The strings in celery are living cells built for stretchy support
Pull a string from a celery stalk and you are holding collenchyma, plant cells with walls thickened at the corners yet free of rigid lignin. They hold young stems upright while still stretching as the stem grows. It is one of many tricks that set plant cells apart from ours, from cellulose walls to giant water-filled vacuoles.
Plant cells have a recognisable toolkit. Their primary walls, built outside the membrane from cellulose, hemicelluloses and pectin, contrast with fungal walls of chitin and bacterial walls of peptidoglycan. Many cells add tougher secondary layers of lignin or suberin, and surface cells secrete cutin to form the waxy cuticle. Walls give tissues their shape yet stay flexible during growth, and tiny channels called plasmodesmata link neighbours, so that membranes and endoplasmic reticulum run continuously from cell to cell, carrying nutrients and hormones.
A large central vacuole, wrapped in a membrane called the tonoplast, keeps the cell pressurised, controls traffic between cytosol and sap, stores nutrients such as nitrogen and phosphorus, and breaks down waste proteins and old organelles. Plastids handle energy and storage: chloroplasts use chlorophyll to turn sunlight, water and carbon dioxide into food, while amyloplasts store starch, elaioplasts store fat and chromoplasts make pigments. Plastids carry their own genomes of about 100 to 120 genes, compared with 37 in mitochondria, a sign they began as bacteria living inside an early ancestor of plants and algae.
Division works differently too. Land plants and a few algal groups build a structure called a phragmoplast that guides construction of a cell plate between the two daughters. Most plant cells lack flagella and centrioles; the only exceptions are the swimming sperm of mosses, ferns, cycads and Ginkgo, while conifers and flowering plants have neither.
All tissues grow from meristem cells, the plant counterpart of stem cells. Parenchyma cells, which make up most of a leaf, handle storage, photosynthesis and nectar production, and some remain able to generate new undifferentiated cells throughout life; potato tubers are packed with the storage kind. Collenchyma provides flexible strength in growing parts, with walls rich in pectin and hemicellulose; in Petasites they may hold as little as 20 percent cellulose.
Source: Plant cell
