Q1.
Cells of plant, fungi, and bacteria have an additional layer around the cell membrane, called the cell wall. What do you think is the necessity of the cell wall in these cells?
Answer
These organisms cannot move away from trouble, so they need a rigid outer casing that gives shape, strength and protection while still letting water and minerals through.
- Mechanical support. Plants stand upright, and leaves and flowers stay firm, because millions of walled cells press against one another. Wind and rain are resisted by the wall, not by the thin membrane.
- Protection against bursting. Root cells sit in dilute soil water, which is hypotonic, so water keeps entering by osmosis. Without a wall the cell would swell until it burst. The wall pushes back and the inward flow stops.
- Shape. A walled cell keeps its box shape even after losing water — this is exactly why onion peel and Rhoeo cells look regular and rectangular under a microscope while cheek cells look irregular.
- Still permeable. The wall is rigid but freely permeable, so water and dissolved minerals pass straight through it. The selection of what enters is left to the membrane just inside.
Plant cell wall is made mainly of cellulose
cellulose = many glucose units linked together (a carbohydrate)
cellulose = many glucose units linked together (a carbohydrate)
Why it happens: an animal solves the same problems in a different way — it has a skeleton and it can walk away from wind, drought or a predator. A plant is fixed in one place, so its support has to be built into every single cell. The trade-off is flexibility: because animal cells have no wall they can change shape, which is what lets muscle contract, white blood cells squeeze between tissues and an embryo fold into an organism.
Did you know? Cellulose in your diet cannot be digested by human enzymes, so it passes through as roughage and helps digestion by adding bulk to the food.