Exam III Flashcards
The cell walls all serve two common primary functions:
Regulating cell volume and determining cell shape
Despite this diversity in cell wall morphology, cell walls commonly are classified into two major types:
primary walls and secondary walls
Primary walls are formed by growing cells and are usually considered to be relatively unspecialized and similar in molecular architecture in all
cell types.
Secondary walls are the cell walls that form after cell growth (enlargement) has ceased. Secondary walls may become highly specialized in structure and composition, reflecting the differentiated state of the cell. Xylem cells, such as those found in wood, are notable for possessing
The middle lamella (plural lamellae)
A thin layer of material. Can usually be seen at the junction where the walls of neighboring cells come into contact. The composition of the middle lamella differs from the rest of the wall in that it is high in pectin and contains different proteins compared with the bulk of the wall. Its origin can be traced to the cell plate that formed during cell division.
The Primary Cell Wall Is Composed of Cellulose
Microfibrils Embedded in a Polysaccharide Matrix. This structure provides both strength and flexibility.
In the case of cell walls, the matrix (plural matrices) consists of two major groups of polysaccharides, usually called hemicelluloses and pectins, plus a small amount of structural protein
These polysaccharides are named after the principal sugars they contain.
For example, a glucan is a polymer made up of glucose, a xylan is a polymer made up of xylose, a galactan is made from galactose, and so on. Glycan is the general term for a polymer made up of sugars.
Hemicelluloses are flexible polysaccharides that characteristically bind to the surface of cellulose.
They may form tethers that bind cellulose microfibrils together into a cohesive network
Pectins form a hydrated gel phase in which the cellulose– hemicellulose network is embedded.
They act as hydrophilic filler, to prevent aggregation and collapse the cellulose network. They also determine the porosity of the cell wall to macromolecules
Cellulose Microfibrils Are Synthesized at the Plasma Membrane
Cellulose is a tightly packed microfibril of linear chains of (1→4)-linked β-D-glucose
The extensive noncovalent bonding between adjacent glucans within a cellulose microfibril gives this structure remarkable properties. Cellulose has a high tensile strength, equivalent to that of steel.
Cellulose is also insoluble, chemically stable, and relatively immune to chemical and enzymatic attack. These properties make cellulose an
excellent structural material for building a strong cell wall.
Matrix Polymers Are Synthesized in the Golgi and
Secreted in Vesicles. The matrix is a highly hydrated phase in which the cellulose microfibrils are embedded.
The major polysaccharides of the matrix are synthesized by membrane-bound enzymes in the Golgi apparatus and are delivered to the cell wall via exocytosis of tiny vesicles
(1→4)-linked β-D-glucose chain —-> Cellulose Microfibril
CesA —-> Rosette Subunit —> Rosette
Possible model of cell wall synthesis in plants
Draw and label
Glucose residues are donated to the growing glucan chains by UDP-glucose (UDP-G). Sucrose synthase may act as a metabolic channel to transfer glucose taken from sucrose to UDP-glucose, or UDP-glucose may be obtained directly from the cytoplasm
