The document summarizes key information about cell walls. It discusses that cell walls provide structural support and protection for plant and prokaryotic cells. The material in cell walls varies by species but generally includes cellulose, hemicellulose, lignin and pectin. Cell walls give cells rigidity and strength while also acting as a semi-permeable filter. Plant cell walls in particular must withstand high internal turgor pressure.

2.  The cellwall is a tough, flexible but sometimes fairly rigid layer that surrounds some types of cells.
 It is located outside the cell membrane and provides these cells with structural support and
protection, in addition to acting as a filtering mechanism.

3.  A major function of the cell wall is to act as a pressure vessel, preventing over-expansion when water enters
the cell. Cell walls are found in plants and prokaryotic cells but not in mycoplasmas.
 The material in the cell wall varies between species, and can also differ depending on cell type and
developmental stage.

4.  The cell wall serves a similar purpose in organisms that possess them.
 The wall gives cells rigidity and strength, offering protection against mechanical
stress.
 The cell wall also limits the entry of large molecules that may be toxic to the cell.

5.  The primary cell wall of most plant cells is semi-permeable and permits the passage of small molecules and small proteins,
with size exclusion estimated to be 30-60 kDa.
 Key nutrients, especially water and carbon dioxide, are distributed throughout the plant from cell wall to cell wall in
apoplastic flow.
 The pH is an important factor governing the transport of molecules through cell walls.

6.  The rigidity of the cell walls is often overestimated.
 In most cells, the cell wall is flexible, meaning that it will bend rather than holding a fixed shape, but has considerable tensile strength.
 The apparent rigidity of primary plant tissues is enabled by cell walls, but not due to the walls' stiffness.
 Hydraulic turgor pressure creates this rigidity, along with the wall structure.
 The flexibility of the cell walls is seen when plants wilt, so that the stems and leaves begin to droop, or in seaweeds that bend in water
currents.

7.  The walls of plant cells must have sufficient tensile strength to withstand internal osmotic
pressures of several times atmospheric pressure that result from the difference in solute
concentration between the cell interior and external water. Plant cell walls vary from 0.1 to several
µm in thickness
Composition
 xylan, 20-35%, a type of hemicellulose
 lignin, 10-25%, a complex phenolic polymer that penetrates the spaces in the cell wall between
cellulose, hemicellulose and pectin components, driving out water and strengthening the wall.

8.  Like plants, algae have cell walls. Algal cell walls contain either polysaccharides
(such as cellulose (a glucan)) or a variety of glycoproteins (Volvocales) or both.
 The group of algae known as the diatoms synthesize their cell walls (also known as
frustules or valves) from silicic acid (specifically orthosilicic acid, H4SiO4). The acid
is polymerised intra-cellularly, then the wall is extruded to protect the cell.

9.  There are several groups of organisms that may be called "fungi".[vague] Some of these groups have
been transferred out of the Kingdom Fungi, in part because of fundamental biochemical
differences in the composition of the cell wall. Most true fungi have a cell wall consisting largely of
chitin and other polysaccharides.True fungi do not have cellulose in their cell walls.