Plant Cells, Chloroplasts, Cell Walls (2024)

Like mitochondria, chloroplasts likely originated from an ancient symbiosis, in this case when a nucleated cell engulfed a photosynthetic prokaryote. Indeed, chloroplasts resemble modern cyanobacteria, which remain similar to the cyanobacteria of 3 million years ago. However, the evolution of photosynthesis goes back even further, to the earliest cells that evolved the ability to capture light energy and use it to produce energy-rich molecules. When these organisms developed the ability to split water molecules and use the electrons from these molecules, photosynthetic cells started generating oxygen — an event that had dramatic consequences for the evolution of all living things on Earth (Figure 1).

Today, chloroplasts retain small, circular genomes that resemble those of cyanobacteria, although they are much smaller. (Mitochondrial genomes are even smaller than the genomes of chloroplasts.) Coding sequences for the majority of chloroplast proteins have been lost, so these proteins are now encoded by the nuclear genome, synthesized in the cytoplasm, and transported from the cytoplasm into the chloroplast.

Like mitochondria, chloroplasts are surrounded by two membranes. The outer membrane is permeable to small organic molecules, whereas the inner membrane is less permeable and studded with transport proteins. The innermost matrix of chloroplasts, called the stroma, contains metabolic enzymes and multiple copies of the chloroplast genome.

Chloroplasts also have a third internal membrane called the thylakoid membrane, which is extensively folded and appears as stacks of flattened disks in electron micrographs. The thylakoids contain the light-harvesting complex, including pigments such as chlorophyll, as well as the electron transport chains used in photosynthesis (Figure 2).

Besides the presence of chloroplasts, another major difference between plant 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 develop turgor pressure, which is the pressure of the cell contents against the cell wall. Plant cells have high concentrations of molecules dissolved in their cytoplasm, which causes water to come into the cell under normal conditions and makes the cell's central vacuole swell and press against the cell wall. With a healthy supply of water, turgor pressure keeps a plant from wilting. In drought, a plant may wilt, but its cell walls help maintain the structural integrity of its stems, leaves, and other structures, despite a shrinking, less turgid vacuole.

Plant cell walls are primarily made of cellulose, which is the most abundant macromolecule on Earth. Cellulose fibers are long, linear polymers of hundreds of glucose molecules. These fibers aggregate into bundles of about 40, which are called microfibrils. Microfibrils are embedded in a hydrated network of other polysaccharides. The cell wall is assembled in place. Precursor components are synthesized inside the cell and then assembled by enzymes associated with the cell membrane (Figure 3).

Plantcells additionally possess large, fluid-filled vesicles called vacuoles within their cytoplasm.Vacuoles typically compose about 30 percent of a cell's volume, but they canfill as much as 90 percent of the intracellular space. Plant cells use vacuolesto adjust their size and turgor pressure. Vacuoles usually account for changesin cell size when the cytoplasmic volume stays constant.

Somevacuoles have specialized functions, and plant cells can have more than onetype of vacuole. Vacuoles are related to lysosomes and share some functionswith these structures; for instance, both contain degradative enzymes forbreaking down macromolecules. Vacuoles can also serve as storage compartmentsfor nutrients and metabolites. For instance, proteins are stored in thevacuoles of seeds, and rubber and opium are metabolites that are stored inplant vacuoles.