Plant cells

Structure

• Plant cells have cell walls composed of cellulose, hemicelluloses, and pectin and constructed outside the cell membrane. Their composition contrasts with the cell walls of fungi, which are made of chitin, of bacteria, which are made of peptidoglycan and of archaea, which are made of pseudopeptidoglycan. In many cases lignin or suberin are secreted by the protoplast as secondary wall layers inside the primary cell wall. Cutin is secreted outside the primary cell wall and into the outer layers of the secondary cell wall of the epidermal cells of leaves, stems and other above-ground organs to form the plant cuticle. Cell walls perform many essential functions. They provide shape to form the tissue and organs of the plant, and play an important role in intercellular communication and plant-microbe interactions. The cell wall is flexible during growth and has small pores called plasmodesmata that allow the exchange of nutrients and hormones between cells.
• Many types of plant cells contain a large central vacuole, a water-filled volume enclosed by a membrane known as the tonoplast that maintains the cell's turgor, controls movement of molecules between the cytosol and sap, stores useful material such as phosphorus and nitrogen and digests waste proteins and organelles.
• Specialized cell-to-cell communication pathways known as plasmodesmata, occur in the form of pores in the primary cell wall through which the cell membrane and endoplasmic reticulum of adjacent cells are continuous.
• Plant cells contain plastids, the most notable being chloroplasts, which contain the green-colored pigment chlorophyll that converts the energy of sunlight into chemical energy that the plant uses to make its own food from water and carbon dioxide in the process known as photosynthesis. Other types of plastids are the amyloplasts, specialized for starch storage, elaioplasts specialized for fat storage, and chromoplasts specialized for synthesis and storage of pigments. As in mitochondria, which have a genome encoding 37 genes, plastids have their own genomes of about 100–120 unique genes and are interpreted as having arisen as prokaryoticendosymbionts living in the cells of an early eukaryotic ancestor of the land plants and algae.
• Cell division in land plants and a few groups of algae, notably the charophytes and the chlorophyte order Trentepohliales, takes place by construction of a phragmoplast as a template for building a cell plate late in cytokinesis.
• The motile, free-swimming sperm of bryophytes and pteridophytes, cycads and Ginkgo are the only cells of land plants to have flagella similar to those in animal cells. The conifers and flowering plants do not have motile sperm and lack both flagella and centrioles.