, while the inner membrane is less permeable and lined with transport proteins. The innermost chloroplast matrix, called the stroma, contains metabolic enzymes and multiple copies of the chloroplast genome. Chloroplasts also have a third inner membrane called the thylakoid membrane, which is widely folded and looks likeon electron micrographs of flattened discs. Thylakoids contain a light-harvesting complex, including pigments such as chlorophyll, as well as electron transport chains used in photosynthesis (Fig. 2). What is a cell wall? Besides the presence of chloroplasts, another important difference between plant cells and animal cells is the presence of a cell wall. The cell wall surrounds the plasma membrane of plant cells and provides tensile strength and protection against mechanical and osmotic stress. It also allows cells to exert turgor pressure, the pressure of the cell contents against the cell wall. Plant cells have a high concentration of dissolved molecules in the cytoplasm, which under normal conditions causes water to enter the cell and cause the cells central vacuole to swell and press against the cell wall. With a healthy water supply, the turgor pressure prevents the plant from wilting. During drought, the plant may wilt, but its cell walls help maintain the structural integrity of its stems, leaves, and other structures despite shrinking, less abundant vacuoles. Plant cell walls are made primarily of cellulose, the most abundant macromolecule on Earth. Cellulose fibers are long linear polymers of hundreds of glucose molecules. These fibers are gathered into about 40 bundles called microfibrils. Microfibrils are embedded in a hydrated network of other polysaccharides. The cell wall will snap back into place. Precursor components are synthesized in the cell and then assembled by enzymes associated with the cell membrane (Fig. 3). What are vacuoles? Plant cells also have large fluid-filled sacs in the cytoplasm called vacuoles. Vacuoles usually make up about 30 percent of a cells volume, but can fill up to 90 percent of the intracellular space. Plant cells use vacuoles to regulate their size and turgor pressure. Vacuoles usually account for changes in cell size when the cytoplasmic volume remains the same.Some vacuoles have specific functions, and plant cells can have more than one type of vacuole. Vacuoles are associated with lysosomes and share certain functions with these structures; for example, both contain degradative enzymes to break down macromolecules. Vacuoles can also serve as storage compartments for nutrients and metabolites. For example, proteins are stored in seed vacuoles, and rubber and opium are metabolites stored in plant vacuoles. Summary Plant cells have some distinctive features, including chloroplasts, cell walls, and intracellular vacuoles. Photosynthesis takes place in chloroplasts; cell walls allow plants to build strong vertical structures; and vacuoles help regulate how cells deal with water and store other molecules. The cell wall is the outer layer that surrounds some cells and is outside the cell membrane. All cells have cell membranes, but usually only plants, fungi, algae, most bacteria, and archaea have cells with cell walls. The cell wall provides strength and structural support to the cell and can somewhat control the type and concentration of molecules that enter and leave the cell. The materials that make up the cell wall vary depending on the type of organism. The cell wall has evolved many times in different groups of organisms. The cell wall has several different functions. It is flexible, but provides strength to cells that helps protect the cell from physical damage. It also gives shape to the cell and allows the body to maintain a generally defined shape. The cell wall can also provide protection against pathogens such as bacteria that try to invade the cell. The structure of the cell wall allows many small molecules to pass through, but not larger molecules that could damage the cell. The main component of the plant cell wall is cellulose, a carbohydrate that forms long fibers and gives the cell wall its rigidity. A group of cellulose fibers is formed

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